LIBRARY
FACULTY OF FORESTRY
UNIVERSITY OF TORONTO
cbc IRural Ueit=J6ooh Series
EDITED in L. 11. BAILEY
SOILS
THEIR PROPERTIES AND MANAGEMENT
Strict
EDITED BY L. H. BAILEY
Carleton, THE SMALL GRAINS.
B. M. Duggar, PLANT PHYSIOLOGY, "with
special reference to Plant Production.
J, F. Duggar, SOUTHERN FIELD CROPS.
Gay, THE BREEDS OF LIVE-STOCK.
Gay, THE PRINCIPLES AND PRACTICE OF
JUDGING LIVE-STOCK.
Goff, THE PRINCIPLES OF PLANT CULTURE,
Revised.
Harper, ANIMAL HUSBANDRY FOR SCHOOLS.
Harris and Stewart, THE PRINCIPLES OF
AGRONOMY.
Hitchcock, A TEXT -BOOK OF GRASSES.
Jeffery, TEXT-BOOK OF LAND DRAINAGE.
Livingston, FIELD CROP PRODUCTION.
Lyon, Pippin and Buckman, SOILS THEIR
PROPERTIES AND MANAGEMENT.
Mann, BEGINNINGS IN AGRICULTURE.
Montgomery, THE CORN CROPS.
Piper, FORAGE PLANTS AND THEIR CULTURE.
Warren, ELEMENTS OF AGRICULTURE.
Warren, FARM MANAGEMENT.
Wheeler, MANURES AND FERTILIZERS.
White, PRINCIPLES OF FLORICULTURE.
Widtsoe, PRINCIPLES OF IRRIGATION PRAC-
TICE.
SOILS
THEIR PROPERTIES AND MANAGEMENT
BY
T. LYTTLETON LYCXN", PH.D.
PROFESSOR OF SOIL TECHNOLOGY, CORNELL UNIVr,i.SITY
ELMER 0. FIPPIN, B.S.A.
EXTENSION PROFESSOR OF SOIL TECHNOLOGY
CORNELL UNIVERSITY
HARRY 0. BUCKMAN, PH.D.
ASSISTANT PROFESSOR OF SOIL TECHNOLOGY
CORNELL UNIVERSITY
A
THE MACMILLAN COMPANY
1920
All rights reserved
COPYRIGHT, 1909 AND 1915,
BY THE MACMILLAN COMPANY.
Set tip and electrotyped. Published September, 1915.
LT1
Norfnooti
J. 8. Gushing Co. Berwick & Smith Co.
Norwood, Mass., U.S.A.
ACKNOWLEDGMENTS
THE authors wish to acknowledge the originals of
the two colored maps as occurring in Bulletins 60 and
96 of the Bureau of Soils, U.S. Department of Agricul-
ture. The remaining illustrations were drawn es-
pecially for this text. Some few are copies in part, or
in whole, from other authors. Acknowledgments not
made in text are due the Bureau of Soils, U. S. De-
partment of Agriculture ; T. C. Chamberlin ; E. W.
Hilgard; F. H. King ; G. P. Merrill ; and H. W. Wiley.
The authors wish also to express their appreciation to
Messrs. A. B. Beaumont and J. H. Bromley for their
careful work in the preparation of the original drawings
and to Miss Lela G. Gross for aid on the manuscript.
Thanks are also due Professor W. D. Bancroft for sug-
gestions as to the theoretical phases of soil colloids
and to Professor J. L. Stone for a like favor regarding
the soundness of the chapter on fertilizer practice.
TABLE OF CONTENTS
CHAPTER I
PAGES
SOME GENERAL CONSIDERATIONS 1-12
Composition of the soil, 1 Factors for plant growth,
2 Plant food elements, 3 Abundance of plant food
elements, 4 Soil-forming rocks, 5 Soil-forming min-
erals, 6 Relative abundance of minerals, 7 Organic
matter, 8 The soil and the plant, 9.
CHAPTER II
SOIL-FORMING PROCESSES ....... 13-30
Water, 10 Wind, 11 Ice, 12 Heat and cold, 13
Frost, 14 Plants and animals, 15 Oxidation and
carbonation, 16 Deoxidation, 17 Hydratkm, 18
Solution, 19 A general statement of weathering, 20
Factors affecting weathering, 21 Law of mineral and
rock decay, 22 Special cases of weathering, 23 Prac-
tical relationships of weathering, 24.
CHAPTER III
GEOLOGICAL CLASSIFICATION OF SOILS . . . 31-45
Residual soils, 25 Distribution of residual soils, 26
Cumulose soils, 27 Colluvial soils, 28 Alluvial
soils, 29 Distribution of alluvial soils, 30 Marine
soils, 31 Characteristics of marine soils, 32 Distri-
bution of marine soils, 33.
CHAPTER IV
'GEOLOGICAL CLASSIFICATION OF SOILS (Continued) . . 46-64
The ice sheet, 34 The American ice sheet, 35
Cause of the ice age, 06 The extension of the ice sheet,
37 The ice as a soil builder, 38 Glacial till soils, 39
vii
v
TABLE OF CONTENTS
Composition of glacial soils, 40 Humus of glacial
soils, 41 Glacial lakes, 42 Lacustrine soils glacial
lake, 43 Lacustrine soils recent lake, 44 JEolian
soils, 45 Loess soils, 46 Distribution of loess, 47
Adobe soils, 48 Sand dunes, 49 Volcanic dust, 60.
CHAPTER V
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS OF SOILS . 65-82
Climatic relationships, 51 Geochemical relationships
of residual and marine soils, 52 Residual and glacial
soils, 53 Effect of glaciation on agriculture, 54 Hu-
mid and arid soils, 55 Soil color, 56 White and
black soils, 57 Red and yellow soils, 58 Agricul-
tural significance of color, 59 Soil and subsoil, 60
Soil and subsoil of humid regions, 61 Soil and subsoil
of arid regions, 62.
CHAPTER VI
SOIL PARTICLE ........ 83-107
Soil separates and mechanical analysis, 63 Princi-
ples of mechanical analysis, 64 Mechanical analysis
by water in motion. Schone elutriator, 65 Hilgard's
churn elutriator, 66 Yoder's centrifugal elutriator, 67
Mechanical analysis by water at rest Osborne's
beaker method, 68 Atterberg's modified Appiani silt
cylinder, 69 Centrifugal soil analysis, 70 Classifica-
tion of soil particles, 71 Bureau of Soils classification,
72 Physical character of the separates, 73 Mineral-
ogical characteristics of the separates, 74 The chem-
ical constitution of soil particles, 75 Value of a
mechanical analysis, 76 Soil class, 77 Determination
of class, 78 The significance of texture and class, 79.
CHAPTER VII
PHYSICAL PROPERTIES OF THE SOIL .... 108-126
Arrangement of soil particles, 80 The absolute spe-
cific gravity of the soil, 81 Apparent specific gravity,
TABLE OF CONTENTS
IX
82 Actual weight of a soil, 83 Pore space in soil, 84
The number of soil particles, 85 Surface exposed
by soil particles, 86 The effective mean diameter of
soil particles, 87.
CHAPTER VIII
THE ORGANIC MATTER OF THE SOIL
The source and distribution of organic matter, 88
Composition of plants, 89 Decay of organic matter in
soils, 90 Composition of soil humus, 91 The work of
Oswald Schreiner, 92 Toxic material in the soil, 93
End products of humus decay, 94 Carbonized mate-
rials of soil, 95 The estimation of soil organic matter,
96 The estimation of soil humus, 97 The organic
content of representative soils, 98 The humus content
of soil, 99 The influence joi the original material on
the resultant Tiuinus, 100 Effects of organic matter on
Maintenance of soil organic matter, 102.
126-152
CHAPTER IX
>-THE COLLOIDAL MATTER OF SOILS 153-169
The colloidal state, 103 The properties of colloids,
104 Colloidal phases, 105 Flocculation, 106 Com-
mon soil colloids and their generation, 107 Prepara-
tion of colloids, 108 Colloids and soil properties, 109
Factors affecting colloids, 110 Estimation of col-
loidal content, 111.
CHAPTER X
L STRUCTURE 170-197
Plasticity, 112 The cause of plasticity, 113 The
importance of plasticity, 114 Cohesion, 115 Methods
of determining cohesion, 116 Factors affecting cohe-
sion, 117 Moisture limits for successful tillage, 118
Control of cohesion and plasticity, 119 Soil tilth, 120
Granulation, 121 Forces facilitating granulation,
122 Wetting and drying, 123 Freezing and thawing,
TABLE OF CONTENTS
124 Addition of organic matter, 125 Action of plant
roots and animals, 126 Addition of lime, 127 Til-
lage, 128 The action of the plow, 129 Resume^ 130.
CHAPTER XI
FORMS OF SOIL WATER AND THEIR MOVEMENT . .
Methods of expressing soil moisture, 131 Kinds of
water in the soil, 132 Hygroscopic water, 133 Effects
of texture and humus on hygroscopicity, 134 Nature
of the film, 135 Effect of humidity and temperature
on hygroscopic water, 136 Determination of hygro-
scopicity, 137 Heat of condensation, 138 Capillary
water, 139 Surface tension and the force developed
thereby, 140 The form of water surfaces between soil
particles, 141 Factors affecting capillary water, 142
Surface tension and the amount of capillary water, 143
Texture and the amount of capillary water, 144
Effect of structure on the amount of capillary moisture,
145 Organic matter and the amount of capillary mois-
ture, 146 Determination of capillary water, 147 The
moisture equivalent of soils, 148 The maximum re-
tentive power of soils, 149 Capillary Movement, 150
Factors affecting rate and height of Capillary move-
ment, 151 Effect of thickness of film on capillary
movement, 152 Surface tension and capillary move-
ment, 153 Effect of texture on capillary movement,
154 Texture and capillary pull of soils, 155 Effect
of structure on capillary movement, 156 Gravitational
water, 157 Pressure and the movement of gravity
water, 158 Effect of temperature on the flow of grav-
ity water, 159 Effect of texture and structure on the
flow of gravity water, 160 Determination of the quan-
tity of free water that a soil will hold, 161 The calcu-
lation of the free water of a soil, 162 Value of studying
flow and composition of gravitational water, 163 The
study of gravity water by means of tile drains, 164
The lysimeter method of studying gravitational water,
165 Thermal movement of water, 166.
198-242
TABLE OF CONTEXTS xi
CHAPTER XII
PAGES
THE WATER OF THE SOIL IN ITS RELATION TO PLANTS . 243-263
Relations of water to the plant, 167 The water
requirement of plants, 168 Factors affecting trans-
piration, 169 Effect of crop and climate on transpira-
tion, 170 Effect of soil moisture on transpiration, 171
Influence of fertility on transpiration, 172 Effect of
texture on transpiration, 173 Actual amounts of water
necessary to mature a crop, 174 Role of capillarity in
the supplying of the plant with water, 176 Influence
of water on the plant, 176 Availability of the water in
the soil, 177 Unavailable soil water, 178 The wilting
coefficient of plants, 179 Determination of the wilting
point, 180 Calculation of the wilting point, 181 Re-
lation of texture to the wilting point, 182 Available
and superavailable water, 183 Optimum moisture for
plant growth, 184.
CHAPTER XIII
THE CONTROL or SOIL MOISTURE ..... 264-288
Run-off losses, 185 Percolation losses, 186 Methods
of checking loss by run-off and leaching, 187 Evapo-
ration losses, 188 Methods of checking evaporation
losses, 189 Mulches for moisture control, 190 Kinds
of mulches, 191 The functions of a mulch, 192 The
soil mulch versus the dust mulch, 193 Formation of a
mulch, 194 Depth of a mulch, 195 Re'sume' of mulch
control, 196 Water saved by a mulch, 197 Effect of
mulches other than on moisture, 198 General useful-
ness of a mulch, 199 Other practices affecting evapo-
ration losses, 200 Fall and early spring plowing, 201
Rolling, 202 Shelters, 203 Level cultivation, 204
Plants, 205 Summary of moisture control, 206.
CHAPTER XIV
SOIL HEAT 289-326
Relation of heat to germination and growth, 207
Chemical and physical changes due. to heat, 208
Xll
TABLE OF CONTENTS
Sources of soil heat, 209 Factors affecting soil tem-
perature, 210 Specific heat, 211 Variations of spe-
cific heat, 212 Specific heat based on volume of soil,
213 Effect of water on specific heat, 214 Absorptive
power of soils for heat, 215 Effect of color on absorp-
tion of heat, 216 Effects of texture and structure on
heat absorption, 217 Radiation of heat by soil, 218
Conductivity and convection of heat in soils, 219
Measurement of conductivity, 220 Effect of texture
on conductivity of heat, 221 Effects of humus and
structure on conductivity, 222 Influence of moisture
on heat conductivity in soil, 223 Effect of evaporation
of water on soil temperature, 224 Effect of organic
decay on soil temperature, 225 Relation of slope to
soil temperature, 226 Heat supply and its effects, 227
Control of soil temperature, 228.
CHAPTER XV
AVAILABILITY OF PLANT NUTRIENTS AS DETERMINED BY
CHEMICAL ANALYSIS ......
Solubility of the soil in various solvents, 229 Com-
plete solution of the soil, 230 Partial solution with
strong acids, 231 Significance of a strong hydrochloric
acid analysis, 232 Relation of texture to solubility,
233 Nature of the subsoil, 234 Calcium carbonate,
235 Deficiency of ingredients and manurial needs, 236
Partial solution with weak acids, 237 Advantages
in the use of dilute acids, 238 The one-per-cent citric
acid method, 239 Usefulness of the citric acid method,
240 Dilute mineral acids, 241 Extraction with an
aqueous solution of carbon dioxide, 242 Extraction
with pure water, 243 Influence of absorption, 244
Other factors influencing extraction, 245 The soil solu-
tion in situ, 246 Devices for obtaining a soil solution,
247 Composition and concentration of the soil solu-
tion, 248 Variability in composition and concentration
of the soil solution, 249 Discussion of the theories
regarding soil solutions, 250.
327-348
TABLE OF CONTENTS
xiu
CHAPTER XVI
THE ABSORPTIVE PROPERTIES OP SOILS ....
Substitution of bases, 251 Time required for absorp-
tion, 252 Insolubility of certain absorbed substances,
253 Influence of size of particle, 254 Causes of ab-
sorption, 255 Zeolites, 256 Chabazite, 257 Pres-
ence of zeolites questioned, 258 Absorption of phos-
phoric acid, 259 Formation of insoluble phosphates,
260 Absorption, 261 Absorption by colloids, 262
Absorptive properties of colloidal matter, 263 Selective
absorption, 264 Absorptive power of colloidal silicates,
265 Absorption by colloids versus absorption by zeo-
lites, 266 Absorption by organic matter, 267 Ab-
sorption of water vapor and of gases by soils, 268
Absorption of ammonia, 269 Absorption of carbon
dioxide, 270 Absorption of nitrogen and oxygen, 271
Relation of temperature to gas absorption, 272
Relation of absorptive capacity to productiveness, 273
Absorption as related to drainage, 274 Substances
usually carried in drainage water, 276 Drainage rec-
ords at Rothamsted, 276 Drainage records at Brom-
berg, 277 Losses of nitrogen and calcium, 278
Composition of surface water, 279.
PAGES
349-374
CHAPTER XVII
ACID OR SOUR SOILS
Nature of soil acidity, 280 Positive acidity, 281
Negative acidity, 282 Production of sour soils, 283
Removal of bases by drainage as a cause for acidity, 284
Removal of bases by plants, 285 Effect of green
manures on acidity, 286 Effect of fertilizers on soil
acidity, 287 Acidity in relation to climate and to for-
mation of soil, 288 Weeds that flourish on sour soils,
289 Crops adapted to sour soils, 290 Crops that are
injured by acid soils, 291 Qualitative tests for acidity,
292 Litmus paper test, 293 Ammonia test, 294
Zinc sulfide test, 295 Litmus paper and potassium
nitrate, 296 Acid test for carbonates, 297 Plants as
376-390
XIV TABLE OF CONTENTS
PAGES
indicators of acidity, 208 Quantitative determinations
of acidity, 299 Potassium nitrate method, 300 Lime-
water method, 301 Resume', 302.
CHAPTER XVIII
ALKALI SOILS 391-403
Composition of alkali salts, 303 White and black
alkali, 304 Effect of alkali on crops, 305 Effect on
different plants, 306 Other conditions that influence
the action of alkali, 307 Accumulation of alkali, 308
Irrigation and alkali, 309 Handling of alkali lands, 310
Eradication of alkali, 311 Leaching with under-
drainage, 312 Correction with gypsum, 313 Scraping,
314 Flushing, 315 Control of alkali, 316 Cropping
with tolerant plants, 317 Alkali spots, 318.
CHAPTER XIX
ABSORPTION OF NUTRITIVE SALTS BY AGRICULTURAL PLANTS 404-420
How plants absorb nutrients, 319 Relation between
root-hairs and soil particles, 320 Liebig and Sachs on
solvent action of plant-roots, 321 Czapek's experiment,
322 Secretion of an oxidizing enzyme by plant-roots,
323 Importance of carbon dioxide as a solvent, 324
Insufficiency of carbon dioxide, 325 The present status
of the question, 326 Possible root action on colloidal
complexes, 327 Why crops vary in their absorptive
powers, 328 Extent of absorbing systems, 329 Ab-
sorptive activity, 330 The absorptive power of cereals,
331 The feeding of grass crops, 332 Leguminous
crops, 333 Root crops, 334 Vegetables, 335 Fruits,
336 Mineral substances absorbed by plants, 337 Re-
lation of plant growth to concentration of nutrient solu-
tion, 338 Quantities of plant food materials removed
by crops, 339 Quantities of plant food materials con-
tained in soils, 340 Possible exhaustion of mineral
nutrients, 341.
TABLE OF CONTENTS
XV
CHAPTER XX
/-"""ORGANISMS IN THE SOIL 421-442
Macroorganisms.
Rodents, 342 Worms, 343 Insects, 344 Large
fungi, 345 Plant root, 346.
Microorganisms.
Plant microorganisms, 347 Plant microorganisms
injurious to higher plants, 348 Plant microorganisms
not injurious to higher plants, 349 Bacteria, 350
Distributionof bacteria, 351 Numbers of bacteria, 352
Numbers as influenced by season, 353 Conditions
affecting growth, 354 Oxygen, 355 Moisture, 366
Temperature, 357 Organic matter, 358 Soil acidity,
359 Functions of soil bacteria, 360 Decomposition
of mineral matter, 361 Influence of certain bacteria
and molds on the solubility of phosphates, 362 Decom-
position of non-nitrogenous organic matter, 363 De-
composition of nitrogenous organic matter, 364.
CHAPTER XXI
; NITROGEN CYCLE 443-474
Decay and putrefaction, 365 Ammonification, 366
Bacteria and substances concerned in ammonification,
367 Nitrification, 368 Effect of organic matter on
nitrification, 369 Effect of soil aeration on nitrification,
370 Effect of sod on nitrification, 371 Depths at
which nitrification takes place, 372 Loss of nitrates
from the soil, 373 Nitrate reduction, 374 Nitrate
assimilating organisms, 375 Denitrification, 376 Ni-
trogen fixation through symbiosis with higher plants,
377 Relation of bacteria to nodules on roots, 378
Transfer of nitrogen to the plant,' 379 Soil inoculation
for legumes, 380 Nitrogen fixation without symbiosis
with higher plants, 381 Nitrogen-fixing organisms, 382
Mixed cultures of nitrogen-fixing organisms, 383
Nitrogen fixation and denitrification antagonistic, 384.
XVI
TABLE OF CONTENTS
Treatment of Soils with Volatile Antiseptics and Heat.
Effects of carbon bisulfide and heat on properties of
soils, 385 Hypotheses to account for effects of carbon
bisulfide and of heat, 386 Koch's theory, 387 Hiltner
and Stormer's theory, 388 Russell and Hutchinson's
theory, 389 Greig-Smith's theory, 390.
CHAPTER XXII
THE SOIL AIR 475-488
Factors that Determine Volume.
Texture, 391 Structure, 392 Organic matter, 393
Moisture content, 394.
Composition of Soil Air.
Analyses of soil air, 395 Sources of carbon dioxide
in soil air, 396 Production of carbon dioxide as affect-
ing composition, 397.
Functions of Soil Air.
Oxygen, 398 Carbon dioxide, 399.
Movement of Soil Air.
Diffusion of gases, 400 Movement of water, 401
Changes in atmospheric pressure, 402 Changes of tem-
perature in atmosphere or in soil, 403 Suction produced
by wind, 404.
Methods for Modifying the Volume and Movement of
Soil Air.
Tillage, 405 Manures, 406 Underdrainage, 407
Irrigation, 408 Cropping, 409.
CHAPTER XXIII
COMMERCIAL FERTILIZERS 489-533
Early ideas of the function of manures, 410 Devel-
opment of the idea of the nutrient function of manures,
411 Classes of manures, 412 Commercial fertilizers,
413 Fertilizer constituents, 414.
Fertilizers Used for their Nitrogen.
Forms in which nitrogen exists in soils, 415 Forms
in which nitrogen is absorbed by plants, 416 Use of
nitrates by plants, 417 Ammonia as a plant food, 418
TABLE OF CONTENTS
XV11
Utilization of humus compounds by plants, 419
Sodium nitrate, 420 Ammonium sulfate, 421 Ferti-
lizers containing atmospheric nitrogen, 422 Cyanamid,
423 Composition of cyanamid, 424 Changes of cal-
cium cyanamid in the soil, 425 The use of cyanamid,
426 Calcium nitrate, 427 Organic nitrogen in ferti-
lizers, 428 Availability of organic nitrogenous ferti-
lizers, 429.
Fertilizers Used for their Phosphorus.
Bone phosphate, 430 Mineral phosphates, 431
Superphosphate fertilizers, 432 Reverted phosphoric
acid, 433 Relative availability of phosphate fertilizers,
434 Changes that occur when superphosphate is added
to soils, 435 Other factors influencing the availability
of tricalcium phosphate, 436 Effect of plants on the
availability of tricalcium phosphate, 437 Effect of
basicity on tricalcium phosphate, 438 Influence of fer-
menting organic matter, 439 Influence of other salts,
440.
Fertilizers Used for their Potassium.
Stassfurt salts, 441 Wood ashes, 442 Insoluble
potassium fertilizers, 443.
Sulfur and Sulfates as Fertilizers.
The use of free sulfur, 444 Sulfur as sulfate, 445.
Catalytic Fertilizers.
Nature of catalytic action, 446 Catalytic action of
soils, 447 Substances used as catalytic fertilizers, 448
Manganese, 449 Physiological role of manganese,
450 Action of manganese as a fertilizer, 451 Forms
of manganese and response of soils, 452.
CHAPTER XXIV
SOIL AMENDMENTS ;
Salts of calcium, 453 Effect on tilth and bacterial
action, 454 Liberation of plant food materials, 455
Influence of lime on the formation of nitrates in soil, 456
Effect on toxic substances and plant diseases, 457
534-545
XV111
TABLE OF CONTENTS
The lime-magnesia ratio, 458 Forms of calcium, 459
Caustic limes, 460 Carbonate of lime, 461 Relative
effectiveness of caustic lime and carbonate, 462 Sul-
fate of calcium, 463 Common salt, 464 Muck, 465.
CHAPTER XXV
FERTILIZER PRACTICE .
546-576
Effects of nitrogen on plant growth, 466 Effects of
phosphorus on plant growth, 467 Effects of potassium
on plant growth, 468 Law of the minimum, 469 Fer-
tilizer brands, 470 Fertilizer inspection and control,
471 Trade values of fertilizers, 472 The buying of
mixed goods, 473 Home-mixing fertilizers, 474 Fer-
tilizers not to be mixed, 475 How to mix fertilizers,
476 Factors affecting the efficiency of fertilizers, 477
Method and time of applying fertilizers, 478 Fertiliz-
ing crops, 479 Systems of fertilization, 480.
CHAPTER XXVI
FARM MANURES
General character and function of farm manures, 481
Variable composition of manures, 482 Litter, 483
Class of animal, 484 Individuality, condition, and age
of animal, 485 Food of animal, 486 Handling of the
manure, 487 Composition and character of farm ma-
nures, 488 Actual plant-food in liquid and solid excre-
ment, 489 Production of manure, 490 Heiden's
formula, 491 Poultry manure, 492 Commercial and
agricultural evaluation of manures, 493 The fermenta-
tion of manure, 494 Aerobic action, 495 Anaerobic
action, 496 Fermentation in general, 497 Gases from
manures, 498 Change of bulk and composition of rot-
ting manure, 499 Fire-fanging of manure, 500
Waste of farm manures, 501 Losses due to digestion,
502 Losses due to leaching and fermentation, 503
Increased value of protected manure, 504 The money
waste of manure, 505 Handling of manures, 506
Care of manure in the stalls, 507 Hauling directly to
577-618
TABLE OF CONTENTS
the field, 508 Cement pit, 509 Covered barnyard,
510 Piles outside, 511 Distribution of manure in the
field, 512 Reinforcement of manure, 513 Benefits
from reinforcing, 514 Lime and manure, 515 Com-
posting, 516 Manure and muck, 517 Effects of ma-
nure on the soil, 518 Residual effect of manure, 519
Place of manure in the rotation, 520 Re'sume', 521.
XIX
PAGES
CHAPTER XXVII
GREEN MANURES
Effects of green-manuring, 522 Quantities of plant
constituents added by green-manuring, 523 Decay of
green manure, 524 Crops suitable for green manures,
525 When to use green manures, 526 When to turn
under green crops, 527 How to turn under green mate-
rial, 528 Green manures and lime, 529 Green manure
and the rotation, 530.
619-626
CHAPTER XXVIII
LAND DRAINAGE .
. 627-<
Extent of drainage needed in humid regions, 531
History of drainage, 582 Effects of land drainage on
the soil, 533 Methods of drainage, 534 Construction
of small open ditches, 535 Construction of large open
ditches, 536 Construction of early types of under-
drains, 537 Stone drains, 538 Tile drains, 589
Quality of tile, 540 Shapes of tile, 541 Protection of
joints, 542 Entrance of roots into tile, 543 Protection
of joints on curves, 544 Foundation for tile, 545
Arrangement of drainage systems, 546 Grade of tile
drains, 547 Depth of drains, 548 Distance between
drains, 549 Construction of drainage trenches for tile,
650 Laying tile, 551 Size of tile, 552 Amount of
water to be removed from land, 553 Carrying capacity
of a tile-drain system, 554 Cost of drainage, 555
Storm channels, 556 Silt basins, 557 Surface intakes,
658 Outlets, 559 Muck and peat soil, 560 Drain-
XX
TABLE OF CONTENTS
age of irrigated and alkali lands, 561 Vertical drainage,
562 Drainage by means of explosives, 563 Re'sume',
564.
CHAPTER XXIX
TILLAGE 663-681
Objects of tillage, 565 Implements of tillage, 566
Effects on the soil, 567 Classes of tillage implements,
568 Plows, 569 Pulverizing action of the plow, 570
Types of plows, 571 Shapes of moldboard plows,
572 Position of the furrow slice, 573 Depth and
width of furrow, 574 Plow sole, 575 Hillside plow,
576 Covering rubbish, 577 Subsoil plow, 578
Cultivators, 579 Cultivators proper, 580 Leveler
and harrow types of cultivator, 581 Seed cultivators,
582 Packers and crushers, 583 Rollers, 584 Clod
crushers, 585 Efficient tillage, 586.
CHAPTER XXX
IRRIGATION AND DRY FARMING .
, 682-717
Relation of irrigation to rainfall, 587 Extent of irri-
gated land, 588 History of irrigation, 589 Develop-
ment of irrigation practice in the United States, 590
Irrigation in humid regions, 591 The Reclamation
Service, 592 Legal, economic, and social effects of
irrigation, 593 Divisions of irrigation, 594 Source of
water for irrigation, 595 Canals, 596 Preparation of
land for irrigation, 597 Methods of applying water,
598 Overhead sprays, 599 Subirrigation, 600
Methods most used in arid regions, 601 Flooding, 602
Furrows, 603 Size and form of furrows, 604 Unit
of measurement, 605 Amount of water to apply, 606
Time to apply water, 607 Conservation of moisture
after irrigation, 608 Sewage irrigation, 609.
Dry Farming.
Practices in dry farming, 610 Storage of water in
the soil, 611 Conservation of moisture, 612 Alternate
cropping, 613 Drought-resistant crops, (514 Soils
TABLE OF CONTENTS XXI
PAGES
associated with dry farming, 615 Extent of dry farm-
ing, 616.
CHAPTER XXXI
THE SOIL SURVEY 718-740
The classification of soils by survey, 617 Factors
employed in classification, 618 Texture, the soil class,
619 Special properties, the soil series,. 620 Source of
material, the soil group, 621 Agency of formation, the
soil province, 622 Climate, 623 The practical classi-
fication of soils in the United States, 624 The soil type
and soil series, 625 The equipment for survey work,
626 Procedure in the field, 627 Collection of soil
samples, 628 The accuracy and detail of the soil sur-
vey, 629 The soil survey report, 630 The soil map,
631 The extent of soil surveys in the United States,
632 Surveys by state institutions, 633 Surveys in
other countries, 634 Use of the soil survey, 635.
SOILS: THEIR PROPERTIES
AND MANAGEMENT
CHAPTER I
SOME GENERAL CONSIDERATIONS
THE broken and weathered fragments of rock that
cover in a thin layer the solid part of the earth and that
furnish the foothold and, in part, the sustenance for plant
life, are termed soil. Soil comes from rock and returns
to rock. It is merely a transitory stage in the change
from one form of rock to another. It is never still. From
the time when the particle leaves the disintegrating rock
until it is again cemented in the skeleton of the earth,
it is subjected to almost constant movement and to the
action of numerous forces that change it chemically and
physically. It is the movement, the strain and the
stress, the hard treatment at the hands of disintegrating
agencies, that make the soil useful to plant life.
It was only the simpler forms of plants, however, that
first throve on the pulverized rock. Tribe after tribe of
plants has invaded the soil. Each has wrested from it
the mineral matter necessary for its growth and develop-
ment. Each has, in the end, left not only the mineral
matter that it obtained from the disintegrated rock, but
also the carbon and the oxygen that had been won from the
2 SOILS: PROPERTIES AND MANAGEMENT
air in the struggle for life. Primitive plants have been
followed by more highly organized ones as the incursions
have gone on, and always to the profit of the soil, until
the soil has accumulated a great store of organic matter
and a teeming population of microscopic life.
This debris of rock and plant residue that has accumu-
lated through the centuries of struggle is the arable soil
from which man obtains his bread. The study of this
soil is a history of strife and struggle, and as the light of
investigation is turned on it, new contestants, new opera-
tions, new results, and new principles are brought to view
and the story must be retold.
1. Composition of the soil. Broadly speaking, the
soil is composed of two general classes of materials, rock
and organic matter. The former usually makes up the
bulk of the soil, while the latter occurs under normal
conditions in relatively small amounts. In spite of this
low proportion, however, its presence is of vital impor-
tance to productivity. The soil has also three general
phases the physical, the chemical, and the biological.
In the physical phase, the size and shape of particle, the
movement of air and water, and other physical proper-
ties are dealt with ; in the chemical phase, the composi-
tion of the particle, of the organic matter, and of the soil
solution is of dominant importance ; in the biological phase,
the soil is seen to be not an inert material, but teeming
with life minute forms of life, to be sure, but of great
importance in the manufacture of food for plants. Under
these three general phases, then, the changes going on in
a soil may be studied, and they are found to be directed
primarily toward the production and maintenance of con-
ditions favorable for plant growth. The soil is not a
simple medium to study, but is extremely complicated
SOME GENERAL CONSIDERATIONS 3
for two reasons : first, because of the complicated nature
of its two general constituents ; and secondly, because of
the action and interaction of these constituents with each
other.
2. Factors for plant growth. The growth and devel-
opment of a plant are largely the result of two sets of
factors, the internal and the external. The former de-
pends on the nature of the plant itself, the latter on its
environment. The external factors of plant growth under
normal conditions may be classified as follows : (1) me-
chanical support, (2) air, (3) heat, (4) light, (5) water, and
(6) food. With the exception of light, the soil supplies,
either wholly or in part, all the conditions named. As
a mere mass of ground-up rock with which are mixed
varying quantities of decayed organic matter, the soil
acts as a medium for root development and thereby pro-
vides a foothold for the plant. Air, heat, and water are
supplied as a consequence of the inherent physical condi-
tion of a soil. The circulation of water serves to bring
food into solution for absorption by the rootlets* Thus
the two prime functions of the soil are realized the
supplying of plant-food and of a foothold for plant life.
3 . Plant-food elements - 1 While the physical condition
of the soil has tremendous influence on plant growth, the
food elements must first be considered, since their avail-
ability is so closely related to the factors that function
in soil formation. Ten elements are usually considered
as absolutely necessary for plant growth. They may be
classified as follows :
1 For a complete discussion of the plant-food elements as re-
lated to the plant, see Russell, E. J. Soil Conditions and Plant
Growth, Chapter II, pp. 30-46. New York City. 2d edition,
19' 5.
4 SOILS: PROPERTIES AND MANAGEMENT
Elements obtained from Elements l coming directly from
air or water the soil itself
Carbon Nitrogen Magnesium
Oxygen Phosphorus Iron
Hydrogen Potassium Sulfur
Nitrogen Calcium
Carbon is obtained very largely by the plant directly
from the air as carbon dioxide (CO 2 ) ; while oxygen
comes directly from the atmosphere or from water, which
is also the source of at least a part of the hydrogen utilized
in vegetative growth. The other elements, except in
the case of leguminous crops, are taken wholly from the
soil solution itself.
While all these elements found in the soil must be
available in order that plants may grow normally, only
a very few ever become limiting factors. The three
elements most likely to be lacking in a soil from a food
standpoint are nitrogen, phosphorus, and potassium.
They may be designated as the primary elements for
plant growth. The other elements are usually present
in amounts many times greater than will ever be needed
by crops. Calcium, w T hile necessary in large quantities
in a soil, is largely an amendment, and very seldom may
limit plant growth because of being in too minute quantity
to supply the food needs of a crop. The liming of a soil
is for other purposes than the supplying of calcium for
plant nutrition. Sulfur is supposed, in certain soils,
to limit plant growth because of its insufficiency, but
ordinarily it is never found in a minimum quantity.
Nitrogen exists in the soil largely as a portion of the
1 Sodium, silicon, and aluminium are found in plants, but are
not essential to proper growth.
SOME GENERAL CONSIDERATIONS 5
artitaHy or wholly decayed organic matter present therein.
SfliTntilized by the plant ordinarily in the form of nitrate.
The atmosphere, composed of four-fifths nitrogen by
volume, has been the original source of this element;
and through natural processes which are continually
at work the nitrogen has been transferred to the soil.
The encouragement of this natural fixation, thus draw-
ing upon the great body of gas surrounding the earth,
has become of great practical importance in agricultural
operations.
Phosphorus has its origin in the mineral apatite and
exists in most soils largely as a tricalcium phosphate
(CaaCPO^). In case of a lack of lime or of the presence
of considerable quantities of humus, phosphorus may be
present as ferric or aluminium phosphates or as organic
phosphoric acid. Phosphorus is probably taken up by
the plant as the mono- or di-calcic phosphate (CaH^PO^
or Ca 2 H 2 (PO 4 )2).
The potassium of the soil exists largely in feldspar
(K 2 O . A1 2 O3 6 SiO 2 ), in mica, or in hydrated aluminium
silicates, which, while rather insoluble, supply potash
to the soil solution in the bicarbonate, chloride, nitrate,
or sulfate forms. It is from such compounds that the
plant draws upon the soil for this element.
4. Abundance of plant-food elements. Having con-
sidered the plant-food elements, especially those of primary
importance, it is of interest to note their distribution in
the earth's crust. Clarke l estimates the composition
of the lithosphere, which makes up 93 per cent of the
known terrestrial matter, as fo n ows :
1 Clarke, F. W. Data of Geochemistry. U. S. Geol. Survey,
Bui. 491, p. 33. 1911.
6 SOILS: PROPERTIES AND MANAGEMENT
. a^ff
Oxygen .... 47.17 Sodium . .
Silicon .... 28.00 Potassium .
Aluminium . . . 7.84 Hydrogen ... .23
Iron 4.44 Carbon 19
Calcium .... 3.42 Sulfur 11
Magnesium ... 2.27 Phosphorus . . . .11
The briefest scrutiny of this table reveals the fact that
the lighter elements are the more abundant hi the earth's
crust. The first four elements make up eighty-seven per
cent, while the primary elements of plant growth either
are lacking or are present only in very small quantities.
5. Soil-forming rocks. As has been stated, ordinary
soil is made up largely of inorganic matter which is derived
from ground-up rock material. Therefore, in any study
of soil origin or formation, however cursory, the atten-
tion must be directed toward geological conditions, not
because of their mere geological interest but because of
their ultimate bearing on soil fertility and crop growth.
In the soil we expect to find, and do find, fragments of
the commonest rocks, because those most exposed and
those present to the largest extent at the earth's surface
must be the ones to break down into soil. Therefore
the commonest soil-forming rocks are the rocks that
are met so commonly in the field. They may be classified
broadly under three heads igneous, sedimentary, and
metamorphic. Some of the common types are as follows :
Igneous Sedimentary Metamorphic
- Granite Limestone Schist
-Syenite Sandstone Gneiss
Diorite Shale Marble
Diabase Dolomite Slate
Gabbro Quartzite
Peridotite
SOME GENERAL CONSIDERATIONS 1
The igneous rocks furnish material for the formation
of the types constituting the other groups. They may
be divided in a general way into two classes one con-
tain ing a high percentage of silica and some free quartz,
the other having a medium or low silica content and no
quartz. The former is designated as acid, and the latter
as basic since it contains a high percentage of the alkalies
and the alkaline earth minerals. Granite and gabbro
are excellent examples, respectively, of these general
groups of rock.
The sedimentary rocks, formed from material derived
from the igneous rocks, have been deposited usually
under fresh- or salt-water conditions. The development
of pressure has in many cases been instrumental in the
consolidation of this material. The limestone and the
dolomite deposited by precipitation may be expected
to be comparatively soluble rocks. Shale is merely a
more or less hardened clay, while sandstone varies ac-
cording to the cementing material which serves to hold
its sand grains together. This cement may be iron
(Fe2O3), calcium carbonate (CaCOs), or silica (Sift).
The action of heat, usually with pressure, on either
igneous or sedimentary rocks, results in the third group,
the metamorphic. Thus, granite on metamorphosis
may form either a gneiss or a schist ; limestone or dolomite
may form marble ; shale may form slate ; and sandstone
may form quartzite.
On examination, ordinary rock is found to be composed
of one or more minerals. In other words, it is a mineral
aggregate. The mineral, in turn, is a natural compound
of approximately a constant chemical composition, usually
displaying a crystalline form and other well-defined
physical properties. In order to illustrate the com-
8 SOILS: PROPERTIES AND MANAGEMENT
plication that may arise, the mineral composition of some
common rocks is given below :
Granite Quartz, orthoclase, and plagioclase with mica
and hornblende.
Syenite Orthoclase and mica with hornblende and
augite.
Basalt Plagioclase and hornblende or augite with
apatite, pyrite, and mica.
Peridotite Olivine with augite, pyrite, mica, and horn-
blende.
Limestone Calcium or magnesium carbonate with traces
of silica and iron.
Sandstone Sand cemented with iron, silica, or calcium
carbonate.
This complex character of rocks has an important
bearing on the question of soil formation, since the presence
or absence of certain minerals may have considerable in-
fluence on the physical or chemical characteristics of the
resultant soil. It is the minerals, therefore, rather than
the rocks themselves, that must be looked to in a study
of the great mass of inorganic matter, some active and
some inactive, which makes up the bulk of ordinary
soils. The question of the composition of a soil thus
becomes more intensely geologic as we proceed.
6. Soil-forming minerals. A great many minerals
have been discovered, studied, and classified, but only a
comparatively few occur in any abundance in the normal
soil. Nevertheless, it may be said that practically all
soils contain all the common rock-forming minerals.
This is to be expected, as fragments of practically all the
common rocks go to make up an ordinary soil. The
SOME GENERAL CONSIDERATIONS 9
following list will give some idea of the minerals normally
present in soils : -
COMMON SOIL-FORMING MINERALS
1. Quartz . . Si0 2
2. Orthoclase K 2 O . A1 2 3 . 6 SiO 2
3. Plagioclase Na 2 O . A1 2 O 3 . 6 SiO 2 , CaO . A1 2 3 . 2 Si0 2
or combinations
4. Hornblende Chiefly Ca(MgFe) 3 Si 4 Oi 2 with *
Na 2 Al 2 SiA 2 and (MgFe) 2 . (AlFe) 2 .
Si 2 Oi 2
5. Augite . . Chiefly CaMgSi 2 O 6 with (MgFe)
(AlFe) 2 Si 2 O 6
6. Muscovite 2 H 2 O . K 2 O . 3 A1 2 3 . 6 SiO 2
ii in
7. Biotite . . (HK) 2 (MgFe) 2 (AlFe) 2 (SiO 4 ) 3
8. Olivine . 2 (MgFe)O . SiO 2
9. Serpentine 3 MgO . 2 SiO 2 . 2 H 2 O
10. Epidote . H 2 O . 4 CaO . 3 (AlFe) 2 O 3 . 6 Si0 2
11. Apatite . 3 Ca 3 P 2 O 8 + (CaFl 2 ) or (CaCl 2 ) or
combinations
12. Zircon . . ZrO 2 . SiO 2
13. Chlorite . H 4 o(FeMg) 23 Al 14 Sii 3 O9o
14. Calcite . CaCO 3
15. Dolomite . CaCO 3 . MgCO 3
16. Gypsum . CaSO 4 .2H 2 O
17. Talc . . H 2 0.3MgO.4SiO 2
18. Hematite . Fe 2 O 3
19. Siderite . FeCO 3
20. Limonite . 2 Fe 2 O 3 . 3 H 2 O
21. Kaolinite . 2 H 2 O . A1 2 O 3 . 2 Si0 2
22. Zeolites . Complex hydrated aluminium silicates of
Ca, K, and Na as Philolite (CaK 2 N 2 )
Al 2 Sii O 24 . 5 H 2 O
10 SOILS: PROPERTIES AND MANAGEMENT
There are certain of these minerals that merit especial
attention because of particular attributes which they may
impart to a soil. Quartz, for example, is very common
in all soils, making up usually from 85 to 99 per cent of
their composition. It is a makeweight material, how-
ever, as it is used to a very slight extent by most plants ;
but it adds a stability to the soil that perhaps the soil
would not otherwise have, and this function is of con-
siderable significance. Of greater importance from the
plant-food standpoint are the feldspars, of which orthoclase
is probably primary because it is the source and store-
house of the soil potash. Acted upon by physical and
chemical agencies, it slowly supplies the soil solution with
potassium, which in turn nourishes the plant. The micas
also may furnish considerable potash for crop growth.
The plagioclase, instead of being rich in potassium, as
the formula indicates, contain the lore basic elements,
calcium and magnesium, as also do the pyroxenes and
amphiboles represented by augite and hornblende. Olivine
and serpentine, also silicates, are particularly rich in
magnesium. Practically all the phosphorus in the soil,
either organic or inorganic, has had its origin in the
mineral apatite ; yet this mineral is present in rocks and
soil usually in very small quantities, making up not more
than 0.6 per cent of the bulk of igneous rocks. More-
over it is a rather insoluble material. This fact, together
with the small quantities occurring in soil-forming rocks,
may account for the need of phosphorus in many otherwise
fertile soils.
Calcium, so important as a basic material in soil, may
be supplied to a certain extent by other minerals besides
those already named calcite, dolomite, and gypsum
being perhaps the most important, especially the calcium
SOME GENERAL CONSIDERATIONS 11
carbonate in either the crystalline or the amor-
phous forms. Plenty of calcium in a soil tends not only
to better physical conditions, but also to improve chemical
reactions and biological activity. The iron of the soil
minerals is of importance in its color relationships, for
when oxidized to the hematite form a bright red may be
imparted, while a yellow may result if limonite is pro-
duced. Color has great significance in a general estimate
of soil productivity and is always an important factor in
soil identification and survey. The ~ tendency of most
iron compounds in the soil is toward the hematite or the
limonite form when subjected to oxidation and hydration.
Kaolinite is a product of rock decomposition and is
considered to be of considerable importance in most clays
or clay loams. It is almost always impure and in this
form is designated as kaolin. Kaolin and the soil zeolites,
which are hydrated aluminium silicates carrying chiefly
calcium, sodium, and potassium, are really the end prod-
ucts of rock decay and therefore are secondary minerals.
Consequently they must always be considered in any
study of soil formation or of soil utilization, particularly
as they may serve to enrich the soil solution in plant-food
held by them in physical and chemical combinations.
7. Relative abundance of minerals. D'Orbigny 1 pre-
sents the following table as a result of his calculation on
the distribution of certain minerals in the earth's crust :
Percentage Percentage
Feldspars 48 Carbonates .... 1
Quartz 35 Hornblende, augite, etc. 1
Mica 8 All other minerals . . 2
Talc 5
1 Hall, A. D. The Soil, p. 16. New York City. 1907.
12 SOILS: PROPERTIES AND MANAGEMENT
This agrees in general with the distribution of these
minerals in the earth's surface and accounts for their
universal presence in all soils.
8. Organic matter. The minerals as listed account
for all the elements of plant-food obtained from the
soil except nitrogen, which, as already indicated, is found
very largely locked up in proteid and other nitrogenous
material. The incorporation of organic matter in any
soil, either by natural or by artificial means, besides
tending to better its physical condition also enriches
it in its total, or gross, nitrogen content. Though this or-
ganic matter is so necessary in a fertile soil, its addition
and thorough incorporation occurs late in the process of
soil formation. Through the agency of bacteria and
other organisms the organic compounds are slowly sim-
plified, new compounds are split off, and nitrogen is in-
troduced into the soil solution mainly as nitrate, which
is one of the principal forms in which it may be used by
plants growing on the soil.
9. The soil and the plant. Observed from the agri-
cultural standpoint, then, the soil becomes purely a
medium for crop production. Its composition, both
mineral and organic, is of vital importance in the further-
ance of such a use. All the physical, chemical, and
biological agencies become directed toward this end.
The study of the soil and a better understanding of its
function will allow the great class of landowners not
only to increase their crops, and consequently their
profits, but at the same time to maintain as far as possible
the fertility of our greatest national resource. A rational
study of the soil should ultimately lead to a study of
conservation in its bearing both to present prosperity
and to the welfare of posterity.
CHAPTER II
SOIL-FORMING PROCESSES
AFTER the first proper estimate of the relations between
the crop and the soil, the next step is toward the mode of
soil formation and the agencies concerned. As might be
expected, this is a complicated problem from the fact
that most rocks are so heterogeneous in their composition.
The question becomes still further involved because of
the many factors that are continually functioning in rock
decay. This process of the breaking down of rock masses
and their gradual evolution into soil is called weathering. 1
Rock weathering may be defined specifically as the changes
that rock masses undergo due to the physical and chemical
activities of atmospheric agents. Everything on the
earth's surface is seeking a more stable condition, and
therefore, from a geological standpoint, is continually
changing. If a soil represents a more stable condition
than the exposed rock, the rock slowly evolves toward
the soil. Again, if a soil presents constituents not wholly
stable, that soil will change by an elimination or an
alteration of these components. The soil, then, is a
geologic unit. It is a transition product from one con-
dition to another.
This weathering, which brings about such changes and
is such a factor in the modifications of our topography, is
1 For a complete discussion of weathering, see Merrill, G. P,
Rocks, Rock Weathering, and Soils. New York. 1906.
13
14 SOILS: PROPERTIES AND MANAGEMENT
very superficial and affects the earth to but relatively
shallow depths However, from the fact that it provides
a medium for crop growth and at the same time is largely
instrumental in maintaining the fertility of this medium,
its agencies and processes become of great significance.
The forces of weathering, while very diverse not only
as to action but also as to product, permit of an outline
so clear that the true relationships at once become ap-
parent. This classification may be made under two
heads, mechanical and chemical, as follows:
Forces of weathering
I. Mechanical changes, or disintegration
A . Erosion and denudation
Water, wind, ice
B. Temperature
Heat and cold, and frost
C. Plants and animals
II. Chemical changes, or decomposition
A. Oxidation and carbonation
B. Deoxidation
C. Hydration
D. Solution
10. Water. The three great agencies of erosion and
denudation are water, wind, and ice. They are instru-
mental not only in the breaking up of rocks, but also in
transporting the resultant materials. Water is especially
of importance, as its denuding effects are very rapid when
viewed over geological periods. It is estimated that
the United States is being planed down at the rate of
one inch in seven hundred and sixty years. This is rapid
enough to dig the Panama Canal in seventy-three days.
SOIL-FORMING PROCESSES 15
The water, in order to be a successful cutting agent, must
be laden with sediment, so that its carrying power largely
determines its power of erosion. In other words, it must
be armed.
From the time when the raindrops beat down on a
surface until they have been gathered into rivulets and
streams and finally discharged into the ocean, they are
engaged in moving the detrital matter already produced.
The Mississippi River is working fast enough at the
present time to reduce the continent of North America
to sea level in four million years. The Appalachian
Mountains, born in Paleozoic times, have lost vastly
more material than now remains for us to view. Our
river and lake soils are due to the cutting and carrying
power of the streams. The deltas, and the marine soils
of the Atlantic and Gulf coasts, afford other examples
of such effects. The continual pounding and grinding
of waves is no mean factor in rock disintegration. The
rounding of the sands is a mute evidence of this great
force.
11. Wind. The wind as a soil-forming agent has,
like water, two phases of action erosive and tran spor-
tive. Sweeping over the land in dry weather, it has the
power of picking up innumerable fine particles which
may abrade rocks very noticeably over a term of years.
The fluting of exposed rocks, especially in arid regions,
the undermining of cliffs, and the polishing of stones
to a smoothness equal to that of glass, are frequent occur-
rences. The roughening of windowpanes in houses near
the seashore during severe storms, and the illegibility
of old tombstones, are of common record. Great areas
of soil have been deposited by winds, especially in the
United States. The loess of the Mississippi Valley and
16 SOILS: PROPERTIES AND MANAGEMENT
the adobe of the Southwest owe their origin, at least
partially, to the carrying power of wind at a time when
aridity existed qver all this area.
12. Ice. When in large bodies, as in glaciers, ice
exerts a tremendous grinding power. Glacial ice, by its
mobility and motility, adapts itself to all topography,
and as it moves slowly forward it grinds and scours and
abrades even the hardest rocks. The great masses of
pebbles and rocks which are picked up and imbedded
by glaciers, especially in their lower surfaces, increase
their cutting power many fold. The effect of glaciers
is of particular interest because of the fact that all of
the northern part of the United States was covered at
one time with a great ice sheet, and our northern soils
are due either directly or indirectly to the advances and
retreats of this ice sheet. Formed in northern latitudes
due to a change in climatic conditions, the ice sheet
slowly covered many thousands of square miles of terri-
tory, and as the ice was usually several thousand feet
thick, hills, and often mountains, were overridden. Their
tremendous weight made the grinding action almost
irresistible. In the retreat, or melting back, of the ice,
a mantle of this ground-up and well-mixed material was
deposited as soil; while the streams flowing from its
front, or into glacial lakes, were furnished with heavy
sediments for distribution in other regions.
13. Heat and cold. The changes in temperature of
the air, and tha soils and rocks, tend vastly to augment
the effect of the denuding agents. Constant expansion
and contraction is productive of weakness and ultimate
physical breakdown. Heat is conducted slowly through
rocks, thus leading to differential heating and unequal
expansion or contraction. Rocks, as already noted, are
SOIL-FORMING PROCESSES 17
usually mineral aggregates, and these minerals vary
in their coefficients of expansion. With every change
of temperature, differential stresses are set up which
ultimately must produce a considerable effect. When
the separate minerals expand they expand differently,
and when they contract they never again assume quite
their former relationships to one another. Thus crevices,
cracks, and rifts are created in rocks, especially those of
heterogeneous mineral composition. The expansion coef-
ficient of granite is .0000048 of an inch to a foot for every
degree Fahrenheit, while that of marble is about .0000056
of an inch. This seems to be very slight, but it must be
remembered that under natural conditions large surface
areas of rock are concerned. A sheet of granite 100 feet
long will expand one-half an inch with a change of 75
Fahrenheit, which is not an uncommon variation of
temperature in arid regions or high altitudes. This
leads to chipping, flaking, and exfoliation. The rock
fragments may range from microscopic sizes to large
blocks, which are often split off with great violence.
14. Frost. Great as is the action of a simple change
of temperature, its effects become many fold more ap-
parent when water is present. We then have the action
of frost. The cracks and crevices made by heat and cold
will in a humid region become filled with water. This
moisture, on freezing, exerts a very great force. The
expansive power of water passing from the liquid to the
solid state is equal to about 150 tons to a square foot,
which is equivalent to the weight of a column of rock
about a third of a mile in height. Moreover, most rocks
contain a certain amount of water in themselves. This
water is recognized in excavation operations as quarry
water. The passage of the quarry water to a solid state
18 SOILS: PROPERTIES AND MANAGEMENT
must result most disastrously to the physical condition
of the rock. This action of frost is by no means com-
plete when the rock is fined mechanically to a soil, but is
continued on the soil itself. Such further fining is of
the greatest importance in bettering the physical condi-
tion of the soil, and is usually designated as a wetting and
drying and freezing and thawing process. It is to such
forces, more than to any other action, that the farmer
owes the good tilth of his soil.
15. Plants and animals. Plants and animals unite
their forces with those already mentioned to bring about
further physical change. Unlike the modifications due to
erosion, denudation, and temperature, these agencies affect
the soil to a greater extent than they affect the parent rock.
In other words, they begin their work after the minerals
have been reduced, at least partially, to the form of a
soil. Simple plants, as mosses and lichens, will develop
readily on rock ledges and coarse rock fragments. They
send their rootlets into the crevices and exert a prying
and loosening effect. They also catch dust, provide
humus, and gradually accumulate a soil in which higher
and still higher species of plants may grow. Their
chemical effects, especially regarding solution and oxida-
tion, aid in this disintegration. The distribution of
organic matter through the soil by the extension and
death of plant roots is of no mean importance in soil
fertility. Bacteria also may be a factor in rock decay,
not only through their action on the humus material
but also through a direct attack on the rocks themselves.
Their influence, however, is probably mostly chemical.
Animals also effect the fining of rock fragments and
soils, from their burrowing and mixing tendencies. Such
rodents as gophers and squirrels open up the soil, thus
SOIL-FORMING PROCESSES 19
providing better circulation of air and water. This
brings about a deeper and more effective action of the
other physical agencies of weathering. Earthworms
produce similar effects. Their holes provide channels
for ready drainage, and large quantities of soil are brought
to the surface yearly by them. Darwin estimates that
this amounts to as much as one or two inches in a
decade.
16. Oxidation and carbonation. The physical and x
chemical forces do not act alone, but, as a general thing,
combine in their effects. Thus one set of factors aids
and accelerates the other. Scarcely has the disintegration t
of a rock begun, then, before its decomposition is also
apparent. Of the chemical forces oxidation is usually,
especially near the surface of the earth, the first to be
noticed. It is perceptibly manifested in rocks carrying
iron, and consists in such a change that the added oxygen
may be accommodated. Sulfides readily succumb and
become oxides, while these same oxides are prone to take
up oxygen to their fullest extent. This oxidation is dis-
closed by a discoloration of the rock, which is first streaked
and stained with iron oxide but at last changed to a
uniform ochre. The change may be exemplified by the
following reactions :
2FeS 2 + 70 2 + 4H 2 = 2 FeO + 4H 2 SO 4
4 FeO + O 2 = 2 Fe 2 O 3 (red)
While not all the minerals contain iron, enough of them
do to impart a fatal weakness to most rocks. The ferrous
oxide (FeO), being soluble, is washed out and the rock is
creviced and crumbled. A way is now open for more
energetic physical and chemical decay.
With the oxidizing action there is also tke influence of
20 SOILS: PROPERTIES AND MANAGEMENT
carbon dioxide (CO 2 ), which is universally a constituent
of air and is a product of the decaying vegetable matter
present in most soils. This means that the water cir-
culating among rock fragments, especially those of a soil,
is- heavily charged with this compound. The carbon a-
tion may be illustrated as follows :
2 FeS 2 + 7 2 + 4 H 2 O + 2 CO 2 = 2 FeCO 3 + 4 H 2 S0 4 or
2 NaOH + C0 2 = Na 2 CO 3 + H 2 O
17. Deoxidation. Deoxidation is an opposite re-
action to oxidation, being a loss of oxygen either to the
air or to some other compound. With hematite it might
take place as follows :
2 Fe 2 O 3 - O 2 = 4 FeO
Under normal conditions, however, it is not a very im-
portant factor, since most rock fragments and soil are
fairly well aerated, at least too well aerated to allow this
reverse process to occur. In poorly drained soil or in
soil very rich in humus and carrying organic acids it
may occur, and is usually manifested by the development
of blue and gray colors, indicating that a reduction has
taken place. The bleaching of sands, sandstones, and
clays may be due partially to this, and also to a removal
of the ferriferous salts in solution. Some subsoils dis-
play this phenomenon. The average farmer, however,
need not concern himself with the injuries that may re-
sult from deoxidation.
18. Hydration. Hydration usually accompanies oxi-
dation, but when occurring at great depths it may be
practically the only change the minerals have undergone.
Minerals, especially feldspars, become clouded and lose
their luster on this assumption of chemically combined
SOIL-FORMING PROCESSES 21
water. There is also a considerable increase in bulk, this
being often as much as 88 per cent during the transition
of a rock to a soil. Hydrated minerals, while apparently
sound, quickly succumb when exposed to forces of weather-
ing which are more superficial in their effects. Car-
bonation and oxidation usually take place as correla-
tive actions with hydration. A simple example of
hy drat ion is shown in the change of hematite to limo-
nite, which occurs in practically every case when iron
is allowed to oxidize from pyrite or a simpler oxide to
the higher forms :
2 Fe 2 O 3 (red) + 3 H 2 O = 2 Fe 2 3 . 3 H 2 (yellow)
19. Solution. As it is now quite evident that weather-
ing, especially the chemical manifestations, is largely a
simplification of compounds, and that water is almost
universally present, some solution must occur. These
simple materials are particularly prone to enter solution
because of the presence of carbon dioxide, which, by
acidifying the soil water, intensifies its solvent action to
a considerable extent and consequently increases its power
as a weathering agent. The atmosphere contains amounts
of this gas ranging from 3.87 to 4.48 parts in 10,000,
while considerable amounts are brought down on the
rocks and the soil in snow and rain. The carbon dioxide
evolved directly into the soil water from decaying organic
matter also aids in keeping the soil charged with this gas.
This means, then, that solution is largely a process of
carbonation, especially after the soluble constituents have
been thrown out into the soil solution. It is evident
that oxidation, carbonation, hydration, and solution act
in unison to bring about the chemical decay of the rock
and the soil. This combined action may be represented
22 SOILS: PROPERTIES AND MANAGEMENT
by showing the various stages that orthoclase may undergo
in producing a residual clay :
KAlSi 3 8 + HOH = HAlSi 3 8 -f KOH
2 KOH + C0 2 = K 2 CO 3 + H 2
HAlSi 3 O 8 - 2 SiO 2 = HAlSiO 4 (kaolinite)
The silica in this case may become quartz or colloidal
silica, or, what is more probable, may unite with certain
elements to produce complex hydrated silicates.
20. A general statement of weathering. The question
of rock weathering is complicated because no one action
can be considered alone. All forces are acting together,
tending to produce a great complex of reaction and inter-
action. No amount of explanation or speculation can
ever fully clarify the question as to the formation of a
soil from a parent rock. Nevertheless, knowing in general
the separate forces and reactions produced, we may formu-
late the phenomenon in a general arid superficial way.
The change that a rock undergoes in the formation of a
residual clay is first a physical breaking-down accom-
panied by chemical changes, which consist in the hydra-
tion of the feldspars, the oxidation of the iron, and the
solution and carbonation of the soluble bases.
21. Factors affecting weathering. It is readily to be
seen that the activity of the various agencies of weather-
ing will be modified by certain factors which determine
not only the kind of rock decay but also its rate. Of
these, climate is probably of the greatest importance.
The difference in the weathering in an arid region as
compared to that in a humid region will illustrate this
point. Under arid conditions the physical forces will
dominate and the resulting soil will be coarse. Freezing
and thawing, heat and cold, the action of the wind, and
SOIL-FORMING PROCESSES 23
the effect of animals, will be almost the sole agents.
In humid regions, however, the forces are more varied
and practically the full quota will be at work. Chemical
decay will accompany the disintegration, and the resultant
product will be finer and more minutely divided. The
separate minerals will show also the change of color and
loss of luster due to the decomposition of some of their
essential elements. The same rocks, then, will behave
differently under different climatic conditions. A granite,
for instance, is a very insoluble rock as compared with a
limestone, and in a humid region where chemical agencies
are dominant it would be markedly more resistant. If,
however, these two rocks are placed under arid conditions
where the physical forces are potent, particularly as re-
gards change of temperature, the comparison is different.
The limestone, being homogeneous, is not affected by
atmospheric changes; but the stresses set up in granite
due to differential contraction and expansion must ulti-
mately reduce it to fragments.
As weathering is confined to the very surface of the
earth, the exposure or position of a rock will determine
the kind and the rate of decay. If the rock is very deep
below the surface, only hydration may occur ; while if
it exists as an exposed ledge, the full force of the weather-
ing agents will be sustained. If the debris of the decayed
rocks is not removed, this serves as a blanket for the
protection of the rocks below. The tran sportive powers
of weathering are important in maintaining a clean sur-
face for action.
The texture of the rock is also a factor. Other things
being equal, a coarsely crystalline rock will disintegrate
and decompose more rapidly than one of finer grain.
The coarser the gram, the larger the amount of interstitial
24 SOILS: PROPERTIES AND MANAGEMENT
space and the greater the encouragement to physical
agencies. As physical changes open the way for chemical
decay, coarse texture will ultimately encourage decomposi-
tion as well as disintegration.
Lastly, the disruptive forces of the rock will be in-
fluenced by the chemical composition of the minerals
and the mineral composition of the rock. A rock made
up of minerals that offer but little resistance to decay
will naturally reduce readily and quickly to a soil.
Rocks that very largely bear minerals which are re-
fractory in their nature, however, may never decompose
far enough or rapidly enough to give a soil of any agri-
cultural significance. The next step, then, in the study
of soil formation is a consideration of the relative resistance
of the minerals and the rocks.
22. The law of mineral and rock decay. Considerable
work has been done on the comparative solubility of
minerals both in pure and carbonated water, but in most
cases it has proved somewhat inconclusive. Neverthe-
less we are able, by consulting the work of Mtiller, 1 Clark, 2
Daubree, 3 and others, to arrange some of the commoner
minerals in the order of their solubility, the most resistant
minerals heading the list :
1. Quartz 6. Epidote 11. Apatite
2. Muscovite 7. Serpentine 12. Olivine
3. Biotite 8. Talc 13. Calcite
4. Orthoclase 9. Hornblende
5. Plagioclase 10. Augite
1 Miiller, R. Solution of Rocks in Carbonated Water. Jahrb.
k-k Geol. Reichsanstalt, Vol. XXVII, p. 25. 1877.
2 Clark, F. W. Data of Geochemistry. U. S. Geol. Survey,
Bui. 330, p. 401. 1908.
3 Daubree, A. Solubility of Orthoclase. Etudes de Geol.
Experimental, pp. 27 and 252. Paris, 1847.
SOIL-FORMING PROCESSES 25
The next step is to deduct some general law which can
be shown to govern the resistance of these minerals.
Such a statement would aid considerably in the making of
general deductions regarding weathering. The siliceous
content of some of these minerals, taken in the order as
above, throws considerable light on this phase :
Per cent of SiO 2 Per cent of SiO 2
Quartz ..... 100 Hornblende .... 45
Orthoclase .... 65 Olivine ..... 41
Plagioclase .... 55 Calcite ..... trace
Another case might be cited in a comparison of the
chemical composition of anorthite, hornblende, and
olivine :
Anorthite . CaAl 2 Si 2 O8
Hornblende . Ca(M g Fe) 2 (SiO 3 ) with
Olivine . . (MgFe) 2 SiO 4
It is to be noted that immediately as the resistance of a
mineral declines, its content of silica decreases and the
percentage of the basic constituents increases. Silica
and aluminium, then, mark resistance to decay; while
calcium, magnesium, sodium, potassium, and iron func-
tion in increasing susceptibility to decay. The law of
mineral resistance may be formulated as : " The more
basic a rock becomes, the more rapid is its decomposi-
tion ; and the more acid, the less marked is its decay." 1
This general law certainly should apply to rocks that
are made up of the minerals listed above. One example
will show this clearly. The igneous rocks, as already
stated, may be divided into two groups, acid and basic.
1 Buckman, H. O. The Formation of Residual Clay. Trans.
Amer. Ceramic Soc., Vol. XIII, p. 362. February, 1911.
26
SOILS: PROPERTIES AND MANAGEMENT
This acidity and basicity is determined by the presence
of silica and the alkalies, respectively, as carried by
certain essential minerals. Suppose we name some
representative igneous rocks in the order of their acidity,
and list some of the minerals carried by them :
1. Granite . . Quartz, orthoclase, and mica
2. Diabase . . Plagioclase, mica, hornblende, or augite
3. Peridotite . Principally olivine
It is to be seen that the minerals contained by granite
are more resistant than those carried by either the diabase
or peridotite, while the olivine of the last group is near
the foot of the list when the minerals are arranged in the
order of their resistance.
The following data 1 bear out the argument presented
above as to the relative resistance of rocks :
PROPORTIONAL AMOUNTS OF FRESH ROCKS SOLUBLE IN BOIL-
ING HYDROCHLORIC ACID AND SODIUM CARBONATE SOLUTIONS
PHONOLITE
DIABASE
GRANITE
SiO 2
21.64
10.85
9.49
Al 2 O 3 Fe 2 O3
12.60
35.65
8.36
CaO
1.07
3.09
.60
MgO . . .
.40
2.20
.71
K 2 O
.28
1.21
1.68
Na 2 O . . .
5 45
50
1.23
41.44
33.50
22.07
It is evident, then, that the law of mineral resistance
applies to rocks as well as to the separate minerals, al-
though its application thereto is much more complex
and difficult to interpret.
1 Merrill, G. P. Rocks, Rock Weathering, and Soils, p. 366,
New York. 1906.
SOIL-FORMING PROCESSES
27
23. Special cases of weathering. The weathering of
granite and limestone under different climatic conditions
has already been compared. The changes that take
place in these rocks as they are evolved to residual clays
may now be considered. The following analyses serve
to show on what elements the losses are likely to be most
serious during the process:
FRESH GRANITE AND ITS RESULTANT CLAY 1
ROCK
CLAY
PERCENTAGE
LOST
SiOo
6069
4531
52 45
A1 2 O,
Fe 2 3
CaO
MgO
K 2 O
16.89
9.06
4.44
1.06
425
26.55
12.18
00.00
.40
1 10
00.00
14.35
100.00
74.70
83 52
Na 2 O
P 2 O 5 . .
2.82
25
.22
47
95.03
0000
Ignition
62
13 75
Gain
VIRGINIA LIMESTONE AND ITS RESIDUAL CLAY 2
ROCK
CLAY
PERCENTAGE
LOST
I
SiO 2
7.41
^ 57.57
j -
27.30
A1 2 O- .
vJ.91
2044
0000
Fe 2 O J
98
7 93
24 89
CaO
MgO
28.29
18 17
.51
1 21
99.83
99 38
K 2 O
1 08
4 91
^7 4Q
Na 2 O
P 2 O 5
.09
03
.23
10
76.04
68 78
CO 2
41.57
38
99 15
H 2 O
57
6 69
Gain
1 Merrill, G. P. Bui. Geol. Soc. Amer., Vol. 8, p. 160. 1879.
2 Dille*, J. S. U. S. Geol. Survey, Bui. 150, p. 385. 1898.
28
SOILS: PROPERTIES AND MANAGEMENT
Soils have resulted in both cases from the decay of
these rocks. In the case of the granite the resulting soil
was a deep red clay, with quartz grains present. The
soil from the limestone was a plastic clay, high in silica
and aluminium. Leaching has probably gone on to a
very great extent in both soils. It is noticeable also
that the basic constituents have suffered the greatest
losses, especially calcium, magnesium, sodium, and
potassium. The carbonate has almost wholly disap-
peared from the limestone clay, showing that a limestone
soil may not necessarily be rich in lime. As a matter of
fact, the chances are that if it is residual it will be lacking
in that compound. When shown diagrammatically (See
Figs. 1 and 2), the changes that the parent rocks have
undergone chemically in forming a clay will become
apparent.
FIQ. 1. Diagrammatic representation of the chemical composition of
fresh granite and its residual clay. See analyses above.
SOIL-FORMING PROCESSES W
As shown by the diagrams, the soil from the granite does
not differ greatly from the original rock, except in loss
of bases, assumption of water, and increase of organic
(jfrew rock
(fy&es/dua/ cfay
FIG. 2. Diagram showing the chemical composition of Virginia lime-
stone and its residual clay. See analyses above.
matter. The residual clay from the limestone presents
greater differences, dne to the almost entire disappear-
ance of calcium carbonate. The diagrams for the two
clays resemble each other fairly closely in spite of their
widely differing sources. Because weathering causes
the persistence and accumulation of silica, aluminium,
and iron, and a loss of the basic materials, all soils as
they weather tend to approach a similar composition.
Yet, owing to a difference in the adjustment of the forces
at work and in the time element, no two soils will ever
be exactly alike. Soils will differ, then, from the original
rock and from one another according to the intensity
30 SOILS: PROPERTIES AND MANAGEMENT
and character of the weathering and the constitution of
the parent minerals.
24. Practical relationships of weathering. Weather-
ing processes result in a general simplification of com-
pounds. Their action first affects the rock, with the
result that a soil is produced ; but they still remain ac-
tive in the soil after it is in a condition to support plants.
The physical agencies especially tend to loosen and fine
the soil, contributing largely to its tilth. The farmer
encourages such influences by plowing his land and by
other operations. Were it not for such weathering
action, the soil would become physically unable to afford
foothold for plants. The continued chemical changes
resulting in solution and carbonation provide a soil water
rich in plant-food nutriment. Weathering, then, by a
slow process over geologic periods has provided us with
soil, and by the same slow process is maintaining the
fertility of this creation. The encouragement and control
of such an agency is of no small importance in agricultural
practice.
CHAPTER III
THE GEOLOGICAL CLASSIFICATION OF SOILS
WEATHERING must be considered as affecting soils both
in situ and in motion. This gives two general classes of
materials those that have not been shifted far from
their place of origin, and those in the formation of which
the transporting agencies have been instrumental. These
two general groups, designated as sedentary and trans-
ported, 1 are subject to considerable subdivision, as
follows :
f Residual
Sedentary ~ ,
Cumulose
Transported
Gravity Colluvial
Alluvial
Water
Marine
Lacustrine
Ice Glacial
Wind ^Eolian
25. Residual soils. This group of soils covers wide
areas of our arable regions and comes from many kinds of
rocks. Residual soils are old soils, the oldest with which
we have to deal in agricultural operations. Since a
1 See Trowbridge, A. C. A Classification of Common Sedi-
ments. Jour, of Geol., Vol. 22, No. 4, pp. 420-436. 1911.
31
32
SOILS: PROPERTIES AND MANAGEMENT
residual soil is formed in situ, the rocks that underlie it,
if sound, show the character and composition of the rocks
from which the soil was actually a product. In such
soils the changes that a rock undergoes in forming a
residual clay are to be studied to the best advantage.
An examination of the various grades of material that are
found overlying the country rock (Fig. 3) in an area where
FIG. 3. The gradual transition of country rock into residual soil by
weathering in situ.
this residual mantle exists, reveals more or less accurately
the gradations from rock to soil. Residual soils, besides
being old soils, are usually nonstratified and present a
heterogeneous mass of material. Since they have been
subjected to leaching over vast periods, a very large
amount of the soluble materials have been washed out,
tending to leave high percentages of the persistent ma-
terials, such as silica, iron, and aluminium. An analysis
of an Arkansas limestone, its residual clay and the cal-
culated percentage loss of the various constituents pres-
ent in the fresh rock, illustrates this point :
GEOLOGICAL CLASSIFICATION OF SOILS
ARKANSAS LIMESTONE AND ITS RESIDUAL CLAY 1
FRESH ROCK
CLAY
PERCENTAGE
LOST
SiO 2
4.13
33.69
.00
AloO 3
4.19
3030
11 35
Fe 2 O 3
2.35
1.99
8956
MnO
433
1498
57 59
CaO . . .
44 79
3 91
9893
MgO
.30
.26
8938
K 2
Na-O
.35
16
.96
61
66.36
53 26
CO 2
34.10
.00
10000
The vast age of such soils tends to bring about great
oxidation, so that most of the iron has changed to hematite
and limonite. Since almost all soils contain considerable
iron, the prevailing colors of residual soils are reds and
yellows, depending on the degree of oxidation and hydra-
tion. Grays and browns may exist, however, where iron
has been lacking or oxidation has been feeble. In texture
such soils usually present very fine conditions. Having
been attacked by both the physical and the chemical
agencies, the particles have been reduced to a very fine
state of division. Over residual areas the heavier soils
predominate, as silts, silt loams, clays, and clay loams.
Very often sand or chert may be present, having been a
constituent of the original rock mass.
An examination of the particles of a residual soil usually
shows them to be in an advanced stage of decay. The
feldspars have lost their luster and have become opaque.
The iron has become oxidized, and the soluble bases have
^enrose, R. A. F.
Vol. I, p. 179. 1890.
D
Ann. Rept. Geol. Survey Arkansas,
34 SOILS: PROPERTIES AND MANAGEMENT
either disappeared or changed their combinations to more
stable forms. The tendency of all soils is toward a con-
dition of equilibrium, and consequently they approach,
but never reach, a common composition. This does not
apply to their productivity, because many other factors
besides chemical composition go to determine cropping
power. Residual limestone soils, therefore, become poorer
and poorer in lime as their age increases. The organic
matter of residual soils largely depends, in amount and
condition, on climatic factors. If rainfall and tempera-
ture, for instance, are favorable for the rapid and con-
tinued development of a natural vegetation, the soil
will be rich in humus, so rich at times as to mask to a
certain extent the red color so characteristic of such soils.
If plants do not grow well on this soil, however, it will be
low in organic matter and probably in poor physical con-
dition, so vital is humus to a proper foothold for plant
life. Two residual soils coming from the same kind of
rocks may vary rather widely in their general character-
istics, especially as to crop productivity.
26. Distribution of residual soils. Residual soils are of
wide distribution in the United States, 1 particularly in the
eastern and central parts. A glance at the soil map of this
country (See Fig. 4) shows four great provinces the Pied-
mont Plateau, the Appalachian Mountains and Plateaus,
the Limestone Valleys and Uplands, and the Great Plains
Region. The age of these soils varies in the order named,
showing that while they are very old as compared with
other soils yet to be discussed, there may be vast periods
of geologic time between their beginnings. As a matter
1 For a full discussion of the origin and characteristics of the
soils of the United States, see Marbut, C. F., and others. U. S.
D. A., Bur. Soils, Bui. 96. 1913.
GEOLOGICAL CLASSIFICATION OF SOILS 35
of fact, there is probably a greater difference in age be-
tween the soils of the Piedmont Plateau and those of
the Great Plains Region than has elapsed since the latter
were formed. The soils of the Piedmont Plateau have
been formed mostly from gneiss and schist. In fact,
the Piedmont Plateau is the remnant of the old continent,
Appalachia, which was in existence in early Cambrian
times. The rocks of the Appalachian Mountains and
Plateaus are limestone, sandstone, and shales. The Great
Plains Region presents limestone, sandstone, and shale
of the Cretaceous, Permian, and Carboniferous ages, be-
sides much unconsolidated material. The soils of these
provinces, extending as they do over great areas, vary
within wide limits due to rock formation, climatic con-
ditions, and age; yet certain common characteristics,
as already pointed out, are exhibited by all.
27. Cumulose soils. This type of soil is of a very
different character from the one just under discussion,
being made up largely of organic matter with the mineral
constituents of secondary importance. At relatively
recent periods shallow lakes, ponds, and basins were
formed, partly by stream action, partly by marsh con-
ditions along sea or lake coasts, or, what is commoner in
the northern part of the United States, by glaciation.
Any basin that contains water throughout the year serves
as a place for the formation of cumulose soil. The highly
favorable moisture relations along the banks and shores
of such standing water encourage the growth of many
plants such as algae, moss, reeds, flags, grass, and the like.
These plants thrive, die, and fall down only to be covered
by the water in which they were growing. The water
shuts out the air to a large extent, prohibits rapid oxida-
tion, and thus acts as a preservative for the rapidly collect-
36 SOILS: PROPERTIES AND MANAGEMENT
ing organic matter. Year after year this process goes
on, and year after year the bed of cumulose material
becomes deeper and deeper. Large shrubs, and even
forests, often grow on such land. Time and the lack of
water are the factors that may limit the depth of such
beds. Accumulations of this nature are found dotted
over the entire country. Their size may vary from a
few acres to several thousand. Along streams the old
abandoned beds offer a common opportunity for the
beginning of such accumulations. Along large bodies
of water, marshes, either salt or fresh, may allow the
process to go on. Shallow basins scraped and gouged
out by advancing glaciers are frequently occupied by
such material. In the last-named case the beds are
more or less independent of topography, and may be
found on hillsides, or even on hilltops, as well as in the
lower lands.
Cumulose materials may be grouped under two heads,
peat and muck. The only difference is in their stage of
decay. In peat the stem and leaf structure of the original
plants can still be detected, and identification is quite
possible. In muck, however, the putrefaction and decay
have gone so far that the plant tissue has lost its
identity as such and is merged into that complicated
and indefinite material called humus. The composition
of peat and muck may be much altered by the wash-
ing-in of mineral matter from above. In some cases
the beds may be from 80 to 85 per cent organic, while
in other cases, due to this foreign material, the percent-
age may drop to as low as 15, giving a black or swamp
marsh mud.
The following analyses illustrate the composition of
representative cumulose soils :
GEOLOGICAL CLASSIFICATION OF SOILS
37
l
2
3
Mineral matter
Organic matter
31.60
6840
24.79
67 63
80.40
15 77
Nitrogen
2 63
203
P 2 5
K 2 O
.20
17
.19
15
.15
65
Moisture
7 58
3 83
1. Muck Pickel, G. M. Muck : Composition and Utili-
zation. Fla. Exp. Sta., Bui. 13. 1891.
2. Muck Kept. Can. Exp. Farms, 1910. Kept, of chem-
ist, p. 160.
3. Marsh mud Kept. Can. Exp. Farms, 1910. Kept, of
chemist, p. 137.
Muck soils, while usually not of large extent, become of
extreme value when drained, especially if they are near
a good market. They are of particular value in trucking
operations, being adapted to such crops as onions, celery,
lettuce, and the like. Usually they must not only be
provided with drainage, but also be treated with fertili-
zers carrying phosphorus and, especially, potash. It is
also a good practice to start vigorous decay by the appli-
cation of barnyard manure, as the nitrogen carried by
muck soils is usually not very readily available to plants.
In many cases muck and peat may be underlaid at vary-
ing depths by marl, which is a soft, impure calcium
carbonate. Before and at the beginning of the organic
accumulation these basins were inhabited by Mollusca,
which at death deposited their shells on the bed of the
inclosure. These shells are now found in a more or less
fragmentary condition, usually mixed with sand and
clay and covered to a varying depth with peat or muck.
Such material, because of its richness in lime, is valuable
38
SOILS: PROPERTIES AND MANAGEMENT
as a soil amendment, and often where it is found pure
enough in quality and in sufficiently large quantities it
is handled commercially. When it contains large amounts
of phosphorus, as it does in some cases, it may be used
as a fertilizer. The following analyses 1 show the general
character of this soil :
l
2
SiO 2
25.28
5.65
A1 2 O 3 Fe 2 O 3
302
330
CaO
MgO
37.52
12
48.51
1 96
K 2 O
.22
.23
Na 2 O
25
30
P 2 O 5
.40
Trace
CO 2
2902
3980
Ignition . ....
4.17
.25
28. Colluvial soils. This class of soil is not of great
importance, first because of its small area and its inac-
cessibility, and secondly because it is usually a coarse,
loose soil, rather unfavorable for plant growth. It is
formed, as its name indicates, in regions of precipitous
topography, and is made up of fragments of rocks de-
tached from the heights above and carried down the
slopes by gravity. Talus slopes, cliff debris, and other
heterogeneous rock detritus are examples of colluvial
soil. Avalanches are made up largely of such material.
As the physical forces of weathering are most active in
the formation of these soils, the amount of solution and
oxidation is small. The upper part of the accumulation
1 Kerr, W. C. Geology of North Carolina, Vol. I, p. 195.
1875.
GEOLOGICAL CLASSIFICATION OF SOILS
39
exhibits this isolated physical action to the greatest extent,
the particles being angular, coarse, and comparatively
fresh ; farther down the slope the material may merge
by degrees into ordinary soil. Such soils are usually
shallow and stony, and approach the original rock in
color unless large amounts of organic matter have ac-
cumulated (Fig. 5).
FIG. 5. Diagram showing the formation of a colluvial soil, (a), bed
rock; (6), dismantled cliff ; (d), coarse unproductive talus; (p), soil
capable of bearing plants.
29. Alluvial soils. In considering the importance of
water as a weathering agent, it was found that it had
both cutting and transporting powers. The alluvial
40 SOILS: PROPERTIES AND MANAGEMENT
soil is a direct result of both these activities. The carry-
ing power of water varies directly as the sixth power of
its velocity; so that a doubling of the velocity increases
the transportive ability sixty-four times. It is estimated 1
that water flowing at the rate of three inches a second
will carry only fine clay, but if this rate is increased to
twenty-four inches a second, pebbles the size of an egg
will be moved along the stream bed. Any checking of
the velocity of a stream will cause it to deposit the ma-
terial carried in suspension, the larger particles first and
the finest when the current becomes very sluggish. This
brings about one of the important characteristics of an
alluvial soil, its stratification. Wherever material is
being laid down by water this phenomenon is exhibited,
due to the rapid changes in velocity. As a stream ap-
proaches nearer and nearer to its outlet, its bed becomes
less and less inclined and the current more and more
sluggish. This tends toward an aggrading of the stream
bottom from the deposited material. Such a condition
naturally increases the probability of overflow in high
water. Overflow at a time when the stream is carrying
its maximum of sediment causes the deposition of a thin
layer of soil over the areas covered by the water. This
soil is stratified according to the conditions under which
it was laid down, the finer particles usually being carried
farther and often deposited in slack water or lagoons.
Also, a stream on a gently inclined bed may begin to
swing from side to side in long, gentle curves, due to the
deposition of alluvial material on the inside of the curve
and the cutting by the current on the opposite bank.
This results in oxbows, lagoons, and similar inclosures,
1 Geikie, A. Text Book of Geology, p. 380. New York. 1893.
GEOLOGICAL CLASSIFICATION OF SOILS 41
ideal for the deposition of alluvial matter. Deltas are
another good example of alluvial deposits, whether oc-
curring in ocean, gulf, or lake. Due to a change in grade,
a stream may cut down through its already well-formed
alluvial deposit, leaving terraces on one or both sides.
Often two, or even three, terraces may be detected along a
valley, marking a time when the stream bed was at
these elevations. On the lower slopes of hills border-
ing valleys, the colluvial deposits may touch or even
mingle with the alluvial, and furnish a stream with some
of its detritus.
Alluvial soils, then, are found as narrow ribbons along
streams. They are always young soils, and are still in
process of formation. Since in most cases they are de-
posited by slowly moving water, the texture of such soils
is fine, the soils being mostly clays, silts, and fine sandy
loams. Found in low lands, alluvial soils need drainage
to a large extent. Because of the favorable moisture con-
ditions these soils usually have a very large amount of
organic material, as vegetation grows readily under such
conditions. Considerable humus is also washed into
alluvial materials at the time of their deposition. The
soil is usually deep, and, because of the high organic con-
tent, universally of good physical condition, although
very heavy stiff clays may be found in certain cases.
The character of the soils and the rocks from which the
detritus has been obtained exerts considerable influence
on its character. For example, a red soil will often
give rise to a reddish alluvial soil, while a soil or a rock
poor in lime will certainly not be parent to a soil very
much richer in that constituent.
30. Distribution of alluvial soils. The distribution
of alluvial soils in the United States is not wide, although
42 SOILS: PROPERTIES AND MANAGEMENT
these soils exist along almost every stream east of the
Great Plains Region. Their best and widest develop-
ment is found, as the map indicates (See Fig. 4) along
the lower Mississippi river, where they may often show
a lateral extension of one hundred miles. Extensions of
this band are noted along the Missouri, Ohio, and Upper
Mississippi rivers. All streams flowing east exhibit
areas of such soils, these areas varying with the size and
velocity of the stream.
The soils of the alluvial province may be divided under
two heads because of topographic differences (1) the
first bottoms, or present flood plains ; and (2) the terraces,
or old flood plains. These soils differ in their elevation,
drainage, and age, but their general characteristics are
similar ; the surface features in both cases vary from a flat
to a gently rolling topography (Fig. 6) . Erosion, especially
in the terraces, may have obliterated some of the out-
FIG. 6. Cross-section of typical alluvial soils. (a), bed rock;
(r), stream ; (6), present flood plain, a recent alluvial soil ; (c), flood
plain terrace ; (d), very old stream terrace, an old alluvial soil.
standing features. Alluvial soils, being very rich, are
particularly adapted to trucking crops, although in most
cases they are utilized for more extensive farming. When
well drained and protected from overflow, they are the
richest and most valuable of soils.
GEOLOGICAL CLASSIFICATION OF SOILS 43
31. Marine soils. The sediments which are con-
tinually being carried away by rivers are eventually
deposited in the sea, the coarser fragments near the
shore, the finer particles at considerable distances. This
layer of material, varying in thickness, consists of stratified
gravels, sands, and clays, and is of a rather recent age
compared with the residual soils. It has not become
consolidated as yet, because of insufficient pressure and
time. When such material becomes raised above the
sea, due to a change in land elevation, it is classified as a
marine soil. It has been worn and triturated by a num-
ber of agencies. First, the forces necessary to throw it
into stream suspension were active, and next it was
swept into the ocean to be deposited and stratified,
possibly after being pounded and eroded by the waves
for years. At last came the emergence above the sea
and the action of the forces of weathering in situ. The
latter effects are not of great moment, since with our
most important marine soils they have been at work
for but a comparatively short time, speaking geologically.
32. Characteristics of marine soils. Marine soils,
while much younger than residual soils, are usually more
worn and ordinarily show a less amount of the important
food elements. This is because of their almost con-
tinuous contact with water from the time when they are
swept into the streams until they rise above the sea level
as a soil. They are generally characterized as sandy
soils, because the forces to which they have been sub-
jected have worn out and dissolved most of the minerals
except quartz. This gives them a coarse texture and
fits them particularly for trucking soils. Sands, sandy
loams, and loams predominate usually in such soil prov-
inces, although clays and silts may occasionally be
44 SOILS: PROPERTIES AND MANAGEMENT
found. These soils are usually low in humus, and con-
sequently must be handled with reference to the possi-
bilities of increasing their organic content. Lack of
humus makes the predominating color of the soil light,
ranging from light gray to brown and dark brown. The
character of these soils is governed to some extent by the
origin of the sediments; different rocks, particularly if
weathered under different climatic conditions, may give
rise to widely different marine soils. The climatic con-
ditions to which marine materials are subjected after
being raised above the sea may also be a considerable
diversifying agency.
33. Distribution of marine soils. Marine soils are
found very widely distributed in the eastern United States,
and make up one of our most important soil provinces.
Beginning at Long Island at the north (See Fig. 4),
they extend southward along the coast in a band ranging
from one hundred to two hundred miles in width. The
western edge of the Atlantic coastal plain is marked by
the great " Fall " Line, or the edge of the old continent
Appalachia. It is from this area that most of the sedi-
ments of the Atlantic marine soils were derived. Proceed-
ing southward, we find that Florida is practically all of
marine origin, together with a great area of the Gulf
coast extending westward to central Texas and having
an average width of two hundred fifty miles. This
gulf marine soil is considerably younger than that of
the Atlantic coast. It is cut into two parts by the allu-
vial soils of the Mississippi, and is covered by a narrow
band of alluvial soil on the eastern bank of that stream
The sediments of the Gulf coastal plain were derived
from the erosion and denudation of the old lands to the
northward.
GEOLOGICAL CLASSIFICATION OF SOILS 45
The soils of the Atlantic and Gulf coastal provinces,
formed as vast outwash plains, are very diversified, due
to source of material, age, and climatic conditions. They
comprise a sufficient range in texture and climate to
support a highly varied agriculture. There are great
tracts of general farming land, besides wide areas of
special-purpose soils adapted to highly specialized in-
dustries. The latter soils require refined and intensive
methods of cultivation. Predominantly sandy, these
soils are easy to cultivate, and they are well drained
except in the lower coastal plain belt. Good aeration is
usually found because of their open structural condition.
Severe leaching occurs in times of heavy rainfall, for the
same reason. When sufficiently supplied with organic
matter, carefully fertilized, and cultivated properly,
these soils support a great variety of crops, such as cotton,
corn, oats, forage crops, and peanuts, besides vegetables
and fruits of many varieties.
CHAPTER IV
GEOLOGICAL CLASSIFICATION OF SOILS
(CONTINUED)
ICE in the form of glaciers has been, as already stated,
a very great factor in soil formation, especially in the
north temperate zones of North America, Europe, and
Asia. Not only was the old mantle of material swept
from the land by the advance of the ice, but a new soil
was laid down as drift material. This drift was some-
times merely ground-up rock, sometimes - rock flour
mixed with the original residual soil, and sometimes
glacial material wholly reworked and considerably strati-
fied by water. Besides this, the streams of water that
issued from under the glaciers were instrumental in
many cases in distributing sediments a considerable
number of miles southward of the ice front. Glacial
lakes, also, when in existence for sufficiently long periods,
furnished basins for the distribution and deposition of
materials derived from the erosive and grinding power
of the ice. The ice also furnished a large amount of
very fine detritus, which was susceptible to wind move-
ment. This material, reworked and deposited as bars
in stream beds, was carried many miles by the prevailing
westerly winds during a period of aridity following the
glacial epoch. It now exists over wide areas, especially
in the Middle West of the United States and in northern
China, in which places it reaches its best development.
GEOLOGICAL CLASSIFICATION OF SOILS 47
It is the important soil of these regions. Glaciation was
instrumental, then, either directly or indirectly, in the
formation of three general classes of materials glacial
drift soils, glacial lake soils, and a certain class of ^Eolian
materials designated as loess and adobe.
34. The ice sheet. 1 If in any region, but more likely
one of some elevation, the temperature and the snowfall
stand in such relationship that the heat of summer does
not offset the winter accumulation of snow, great snow
fields form. As this condition persists year after year,
and the snow becomes deeper and more widely spread,
the temperature is reduced to such an extent as to in-
crease the proportion of the precipitation which persists
through the summer's heat. The pressure of the over-
lying snow, and the water from the melting surface, bring
about a change of the snow into ice. Often a recrystalli-
zation appears to occur without a melting and refreezing.
As the depth of ice increases, the phenomenon of move-
ment is inaugurated as the thickness of the ice at the
center develops strong lateral pressure. Ice, when under
great stress, exhibits a plasticity which it does not or-
dinarily possess. As it moves slowly forward under
this tremendous pressure, and with a thickness of develop-
ment almost incredible, it conforms itself to every un-
evenness of the surface it may be invading. It rises over
hills, or shapes itself to valleys and even small depressions,
with surprising ease. The rate of advance or retreat of
a glacier is determined by the rate at which its edges
are wasting or melting away. If the melting is slow, the
ice front advances ; if it just balances the advance, the
1 For a complete discussion of glaciers and glaciation, see
Salisbury, R. D. The Glacial Geology of New Jersey. Geol.
Survey of New Jersey, Vol. 5. 1902.
48
SOIL 8: PROPERTIES AND MANAGEMENT
ice front is at a standstill; but if the wasting is rapid,
and the advance of the glacier is not fast enough to re-
place this waste, the ice is said to be retreating. A great
ice sheet may exhibit all three of these conditions many
times in its history.
FIG. 7. Map of North America, showing the area covered by the
great ice sheets, the three centers of accumulation, and the approxi-
mate southward extension.
35. The American ice sheet. The northern part of
the American continent was at one time covered by a
GEOLOGICAL CLASSIFICATION OF SOILS 49
great sheet of ice possessing all the properties described
above. Accumulation seems to have occurred in three
well-defined centers, from which, over long geologic
periods, the ice slowly moved southward, encroaching
upon and covering thousands of square miles. The
ice cap of Greenland is a very good example of the con-
ditions then existing in the northern part of the United
States. The area covered by glaciers in North America
at the time of the greatest extension of the ice is estimated
as 4,000,000 square miles. The thickness of the sheet
was probably very great, ranging from a few feet at the
margin to probably a mile or more toward the centers;
at least it was thick enough to override some of the
highest mountains of the New England ranges. Local
glaciation also occurred on the hill and mountain tops,
which tended to increase the apparent thickness of the
ice mantle.
36. Cause of the ice age. The ice age was not one
unbroken invasion and retreat of the ice cap, but was,
as is conceded by all authorities on glaciation, really
divided into epochs. Five great invasions appear to
affect at least the central part of the United States,
possibly without bringing about a disappearance of the
ice across the Canadian border. These interglacial
periods are shown by forest beds, accumulations of
organic matter, and evidences of erosion between the
drift deposited by the successive ice sheets. Some of
the interglacial periods evidently were times of warm,
and even semitropical, climate. Just exactly what was
the cause of the ice age is still under dispute. The most
probable theory, both as to its occurrence and as to its
disappearance, is that a change in the carbon dioxide
content of the atmosphere took place. It is believed
E
50 SOILS: PROPERTIES AND MANAGEMENT
that doubling the amount now present would bring about
tropical climate in the temperate zones, while halving
it would cause frigid conditions and a probable return
of the great ice fields.
37. The extension of the ice sheet. While the
advancing ice in general exhibited well-defined viscosity,
certain parts were more or less rigid. This was especially
true of the parts near the edges of the sheet. These
parts had become filled in their advance, particularly
near the bottom, with earthy and stony material, which
aided the erosive processes to a very great degree. The
eroding and denuding power of the glaciers is shown
everywhere by the gouged-out valleys and by the scratches,
or striae, on exposed rocks. As this sheet of ice slowly
advanced a few inches or a few feet a day, the mantle of
residual soil was carried away or mixed with the rock flour
constantly formed by the moving ice. The original soil
was really an instrument for more effective ice action.
The scouring effect is observed now to the best advantage
in valleys which lay longitudinally to the ice movement,
as did the valleys of the Finger Lakes of central New
York. Valleys lying at right angles to the ice were very
often partially or wholly filled with debris, and the en-
tire topography was altered. Rivers flowing under the
ice often left large amounts of materials designated now
as eskers and kames. The mixing, grinding, trans-
porting, and stratification that went on emphasizes
again the great influence of glaciation on general topog-
raphy and soils.
The greatest southward extension of the ice in the United
States is marked by a great terminal moraine (Fig. 8).
It is supposed that the margin of the sheet was stationary
at this point for a sufficiently long period to allow this
GEOLOGICAL CLASSIFICATION OF SOILS
51
52 SOILS: PROPERTIES AND MANAGEMENT
narrow band of material to collect by the continual
melting of the ice and a consequent dumping of its load
of debris. This moraine is by no means continuous,
and for miles across the continent no trace of it can be
found. It extends, roughly, eastward from the Canadian
border in Washington to the upper sources of the Missouri
River, then down that river to St. Louis, up the Ohio
River, northeastward until the southwest border of
New York is reached, and then southeast to New York
City and Long Island. Many other moraines are found
to the northward, marking points where the ice became
stationary for a time during its retreat.
38. The ice as a soil builder. It was during these
retreats that the ice acted as a soil-forming agent. Ma-
terial gathered and ground by the ice as it pushed to the
southward was finely pulverized and it is -only natural
to suppose that this debris was deposited as the ice slowly
retreated by the melting back of its margins. The
material laid down as a great mantle over the glaciated
areas is called drift. Some of this has been reworked
and stratified by water, but a very large proportion has
remained untouched since it was laid down by the melt-
ing ice. It presents in most cases except at the very
surface, where weathering may have occurred or organic
matter accumulated exactly the same condition as
when deposited. This mass of un stratified material is
heterogeneous, both as to size of the particles that make
it up and as to its rock composition. It may be coarse
and bowldery, especially in mountains or where there
are gneisses or schists, or it may be very fine where the
rocks are soft. Bowlder clay is a term sometimes used
in describing the matrix of this glacial deposit. In some
cases foliation occurs, and often coarse and fine layers
GEOLOGICAL CLASSIFICATION OF SOILS 53
of till may alternate. This great mantle of material,
varying in thickness and constitution according to the
underlying rocks and the strength of glaciation, gives a
great soil province in northern United States which
may be designated as glacial till soil.
39. Glacial till soils. The glacial till soils may be
characterized physically as heavy or relatively heavy
soils. The tremendous force of the grinding has pro-
duced fine particles, and as a consequence clay loams
and silt loams predominate. Such soils usually have a
subsoil which is finer than the surface material and may
be so impervious to water as to produce bad drainage
conditions. The individual particles of a glacial soil
are found to be unweathered to a great degree unless the
soil is mixed with some of the old mantle of residual
material which once overspread all our glaciated areas.
The particles are jagged and unrounded ; the feldspars
retain all of their luster, and the iron stains so common
in residual soils are almost absent. As the glacial soils
are young soils, their colors are seldom made up of reds
and yellows, but grays and browns prevail. Red may
occur, however, where red sandstones have been glaciated
or where red residual soil has become incorporated in
the till. Where considerable organic matter has accu-
mulated the soil is usually very black. The subsoils
in the glaciated areas usually present colors ranging
from light grays to light browns. Blue or mottled clay
or clay loam is often found, due to a lack of aeration in
the soil ; to the soil expert such a condition near the sur-
face indicates a need of drainage.
40. Composition of glacial soils. The chemical com-
position of glacial soil approaches more nearly than that
of any other soil the composition of the original rock.
54 SOILS: PROPERTIES AND MANAGEMENT
This close resemblance to the parent rock is not sur-
prising, since glacial soil is ground-up rock material of
recent formation on which the weathering agencies have
as yet had little time for action. Therefore the amounts
of the important constituents in such soil are governed
largely by the composition of the original rock. The
lime content is due to such a relationship, and the agri-
cultural value of the soil is greatly influenced thereby,
since large amounts of calcium are of great importance
to soil fertility. The hill soils of central New York
(Volusia series) come from shales poor in lime, and the
soil owes its properties very largely to this lack, which
is traceable to the parent rock. On the other hand, cer-
tain glacial soils of the Mississippi Valley (Miami series)
formed from sandstones and limestone, contain plenty
of lime due to the nature of their rock origin. Glacial
soils from limestone always contain plenty of lime, a
condition that is far from true with residual soils.
41. Humus of glacial soils. The humus content of
glacial soils depends to a large extent on the climatic
conditions under which the soil has existed since its
formation. If environmental factors have been such
as to encourage the accumulation of organic matter,
these soils will exhibit the deep black color that arises
from the presence of such material. If, however, con-
ditions do not encourage the natural growth of a heavy
vegetation, the amount of organic matter in such virgin
soil will be low. Lime may be a very great factor in
such soils, not only in the encouragement of plant growth,
but also in the proper decay of the plant tissue after
it has become incorporated with the soil.
Glacial till soils are found distributed over all the area
north of the great terminal moraine, and stretch, roughly,
GEOLOGICAL CLASSIFICATION OF SOILS 55
from New England to the Pacific coast (see Fig. 4).
They comprise a great variety of soils, not only as to their
physical character, but also as to fertility. They are
adapted to many crops, but general farming is practiced
on them to the greatest degree. This means extensive,
rather than intensive, operations. In some localities
dairying has been developed to a large extent, and has
proved to be not only a means of obtaining paying re-
turns from such soils, but at the same time a method of
keeping up their fertility.
42. Glacial lakes. Such great masses of ice could
not advance and retreat, again and again, on such an
extensive scale, without causing the formation of great
torrents of water. It is more than probable that at all
times great streams gushed from the ice front, laden
with much sediment. Often these streams were under
pressure, which when released caused an immediate
deposition of material. As long as the ice front stood
south of the east and west divide, this water found ready
egress and flowed rapidly away to deposit its load as
gravelly outwash, river terraces, valley trains, and allu-
vial fans. These formed alluvial soils of varied character,
depending on the size of the materials carried. There
came a time, however, in the retreat of the ice, when the
front stood north of the divide and only a small pro-
portion of the water found itself free to flow over the divide
and away to the southward. The remaining water was
ponded between the ice front and the old divide. Thus
glacial lakes were produced, of large or small extent,
according to the position of the ice. The location of
such lakes is shown on the soil map of the United States.
The ponded water remained in this condition for many
years, subject, of course, to changes concordant with
56 SOILS: PROPERTIES AND MANAGEMENT
the oscillation of the ice front. With the ice melting
rapidly on the hilltops, these lakes were constantly fed
by torrents from above which were laden with sediment
derived not only from under the ice, but also from the
unconsolidated till sheet over which it flowed. As a
consequence, there were in the glacial lakes deposits rang-
ing from coarse delta materials near the shore to fine silts
and clay in the deeper and stiller water. Such materials
now cover large areas (see Fig. 4), not only in New York
State and along the Great Lakes, but also in the Red
River Valley and in the northerly inclined valleys of the
Rocky Mountains and the Cascade and Sierra Nevadas.
They make up by far the most important lacustrine
soils.
43. Lacustrine soils glacial lake. Glacial lake soils
probably present as wide a variation in physical char-
acteristics as any of our great soil provinces. Being
deposited by water, they have been subject to much
sorting and stratification, and range from coarse gravels
on the one hand to fine clays on the other. They are
generally found as the lowland soils in any region, al-
though they may occur well up on the hillsides if the
shores of the old lakes encroached thus far. The color
of such soils varies from gray to black, according to
the degree of organic matter present. The humus con-
tent of such soils, as with the glacial till, varies with
climate, and may be high, low, or medium according
to conditions. The thickness of glacial lake deposits is
variable, ranging from a few feet to many feet. In
chemical composition they closely approximate the soil
from which they are derived. This is particularly true
as regards the presence of lime. The Dunkirk soil
of southern New York, a wash from the lime-poor
GEOLOGICAL CLASSIFICATION OF SOILS 57
Volusia series of the highlands, is low in lime; while
the same soil just south of Lake Ontario, obtaining its
wash from a limestone till (Ontario series), is rich in
lime. As may be inferred from the above comparison,
the glacial lake soils of the United States are variable
in their fertility.
The distribution of the glacial lake deposits, as seen
from the soil map of the United States, is fairly wide.
Such soils are found in areas large enough to be of great
agricultural influence, extending from New England
\\v-stward along the Great Lakes until their greatest
expanse is reached in the Red River Valley. These de-
posits make up some of the most important soils of the
northern states. They are valuable not only for ex-
tensive cropping with grain and hay, but also for fruit
and trucking crops. The Ice Age was certainly not
in vain as far as the production of fertile soils is con-
cerned.
44. Lacustrine soils recent lake. There is yet
another lacustrine soil to be considered besides the one
just discussed recent lake soil. While the glacial lake
deposits were formed many thousands of years ago, the
lake soils of the second group are in process of construc-
tion. It is a well-known fact in physical geography
that lakes are only enlarged stream beds, and are doomed
ultimately to be filled by river sediments. Such soils
have been reclaimed to a certain extent, but their acreage
is not large enough to give them the importance of
the glacial lake soils. The lake soil is usually of a fine
character, rich in humus, of good tilth. If properly
drained, it is almost invariably highly productive, and
is adapted to a variety of crops depending on climatic
conditions.
58 SOILS: PROPERTIES AND MANAGEMENT
45. .flSolian soils. During glaciation much fine ma-
terial was carried miles below the front of the glaciers
by streams that found their sources therein. This fine
sediment was deposited over wide areas by the over-
loaded rivers. The accumulations occurred below the
ice front at all points, but seem to have reached their
greatest development in what is now the Missouri Valley.
There, too, the sediment seemed finest, and, coming
mainly from glaciated limestones, was very rich in cal-
cium. It is generally agreed by glacialists that a period
of aridity, at least as far as this particular region is con-
cerned, immediately followed the retreat of the ice.
The low rainfall of this period was accompanied by
strong westerly winds. These winds, active perhaps
through centuries, were instrumental in the picking-up
and distributing of this fine material over wide areas
of the Mississippi, Ohio, and Missouri valleys. One
strong argument for this^Eolian origin is that the soil is
found in its deepest and most characteristic development
along the eastern banks of the large streams. Especially
noticeable is the extension down the eastern side of the
Mississippi River almost to the Gulf of Mexico. This
wind-blown material, called loess, is found over wide
areas in the United States, in most cases covering the
original till mantle. It covers eastern Nebraska and
Kansas, southern and central Iowa and Illinois, northern
Missouri, and parts of Ohio and Indiana, besides a wide
band, as already noted, extending southward along the
eastern border of the Mississippi River. Due to its
mode of origin, its depth is always greatest near the
streams and gradually becomes less farther inland. In
places, notably along the Missouri and Mississippi rivers,
its accumulation has given rise to great bluffs which
GEOLOGICAL CLASSIFICATION OF SOILS 59
bestow a characteristic topography to that region. The
loess soil is found also covering the great areas of China
and Siberia, and thus it is one of the important soils of
the world. Another soil, made up, at least partially, of
wind-blown material and found in Arizona and New
Mexico, is called adobe. Volcanic soils of the western
United States and elsewhere are to some extent of wind
origin. Sand dunes are of ^Eolian origin, but these sink
into insignificance as to agricultural value when com-
pared with the soils named above, especially loess.
46. Loess soils. Loess is usually a fine calcareous
silt or clay, of a yellowish or yellowish buff color. While
it may be readily pulverized when subjected to cultiva-
tion, it possesses remarkable tenacity in resisting ordinary
weathering. The vertical walls and escarpments formed
by this soil show one of its striking physical character-
istics. In China l caves that house thousands of persons
are dug in the defiles and canons existing in this deposit.
Another feature of loess is the presence of minute vertical
canals lined with a deposit of calcium carbonate. These
canals are supposed to give the soil its vertical cleavage
and its tenacity. The particles of loess are usually un-
weathered and angular. Quartz seems to predominate',
but large quantities of feldspar, mica, hornblende, augite,
calcite, and other substances are found.
A few typical analyses 2 are given below :
1 Richthofen, F. Chinese Loess. Geol. Mag., May, 1882,
p. 293.
2 Clark, F. W. The Data of Geochemistry. U. S. Geol. Sur-
vey, Bui. 491, p. 486. 1911.
A. From near Dubuque, Iowa.
B. From Vicksburg, Mississippi.
C. From Kansas City, Missouri.
D. From Cheyenne, Wyoming.
60
SOILS: PROPERTIES AND MANAGEMENT
A
B
c
D
SiO 2
A1 2 3
Fe 2 O 3
72.68
12.03
3 53
60.69
7.95
2 61
74.46
12.26
3 25
67.10
10.26
2 52
MgO
CaO
Na 2 O
K 2 O
1.11
1.59
1.68
2 13
4.56
8.96
1.17
1 08
1.12
1.69
1.43
1 83
1.24
5.88
1.42
2 68
P 2 O 5
.23
.13
09
.11
C0 2
H 2 O . .
.39
2.50
9.64
1.14
.49
2.70
3.67
5.09
It is immediately noticeable that the lime content of
these soils is high, as is also the phosphoric acid. In
fact, all the more soluble constituents are present in
relatively large quantities, as would naturally be ex-
pected from the mode of origin of such soils they having
been subjected to aridity and then deposited by the wind
at a relatively recent period. It is maintained by some
geologists l that the deposition of loess is still going on
in certain parts of the world, but that the rate of accumu-
lation is so exceedingly slow that it escapes the notice
of all but trained observers. The lack of fossils, par-
ticularly those of plants, is accounted for by this slow
rate of formation, which allows sufficient time for all
organic matter to become fully oxidized before being
covered by the drifting material. Snail shells are often
found, but as they are of land species they argue against
a water origin of loess.
1 Merzbacher, G. The Question of the Origin of Loess. Mitt.
Justus Perthes' Geogr. Anst., 59, pp. 16-18, 69-74, and 126-130.
1913.
GEOLOGICAL CLASSIFICATION OF SOILS 61
47. Distribution of loess. Not only is loess found
over thousands of square miles in the central part of the
United States, but it occurs elsewhere in large areas.
It is greatly developed in northern France and Belgium,
and along the Rhine in Germany, where it is an important
soil in all the valleys that are tributary to that river.
Silesia, Poland, southern Russia, Bohemia, Hungary,
and Roumania, all have deposits of this highly fertile
material. In Europe it extends from sea level to eleva-
tions of 5000 feet, showing its independence of water
as a formative agent. In China it is found over a very
large part of the valley of the Hoangho, a region prob-
ably larger in area than France and Germany combined.
The thickness of the deposit is variable, ranging from a
few feet to several thousand feet in certain places. The
depth is practically always sufficient for any form of
agricultural operations.
Wherever moisture relations are favorable loess is an
exceedingly fertile soil, due to its rich stores of potash,
phosphorus, and lime. Its organic content is usually
medium to high, depending on conditions. In general
it may be classified as the richest soil in the world,
considering its wide extension and the great variety
of climate and of crops to which it is subjected. In
the United States it occurs in the Corn Belt region,
and might be called the great corn soil of the Mississippi
Valley.
48. Adobe soils. The term adobe is a name applied
to a fine calcareous clay or silt formed in a manner some-
what like that in which loess is formed. It is supposed
that, while part of the deposit came from the waste of
talus slopes as mountains were weathered under conditions
of aridity, the remainder had an origin similar to that of
62
SOILS: PROPERTIES AND MANAGEMENT
loess. 1 Certain characteristics also seem to indicate that
the valley adobe might have been deposited by water. 2
It appears, therefore, that, while the physical characters
of all adobe are somewhat similar, its mode of origin and
chemical composition may be variable. Below are the
analyses 3 to two typical adobe soils :
A
B
SiO 2
66.69
4464
AloO 3
14 16
13 19
Fe 2 O 3
4.38
5 12
CaO
249
13 91
MgO
1.28
296
K 2
NaaO
1.21
67
1.71
59
CO 2
.77
8.55
P 2 O 5
29
94
Organic matter
2.00
3.43
Like the loess, adobe is an exceedingly rich soil, but
it occurs in an arid or a semiarid region. When irrigated,
its fertility seems inexhaustible. It is found in Colorado,
Utah, southern California, Arizona, New Mexico, and
Texas. It has an especially wide distribution in New
Mexico. Like loess its elevation is variable, ranging
from sea level in California and Arizona to 6000 feet
along the eastern border of the Rocky Mountains. Its
maximum thickness cannot be estimated, as it is very
1 Russell, I. C. Subaerial Deposits of the Arid Regions
of North America. Geol. Mag., August, 1889, pp. 342-350.
2 Hilgard, E. W. Relations of Soil to Climate. U. S. Weather
Bur. Bui. 3. 1892.
3 Merrill, G. P. Rocks, Rock Weathering, and Soils, p. 321.
New York. 1896.
GEOLOGICAL CLASSIFICATION OF /SOILS 63
little eroded and is supposed to be still accumulating.
Some valleys are known to be filled to a depth of 3000
feet with this material. Its characteristics are its fine
texture, its great depth, its wide distribution, and its
great fertility when moisture conditions are suitable for
crop growth.
49. Sand dunes. Sand dunes are the outgrowth
of two conditions a large quantity of sand and a
wind that blows in a more or less prevailing direction.
Under such conditions the sand and other fine material
not only is blown into heaps, but also tends to move in
the direction of the prevailing wind. Such heaps or
mounds of sand may travel several feet a day by the
continual movement of the sand grains up the windward
side of the dune, only to be deposited again on the lee-
ward side. Sand dunes may often assume gigantic
proportions, being sometimes several hundred feet high
and twenty or thirty miles long. In such proportions
they become a grave menace to agriculture, not only
because they are an absolutely valueless medium for plant
growth, but also because they cover fertile lands and
entirely blot out all plant growth. The particles of this
wind-blown sand are usually round, from the continual
abrasion that they receive. A great many minerals
may be represented, but quartz is the commonest, es-
pecially if the dune originally had its origin on a lake or
a seashore.
50. Volcanic dust. From early geologic times de-
posits of the very fine material that is continually being
ejected from volcanoes have been distributed over the
earth's surface. These deposits are usually flour-like,
and while at one time they probably covered many
square miles of territory, they have succumbed very
64 SOILS: PROPERTIES AND MANAGEMENT
largely to erosion and denudation, and only remnants
are found at the present time. Such material may be
found in Montana, Nebraska, and Kansas. ^Eolian
deposits of this character are usually rather porous and
light, and are likely to be highly siliceous. They are
not of great agricultural importance.
CHAPTER V
CLIMATIC AND GEOCHEMICAL RELATION-
SHIPS OF SOILS
ALTHOUGH during the process of weathering the tendency
of all soil is toward a common composition, such a con-
dition is never reached, due to different kinds and varying
intensities of decay and disintegration. Soils lend them-
selves readily to a geological classification because of this
difference in mode of formation. Such a classification
really signifies a variation in composition. A difference
in age, a preponderance of physical agencies over chemi-
cal or vice versa, a difference in the transportive agencies,
or a variation in climatic conditions after a soil is once
formed, will assuredly give a different product, not only
chemically, but physically and biologically as well.
51. Climatic relationships. It is evident that climate
is a factor in all geochemical relationships of soils. Not
only does climate determine the kind of weathering and
its intensity, but in many ways it influences very largely
the characteristics of the soils of different provinces and
sections. Climate must be considered also in the geo-
logical classification of soils, since it plays such an im-
portant role in determining the kind and intensity of the
formative agents. In any scheme of grouping for the
systematic survey and mapping of soils, climate is the
very first factor to be considered. It gives three great
groups tropical, subtropical, and temperate. These
may in turn be subdivided into arid, semiarid, and humid.
P 65
66
SOILS: PROPERTIES AND MANAGEMENT
In the utilization of soil, climate, particularly as regards
rainfall and temperature, plays an important part. Crop
adaptation is really more of an adaptation to climate than
to soil, although the latter also should be very carefully
studied. The climatic relationships in soil formation,
in soil chemistry, and in geochemistry in general, cannot
be too strongly emphasized, whether the viewpoint be
technical, practical, or merely educational.
52. Geochemical relationships of residual and marine
soils. It is evident from the above that coastal plain,
residual, and glacial soils should exhibit certain well-
defined general differences due to their mode of formation.
The following analyses, which are representative of the
provinces in question, illustrate the chemical differences
of coastal plain and residual soils :
ANALYSES OF TYPICAL COASTAL PLAIN AND RESIDUAL SOILS
LIGHT SANDY
LOAM PROM
MARYLAND
AVERAGE OF
5 SAMPLES 1
CORN AND
WHEAT CLAY
LOAM SOIL
AVERAGE OP
3 SAMPLES^
RESIDUAL
SOIL PROM
VIRGINIA
GNEISS 2
RESIDUAL 1
SOIL FROM
VIRGINIA
LIMESTONE *
SiO 2
92.30
80.55
45.31
57.57
A1 2 O 3
320
882
2655
2044
Fe 2 O 3 ....
P 2 5
CaO
.91
.05
41
2.67
.42
.47
12.18
.47
trace
7.93
.10
.51
C0 2 .....
MgO
.08
35
.05
29
trace
.40
.38
1.20
Na 2 O. . . . .
K 2 O . .
.50
70
.49
1 22
.22
1 10
.23
4.9J
1 Veitch, F. P. The Chemical Composition of Maryland
Soils. Maryland Agr. Exp. Sta., Bui. 70, pp. 71 and 73. 1901.
2 Merrill, G. P. Weathering of Micaceous Gneiss in Albemarle
County, Virginia. Bui. Geol. Soc. Amer., Vol. 8, p. 160. 1879.
3 Diller, G. S. The Educational Series of Rock Specimens.
U. S. Geol. Survey, Bui. 150, p. 385. 1898.
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 67
It is to be noted, in the first place, that silica exists in
large quantities in the coastal plain soils, due to the fact
that quartz is such a resistant mineral. The constant
washing that this soil has undergone has very largely
decomposed the silicates. The aluminium and iron are
rather low on the average in such soils, even in those of
the richer type. It is to be noted also that the amounts
of phosphoric acid, calcium, potash, magnesia, and sodium
are much less in the marine soils, due to the excessive
washing that they have received. These figures would
lead to the belief that in general the marine soils are
lower in the mineral plant-food constituents than soils
formed in situ. The amount of organic matter that they
may contain depends entirely on their location and
climatic conditions. They may or may not be rich in
humus, according to circumstances. It is generally con-
sidered, however, that they are not so well supplied with
the organic elements as are other soils.
53. Residual and glacial soils. A comparison of
residual and glacial provinces cannot be made with such
assurance, because of the many kinds of rocks that may
have been parent to the soils and because of the great
variety of climatic conditions under which the soil-form-
ing processes may have gone on. Such a comparison is
best made in a region where both residual and glacial soils
are found, as nearly as it is possible to judge, coming
from the same rocks. Analyses of soils under such con-
ditions are available, from the driftless and glaciated
parts of Wisconsin. The original rock was limestone.
The analyses 1 are as follows :
1 Chamberlin, T. C., and Salisbury, R. D. The Drift-
less Area of the Upper Mississippi. Sixth Ann. Kept., U. &
Geol. Survey, pp. 249-250. 1885.
68
SOILS: PROPERTIES AND MANAGEMENT
ANALYSES OF RESIDUAL AND GLACIAL CLAYS FROM THE DRIFT-
LESS AND GLACIATED AREAS OF WISCONSIN
RESIDUAL
GLACIAL
1
2
3
4
SiO 2 .
71.13
12.50
5.52
.38
.85
2.19
1.61
.02
.43
4.63
49.13
20.08
11.04
1.92
1.22
1.33
1.61
.04
.39
11.72
40.22
8.47
2.83
7.80
15.65
.84
2.36
.05
18.76
1.95
48.81
7.54
2.53
7.95
11.83
.92
2.60
.13
15.47
2.02
A1 2 3
Fe9(X
MgO ....
CaO
Na 2 O ....
K 2 O
P 2 O 6 . . .
C0 2 .....
H 2 O . . .
These analyses illustrate to very good advantage the
beliefs entertained by Chamberlain and Salisbury regard-
ing the differences between residual and glacial clays.
Residual clay is designated by them as " rock rot/' and
glacial clay as " rock flour." The latter, being less
weathered, retains a larger proportion of its easily soluble
materials. It is to be noted here, as in the comparison
of marine and residual soils, that silica, aluminium, and
iron are lower in the soil subjected to the less amount of
leaching, which in this case is the glacial clay. This in
itself would serve to indicate that the important plant-
food constituents are generally present in larger quanti-
ties in the glacial clay. In fact, it would be expected
that the glacial soils would approximate very closely the
rock or rocks from which they came. The phosphoric
acid, lime, sodium, magnesium, and potash of the residual
soils in this case amount on the average to 5.73 per cent,
while that of the glacial clays reaches the high figure of
24.61 per cent. This is due largely to the great amount
CLIMATIC AND GEOCTJEMICAL RELATIONSHIPS 69
of lime present, and again emphasizes the point that,
while a glacial soil from a limestone is rich in lime, a
residual soil from the same rock is usually poor in that
constituent. Even loess, which has been subjected to
some washing before being deposited, is a considerably
richer soil than those of residual origin.
It must be remembered, however, that these comparisons
are of a general character and do not apply to all cases,
since many glacial soils may be very much poorer in the
plant-food constituents than some of the representative
residual soils. Moreover, the physical condition of a
soil is a great factor in productivity. As a matter of
fact, the mere presence of plant-food is but one of a
considerable number of factors that determine the crop-
producing power of a soil. Also, the humus content of
the soils of various provinces may be variable, due to
climatic conditions. Neither are all glacial soils rich in
lime, as that constituent is determined largely by the
amount in the parent minerals. A rock poor in lime,
therefore, must from necessity give rise, when glaciated,
to a soil deficient in lime. This is well illustrated by the
average analyses 1 of the loam soils of Ashtabula County,
Ohio, originating from the glaciation of the lime-poor
shales of that region :
CaO 25
MgO 61
P 2 5 04
K 2 O 1.87
N 15
Humus 1.70
1 Ames, J. W., and Gaither, E. W. Soil Investigations. Ohio
Agr. Exp. Sta., Bui. 261. 1913.
70
SOILS: PROPERTIES AND MANAGEMENT
However, our major premise does seem to stand in a
general way that a glacial soil, other things being
equal, contains a larger amount of the mineral plant-food
constituents, and ordinarily a smaller amount of such
materials as silica, iron, and aluminium, than does a
corresponding soil of residual origin.
The following data 1 bring out the points already dealt
with in their fullest significance:
PERCENTAGE OF P 2 O 5 , CAO, MaO, AND J 2 O IN SOILS OF DIF-
FERENT PROVINCES
SOILS
P*Os
CaO
MgO
K 2 O
TOTAL
7 Coastal plain ....
.07
.14
.16
.70
1.07
3 Residual (crystalline) .
.25
.67
.75
2.08
3.75
10 Glacial
.22
1.36
.79
2.08
4.45
54. Effect of glaciation on agriculture. These differ-
ences between residual and glacial soils reflect on the
general fertility of the soils. In a comparison of thr
driftless area of Wisconsin with the glaciated parts, 2
only 43 per cent of the former is improved as against
61 per cent of the latter, while the value of the farms
on the glaciated soil averages 50 per cent higher. The
same general differences appear between the glacial and
residual soils of Indiana 3 and Ohio. 4
1 Failyer, G. H., and others. The Mineral Composition
of Soil Particles. U. S. D. A., Bur. Soils, Bui. 54. 1908.
2 Whitbeck, R. H. The Glaciated and Driftless Portions of
Wisconsin. Bui. Geog. Soc. Phil., Vol. IX, No. 3, pp. 10-20. 1911.
3 Von Engeln, O. D. Effects of Continental Glaciation on
Agriculture. Bui. Amer. Geog. Soc., Vol. XLVI, p. 246. 1914.
4 Ames, J. W., and Gaither, E. W. Soil Investigations.
Ohio Agr. Exp. Sta., Bui. 261. 1913.
CLIMATIC AND GEOCUEMICAL RELATIONSHIPS 71
Von Engeln, 1 in a comparison of glaciated soils with
corresponding residual areas, was able to point out cer-
tain general differences. The agricultural condition
within the zone of glaciation was always consistently
higher than that beyond the regions of drift accumulation.
The extensive leveling due to glacial erosion and dep-
osition had almost always resulted favorably for agri-
cultural operations. Even the thickness of the drift
was found to conserve the ground water supply. Not
only did this author conclude that glacial soils were
richer in soluble plant-food constituents than residual
soils, but he also showed that glacial soils had a greater
crop-producing power and a higher agricultural value.
The dominant textural quality of glacial soils seems
adapted to certain staple food crops, and, due to their
intermingling, a considerable opportunity for diversified
and intensified farming is offered. It is therefore evident
that in any study of soils, particularly those of the United
States, a careful consideration of the effects of glaciation
is necessary. The great ice sheet has been responsible
in some cases for the rejuvenation of our soils, in others
for the production of an entirely new soil mantle. Even
the alterations in topography are factors not to be ignored.
55. Arid and humid soils. 2 - - This distinction between
soils due to differences in the formative process is always
evident, but is particularly striking in a comparison of arid
and humid regions. In areas of light rainfall the physical
agents are dominant, and disintegration goes on very
largely without decomposition. Under humid conditions,
1 Von Engeln, O. D. Effects of Continental Glaciation on Agri-
culture. Bui. Amer. Geog. Soc., Vol. XLVI, pp. 353-355. 1914.
2 For a more complete discussion of this subject, see Hilgard,
E. W. Soils, Chapters XX and XXI. New York, 1911.
72 SOILS: PROPERTIES AND MANAGEMENT
however, the chemical forces are the determining factor
as to the character of the soil. Arid soils are therefore
usually coarser soils and their color is very likely to be
light. Such soils are deep and uniform, there being
but little difference between the surface and the subsoil.
The soils of the humid regions are usually of fine texture,
particularly in residual regions, since the chemical agencies
have been so active. Various colors may develop because
of oxidation, hydration, and the presence of organic matter.
Such soils usually are not excessively deep, and are likely
to be underlaid by subsoils heavier than the surface.
The general physical condition and tilth of arid soil is
uniformly better than that of regions of plentiful rainfall.
Chemically, because of less leaching the arid soils con-
tain more of the important mineral plant-food elements.
The following analyses bring out the differences in a
striking manner :
ARID SOILS
AVERAGE OP
573 SAMPLES 1
HUMID SOILS
AVERAGE OF
696 SAMPLES 1
AVERAGE
COMPOSITION OF
LlTHOSPHERE*
Insoluble residue and soluble
SiO 2 ....
7587
88.21
59.36 (SiO 2 )
A1 2 O 3
7.21
3.66
14.81
Fe->CK
548
3.88
6.34
P 2 O 6
.16
.12
.29
CaO
1 43
.13
4.78
MgO
1.27
.29
3.74
Na 2 O
.35
.14
3.35
K 2 O
.67
.21
2.98
Water and ignition . . .
Humus ....
5.15
1.13
4.40
1.22
1 Hilgard, E. W. The Relation of Soil to Climate. U. S.
Weather Bur., Bui. 3. 1892.
2 Clarke, F. W. Data of Geochemistry. U. S. Geol.
Survey, Bui. 491, p. 33. 1911.
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 73
It is immediately apparent that the arid soil is poorer
in silica than the humid soil, but richer in iron and alumin-
ium, indicating a less weathered condition of the feldspars.
Due to a greater amount of leaching, the humid soil is
much lower in phosphoric acid, lime, magnesium, sodium,
and potassium. The humus in arid soils is somewhat
lower than in the soils under better conditions of rainfall,
as one would naturally expect. The amount of easily
soluble material is higher in arid regions, due to the lack
of heavy rain and the tendency for soluble salts to accumu-
late. A comparison of the analyses above with Clarke's
estimate regarding the composition of the earth shows
that the humid-region soil has moved farther away from
the average soil-forming rock than the soil produced
under conditions of aridity.
Biologically, organisms are found active at greater
depths x in arid regions than in humid regions, because
of the loose structure of arid soils and because of their
good aeration. Such soils are seldom water-logged. In
humid regions bacterial action is limited very largely
to the surface foot of soil, since only there are the aeration
and the food conditions adequate. The intensity of
biological activity in arid soils is very largely governed
by moisture, and when moisture conditions are satisfied
bacterial changes may be expected to take place rapidly.
Cases are on record in which the soluble salts due to
bacterial action have become of such concentration as
to be toxic to plants.
56. Soil color. Another characteristic of soil is its
color, which has originated during the processes of soil
1 Lipman, C. B. The Distribution and Activities of Bacteria
in Soils of the Arid Region. Univ. of Calif., Pub. in Agr. Sci.,
Vol. I, No. 1, pp. 1-20. 1912.
74 SOILS: PROPERTIES AND MANAGEMENT
formation, largely through natural weathering agencies.
This is really a phase of geochemistry, particularly as
regards those tints that originate from the oxidation of
the iron. Color has long occupied the attention of
geologists and agriculturists, in the first place because
it gives a clew to the mode of soil formation, and in the
second place because it is to a certain extent an index to
agricultural value. At the outset it must be understood
that soil colors are not pure colors, although spoken of
as such, but tints and shades. In soils it is possible to
find almost any conceivable color, ranging from white
sands to black swamp muds or the blood-red clays of the
Piedmont region. The three coloring matters of soil
may be classified as (1) the color arising from the mineral,
(2) the color given by the humus present in the soil and
around the particles, and (3) the reds or the yellows due
to oxidization of the iron. These three primary, or
basal, colors may be represented for convenience as
follows :
Wtt/TE
BLACK
FIG. 9. A triangular representation of the three primary soil colors
and their mixtures.
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 75
A soil low iii humus, and with the iron either absent or
unoxidi/ed, will be of a light color. Sea sands are good
illustrations of this condition. A well-drained soil con-
taining large quantities of organic matter will present a
deep black color in spite of the oxidized iron, as the latter
will be masked to a large extent. If humus is low or
lacking and the iron -is oxidized, a red or a yellow color
may characterize the soil. As might be expected, there
are blendings of these three primary colors, and grays,
browns, and yellows of varying intensities are common.
57. White and black soils. The light colors in soils
are not due to the agencies of weathering, but rather to
a lack of such action. The cause of such coloration is
therefore not hard to explain. The development of the
black or dark colors and tints, being due to the accumula-
tion of organic matter, indicates the operation of two
favoring conditions: first, climatic agencies that stimu-
late the luxuriant development of plants ; and, secondly,
sufficient aeration to promote a favorable decay of such
tissue. It is a well-recognized fact that in order to
develop a black color from decaying vegetable matter,
fairly good aeration must be provided. If such a con-
dition does not prevail, the decayed material has a lighter
hue and may exhibit toxic properties which will check
or inhibit plant growth. The development of the black
color, therefore, in a normal well-drained soil, is an in-
dication of good soil sanitation.
58. Red and yellow soils. The presence of iron, as
already noted, is a very important factor in rock weather-
ing, and the discoloration due to its presence is an unfailing
indication of chemical decay. The iron in minerals
occurs usually as ferrous oxide, which is soluble, especially
if the water circulating among the rock fragments carries
76 SOILS: PROPERTIES AND MANAGEMENT
carbon dioxide. As this water comes in contact with the
air, its excess of carbon dioxide is discharged and the
oxides and carbonates of iron are deposited. Under
this condition oxidation goes on rapidly, and the iron
passes to the ferric state and becomes insoluble. Thus
it may be seen that iron imparts a fatal weakness to rocks
and minerals in which it may exist,- due to its solubility ;
yet from the oxidation that it undergoes, it tends to
persist and accumulate in soils. A corollary might be
added to the law of mineral resistance, to the effect that
" the more iron a mineral contains, the more susceptible
it is to the weathering agencies."
Therefore, from the geochemical standpoint, the de-
velopment of the red and yellow colors in soils has been
the subject of considerable dispute from time to time.
The red and yellow soils of the Cotton States frequently
excite comment, especially as a difference in fertility is
popularly recognized ; the red surface soil with a red
subsoil being considered more fertile than a similar soil
with a yellow subsoil. Crosby l believes that the differ-
ence in color is due to a difference in hydration of the
iron oxides. The soil temperatures, particularly in
tropical and subtropical regions, have first tended to
fully oxidize and hydrate the iron, and then to dehydrate
the soil at the surface into the deep red color, leaving the
subsoil yellow and causing the contrasts so markedly
evident. The ultimate product of both oxidation and
hydration would be limnite, a yellow mineral; while
if only oxidation were active, hematite, which imparts
a red color, would result as a final product. A dehydra-
tion of the limnite would cause the formation of hematite
1 Crosby, W. O. Colors of Soils. Proc. Boston Soe. Nat. Hist.,
Vol. 23, pp. 219-222. 1875.
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 77
or some intermediate product. The composition of the
common iron oxides found in soils tends to support
Crosby's explanation :
Hematite .... Fe 2 O 3 Red
Turgite .... 2 Fe 2 O 3 . H 2 O
Goethite .... Fe 2 O 3 . H 2 O
Limonite .... 2 F 2 O 3 . 3 H 2 O
Xanthosiderite . . Fe 2 O 3 . 2 H 2 O
Limnite .... Fe 2 O 3 . 3 H 2 O Yellow
Merrill l holds the same idea, but thinks that the sur-
face soil may contain relatively more iron than the sub-
soil. He considers that the ferric iron oxides, because
of their insoluble nature, tend to accumulate at the
surface, and because of their large quantities and because
they are there subjected to more vigorous weathering
action the vivid red colors tend to develop.
The iron coloring matter usually exists as a coating 2
on the soil particles, although it may sometimes occur
as concretions. It is found also that in general, but not
always, the intensity of the color varies with the amount
of iron present. From a large number of analyses com-
piled by Robinson and McCaughey, 3 the following figures
may be obtained showing the authority for such a state-
ment :
AVERAGE IRON CONTENT OF PERCENT OF
SOILS FERRIC IRON
Deep reds to light reds 14.40
Ochre yellow to yellow 8.85
1 Merrill, G. P. Rocks, Rock Weathering, and Soils, p. 375.
New York. 1906.
2 Van Bemmelen, J. M. Beitrage zur Kenntnis der Ver-
witterungsprodukte der Silicate in Ton-, Vulkanischen-, und Lat-
erite-Boden. Zeit. Anorg. Chem., Bd. 42, Seite. 290-298. 1904.
3 Robinson, W. O., and McCaughey, W. J. The Color of
Soils. U. S. D. A., Bur. Soils, Bui. 79, p. 21. 1911.
78 SOILS: PROPERTIES AND MANAGEMENT
This being true, the thicker the film, the greater is the
intensity of the color. The same quantity of iron, there-
fore, would make a greater showing in a sandy soil than
in clay, as the amount of internal surface of the former is
comparatively low and the film of iron oxide would there-
fore be thicker.
It is evident from the data already presented that
the intensity of color arising from iron in the soil is due
to several conditions. Without a doubt the oxidation
that occurs is of primary importance, but the hydration
that very often takes place is a powerful modifying agent.
The thickness of the film, as determined by the amount
of iron present or by the texture of the soil, is probably
a factor having to do particularly with the intensity of
the coloration, although the color or tint itself may be
modified to a certain extent thereby.
59. Agricultural significance of color. The white or
the black color of a soil indicates the lack or the presence
of an important constituent, namely, organic matter.
This matter not only tends to keep the soil in good
physical condition, but also acts both as a plant-food
and as a source of energy for bacteria and other soil
organisms. A dark soil, provided its drainage and
climatic conditions are favorable, is usually a rich soil.
The dark color is no mean factor in temperature relation-
ships, since not only does a dark soil absorb heat faster
than a light soil, but the tendency of the former toward
reflection and radiation is much restricted. This is
important with crops which must go into the soil early
in the season, or which need to be pushed rapidly to
maturity. A dark color, with virgin soils especially, is
an excellent guide to fertility and general agricultural
value.
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 79
Red colors in soil often show low or medium organic
content. Besides this, the presence of oxidized iron is
always an indication of age. The residual soils of the
Piedmont Plateau are especially characterized in this
way. Age gives opportunities for leaching, and conse-
quently a lack of the soluble bases may be expected in
such soils. The reds and the yellows are characteristic
of residual soils, or of soils that have arisen from them
by erosion or glaciation. A red color is almost as efficient
in the absorption of heat as is black; so that the early-
growth and quick-maturing tendencies of crops on a
red soil, other things being equal, are about the same as
on a dark soil. Hilgard, 1 who lays great stress on the
agricultural significance of color, considers the mottled
yellows and reds as indications of poor drainage, since
such a condition shows that oxidation has been both
unequal and insufficient. A soil that has a heavy blue
or mottled blue clay as a subsoil will in most cases be
greatly benefited by some form of drainage.
60. Soil and subsoil. A common distinction is made
between the surface soil and that which is some distance
below the surface. This is natural, as the forces of soil
formation have served to bring about certain distinctions,
especially in humid regions, which are of importance in
any consideration of soil fertility and crop growth. Cli-
matic agencies acting on soil after it has been formed have
served to intensify these distinctions. The term soil
is used to designate the top layer of earth, which usually
extends to the plow line or even deeper. The soil below
is spoken of as the subsoil, and may be rather variable in its
depth (Fig. 10) . Often the subsoil is divided into the upper
1 Tfilgard, E. W. Soils, pp. 283-285. New York. 1906.
80
SOILS: PROPEETIES AND MANAGEMENT
and the lower subsoil, the upper subsoil being considered
to extend to about the depth of three feet below the sur-
face. Usually, especially in humid regions, there is a
FIG. 10. Soil and subsoil, (a), Surface soil with many plant roots;
(6), subsoil; (c), country rock.
sharp line of demarcation between the soil and the sub-
soil, due to differences in the humus content. As organic
matter accumulates faster at the surface, the soil there
tends to assume a darker color. Whether the land has
been tilled or not, this line of separation is fairly marked
and can usually be located with little difficulty. On
tilled land, where the surface soil extends to about the
depth of plowing, the plow line marks the separation of
surface and subsoil. Where soil samples are being taken
for soil survey or soil analysis, some arbitrary depth,
depending on circumstances, is usually established for
the surface soil. This depth varies from six to twelve
inches.
61. Soil and subsoil of humid regions. In humid cli-
CLIMATIC AND GEOCHEM1CAL RELATIONSHIPS 81
mates there are usually certain well-defined differences be-
tween the surface soil and the subsoil, besides the organic
content already spoken of." The subsoil is usually of a finer
and heavier character than the surface soil, due to the
downward movement of the small particles. This tends
to give the subsoil high retentive power, and may make
it rather impervious to water. Poor drainage conditions
may result. A certain amount of retentive power in a
subsoil is of considerable advantage, in that it aids in
the storage of water and prevents the excessive leaching
away of soluble plant-food. Moreover, almost all the
bacterial activities so important in the simplification of
compounds carrying food constituents are. restricted
to the surface soil. The subsoil, being protected by the
layers above, has not been subjected to such vigorous
weathering, and as a consequence its mineral constituents
are not so available for the use of the crop. The deepen-
ing of the plow line and a consequent turning-up of the
subsoil must be carried out very cautiously for the above
reason. The cropping power of a soil may be markedly
reduced by the presence of too much of such material on
the surface at one time.
The root distribution is restricted largely to the surface
soil, and this condition determines to some extent the
larger accumulation of humus therein and also its better
aeration and drainage. Experiments conducted in Utah 1
show that with barley, corn, and clover, from 90 to 96
per cent of the roots grow in the upper seven inches of
soil. From experiments made in Kansas 2 and in North
1 Sanborn, J. W. Roots of Farm Crops. Utah Agr. Exp.
Sta., Bui. 32. 1894.
2 Ten Eyck, A. M. The Roots of Plants. Kansas Agr. Exp.
Sta., Bui. 127. 1904.
G
82 SOILS: PROPERTIES AND MANAGEMENT
Dakota, 1 the roots of such crops as alfalfa were found
to penetrate to a depth of ten feet, while the small grains
often showed an extension of their roots to four feet
below the surface. It must be borne in mind, however,
that, while some plant roots may penetrate far into the
subsoil, the main feeding rootlets are restricted largely
to the surface soil. This is natural, as there they find
the aeration and drainage essential to normal growth.
Hilgard 2 has shown that plants in arid regions have a
root extension far beyond that of the same crops under
humid conditions. The physical conditions of the arid
subsoil, the larger amount of plant-food, and the better
aeration, account for such differences.
62. Soil and subsoil of arid regions. The subsoils
in arid or semiarid regions do not exhibit such marked
contrast to the surface soil as are observed in humid
climates. In arid soils there is generally no sharp line
of demarcation between soil and subsoil, the latter being
as high in humus and in agricultural value as the former.
Nor is any great textural variation to be observed. The
latter condition is due to the fact that physical weather-
ing is dominant in such a region. As a consequence, arid
soils may be leveled, often excessively, in establishing
an even surface for the application of irrigation water,
without any danger of lowering the fertility thereby.
Such a practice in humid regions would be fatal to the
further growing of successful crops, at least for a con-
siderable period of years.
1 Sheppard, J. H. Root Systems of Field Crops. N. Dak.
Agr. Exp. Sta., Bui. 64. 1905.
2 Hilgard, E. W. Soils, Chapter X, pp. 161-187. New York,
1911.
CHAPTER VI
THE SOIL PARTICLE
THE soil formed by the grinding-up of rocks and the
intermixing therewith of small quantities of organic
matter must be studied physically from the standpoint
of its particles. These particles, varying in size from
coarse gravel easily discernible by the naked eye to
particles so fine as to be invisible under the ultramicro-
scope, determine very largely the different relationships
of the soil to the plant. The movement of air in the soil,
the circulation of water, the rate of oxidation and hydra-
tion, and the presence and virility of various organisms,
are determined very largely by the size of the particles
making up the soil. Texture is the term used to express
this size of particle. Thus a soil texture may be coarse,
medium, or fine, indicating that the particles making up
that soil conform in general to such description. Tex-
ture is of great importance in soil study and utilization.
There is hardly any condition exhibited by the soil
that is not influenced, if not directly determined, by the
size of the soil particles. A study of plant conditions,
whether physical or chemical, ultimately leads either
directly or indirectly to a consideration of soil texture.
Texture, however, is an element which can be but little
modified under normal conditions. We have seen how
a rock can be disintegrated and decomposed into a soil.
A change in texture has been wrought, but such a process
demands geologic ages for its fulfillment. In the time
83
84 SOILS: PROPERTIES AND MANAGEMENT
covered by the life of man the necessary forces are not
active enough to have this effect; consequently, as far
as the farmer is concerned the texture of the soil in his
field is subject to but slight alteration. A sand remains
a sand and a clay remains a clay, as far as practical con-
siderations are concerned. Changes in texture may be
made on a small scale by mixing two soils, but this is not
practicable in the field.
63. Soil separates and mechanical analysis. The
soil particles, varying in size as they do, may be separated
into arbitrary divisions, according to their diameters.
The various groups are designated as soil separates, and
the process of making the separation and determining
the percentage of each group present is called mechanical
analysis. There are a large number of classifications, or
groupings, of the soil particles, as well as several methods
of bringing about the actual separation. The grouping
and method of mechanical analysis most generally used
in this country is that devised by the United States Bureau
of Soils. 1 Other methods 2 are more nearly accurate,
but speed as well as precision is necessary in this work.
A Swedish classification 3 of soil particles has been adopted
by the Committee on Mechanical Soil Analysis, 4 appointed
1 Briggs, L. J., and others. The Centrifugal Method of Soil
Analysis. U. S. D. A., Bur. Soils, Bui. 24. 1904.
2 For a detailed discussion of all methods of mechanical
analysis, see Wiley, H. W. Agricultural Analysis, Vol. I,
pp. 195-276. Easton, Pa. 1906.
3 Atterberg, A. Die Mechanische Bodenanalyse und die
Klassifikation der Mineralboden Schwedens. Internal. Mitt,
f. Bodenkunde, Band II, Heft 4, Seite 312-342. 1912.
4 Schucht, F. Uber die Sitzung der Internationalen Kom-
mission fur die Mechanische und Physikalische Bodenunter-
suchung in Berlin am 31, October 1913. Internal. Mitt, f
Bodenkunde, Band IV, Heft I, Seite 1-31, 1914.
THE SOIL PARTICLE
85
by the Second International Agro-Geological Congress,
which met in Stockholm in 1910. In simplicity and
facility of interpretation the last-named grouping seems
at least equal to that of the Bureau of Soils. Since a
number of methods of mechanical analysis have been
devised during the evolution and study of soil separation,
it is necessary to be conversant with the principles in-
volved and with at least two or three of the most successful
modes of procedure.
64. Principles of mechanical analysis. The various
methods of mechanical analysis may be grouped according
to the agents employed in the separation. The outline is
as follows :
1. Sieve
Outline of systems of mechanical analysis
Wet
or (Used to separate sands in practically all
-Q methods)
2. Air (Cushman's air elutriator)
Gravity (Schone's elutriator and
3. Water
In motion
At rest
Hilgard's churn elutriator)
Centrifugal (Yoder's centrifugal
elutriator)
Gravity (Osborne's beaker method
and Atterberg's modified silt
cylinder)
Centrifugal (Bureau of Soils
method)
In the consideration of such an outline, certain of the
general methods proposed may be dismissed without
further parley since they are inadequate for the separation
86 SOILS; PROPERTIES AND MANAGEMENT
in question. Sieves of all kinds have the one great dis-
advantage that their meshes cannot be made small enough
to separate the finer grades of soil. When one considers
that many soil particles are less than .005 millimeter in
diameter, the inadequacy of sieve separation becomes
apparent. However, sieves may be used in connection
with other methods as an easy way of dealing with the
larger soil particles. Air in motion x is inadequate, as
it can be used only for very fine particles. Even with
these the separation is slow and inaccurate, because of
the tendency of the dry particles to cohere. These two
methods have therefore been largely abandoned as dis-
tinct methods, and water is used as the medium of separa-
tion in all the modern systems of mechanical analysis.
The principle involved in the subsidence of soil particles
in water, whether the force of gravity or centrifugal
force is utilized, is recognized by every one. When
fragments of rock or soil are suspended in water, they
tend to sink slowly, and it is a well-recognized fact that
other things being equal, the rate of settling depends
on the size of the particle. As the particle is decreased
in size, its weight decreases faster than 'the surface ex-
posed to the buoyant force of the water. As a conse-
quence, the rapidity with which the soil particles settle
is proportional to their size. The suspension of a sample
of soil would therefore be the first step in mechanical
separation by water ; the second step would be subsidence
and the withdrawal of each successive grade of particles
as it slowly settled ; the third step would be determina-
tion of the percentage of each grade, or group, of particles
1 Cushman, A. S., and Hubbard, P. Air Elutriation of
Fine Powders. Jour. Amer. Chem. Soc., Vol. 29, No. 4, pp.
589-597. 1907.
THE SOIL PARTICLE
as based on the original sample. This is
every method of mechanical analysis in
utilized aims to do, although often the
technique are excessively complicated.
65. Mechanical analysis by water in
motion. Schbne's elutriator. Any ap-
pliance that is designed to separate
particles of different sizes by water in
motion may be designated as an elutria-
tor. One of these, commonly used in
F,urope, is called Schone's elutriator. 1
This utilizes hydraulic force. In it the
upward current of water ascends a coni-
cal glass tube (see Fig. 11) from a
narrow curved inlet tube below. The
soil sample present in the inlet tube is
kept agitated by the current. It is evi-
dent that by regulating the rate of flow
of the water, different sizes of particles
will be carried away over the top of
this conical glass tube. Thus by a gentle
flow only fine grades will be separated,
while by increasing the current larger
and still larger particles will be carried
upward against the force of gravity.
There are three objections to this
method : first, the entrance tube may
become clogged, and, unless a very small
precisely what
which water is
apparatus and
/
FIG. 11. Schone's
elutriator for me-
chanical soil
analysis with
water in motion.
1 Schone, E. Ueber Schlammanalyse. Bui. Soc. imperiale
d<s Naturalistes de Moscow, 40, Part 1, p. 324. 1867. Uber
Schlammanalyse und einen neuen Schlammapparat. Berlin,
1S()7. Also see Wiley, H. W. Agricultural Analysis, Vol. I,
pp. 231-241. Easton, Pa. 1906.
88
SOILS: PROPERTIES AND MANAGEMENT
quantity of soil is used, the mass is not kept properly
agitated; secondly, convex currents are set up in the
conical glass tube, which vitiates the results ; and, thirdly,
the separate particles tend to coalesce into granules. It
is evident that in any separation of soil particles all
granulation must be avoided. This is usually accom-
plished by shaking or boiling the
sample previous to the determi-
nation. The tendency toward
granulation during the process
of separation itself is fatal to
accuracy, as compound particles
carrying a large number of small
grains would fail to pass over
at water-current velocities cor-
responding to their component
parts.
66. Hilgard's churn elutriator.
The errors of the Schone ap-
paratus are obviated to some
extent by Hilgard's, 1 the prin-
ciple of operation remaining ex-
actly the same. In Hilgard's
elutriator the deflocculated soil
sample is introduced into the
FIG. 12. Hilgard's ehum base of a cylindrical tube (see
elutriator for mechanical pj 12 ) in which is placed a
soil analysis of particles . .
above .01 mm. in diameter, rapidly revolving stirrer. IhlS
(), intake; (p), stirrer; j s designed to counteract convex
(c), screen; (a), separating
chamber; (o), outlet tube, currents and to prevent the
1 Hilgard, E. W. Methods of Physical and Chemical Soil
Analysis. Ann. Kept. California Agr. Exp. Sta., pp. 241-257.
1891-1892.
THE SOIL PARTICLE 89
formation of compound particles. A screen placed just
above the stirrer serves to prevent the whirling motion
from being communicated to the ascending column of
water in which the separation occurs. The various grades
in the separation are regulated by the rate of water flow.
With this apparatus it is necessary to remove the finer
particles below .01 mm. in diameter by subsidence
previous to the determination.
While this method is very nearly accurate and will
give a separation of the various grades such as is impossible
with most other methods, it is impracticable in ordinary
soil work. The large quantity of water which is used
in carrying over each grade, and which of course must
be evaporated before the sample can be weighed, is the
first objection. The length of time necessary for the
separation, and the cost of the apparatus, are two addi-
tional objections urged against it. As mechanical analysis
is used largely in determining soil texture, rapidity and
ease of operation are of more importance than the ex-
tremely accurate separation of the particles.
67. Yoder's centrifugal elutriator. One of the ob-
jections to the methods already described is the length
of time necessary for a determination. This is due to
the fact that very fine particles subside in water very
slowly. In order to hasten the separation, Yoder l
devised a machine in which hydraulic force may be
supplemented by a centrifugal pull. This ingenious
apparatus consists of an elutriator bottle (see Fig. 13)
mounted in a centrifuge. The muddy water is introduced
into the bottle at the center of the centrifuge. It then
passes to the bottom of the bottle and back again to the
1 Yoder, P. A. A New Centrifugal Soil Elutriator. Utah
Agr. Exp. Sta., Bui. 89. 1904.
90
SOILS: PROPERTIES AND MANAGEMENT
outlet, carrying with it a sediment the size of which
depends on the rate of water flow and the strength of
the centrifugal force. The bottle is so designed that
particles in all parts of the separating chamber are sub-
jected to the same force, no matter what their distance
FIG. 13. Separately bottle of Yoder's centrifugal elutriator. (), Bottle ;
(e), intake ; (a), tube for conducting liquid to bottom of separately
bottle; (o), outlet; (C), centrifuge ; (w), counterpoise.
from the center of the centrifuge may be. The apparatus
can be used only for separating particles less than .03
millimeter in diameter. It is open to the same objections
that apply to Hilgard's machine, besides being very much
more complicated and delicately adjusted. It is too
costly an apparatus for ordinary work.
68. Mechanical analysis by water at rest. Osborne's
deaker method. One of the earliest and most nearly
accurate methods to be perfected was the separation of
the various grades of soil by simple subsidence in a column
of still water. This is commonly spoken of as the Os-
borne beaker method. 1 The determination is very
simple. The soil sample is first fully deflocculated and
thrown into suspension, each particle functioning sepa-
rately. Beakers are commonly used as containers, but
1 Osborne, T. B. Methods of Mechanical Soil Analysis. Ann.
Kept. Connecticut Agr. Exp. Sta., 1886, pp. 141-158; 1887, pp.
144-162; 1888, pp. 154-157.
THE SOIL PAliTlCLE 91
any vessel that is relatively deep will do for the deter-
mination. The larger particles, or sand grains, will of
course settle first, and the finer silts and clays may be
dec-anted off. As the sands carry finer particles down
with them, the suspension and subsidence must be re-
peated a number of times. The finer particles, separated
thus and decanted, may be further subdivided in the
same manner. The time necessary for such decantation
as will leave in suspension only particles below a given
size is determined by the examination of a drop of the
suspension under a microscope fitted with an eyepiece
micrometer. In this way the size of the particles decanted
may be accurately measured.
The three steps in this method of separation are :
deflocculation of the sample; separation by successive
subsidence and decantation; and evaporation to dryness
of the separates and their calculation to a percentage
based on the original sample. The method, however, is
slow, as the time necessary for each subsidence of the
finer particles is very great and the number of individual
subsidences is large. Neither is the method capable of
the refinement of separation which is possible with cer-
tain of the elutriators. As a consequence it has been
superseded by methods that utilize centrifugal force for
the finer separations while retaining gravity for removing
the various grades of sand.
69. Atterberg's modified Appiani l silt cylinder (Fig.
14). This method 2 is similar to the beaker method in
1 Appiani, G. Ueber einen Schlammapparat fiir die Analyse
<i<T Boden- und Thonarten. Forsch. a. d. Gebiete d. Agri-
Physik, Band 17, Seite 291-297. 1894.
- Atterberg, A. Die Mechaniscke Bodenanalyse und die
Klassifikation der Mineralboden Schwedens. Internat. Mitt,
f. Bodenkunde, Band II, Heft 4, Seite 312-342. 1912.
92
SOILS: PROPERTIES AND MANAGEMENT
general principle, but a special apparatus is employed by
means of which the various grades obtained by sedimenta-
tion are siphoned off instead of decanted. The cylinder is
really a modified Wahnschaffe cylinder, 1 such as was
used in early soil analyses for drawing off the various
suspensions except that the siphon is placed outside the
cylinder instead of inside.
The cylinder (die Schlammapparat)
as used by Atterberg is about 25 cen-
timeters high, with a glass pedestal
and a ground glass stopper. It is
graduated at 5, 10, 15, and 20 cen-
timeters upward from the bottom.
The same distance is divided also
into 16 divisions at the left of the
first graduation. The latter gradua-
tion is used in the separation of the
clay (Schlamm), so that the height
of the sedimenting column may be
regulated according to the time
available for the settling process.
An outside siphon, 4 to 5 milli-
meters wide, is attached to the cylin-
der at the bottom for the drawing
of the liquid when the sedimenta-
FIG. 14. Atterberg s . P i
silt cylinder for the tlOn IS Complete. Hie top OI this
mechanical analysis s i pnon j s opposite the 5-centimeter
of soil by subsidence. . .
mark on the cylinder. Cylinders
of this size are used only for the separation of particles
below .2 millimeter in diameter; for larger particles a
somewhat taller cylinder is used, with a siphon of the
1 Wiley, H. W. Agricultural Analysis, pp. 205-207. Eastern,
Pa. 1906.
TUK SOIL PARTICLE 93
same width as for the finer particles. The graduation of
the cylinder and its diameter are the same as described
above.
A 20-gram sample of soil is used with this apparatus,
and deflocculation is brought about by means of a stiff
brush. The sample is reduced to a paste in a porcelain
dish, and then, by alternate working with the brush
and decanting, all the particles are thrown into separate
suspension. A deflocculating chemical is used in humus
soils, in order to hasten the process and counteract the
effect of the organic matter. As in the beaker method,
the size of the various grades of separation may be varied
according to the will of the operator.
70. Centrifugal soil analysis. Of the centrifugal
methods used in mechanical analysis, that employed by
the United States Bureau of Soils 1 is the most successful.
A 5-gram sample of well-pulverized soil is put into a
shaker bottle of about 250 cubic centimeters capacity
(see Fig. 15). This bottle is filled about two-thirds
full of water, so that in shaking the disintegrating force
of the liquid may be utilized. A few drops of ammonia
are added, to dissolve the organic matter and to make
deflocculation easier. The sample is then agitated in
the bottle until disintegration is complete. This period
ranges from five to twenty hours, depending on the
sample.
The separation of the silt and the clay from the sands
is made in the shaker bottle by simple subsidence, the
time for decantation being determined by a microscopic
examination of a drop of the suspension. The silt and
1 Fletcher, C. C., and Bryan, H. Modifications of the
Method of Soil Analysis. U. S. D. A., Bur. Soils, Bui. 84.
1912.
94
SOILS: PROPERTIES AND MANAGEMENT
the clay are decanted directly into a test tube fitted into
a centrifuge (see Fig. 15). Whirling at the rate of 800 to
1000 revolutions a minute will cause the subsidence of
the silt to the bottom of the test tube in a few minutes.
The clay is then decanted. The microscope is necessary
here, in order to determine when the settling of the silt
is complete. As small particles tend to cling to the
larger particles, the entire operation must be repeated
FIG. 15. Apparatus for centrifugal mechanical analysis of soil, show-
ing shaker bottle, shaker, centrifuge, and test tube.
several times; therefore the processes of gravity sub-
sidence and centrifugal subsidence are carried on side by
side, material being constantly poured from the shaker
bottle into the centrifuge tubes and from the test tubes
into the receptacles for the clay.
The centrifuge is usually large enough to allow the
separation of several duplicate samples at once. The
various separates made by this method are dried and
THE SOIL PARTICLE 95
The sands, which are obtained in bulk, are
further separated by sieves into the grades desired.
Where a large quantity of organic matter is present, it
must be determined and included in the final report on
the sample.
This method of mechanical analysis as perfected by
the Bureau of Soils has been very generally adopted by
si >il workers. It has many advantages over other methods.
In the first place, it is rapid, often requiring only hours
where other methods take days for completion ; secondly,
it is simple, and the technique of the separation is easily
acquired; thirdly, in the decantations no very large
amount of water is accumulated with the separates,
except for the clay, and thus the time and cost of evapora-
tion is eliminated. The clay, moreover, may be as ac-
curately determined by difference as by direct methods,
thus allowing a further saving of time. The cost of the
equipment for this method is low. The apparatus itself
iinple, and is carried by all standard chemical com-
panies. The same cannot be said of the various elutriator
mechanisms. While the method is accurate only within
one per cent, it is sufficiently precise for practical pur-
poses especially in class determination, for which me-
chanical analysis is generally utilized.
71. Classification of soil particles. With the large
number of different methods of mechanical soil analyses
there has arisen a large variation in textural groupings
expressed in diameter of particles. This would naturally
occur because of the differences in degree of refinement
which the various methods of separation allow, and also
l>rc;iii..e of the uses which the investigators wished to
make of such analyses. Some of the best-known group-
ings arc given below:
96
SOILS: PROPEPT1ES AND MANAGEMENT
VARIOUS TEXTURAL CLASSIFICATIONS USED IN THE MECHANI-
CAL ANALYSES OF SOILS. EXPRESSED IN DIAMETER OF
PARTICLES IN MILLIMETERS
SEPARATE
OSBORNE l
HlLGARD 2
BUREAU OP
SOILS s
ENGLISH 4
ATTERBERG 5
1
3.000
3.000
2.000
1.000
20.000
2
1.000
1.000
1.000
.200
2.000
3
.500
.500
.500
.040
.200
4
.250
.300
.250
.010
.020
5
.050
.160
.100
.002
.002
6
.010
.120
.050
7
.072
.005
8
.047
9
.036
10
.025
11
.016
12
.010
Of these classifications only three need claim our atten-
tion that of the Bureau of Soils, that of Hall and
Russell (the English classification), and that devised by
Atterberg. These represent the groupings used in ex-
pressing mechanical soil analyses in the United States,
1 Osborne, T. B. Methods of Mechanical Soil Analysis.
Ann. Kept. Connecticut Agr. Exp. Sta., 1886, pp. 141-158;
1887, pp. 144-162 ; 1888, pp. 154-157.
2 Hilgard, E. W. Methods of Physical and Chemical Soil
Analysis. Ann. Kept. California Agr. Exp. Sta., 1891-1892,
pp. 241-257.
3 Briggs, L. J., and others. The Centrifugal Method of
Soil Analysis. U. S. D. A., Bur. Soils, Bui. 24. 1904.
4 Hall, A. D., and Russell, E. J. Soil Surveys and Soil
Analyses. Jour. Agr. Science, Vol. IV, part 2, pp. 182-223.
1911.
6 Atterberg, A. Die Mechanische Bodenanalyse und die
Klassifikation der Mineralboden Schwedens. Internat. Mitt,
f. Bodenkunde, Band II, Heft 4, Seite 312-342. 1912.
TI1K SOIL PARTICLE
97
in England, and in Continental Europe, respectively.
As to which is the best for an interpretation and ex-
pression of textural qualities it is difficult to say. They
are all arbitrary, yet they are all extremely useful.
It therefore seems immaterial which one is employed.
It would be better, of course, if the classification were
uniform for all countries; correlation of soil properties
would then be easier.
72. Bureau of Soils classification. As the grouping
established by the United States Bureau of Soils is met
with in all of our soil literature, and as it is really the
standard classification for this country, a closer considera-
tion of it may be profitable. The discussion of the
properties exhibited by the various separates, and of the
interpretation and value of a mechanical analysis, will
therefore be made with this classification as a basis. By
way of illustrating the grouping and the mode of ex-
pressing a mechanical analysis the results obtained from
two distinctly different soils are given below :
MECHANICAL ANALYSES x OF A DUNKIRK FINE SANDY LOAM
AND A DUNKIRK CLAY
SEPARATE
SIZE IN
MILLIMETERS
FINE SANDY
LOAM
CLAY
Fine gravel
2-1
%
1
%
1
Coarse sand
Medium sand
Fine sand
1-.5
.6-.2S
.25- 10
2
3
22
2
2
6
Very fine sand
Silt
.10-.05
05- 005
35
27
7
39
Clay
below .005
10
43
1 Soil Survey Field Book, pp. 152, 154.
Soils. 1906.
U. S. D. A., Bur.
98 SOILS: PROPERTIES AND MANAGEMENT
73. Physical character of the separates. It is im-
mediately apparent that as these groups vary in size they
must exhibit properties, especially physical ones, which
are widely different. These properties should in turn
be imparted to the soil of which the separates form a
part. If we are conversant with these various values, a
mechanical analysis should reveal to us at a glance cer-
tain soil conditions which may or may not be conducive
to the best plant growth.
The clay particles are very minute ; many of them are
so small as to be invisible under the ultramicroscope.
They are really shreds and fragments of minerals, and
are jagged and angular in outline. They are highly
plastic, and when rubbed together they become sticky
and impervious. They shrink much on drying, with the
absorption of considerable heat. On being wet again
they swell with the evolution of the heat already taken
up. Many of the particles exhibit the Brownian move-
ment and will remain in suspension for an indefinite
period. The finer part of the clay makes up a portion of
that indefinite group of material in the soil called colloids,
which because of their fineness of division (molecular
complexes) exhibit certain well-defined properties, of
which adsorption of moisture and salts in solution, and
high plasticity and cohesion, are the most important
from a soil standpoint. Silt exhibits the same qualities
as clay, but to a much less marked extent. The presence
of clay in a soil imparts to it a heavy texture, with a
tendency to slow water and air movement. Such a soil
is highly plastic, but becomes sticky when too wet and
hard and cloddy when too dry. The expansion and the
contraction on wetting and drying are very great. The
water-holding capacity of a clay soil is high.
THE SOIL PARTICLE 99
The sands and the gravel, because of their sizes, func-
tion as separate particles. They are irregular and rounded,
the continual rubbing that they have received being
sufficient to have effaced their angular character. They
exhibit very low plasticity and cohesion, and as a con-
sequence are little influenced by changes in water content.
Their water-holding capacity is low, and because of the
large size of the spaces between each separate particle
the passage of water is rapid. They therefore facilitate
drainage and encourage good air movement. In all
the grades of sand the separate particles are visible to
the naked eye, a condition impossible with the silt and
clay groups. Soil containing much sand or gravel,
therefore, is of an open character, possessing good
drainage and aeration, and is usually in a loose friable
condition.
74. Mineralogical characteristics of the separates. -
From the mineralogical standpoint there are usually
considerable differences in the soil separates. These
differences would naturally be expected to occur par-
ticularly in residual soils, because of the differentiating
tendencies of weathering. Quartz would naturally per-
sist, and because of its slow solubility would very soon
make up most of the larger soil grains. Other minerals,
such as the feldspars, hornblende, augite, and the like,
being less persistent as the law of mineral resistance
has already taught us, would be worn to fine shreds
and be found as the main constituents of the silts
and the clays. The following data sustain this as-
sumption regarding the mineralogical characteristics of
some of the soil groups as designated by the Bureau of
Soils as well as furnish some interesting comparisons of
some important soil provinces :
100 SOILS: PROPERTIES AND MANAGEMENT
GENERAL MINERALOGICAL COMPOSITION OF THE SANDS AND
SILTS OF VARIOUS SOIL PROVINCES OF THE UNITED STATES *
SOIL
No. OP
SAMPLES
MINERALS OTHER THAN
QUARTZ IN
Sands
Silts
Residual
12
6
4
3
15%
12%
5%
37%
21%
15%
8%
42%
Glacial and loessial . .
Marine
Arid .
It is to be seen immediately that in every case the
silt carries a larger quantity of the important soil-forming
minerals and ^a smaller quantity of quartz than does the
sand. This reveals at least one of the reasons for the
greater fertility and lasting qualities of fine-textured soils
as far as agricultural operations are concerned. It is
important to note, however, that, although quartz is
the predominating mineral in sands, all the common
soil-forming minerals are usually accessory. This merely
serves to again emphasize the fact that all soils contain
all the common minerals found in soil-forming rocks.
It is also interesting to note the general differences
exhibited by the various soil provinces. The residual,
glacial, and coastal plain soils possess minerals other
than quartz in the order named. In the marine soils,
in particular, this difference has largely come about by
the disintegration and leaching that these soils have
undergone during their formation. The arid soils, due
to the suppression of chemical weathering and the activity
1 McCaughey, W. G., and William, H. F. The Microscopic
Determination of Soil-Forming Minerals. U. S. D. A., Bur.
of Soils, Bui. 91. 1913.
THE SOIL PARTICLE
101
of the physical agents, exhibit smaller quantities of free
quartz. The silica in such soils is held as complex sili-
cates, which very largely carry the elements that are so
important in plant development.
Although these data are based on but a few samples,
they are so concordant with what would naturally be
expected that these general conclusions cannot be avoided.
75. The chemical constitution of soil particles. The
mineralogical examination of soils has revealed a larger
percentage of such minerals as feldspars, mica, horn-
blende, and the like, in the finer separates. A larger
percentage of the important plant-food elements would
therefore be expected in those groups. The following
data, 1 compiled from work performed by the United
States Bureau of Soils, substantiate this assumption :-
CHEMICAL COMPOSITION OF VARIOUS SOIL SEPARATES
NUM-
PERCENTAGE OF
PERCENTAGE OF
PERCENTAGE OF
BEROF
P 2 Ofi IN
K 2 O IN
CaO IN
SOILS
q . M
PLE8
Sand
Silt
Clay
Sand
Silt
Clay
Sand
Silt
Clay
Crystalline residual
3
.07
.22
.70
1.60
2.37
2.86
.50
.82
.94
Limestone residual
3
.28
.23
.37
1.46
1.83
2.62
12.26
10.96
9.92
Coastal plain
7
.03
.10
.34
.37
1.33
1.62
.07
.19
.55
Glacial and loessial
10
.15
.23
.86
1.72
2.30
3.07
1.28
1.30
2.69
Arid soils
2
.19
.24
.45
3.05
4.15
5.06
4.09
9.22
8.03
It is seen that on the average the soils with finer particles
are richer in phosphoric acid, potash, and lime, than
those of coarser texture, the only exception in this case
being in the lime of the residual limestone soils. The
arid soils present a less marked difference in the sands,
1 Failyer, G. H., and others. The Mineral Composition
of Soil Particles. U. S. D. A., Bur. Soils, Bui. 54. 1908.
102 SOILS: PROPERTIES AND MANAGEMENT
silts, and clays than the representatives of the other
soil provinces; this is true also of the glacial soils, but
to a less degree. Under such conditions of weathering
the sands have not as yet been depleted of their stores
of essential elements. Average data compiled from a
number of soil analyses by Hall, 1 presented below, tend
to corroborate the data already noted and that obtained
by Loughridge 2 of California :
COMPOSITION OF SOIL SEPARATES
SiOz
AW)i
Fe 2
CaO
MgO
K 2
P 2 0s
Coarse sand (1-.2 mm.) .
93.9
1.6
1.2
.4
.5
.8
.05
Fine sand (.2-.04 mm.)
94.0
2.0
1.2
.5
.1
1.5
.1
Silt (.04-.01 mm.) . . .
89.4
5.1
1.5
.8
.3
2.3
.1
Fine silt (.01-.002 mm.) .
74.2
13.2
5.1
1.6
.3
4.2
.2
Clay (Below .002 mm.) .
53.2
21.5
13.2
1.6
1.0
4.9
.4
76. Value of a mechanical analysis. It is now evi-
dent that the proper interpretation of a mechanical
analysis throws considerable light on the probable physical
and chemical properties of a soil. To the trained ob-
server the preponderance of sand or clay signifies certain
physical properties which may affect the plant not only
mechanically, but physiologically as well, through varia-
1 Hall, A. D., and Russell, E. J. Soil Surveys and Soil
Analyses. Jour. Agr. Science, Vol. IV, Part 2, p. 199. 1911.
Also a Report of the Agriculture and Soils of Kent, Surrey, and
Sussex. Board of Agriculture and Fisheries. 1911.
2 Loughridge, R. H. On the Distribution of Soil Ingredi-
ents among Sediments Obtained in Silt Analyses. Amer.
Jour. Sci., Vol. VII, p. 17. 1874.
In this connection see also Puchner, Dr. Uber die Ver-
tielung von Nahrstoffen in den Verschieden Feinen Bestandteilen
des Boden. Landw. Ver. Stat., Band 66, Seite 463-470. 1907.
THE SOIL PARTICLE 103
tions in air and water movements. The chemical phases
of such an interpretation are also worthy of considera-
tion, as the proportion of the various separates determines
whether the essential plant-food elements will be present
in sufficient quantities to permit normal crop growth.
Thus in a general way the mechanical analysis of a soil
not only enlightens us as to the general properties of a
given soil, but is to some extent a criterion of agricultural
value and crop adaptation. Some authors 1 maintain
that in the investigation of any soil a mechanical analysis
should first be made, as such an analysis throws so much
light on the general qualities of a soiL,
77. Soil class. Class is a term used in relation to
the texture, or size of particles, of a soil. Class differs
from texture, however, in that it has reference rather to
the particular properties exhibited by a soil than to any
absolute grain size. As soils are not made up of particles
of the same size, a blanket term is needed which will not
only give an idea of the texture of the soil, but also name
it in such a manner as to reveal general peculiarities and
properties. We may have any number of classes, de-
pending on the sizes of the soil grains carried.
These class names have originated through long cen-
turies of agricultural operations, but of late they have
been more or less standardized because of the necessity
of a definite nomenclature. In general the names used
for the soil classes are the same as are used in mechanical
analyses to designate the soil separates. This is rather
unfortunate, but it obviates the increase of technical terms
and a little care will prevent confusion in this regard.
Another word introduced by common usage is loam.
1 Hall, A. D., and Russell, E. J. Soil Surveys and Soil
Analyses. Jour. Agr. Science, Vol. IV, Part 2, p. 199. 1911.
104 SOILS: PROPERTIES AND MANAGEMENT
Loam, from the technical standpoint, refers to a soil
possessing in about equal amounts the properties im-
parted by the various separates. If, however, we have
practically the same condition but with one size of particle
predominating, the name of that particular separate is
prefixed, giving still more data regarding the soil in
question. Thus a loam in which clay is dominant will
be classified as a clay loam. In the same way we may
have a sandy loam, a sandy clay loam, a gravelly sandy
clayey loam, and so on. The number of soil classes that
may occur is therefore rather large, ranging from coarse
gravel, through the various grades of sands, to silts and
clays.
A few of the common classes, with their mechanical
analyses, are listed below :
MECHANICAL COMPOSITION OF VARIOUS SOIL CLASSES l
NUMBER OF
SAMPLES
j
H
>
5
1
&
Q
Z
!
GO
W
I
MEDIUM SAND
Q
Z
<
CQ
H
fe
g
1
s
>
3
o5
j[
Coarse sands . . .
135
12
31
19
20
6
7
5
Sands
401
2
15
23
37
11
7
5
Fine sands
511
1
4
10
57
17
7
4
Sandy loams . . .
1141
4
13
12
25
13
21
12
Fine sandy loams .
934
1
3
4
32
24
24
12
Loams
659
2
5
5
15
17
40
16
Silt loams ....
1268
1
2
1
5
11
65
15
Sandy clays . . .
162
2
8
8
30
12
13
27
Clay loams . . .
718
1
4
4
14
13
38
26
Silty clay loams . .
765
2
1
4
7
61
25
Clays
1970
1
3
2
8
8
36
42
1 Whitney, M. The Use of Soils East of the Great Plains
Region. U. S. D. A., Bur. Soils, Bui. 78, p. 12. 1911.
THE SOIL PARTICLE 105
It is evident that a mechanical analysis of a soil is
nothing more or less than an expression of class, and
the inferences that may be derived from either are
the same. This leads to a consideration of class deter-
mination.
78. Determination of class. The common method
of class determination is that employed in the field. It
consists in examination of the soil as to color, an estima-
tion of its humus content, and, especially, a testing of
the " feel " oi the soil. Probably as much can be judged
as to the texture and class of a soil by merely rubbing it
between the thumb and the fingers as by any other
superficial method. This is a method used in all field
operations, especially in soil survey work. The accuracy
of the determination depends largely on experience.
Inaccuracies are likely to occur in distinguishing between
the various finer grades of soil; for this reason more
nearly exact methods are necessary at times, especially
in checking soil survey work or in carrying out investiga-
tions in which absolute accuracy is required.
As a mechanical analysis of a soil is really a percentage
expression of texture, it presents an exact method for
class determination. For detailed work somewhat com-
plicated tables * have been arranged ; but the following
diagram (Fig. 16), devised by Whitney, 2 presents a simple
method for the identification of a soil from a mechanical
analysis. The convenience of this triangular representa-
tion may be tested by the use of the average analyses,
already presented on a previous page.
1 Bur. of Soils, Soil Survey Field Book, p. 17, U. S. D. A.,
Bur. Soils. 1906. Also, Bur. Soils, Bui. 78, p. 12. 1911.
2 Whitney, M. The Use of Soils East of the Great Plains
Region. U. S. D. A., Bur. Soils, Bui. 78, p. 13. 1911.
106 SOILS: PROPERTIES AND MANAGEMENT
FIG. 16. Diagram for the determination of soil class from a mechani
cal analysis.
79. The significance of texture and class. Soil
texture and class are the basis for all soil consideration,
whether regarding some specific property or a general
condition such as crop adaptation. No matter what the
phase of soil study may be, texture and class are sure
to have some important influence and must be included
in the investigation. From observations in practice,
certain crops have been found to be adapted to certain
kinds of soil as clay loam for wheat, silt loam for
corn, loam or sandy loam for potatoes, clay or clay loam
for timothy, and so on. Two authors l have determined
the mechanical qualities of soils well adapted to certain
crops. An average of their analyses is given below:
1 Hall, A. D., and Russell, E. J. Soil Surveys and Soil
Analyses. Jour. Agr. Science, Vol. IV, Part 2, p. 207. 1911.
THE SOIL PARTICLE
107
THE MECHANICAL ANALYSIS OF SPECIFIC CROP SOILS
WHEAT
(9 samples)
BARLEY
(9 samples)
POTATO
(8 samples)
HOP
(7 samples)
FRUIT
(6 samples)
Fine gravel
1.4
1.2
.9
1.2
1.0
Coarse sand .
3.7
18.3
20.1
4.8
6.8
Fine sand . .
24.5
32.0
43.5
33.8
42.0
Silt ....
23.0
18.2
11.0
28.8
23.3
Fine silt . .
12.8
8.0
6.4
8.7
7.3
Clay . . .
20.0
11.9
9.7
12.1
10.9
The soil particle can thus be seen to function in no
insignificant manner regarding plant conditions. Its
size, its physical relationships, its chemical composition,
and the conditions imposed by a preponderance or a
limitation in its various grades, are of vital importance.
Soil texture and soil class, therefore, are terms of constant
usage in soil discussion and study, whether the viewpoint
is practical or purely theoretical.
CHAPTER VII
SOME PHYSICAL PROPERTIES OF THE SOIL
WHILE texture is of great importance in the determina-
tion of the physical and chemical nature of a soil, it is
evident that the arrangement of the particles also exerts
considerable influence. The term texture refers to the
size of the soil particles; the term structure is used in
reference to their arrangement, or grouping. It is at
once apparent that certain conditions such, for ex-
ample, as air and water movement, heat transference,
and the like will be as much affected by structure
as by texture. As a matter of fact, the great changes
wrought by the farmer in making his soil better suited
as a foothold for plants are structural changes rather
than changes in texture. The compacting of a light
soil or the loosening of a heavy soil is merely a change
in arrangement of the soil grains. It is of interest, there-
fore, to ascertain the probable arrangement of the particles
in any soil.
80. Arrangement of soil particles (Fig. 17). In any
consideration it is the easier way to advance from the
simple to the complex. Therefore in the explanation of
structural relationships a theoretical condition will be dealt
with first, after which the discussion will proceed to the
intricate condition existing in the soil. Assuming that
this theoretical condition consists in spherical particles
all of the same size, we find these particles susceptible
108
SOME PHYSICAL PROPERTIES OF THE SOIL 109
to two different arrangements: (1) in columnar order,
with each particle touched in four points by its neighbors ;
and (2) the oblique, in which each particle is in contact
with six of its neighbors. The possible pore space in
the first case is 47.64 per cent, while that in the second
case is 25.95 per cent. The amount of this pore space
is uninfluenced by the size of the particles, provided they
are round and all of the same volume.
FIG. 17. Ideal arrangements of spherical particles, showing, from left
to right, columnar, oblique, compact, and granular orders.
To any one of practical experience it is a well-known
fact that the soil particles are not homogeneous as to
size, and neither do all the particles function as simple
grains, being gathered together in groups called granules,
or crumbs. A small particle of soil may be made up of
a number of very small grains. This will modify the
ideal condition as described above, giving two additional
conditions first, a mixture of spherical grains of differ-
ent sizes, and, secondly, a condition in which the large
grains are complexes made up of numerous small particles.
A mixture such as is presented by the first of these con-
ditions, in which the small grains fit in between the
larger ones, will result in a reduction of pore space. The
pore space will fall below 25.95 per cent and approach
zero. A real soil having such restricted pore space is
110 SOILS: PROPERTIES AND MANAGEMENT
designated as being in a puddled condition. This con-
dition is detrimental to plant growth, for it not only
impedes root development and extension, but also pre-
vents the circulation of air and water, a function necessary
for proper soil sanitation. In the second condition an
increase in pore space must occur, as each large grain
presents considerable internal air space. If the granules
as well as their component particles were arranged in
columnar order, the pore space would reach the high
percentage of 72.58. Under natural conditions, then,
the pore space might range from zero plus to about 72
per cent.
However, not only are the particles of a normal soil
not of the same size, but they are far from round. A
soil, as already demonstrated, ordinarily presents varying
amounts of particles, ranging in size from stone and
coarse gravel to the very finest clay. These particles
may also differ in shape, varying from almost perfect
spheres to flakes, chips, and fragments of every con-
ceivable form. Therefore the laws that apply to the
ideal condition will hold only in a general way in a normal
soil. It is evident, first, that the more compact the
soil, the less is the pore space ; secondly, that it is possible
to so manipulate a soil as to work the small particles in
between the larger ones and create an impervious or
puddled condition; and, thirdly, that by the forming
of granules the pore space of a soil may be increased to
a high percentage.
From the standpoint of size and arrangement of particles
there are really two classes of soils, those of single grain
structure and those that are granular. In the former
each particle functions separately. In order to do this
the particles must be large. This condition is found in
SOME PHYSICAL PROPERTIES OF THE SOIL 111
sand. In such soil would naturally be found also a
medium to low pore space, just as has been exemplified
in the ideal condition described above. In granular
soils, the granules, being made up of small particles,
present much internal pore space. This condition occurs
only in fine soils, such as loams, silts, and clays, since
large particles will not cohere firmly enough to produce
a crumb structure. A fine-textured soil, which will
puddle more readily than a coarser one, is thus saved
from a semi-impervious condition by this tendency toward
granulation.
The ideal soil condition might be considered to be most
likely to occur in a loam (Fig. 18). In loamy soil some
of the particles are large and
function separately; others are
medium in size and tend to form
the nuclei around which smaller
particles may cluster to form
granules, or crumbs. There are
thus a few large pore spaces
which facilitate drainage, and
numberless small openings in
which water is retained. Air
therefore finds easy movement
and sanitation is promoted. In
such a condition the organic mat-
ter plays an important part.
This exists usually as dark, par-
tially decayed material. It pushes
apart the grains and lightens the soil, and contributes
much in bringing about the loamy condition so favorable
to plant development. Because of its water-holding
capacity also it proves a valuable addition. Thus with
FIG. 18. The arrange-
ment of particles in loamy
soil of good structural
condition. (a). Large
granule ; (6), small sand
particle ; (c) , large pore
space ; (d) small granules
with small interstitial
spaces; (e), large sand
grain.
112 SOILS: PROPERTIES AND MANAGEMENT
particles of varying sizes, of a structure partly single-
grain and partly granular, to which has been added by
natural means sufficient organic matter in an advanced
stage of decomposition, we have the ideal soil conditions
for plant development. Yet the same laws govern here
in a general way as were found to function with homo-
geneous grains of spherical shape.
81. The absolute specific gravity of the soil. The
structural condition of any soil, be it single-grain, granular,
or a favorable combination of the two, has considerable
influence on certain other physical conditions. One of
those most affected is the weight. The weight of a soil
is determined by two factors : the weight of the individ-
ual particles, or the absolute specific gravity; and the
amount of actual space taken up by such particles in any
given volume. The latter is really a structural condition
and is independent to some extent of the size of particles.
The absolute specific gravity, or weight compared with
an equal volume of water, of some of the common minerals
is as follows :
Quartz .... 2.7 Apatite ... 3.2
Orthoclase . . 2.6 Gypsum ... 2.3
Plagioclase . . 2.7 Hematite . . 5.2
Mica .... 3.0 Limonite ... 4.0
Olivine .... 3.4 Serpentine . . 2.6
Calcite .... 2.7 Chlorite ... 2.2
Dolomite ... 2.9 Talc .... 2.7
Although a great range is observed in the absolute
specific gravities of these common soil-forming minerals,
it must be remembered that such minerals as quartz
and feldspar usually make up the bulk of a soil. As a
consequence it has been found that the absolute specific
SOME PHYSICAL PROPERTIES OF THE SOIL 113
gravity of a purely mineral soil varies only between
narrow limits, these being from 2.6 to 2.8. Nor has
fineness any appreciable effect, as shown below by Whit-
ney's 1 determinations on the various separates :
SPECIFIC GRAVITY
Fine gravel (2-1 mm.) 2.647
Coarse sand (1-.5 mm.) 2.655
Medium sand (.5-25 mm.) 2.648
Fine sand (.25-.10 mm.) 2.659
Very fine sand (.10-.05 mm.) .... 2.680
Silt (.05-.005 mm.) 2.698
Clay (below .005 mm.) 2.837
The only marked variation here observed is in the clay
separate, and this may be due to the concentration of the
iron-bearing silicates in this grade. However, for all
practical purposes the average absolute specific gravity
of a mineral soil may be placed at about 2.70. One
condition that may vary this is the quantity of organic
matter present. As the specific gravity of the soil humus
usually ranges from 1.2 to 1.7, the more humus jthere is
present, the lower will be the absolute figure for a given
soil. A purely organic soil, such as muck or peat, pre-
sents a variable absolute specific gravity ranging from
JJLto 2.Q, accordingjtQ the_amQiint of wash it has received
from external sources. Some humus-loam soils may
drop as low as 2.1. Nevertheless for general calculations
the average arable soil may be considered to have an
absolute specific gravity of about 2.70.
82. Apparent specific gravity. Since all soils contain
more or less pore space, depending on textural and struc-
1 Whitney, M. Some Physical Properties of Soils. U. S.
D. A., WeMher Bur., Bui. 4, p. 34. 1892.
i
114 SOILS: PROPERTIES AND MANAGEMENT
tural conditions, the actual weight of the absolutely dry
soil in any volume is of great importance. This is ex-
pressed as is the absolute specific gravity of any material,
the weight of an equal volume of water being used as a
unit. Because of their tendency to granulate, fine soils
have a very large amount of pore space, as has been shown
in the discussion of structure ; it is to be expected, there-
fore, that they will weigh less in any particular volume
than will soils made up of large particles. Coarse soils
are heavy soils, as far as weight is concerned. Mineral
soils may range in apparent specific gravity 1 from 1.10
to 1.20 for clay to 1.65 to 1.75 for sand. Humous loams
may drop as low as 1.00, and muck often reaches the
low figure of .40. The appar-
ent specific gravity is always
expressed on the basis of abso-
lutely dry soil.
In the field the apparent
specific gravity of a soil may
be determined by driving a
cylinder of known volume into
the ground and obtaining
thereby a core of natural soil
(see Fig. 19). By weighing
the soil and then determining
the amount of water that it
holds, the amount of absolutely
dry soil may be ascertained.
Dividing this by the weight
of an equal volume of water
gives the apparent specific
1 See Whitney, M. Some Physical Properties of Soils.
U. S. D. A., Weather Bur., Bui. 4. 1892.
FIG. 19. Cylinder for deter-
mining the apparent specific
gravity of soil in the field.
The cutting edge at (6) is
drawn in somewhat to pre-
vent excessive friction be-
tween the sides of the cylin-
der and the entering soil core.
SOME PHYSICAL PROPERTIES OF THE SOIL 115
gravity for that soil. A laboratory determination may be
made by putting the soil into a receptacle of known volume
and weighing it. From the weight of the absolutely
dry soil and the weight of an equal volume of water, the ap-
parent specific gravity may be calculated. This method
will give only approximate results, however, as the struc-
tural relationships are more or less artificial. The only
reliable method is the one first described.
83. Actual weight of a soil. With the apparent
specific gravity of a soil known, its weight in pounds to
the cubic foot may be found by multiplying by 62.42.
Soils may vary in weight from 68 to 80 pounds for clays
and silts to 100 to 110 pounds for sand. The greater the
humus content, the less is this weight to the cubic foot.
A muck soil often weighs as little as 25 or 30 pounds.
This weight, of course, is for absolutely dry soil and does
not include the water present, which may be much or
little, according to circumstances. The actual weight
of a soil is often expressed in acre-feet. An acre-foot of
soil refers to a volume of soil one acre in extent and one
foot deep. In the same way we may have an acre-eight-
inches or an acre-six-inches. The weight of an acre-foot
of soil usually varies from 3,500,000 to 4,000,000 pounds ;
granulation and organic matter may modify this con-
siderably. The value of knowing the actual_weight of
a soil lies in the possibility of calculating thereby the
amount of water, the amount of humus, or the actual
number of pounds of the mineral constituents present in
the soil. Such information affords a ready means of
comparing two soils as to their crop-producing capabilities.
84. Pore space in soil. The pore space in soil is
due largely to structural conditions. As already em-
phasized, the coarser the soil, the smaller is the aggregate
r
116 SOILS: PROPERTIES AND MANAGEMENT
amount of internal space. Each individual space is
larger under such conditions, and this accounts for the
ready movement of water and air through such soils.
A clay soil, while containing a very large amount of pore
space, has the disadvantage of very minute individual
pores. The large amount of space occurs because of
the lightness of the particles and the tendency toward
granulation. The small size of the individual spaces
is a direct function of size of particle, or texture.
A very simple formula may be used for a determination
of the percentage of pore space in any soil, provided the
absolute and the apparent specific gravities are known :
TAp. Sp. Gr ^ 1001
Percentage or pore space = 100 77
LAb. bp. Gr. 1 J
Thus a soil having an apparent specific gravity of 1.60
and an absolute specific gravity of 2.60 has 38.5 per cent
of pore space ; while in a soil in which the above figures
are 1.10 and 2.50, respectively, the percentage of pore
space is 56. The following figures, taken from King, 1
illustrate the relation that texture holds to total pore
space in soils : - PERCENTAGE OF
PORE SPACE
Sandy soil 32.49
Loam 34.49
Heavy loam 44.15
Loamy clay soil 45.32
Clayey loam 47.10
Clay 48.00
Very fine clay 52.94
1 King, F. H. Physics of Agriculture, p. 124. Published
by the author, Madison, Wisconsin. 1910.
SOME PHYSICAL PROPERTIES OF THE SOIL 117
The pore space in any of these soils is, of course, subject
to considerable fluctuation, especially in the surface
soil, due to tillage and the incorporation of organic matter ;
hence a sandy loam might under certain conditions present
more pore space than a silt or a clay loam. When soils
are in the physical condition for the best plant growth,
however, the rule holds that, the finer the soil, the greater
is the pore space. The differences in pore space between
the surface soil and the subsoil in Wisconsin are shown
by King 1 as follows:
WEIGHT PER
CUBIC FOOT
PERCENTAGE OP
PORE SPACE
First foot
79 0-
52 2
Second foot
92 6
440
Third foot
Fourth foot
104.6
1062
36.8
358
Fifth foot
Sixth foot
111.0
111.1
32.9
32.8
The pore space in a normal soil is occupied by water
and air. If the water content is low, the air space is
large, and vice versa. Thus the relationships of the total
pore space and the size of the individual spaces to the
amount of air and water contained, to their movement
through the soil, to soil sanitation, to root extension, to
bacterial action, and to cropping conditions in general,
become apparent. It -is the regulation of this pore space
that is really studied in any structural consideration.
The effect on plant growth of a change in pore space is
the final test of its advisability.
1 King, F. H. Physics of Agriculture, p. 111. Published
by the author, Madison, Wisconsin. 1910.
118 SOILS: PROPERTIES AND MANAGEMENT
85. The number of soil particles. The number of
particles in any given volume of soil is really determined
by texture, and, as this number determines very largely
the probable arrangement of the soil grains, structure
becomes in turn dependent on the size of grain. Since
soil particles run to very small diameters, the number
in any given volume is very large, especially when we
are dealing with fine-textured soils or with soils of com-
pact structural condition. Any calculation of the number
of particles present in a soil is open to considerable in-
accuracy, first, because it is impossible to get a correct
figure for the average diameter of the particles of any
soil or of the various groups of separates that go to make
it up, and, secondly, because it must be assumed in the
calculation that the particles are spherical. This assump-
tion is of course incorrect, as has already been demon-
strated ; but it must be entertained in order to obtain
approximate ideas as to the number of grains in any soil.
The number of particles in any soil sample may be
arrived at from a mechanical analysis and the diameters
that limit each group. Using the average diameter of
each group together with the percentage of the groups
in a given sample, the number of particles may be cal-
culated by the following formula :
Number of particles in a _ Weight of sample in grams
sample of soil 1/6 77 D s X 2.70
The formula 1/6 TT D 3 is that used for determining the
volume of a sphere, the diameter in this case being ex-
pressed in centimeters. The volume of the sphere, then,
is obtained in cubic centimeters, which must be multiplied
by the absolute specific gravity of soil minerals, or 2.70,
SOME PHYSICAL PROPERTIES OF THE SOIL 119
in order that the weight in grams of a single soil grain
may be obtained. A calculation by this method of the
number of particles in a sandy loam is given below :
SEPARATES
LIMITS
NUMBER OP PARTI-
CLES TO THE GRAM
OP EACH SEPARATE
M
*" xo a3
w j 3
5 < >< 2
s ^
APPROXIMATE
NUMBER OP
PARTICLES IN ONE
GRAM OP SANDY
LOAM
Fine grav-
el . .
2-1 mm.
209
1
2
Coarse
sand .
X-.5 mm.
1,670
4
67
Medium
sand .
.5-.25 mm.
13,410
25
3,352
Fine
sand .
.25-. 10 mm.
131,900
35
46,165
Very fine
sand .
.10-.05 mm.
1,676,500
20
335,300
Silt . .
.05-.005 mm.
35,934,000
10
3,593,400
Clay . .
below .005 mm.
45,632,000,000
5
2,281,600,000
2,285,578,286
A very great error is introduced by this method, es-
pecially in assuming that the average size of particles
for the clay separate is .0025 millimeter. As the clay
particles may become molecular complexes and conse-
quently are very, very small, it stands to reason that
such an assumption is far from correct. Nevertheless,
it gives a very good idea as to the immense number of
grains that we have to deal with, even in the coarsest of
soils. A few figures as to the approximate number of
particles in various average soil classes of the United
States, 1 as reported by the Bureau of Soils, are given
below :
1 For mechanical analysis of the classes, see Chapter VI, p. 104.
120 SOILS: PROPERTIES AND MANAGEMENT
APPROXIMATE NUMBER OF PARTICLES TO THE GRAM IN
VARIOUS CLASSES OF SOIL IN THE UNITED STATES
CLASS
APPROXIMATE NUMBER OF
PARTICLES
Coarse sands . .
Sands ....
Fine sands . .
Sandy loams
Fine sandy loams
Loams ....
Silt loams . . .
Sandy clays . .
Clay loams . .
Silty clay loams
Clays ....
2,299,145,360
2,287,251,842
1,826,176,893
5,483,797,920
5,485,069,147
7,332,679,042
6,868,546,664
12,324,914,033
11,877,875,092
11,430,037,544
19,177,571,994
86. Surface exposed by soil particles. Besides giving
an actual numerical figure and an insight into the prob-
able structural relationships of a soil, the approximate
number of particles may serve still another purpose -
that of enabling us to calculate the aggregate internal
surface exposed by the soil grains. The surfaces of the
grains hold more or less water according to their area,
and they increase the amount of chemical and biological
activities functions so necessary to a continuous re-
placement in the soil solution of the elements withdrawn
by the plant. The minerals in the soil are all very re-
sistant to solution ; if they were not, they would long
ago have been leached away. Such materials, while
almost insoluble, allow the amount of material going
into solution to be notably increased by fineness of tex-
ture, although their solubility remains the same. The
fineness of the particles, then, presents another significant
feature besides those already pointed out.
SOME PHYSICAL PROPERTIES OF THE SOIL 121
Another important property of the surface of the
grains is the tendency toward the retention of soluble
material in a partially or wholly available condition for
plant use. This power, designated as adsorption, is
one exhibited to a high degree by fine soils, in which
the individual pore spaces are small and the amount of
surface exposed is large. It is an important factor to
be observed in the addition to the soil of soluble fertilizing
constituents. Adsorption may also, by bringing materials
into closer contact, hasten or retard certain chemical
actions. Reactions may thus be expected to go on in the
soil that would not take place in the laboratory beaker.
The relation of this adsorption to bacterial activity also
cannot be overlooked.
The aggregate area presented by soil particles is very
large, even for the coarser soils. With the finer soils,
because of the immense number of particles, a figure is
reached that is almost beyond comprehension. When
the approximate number of particles and their sizes
in any given weight of soil are known, the internal surface
may be calculated by the following formula :
Surface = TT D 2 X number of particles
As the estimation of the number of particles in a soil is
so inaccurate, it is evident that a calculation of the
surface exposed based on such a figure must also be in
error.
However, to give some idea of the internal surface ex-
posed by ordinary soils, the calculations made on a few of
the average soil classes of the United States, already
presented, 1 are given in the table on the following page,
1 See Chapter VI, p. 104.
122 SOILS: PROPERTIES AND MANAGEMENT
APPROXIMATE INTERNAL AREA EXPOSED BY AVERAGE CLASSES
OF UNITED STATES SOILS
SQUARE INCHES
PER GRAM
SQUARE FEET
PER POUND
ACRES PER
ACRE-FOOT OF
3,500,000 POUNDS
Coarse sands ....
Sands
91
89
286
280
23,055
22 549
Fine sands ....
Sandy loams ....
Fine sandy loams . .
Loams
79
213
222
294
248
671
699
926
20,014
53,965
56,180
74,410
Silt loams
307
967
77 700
Sandy clays ....
Clay loams . - . . .
Silty clay loams . . .
Clays
417
430
458
653
1313
1354
1442
2057
105,540
108,830
115,910
165,270
It is at once apparent that the amount of surface
exposed by the soil grains of even a sand is tremendous.
It is not to be wondered at that the slowly soluble minerals
are able to supply sufficient food to the crop growing on
the soil, when such a large amount of surface is continually
available for chemical action. The figures presented for
an acre-foot of soil are almost too large for adequate com-
prehension. It is quite evident that the finer the soil,
the greater is the amount of internal surface. For ex-
ample, a sandy loam weighing 90 pounds to the cubic
foot would present 60,390 square feet of surface, while
a clay weighing 75 pounds to the cubic foot would expose
about 154,275 square feet. This is equivalent to 1.39
and 3.54 acres, respectively.
87. The effective mean diameter of soil particles. -
It is very evident that the calculations presented above,
both as to the number of particles and as to the internal
SOME PHYSICAL PROPERTIES OF THE SOIL 123
surface exposed, are far from correct, as we can arrive
at no definite figure as to the average size of grain. Neither
do we know the actual structural conditions. In con-
sidering these inaccuracies King 1 decided that we were
in need of a single term which not only would give an
indication regarding the size of gram, but also would
carry with it definite ideas as to the arrangement of the
particles, particularly as to the rate at which they would
allow air and water to pass through. This would bring
the considerations nearer to the plant, as permeability
very largely determines the conditions for plant develop-
ment. King, while he could obtain neither the mean
diameter of particle nor the actual internal surface, found
that he could determine with considerable accuracy,
particularly in sands, the diameter of grain which if sub-
stituted for the actual one would permit under like con-
ditions the same rate of air and water movement. This
size of grain he designated as the effective mean diameter
of particle for that particular soil.
The theory of the method is presented by Schlicter, 2
and is based on the flow of fluids through capillary tubes.
From the observed rate of the flow of air through a soil
column under controlled conditions, it is possible to
calculate the effective diameter of the interstitial spaces.
From these data the size of the spherical grains which
would be necessary to form such pore spaces, or capillary
tubes, is computed by appropriate formulae. Such a
figure represents the effective mean diameter of the soil,
1 King, F. H. Physics of Agriculture, pp. 119-120. Pub-
lished by the author, Madison, Wisconsin. 1910.
2 Schlicter, C. S. Theoretical Investigation of the Move-
ment of Ground Waters. U. S. Geol. Survey, 19th Ann. Kept..
Part II, pp. 301-384. 1899.
124 SOILS: PROPERTIES AND MANAGEMENT
from which the effective surface exposed can be deter-
mined. Thus, designating a soil as having an effective
mean diameter of particle of .0052 millimeter merely
indicates that this particular soil shows an air and
water movement the same as would be shown by a
homogeneous soil with spherical particles of that diameter.
The apparatus 1 for the de-
termination consists of a cylin-
der in which is placed a sample
of air-dry soil, the pore space
being carefully determined by
weighing. The rate of air
movement is then determined
by connecting with an aspi-
rator, the temperature and
the pressure being constantly
under control. The reading is
usually calculated to a tem-
perature of 20 C. The fact
that the structural condition
of the soil is likely to be dis-
turbed in placing the sample
in the aspirator tube detracts
from the accuracy, especially
in fine soils. Nevertheless,
King found his results fairly
accurate, and showed that
the calculated and the ob-
served flow of water through
FIG. 20. King's aspirator for
the determination of the rate
of air movement through
soils. (GO , Pressure gauge ;
(S) , soil column ; (L) , water ;
(A), aspirator; (W), weight.
1 King, F. H. Principles and Conditions of the Movements
of Ground Water. U. S. Geol. Survey, 19th Ann. Kept., Part
II, pp. 222-224. 1899. A complete discussion is given of
King's ideas in this article, pp. 67-294.
SOME PHYSICAL PROPERTIES OF THE SOIL 125
ds 1 agreed rather closely (see Fig. 20). The effective
diiimeter of the particles of some of our common soils, to-
gether with the effective surface exposed, is given
below : 2
SOIL
EFFECTIVE
DIAMETER
PERCENTAGE
OF PORE
SPACE
EFFECTIVE SUR-
FACE EXPOSED IN
ONE CUBIC FOOT
OF SOIL
Coarse sandy soil . .
Sandy soil
.1432 mm.
.0755 mm.
34.9
34.4
8,318 sq. ft.
15,870 sq. ft.
Sandy loam
.0303 mm.
38.8
36,880 sq. ft.
Loam
.0219 mm.
44.1
46,510 sq. ft.
Loamy clay soil .
Fine clay soil ....
.0140 mm.
.0086 mm.
45.3
48.0
71,316 sq. ft.
110,500 sq. ft.
Very fine clay . . .
.0049 mm.
52.9
173,700 sq. ft.
The method of King has certain advantages, besides
giving an idea as to the number of particles, their internal
surface, and the relation of this internal surface to soil
conditions. In the first place, a single figure is used
to express the size of particle ; secondly, from this effec-
tive size of particle the probable rate of air and water
movement may be calculated ; and, thirdly, the number
of particles and the internal surface calculated therefrom
have a fairly definite relationship to the plant, as such
figures are so closely correlated to the circulation of air
and water.
1 King, F. H. Physics of Agriculture, p. 123.
by the author, Madison, Wisconsin. 1910.
2 Ibid., p. 124.
Published
CHAPTER VIII
THE ORGANIC MATTER OF THE SOIL
ONE of the essential differences between a soil and a
mass of rock fragments lies in the organic content of the
former. Organic matter is a necessary constituent in
order that finely ground mineral material may be desig-
nated as a soil and that it may grow crops successfully.
Physical condition depends largely on the presence,
and chemical reaction is greatly accelerated by the de-
cay, of organic matter. In the process of soil formation
its addition is more or less a secondary step. In residual
debris the amount of organic matter held by the growing
soil increases as the process of weathering goes on ; in
glacial soils, however, the matrix, or skeleton of the soil,
is already formed before there is an opportunity for humus
to become incorporated therein. The final result from
the mixing of the minerals carrying numerous weathered
and altered products with the decayed or partially de-
cayed organic matter that is sure to accumulate, must
be a mass much more complicated than either of the
original constituents. It is hardly necessary to further
emphasize the complexity of the average soil, the reasons
therefor, and the difficulties in studying the question.
88. The source and distribution of organic matter. -
The source of practically all soil organic matter is plant
tissue. Some of this matter accumulates from the above-
ground parts of plants that have died and fallen down
126
THE ORGANIC MATTER OF THE SOIL
127
to become mixed with the surface soil; the remainder
is a result of root extension and subsequent decay. The
organic matter of the surface soil is derived from the
tops and the roots of plants growing on it, while that of
the subsoil is very largely a result of root extension and
subsequent decomposition. The relationship between the
humus content of three soils and the roots developed is
shown by the following data presented by Kostytscheff 1
and quoted by Hilgard 2 and Wollny 3 :
ROOT CONTENT AND PERCENTAGE OF HUMUS IN THREE
RUSSIAN SOILS
i
2
3
DEPTH
(inches)
ROOTS
HUMUS
ROOTS
HUMUS
ROOTS
HUMUS
6
100
5.4
100
8.1
100
9.6
12
89
4.8
64
5.2
80
7.7
18
67
3.6
48
3.9
70
6.7
24
47
2.5
35
2.8
58
5.6
30
47
2.5
26
2.1
38
3.6
36
35
1.8
18
1.5
33
3.1
42
24
1.3
6
.5
16
1,5
48
14
.8
54
7
.3
89. Composition of plants. It is usual, in classifying
the materials composing plant tissue, to group them under
three heads carbohydrates, fats and oils, and proteins.
1 Kostytscheff, M. P. Les Terres Noires de Russie. Annales
de la Sci. Agron., Tome II, pp. 165-191. 1887.
- Hilgard, E. W. Soils, p. 130. New York. 1906.
3 Wollny, E. Die Zersetzung der Organischen Stoffe, p. 194
Heidelberg. 1897.
128 SOILS: PROPERTIES AND MANAGEMENT
The carbohydrates, having the general formula of
C x (H 2 O) n , include such compounds as glucose, starch,
cellulose, dextrose, cane sugar, and the like. The fats
and oils may be represented in plants by such glycerides
as butyrin, stearin, olein, palmitin, and the like. The
proteins are by far the most complicated of the three
principal compounds, as they may carry not only carbon,
hydrogen, oxygen, and nitrogen, but also mineral elements
such as sulfur, phosphorus, lime, iron, and other elements.
They are compounds of high molecular weight and are
mostly of unknown constitution. Simple proteins, such
as albumin, globulin, protamins, and others, are found
in plants, besides certain derived proteins such as
proteoses and peptones. In addition to all these, there
is a host of other compounds that have no small influence
on the composition of the soil organic matter. Among
these are the alkaloids, waxes, tannins, phenols and their
derivatives, hydrocarbons, resins, acids, aldehydes, and
others.
The original plant tissue, therefore, while fairly well
known, as to chemical constitution, is far from simple.
The degradation of such material, especially in the pres-
ence of complex mineral products, will evidently give rise
at first to compounds no simpler ; in fact, the chances
are that the resulting compounds will be much more com-
plicated. It is only later in the processes of decay that
simple products result.
90. Decay of organic matter in soils. From the fact
that weathering in general is a process of simplification,
and since it is evident that the plant tissue as it enters
the soil is so very complex, the general change that the
organic matter undergoes must be one of simplification.
This simplification, however, is very slow, and many of
THE ORGANIC MATTER OF THE SOIL 129
the products built up are more complex than the original
tissue. Most of this decay and simplification is due to
that great group of organisms so universally present in
soil, called bacteria. Some of these are putrefactive in
their action, while others deal to a large extent with the
products of the decomposition. All, however, exert a
general simplifying influence. The action of such or-
ganisms may be direct, but is more likely to be enzymotic
in its nature, and may take place either within or outside
of the cell. A cycle is therefore set up, in which the higher
plants and animals are occupied in building up, while
bacteria are tearing down and reducing the residue of
plant action to simple forms, such as can be ultimately
utilized again in plant nutrition. The great importance
of bacteria is thus evident, and the encouragement of
their growth and function is clearly a part of good soil
management.
When the complex molecules that make up plant
tissue break down, they split along definite lines of cleav-
age, depending on the structure of the original molecule.
These bodies, which are usually simpler in nature than
those from which they have sprung, are called cleavage
products, and without a doubt they are the primary
products of the first step in organic decay. These com-
pounds are subject to still further change, and because
of the great number of agencies at work the secondary
products that result may be simpler or more complex,
according to conditions. Bacteria have a tendency,
while tearing down organic matter, to construct certain
built-up products which present a very complicated
molecule until they are in turn degraded. The secondary
products therefore vary widely because of differences in
temperature, moisture, aeration, and other conditions.
130 SOILS: PROPERTIES AND MANAGEMENT
The character of the secondary products probably ex-
hibits a greater variation than does that of the original
plant tissue. In the process of decay these products
become black or brown in color, and are usually designated
as humous materials in the soil. Organic matter, then,
covers all the material of organic origin in the soil, and
may refer not only to the original plant tissue, but also
to that which has lost its identity in the secondary prod-
ucts. Humus refers specifically to the primary and the
secondary products of decay, and may be simple or com-
plex, according to conditions.
As the process of decay goes on, certain end products
result. These are partially solid and partially gaseous.
Carbon dioxide is a universal product of bacterial activity
of all kinds, as is also water. Besides these, urea, am-
monia, nitrites, and nitrates may result from nitrogenous
decay. The three general classes of organic matter found
in soil may be illustrated by the following diagram :
PLANT TISSUE I HUMUS I END PRODUCTS
TJNDECOMPOSED MATTER | SECONDARY AND INTERMEDIATE | SIMPLE MATERIAL
FIG. 21. Diagram illustrating the three general classes of organic
matter found in soils.
It is therefore possible to have present, besides the
original organic constituents which are mostly of plant
origin, not only their primary and secondary degradation
products, but also compounds either torn down or built
up from these. An attempt to enumerate even the
original compounds in the plant tissue, or even the simple
end products of complete decay, would result in a long
list of materials representing almost every known class
of organic compound. Such a procedure is possible,
but is unnecessary as the important ones have already
THE ORGANIC MATTER OF THE SOIL 131
been mentioned. It is to be kept in mind that the simpler
products of decay are the ones utilized by crops, although
it is a well-established fact that some of the secondary
and intermediate compounds may be taken up by certain
plants and probably are of some importance from the
standpoint of use as plant-food.
91. Composition of the soil humus. It is evident
that the most complicated parts of the organic matter
in the soil are the primary and the secondary products
of decay, or the so-called soil humus. The study of this
matter is difficult and calls for the very highest knowledge
of organic chemistry. This is true for two reasons:
first, because of the complexity of these compounds;
and, secondly, because they are continually changing.
A certain compound present in the soil one week may
be altered the next week. Moreover, while some of the
soil humus is soluble in water and may circulate in the
soil solution, the bulk of it is insoluble. This in itself
presents difficulties. When the soil humus is treated
with the various extractive agents, reactions may be
induced which would not take place in a normal soil.
Compounds are then formed which not only would be
abnormal, but would probably not exist under natural
conditions.
A great many chemists have worked on the problem of
the constitution of the organic matter of the soil and
have published their results. The ideas of the early
workers are really embodied in the conclusions advanced
by Mulder, 1 who was in many ways far in advance of his
1 Mulder, T. J. Die Organischen Bestandtheile im Boden.
Chemie der Ackerkrume, I, pp. 308-360. Berlin, 1863. Also,
Wiley, H. W. Agricultural Analysis, Vol. I, p. 53. Easton,
Pa. 1906.
182 SOILS: PROPERTIES AND MANAGEMENT
time. Mulder contended that the organic matter con-
sisted of seven distinct compounds, as follows :
1 and 2. Ulmic acid and ulmin 5. Geic acid
3 and 4. Humic acid and humin 6. Apocrenic acid
7. Crenic acid
These bodies he considered as arising from one another
by oxidation; thus, ulmic acid (C^HuO^) gave humic
acid (C 4 oHi 2 Oi 2 ), which in turn yielded geic acid
(C 4 oHi 2 Oi 4 ), followed by apocrenic acid (C^gH^C^),
and finally by crenic acid (C24Hi 2 Oi6). Such a classi-
fication seems very simple, but certain flaws are at once
noticeable. In the first place, nitrogen does not find a
place in any of these formulae ; secondly, the compounds
are simpler than most plant tissue, which is not what
would be expected, especially with some of the degrada-
tion compounds; thirdly, none of these products have
united with the bases in the soil, a reaction that would
be very likely to take place especially with acid compounds.
Even the investigators l of Mulder's time obtained dis-
cordant results, but these were explained for the time
being by assuming that the discrepancies occurred because
of added molecules of water.
Later investigators, while progressing only slightly
toward definite results, did accomplish one thing of im-
portance, and that was throwing considerable doubt
on the old ideas of the Mulder school of chemists. This
again opened up the question as to the composition of
the soil organic matter, especially the humous constituents.
Thus, while it is evident that no such compounds as geic
1 See Schreiner, O., and Shorey, E. C. The Isolation of
Harmful Organic Substances from Soils. U. S. D. A., Bur.
Soils, Bui. 53, pp. 15-16. 1909.
THE ORGANIC MATTER OF THE SOIL 133
acid, humic acid, or crenic acid exist in the soil, one name
has persisted in soil literature that of humus and
humic acid. The word humus, as already indicated,
does not relate to any definite compound, but to the
great mass of primary and secondary products of bio-
logical and chemical organic decay taking place in the
soil. One of the men whose work established beyond
a doubt the fact that humus was not a definite compound
was Van Bemmelen. 1 His investigations still further
showed that the soil humus was largely in a colloidal
condition, and therefore exhibited properties quite dis-
tinct from those shown by crystalloids.
In recent years investigation has again been directed
toward the immense field opened by the overthrow of
the Mulderian school. Baumann, 2 by his researches,
has shown freshly precipitated humus to possess properties
which are largely colloidal in nature. Among these char-
acteristics are high water capacity, great adsorptive power
for certain salts, ready mixture with other colloids, power
to decompose salts, great shrinkage on drying, and coagula-
tion in the presence of electrolytes. Jodidi 3 has studied
the composition of the acid-soluble organic nitrogen in
peat and in mineral soils. The nitrogenous compounds
thus obtained can be divided into the following
groups :
1 Van Bemmelen, J. M. Die Absorptions Verbindungen
und das Absorptionsvermogen der Ackererde. Landw. Ver-
suchs. Stat., Band 35, Seite 67-136. 1888.
2 Baumann, A. Untersuchungen iiber die Humussauren.
Mitt. d. K. bayr. Moorkulturanstalt, Heft 3, Seite 53-123.
1909.
3 Jodidi, S. L. Organic Nitrogenous Compounds in Peat
Soils I. Mich. Agri. Exp. Sta., Tech. Bui. 4, November, 1909.
Also, The Chemical Nature of the Organic Nitrogen in Soil.
Iowa Agr. Exp. Sta., Research Bui. I, June, 1911.
134 SOILS: PROPERTIES AND MANAGEMENT
1. Nitric nitrogen 3. Diamino aeids
2. Ammoniacal nitrogen 4. Acid amides
5. Monamino acids
The two latter constituents were found to make up the
bulk of the organic nitrogen, but quantitative determina-
tions proved uncertain. These compounds produced
ammonia readily, the rate depending on their chemical
structure.
92. The work of Oswald Schreiner. Of the chemists
who have been most active and most successful, Schreiner l
deserves especial mention. Our present knowledge of the
chemical constitution of the organic matter of the soil is
very largely due to his efforts. While he realized that
the isolation of specific compounds from the soil was likely
to present insurmountable problems, and that the iden-
tification of such compounds after they were obtained
might be very difficult, he undertook a systematic ex-
traction of the soil. As a result of several years of work
he was able to isolate and identify a number of com-
pounds. The complexity and varied character of these
compounds is revealed by the following list of the more
important bodies isolated :
LIST OF COMPOUNDS ISOLATED FROM SOIL ORGANIC
MATTER BY SCHREINER, SHOREY, SKINNER, REED,
AND OTHERS, OF THE U. S. BUREAU OF SOILS
Hentriacontane, C 3 iH 6 4 Picoline carboxylic acid,
Dihydroxystearic acid, C 7 H 7 O 2 N
Ci 8 H 3 6O 4 Histidine,
1 Schreiner, O., and Shorey, E. C. The Isolation of Harm-
ful Organic Substances from Soils. U. S. D. A., Bur. Soils,
Bui. 53, 1909 ; also, Buls. 47, 70, 74, 77, 80, 83, 87, 88, and 90.
THE ORGANIC MATTER OF THE SOIL
135
Monohydroxystearic acid,
Agroceric acid, C 2 iH 42 O 3
Agrosteral, C 2 2H 2 2O . H 2 O
Paraffinic acid, C 24 H 48 O 2
Lignoceric acid, C 2 4H 48 O 2
rhytosterol, C 26 H 44 O . H 2 O
Pentosan, C 5 H 8 O 4
Oxalic acid, C 2 H 2 O 4
Succinic acid, C 4 H 6 O 4
Sacharaic acid,
Acrylic acid, C3H 4
Mannite, C 6 Hi 4 O 6
Rhamnose, CeHnO
Salicylic aldehyde,
C 6 H 4 OHCOH
Arginine, C6
Cytosine, C 4 H 6 ON 3 . H 2 O
Xanthine, C 5 H 4 O 2 N 4
Hypoxanthine, C 5 H 4 ON 4
Tysine, C 6 H 14 O 2 N 2
Adenine, C 5 H 5 N 5
Choline, C 5 H 15 2 N
Trimethylamine, C 3 H 9 N
Quanine, CH 5 X 3
Creatinine, C 4 H 7 ON 3
Creatine, C 4 H 9 O 2 N 3
Nucleic acid (constitution
unknown)
Trithiobenzaldehyde,
(C 6 H 6 CSH) 3
From a chemical standpoint these compounds may be
classified under four heads : (1) those containing carbon
and hydrogen; (2) those containing carbon, hydrogen,
and oxygen ; (3) those containing carbon, hydrogen, and
nitrogen, or carbon, hydrogen, oxygen, and nitrogen;
(4) those containing sulfur in combination with the
elements listed above. With the possible presence in
soils of compounds containing five elements, it is little
wonder that the subject is a complicated one. It is
evident, moreover, that the list given above is only a
partial one, and many other compounds of an even more
intricate composition will later be isolated.
So far as the plant is concerned, the compounds may
be divided into three groups those that are beneficial,
those that are neutral, and those that are toxic, or harm-
ful, in their effects. As an example of the first group,
136 SOILS: PEOPERTIES AND MANAGEMENT
histidine and creatinine 1 may be mentioned. Here is
a case in which the compounds found in the soil humus
may exert a stimulating effect on plant growth, and
may also be a source of plant-food, supplementing the
nitrates 2 to a certain extent. That the nitrogen of the
soil organic matter may be utilized by plants is well sum-
marized by the publications of Hutchinson and Miller. 3
As an example of a harmful compound arising from the
decomposition of the organic matter, dihydroxystearic
acid 4 may be mentioned as one of the best known. This
compound was the first to be isolated and identified by
Schreiner, and is very toxic.
93. Toxic material in the soil. The discovery of
such compounds in the soil has revived the old theory
of toxicity, 5 by which the infertility of certain soils is
accounted for. Root excretions are also held to be
detrimental to succeeding crops of the same kind. The
1 Skinner, J. J. Effect of Histidine and Arginine as Soil
Constituents. Eighth Internat. Cong. App. Chem., Vol. XV,
pp. 253-264. 1912. Also, Beneficial Effects of Creatinine
and Creatine on Growth. Bot. Gaz., Vol. 54, No. 2, pp. 152-
163. 1912.
2 Schreiner, O., and Skinner, J. J. Nitrogenous Soil Con-
stituents and Their Bearing upon Soil Fertility. U. S. D. A.,
Bur. Soils, Bui. 87, p. 68. 1912. Also, Schreiner, O., and others.
A Beneficial Organic Constituent of Soils; Creatinine. U. S.
D. A., Bur. Soils, Bui. 83, p. 44. 1911.
3 Hutchinson, H. B., and Miller, N. H. J. The Direct
Assimilation of Inorganic and Organic Forms of Nitrogen by
Higher Plants. Jour. Agr. Sci., Vol. 4, Part 3, pp. 282-302.
1912.
4 Schreiner, O., and Skinner, J. J. Some Effects of a Harm-
ful Organic Soil Constituent. U. S. D. A., Bur. Soils, Bui. 70.
1910.
5 See Schreiner, O., and Reed, H. S. Some Factors Influ-
encing Soil Fertility. U. S. D. A., Bur. Soils, Bui. 40, pp. 36-
40. 1907.
THE ORGANIC MATTER OF THE SOIL 137
toxic materials of the soil humus largely originate under
conditions of poor drainage and aeration, and conse-
quently are biological in their genesis. The toxicity of
such compounds as dihydroxystearic acid, picoline car-
hnxylic acid, 1 and aldehydes 2 may therefore be overcome
by oxidation, 3 so that good soil aeration is a factor in
dealing with such conditions. Insufficient sanitation
of the soil seems to account very largely for the presence
of soil toxines. Fertilizers, according to Schreiner and
Skinner, 4 seem to decrease the harmful effects of such
compounds; nitrogenous fertilizers overcoming some
toxic materials, and phosphorus" or potash neutralizing
others. For example, in water solution and sand culture,
nitrogen seems especially efficacious in correcting such
toxic substances as dihydroxystearic acid and vanillin,
phosphorus is particularly powerful in counteracting
oumarine, and potash has considerable influence on
quinone.
While the real importance of the toxic material generated
in the soil cannot be fully discussed at this point, it is
quite evident that such constituents do tend to develop
under insanitarv conditions and must be considered in
1 Schreiner, O., and Skinner, J. J. The Isolation of Harm-
ful Organic Substances from Soils. U. S. D. A., Bur. Soils,
Bui. 53, pp. 46-49. 1909.
2 Schn-inor, O., and Skinner, J. J. Harmful Effects of
Aldehydes in Soils. U. S. D. A., Bui. 108 (Professional Paper).
1914,
" Schreiner, O., and, others. Certain Organic Constituents
of Soils in Relation to Soil Fertility. U. S. D. A., Bur. Soils,
Bui. 47, p. 52. 1907. Also, Schreiner, O., and Reed, H. S.
The Role of Oxidation in Soil Fertility. U. S. D. A., Bur.
Soils, Bui. 56, p. 52. 1906.
4 Sr-hreiner, O., and Skinner, J. J. Organic Compounds and
Fertilizer Action. U. S. D. A., Bur. Soils, Bui. 77. 1911.
138 SOILS: PROPERTIES AND MANAGEMENT
the discussion of the composition of that great group oi
intermediate compounds, called humus, arising from the
decay of the organic matter of the soil. While Sehreiner
found twenty soils, out of a group of sixty taken in eleven
States of this country, to contain dihydroxystearic acid,
this does not necessarily mean that this compound in
itself is a serious detrimental factor. It is very likely
that such compounds are merely products of improper
soil conditions, and are to be considered as concomitant
with depressed crop yields. When such conditions are
righted, the so-called toxic matter will disappear. Good
drainage, lime, tillage, a balanced food ration, promoted
aeration and oxidation, are so efficacious in this regard
that permanent soil toxicity need never be feared by the
farmer.
94. End products of humus decay. As the processes
of chemical and biological decay of the soil organic matter
proceed, the simple compounds already noted begin to
appear. This change is of course coordinate with a
certain amount (5f synthetic action, but compounds thus
built up must ultimately succumb to the agencies at
work and suffer a splitting-up and reduction to simple
bodies. Carbon dioxide is one of the most important of
these compounds, being always a product of bacterial
activity. Its importance has already been noted in the
discussion of weathering. Here it heightens the solvent
power of water and tends to increase the amount of plant-
food carried in the soil solution. Carbonation is a direct
result of its presence. Carbon dioxide may also tend to
flocculate colloidal matter in soils, and thus benefit the
physical conditions. With increased organic matter in
any soil, there greater bacterial action and an increase
in the carbon dioxide evolved may well be expected. In
THE ORGANIC MATTER OF THE SOIL
139
fact, the carbon dioxide production of a soil is considered
by some authors 1 to be a measure of bacterial activity.
\Vith this increase in carbon dioxide the soil air becomes
more heavily charged and an alteration in bacterial and
plant relationships may thereby be induced. The fol-
lowing figures, by Wollny, 2 show the composition of the
soil atmosphere and the effects of additional humous
material on the carbon dioxide content:
PERCENTAGE BY
VOLUME OF
C0 2
Soil air (average of 19 analyses) ....
Atmospheric air
2.54
.04
1.06
9.74
18.33
20.96
19.72
10.35
A sandy soil
A sandy soil plus manure
While carbon dioxide may be evolved by the splitting-up
of both carbohydrate and nitrogenous bodies, ammonia
results only from the latter. It is really the first ex-
tremely simple nitrogenous body produced. It can be
utilized by some plants as a source of nitrogen, as is also
true with certain simple humic bodies, but ordinarily
it must undergo oxidation. This oxidation results in
nitrites (XO 2 ) and ultimately in nitrates (NO 3 ), the
latter being usually considered as the chief source of
the nitrogen utilized by plants.
1 Stoklasa, J., and Ernest, A. Ueber den Ursprung, die
Menge, und die Bedeutung des Kohlendioxyds im Boden.
Centrlb. Bakt., II, 14, Seite 723-736. 1905.
2 Wollny, E. Die Zersetzung der Organischen Stoffe,
Seite 2. Heidelberg, 1897.
140 SOILS: PROPERTIES AND MANAGEMENT
Other end products, such as methane (CH4), hydrogen
disulfide (H 2 S), free nitrogen (N), sulfur dioxide (SO 2 ),
carbon disulfide (CS 2 ), and the like, may also result.
They are relatively unimportant, however, as regards
the plant, in comparison to the role played by carbon
dioxide, ammonia, the nitrites, and the nitrates. The
production of the nitrates from ammonia particularly
is very clearly correlated with good soil conditions, es-
pecially optimum moisture and adequate aeration. The
proper handling of the soil, then, not only will tend to
eliminate toxic matter and prevent its further formation,
but will encourage the proper decay of the soil humus and
the production of end products which will function directly
or indirectly as plant foods.
Snyder 1 found that when humus was extracted with an
alkali and then precipitated with an acid, it yielded from
five to tw T enty-five per cent of a reddish brown ash. This
ash contained silica, iron, and alumina, as well as mag-
nesia, potash, phosphorus, sulfur, sodium, and calcium.
While part of these mineral constituents may be chemically
combined with humus, it is probable that some may be
present because of the adsorptive capacity of the organic
colloids which are always present in humus generated
under normal conditions. Snyder has estimated that
in an ordinary soil containing a fair amount of organic
matter, one-sixth of the phosphorus and one-twelfth of
the potash may be present in such a state. They are
then fairly available, and are yielded much more readily
to the plant than if of a strictly inorganic nature.
95. Carbonized materials of soil. After the extrac-
tion of the soil for the study of the ordinary humus com-
1 Snyder, Harry. Production of Humus from Manures.
Minnesota Agr. Exp. Sta., Bui. 53, pp. 29-30. 1897.
THK ORGANIC MATTER OF THE SOIL 141
pounds, a considerable mass of material remains, which
is insoluble in water, alkali, and other ordinary solvents.
By the extraction of a large amount of soil, Schreiner l
was able to study this material. He found it susceptible
to division into six groups, as follows: (1) plant tissue,
(2) insect and other organized material, (3) charcoal
particles, (4) lignite, (5) coal particles, and (6) materials
resembling natural hydrocarbons, as bitumen, asphalt, and
the like. Such material was found not only near the sur-
face of the soil, but at depths of fifteen or twenty feet be-
low. All the groups above listed were found by Schreiner
to be represented in the thirty-four soils collected from
all parts of the United States and subjected to rigid test.
The exact origin of such material is problematical.
Forest and prairie fires, infiltration, mild oxidation, and
nullification might be mentioned. Of a certainty, the
agencies of distribution are the natural forces engaged
in physical weathering. This carbonized material is
important, as it makes up no inconsiderable part of the
soil humus. It is very resistant, and consequently lends
stability to the soil organic matter. It can be divided
into two general groups, organized and unorganized;
in the former the original structure remains intact, while
in the latter the original features have been obliterated.
The study of such material and the changes that it under-
goes not only increases the list of known organic com-
pounds existing in the soil, but throws considerable light
on the nature of the soil organic matter as a whole.
96. The estimation of the soil organic matter. -
Many methods have been proposed for the determination
1 Schreiner, O., and Brown, B. E. Occurrence and Nature
of Carhoni/rcl Material in Soils. U. S. D. A., Bur. Soils,
Bui. 90. 1912.
142 SOILS: PROPERTIES AND MANAGEMENT
of the organic matter in soils, but none have proved
entirely satisfactory, since the composition of this ma-
terial is so complicated and so likely to change while
under investigation. Other soil constituents also tend
to interfere with the determination. Two general methods
seem worthy of mention, as they have been used very
widely in soil analyses and at least give comparative, if
not absolutely accurate, results.
Loss on ignition. 1 This is a simple method which
designs to burn off the organic matter and determine
its loss by difference. Five grams of dry soil are
placed in a platinum dish and ignited at a low red
heat until the organic matter is all oxidized. The
cold mass is moistened with ammonium carbonate
and heated to a temperature of 150 C. in order to
expel the excess of ammonia. The loss is rated as or-
ganic matter.
This method is open to the objection that, besides the
loss of organic matter, a certain small amount of water
of combination, together with all ammoniacal compounds,
nitrates, all carbon dioxide, and some alkali chlorides if
the temperature is carried too high, is driven off. The
method therefore gives high results, especially in the
presence of large amounts of hydrated silicates. An
attempt to replace the carbon dioxide is made in the
treatment of the cold mass with ammonium carbonate.
Notwithstanding these objections, this method is one of
the best and is very generally used all over the world
in estimating the organic matter of the soil. Very often
1 Houston, H. A., and McBride, F. W. A Modification of
Grandeau's Method for the Determination of Humus. U. S.
D. A., Div. Chem., Bui. 38 (edited by H. W. Wiley), pp. 84-92.
1893.
THE OEGANIC MATTER OF THE SOIL 143
the combustion is carried on in a current of oxygen over
hot copper oxide. The organic carbon may thus be
determined very accurately, and the organic matter
calculated by multiplying the carbon found by the factor
1.724.
Chromic acid method. 1 This method, proposed by
Wolff, has been modified and improved by various chem-
ists. Warington and Peake 2 have perhaps done more
with the method than any other investigators. In the
United States the modification of Cameron and Brea-
zeale 3 has been very generally accepted. It consists
in the treatment of the soil sample with sulfuric acid
and chromic acid or potassium bichromate. The organic
matter, in the presence of the sulfuric acid and an oxidizing
agent, evolves carbon dioxide until, if the mixture is
boiled, practically all of the carbon is thus driven off.
This gas is draw r n through a train of absorption bulbs,
caught in a solution of potassium hydrate, and thus
weighed. On the supposition that organic matter is
58 per cent carbon, it is very easy to make the calcula-
tion. The carbon found may be multiplied by 1.724,
or the carbon dioxide by .471. The product is considered
as soil organic matter. The results thus obtained are
usually lower than with combustion or ignition methods,
1 For comparison of methods, see Wiley, H. W. Principles
and Practices of Agricultural Analysis, Vol. I, pp. 347-356.
Easton, Pa. 1906.
2 Warington, R., and Peake, W. A. On the Determina-
tion of Carbon in Soils. Jour. Chem. Soc. (London) Trans.,
Vol. 37, pp. 617-625. 1880.
3 Briggs, L. J., and others. The Centrifugal Method of
Mechanical Soil Analysis. U. S. D. A., Bur. Soils, Bui. 24,
pp. 33-38. 1904. Also, Cameron, F. K., and Breazeale, J. F.
The Organic Matter in Soils and Subsoils. Jour. Amer.
Chem. Soc., Vol. 26, pp. 29-45. 1904.
144 SOILS: PROPERTIES AND MANAGEMENT
due to the resistance to oxidation 1 by the carbonized
matter, already discussed. This material, while it suc-
cumbs to ignition, resists the action of the sulfuric and
chromic acids to a very large degree.
97. The estimation of soil humus. The common
method of humus estimation is that proposed by Grand-
eau. 2 The sample of soil is first washed with acid in order
to remove all bases. It is next treated with ammonia,
which will then dissolve out the humous materials. By
catching this percolate, evaporating it to dryness, and
weighing it, the percentage of humus may be calculated.
The dark humous extract obtained with the ammonia
is called the Matiere Noire.
This method has undergone several modifications, 3 of
which that of Hilgard 4 and that of Houston and McBride 5
seem the most promising. The method of the latter
chemists has been adopted by the Association of Official
Agricultural Chemists and is considered as the official
method. In the procedure an attempt is made to keep
the concentration of the ammonia in contact with the
soil constant during the extraction. Consequently the
sample, after treatment with the acid, is washed into a
1 Schreiner, O., and Brown, B. E. Occurrence and Nature
of Carbonized Material in Soils. U. S. D. A., Bur. Soils,
Bui. 90, pp. 19-21. 1912.
2 Grandeau, L. Traiti d'Analyse de Matieres agricoles. I,
p. 151. 1897.
3 A comparison of the various methods is found as follows :
Alway, F. J., and others. The Determination of Humus.
Nebr. Agr. Exp. Sta., Bui. 115. June, 1910.
4 Hilgard, E. W. Humus Determination in Soils. U. S.
D. A., Div. Chem., Bui. 38 (edited by H. W. Wiley), p. 80.
1893.
5 Wiley, H. W. Official and Provisional Methods of Analy-
sis. U. S. D. A., Bur. Chem., Bui. 107, p. 19. 1908.
THE ORGANIC MATTER OF THE SOIL 145
500 cubic centimeter flask, which is filled to the mark
with 4 per cent ammonia. Digestion is allowed to pro-
ceed for twenty-four hours, with frequent shakings, and
and an aliquot portion of the supernatent liquid is taken
for analysis. This method with its modifications is
practically the only one that we have for the estimation
of soil humus. It is based on the fact that when a
soil is lacking in active basic material, the humous
matter may be extracted with ammonia. A modification
of this method may be used as a test for soil acidity,
as any soil of humid regions allowing the extraction of
humus by ammonia alone must lack basic materials.
The composition of the ash constituents of the matiere
noire is given by Snyder * as follows, the data being the
average of eight analyses :
THE ASH FROM THE HUMUS OF MINNESOTA PRAIRIE SOILS
Percentage
Insoluble 61.97
Fe 2 O 3 3.12
A1 2 O 3 3.48
K 2 7.50
Na 2 O 8.13
CaO 09
MgO 36
P 2 O 5 12.37
SO 3 98
CO 2 1.64
The relatively high percentage of phosphoric acid is
immediately noticeable in this analysis. This indicates
1 Snyder, Harry. Soils. Minnesota Agr. Exp. Sta., Bui. 41,
p. 30. 1895.
146 SOILS: PROPERTIES AND MANAGEMENT
that no mean portion of the soil phosphates is held in
organic combination. The promotion of favorable hu-
mous decay must thus liberate a considerable amount of
phosphorus for plant utilization.
98. Organic content of representative soils. The
organic content of soils varies widely according to climatic
conditions. The following average data show the limits
of variation as well as the comparative content of the
important soil sections of the United States :
ORGANIC CONTENT OF UNITED STATES SOILS
SANDY SOILS
CLAY LOAMS AND LOAMS
Soil
Subsoil
Soil
Subsoil
North Central States .
1.84
.76
3.06
1.07
Northeastern States
1.66
.60
3.73
1.35
South Central States .
1.16
.55
1.80
.65
Southeastern States
.93
.41
1.53
.73
Semiarid States . . .
.99
.62
2.64
1.11
Arid States ....
.89
.64
1.05
.62
It is at once apparent that the subsoil contains con-
siderably less organic matter than do the surface layers.
Also, the areas of the United States that have been
glaciated are noticeably richer in organic material than
the residual, coastal plain, and arid regions. This is
largely a climatic and geochemical relationship. Some
soils, particularly alluvial soils, very often run higher
than the average data given above. An organic content
of 5 or 6 per cent is not an uncommon figure with such
materials. Muck and peat soils are of course not to be
classified with the above, as their organic content may
THE ORGANIC MATTER OF THE SOIL
147
range from 35 to 85 per cent, according to the admixture
of mineral matter from extraneous sources.
99. The humus content of soils. The humus content
of soils is of course lower than the organic matter therein
contained. It likewise varies according to climate and
region, not only in amount, but also in composition. The
following data, compiled from Hilgard, 1 illustrate this
point :
THE HUMUS OF ARID AND HUMID SOILS
HUMUS IN
SOIL
NITROGEN IN
HUMUS
NITROGEN IN
SOIL
(Percentage)
(Percentage)
(Percentage)
41 Arid uplands soils
15 Subirrigated arid soils . .
24 Humid soils
.91
1.06
4.58
15.23
8.38
4.23
.135
.099
.166
It is evident that humid soils not only contain the
greatest amounts of organic matter, but also excel in
humus. The humus of the arid regions, however, is
richer in nitrogen, due to the peculiar decomposition going
on. As a consequence the nitrogen in the soil of humid
regions is not greatly in excess of that in the soils of drier
climates.
The percentage of humus not only decreases as the
lower depths of soil are examined, but also changes in
composition, becoming poorer in nitrogen the deeper
the soil is penetrated. The following data on a Russian
river soil, quoted by Hilgard, 2 may be cited as an in-
stance :
1 Hilgard, K". W. Soils, pp. 136-137. New York. 1911.
2 Ibid., p. 139.
148 SOILS: PROPERTIES AND MANAGEMENT
THE HUMUS OF A RUSSIAN ALLUVIAL SOIL
DEPTH IN FEET
PERCENTAGE OF
HUMUS
PERCENTAGE OF
NITROGEN IN
HUMUS
PERCENTAGE OP
HUMOUS NITROGEN
IN SOIL
1
1.21
5.30
.064
2
1.16
4.32
.054
3
1.14
3.87
.044
4
1.17
3.76
.344
5
.74
2.16
.016
6
.60
2.66
.016
7
.47
2.54
.012
8
.78
1.54
.012
9
.54
2.24
.012
10
.52
1.15
.006
11
.53
1.51
.008
12
.44
1.81
.008
Other depth relationships, especially regarding the
proportions of carbon, humus, and nitrogen, are brought
out in the following data, obtained by Alway and Vail l
in the study of Nebraska Soils :
COMPOSITION OF A NANCE COUNTY, NEBRASKA, SOIL NEAR
GENOA
DEPTH
IN FEET
PERCENT-
AGE OF
NITROGEN
PERCENT-
AGE OF
CARBON
PERCENT-
AGE OF
HUMUS
PERCENT-
AGE OF
ASH IN
HUMUS
RATIO OF
C
N
H
N
C
H
1
.255
2.61
2.47
1.61
10.2
9.7
1.0
1
.102
.85
1.00
.90
8.3
9.8
.9
3
.056
.31
.40
.52
5.5
7.1
.8
4
.042
.24
.30
.64
5.7
7.1
.8
5
.034
.17
.19
.33
5.0
5.6
.9
6
.027
.14
.16
.36
5.2
5.9
.9
1 Alway, F. J., and Vail, C. E. The Relative Amounts
of Nitrogen, Carbon, and Humus in Some Nebraska Soils.
Nebraska Agr. Exp. Sta., 25th Ann. Kept., p. 155. 1912.
THE ORGANIC MATTER OF THE SOIL
149
100. Influence of the original material on the resultant
humus. It is evident that the source from which any
humus material is derived will exert a profound influence
on its composition, especially its nitrogen content.
Snyder 1 has investigated this by mixing certain materials
with a soil poor in humus and allowing the process of
decay to proceed for a year under favorable conditions.
At the end of the period the humus was extracted by the
Grandeau method. The results are given below :
THE COMPOSITION OF HUMUS PRODUCED FROM VARIOUS OR-
GANIC MATERIALS
C
H
o
N
Sugar
57.84
3.04
39 04
08
Sawdust
4928
3 33
47 07
32
Oats straw ....
54.30
2.48
40.72
2.50
Wheat flour ....
51.02
3.82
40.14
5.02
Cow manure ....
41.93
6.26
45.63
6.16
Green clover ....
54.22
3.40
34.14
8.24
Meat scrap ....
48.77
4.30
35.97
10.96
Although the humification may not have reached
completion in this case, the great variation in nitrogen
is striking. Existing, as it probably does, mostly as
acid amides and monamino acids, it will change readily
to ammonia and exert a marked effect on plant growth.
Possibly the variation of the nitrogen in soil humus is
the most potent factor in the nutritive functionings of
this material. The variability of the carbon, hydrogen,
and oxygen of the soil humus is not such an important
factor, as these elements can easily be supplied to the
1 Snyder, Harry. Production of Humus from Manures,
Minnesota Agr. Exp. Sta., Bui. 53, p. 25. 1897.
150 SOILS: PROPERTIES AND MANAGEMENT
soil by the plowing-imder of green materials or of barn-
yard manures. In general, the percentage content of
inorganic matter increases as the organic matter decays.
101. Effects of organic matter on soil. The effects
of the organic matter on soil and plant conditions are as
numerous as they are complex. Some of the influences
are direct, others are indirect. j\s the specific gravity of
organic matter is low, the first effect of its addition would
be to lower the absolute and the apparent specific gravity
of the soil. As the water capacity of humus is very high,
a soil rich in organic constituents usually possesses a
high water-holding power. This makes possible greater
volume changes both on drying and in the presence of
excessive moisture. The granulating effects of wetting
and drying and freezing and thawing are therefore ac-
celerated. The organic matter tends also to spread
the individual particles of soil farther apart, especially
in a clay. Its loosening effects are immediately ap-
parent in such soil. On the other hand, because organic
matter has a higher cohesive and adhesive power than
sand, it performs the function of a binding material
with the latter soil, a condition much to be desired in a
material possessing such textural characteristics.
The better tilth induced by the presence of organic
matter in any soil tends to facilitate ease in drainage
and to encourage good aeration. These tw T o conditions
are of course necessary for the promotion of soil sanita-
tion. Root extension and bacterial activity are thus
increased. It is of especial importance that the splitting-
up of the organic matter shall take place in the presence
of plenty of oxygen, in order that toxic compounds may
not be generated and that a humus highly favorable to
plant growth shall be produced. The increased water
THE ORGANIC MATTER OF THE SOIL 151
capacity of the soil resulting from the presence of organic
matt-rials is of some importance in drought resistance,
while the black color imparted by the humus tends to
raise the absorptive power of the soil for heat.
The soil organic matter, however, functions in other
ways than those strictly physical. The humus or its
degradation products may serve as plant-food. Bacteria
and other soil organisms are also furnished a source
of energy thereby, and the production of carbon dioxide
is much increased. This carbon dioxide, as well as the
organic acids generated, tends to raise the capacity of
the soil water as a solvent agent, and thus the amount of
mineral plant food available to the crop is greatly in-
creased. The general effect of organic matter, then, is
to better the soil as a foothold for plants, and to increase
cither directly or indirectly the available food supply for
the crop.
102. Maintenance of soil organic matter. The main-
tenance of a proper supply of organic matter in a soil is
a question of great practical importance, as productivity
is governed very largely by the humus content of the soil.
This maintenance of the soil humus depends on two
factors the source of supply and methods of addition,
and the promotion of proper soil conditions in order that
the organic matter may perform its legitimate functions.
The organic matter of the soil may be increased in a
natural way by the plowing-under of green crops. This
is called green-manuring and is a very satisfactory prac-
tice. Such crops as rye, buckwheat, clover, peas, beans,
and vetch lend themselves to this method of soil im-
provement. Not only do these crops increase the actual
carbohydrate content of a soil, but in the case of legumes
the nitrogen also is increased in amount, due to the
152 SOILS: PROPERTIES AND MANAGEMENT
symbiotic action of the nodule bacteria. Green-manure
crops may also protect the soil from loss of plant-food by
leaching. The addition of barnyard manure is a common
method of raising the organic content from external
sources, and on decaying this manure performs the same
function as natural soil humus. Muck, peat, straw, or
leaves may be used in a similar manner.
Improper soil conditions not only prevent the proper
decay of organic matter, but also tend to encourage the
production of products inimical to plant growth. There-
fore, in order that organic materials added to any soil
may produce the proper humous constituents and per-
form their normal functions, soil conditions in general
must be of the best. Tile drainage should be installed,
if necessary, in order to promote aeration and granulation.
Lime should be added if basic materials are lacking, for
it promotes bacterial activity as well as plant growth.
The addition of fertilizers will often be a benefit, as will
also the establishment of a suitable rotation. The. rota-
tion of crops not only prevents the accumulation of
toxic materials, but also, by increasing crop growth,
makes possible a larger addition of organic matter by
green-manuring.
Good soil management seeks to adjust the addition
of organic matter, the soil conditions, and the losses
through cropping -and leaching, in such a way that paying
crops may be harvested without impairing the humus
supply of the soil. Any system of agriculture that tends
to permanently lower the organic matter of the soil is
impractical, and improvident, as well as unscientific.
CHAPTER IX
THE COLLOIDAL MATTER OF SOILS 1
IT has already been noted, in the discussion of the clay
separate of any soil, that in the presence of water
certain of the very finest particles, even though apparently
dissolved, assume particular and important properties,
such as high adsorption, nondiffusion through mem-
branes, and the Brownian movement. Such material
has been designated as colloidal in nature. It must
be understood from the beginning, however, that
1 Some of the following general references may prove of in-
terest :
Freundlich, H. Kapillarchemie. Leipzig, 1909.
Zsigmondy, R. Kolloidchemie. Leipzig, 1912.
Bancroft, W. D. The Theory of Colloid Chemistry.
Jour. Phys. Chem., Vol. 18, No. 7, pp. 549-558. 1914.
Xiklas, H. Die Kolloidchemie und ihre Bedeutung fur
Bodenkunde, Geologic, und Mineralogie. Internat. Mitt,
fiir Bodenkunde, Band II, Heft 5, Seite 383-403. 1913.
Ramann, E. Kolloidstudien bei Bodenkundlichen Arbei-
ten. Kolloidchemische Beihefte, Band II, Heft 8/9, Seite 285-
MO:*. 1911.
Konig, J., Hasenbaumer, J., und Hassler, C. Bestimmung
der Kolloide im Ackerboden. Landw. Ver. Stat., Band 76,
Heft 5/6, Seite 377-^41. 1912.
Noyes, A. A. The Preparation and Properties of Colloidal
Mixtures. Jour. Amer. Chem. Soc., Vol. 27, pp. 85-104. 1905.
Thaer, W. Der Einfluss von Kalk und Humus auf die
Mechanische und Physikalische Beschaffenheit von Ton-,
Lehm-, und Sandboden. Jour. f. Landw., Band 59, Heft 1.
Seite 9-57. 1911. Also, Kolloidchemische Studien. Jour,
f. Landw., Baud 60, Heft 1, State 1-18. 1912.
153
154 SOILS: PROPERTIES AND MANAGEMENT
colloidal material does not differ from crystalloidal
in chemical composition, but the distinction is merely
one of size of particles. For example, if large particles
are suspended in water, they will immediately sink,
since their weight is so much greater than the sur-
face that is exposed for buoyance. When these particles
are decreased in size, their weight decreases much faster
than the surface exposed. It is therefore evident that
a point will at last be reached at which the particles,
because of their minute size, will form a homogeneous
solution. The upper limit of the colloidal state has then
been entered.
103. The colloidal state. The colloidal state in which
these particles are now found is a peculiar one, and ex-
hibits much diversity, not only in properties, but also,
in the size of particle in which the material exists. The
upper limit of the clay group as designated by the classi-
fication of the United States Bureau of Soils is .005 milli-
meter, while the upper limit of the particle existing in a
colloidal state is estimated to be below .005 of a micron,
or .000005 millimeter. Indeed, so small are the colloidal
particles that they become molecular complexes, that is,
a few molecules may go to make up a particle.
The various colloids, or the same colloid under different
conditions, may exhibit greatly differing sizes of particles.
Some colloidal particles are very large, approaching the
upper limit already set for material in such a state. Other
particles are finer. It is evident that a gradation must
exist until a particle is reached which consists of only one
molecule. The solution then ceases to be a molecular
complex and becomes a true solution. The colloidal
state thus grades into the true solution, just as an
ordinary suspension grades into a true, or colloidal ,
THE COLLOIDAL MATTER OF SOILS 155
suspension. While this method of comparison fails to
recognize the various phases that colloidal materials
may exhibit and is therefore faulty in this regard, it
does lay emphasis on the differences as to size of par-
ticle that exist between colloidal bodies and materials
as they are ordinarily recognized. This relationship
is shown by the following diagram:
ORDINARY SUSPENSION | COLLOIDAL STATE | TRUE SOLUTION
MOLECULAR COMPLEX
FIG. 22. Diagram showing the relationship of the colloidal state
(molecular complex) to ordinary suspensions and true solution^.
Since colloidal particles vary in size from .005 of a
micron to a molecule, the range must be very great.
Just how great cannot be very accurately stated. It
is interesting to note, however, that this rang-e is much
greater in proportion than is exhibited between the fine
gravel and the ordinary clay particles found in soil. With
this possible difference, it is no great wonder that the
various colloids exhibit with different intensities the
characteristics so peculiar to them and of such great im-
portance in everyday life. The particles in the upper
range of the colloidal field can be seen with the ordinary
microscope. As such particles become smaller they cease
to be visible under the ordinary microscope and can be
detected only by the ultramicroscope. It is probably
true that by far the greater proportion of the particles
of material in a colloidal state cannot be detected by
microscopic means. This gradation of colloidal materials
and the extreme fineness of the particles is well illus-
trated by the following diagram (Fig. 23), although it
fails to convey any idea regarding the various phases
that colloids may occupy.
156 SOILS: PROPERTIES AND MANAGEMENT
FIG. 23. Diagram showing the possible range in the size of colloidal
particles.
104. The properties of colloids. In general there are
certain properties which materials in a colloidal state
exhibit and by which they are distinguished from true
solutions. In the first place, since they are not in
true solution they exert little effect on the freezing
point, on vapor tension, and on vapor pressure. Some
colloids have absolutely no effect on these conditions,
while others, as they allow a certain small amount
of true solution to take place, do possess such
THE COLLOIDAL MATTER OF SOILS 157
influences to a slight degree. Secondly, colloids do not
pass readily through semipermeable membranes, as
parchment paper, while crystalloids do. This serves as
a very easy way of separating colloidal and crystalloidal
material. As a matter of fact, the membrane is itself a
colloid. Thirdly, heat and the addition of electrolytes
will serve to coagulate or precipitate certain colloids, a
property which again serves to distinguish them sharply
from a true solution. Fourthly, colloidal material has great
adsorptive power, not only for water, but also for materials
in solution, a quality of extreme importance in soil studies.
It has been shown that a colloid is a material in a
certain state of division, in which it exhibits properties
not possessed by an ordinary suspension or by a true
solution. It is therefore proper to speak of matter so di-
vided as being in the colloidal state, or colloidal condition.
It is not to be inferred, because the colloidal phase is
contrasted with the crystalloidal, that colloids are amor-
phous. They may or may not be in such a condition.
Moreover, the same material may exist without chemical
change either in the colloidal or non-colloidal state. For
example, silicic acid, ferric hydrate, gold, carbon black,
and other materials, may or may not be colloidal, ac-
cording to circumstances. The fineness of division is
the explanation of colloidal properties. In order to
place such a discussion on a more understandable basis,
a few illustrations of the colloidal state w r ill not be amiss.
The following materials, which may exist as colloids, may
be for convenience grouped under two general heads,
organic and inorganic :
Organic: Gelatin, agar, caramel, albumin, starch, jelly,
humus, carbon black, tannic acid, etc.
158 SOILS: PROPERTIES AND MANAGEMENT
Inorganic: Gold, silver, iron, ferric hydrate, arsenious
oxide, zinc oxide, silver iodide, Prussian
blue, etc.
105. Colloidal phases. In general, two conditions
are necessary for the colloidal state a dispersive medium,
and a material that will disperse, the latter being usually
designated as the disperse phase. Three materials may
function as a dispersive medium a liquid, a solid, or a
gas. In the same way, with each dispersive medium there
may be three disperse phases a liquid, a solid, or a gas.
This gives nine general phases to be considered in colloidal
chemistry. From the soil standpoint, the liquid-solid
and the liquid-liquid phases are by far the most important
and will be the only ones to receive detailed attention
here. In the liquid-solid phase, as with colloidal gold
or ferric hydrate, the particles are suspended in water
as the dispersive medium. In the case of gelatin, another
liquid-solid example, the jelly surrounds the dispersive
medium, or liquid. An emulsion may exhibit the liquid-
liquid phase, and possibly exists in soils rich in humus.
In these colloidal phases under discussion and of such
particular interest in soil study, two general classes of
materials are found, which seem to differ radically from
each other and yet are likely to lead to considerable con-
fusion unless special pains are taken to distinguish between
them. As types, gelatin and a colloidal suspension of
ferric hydrate may be cited. The gelatin is considerably
more viscous than water, while the ferric hydrate does
not differ from water in this respect. The former gelatin-
izes on cooling or on loss of moisture, but will become
dispersed again on the addition or presence of water. In
other words, it will pass again and again, back and forth,
THE COLLOIDAL MATTER OF SOILS 159
from a sol to a gel. It is what might be called a reversible
colloid. Moreover, it is not coagulated by ordinary addi-
tions of salt or by heating. The ferric hydrate colloid,
on the other hand, when precipitated or agglutinated by
any means may not easily be brought back again to the
sol state. It is a so-called irreversible colloid. Moreover,
it is thrown down by the addition of electrolytes. There
exist, then, the viscous, gelatinizing, reversible colloids, and
the non-viscous, non-gelatinizing, easily coagulable, and
non-reversible colloids, besides all gradations and variations
between the two. In the ordinary clay soil, both types
of these materials probably exist and play important parts
in the physical and chemical characteristics exhibited.
106. Flocculation. While the gelatinous colloids of
the soil, such as some of the humic materials, are not
agglutinated by the addition of electrolytes, most of the
colloids of a nature similar to colloidal silicic acid and
ferric hydrate are thrown down by this treatment. This
phenomenon is often spoken of as fiocculation. A very
good example is afforded by treating a clay suspension
with a little caustic lime. The tiny particles almost
immediately coalesce into floccules, and, because of their
combined weight, sink to the bottom of the con-
taining vessel, leaving the supernatant liquid clear.
The same action will take place in the soil itself, but of
course with less rapidity and under conditions less notice-
able to the eye. The colloids thus thrown down, being
largely irreversible, cannot again assume their former
attributes and thus lose their distinguishing character-
istics. In general, acids bring about fiocculation while
alkalies do not, calcium oxide and calcium hydrate being
the best-known exceptions to the latter. Ammonia is
an intense deflocculator.
160 SOILS: PROPERTIES AND MANAGEMENT
Just how this phenomenon of flocculation or agglutina-
tion may be accounted for theoretically it is rather difficult
to state. The general theory is one of electrification. It
is found that certain colloids, when subjected to the
proper electric current, will migrate to either the positive
(anode) or the negative (cathode) pole. These particles
evidently carry a charge of electricity. Ferric hydrate,
aluminium hydrate, and basic dyes, for example, move
toward the cathode and carry a positive charge ; while
arsenious sulfide, silicic acid, gold, silver, and acid dyes
move toward the anode and are negative. It is assumed
that as long as the colloidal particles remain charged
they repel each other and the colloidal state persists.
When an electrolyte is added the ionization is supposed
to cause a discharge of the repellent electricity carried
by the colloidal particles, and flocculation or agglutina-
tion immediately takes place.
Certain colloids may flocculate certain others, as the
gelatinization of silicic acid by ferric hydrate. At times
one colloid may protect another, probably by surrounding
it with a protective film. Such a case may be shown by
adding gelatin to a clay suspension. When a colloid such
as ferric hydrate is flocculated, it loses to a certain extent
its peculiar properties, and assumes the characteristics of
ordinary materials. It is evident, therefore, that if the
properties exhibited by colloidal materials become either
directly or indirectly detrimental to plants, their floccula-
tion would be beneficial. In field practice this is usually
accomplished by the addition of lime. The colloidal
material existing in a normal soil and possessing a gelat-
inous nature, similar in general to gelatin, is probably
not all flocculated by the addition of ordinary amounts
of electrolytes. This material may be influenced by
THE COLLOIDAL MATTER OF SOILS 161
drying, whereby it slowly gives off water, becomes more
and more viscous, and at last may lose its gel qualities
and become hard and irreversible. It is evident, there-
fore, that wetting and drying, frost, and the like, become
factors in dealing with this form of colloidal matter.
107. Common soil colloids and their generation. -
The common soil colloids may, for convenience, be dis-
cussed under two heads, organic and inorganic. Of the
former, the so-called humic acid stands as the example;
of the latter, silicic acid, ferric hydrate, and amorphous
zeolitic silicates are the commonest.
Organic colloids. The humic colloids in a normal
fertile soil probably make up the bulk of the colloidal
matter. Such material is very heterogeneous, very
complex, and constantly changing. As yet very little
study of the organic soil colloids has been made because
of the difficulties presented by the problem. Humic
colloids may be viscous or non- viscous, as the case may
be, and may or may not be thrown down by lime. The
adsorptive power of these colloids for water, gases, and
such materials as calcium, magnesium, and potash, is
*Van Bemmelen, J. M. Die Absorption. Seite 114-115.
Dresden, 1910. Also, Die Absorptionsverbindungen und das
Absorptions vermogen der Ackererde. Landw. Ver. Stat.,
Band 35, Seite 69-136. 1888.
Way, J. T. On Deposits of Soluble or Gelatinous Silica
in the Lower Beds of the Chalk Formation. Jour. Chem. Soc.,
Vol. 6, pp. 102-106. 1854.
Warington, R. On the Part Taken by Oxide of Iron and
Alumina in the Adsorptive Action of Soils. Jour. Chem. Soc.,
LM ier., Vol. 6, pp. 1-19. 1868.
Cushman, A. S. The Colloid Theory of Plasticity. Trans.
Amor. Cer. Soc., Vol. 6, pp. 65-78. 1904.
Ashley, H. E. The Colloid Matter of Clay and its Meas-
urements. U. S. Geol. Sur., Bui. 388. 1909.
162 SOILS: PROPERTIES AND MANAGEMENT
very highly developed more so, probably, than that
of the inorganic colloids. These organic colloids are
formed during the tearing-down and splitting-off processes
of bacterial activity. Some of the humic materials are
thrown off in a sufficiently fine state of division to assume
the condition that has been designated as colloidal. Of
course the chemical forces of weathering are also operative
in this process of organic colloidal production.
Mineral colloids. The inorganic soil colloids, especially
ferric oxide and silicic acid, are less complex than the
organic and have been more thoroughly studied. Such
colloids are generated during the operation of the ordinary
forces of weathering, especially the chemical phase. For
example, when a feldspar undergoes decomposition, the
following reaction may be" used to illustrate the possible
change that takes place : -
2 KAlSi 3 O 8 + 2 H 2 O + CO 2 = H 4 Al 2 Si 2 O 9 + 4 Si0 2 +
K 2 CO 3
Kaolin practically always has its origin in this way,
together with an alkali carbonate and silica. The process
is essentially one of hydration and carbonation ; the CO 2
by reacting with the alkali permits the process to go on.
The silica may go in three directions, according to con-
ditions to free quartz, to hydrated silicates, and to
colloidal silica. Similar reactions may be written for
iron and aluminium, but they can only show, as does
the above, the general trend of the change. In general it
can be concluded that most inorganic colloids arise from
ordinary chemical weathering, together with secondary
minerals of various kinds. Such colloids must be very
dilute and are difficult to study because of their reaction
among themselves.
THE COLLOIDAL MATTER OF SOILS 163
108. Preparation of colloids. There are a number of
methods that may be used in the preparation of artificial
colloidal solutions, but the description of only one will
suffice in the present discussion. This is the use of a
gemipenneable membrane. It has already been men-
tioned that crystalloids pass with ease through a membrane
such as parchment paper, while colloids do not. It is
reasonable to expect, then, that these materials may be
thus separated by proper adjustments. As a matter of
fact, such a procedure is employed in many cases. The
operation is called dialysis, and the membrane, itself a
colloid, is designated as the dialyzing membrane.
For example, if a solution of ferric chloride to which
some ammonium carbonate has been added is placed in a
dialyzer with pure water on the outside, the hydrochloric
acid and other impurities gradually pass through the
membrane and a more or less pure colloidal solution of
ferric hydrate is left behind. The objection to this
method lies in its extreme slowness. Nevertheless, since
the cells of plants present a semipermeable membrane,
this method of preparation serves to explain many actions
that go on between soil and plant during the processes
of nutrition. In the soil the formation of colloidal ma-
terial is entirely a natural exertion of chemical and bio-
logical forces under such conditions that the particles
split off are in that state of division which has been desig-
na^ed as colloidal.
It may be inferred that the quantity of colloidal matter
in an average soil is large, but as a matter of fact this is
not the case. The proportion of the soil in a colloidal
state at any one time is very small. It must be remem-
bered, however, that material is continually being thrown
out of the colloidal condition and at the same time more
164 SOILS: PROPERTIES AND MANAGEMENT
is generated; thus the effects may be marked, although
the amount present at any one time is extremely minute.
109. Colloids and soil properties. As may naturally
be inferred, the influence of the colloidal matter on soil
conditions, especially as related to plants, is extremely
important. This influence is exerted in two ways. First,
on cohesion and plasticity ; and, secondly, on the adsorp-
tive power of the soil. Both these qualities must be con-
sidered, not only in the physical, but also in the chemical
and the biological, study of the soil as a medium for crop
production.
In general it is found that, other conditions being equal,
an increase of colloidal matter increases plasticity; in
other words, the ease with which a soil may be worked
into a puddled condition becomes greater. This is a rather
undesirable quality when too pronounced, and in clays
in which it is most likely to be developed some means
of decreasing the colloidal influence is advisable. This
great plasticity is developed because the colloids, espe-
cially those of a gelatinous or viscous nature, facilitate
the ease with which the particles may move over one an-
other and yet cohere sufficiently to prevent disruption
of the mass. In general, also, the greater the plasticity
of a soil, the greater is the cohesion when dry. In soils,
then, in which the colloidal material is very high, clodding
may occur if the soil is tilled too dry because of the great
tendency of the particles to cohere. This cohesion and
plasticity, as factors in soil structure, soil granulation,
and tilth, will be discussed in the succeeding chapter.
It is sufficient at this point only to observe the relation-
ship of colloidal materials to the development of such
qualities.
The second important attribute imparted to soil by
THE COLLOIDAL MATTER OF SOILS 165
colloid development is high adsorptive power. This
power extends not only to condensation of gases, but also
to water and to materials in solution. The water of
condensation on dry soil particles when exposed to a
saturated atmosphere is largely determined by the col-
loidal content. In other words, the surface exposure of
colloidal matter is so preponderant in water condensation
as in a general way to allow the one to be a relative meas-
ure of the other. Again, colloids exert adsorptive power
for material existing in the soil water, and to a limited
extent compete with the plant for food. Until the col-
loids are satisfied the soil solution may not reach its
maximum concentration for crop growth. This adsorp-
tive power is exerted especially on the basic materials,
such as calcium, and unless the existing colloids are fully
satisfied the soil tends to become lacking in available
bases. This condition is generally termed soil acidity.
It may readily be seen that the concentration of the soil
solution is governed to a considerable extent by the col-
loidal content of the soil, and that the adjustments in
concentration are always toward an equilibrium between
the two. Colloidal matter does not exert the same adsorp-
tive power for all materials, but is capable of what might
be called selective adsorption. For example, if ammonium
sulfate is added to a soil, the ammonia is strongly taken
up, which tends to release the sulfate. The continuous
use of such a fertilizer on a soil poor in lime will ultimately
result in the presence of free sulfuric acid. This example
is sufficient to emphasize the relationship of adsorptive
powers to fertilizer practice.
110. Factors affecting colloids. It must not be in-
ferred from the preceding discussion that the generation
of colloids is detrimental to soil conditions. In light
166 SOILS: PROPERTIES AND MANAGEMENT
soils the presence of such material is extremely necessary,
as it tends to bind the soil together, facilitates granula-
tion, and prevents loss of plant food by leaching. It is
only in heavy soils in which such material is excessive
that a detrimental condition is likely to exist. This occurs
because of a high cohesion and plasticity, because of the
competition for food that is likely to arise with the crop,
and because of the tendencies toward acidity. Where
lime is low or lacking, the situation has a tendency to
become still more aggravated by further colloidal devel-
opment.
In general, the practice of underdrainage by allowing
the wetting and drying of the soil to proceed, is the first
step not only for the curbing of excessive and improper
colloidal influence, but also for the encouragement of
just the right development thereof. The freezing of
winter, tillage at proper times, the addition of humus,
and the application of lime are all practices that aid
in the control of colloidal conditions. Since this control
and utilization of colloidal influences is only a phase of
soil structure as related to tilth and granulation, a further
discussion of the subject will be reserved for later consid-
eration.
111. Estimation of colloidal content. The colloids
in the soil are so complex, so numerous, so variable in
function, and so susceptible to change, that an exact
determination of their amount is impossible. The knowl-
edge of colloidal material in general is so meager that it
is not surprising that such slight advances have been
made in fully and clearly determining their character in
a complicated material, as the soil undoubtedly is. The
important methods of estimating the colloid content of
the soil depend for their expression on the intensity of
THE COLLOIDAL MATTER OF SOILS 167
certain qualities, supposed to be developed largely by
colloid content. This indicates that the methods are
largely comparative, rather than exact or strictly analyti-
c-ill in nature. These important methods 1 are three in
number : Van Bemmelen's, Ashley's, and Mitscherlich's.
]'an Bemmelen. The first investigator to advance
a method for colloid estimation was Van Bemmelen, 2
who considered that the amount of silica dissolved from
a soil by digestion with hydrochloric or sulfuric acids
was a measure of its colloidal content. It is now known
that some materials, such as crushed rock, may yield
as much silica with this treatment as a highly colloidal
clay. This method is not of great importance at the
present time, except as to the information that it gives
regarding the evolution of colloidal soil study.
Ashley. A second method, and one of much more
value, has been evolved by Ashley. 3 He found that the
adsorption of certain dyes by soils afforded a very good
index to colloidal content. The difficulty in this method,
however, lies in choosing the most effective dye and regu-
lating its concentration. Moreover, different colloids
vary so much in adsorptive capacity for the same dye,
that only roughly comparative results have thus far been
possible.
Mitscherlich. The third, and as yet the most valu-
1 A comparison of these methods is found as follows :
Stremme, H., and Aarnio, B. Die Bestimmung des Gehaltes
anorganischer Kolloide in Zersetzten Gesteinen und deren
tonigen Unlagerungsprodukten. Zeitsch. f. Prak. Geol.,
Band 19, Seite 329-349. 1911.
2 Van Bemmelen, J. M. Die Adsorptionsverbindungen
und das Absorptionsvermogen der Ackererde. Landw. Ver.
Stat., Band 35, Seite 69-136. 1888.
3 Ashley, H. E. The Colloid Matter of Clay and Its
Measurement. U. S. Geol. Sur., Bui. 388. 1909.
168 SOILS: PROPERTIES AND MANAGEMENT
able, mode of colloidal estimation is that of Mitscherlich, 1
in which the adsorptive capacity of the soil is again made
the comparative index. Water instead of dye is used as
the adsorbed material. In this method the air-dry soil
in a thin layer is brought to absolute dryness over phos-
phorus pentoxide. It is then placed in a desiccator over
a 10 per cent solution of sulfuric acid and the condensa-
tion is hastened by a partial vacuum. The sulfuric acid
is used in order to prevent the deposition of dew on the
soil. After exposure for at least twenty-four hours the
soils are found to have taken up their maximum moisture
of condensation, which is called the hygroscopic water.
The soil is then weighed, and the increase, figured to a
percentage basis, is taken as a measure of colloidal con-
tent. The reverse process may also be followed, by
exposing air dry soil in a saturated atmosphere and
afterwards drying over phosphorus pentoxide. The
hygroscopicity of the soil, or its hygroscopic coefficient,
is thus the basis for colloidal comparison. It is now
clear why the term colloidal estimation is employed in
this discussion, rather than colloidal determination.
An objection to the Mitscherlich method is advanced
by Ehrenberg and Pick, 2 who claim that the drying over
1 Rodewald, II., und Mitscherlich, A. E. Die Bestimmung
der Hygroskopizitat. Landw. Ver. Stat., Band 59, Seite
433-441. 1903. Also, Mitscherlich, E. A., und Floess, R.
Ein Beitrage zur Bestimmung der Hygroskopizitat und zur Bewer-
tung der physikolischen Bodenanalyse. Internat. Mitt. f. Boden-
kunde, Band I, Heft 5, Seite 463-480. 1912.
2 Ehrenberg, P., und Pick, II. Beitrage zur physikalischen
Bodenuntersuchung. Zeit f. Forst- und Jagdwesen, Band
43, Seite 35-47. 1911. Also, Vageler, P. Die Rodewald-
Mitscherlichsche Theorie der Hygroskopizitat vom Standpunkte
der Colloidchemie und ihr Wert zur Beurteilung der Boden.
Fuhling's Landw. Zeit., Band 61, Heft 3, Seite 73-83. 1912.
THE COLLOIDAL MATTER OF bOILS 169
phosphorus pentoxide will coagulate certain colloids and
lower their adsorptive power, thus causing the hygro-
scopic coefficient to become an unreliable comparative
figure. They suggest first the exposure of the field
soil over the water and sulfuric acid, and then the ex-
traction of the hygroscopic water over phosphorus pent-
oxide. Since this modification is very slow, the original
Mitscherlich method, in spite of its faults, remains the
most valuable up to the present time.
CHAPTER X
SOIL STRUCTURE
WHILE texture is the term used in reference to the size
of the particles in a soil mass, the word structure is em-
ployed in reference to the arrangement of the grains.
The structural condition of the soil is very important to
plant growth, since the circulation of air and water are
so necessary to normal development. The structural
condition may be loose or compact, hard or friable, granu-
lated or non-granulated, as the case may be. Of these
conditions, granulation, especially in heavy soils, is of
vital importance, since it is really a summation of all
favorable structural conditions. By granulation is meant
the drawing together of the small particles around a
suitable nucleus, so that a crumb structure is produced.
The grains thus cease to function singly. The impor-
tance of such a structural condition on a heavy soil is
very obvious. The soil becomes loose because of the
larger units, air moves more freely, and water not only
drains away readily when in excess, but responds with
celerity to the capillary pull of the plant. Before the
promotion of granulation and the factors that function
therein may be clearly discussed, however, two properties
of particular importance, especially in soils of fine tex-
ture, must be considered. These properties are plasticity
and cohesion.
112. Plasticity. Any material which allows a change
of form without rupture, and which will retain this form
170
SOIL STRUCTURE 171
not only when the pressure is removed, but also when dry,
is said to be plastic. Putty with a proper admixture of
oil is a very good example of a plastic body. As is well
known, the various plastic materials differ in their plastic-
ity. Not only this, but such substances as clay or some
other soils vary in plasticity with their moisture content,
their granulation, and their texture. The great diffi-
culty in the study of plasticity has been in finding a
means of estimation allowing an exact numerical expres-
sion. The amount of hygroscopic water that a soil will
hold has been used as an expression of plastic qualities,
as well as shrinkage on drying, the ability to adsorb dyes,
tensity, and other characteristics. None of these has
proved satisfactory, since one quality of a clay or other
soil is used as a measure of another quality.
Atterberg 1 has suggested that the difference in mois-
ture content of a clay at the point at which it ceases to
be plastic, as compared with the moisture content at
which it becomes viscous, might be used as an expres-
sion of plasticity. He has called this figure the plasticity
coefficient. Thus, a soil may cease to be plastic at 20
per cent of moisture and may flow at 40 per cent. The
plasticity coefficient would then be 20. While this is one
of the latest methods, it is open to the objection already
stated that one quality of a soil is used as a measure
of another. Two soils showing the same plasticity coeffi-
cient by this method may exhibit undoubted differences
in actual plasticity. Kinnison, 2 in testing several methods
of expression, found Atterberg's no better than others
1 Atterberg, A. Die Plastizitat der Ton. Internal.
Mitt. f. Bodenkunde, Band I, Heft 1, Seite 10-43. 1911.
2 Kinnison, C. S. A Study of the Atterberg Plasticity
Method. Trans. Amer. Cer. Soc., Vol. 1G, pp. 472-484. 1914.
172 SOILS: PROPERTIES AND MANAGEMENT
already in use. For all practical purposes in soil dis-
cussions, general descriptive terms may be employed.
113. The cause of plasticity. Exactly what may be
the cause of plasticity has long been under discussion.
The various theories advanced may be grouped under the
following heads : 1
A. Structure of clay particles
1. Fineness of grains
2. Plate structure
3. Interlocking particles
4. Sponge structure
B. Presence of hydrous aluminium silicates
C. Molecular attraction between particles
D. Presence of colloidal matter
Of these theories accounting for the plasticity of cer-
tain bodies, that of colloid content seems the most rea-
sonable. 2 The presence of gelatinous colloidal matter,
with a certain optimum amount of water, seems to facili-
tate the ready movement of the particles while at the
same time exerting sufficient force to prevent the body
from splitting apart at the time of movement, or when
the pressure is removed or the material dried. Thus,
in general, other conditions remaining equal, materials"
become more plastic the greater the content of colloidal
matter. In general the colloids function as a measure
of plasticity. The consideration of shrinkage, hygro-
1 Davis, N. B., The Plasticity of Clay. Trans. Amer.
Cer. Soc., Vol. 16, pp. 65-79. 1914.
2 Cushman, A. S. The Colloid Theory of Plasticity.
Trans. Amer. Cer. Soc., Vol. 6, pp. 65-78. 1904. Also, Ashley,
H. E. The Colloid Matter of Clay and Its Measurement.
U. S. Geol. Survey, Bui. 388. 1909.
SOIL STRUCTURE 173
scopic water, and dye adsorption as an expression of
plasticity become logical on this basis.
114. The importance of plasticity. Plasticity assumes
considerable importance in a soil when it becomes highly
developed, since it promotes ease in puddling. The
more plastic a soil is, the more likely it is to become pud-
dled by tillage, especially if it has a high moisture con-
tent. Thus a clay cannot be plowed wet, since this
would allow its particles to be worked into that very intri-
cate condition so detrimental to plant growth. A sand,
on the contrary, may be stirred even when saturated,
and still its structural condition will not be impaired
since its plasticity is low or nihil. A very plastic soil
is also likely to become exceedingly hard when dry unless
it is well granulated, which shows the great care demanded
by soils having high plasticity coefficients.
The three factors that affect plasticity to the greatest
extent are texture, granulation, and moisture. In gen-
eral, the finer the texture of the soil, the higher is the
maximum plasticity thereof. The more granular a soil,
the lower is the plasticity or the tendency to puddle
when plowed. The amount of water is the third vital
factor. A soil will exhibit its maximum plasticity at a
definite moisture content. This point will lie somewhere
between the flowing, or viscous, condition and the point
at which a soil refuses to mold, or, in other words, to be-
come crumbly. With a soil such as a clay, in which the
plasticity is high, plowing should be done when the
moisture condition is such that there is no likelihood of
puddling, and yet the soil will turn over with a maximum
granulating effect.
115. Cohesion. Very closely correlated with plas-
ticity, but not in exact similarity, is cohesion. By the
174 SOILS: PROPERTIES AND MANAGEMENT
cohesion of a soil is meant the tendency that its particles
exhibit in sticking together and in conserving the mass
intact. In general, the greater the plasticity of a soil,
the higher is its cohesion, especially when it is dry or
only slightly moist. For that reason, cohesion might
be made a rough measure of plasticity. Cohesion of a
soil occurs under two general conditions, the wet and the
dry. When a soil is moist its cohesion is developed by the
moisture films and the colloidal materials that may be
present. This form of cohesion is often spoken of as
tenacity. When a soil is dry its cohesion is developed to
some extent by the interlocking of its grains and the dep-
osition of cementing salts. The greatest force is devel-
oped, however, by the drying and shrinking of the
gelatinous colloidal matter. As a general rule, the greater
the amount of colloidal material, the more firmly the_soii.
is bouiid.lo^ether_when dry, or, in other words, the greater
is its cohesion.
Cohesion is important in tillage operations, in that
soils having a high coefficient of cohesion tend to become
cloddy when plowed and may thus be rendered poor in
physical condition. This may be avoided by timing the
operation so that the moisture content is somewhere
above the point at which excessive cohesion is exerted.
As cohesion is not greatly developed, except in a heavy
soil, it is only where fine texture is found that such a
danger exists. As already shown, the danger is a double
one, for, since high plasticity and high cohesion go to-
gether, a soil plowed too wet may puddle while one
plowed too dry may clod.
116. Methods of determining cohesion. A number
of methods have been devised for determining the co-
hesion of clays and other soils. One of the earliest was
SOIL STRUCTURE 175
Schiibler's, 1 in which was tested the resistance of rec-
tangular prisms of dry soils to penetration by a steel
blade. The apparatus consisted of a beam supported
on a fulcrum more than one third of the distance
from the end. A pan for holding the weights added
for causing the crushing was hung at the end of the
long arm, while a counterpoise on the short end of the
beam acted as a balance. The steel knife was placed
on the long end of the arm near the fulcrum. The dry
soil prism was placed under the knife and weights were
added to the pan until crushing occurred. The weight
necessary was designated as the cohesion coefficient of
that soil.
Haberlandt 2 measured cohesion by crushing soil
cylinders of a definite size. A glass vessel was placed
on the top of the column and water was added until the
column gave way. The weight necessary to bring this
about was designated as the absolute cohesion of the
sample. Haberlandt also measured cohesion of soil
cylinders of a definite size by determining the resistance
to breaking under a transverse load, the soil column being
placed across supports six centimeters apart. On the
column midway between the ends a scale pan was sup-
ported, into which weights were put until breaking oc-
curred. The figure thus obtained was called the relative
cohesion.
1 A good description of Schiibler's apparatus is found on page
104 of Bodenkimdo, by E. A. Mitscherlich, published by Paul
Parey, Berlin, in 1905.
2 Haberlandt, H. Ueber die Koharescenz Verhaltnisse
verschiedener Bodenarten. Forsch. a. d. Gebiete d. Agri.-
Physik., Band I, Seite 148-157. 1878. Also, Wissenschaftlich
praktische Untersuchungen auf dem Gebiete des Pflanzenbaues,
Band I, Seite 22. 1875.
176 SOILS: PROPERTIES AND MANAGEMENT
Puchner 1 used a penetration apparatus, consisting
of a vertical shaft held by metal guides and counterpoised
by a weight hung over a pulley. The shaft was armed
at the lower end with a cutting blade, while the upper
end carried a scale pan for holding the necessary weights
for penetration. The cohesion coefficient was the weight
necessary to force the blade a certain distance into the
soil. All important apparatus have been modeled after
either Puchner's or Schiibler's. That of Atterberg 2 (see
Fig. 24) follows the former, while that of the Bureau of
Soils 3 (see Fig. 25) resembles the latter.
FIG. 24. Atterberg' s apparatus for determining the cohesion of soil
prisms. The soil prism is placed between the cutting edge (K) and
the sharp plunger (P). Weights are added at (TF). (<J) is a coun-
terpoise.
1 Puchner, H. Untersuchungen iiber die Kohareszenz der
Bodenarten. Forsch. a. d. Gebiete d. Agri.-Physik., Band 12,
Seite 195-241. 1889.
2 Atterberg, A. Die Konsistenz und die Bindigkeit der Boden.
Internal. Mitt. f. Bodenkunde, Band II, Heft 2-3, Seite 149-
189. 1912.
* Cameron, F. K., and Gallagher, F. E. Moisture Content
and Physical Condition of Soils. U. S. D. A., Bureau of Soils.
Bui. 50. 1908.
SOIL STRUCTURE
177
Recently Puchner 1 has found a machine for measuring
the crushing strength of dry soil cylinders to be of value
in determining the absolute, or maximum, cohesion of
soils. The results, as he has already demonstrated in
his previous work, are comparable, in relative value at
least, to those obtained by simpler methods.
The great difficulty encountered in measuring the
natural cohesion of a soil, either wet or dry, is not so
FIG. 25. The apparatus used by the United States Bureau of Soils
for determining the penetration of soils. (C) is mechanically packed
soil ; (A), steel point; (P), pail for receiving sand from funnel (F).
(W) is a counterpoise.
much in the accuracy of the determination as in controlling
physical conditions. The cohesion of a soil depends
very much on the handling it has received in the prepara-
tion, in the amount of water that is added, and in the
amount and length of time of drying. Natural granu-
lation cannot be secured. The Bureau of Soils attempted
to obviate these difficulties by mechanical sifting and
packing, but the results were abnormal, due to the sift-
ing process. As a consequence, most cohesion results
1 Puchner, H. Vergleichende Untersuchungen liber die
Kohiireszenz verschiedener Bodenarten. Internal. Mitt, fur
Bodenkunde, Band III, Heft 2-3, Seite 141-239. 1913.
178 SOILS: PROPERTIES AND MANAGEMENT
have been determined either on samples that have been
worked to a maximum plasticity and then brought to
the required moisture content, or on uniformly compacted
samples that have been allowed to take up water by capil-
larity. In general the curve that would occur with normal
granulation, while lower, will follow the direction of a
maximum plasticity curve.
117. Factors affecting cohesion. It is obvious, from
what has already been said regarding the general char-
acteristics of soils, that texture must play an important
role in the determination of the cohesion factor. In
general, the finer the texture, the greater is the cohesion,
since, whether the soil is wet or dry, the forces that tend
to hold the particles together are stronger than in a coarser
soil. The drier the soil, however, the greater is the tex-
tural influence in this regard, due to the very great in-
crease in the binding capacity of the colloidal matter
on drying. In a coarse soil this binding effect is small or
entirely absent.
Another factor is the granular condition of the samples.
In general granulation may be said to be due to an exer-
tion of cohesion between a limited number of particles,
resulting in a crumb, or granular, structure. This granu-
lation, by loosening the soil mass, lowers not only plastic-
ity but cohesion also. The__addition of organic matter
to a jjoil, by hastening and increasing granulation, will
tend to lower cohesion at every moisture content ranging
]Tom^a__dry to a saturated condition. The following
data, taken from Puchner, 1 bring out the points just
discussed :
1 Puchner, H. Untersuchungen liber die Kohareszenz der
Bodenarten. Forsch. a. d. Gebiete d. Agri.-Physik., Band 12,
Seite 195-241. 1889.
SOIL STRUCTURE
179
EFFECTS OF TEXTURE, GRANULATION, HUMUS, AND MOISTURE
ON COHESION OF SOILS. (PUCHNER.)
SOIL
PENETRATION IN GRAMS AT VARIOUS
MOISTURE CONTENTS
100
per
cent
80
per
cent
60
per
cent
40
per cent
20
per cent
per cent
('lav
114
30
85
167
44
59
115
35
58
2,404
437
1,887
2,937
754
1,010
1,414
272
85
9,537
6,304
3,803
4,237
4,704
1,704
1,904
775
115
11,870
12,370
10,003
5,137
7,204
4,204.
1,804
1,408
492
15,037
13,204
13,703
8,370
9,537
5,070
870
8,125
808
20,037
15,704
6,057
2,370
12,037
1,637
487
12,358
1,342
1* day + 1 quartz
1 chiy + 2 quartz
Oiiiirtz
2 clay + 1 humus
1 clay + 2 humus
Humus
Pulverized loam . .
Granulated loam
(granules .5-9 mm.
diam.) ....
Z0OOO
7
\
FIG. 2G.
The effects of texture, humus, and moisture on the cohesion
of soils.
180 SOILS: PROPERTIES AND MANAGEMENT
The relationships already spoken of are especially well
shown by the curves (see Fig. 26), particularly the effect
of the moisture content on cohesion. In a heavy soil the
cohesion increases steadily from a saturated condition until
dryness is reached, the increase becoming accelerated as
the percentage of moisture decreases. This is because
30\
FIG. 27. The cohesion curves of clay and fine sandy loam at various
moisture contents. (C), point at which soil changes in color.
SOIL STRUCTURE 181
the binding power of the colloidal material is greatly
augmented by desiccation. In a coarse soil such as
quartz, in which there is very little colloidal matter, the
en! lesion is developed principally by the water film. As
this thins, its pulling power increases and the curve
-lids; but when the soil dries this film disrupts and
the curve drops again, having no colloidal binding ma-
terial. The same relationships are shown by curves (see
Fig. 27) adopted from recent determinations by Atter-
berg. 1
118. Moisture limits for successful tillage. In heavy
soils, in which the colloidal content is usually high, plas-
ticity and cohesion also are high. This means that the
soil when too moist will be puddled by tillage implements,
especially such as the plow, and when too dry clodding
will occur because of very high cohesion. A moisture
limit must therefore exist on a heavy soil, within which
successful plowing may be done and maximum granu-
lation results may be secured. That this moisture limit
is narrow is obvious, since high cohesion and high plas-
ticity bound it so closely on either hand. Such a rela-
tionship must be kept in mind not only by the farmer
but by the technical man as well, since so much depends
in any work upon good soil tilth. The relationship is
clearly shown by the following curves (Fig. 28) partially
adopted from Atterberg. 1 The cohesion and plasticity
curves are seen to cross near the center of the diagram
and indicate the existence of a zone where neither is
exceedingly high or low.
In a clay soil a study of the optimum moisture condi-
1 Atterberg, A. Die Konsistenz Kurven der Mineralboden.
Internat. Mitt. f. Bodenkunde, Band IV, Heft 4-5, Seite 418-
431. 1914.
182 SOILS: PROPERTIES AND MANAGEMENT
PLASTICITY
\
\
\
FIG. 28. Diagram showing the moisture limits for successful plowing
in soils of different class, cc and hh' represent the moisture limits
within which successful plowing may be accomplished on a clay and
humus clay respectively.
tions is necessary in order to determine what is just the
right moisture content for good plowing. That this
condition must be carefully gauged and immediate use
made of the advantages it offers is shown by its narrow
limit- A few days may suffice for the moisture to drop
through such a narrow area of fluctuation. A clay soil is
so difficult to handle at best that no opportunities such as
are offered by optimum moisture conditions should be lost.
Moreover, a heavy soil plowed too dry or too wet does not
regain its normal granular condition for several seasons.
In a sandy soil no such difficulties are encountered.
SOIL STRUCTURE 183
Since the soil has low cohesion, plowing when it is too
dry will not clod, while, because of low plasticity, little
puddling will occur if tillage is in progress when the soil
is wet. Being always rather loose, such a soil is often
benefited by plowing when it is slightly wet, the parti-
cles being brought into closer contact and excessive per-
colation being stopped while at the same time the water
capacity is raised.
119. Control of cohesion and plasticity. It is evi-
dent not only that cohesion and plasticity control the
successful tillage of the land, especially where the soil
texture is fine, but also that these same factors vary with
the moisture and the granular structure of the soil. It
has been shown that there is a moisture zone in all soils
this being narrower the finer the soil texture at which
neither cohesion nor plasticity is excessive. In this zone
a heavy soil may be successfully plowed, with results
favorable to the structural condition of the soil. Since
the processes of granulation have already been shown to
lower cohesion and plasticity, it is evident that as a crumb
structure is developed the moisture zone for proper plow-
ing will be widened, especially in a heavy soil. This is a
very important point in the handling of clays and clay
loams, since it not only opens a way for the elimination
of the dangers of bad structural relationships, but also
provides for putting the soil in a condition for easier and
more convenient tillage. In a sandy soil, particularly
where humus is the granulating agent, a soil with
more cohesion, more plasticity, and a greater water-
holding power is developed, all of which tend toward a
better medium for plant growth. Methods of develop-
ing this granulation thus become the logical topic for
further discussion.
184 SOILS: PROPERTIES AND MANAGEMENT
120. Soil tilth. The previous data and discussion
have clearly shown the very great importance of a crumb
structure in the working of the soil in the field. Since
good physical condition will reflect itself on crop yield,
it is evident that structure must ultimately be considered
in relation to all plant growth. This relationship is
usually expressed by the term tilth. While structure
refers to the arrangement of the particles in general, and
granulation to a particular aggregate condition, tilth
goes one step further and includes the plant. Tilth, then,
refers to the physical condition of the soil as related to
crop growth. It may be poor, medium, good, or excel-
lent, according to circumstances. Good tilth may de-
mand in some soils maximum granulation, in others
only a medium development. Maximum tilth always
implies the presence of water, since the best physical
relationships cannot be developed without optimum
moisture conditions.
From the curves already presented, it is evident that
an optimum moisture condition exists for the proper
tillage of a soil, especially one of a heavy character. Also,
an optimum moisture condition must exist for proper
tilth, and therefore for proper plant development, since
adequate tilth is the best physical condition for crop
growth. Practical experience and theoretical evidence 1
have shown that these two optimum conditions are
identical in nearly all cases, a happy coincidence in the
practical management of a soil. The optimum moisture
condition for plant growth, then, is the proper moisture
condition for effective plowing. The optimum condition
1 Cameron, F. K., and Gallagher, F. B. Moisture Content
and Physical Condition of Soils. U. S. D. A., Bur. Soils, BuL
50, p. 8. 1908.
SOIL STRUCTURE 185
for developing a favorable crumb structure is obviously
the optimum moisture content for the development of
the highest tilth. In fact, it can "be stated with certainty
that the optimum moisture condition for plant growth
is the optimum for all favorable soil activities, whether
physical, chemical, or biological. Granulation, then,
becomes the vital factor in placing the soil in a physical
condition such that the highest tilth, that physical criterion
which every farmer should strive for, may be developed
in any soil. Until proper granulation is reached, no soil
can be expected to yield maximum paying returns.
121. Granulation. While it is possible to list the
factors that bring about granulation in a soil, it is diffi-
cult to state specifically just why this phenomenon takes
place. It has been suggested that much of the granule
formation in the soil is due to the contraction of the
moisture film around the particles when, for any reason,
the moisture content is reduced (see Fig. 29) . It is known
FIG. 29. A puddled and a well-granulated soil.
that the soil particles tend to be drawn together by this re-
duction in the soil moisture, due to the pulling pow r er of the
thinned film. If to this condition is added a material which
tends to exert not only a drawing power on loss of mois-
ture, but also a binding and cementing power when dry, all
186 SOILS: PROPERTIES AND MANAGEMENT
the essentials for successful granulation are present. This
second force is found in the colloidal material present in
considerable quantities iTi heavy soils. These are the same
forces that have already been shown to determine the
cohesion and plasticity of the soil, except that in granu-
lating operations they are localized at numberless foci,
and clodding or puddling is thereby prevented. It is
evident that if cohesion and plasticity forces are to func-
tion for granulation or, in other words, locally in the
soil instead of generally and uniformly as when clodding
or puddling occurs a certain moisture content must
be maintained. From what has already been shown,
it is hardly necessary to restate that this moisture condi-
tion is near the optimum moisture content for plant
growth.
Warington * attributes granulation to unequal expan-
sion and contraction of the soil mass, due to the imbibi-
tion and loss of water. In a soil subject to such a condi-
tion, the cohesive forces being localized, the internal
strains and pressures are unequal and a tendency arises
for the mass to divide along lines of weakness into groups
of particles. The binding capacity of colloidal material,
as well as of salts deposited from the soil solution, tends
to make such a crumb structure more or less permanent.
Tillage operations, development of roots, burrowing
of animals and insects, the presence of humus, and the
formation of frost crystals, may assist in further develop-
ing these lines of weakness in the soil mass, on which the
tension of the moisture films around the soil particles is
brought to bear. The flocculation of soil particles may
also develop lines of cleavage by their aggregation around
1 Warington, R. Physical Properties of Soils, pp. 36-41.
Oxford. 1900.
SOIL STRUCTURE 187
certain centers. This movement of the soil particles is
in every case facilitated by the presence of a moderate
amount of moisture.
122. Forces facilitating granulation. Granulation is
nothing more or less than a condition brought about by
the force exerted by a variable water film and the pull-
ing and binding capacities of colloidal material, operating
at numberless localized foci. It is evident that any influ-
ence or change in the soil which will cause a greater locali-
zation of these operative forces will promote increased
granulation. The addition of materials from extraneous
sources is also a practice that may tend to develop lines of
weakness and thus cause a more intense localization of
the forces at work.
The conditions, additions, and practices tending to
develop or facilitate a granular structure in soils may be
listed under six heads : (1) wetting and drying of the soil,
(2) freezing and thawing, (3) addition of organic matter,
(4) action of plant roots and animals, (5) addition of
lime, and (6) tillage.
123. Wetting and drying. The drying of a soil has
been shown to result in a drawing together of the particles
into aggregates. When this process is repeated again
and again by alternate wetting and drying, the influence
on granulation becomes marked.
In drying, the small particles are moved into the spaces
between the larger ones, thereby reducing the volume
as is shown by the checks produced. These checks that
result from shrinkage are due to the unequal contraction.
There comes a time when the general film around the
whole mass must rupture, and it breaks along the lines
of least resistance. If the soil mass is very uniform, there
will be few breaks and the shrinkage will be mainly around
188 SOTLS: PROPERTIES AND MANAGEMENT
a relatively few centers. This process produces clods,
or " overgrown " granules. If there are numerous lines
of weakness, however, there will be many centers of con-
traction, and consequently a larger number of small
clods, or granules, will be formed. This is the desirable
condition and constitutes good tilth that is, the most
favorable physical condition for plant growth.
Just what may be the effects of wetting and drying on
the colloidal matter of soil is a question. In general,
desiccation tends to flocculate colloids and in many cases
their binding power becomes highly developed thereby.
If such colloids are irreversible, as many in the soil un-
doubtedly are, this binding becomes more or less per-
manent, which explains the tendency for a crumb struc-
ture to persist. Wetting, on the other hand, tends to
develop colloidal matter which will become binding mate-
rial on the next drying. The desiccation and throwing
down of colloids, as well as their generation, thus be-
comes a very important factor in the wetting and drying
as related to granulation.
The following figures 1 represent the relative force
necessary to penetrate puddled clay dried once, as com-
pared with the same puddled soil wet and dried twenty
times. The relative hardness may be taken as a rough
measure of granulation :
Percentage of pene-
tration
1. Puddled clay dried once .... 100.0
2. Puddled clay dried twenty times . 31.4
3. Puddled clay dried twenty times . 30.6
4. Puddled clay dried twenty times . 32.0
1 Fippin, E. O. Some Causes of Soil Granulation. Trans.
Amer. Soc. Agron., Vol. 2, pp. 106-121. 1910.
SOIL STRUCTURE 189
The fact illustrated above has many practical appli-
cations. It should be observed that the change in struc-
ture is not associated with continual wetness, nor is it
identified with a continued dry state. In neither condi-
tion is any force brought to bear on the particles. The
force is exerted only during the drying process and the
wetting process. It is a well-known fact that soils which
are continually wet are usually in bad physical condition.
In the drainage of wet land, it is found that the soil is at
first very refractory; but when good drainage is estab-
lished there is a gradual amelioration of the physical
condition, which is primarily a change in structure. On
the other hand, in a soil continually in a dry state there
is no change in granulation. The improvement of soil
structure, as a result of changes in the moisture content,
is dependent largely on lines of weakness in the soil mass.
Some of these are produced in the process of drying, and
others in the ways already listed.
124. Freezing and thawing. As will be seen in
the consideration of soil moisture, the water is distributed
in the fine pores of the soil. When it freezes it forms long,
needle-like crystals. This crystallizing force is very
great, amounting to about 150 tons when a cubic foot of
water changes to ice. In freezing, the crystals gradually
grow first in the larger spaces. During this process there
is a marked withdrawal of moisture from the smallest
spaces, so that the ice crystals in the large spaces may be
built up. The soil mass is separated by the crystals, and
as the result of even a single hard freeze a wet, puddled soil
is shattered into pieces. The repetition of this process by
subsequent freezing and thawing will further break up the
soil by creating new lines of weakness. The granulating
power of freezing and thawing is shown in the following
190 SOILS: PROPERTIES AND MANAGEMENT
figures, 1 expressed as the relative force necessary to
penetrate a puddled clay treated in various ways :
Percentage
penetration
1. Puddled clay dried once 100.0
2. Puddled clay frozen once and dried once . . 30.3
3. Puddled clay frozen three times and dried once 27.3
4. Puddled clay frozen five times and dried once . 21.8
Freezing probably affects the colloidal material in
the same general way as does drying. This has been
indicated by the work of certain investigators, 2 in which
it was found that lowering the temperature of a soil below
freezing lowered the hygroscopic coefficient.
125. Addition of organic matter. Soils rich in humus
or decomposed organic matter are generally in better
physical condition than soils low in organic content.
The marked effect of the absence of this material in many
long-cultivated soils is well known. For example, in
much of the southern New York hill regions, the soils
are now recognized to have a very different relation to
crop growth from what they had for a few years after
they were cleared. Their color has become lighter, and
with the decay of the humus a decided physical change
has taken place in the soil, which is to some extent cor-
rected by the restoration of the organic content. In
certain prairie soils the effect of humus depletion on struc-
1 Fippin, E. O. Some Causes of Soil Granulation. Trans.
Amer. Soc. Agron., Vol. II, pp. 106-121. 1910.
2 Czermak, W. Ein Beitrag zur Erkenntnis der Veran-
derungen der Sog. physikalischen Bodeneigenshaften durch
Frost, Hitze, und die Beigabe einiger Salze. Landw. Ver.
Stat., Band 76, Heft 1-2, Seite 73-116. 1912. Also, Ehrenberg,
P., und Romberg, G. F. von. Zur Frostwirkung auf den Erd-
boden. Jour. f. Landw., Band 61, Heft 1, Seite 73-86. 1913.
SOIL STRUCTURE 191
ture is even more marked. While the actions of humus
are mam;, as has already been shown, its relationship to
physical condition is always particularly emphasized.
Humus contains much colloidal material and in this
way possesses a certain degree of plasticity. It is, how-
ever, of a very loose structure and the large spaces con-
stitute lines of weakness. Another property of humus is
that it undergoes great change in volume when dried
out a property akin to the fineness of the soil, producing
larger shrinkage cracks. This is noticeable in many
.black clay soils, which check excessively. The great
capacity of humus for moisture permits a wide range in
moisture content, which produces corresponding physical
alteration. This wide swing from one extreme to another
is a potent factor in granulative influences. The color
of the humus affects the color of the soil, and thereby
increases the rate of change from the wet to the dry state
by increased evaporation of moisture. The relative
effects of crude muck, and the ammonia extract from the
same muck, on the cohesion of a puddled clay, as indi-
cated by the force required for a uniform penetration of
a knife-edge, is shown in the following table ; the samples
were dried and rewetted twenty times :
PUDDLED CLAY PLUS MUCK 1
Percentage
of penetration
1. Clay 100
2. Clay plus 5 per cent of muck .... 82
3. Clay plus 15 per cent of muck .... 73
4. Clay plus 25 per cent of muck .... 58
5. Clay plus 50 per cent of muck .... 50
1 Fippin, E. O. Some Causes of Soil Granulation. Trans,
Amer. Soc. A-jron., Vol. 2, pp. 10G-121. 1910.
192 SOILS: PROPEETIES AND MANAGEMENT
PUDDLED CLAY PLUS MUCK EXTRACT l
Percentage
of pene-
tration
1. Clay 100
2. Clay plus 1 per cent of extract 85
3. Clay plus 2 per cent of extract 76
4. Clay plus 4 per cent of extract 69
126. Action of plant roots and animals. The exten-
sion of plant roots changes the soil structure by forcing
the particles apart at each growing root point, and possibly
also by some action yet to be explained. Crops differ
greatly in their effect on soil structure. Grass, millet,
wheat, and other plants with fine roots are more beneficial
to tilth than coarse or tap-rooted plants such as corn,
oats, and beets. Grass affects structure also by protecting
the surface of the ground. It is advisable to establish
a rotation on clay soil, that plowing may be done at fre-
quent intervals and that plants with different root devel-
opments may be given an opportunity to exert their
influences. The organic matter left in the soil by
decaying roots is always in very intimate contact
with the soil grains and has much to do with accelerat-
ing granulation.
Animals also affect soil structure. Earthworms, by
carrying materials to the surface, exert a mixing effect,
while the lines of seepage and zones of weakness developed
through their burrowing proclivities are of no mean im-
portance. Insects, especially ants and other burrowing
creatures, aid in this and in other ways.
Pippin, E. O. Some Causes of Soil Granulation. Trans.
Amer. Soc. Agron., Vol. 2, pp. 106-121. 1910.
-SOIL STRUCTURE 193
127. Addition of lime. One of the important effects
of lime is in its flocculating action. This agglomeration,
as already explained under colloids, is the drawing to-
gether of the finer particles of a soil mass into granules.
When caustic lime is mixed with water containing fine
particles in suspension, there is almost immediately a
change in the arrangement of the particles. They appear
first to draw together in light, fluffy groups, or floc-
cules, which then rapidly settle to the bottom so that
the supernatant liquid is left clear or nearly so. This
phenomenon is termed flocculation, because of the
groups of particles. It is not an action limited to
caustic lime alone, however, but because of the useful-
ness of this compound in other ways, and because of
its very strong action, it is ordinarily used on soils.
This flocculating tendency when caustic lime is added
goes on in the soil as well as with suspensions, although
more slowly. In general the lime serves to satisfy the
adsorptive capacity of the colloidal material, and by
throwing down these colloids develops lines of weakness.
The cohesive power of the soil is thus localized and
granulation must necessarily occur.
The various forms of lime differ in their granulating
capacities, calcium oxide and calcium hydrate being very
active while calcium carbonate is relatively inactive in
this regard. For this reason, if flocculation effects are
desired, the oxide or hydrate combinations are added.
The relative influences of lime on puddled clay as measured
by penetration is shown in the following table ; 1 the soil
was dried once and the untreated soil was used as 100
per cent :
1 Fippin, E. O. Some Causes of Soil Granulation. Trans.
Amer. Soc. Agron., Vol. 2, pp. 106-121. 1910.
o
194 SOILS : PROPERTIES AND MANAGEMENT
Percentage
of pene-
tration
1. Puddled clay 100
2. Clay plus 2 per cent CaO 56
3. Clay plus 4 per cent CaO ...... 43
4. Clay plus 6 per cent CaO 33
5. Clay plus 5 per cent CaCO 3 98
6. Clay plus 10 per cent CaCO 3 Ill
7. Clay plus 25 per cent CaCO 3 95
In the soil the oxide and hydrate revert to the carbonate,
but before this change occurs the flocculating effects have
been exerted and the lines of weakness so essential to
granulative processes have been developed. Lime really
does not produce granulation in a normal soil through
its own action alone, but is aided by the other influences
already discussed.
Warington l reports a statement of an English farmer
to the effect that by the use of large quantities of lime on
heavy clay soil he was enabled to plow with two horses
instead of three. It is generally true that soils rich in
lime are well granulated, and maintain a much better
physical condition than soils of the same texture that
are poor in lime.
128. Tillage. The effect of tillage on soil structure
is to produce lines of cleavage, and these, when produced
by plowing, are multitudinous and fairly uniformly dis-
tributed. Plowing, when the moisture content is suit-
able, tends to break the soil into thin layers, which move
one over the other like the leaves of a book when the pages
1 Warington, R. Physical Properties of Soil, p. 33.
Oxford. 1900.
SOIL STRUCTURE 195
are bent. This disturbance of the existing arrangement
of particles puts in motion the two forces that have al-
ready been discussed, the pull of the water film and the
binding power of the colloidal matter. The strength
of cohesion between small particles, such as clay, can be
reali/ed when one considers the tenacity with which these
particles are held together in dried puddled soil. This
cohesive attraction is inversely proportional to the square
of the distance between the centers of the attracting bodies.
Particles that can be brought as closely together as can
clay particles may be thus held with great firmness.
The effect of tillage when an excess of water is present
is to force the particles into large masses and bring about
a generalized exertion of the forces of plasticity. The
soil then becomes puddled. Tillage when the soil is
too dry results either in clodding or in the soil's becom-
ing so pulverized that it becomes puddled on wetting.
As already emphasized, proper pulverization by tillage,
especially by plowing, may occur only when the soil is
in optimum moisture condition.
129. The action of the plow. The plow brings about its
effects because of the differential stresses set up in the fur-
r ow slice as it passes over the share and the moldboard.
The soil in immediate contact with the plow surface is re-
tarded by friction, and the layers above tend to slide over
one another much as the leaves of a book when they are
bent. If the soil is in just the right condition, maximum
granulation results ; but if the moisture is too high or too
low, puddling or clodding may follow, especially on a
heavy soil. Not only does a shearing occur, but this
shearing is differential, due to the slope of the share and
especially to the curve of the moldboard. Where the
soil is to be turned over with the least expenditure of
196 SOILS: PROPERTIES AND MANAGEMENT
force, the share is sloping and is set to deliver a slanting
cut, and the moldboard is long and gently inclined. This
allows the furrow slice to be turned with little granulation
and a minimum expenditure of energy. When maximum
granulation and pulverization are desired, the moldboard
is short and sharply turned, and the share is less sloping
and the cutting edge is less slanting. Such conditions
make for the development of more friction and the genera-
tion of those internal twisting and shearing stresses neces-
sary for good granulation. The sharper the bending of
the furrow slice, the greater are the internal stresses set
up. While the plow is the very best pulverizing agent
when optimum soil moisture conditions prevail, it is also
a most effective puddling agent when the soil is wet.
Therefore care in the judging of optimum conditions for
plowing is a most important feature in the maintenance
and encouragement of soil granulation and tilth.
130. Resume. The factors controlling the struc-
tural condition of any soil are found to be plasticity and
cohesion. As these increase, the tendencies of a soil to
puddle when wet and to clod when dry are augmented.
Therefore, in heavy soils a decrease in these factors is
advisable, through a careful control of moisture and a
bettering of the granular structure of the soil. Granu-
lation, while due to some extent to the localized influence
of the water film, is traceable largely to the colloidal
matter which acts as a binding agent. It is really a
concentration of the forces of cohesion and plasticity
around numberless localized foci. Granulation takes
place under the influence of wetting and drying, freezing,
plants and animals, addition of humus and lime, and
tillage operations, especially plowing. Due to the high
cohesion and plasticity of heavy soils, the moisture zone
SOIL STRUCTURE 197
for successful plowing is relatively narrow. The ability
to detect when this zone has been reached in a clay soil
is one of the essentials of successful soil management.
Another essential is the effective widening of such a
zone by granulation operations. The optimum mois-
ture condition for tillage is also the optimum condition
for plant growth a happy coincident, since by regu-
lating the moisture content for plant development condi-
tions are rendered most favorable for all soil activities.
It is thus possible to realize that criterion in all soil physi-
cal operations, a maximum tilth.
CHAPTER XI
THE FORMS OF SOIL WATER AND THEIR
MOVEMENT
UNDER all normal conditions the soil bears a certain
amount of moisture, which must be reckoned with in
any study whether of a practical or of a theoretical na-
ture. Moreover, the amount of water varies in its char-
acteristics according to its position. It also has move-
ment, which goes far in determining its usefulness to plants.
Before a discussion of the different forms of water, their
movement, and their availability to plants, may be entered
into, however, some way of quantitatively stating the
amounts present must be determined upon.
131. Methods of expressing soil moisture. During
the many years of soil investigation, especially where the
problems had to deal either directly or indirectly with
moisture, five methods of water expression have been
evolved, their use depending on the nature of the work
and on the points to be expressed. These may be listed
under two general heads :
A. Percentage expression
1. Percentage on a wet basis
2. Percentage on a dry basis
B. Volume expression
1. Cubic inches to the cubic foot of soil
2. Percentage by volume
3. Surface inches
198
THE FORMS OF SOIL WATER 199
The simplest way of explaining the application of these
methods for the expression of the amount of water in a
soil is by a specific case. Suppose a certain soil in field
condition weighs 100 pounds to a cubic foot and carries
10 pounds of water. Obviously it would contain 10
per cent of water by the wet method of calculation, or
11.1 per cent of water, using the absolutely dry soil as
a basis. A pound of water contains 27.6 cubic inches;
therefore the amount of water carried by this soil expressed
by volume would be 276 cubic inches for every cubic
foot of soil. The percentage by volume would equal
(276 -f- 1 728) X 100, or about 16 per cent. An inch .of water
covering the top of a cubic foot weighs 5.2 pounds. Ob-
viously the number of surface inches which this 10 pounds
of water would occupy if placed on the top of the cubic
foot of soil would be 10 -r- 5.2 or 1.92 surface inches.
The first method of moisture expression, as percentage
on a wet basis, is open to two serious objections. In the
first place, two different percentages of water in different
samples of the same soil do not represent the same degrees
of wetness as are expressed by the percentages. For
example, 100 grams of wet soil containing 5 per cent of
water would consist of 5 grams of water and 95 grams of
soil, a ratio of 1 to 19. If the soil contained instead 25
per cent of water, the ratio would be 1-3 instead of 1-3.8,
as the ratio of the percentages would naturally lead one
to expect. The second objection is just as serious and
arises from the fact that soils have different apparent
weights. For example, 5 per cent of water on the wet
basis for a clay weighing when dry 70 pounds to the cubic
foot would equal 3.68 pounds, while 5 per cent on a sand
weighing 100 pounds would give 5.26 pounds of the same
volume. The error of such a method of expression is
200 SOILS: PROPERTIES AND MANAGEMENT
obvious, not only in comparing the water content of the
same soil, but in comparing different soils as well.
In using a percentage of moisture based on the dry
soil instead of on the wet, the first of the above objections
is eliminated. Consequently this method of expression
is perfectly legitimate as long as soils having about the
same apparent specific gravity are compared. As soon
as soils of different weights are considered, however, a
more nearly accurate method must be employed. Ob-
viously, then, the only really rational mode of moisture
statement is by the volume method. In ordinary calcu-
lations of water, however, the percentage by dry weight
is generally used because of its simplicity and the facility
of expression that it affords. It is also much easier to
establish than a percentage based on volume.
The first and second methods of volume expression are
of about equal value as far as direct comparison goes. For
the actual water present the number of cubic inches to a
cubic foot of soil is perhaps preferable, as it shows the exact
amount of water contained and may easily be converted
to pounds to a cubic foot or tons to an acre as the case
may be. The third volume statement is generally used in
field practice, especially in irrigated regions, where water
is measured in inches in depth to an acre of area. Such
a statement of the available water in a soil not only is
convenient, but also gives a direct comparison with the
probable rainfall of the growing season.
132. Kinds of water in the soil. As has already been
demonstrated, a soil of a definite volume weight has a
definite pore space which may be occupied by air or by
water, or shared by both, as the case may be. Of course,
an ideal soil for plant growth is one in which there is
both air and water, the proportions depending on the
THE FORMS OF SOIL WATER 20l
texture and the structure of the soil and the character
of the crop. Assuming for the time being, however, that
the pore space is entirely filled with water, or, in other
words, that the soil is saturated, three forms of water
are found to be present hygroscopic, capillary, and
free, or gravitational. These forms differ, not in their
composition, but in the position that they occupy in rela-
tion to the soil particles.
The hygroscopic and capillary w r ater are both film
forms ; that is, they surround the soil particle, being held
partly by the attraction of the particle and partly by the
molecular attraction of the liquid for itself. The hygro-
scopic film is very thin, being water of condensation, or
adsorption. When this film is satisfied and moisture is
still present, the capillary water film begins to form. The
line of demarcation between hygroscopic and capillary
water is not sharp. The general difference between the
two forms may be considered as being not only one of
position, but also one of movement, this power being pos-
sessed only by the capillary film. With a change in any
controlling condition, such as temperature, hygroscopic
water may change to capillary, or capillary water to
hygroscopic, as the case may be. As the capillary water
continues to increase and the film becomes thicker and
thicker, a point is at last reached at which gravity over-
comes the surface tension of the liquid and drops of water
form which tend to move downward through the air
spaces, being now subject to movement by the attrac-
tion of gravity. Free, or gravitational, water then also
becomes present in the soil. If water is still added, the
gravitational water continues to increase until the air
is almost entirely displaced and a saturated condition
results. There may be a change of capillary to free water
202 SOILS: PROPERTIES AND MANAGEMENT
or of free water to capillary with a change of structure,
temperature, or pressure, as was seen to be the case be-
tween the hygroscopic and capillary moisture. The forms
of water present in a saturated soil may be conveniently
represented by the following diagram : -
HYGROSCOPICI CAPILLARY | FREE
FIG. 30. Diagram representing the three forms of water that may
be present in a soil.
133. Hygroscopic water. The hygroscopic water
in a soil has been spoken of as the water of condensation,
or adsorption. It is, however, quite distinct from water
condensed on a surface colder than the atmosphere in
which it is placed. All bodies possess the power, to
a greater or less degree, of adsorbing water even when
at the same temperature as the air with which they are
in contact, provided, of course, that the air contains water
vapor. The hygroscopic film may be continuous or only
partly continuous, depending on the condition of the
surface. In fact, the movement of water over surfaces
is often greatly facilitated by an already existing hygro-
scopic film. External conditions being constant, the
amount of hygroscopic water of various materials is
determined by two factors : (1) the characteristics of the
material itself, and (2) the amount of surface it exposes.
It is a well-known fact that various materials differ
in the amount of hygroscopic water they will hold, due
to the attraction of the substances themselves for water.
The differences in the thickness of the film is so slightly
altered, however, by differences in materials, that, other
factors being constant, the hygroscopic water becomes
a function almost entirely of surface. Glass become?
THE FORMS OF SOIL WATER
203
far more hygroscopic when pulverized. Porous bodies
are especially high in hygroscopic water, sometimes
holding as much as 20 to 30 per cent of moisture. The
following data, drawn from Ammon l and von Dobeneck, 2
although no doubt faulty, illustrate the differences in
hygroscopicity of materials commonly found in soils
and make plain the complexity of the question when
applied to soil phases : -
PERCENTAGE OF HYGROSCOPICITY OF DIFFERENT SUBSTANCES
AT 20 C. WHEN EXPOSED FOR ONE DAY TO SATURATED
AIR
AMMON
VON DOBENECK
Humus
15.96
18.04
Ferric oxide
Kaolin
19.76
.47
20.41
3.55
Limestone
Quartz
.29
.07
.32
.17
One of the characteristics peculiar to colloids in partic-
ular is a high adsorptive power for moisture, this giving
them properties not usually possessed by crystalloids.
Gelatinous precipitates of silica, ferric oxide, and alumin-
ium oxide are good examples. Colloidal humus, gela-
tin, and agar are noted for their adsorptive powers. The
water in such cases is not simply adsorbed on the external
1 Ammon, Georg. Untersuchungen iiber das Condensa-
tionsvermogen der Bodenconstituenten fur Gase. Forsch.
a. d. Gebiete d. Agri.-Physik, Band II, Seite 1-46. 1879.
2 Dobeneck, A. F. von. Untersuchungen iiber das Absorp-
tdonsvermogen und die Hygroskopizitat der Bodenkonstitu-
fiiton. Forsch. a. d. Gebiete d. Agri.-Physik, Band XV,
Seite 163-228. 1892.
204 SOILS: PROPERTIES AND MANAGEMENT
expanses, but is distributed over the great internal sur-
face exposure. Such water cannot be expelled by ordi-
nary drying, but the material must be subjected to a high
heat in order to drive off even a part of the water so held.
The question is greatly complicated also by the fact that
some bodies have a chemical affinity for water. This
results in the formation of hydrates and other salts. Such
water cannot be expelled without the breaking-up of the
compounds.
Ordinary soil possesses to an extraordinary degree
the three characteristics already cited : that is, it exposes
a very large amount of free surface ; it tends to generate
continuously large amounts of colloidal material such as
ferric hydrate, aluminium hydrate, silicic acid, and espe-
cially humic materials in a colloidal state ; and it always
has present compounds having an affinity for water.
However, since these compounds are easily satisfied, and
also since the adsorptive power of colloids is due to the
surface exposed, it may be considered that, other condi-
tions being equal, the hygroscopicity of the soil is essen-
tially a surface phenomenon. Although for all practical
purposes hygroscopicity may be considered as having
special relation to surface, exact correlation is not easy
partly because of the difficulty of accurately determining
the surface exposed by a normal soil.
134. Effect of texture and humus on hygroscopicity. -
The question being thus reduced to a surface consideration,
it is evident that the texture of the soil, external factors
being under control, is the determining factor. The fol-
lowing figures from Loughridge, 1 by whom the hygroscopic
1 Loughridge, R. H. Investigations in Soil Physics.
California Agri. Exp. Sta., Kept, of Work of the Agri. Exp.
Stations of California for 1892-3-4, pp. 76-77.
THE FORMS OF SOIL WATER
205
moisture was determined by exposing the air-dry soil at
15 C. to a saturated atmosphere and then drying at 100
C., illustrate this point :
HYGROSCOPIC CAPACITY OF VARIOUS SOILS
SOILS
PER CENT CLAY
MATERIAL REMAIN-
ING IN SUSPENSION
AFTER STANDING
FOR 24 HOURS
HYGROSCOPIC
WATER EXPRESSED
IN PERCENTAGE
15 clays
31 97
1045
7 clay loams
17.15
6.06
9 loams
4 sandy loams
4 sands
12.06
7.39
2.93
5.18
2.50
2.21
Apparently, the finer the soil, the greater is the hygro-
scopicity. The finer the soil, the higher also is the per-
centage of clay, and consequently the greater is the amount
of material likely to be present in a colloidal state. As a
matter of fact, the hygroscopic moisture as shown above
is roughly proportional to the clay; and as clay, espe-
cially the finer forms, is largely colloidal in nature, the
colloidal content of a soil practically determines the hygro-
scopic content. This fact is the basis for Mitscherlich's 1
method of colloid estimation, in which hygroscopic mois-
ture determined under certain controlled conditions is
used as a relative measure of colloidal content. The vari-
ous grades of particles constituting the textural make-up
of a soil, then, do not possess the same weight in the deter-
mination of hygroscopicity, the dominant grade being
clay, especially that part which has, by either physical
1 Mitscherlich, E. A. This text, paragraph 111.
206 SOILS: PROPERTIES AND MANAGEMENT
or chemical means or both, been thrown into a colloidal
condition. Especially do the humous colloids, as has
already been shown, function in this regard, so that the
organic matter must be of very great importance in deter-
mining the hygroscopic capacity of any soil. The finer
the soil, the greater is the amount of hygroscopic water
merely because of the large area of surface exposed.
Also, any practice that will increase the colloidal material
the humous colloids being very susceptible to increase
by proper soil management the higher will be the per-
centage of this hygroscopic moisture. Texture and humus,
then, govern the hygroscopicity of most soils.
135. Nature of the film. The nature of this thin film
which is designated as hygroscopic water has not as yet
been determined. Held so strongly by a molecular force
averaging probably 10,000 atmospheres, generated by
adhesion and cohesion, it is not definitely known whether
the film exists as a liquid or a vapor. Consequently it
cannot be expected to conform to the laws that are gen-
erally found to apply to capillary films. In many cases
the film may not be continuous, and being so very, very
thin, it may even possess a negative surface tension. The
radius of influence of a particle in water has been shown
by Chamberlain 1 to be about 1.5 X 10~ 7 centimeters.
Within this zone the molecules of water are much restricted
in their motions. The thickness of the hygroscopic film on
quartz particles as calculated by Briggs 2 is 2.66 X 10~ 6
centimeters, showing that the outer edge of the hygroscopic
1 Chamberlain, C. W. The Radius of Molecular Attrac-
tion. Physical Review, Vol. 31, pp. 170-182. 1910.
2 Briggs, L. J. On the Adsorption of Water Vapor and of
Certain Salts in Aqueous Solution by Quartz. Jour. Phys.
Chem., Vol. 9, pp. 617-641. 1905.
THE FORMS OF SOIL WATER
207
film, where the water to a large extent loses its movement,
is considerably without this zone of influence. In order
to give some idea of the extreme minuteness of the hygro-
scopic film, it may be said that its thickness is less than
the diameter of the smallest known soil bacterium. In
moving from the surface of a particle outward through
an ordinary water film, passage is first made through the
zone of influence. When the edge of this is reached, an
area is passed through which continues with constantly
increasing capacity for molecular motion until the outer
edge of the hygroscopic film is crossed, where molecular
activity reaches its maximum.
136. Effect of humidity and temperature on hygro-
scopic water. Two external conditions seem to affect
the amounts of hygroscopic water that a soil may hold
under definite conditions humidity and temperature.
As a general rule, the higher the humidity, the higher is
the hygroscopic moisture. The experiments of von Dobe-
neck 1 with quartz and humus illustrate this point :
PERCENTAGE OF HYGROSCOPIC WATER HELD AT VARIOUS HUMID-
ITIES AFTER AN EXPOSURE OF TWENTY-FOUR HOURS AT
20 C.
30
Per cent
50
Per cent
70
Per cent
90
Per cent
100
Per cent
Quartz . . .
Humus . . .
.045
4.055
.053
7.765
.076
10.589
.119
15.676
.175
18.014
The results as to the effects of a rise in temperature
on the hygroscopic film are not so definite. Most in-
1 Dobeneck, A. F. von. Untersuehungen iiber das Absorp-
tionsvermogen und die Hygroskopizitat der Bodenkonstitu-
enten. Forsch. a. d. Gebiete d. Agri.-Physik, Band XV, Seite
163-228. 1892.
208 SOILS: PROPERTIES AND MANAGEMENT
vestigators l find that as the temperature is increased
the hygroscopicity becomes lowered, thus following the
general laws of adsorption. Hilgard, however, obtained
opposite results when the air was saturated, although his
data agreed with previous results when hygroscopicity
was studied in an atmosphere unsatisfied as to its capac-
ity for water vapor. King 2 explains this discrepancy
as being due to the very high vapor pressure generated
by a saturated atmosphere at high temperatures, causing
a more rapid taking-up of water by the soil than was
lost from its surface. The time necessary for a soil to
assume its maximum thickness of adsorbed water is un-
certain. Hilgard 3 used seven hours in his determina-
tions, while Mitscherlich 4 exposed his soil for several
days. A soil continues to increase in weight slowly as
its time of exposure to moist air is increased, so that a
sharp line of demarcation between capillary and hygro-
scopic water is difficult to establish. Capillary water
may even be present in the minute interstices before the
hygroscopic film is elsewhere satisfied. 5
137. Determination of hygroscopicity. The method
of the determination of the maximum hygroscopicity of a
soil, or, in other words, the hygroscopic coefficient, is
simple in outline. The soil, in a thin layer, is exposed
1 Patten, H. E., and Gallagher, F. E. Adsorption of Vapors
and Gases by Soils. U. S. D. A., Bur. Soils, Bui. 51, p. 33.
1908. 1 1?
2 King, F. H. Physics of Agriculture, pp. 179-180.
Published by the author, Madison, Wisconsin, 1910.
. 3 Hilgard, E. W. Soils, pp. 196-201. New York. 1911.
4 Mitscherlich, E. A. Bodenkunde, pp. 56-58. Paul
Parey, Berlin. 1905.
5 Briggs, L. J. The Mechanics of Soil Moisture. U. S.
D. A., Bur. Soils, Bui. 10, p. 12. 1897.
THE FORMS OF SOIL WATER 209
to an atmosphere of definite humidity under conditions
of constant temperature and pressure. Complications
arise from the necessity of using a very thin layer of soil,
from the difficulty of controlling humidity, and from the
tendency of capillary water to form in the soil interstices
before the hygroscopic film is satisfied. The question of
how long the exposure should take place is a very serious
factor, as has already been pointed out. In the drying
of the soil after exposure a vexing condition also is en-
countered, in that as the temperature is raised, the giving-
off of water vapor continues. It is evident, therefore,
that not only must any method be more or less arbitrary,
but that its value can be only comparative. The method
of Mitscherlich, as already described, 1 is probably the
most nearly accurate. He exposes the dry soil under
partial vacuum over 10 per cent sulfuric acid and water.
The partial vacuum is to hasten adsorption, and the acid
to prevent a fully saturated air, thereby cutting down
chances of dew deposition.
138. Heat of condensation. The amount of energy
necessary to expel the hygroscopic film from around a
soil particle is very great, since its only movement is
thermal. As a matter of fact, it is really impossible to
divest the soil grain entirely without causing the loss of
moisture other than that simply adsorbed. As so much
energy is expended in removing this film, it is reasonable
to expect that a certain amount of heat of condensation
when the film is resumed would become apparent. Pat-
ten 2 offers the following quantitative data concerning
this point :
1 Mitscherlich, A. E. This text, paragraph 111.
2 Patten, H. E. Heat Transference in Soils. U. S. D. A.,
Bur. Soils, Bui. 59, p. 34. 1909.
210 SOILS: PROPERTIES AND MANAGEMENT
HEAT EVOLVED BY WETTING SOILS DRIED AT 110 C.
SOIL
CALORIES P
KILO OF DRY
ER
SOIL
Coarse quartz
150
Podunk fine sandy loam
200
Norfolk sand
347
Hagerstown loam
1108
Galveston clay
3785
Muck soil (25 per cent organic matter) . . .
6413
139. Capillary water. It has been shown in the pre-
vious discussion that a large proportion of the hygroscopic
film is beyond the radius of influence of the particle and
is not held so rigidly as is the inner portion. In other
words, in this film a certain amount of molecular move-
ment is possible, this movement depending on the dis-
tance from the particle. As soon, how-
ever, as the boundary of the hygroscopic
film is crossed, a comparatively thick
film of moisture is reached in which
molecular movement, except for the
influence of viscosity, is perfectly free
and unimpeded. These two zones (see
Fig. 31) one in which capillary move-
ment is more or less free, and a com-
paratively thin film in which molecular
movement becomes increasingly slug-
gish as the radius of influence of the
soil grain is approached are there-
fore clearly differentiated. The capillary water differs
from the hygroscopic moisture (l) in that it is largely in
a liquid state and consequently is governed by the ordi-
nary laws of liquids ; (2) in that it evaporates at ordinary
FIG. 31. Diagram
showing the rela-
tionship of the
hygroscopic and
capillary water
films surrounding
a soil particle,
(s), particle;
(t), zone of influ-
ence of particle;
(w), outer edge of
hygroscopic zone ;
(c), capillary film.
THE FORMS OF SOIL WATER 211
temperatures, being held with less tenacity ; and (3) in
that it has the power of movement from place to place
within the film, hence the name capillary water.
140. Surface tension and the force developed thereby.
- The power that tends to hold this capillary water in
place against the force of gravity, a constant, depends
on the surface tension of the liquid. This phenomenon
of surface tension is due to the existence of certain molec-
ular forces acting from within. In a drop of water, for
example, the particles are attracted equally in all direc-
tions and consequently are able to move with perfect
freedom. The molecules on the surface of the drop,
however, are not in such an equilibrium of attraction,
since the pull of the water particles within is greater than
that of the air particles without. The resultant attrac-
tion is therefore inward, and is directed along a line per-
pendicular to the surface at that point. The result is
the development of a more or less ideal membrane, the
effective force of which is not affected by the amount of
the surface, but by the curvature. In a sphere the force
or pressure developed by surface tension is equal to twice
the surface tension divided by the radius. This increase
of the effective force by curvature of film is very impor-
tant as regards soil water, since, as will be shown later, it
governs the movement of capillary water from one particle
to another, the direction of the movement being deter-
mined by a difference in pressure as developed by un-
equal curvatures of film surfaces.
As a result of this force developed by surface tension,
the water film around a soil particle tends to equalize
itself until this pressure is everywhere the same. On
this force depends also the thickness of the capillary film.
Under any given condition this capillary film will con-
212 SOILS: PROPERTIES AND MANAGEMENT
tinue to thicken until the mass of the water is so great
as to allow gravity to come into play and pull enough
water away to again restore the equilibrium. The soil
particle would at this point be maintaining its maximum
thickness of capillary film. It is also quite evident that
as the capillary film is thinned as, for example, by
evaporation the force developed by surface tension
would be increased, due to increased curvature of the
film, and the difficulty of removing the external layers
of the film would naturally become greater.
141. The form of water surfaces between soil particles.
- In the case of a soil, however, the question of the
capillary film becomes more complex, since a great num-
ber of different-sized particles are present in more or less
close contact with one another. This means that under
normal soil conditions the capillary film is continuous
from one particle to another a very different question
to consider from that of a film about a single isolated soil
grain more or less
spherical in shape.
Suppose, for example,
that two particles,
each carrving a capil-
x 1 IG. 62. Diagram snowing the coalescence
and readjustment to the capillary film of
two soil particles when brought in con-
tact. At left is shown the condition be-
fore adjustment with a sharp angle at B ;
at right the films are shown in equi-
librium with a great thickening at B.
lary water film, be
brought into such
contact that the films
coalesce. There are
now two distinct sur-
faces that at A, A' (see Fig. 32), with the curvature
of the original film, and that at B, which is very acute
and which naturally must exert a very great outward
pull. Under the stress of this pull developed by the
surface tension acting in this film of very great curvature,
THE FORMS OF SOIL WATER 213
the water is drawn into the space between the particles,
where it becomes thicker than the capillary film about
the particles. This readjustment continues until the
forces developed by the two films become equal. An
equilibrium is now established. It is evident, then, that
as the capillary water becomes less in a soil from any
cause, the moisture collected in the spaces between the
particles becomes less and less, but still remains thicker
than the films about the particles themselves. What
percentage of the capillary water is held in the thickened
waists of the soil grains cannot be calculated, but it is
probable that this moisture makes up the major part
of the capillary water of any soil. One of the errors
in the determination of the hygroscopic coefficient of a
soil, as already pointed out, arises from the tendency
toward the formation of capillary water in these angles
between the soil particles before the hygroscopic film on
the grains themselves becomes satisfied.
142. Factors affecting amount of capillary water. -
As might naturally be expected, the factors that tend to
vary the amount of capillary water in a soil are several,
and their study is more or less complex, due to the second-
ary influences that they may generate. These factors
may be discussed under four heads: (1) surface tension,
(2) texture, (3) structure, and (4) organic matter.
143. Surface tension and the amount of capillary
water. Any condition that will influence surface ten-
sion will obviously influence the thickness of the capillary
film, because of a variation in the forces thereby de-
veloped. A rise in temperature, by lowering the surface
tension, would consequently lower the capillary capacity
of the soil, and if the soil were capillarily saturated would
allow some of the water to become gravitational in its
214 SOILS: PROPERTIES AND MANAGEMENT
nature. A lowering of the temperature would cause a
change in the opposite direction. This theory has been
verified by certain experiments by King, 1 in which he
found, other conditions being constant, a very decided
influence on capillary water through change of tempera-
ture. Wollny 2 has shown that a depression of from .65
per cent in sand to as high as 3.7 per cent in kaolin may
occur from a rise in temperature of twenty degrees. The
surface tension of a liquid may also be greatly changed
by the addition of salts, and, since the soil always carries
some material in solution, the 'surface tension, and conse-
quently ,the capillary capacity, might be expected to
increase. As a matter of fact, the soil solution is very
dilute, and even if large amounts of fertilizer salts were
added the adsorptive power of the soil would tend to
maintain a very dilute soil water at the surface of the
films. Again, as humus decay is continuously going on,
oily materials are probably produced which would tend
to spread over the capillary films and greatly reduce their
surface tension. Therefore, as far as is now known of the
two varying influences, temperature change is by far the
more potent in its influence on capillary capacity.
144. Texture and the amount of capillary water. -
The finer the texture of a soil, jthe greaterjs the number
of angles between the particles in which a film of capillary
water may be held; also, the actual amount of surface
exposed by the particles is immensely larger than in a
1 King, F. H. Fluctuations in the Level and Rate of Move-
ment of Ground Water. U. S. D. A., Weather Bur., Bui. 5,
pp. 59-61. 1892.
2 Wo liny, E. Untersuchungen uber die Wasserkopacitat der
Bodenarten. Forsch. a. d. Gebiete der Agri.-Physik, Band 9,
Seite 361-378. 1886.
THE FORMS OF SOIL WATER 215
.rse soil. Due to these two conditions, a soil oj^ fine
texture will contain considerably more capillary water
than one of which the texture is coarse. The maximum
capillary capacity of a soil is not directly proportional to
the surface, as was roughly proved to be the case with
the hygroscopic coefficient. This is probably because
the angle exposures between the grains increase in number
as the texture becomes finer much faster than the actual
surfaces developed by the particles are generated. The
capillary water in any soil varies with the height of the
column. This comes about from the gravity effects
on the liquid surrounding the particle. If the liquid had
no weight, gravity would not be a factor and the same
thickness of film would be found at
any point in a soil column. Such a
condition would greatly simplify the
study of soil moisture. If a number of
particles (see Fig. 33) carrying maxi-
mum capillary films are brought together
vertically, the weight of the. whole con-
ducting film is thrown momentarily on
the capillary surfaces at the top. The
capillary spaces at this point immediately
lose water downward, so that they may showing the ad-
ume a greater curvature and thus justment of the
7 capillary film in a
support this extra weight thrown on long column and
them. This curvature must be sufficient * he appearance of
. free or gravita-
to balance the curvature pressure of the tional water if the
particles below plus the weight of the wei ^ h * *? to
. great for the sup-
Water in the connecting films. Ihe par- porting films.
tides beneath are at the same time un-
dergoing a similar adjustment with a set of particles still
farther below, losing water in order to allow a change of
216 SOILS: PROPERTIES AND MANAGEMENT
curvature. A thinning of these films results, but not to
such an extent as in the particles above. The action
continues in this manner through each capillary surface
until equilibrium is established, the change in thickness
of film being less and less in each case due to the cumu-
lative support of the films above. If the amount of
capillary water present is too great to be supported by
the films, enough is lost by gravity at the bottom to
bring about an equilibrium. The film is at its maximum
at the bottom of the column, but decreases in thickness
as the column is ascended, not only on the particles
themselves, but in the angle interstices as well. This is
necessary, as each successive film must support an in-
creased weight of water. It is, therefore, evident that
it is impossible to assign any definite figure as to the
capillary water capacity of a soil. Only relative or
comparative data may be quoted. The following diagram
FIG. 34. Diagram showing the distribution of moisture in capillary
columns of soil of different textures. The end of each soil column
rests in free water.
THE FORMS OF SOIL WATER 217
(see Fig. 31) from Buckingham 1 makes clear not only
the influence of texture on capillary water, but also the
distribution of water in a capillary column.
The final mean water content of these soils was 10,
15, and 20 per cent, respectively, for the fine sand, the
sandy loam, and the clay; showing that as the texture
becomes finer, the greater is the average capillary content
even after allowing for the differences in hygroscopic
moisture.
145. Effect of structure on the amount of capillary
moisture. The structure of the soil, or, in other words,
the arrangement of the particles, will become a factor in
capillary capacity in so far as it affects the amount of
effective capillary surface. Any arrangement of parti-
cles that will increase the number of angles of contact will
evidently increase the amount of capillary water. The
compacting of a loose soil will increase the possible capil-
lary moisture until all the interstitial space becomes
capillary in its nature; further compacting will then
cause a marked decrease. The granulation of a clay soil,
by producing a crumb structure and by actually increas-
ing the effective surface exposure, tends to increase its
water-holding capacity. At the same time the compacting
of a sand, by increasing not only the actual effective sur-
face, but also the number of angles possible for capillary
concentration, will cause a rise in the capillary capacity
of that soil.
In a study of this kind it is very evident' that the aver-
age data of a long column should be considered, since
the percentage of moisture at any one point is not in-
dicative of the true capillary capacity of a soil. Such
1 Buckingham, E. Studies on the Movement of Soil
Moisture. U. S. D. A., Bur. Soils, Bui. 38, p. 32. 1907.
218 SOILS : PROPERTIES AND MANAGEMENT
figures have been obtained by Buckingham 1 in his study of
loose and compact soils. The following curves repre-
sent the general trend of his results :
FIG. 35. Diagram showing the effect of a compaction upon the distri-
bution of moisture in capillary columns. (L), loose sandy loam;
(Z/), compact sandy loam ; (C), compact clay ; (C'), loose clay.
While it is evident that the mean water content of the
compact sandy loam is greater than that of the less com-
pact, the latter showed a higher percentage of moisture
up to about the tenth inch. The clay shows a more
marked effect from compacting, dropping in the compact
sample almost as low as the sand, on the average, and
showing at about ten inches from the end of the column
a percentage of moisture considerably below that of either
the loose or the compact sand. It is obvious that the
farmer may do much in the control of capillary water by
promoting a proper physical condition of his soil.
146. Organic matter and the amount of capillary mois-
ture. Organic matter, especially when it has been
reduced to the form of humus, has great capillary capac-
ity, far excelling in this regard the mineral constituents of
the soil. Its porosity affords an enormous internal sur-
1 Buckingham, E. Studies on the Movement of Soil Mois-
ture. U. S. D. A., Bur. Soils, Bui. 38, pp. 34-35. 1907.
THE FORMS OF SOIL WATER 219
face, while its colloids exert an affinity for moisture which
raises its water capacity to a very high degree. Its ten-
dency to swell on wetting is but a change in condition
incident to an approach to its maximum moisture con-
tent. The following data, taken from a compilation by
Stnrer, 1 give an idea of the capillary capacity of the soil
organic matter :
Percentage
of water
1. Humous extract from peat 1200
2. Non-acid extract from peat 645
3. Vegetable mold 309
4. Peat 190
5. Garden loam, 7 per cent humus .... 96
6. Illinois prairie soil 57
7. Field loam, 3.4 per cent humus .... 52
8. Mountain valley loam, 1.2 per cent humus . 47
Even after allowance has been made for the increased
hygroscopic coefficient incident to an increase in organic
matter, the effect of the latter is very strongly evident
on the capillary capacity of a soil. Besides this direct
effect, organic matter exerts a stimulus toward better
granulation, a condition in itself favorable to increased
water-holding power.
147. Determination of capillary water. The capillary
water in a sample of field soil may be determined by mak-
ing a moisture test in the ordinary way for the total water
contained. This represents the hygroscopic plus the
capillary water. A determination of the hygroscopic
coefficient on another sample yields a figure which when
^torer, F. H. Agriculture, Vol. I, p. 106. New York.
1910.
220 SOILS: PROPERTIES AND MANAGEMENT
subtracted from the total water will give the capillary
water present in the sample. The capillary water at
various points in a soil column may be obtained by sub-
tracting the hygroscopic coefficient from the various
percentages of moisture present, since the thin hygroscopic
film is not influenced by height of column or ordinary
structural conditions. In ordinary soils, however, the
differences in hygroscopicity are not so great but that
the total water retained in a soil column against gravity
serves as a very good measure of relative capillary
capacity.
148. The moisture equivalent of soils. Briggs and
McLane l have perfected a method of comparing soils
on the basis of their capacity to hold water against a
definite and constant centrifugal force of one to three
thousand times the force of gravity. The soils, in thin
layer, are placed in perforated brass cups which fit into
a centrifugal machine capable of developing the above
force, and are whirled until equilibrium is reached. The
resultant moisture percentage is designated as the mois-
ture equivalent. It really represents the capillary
capacity of a soil of minimum column length when
subject to a constant and known force or pull. The
finer the soil, the greater of course is the moisture
equivalent. The authors also found that 1 per cent of
clay or organic matter represented a retentive power of
about .62 per cent, while 1 per cent of silt corresponded
to a retention of .13 per cent. Representative data
which show the correlation of the moisture equivalent
to the textural properties of the various types are given
in the table on the following page.
1 Briggs, L. J., and McLane, J. W. The Moisture Equiv
alents of Soils. U. S. D. A., Bur. Soils, Bui. 45. 1907.
THE FORMS OF KOIL WATER
221
PKH-
CENT-
PER-
PER-
PER-
MOIS-
SOIL
AGE OF
OR-
GANIC
AGE
OP
CENTAGE
OP SILT
CENTAGE
OP
CLAY
TURE
EQUIVA-
LENT
MATTER
1. Norfolk coarse sand .
.9
87.9
7.3
4.8
4.6
2. Norfolk fine sandy loam
1.3
73.4
18.1
8.5
6.8
3. Yazoo loam ....
1.3
25.8
64.1
10.1
18.9
4. Waverly silt laom . .
2.0
14.9
62.9
22.2
24.4
5. Houston clay loam
3.7
30.9
42.5
26.6
32.4
6. Houston clay "...
1.4
10.0
56.6
33.4
38.2
149. The maximum retentive power of a soil. An-
other determination has been devised by Hilgard 1 and
used to considerable extent by other investigators. 2 It
is designated as the maximum retentive power of a soil.
mall perforated brass cup is used, having a diameter
of about 5 centimeters and capable of containing a soil
column 1 centimeter in height. A short column is used,
since it is only under such conditions that a soil may re-
tain against gravity the greatest amount of water. Also,
the soil is able to expand or contract, as the case may be,
on the assumption of water until an equilibrium is reached.
A filter-paper disk is placed in the metal cup, and the soil
is poured in, gently jarred down, and stroked off level
with the top of the cup. The cup is then set in water
and the soil is allowed to take up its maximum moisture.
After draining, the weight of the wet soil plus the cup,
together with the weights previously obtained, will allow
the calculation of the total water contained by the soil.
150. Capillary movement. It has already been shown
how different thicknesses of films on two particles tend
1 Hilgard, E. H. Soils, p. 209. New York. 1911.
2 This text, paragraph 181.
222 SOILS: PROPERTIES AND MANAGEMENT
to become equal, due to the pulling force developed by
the angle of curvature between the particles. It is evi-
dent that differences in curvature must be the motive
force in the capillary movement
of soil water. Let it be supposed,
for convenience, that three equal
spheres when brought in contact
FIG. 36. Diagram show- contain unequal amounts of water
ing the mechanics of the [ n the angles of curvature (see
capillary movement of -p. , T , . ,
water in soil. The read- *ig- 36). In this case the greater
justment takes place in pu n would exist at A. since the
the direction of (A) due , ,
to the high tension devel- angle here is more acute. Conse-
oped by the sharp film quently water must move through
curvature at this point. . , , ., ,,
the connecting film until the pull
at A and that at B become the same. Such an adjust-
ment might go on over a large number of films, and if
one end of the column was exposed to an evaporation
of just the right rate and the other end was in contact
with plenty of moisture, large quantities of water would
be pumped by capillarity.
This capillary movement may go on in any direction in
the soil, since it is largely independent of gravity; yet
under natural field conditions the adjustment tends to
take place very largely in a vertical direction. When
a soil is exposed to evaporation the surface films are
thinned and water moves upward to adjust the ten-
sion. This explains why such large quantities of soil
water may be lost so rapidly from an exposed soil.
Capillary adjustment may go on downward, also, as is
the case after a shower. Here the rapidity of the ad-
justment is aided by the weight and movement of the
water of percolation.
The capillary adjustment in a soil may go on under
THE FORMS OF SOIL WATER 223
two conditions : (1) if the soil column is in contact with
free water; and (2) if no gravity water is present, the
movement being merely from a moist soil to a drier one,
an inexhaustible supply of water not being present. In
the first case the lower portion of the soil is entirely
saturated for a short distance above the free water sur-
face, due to the functioning of the pore spaces as true
capillary tubes; above this the film movement becomes
dominant. The second condition of capillary adjustment
is the one most commonly found in a normal soil, since a
water table a short distance below the surface is not
usually conducive to the best crop growth. In studying
the rate and height of capillary rise in any soil, however,
the maintenance of a supply of free water at the lower end
of the column is usually provided for, since this allows a
near approach to the maximum capillary capacity for any
point in the column.
151. Factors affecting rate and height of capillary
movement. To persons familiar with the habits of grow-
ing plants it is evident that capillary movement must
play an important part in their nutrition, since the root-
lets are unable to bring their absorptive surfaces in con-
tact with all the interstitial spaces where the bulk of the
available water is held. Consequently a consideration
of the movement of capillary moisture is necessary, not
only as to its mechanics, but also as to the factors influ-
encing its rate and height of movement. These factors
are four in number: (1) thickness of water film; (2) sur-
face tension ; (3) texture ; and (4) structure.
152. Effect of thickness of water film on capillary
movement. It has been repeatedly noticed, in the
study of the capillary adjustment between two soils, that
the lower the percentage of water, the slower is the rate
224 SOILS: PROPERTIES AND MANAGEMENT
of movement. This indicates that the thickness of the
film covering the particles and connecting the interstices
containing the bulk of the capillary water is, within
certain limits, a dominant factor in rate of movement at
least. Let it be supposed that a withdrawal of water
occurs at A (see Fig. 37), the interstitial space between
two particles, the water surface being represented by the
dotted line aa ' . There is an immediate increase in the
curvature of this surface, and
water tends to flow through
the capillary film spaces at c
and c ', toward this area of
greater tension. If water con-
tinues to be withdrawn at A,
this adjustment continues
with considerable ease until
the film channel at c and c'
becomes so thin as to cause
its surface (bb ') to approach the edge of the hygroscopic
film surrounding the particle. The viscosity of the
water gradually becomes a factor at this point, imped-
ing the capillary adjustment toward A. This point of
sluggish capillary movement has been designated by
Widtsoe l as the point of lento-capillarity.
The amount of capillary water delivered at any one
point, therefore, will obviously be influenced by the
thickness of the film and may consequently be taken
as a measure of rate of rise. A short soil column
should deliver more water from a constant source
than a longer one, due to the thicker films at the sur-
1 Widtsoe, J. A., and McLaughlin, W. W. The Movement
of Water in Irrigated Soils. Utah Agr. Exp. Sta., Bui. 115,
pp. 223-231. 1912.
FIG. 37. Diagram for the ex-
planation of the effect of
thickness of water film about
soil particles upon ease of
capillary movement.
Till': FOllMS OF SOIL WATER 225
face of the former column. King l shows this by the
following data :
EVAPORATION FROM THE SURFACE OF SAND COLUMNS OF DIF-
FERENT LENGTHS, THEIR BASE BEING IN CONTACT WITH
FREE WATER
LENGTH OF COLUMN IN INCHES
EVAPOEATION AT SURFACE
IN INCHES A DAY
6
.114
12
.111
18
.080
24
.034
30
.019
Briggs and Lapham 2 found, in comparing the evapo-
ration from tubes of different lengths (85 and 165 centi-
meters, respectively) of Sea Island soil, that the shorter
column showed over five times as much evaporation in a
period of forty-two days. This diminished flow with the
thinner films is a vital point in plant production, since
wilting must occur as soon as capillary movement becomes
too sluggish to supply moisture fast enough for normal
development.
The thickness of film is important also in a considera-
tion of the height of rise in dry and moist soil respectively.
It is evident that the rate would be much more rapid in
the latter, but what as to total rise ? Stewart, 3 in study-
1 King, F. H. Principles and Conditions of the Movements
of (J round Water. U. S. Geol. Sur., 19th Ann. Kept., Part II,
p. 92. 1897-1898.
2 Briggs, L. J., and Lapham, M. H. Capillary Studies.
U. S. D. A., Bur. Soils, Bui. 19, pp. 24-25. 1902.
3 Reported by Briggs, L. J., and Lapham, M. H. Capillary
Studies. U. S. D. A,, Bur. Soils, Bui. 19, p. 26. 1902.
Q
226 SOILS: PROPERTIES AND MANAGEMENT
ing the capillary limits as to the height of rise in dry and
moist Michigan soils, found this limit much greater where
the soil was damp. This vertical rise from a water table
was almost three times greater, on the average, in the soil
in which the films were originally thicker. Briggs and
Lapham l found this ratio in Sea Island soil to be as high
as four and one-half ; while Wollny 2 has shown sand with
9.5 per cent of moisture to raise moisture from a water
table one-half higher in six days than did the same sand
dry. It is evident, therefore, that a soil with a thick
capillary film will carry moisture faster than one with a
thinner film, and also will raise the moisture higher when
the final film adjustment has taken place.
In an air-dry soil it is obvious that before capillarity
may take place a thicker film than has already existed
must be established. This is often difficult because of the
presence of oily materials deposited on the surface of the
particles during the process of drying out. Such a condi-
tion probably accounts, at least partially, for the differ-
ence in total rise of capillary water in a dry and in a moist
soil, since, theoretically, if time enough were given for
adjustment, the total height should be the same in both
columns. This resistance of dry soil to the resumption
of a capillary film is made use of in soil mulches, where a
dry surface layer of the soil checks evaporation by imped-
ing capillary rise. It is also obvious that in a study of the
rate and height of capillary movement and the factors
affecting it, moist columns should be used, as this is a
1 Briggs, L. J., and Lapham, M. H. Capillary Studies.
U. S. D. A., Bur. Soils, Bui. 19, p. 26. 1902.
2 Wollny, E. Untersuchungen iiber die Kapillare Leitung
des Wassers im Boden. Forsch. a. d. Gebiete d. Agri.-Physik,
Band 7, Seite 269-308. 1884.
THE FORMS OF SOIL WATER 227
near approach to the conditions of a field soil. Since this
is rather a difficult study to carry out, most of the rate
and height data on capillary movement have been largely
obtained with dry columns in contact with free water at
the bottom. Such data are comparative, but are far
from quantitative as regards the performance of any soil
under normal conditions.
153. Surface tension and capillary movement. As
has already been shown, the thickness of a maximum
capillary film is largely determined by surface tension;
and as surface tension with any given curvature exerts a
definite pressure, it is evident that this pressure may be-
come greater or smaller with variations in the surface
tension. One of the most potent factors having to do
with this variation is temperature. If the temperature
of^a soil column in_ capillary equilibrium and containing
it> maximum capillary moisture should be raised, some
of the water would be lost as free water, since the pulling
powerof the films would be decreased. In the same
way, the capillary capacity would be increased by a lower-
ing of the temperature, which of course would mean a
higher capillary rise in either a dry or a wet soil. The
rate of movement, 1 however, would be facilitated in the
first case, since the viscosity of the water would be much
reduced, allowing the movement in the film channels to
take place with less friction.
King 2 has verified these conclusions in his experiments
1 Wollny, E. Untersuchungen iiber die Kapillare Leitung
des Wassers im Boden. Forsch. a. d. Gebiete d. Agri.-Physik,
Band 8, Seite 206-220. 1885.
2 King, F. H. Fluctuations of the Level and Rate of Flow
of Ground Water. U. S. D. A., Weather Bur., Bui. 5, pp. 59-
61. 1892.
228 SOILS: PROPERTIES AND MANAGEMENT
with the fluctuations of the ground water of a soil held
in a large cylindrical tank. He found that with a lower-
ing of temperature the ground water was lowered, due
to the increased capillary capacity of the soil generated
by a higher surface tension. A consequent upward move-
ment of water took place. When the temperature was
raised, however, there was a reverse movement, due to a
change of capillary water to free water brought about by
a lowered surface tension.
The surface tension may also be varied by materials in
solutions, most salts tending to cause increased tension.
The addition of soluble fertilizer salts to a soil would
therefore be expected to exert some influence. It must
be remembered in this connection that all soils contain a
certain amount of oily substances, produced during the
processes of organic decay. It is probable that the lower-
ing effect of such material would largely overbalance
any marked influence from fertilizer salts. Moreover,
as such salts are strongly adsorbed by the soil particles,
their effect on the concentration of the surface film would
probably be light even if undisturbed by the soil resins.
Wollny l has shown that adsorbed salts produce little
effect on capillarity, while non-adsorbed salts cause a
depression increasing with concentration.
Briggs and Lapham 2 found that with Sea Island soil
dissolved salts in dilute solution had no appreciable effect
except in the case of sodium carbonate. The increased
rise in this case they ascribe to the saponification of the
1 Wollny, E. Untersuchungen iiber die Kapillare Leitung
des Wassers. Forsch. a. d. Gebiete d. Agri.-Physik, Band 7,
Seite 269-308. 1884.
2 Briggs, J. B., and Lapham, M. H. Capillary Studies.
U. S. D. A., Bur. Soils, Bui. 19, pp. 5-18. 1902.
THE FORMS OF SOIL WATER 229
oils on the particles, and a consequent exposure of clean
surfaces for capillary movement. These authors found
also that concentrated solutions reduced the rate of
capillary movement. Davis, 1 in working with a silt loam,
obtained variable results, some salts depressing and some
accelerating capillary rise. Potassium acid phosphate
caused the maximum retardation, while ammonium ni-
trate most markedly increased the rate. Since only one
soil was used and the greatest observed capillary rise was
less than twelve inches, additional data must be presented
before it is clear that the concentration of salts may be-
come a very important factor in humid soils. In alkali
soils, in which the concentration of the salts is very great,
there is no doubt that considerable retardation may occur.
154. Effect of texture on capillary movement. In
soils of fine texture, not only is the amount of film surface
exposed greater than in coarse soils, but the curvature of
the films is also greater, due to the shorter radii. The
effective pressure exerted by the films is consequently
much higher in fine-grained soil. The greater exposure
of surface and the increased pressure both serve to raise
the friction coefficient and retard the rate of flow. The
finer the texture of the soil, other factors being equal, the
slower is the movement of capillary water. Water should
therefore rise less rapidly from a water table through a
column of clay than through a sand or a sandy loam.
The height to which water may be drawn by the effec-
tive capillary power of a soil, equilibrium being estab-
lished, depends on the number of interstitial angles. The
greater the number of angles, the greater is the total
1 Davis, R. O. E. The Effect of Soluble Salts on the Physi-
cal Properties of Soils. U. S. D. A., Bur. Soils, Bui. 82, pp.
23-31. 1911.
230 SOILS: PROPERTIES AND MANAGEMENT
supporting power of the films. As a silt soil contains a
larger number of such angles, its capillary pull is greater
than that of a sand, and consequently the ultimate move-
ment would be of greater scope. The finer the texture,
then, the slower is the rate of capillary movement but the
greater is the distance. 1
The relation of texture to rate and height of capillary
movement in dry soil is shown by the following un-
published data, obtained in the laboratory of the Depart-
ment of Soil Technology, Cornell University :
EFFECT OF TEXTURE ON RATE AND HEIGHT OF CAPILLARY
RISE FROM A WATER TABLE THROUGH DRY SOIL
SOIL
1 HOUR
1 DAY
2 DAYS
3 DAYS
4 DAYS
5 DAYS
Inches
Inches
Inches
Inches
Inches
Inches
Sand . . .
3.5
5.0
5.9
6.8
6.8
6.9
Clay . . .
Silt ....
.5
2.5
5.7
14.5
8.9
20.6
10.9
24.2
12.2
26.2
13.3
27.4
It is seen that the movement in sand is rapid, one-half
of the total rise being attained in one hour. The maxi-
mum height is reached in about three days. The silt in
this case seems to be of just about the right textural con-
dition for a fairly rapid rise, yet it exerts enough capil-
lary pull to attain a good distance above the water table.
The friction in the clay is greater, however, and this
results in a slower rate. Whether the clay would ever be
able to exhibit a rise comparable with its tremendous pull-
1 Wollny, E. Untersuchungen iiber die Kapillare Leitung
des Wassers im Boden. Forsch. a. d. Gebiete d. Agri.-Physik,
Band 7, Seite 269-308. 1884. Also, Forsch. a. d. Gebiete d.
Agri.-Physik, Band 8, Seite 206-220. 1885.
THE FORMS OF SOIL WATER 231
ing capacity is doubtful, because of the resistance offered
by the dry soil.
155. Texture and the capillary pull of soils. An ingen-
ious method for measuring quantitatively the capillary
pull exerted by a moist soil has been devised by Lynde
and Dupre. 1 The apparatus consists of a glass funnel
joined to a thick-walled capillary tube by means of a piece
of rubber tubing, a water seal being used at this point.
The lower end dips into mercury. The soil to be studied
is placed in the funnel, and after being saturated is con-
nected by means of a wick of cheesecloth or filter paper
to the water column previously established in the capil-
lary tube. If no break occurs between the soil and the
capillary water column, the apparatus is ready for use.
The excess water having drained away, there is a
thinning of the films on the soil surface due to evapora-
tion. Equilibrium adjustments now take place, which
result in the drawing upward of the water column. The
mercury follows, and the strength of the pull may be meas-
ured by the height of the mercury column. The old
method of measuring capillary power by the water move-
ment through a dry soil is vitiated by two conditions
-the length of time necessary, and the fact that the
maximum lift cannot be obtained due to excessive fric-
tion. This new method uses a wet soil, requires only
a short time, and gives a more nearly accurate idea of the
power of the capillary pull. It does not, however, yield
data regarding rate of movement, a factor of vital
importance to plant growth, as will be shown later.
Lynde and Dupre, in their results, confirm the state-
1 Lynde, C. J., and Dupre, H. A. On a New Method of
Measuring the Capillary Lift in Soils. Jour. Amer. Soc. Agron.,
Vol. 5, No. 2, pp. 107-116. 1913.
232 SOILS: PROPERTIES AND MANAGEMENT
ments already made regarding the relation of texture
to capillary power : -
THE CAPILLARY LIFT OF SOIL SEPARATES AS DETERMINED BY
LYNDE AND DUPRE
SOIL
DIAMETER OF GRAIN,
IN MILLIMETERS
LIFT OF WATER
COLUMN, IN FEET
Medium sand ....
Fine sand
.5 -.25
25 - 10
.98
1 78
Very fine sand ....
Silt
.10 -.05
05 -005
4.05
9 99
Clay
.005-
26.80
The capillary pull may also be established, at least
comparatively, by the height of the wetted soil and the
amounts of water at various points in a soil column that
has reached a capillary equilibrium when its base is in
contact with a constant supply of water. The curves
from Buckingham l (Fig. 34, p. 216) determined after the
soil had stood for sixty-eight days, illustrate this.
156. Effect of structure on capillary movement. -
Structure has already been shown to affect capillary
capacity by its influence on the angle interstices. Evi-
dently, therefore, it may alter both the rate and the height
of capillary rise. The loosening of a clay soil or the
compacting of a sandy soil will lessen the effective film
friction, while at the same time it will strengthen the
capillary pull resulting in a faster and a higher capillary
flow of water. The exact structural condition of any soil
in which this result is realized to its highest efficiency it
is impossible to judge exactly. In general, however, this
1 Buckingham, E. Studies on the Movement of Soil Mois-
ture. U. S. D. A., Bur. Soils, Bui. 38, p. 32. 1907.
THE FORMS OF SOIL WATER 233
point is approached when the soil is in the best physical
condition for crop growth. Tillage operations in general,
tile drainage, and the addition of lime and organic matter,
operate toward this result by their granulating tend-
encies; while rolling, by compacting a too loose surface,
may accomplish the same effect but by an opposite process.
At certain seasons of the year capillarity must be
impeded near the surface, as it continually pumps val-
uable water upward to be lost by evaporation. This
movement may be checked by producing on the soil sur-
face, by appropriate tillage, a layer of dry, loose soil.
This layer, called a soil mulch, affords much resistance
to wetting because of its dryness, w r hile at the same time
it affords but little surface and few angle interstices for
effective capillary pull. Thus it is that a farmer, in order
to meet his immediate or future needs, may alter and
control capillary movement by careful attention to phys-
ical conditions, especially those at the surface where
evaporation is always active.
157. Gravitational water. As soon as the capillary
capacity of a soil column is satisfied, further addition of
moisture will cause the appearance of free water in the air
spaced By the attraction of gravity, this water moves
downward through the earth at a rate varying with soil
and climatic conditions. In general the flow is governed by
four factors pressure, temperature, texture, and structure.
An understanding of the operation of these forces is im-
portant, since the rapid elimination of free water from the
soil is necessary for optimum plant growth. The actual
procedure, however, is considered under the head of " Land
Drainage," a distinct phase of soil management in itself.
158. Pressure and the movement of gravity water. -
It is very evident that any pressure exerted on a water
SOILS: PROPERTIES AND MANAGEMENT
column will accelerate the rate of flow. Under normal
conditions pressure may arise from two sources, baro-
metric pressure and the weight of the water column.
Changes in barometric pressure are communicated to
gravitational water through a movement of the soil air.
As the mercury column rises, more air is forced into the
soil and the pressure on the soil water increases. Such
a change has been observed by King 1 to produce as high as
a 15 per cent decrease in the flow of drains. King observed
also that the level of wells fluctuated from time to time for
the same cause. The expansion of the air of the soil due to
daily heatings was also observed to produce diurnal oscil-
lations in the level and the rate of flow of ground water.
Perhaps of greater import in the rate of percolation of
water is the pressure produced by the weight of the free-
water column. Working along this line, Welitschkowsky 2
has shown rather conclusively that with an ideal length of
column the flow varies directly with the pressure. Plis
ideal column with the sand with which he experimented
was 75 centimeters in length. With a longer column the
flow did not increase as fast as the pressure ; while with
a shorter column, doubling the pressure more than doubled
the flow. These results have been verified by Wollny 3
and ably reviewed by King. 4
1 King, F. H. The Soil, p. 180. New York. 1906.
2 Welitschkowsky, D. von. Experimentelle Untersuchun-
gen iiber die Permeabilitat des Bodens fur Wasser. Archiv f.
Hygiene, Band II, Seite 499-512. 1884.
3 Wollny, E. Untersuchungen iiber die Permeabilitat des
Bodens fur Wasser. Forseh. a. d. Gebiete d. Agri.-Physik,
Band 14, Seite 1-28. 1891.
4 King, F. H. Principles and Conditions of the Movements
of Ground Water. U. S. Geol. Survey, 19th Ann. Kept., Part
II, pp. 67-206. 1897-98.
THE FORMS OF SOIL WATER 235
159. Effect of temperature on the flow of gravity water.
A rise in temperature of the soil not only varies the
amount of capillary water and thus increases the possible
free water present, but at the same time it increases the
fluidity and thus facilitates percolation. The expansion
of the soil air also tends to increase such movement. This
can be noticed in the operation of a tile drain in early
spring as compared with summer conditions. Calculated
effects of temperature change have been verified by con-
trolled experimental results.
160. Effect of texture and structure on the flow of
gravity water. Of much more practical importance
than temperature, in the flow of gravitational water,
are the size and the arrangement of the soil particles.
In working with sands of varying grades, Welitschkowsky, 1
Wollny, 2 and others have shown that the flow of water
varies with the size of particle, or texture. King 3 has
demonstrated that in general with such materials the
rate of flow is directly proportional to the square of the
diameter of the particle. By the use of the effective
mean diameter 4 of a sand sample, he was able to calculate
a theoretical flow which compared very closely to observed
percolations. In sandy soils this law holds in a very
general way, but in clays it fails entirely. For instance,
1 Welitschkowsky, D. von. Experimentelle Untersuchungen
iiber die Permeabilitat des Bodens fur Wasser. Archiv f.
Hygiene, Band II, Seite 499-512. 1884.
2 Wollny, E. Untersuchungen iiber den Einfluss der
Struktur des Bodens auf dessen Feuchtigkeits- und Tempera-
turverhaltnisse. Forsch. a. d. Gebiete d. Agri.-Physik, Band
5, Seite 167. 1882.
3 King, F. H. Principles and Conditions of the Movements
of Ground Water. U. S. Geol. Survey, 19th Ann. Kept., Part
II, pp. 222-224. 1897-98.
4 This text, paragraph 87.
236 SOILS: PROPERTIES AND MANAGEMENT
if such a law was in force a sand having a diameter of
.5 millimeter would exhibit a flow 10,000 times greater
than that through a clay loam with a diameter, say, of
.005 millimeter; whereas the actual ratio, as observed
experimentally by King, was less than 200.
Evidently, therefore, while it can be stated as a general
thesis that the flow varies with the texture, no governing
law can be deduced for soils since structure exerts such a
modifying influence. The percolation in a heavy soil
takes place largely through lines of seepage, which are
really large channels developed by various agencies.
If in the drainage of average soil, the farmer depended
on the movement of water through the individual pore
spaces, the soil would never be in a condition for crop
growth. These lines of seepage are developed by the
ordinary forces that function in the production of soil
granulation, as freezing and thawing, wetting and drying,
lime, humus, plant roots, and tillage operations.
A clear understanding of the factors governing the
flow of gravitational water is of especial importance in
tile drainage operations, particularly regarding the depth
of and interval between tile drains. Since percolation
is so slow in a heavy soil, it is evident that the tile must
be near the surface in order to secure efficient drainage.
In a sand the depth may be increased, because of the
slight resistance offered to water movement. The depths
for laying tile in a heavy soil range from one and a half
to two and a half feet, while in a sand the tile may often
be placed as deep as four feet below the surface. It is
evident also that the less deep a tile drain is laid, the
less distance on either side it will be effective in removing
the water; consequently on a clay soil the laterals must
be relatively close, as compared to the interval generally
THE FORMS OF SOIL WATER 237
recommended for a sandy soil. A rational understanding
of the movements of gravitation water is clearly necessary
in the installation of tile drains, not only that the system
may be fully effective, but also that a minimum effective
cost may be realized.
161. Determination of the quantity of free water that
a soil will hold. While there is no particular advantage
in finding the quantity of gravitational water that a soil
will hold, since a normal soil should never remain saturated
for any length of time, it is nevertheless of interest to
know by what methods such data may be obtained. One
method is to saturate a column of known weight, and
then, by exposing it to percolation, measure the amount
of water that is lost. The gravitational water can then
be expressed in terms of dry soil. The disadvantage in
this method lies in the fact that it is extremely difficult
to free a soil entirely of air, so that a determination made
in this way would yield low results. Again, a very long
time must elapse before a soil will give up all its gravita-
tional water. King 1 found that with even a sand the
draining-away of the free water continued over a space
of two and one half years. It must also be noted here
that because of the lessening of the capillary water as a
column of soil is ascended, the space for possible free
water increases, thus accounting for the ready entrance
of rain into a soil which on the average may contain a
relatively high water content.
162. The calculation of the free water of a soil. A
more nearly accurate idea of the possible free-water
capacity of soil may be obtained by calculation. If the
absolute and the apparent specific gravity of a soil, and
1 King, F. H. Physics of Agriculture, pp. 134-135. Pub-
lished by the author, Madison, Wisconsin. 1910.
238 SOILS: PROPERTIES AND MANAGEMENT
its percentage of moisture when capillarily satisfied, are
known, the following formulas may be used :
Percentage of air space when 1 J^ rcenta S e of pore space
capillarily saturated " (percentage of H 2 O
J I X ap. sp. gr.)
Percentage of free water pos- 1 _ [percentage of air space
sible J ( ap. sp. gr.
163. Value of studying flow and composition of gravita-
tional water. While the determination of the possible
free water that a soil will hold is of little real value, a
knowledge of its movement and its composition is of
vital importance. It has already been shown how the
rate of movement of such water is a factor in efficient
drainage. The amount likely to be thus lost is of interest
in plant production from two standpoints: first, the
role that water plays as a food and a regulator; and
secondly, the losses of nutritive elements that always
occur with drainage. It is quite evident that these
questions should be studied only on soil in a normal field
position. Consequently two methods of procedure are
open the use of an efficient system of tile drains, and
the construction of lysimeters.
164. The study of gravity water by means of tile drains.
In the first method an area should be chosen where
the tile drain receives only the water from the area in
question and where the drainage is efficient. A study
of the amounts of flow throughout a term of years will
yield much valuable data concerning the factors already
discussed. An analysis of the drainage water will throw
light on the ordinary losses of plant-food from a normal
soil under a known cropping system. The advantage
THE FOE MS OF SOIL WATER
239
of such a method of attack lies not only in the fact that
a large area of undisturbed soil is considered, but also
in the opportunity to study practical field treatments
in relation to the movement and composition of drainage
water.
165. The lysimeter method of studying gravitational
water. The lysimeter method, however, has been the
usual mode of attacking these problems. In this method
a small block of soil is used, being entirely isolated by
appropriate means from the soil surrounding it. Effective
and thorough drainage is provided. The advantages of
this method are that the variations found in a large field
are avoided, the work of carrying on the study is not so
great as in a large field, and the experiment is more easily
controlled. One of the best-known sets of lysimeters
was that established at the Rothamsted Experiment
FIG. 38. Cross section of a lysimeter at the Rothamsted Experiment
Station, England, (n), soil column under study; (p), outlet for
collected drainage water.
240 SOILS: PEOPEETIES AND MANAGEMENT
Station 1 in England. (See Fig. 38.) Here blocks of
soil one one-thousandth of an acre in surface area were
isolated by means of trenches and tunnels, and, supported
in the meantime by perforated iron plates, were separated
from the surrounding soil by masonry. The blocks of soil
were twenty, forty, and sixty inches in depth, respectively.
Facilities for catching the drainage were provided under
each lysimeter. The advantage of such a method of
construction lies in the fact that the structural condition
of the soil is undisturbed and consequently the data are
immediately trustworthy.
FIG. 39. Cross section of a soil tank at Cornell University, New
York, (a), soil under investigation; (p), outlet of drainage pipe.
1 Lawes, J. B., Gilbert, J. H., and Warington, R. On the
Amount and Composition of the Rain and Drainage Waters
Collected at Rothamsted. Jour. Roy. Agr. Soc., Ser. II, Vol.
17, pp. 269-271. 1881.
THE FORMS OF SOIL WATER 241
At Cornell University 1 a system of cement tanks sunk
in the ground has been constructed.. Each tank is about
four and a sixth feet square and four feet deep. A sloping
bottom is provided, with a drainage channel opening into
a tunnel beneath and at one side. As the tanks are ar-
ranged in two parallel rows, one tunnel suffices for both.
(See Fig. 39.) The sides of the tanks are treated with
asphaltum in order to prevent solution. The soil must
of course be placed in the tanks, this causing a disturb-
ance of its structural condition. As a consequence data
as to rate of flow and composition of the drainage water
are rather unreliable for the first few years. Such an
experiment must necessarily be one of long duration.
166. Thermal movement of water. Little has been
said as yet regarding this third mode of water movement,
the vapor flow, which is not peculiar to one form of soil
water but affects all alike. It is at once apparent that
the movement of water vapor can be of little importance
within the soil itself, since it depends so largely on the
diffusion and convection of the soil air. While the soil
air is no doubt practically always saturated with water
vapor, the loss of moisture by this means is slight. Buck-
ingham 2 has shown that, while sand allows such a move-
ment to the greatest degree, the loss occurring in a soil
with any appreciable depth of layer is almost negligible.
The question of the thermal movement of water at the
soil surface, however, is vital in farming operations. At
this point the water films are exposed to sun and wind,
and drying goes on rapidly, the free, capillary, and a
1 Lyon, T. L. Tanks for Soil Investigation at Cornell Uni-
versity. Science, N. Ser., Vol. 29, No. 746, pp. 621-623. 1909.
2 Buckingham, E. Studies on the Movement of Soil
Moisture. U. S. D. A., Bur. Soils., Bui. 38, pp. 9-18. 1907.
R
242 SOILS: PROPERTIES AND MANAGEMENT
part of the hygroscopic water vaporizing in the order
named. If the loss of the surface moisture were the
only consideration, the problem would not be serious;
but the capillarity of the soil must be considered also.
As the films at the surface become thin a capillary move-
ment begins, and if the evaporation is not too rapid a
very great loss of water may occur in a short time.
The evaporation from a bare soil in the Rothamsted
lysimeters 1 averaged about seventeen inches a year,
with a rainfall ranging from twenty-two to forty-two
inches. This means that from one-third to one-half of
the effective rainfall was entirely lost as thermal water.
The necessity of checking such a loss becomes apparent,
especially in regions where rainfall is slight or drought
periods are likely to occur. As no country is free from
one or the other of such contingencies, the great promi-
nence that methods of moisture conservation hold in
systems of soil management is understandable. While
means of checking losses by leaching and run-off are
advocated, effective retardation of surface evaporation
is always particularly emphasized.
iWarington, R. Physical Properties of the Soil, p. 109.
Clarendon Press, Oxford. 1900.
CHAPTER XII
THE WATER OF THE SOIL IN ITS RELA-
TION TO PLANTS
WATER, as has already been shown, is one of the external
factors in plant growth in that it is necessary in the
processes of weathering, which results in the simplifica-
tion of compounds for plant utilization. It also func-
tions as an internal factor in plant development, inasmuch
as it maintains the turgidity of the plant cells, acts as a
carrier of food materials, functions as a regulator, and
can actually be utilized as a source of hydrogen and
oxygen. These direct or indirect relations of water
to plant growth may be considered under three heads,
as follows :
167. Relations of water to the plant.
1. Water acts as a solvent and a carrier of plant-food
materials. It is therefore a medium of transfer
for the mineral and gaseous elements from the
soil to their proper places within the plant.
2. As a food water either becomes a part of the cell
without change, or is broken down and its ele-
ments are utilized in new compounds.
3. Water in maintaining turgidity, in equalizing the
temperature by evaporation from the leaves, and
in facilitating quick shifts of food from one part
of the plant to another, acts as a regulator during
assimilation and while synthetic and metabolic
processes are going on.
243
244 SOILS: PROPERTIES AND MANAGEMENT
Soil moisture, therefore, in proper amounts, becomes
one of the controlling factors in crop growth and must
be looked to before the maximum utilization of the
primary elements can be expected. The amount of
water held within the plant is not large, however, in
comparison with the amount lost by transpiration, al-
though green plants contain from 60 to 90 per cent of
moisture. Although the main cause of the high trans-
piration of most crops is not traceable to the dilute con-
dition of the soil solution, certain regulatory functions
may, however, also come into play.
Because of the readiness with which moisture passes
from plants into the atmosphere, large quantities of
water must be taken from the soil in order that
the plant may maintain its proper turgidity. This
excess water is largely lost or disposed of by trans-
piration, at the same time performing its regulatory
functions.
168. The water requirement of plants. As might be
expected, the pounds of water transpired for every pound
of dry matter produced in the crop is very large. This
figure, called the transpiration ratio, or water require-
ment, ranges from 200 to 500 for crops in humid regions,
and almost twice as much for crops in arid climates.
An accurate determination of the transpiration ratio of
a crop is somewhat difficult, due to the methods of pro-
cedure necessary and also to the difficulty of controlling
the numerous factors that may vary the transpiration.
For really reliable figures the plants must be grown in
cans or pots, in order that the water lost may be deter-
mined accurately by weighing. If there is no percolation,
the water ordinarily lost from a cropped soil includes
both that evaporated from the soil surface and that
WATER OF SOIL IN ITS RELATION TO PLANTS 245
transpired from the leaves. The former loss may be
eliminated from calculations in two ways : (1) by covering
the soil in some way so that evaporation is absolutely
checked and the only loss is by transpiration; or (2) by
determining the evaporation from a bare pot and, by
subtracting this from the total water loss from a cropped
soil, finding the loss due to transpiration alone.
An objection to the former method is that any covering
which interferes with evaporation interferes with proper
soil aeration also and may render soil conditions abnormal.
In the second method, however, an even more serious
error enters, since the evaporation from a bare soil is
not the same as that from a soil covered by vegetation
because of the shading effects. Also, due to the action
of the roots, less water is likely to be allowed to move
to the surface by capillary attraction in the cropped soil.
Therefore, any data that, may be quoted can be only
general in its application, not only because of the errors
of determination but also because of the great number of
factors that under normal conditions may vary the
transpiration ratio. The data on the following page,
drawn from various investigators working by the gen-
eral methods 1 already outlined, give some idea of the
water transpired by different crops, due allowance being
made for various disturbing factors. Below the data
regarding transpiration will be found the citations to
the work of the various authors as well as a few
notes regarding their experimental procedure.
1 A brief discussion of the various methods is found as follows :
Montgomery, E. G. Methods of Determining the Water
Requirements of Crops. Proc. Amer. Soc. Agron., Vol. 3,
pp. 261-283. 1911. Also Briggs, L. J., and Shantz, H. L.,
The Water Requirement of Plants. U. S. D. A., Bur. Plant
Jnd., Bui. 285. 1913,
246 SOILS: PROPERTIES AND MANAGEMENT
WATER REQUIREMENTS OF PLANTS BY DIFFERENT
INVESTIGATORS
CROP
LAWES 1
HARPENDEN
ENGLAND 1850
WOLLNY 2
MUNICH
GERMANY 1876
HELLRIEGEL 3
DAHME
GERMANY
1883
KING 4
MADISON
WISCONSIN
1895
K
BRIGGS AND
SHANTZ 6
AKRON, COLO.
1911-1913
Barley
258
774
310
464
468
534
Beans
209
282
736
Buckwheat . . .
646
363
578
Clover
269
310
576
797
Maize
233
271
337
368
Millet
447
310
Oats . " . . . .
665
376
503
469
597
Peas
259
416
273
477
563
788
Potatoes . . . .
385
636
Rape
912
441
Rye
353
685
Wheat
247
338
544
513
1 Lawes, J. B. Experimental Investigation into the Amount
of Water Given off by Plants during their Growth. Jour.
Hort. Soc. London, Vol. 5, pp. 38-63. 1850.
Pots holding 42 pounds of field soil were used. Evaporation
from soil was reduced to a very low degree by perforated glass
covers cemented on the pots. The figures quoted are from
unfertilized soil.
2 Wollny, E. Der Einfluss der Pflanzendecke und Beschat-
lung auf die Physikalischen Eigenschaften und die Frucht-
barkeit des Bodens, Seite 125. Berlin, 1877.
Wollny grew plants in humous sand in amounts ranging from
5 to 12 kilograms. Evaporation was reduced to a very low
degree by perforated covers. Actual evaporation from un-
cropped cans was observed, howerer.
3 Hellriegel, H. Beitrage zur den Naturwissenschaftlichen
Grundlagen des Ackerbaus, Seite 663. Braunschweig, 1883.
Hellriegel grew plants in 4 kilograms of clean quartz sand
and supplied them with nutrient solutions. The loss by evap-
oration from uncropped pots was used in determining losses
WATER OF SOIL IN ITS RELATION TO PLANTS 247
169. Factors affecting transpiration. These figures
serve to indicate not only the variation between crops,
but also the great effect of climate and soil on transpira-
tion. 1 The factors may be listed under three heads, as
follows :
1 A complete review of the literature concerning the climatic
and soil factors in their effect on transpiration may be found
as follows: Briggs, L. J., and Shantz, H. L. The Water
Requirement of Plants. U. S. D. A., Bur. Plant Ind., Bui.
285. 1913.
by transpiration. In later experiments covers were used in
order to cut down evaporation.
4 King, F. H. Physics of Agriculture, p. 139. Published
by author, Madison, Wisconsin, 1910. Also, The Number of
Inches of Water Required for a Ton of Dry Matter in Wis-
consin. Wisconsin Agr. Exp. Sta., llth Ann. Rept., pp. 240
248. 1894; and The Importance of the Right Amount and
Right Distribution of Water in Crop Production. Wisconsin
Agr. Exp. Sta., 14th Ann. Rept., pp. 217-231. 1897.
King used cans holding about 400 pounds of soil. Some were
set down into the earth while others were not. Part of the
work was carried on in the field ; the remainder was run in
vegetative houses. Normal soils were used. Evaporation
from soil was very low, water being added from beneath. The
data quoted are the average of a large number of tests.
5 Leather, J. W. Water Requirements of Crops in India.
Memoirs, Dept. Agr., India, Chem. Series, Vol. I, No. 8, pp.
133-184, 1910, and No. 10, pp. 205-281. 1911.
Jars containing from 12 to 48 kilograms of soil were used.
Loss by evaporation was determined on bare pots. The plants
were grown in culture houses or in screened inclosures.
6 Briggs, L. J., and Shantz, H. L. Relative Water Require-
ment of Plants. U. S. D. A., Jour. Agr. Research, Vol. Ill,
No. 1, pp. 1-63. 1914. Also, The Water Requirements of
Plants. U. S. D. A., Bur. Plant Ind., Bui. 284. 1913.
Plants were grown in cans holding 250 pounds of soil. Evap-
oration from soil was prevented by means of a paraffin covering.
Work was conducted in screened inclosures. The data are the
average of several years' work.
248 SOILS: PROPERTIES AND MANAGEMENT
1. Crop. Differences due to different crops and to
variations of the same crop.
2. Climate. Rain, humidity, sunshine, temperature,
and wind.
3. Soil. Moisture and general fertility.
170. Effect of crop and climate on transpiration. -
Not only do different crops show a variation of tran-
spiration in the same season, but the same crop may give
a totally different transpiration in different years. This
is due in part to inherent differences in the crop itself.
For example, leaf surface or root zone would totally
alter the transpiration relationship under any given
condition. However, a great deal of the variation ob-
served in the ratios already quoted arises from differences
in climatic conditions. As a general thing, the greater
{he rainfall, the higher is the humidity and the lower is
the relative transpiration. This accounts for the high
figures obtained by Widtsoe x in arid Utah. Mont-
gomery 2 found, in studying the water requirements of
corn under greenhouse conditions, that an increase in
the percentage humidity from 42 to 65 lowered the tran-
spiration ratio from 340 to 191. In general, temperature,
sunshine, and wind vary together in their effect on tran-
spiration. That is, the more the sunshine, the higher is
the temperature, the lower is the humidity, and the
1 Widtsoe, J. A. Production of Dry Matter with Differ-
ent Quantities of Irrigation Water. Utah Agr. Exp. Sta.,
Bui. 116. 1912. Also, Irrigation Investigations. Factors In-
fluencing Evaporation and Transpiration. Utah Agr. Exp.
Sta., Bui. 105. 1909.
2 Montgomery, E. G., and Kiesselbach, T. A. Studies in
Water Requirements of Corn. Nebraska Agr. Exp. Sta.,
Bui. 128, p. 4. 1912.
WATER OF SOIL IN ITS RELATION TO PLANTS 249
greater is likely to be the wind velocity. All this would
tend to raise the transpiration ratio.
171. Effect of soil moisture on transpiration. From
the soil standpoint, however, the factors inherent in the
soil itself are of more vital importance as regards tran-
spiration, since they can be controlled to a certain extent
under field conditions. An increase in the moisture con-
tent of a soil usually results in an increased transpiration
ratio. The work of Hellriegel 1 with barley grown in
quartz sand containing a nutrient solution may be cited
in this regard, together with the data obtained by Mont-
gomery 2 at Lincoln, Nebraska, with corn grown in a
loam soil :
EFFECT OF SOIL MOISTURE ON TRANSPIRATION
BARLEY HELLRIEGEL
CORN M ONTG OME R Y
Soil Moisture Per-
centage of Total
Capacity
Transpiration
Ratio
Soil Moisture Per-
centage of Total
Capacity
Transpiration
Ratio
80
277
100
290
60
240
80
262
40
216
60
239
30
223
45
229
20
168
35
252
10
180
These data show clearly that an excessive amount of
water in the soil is not a favorable condition for the
1 Hellriegel, H. Beitrage zu den Naturwissenschaftlichen
Grundlagen des Ackerbaus, Seite 639. Braunschweig. 1883.
2 Montgomery, E. G. Methods of Determining the Water
Requirements of Crops. Proc. Amer. Soc. Agron., Vol. 3,
p. 276. 1911.
250 SOILS: PROPERTIES AND MANAGEMENT
economic use of water, as the plant, in order to supply
itself properly with food, must transpire excessive amounts
of water. As soil moisture may be controlled, this waste
may to a certain extent be eliminated.
172. The influence of fertility on transpiration. The
amount of available plant-food is also concerned in the
economic utilization of water. In general the data along
these lines show that the richer the soil, the lower is the
transpiration ratio. Therefore a farmer, in raising the
general fertility of his soil by drainage, lime, good tillage,
green manures, barnyard manures, and fertilizers, provides
at the same time for a greater amount of plant production
for every unit of water utilized. Again, quoting from
Hellriegel and Montgomery, the following figures are
available :
EFFECT OF THE SUPPLY OF PLANT-FOOD MATERIALS ON THE
TRANSPIRATION RATIO OF BARLEY GROWN IN QUARTZ SAND
WITH A NUTRIENT SOLUTION; CALCIUM NITRATE BEING IN
THE MINIMUM. HELLRIEGEL *
UNITS* OF Ca(NOa)2
APPLIED
DKY MATTER PRODUCED
PER POT (GRAMS) .,.
TRANSPIRATION RATIO
1,111
724
4
8,479
399
8
13,936
347
12
18,288
345
16
23,026
302
20
25,504
292
1 Hellriegel, H. Beitrage zu den Naturwissenschaftlichen
Grundlage des Ackerbaus, p. 629. Braunschweig. 1883.
2 A unit of Ca(NO 3 ) 2 equals 1 mg. -equivalent. A mg.-
equivalent of Ca(NO 3 ) 2 equals 82.1 mg.
\VATER OF SOIL IN ITS RELATION TO PLANTS 251
RELATIVE WATER REQUIREMENTS OF CORN ON DIFFERENT
TYPES OF NEBRASKA SOILS, 1911. MONTGOMERY l
SOIL
DRY WEIGHT OP PLANTS
IN GRAMS PER POT
TRANSPIRATION RATIO
Manured
Unmanured
Manured
Unmanured
Poor (15 bushels) . .
376
113
350
549
Medium (30 bushels) .
413
184
341
479
Fertile (50 bushels) . .
472
270
346
392
173. Effect of texture on transpiration. The effects
of texture have been investigated by a number of men,
the work of Von Seelhorst 2 and of Widtsoe 3 being per-
haps the most reliable. While these investigators found
in general that crops on heavy soils exhibited a lower
transpiration ratio, hasty conclusions must not be drawn.
Since the fine-textured soils contain more plant-food
materials, it is probable that this is the balancing factor
rather than texture alone.
174. Actual amounts of water necessary to mature a
crop. Although it can be seen from the transpiration
ratio that the amount of water necessary to bring an
average crop to maturity is very large, a concrete example
may be cited to advantage. A fair estimate of the dry
matter produced in raising a forty-bushel crop of wheat
would be about two tons. Assuming the transpiration
1 Montgomery, E. G. Water Requirements of Corn.
Nebraska Agr. Exp. Sta., 25th Ann. Kept., p. xi. 1912.
2 Seelhorst, C. von. Uber den Wasserverbrauch von
Roggen, Gerste, Weizen, und Kartoffeln. Jour. f. Land-
wirtschaft, Band 54, Heft 4, Seite 316-342. 1906.
3 Widtsoe, J. A. Irrigation Investigations. Factors Influ-
encing Evaporation and Transpiration. Utah Agr. Exp. Sta.,
Bui. 105. 1909.
252 SOILS: PROPERTIES AND MANAGEMENT
ratio to be 300, the amount of water actually used by
the plant would amount to 600 tons to the acre, or about
5.2 inches of rainfall. This does not include the evapora-
tion that is continually going on from the soil surface,
which might very easily amount to as much more. More-
over, this draft on the soil water is not a uniform one, but
increases gradually as the crop develops, until at heading
time great quantities must be supplied in a short period.
The necessity of moisture conservation in order to meet
the plant requirements and preserve its normal develop-
ment, even in humid regions, becomes obvious.
175. Role of capillarity in the supplying of the plant
with water. A query arises at this point regarding the
mode by which this immense quantity of water is supplied
to the plant. The plant rootlets, especially their absorb-
ing surfaces, are few in number as compared with the
interstitial angles that contain most of the water retained
in the soil. How, then, does the plant avail itself of
water not in immediate contact with its rootlets? This
question has been anticipated in the discussion concern-
ing the capillary equilibrium which tends to occur in all
soils. As soon as the rootlet begins to absorb at one
point, the film in that interstitial angle (see Fig. 36) is
thinned. A considerable convexity of the water surface
occurs at that point, resulting in a great outward pull
which causes the water to move in all directions toward
that point. Thus, a feeding rootlet, by absorbing some
of the soil solution with which it is in contact, creates a
condition of instability which results in considerable
film movement. It can therefore be said that capillarity
is the important factor in any soil in supplying the plant
with proper quantities of moisture.
Many of our early investigators have overestimated
H'ATER OF SOIL IN ITS RELATION TO PLANTS 253
the distances through which this movement may be
effective in properly supplying the plant. 1 It must be
understood, however, that the rate of water supply is
the controlling factor in plant nutrition. It has been
shown also that the longer the capillary column, the less
is the amount of water delivered from a water table to
any given point. Therefore capillarity, although it
may act through a distance of ten feet, may be important
for only three feet as far as plant nutrition is concerned,
since water beyond that point is moved too slowly to be
of any great value in time of need. No reliable data are
available as to this particular phase, but the knowledge
of the factors governing capillary movement clearly
indicates that capillarity of the soil is of greatest im-
portance in a restricted zone immediately around each
absorbing root surface.
176. Influence of water on the plant. 2 - - In general,
as the amount of water available to a crop is increased,
the vegetative growth also is increased, the plant be-
coming more succulent. The percentage of moisture in
the crop, even at harvest time, is usually high. Quality
practically always suffers with such a stimulation of
vegetative activity. This is especially noticeable with
such crops as barley and peaches. Shipping qualities
also are depressed with increased water, especially if
the water available is excessive. With an enlargement
of the plant cell a change probably occurs in the cell
contents, tending toward a greater susceptibility to
disease. Ripening is delayed, tillering is diminished,
1 Warington, R. Physical Properties of Soil, p. 105. Ox-
ford. 1900.
2 Mitscherlich, E. A. Das Wasser als Vegetationsfaktor.
Landw. Jahr., Band 42, Seite 701-717. 1912.
254 SOILS: PROPERTIES AND MANAGEMENT
and the percentage of protein content of the crop is de-
creased. It is a curious fact, as will be shown later, that
many of the general and morphological effects of large
quantities of available water on plant growth are the
same as those from the presence of too much soluble
nitrogen. In cereals the stimulation of increased water
is shown especially in the ratio of grain to straw. Widt-
soe's 1 results in this regard are representative of the
data 2 available on this point :
DISTRIBUTION OP DRY MATTER BETWEEN GRAIN AND STRAW
WITH VARYING AMOUNTS OF WATER
INCHES OF WATER
GRAIN IN PERCENTAGE OF
TOTAL DRY MATTER
5
44.4
1\
43.2
10
42.8
15
40.8
25
38.6
35
37.5
50
32.9
As a general rule this depression of the ratio of grain
to straw is not due to an actual decrease in the grain, but
to a correspondingly greater production of dry matter in
1 Widtsoe, J. A. The Production of Dry Matter with
Different Quantities of Irrigation Water. Utah Agr. Exp.
Sta., Bui. 116, p. 49. 1912.
2 Bunger, H. Uber den Einfluss Verschieden Hohen Was-
sergehalts des Bodens in den Einzelhen Vegetationsstadien
bei Verschiedenem Nahrstoffreichtum auf die Entwicklung
des Haferpflanzen. Landw. Jahrb., Band 35, Seite 941-1051.
1906. Also, Seelhorst, C. von, und Freckmann, W. Der
Einfluss des Wassergehaltes des Bodens auf die Ernten und
die Ausbilding Verschiedener Getriedevarietaten. Jour, f .
Landw., Band 51, Seite 253-269. 1903.
WAT Eli OF SOIL IN ITS RELATION TO PLANTS 255
the vegetative parts. As available water increases this
dry matter ascends until a maximum is reached. The
general relationships are well exemplified by data from
/6
L
!
FIG. 40. The effect of increased water supply on the production of
dry matter by various crops.
Widtsoe, 1 tabulated on the following page, although
other equally valuable figures may be obtained from Von
Seelhorst 2 and Atterberg. 2 The curves above (Fig. 40)
illustrate Widtsoe's data and the general trend in the
dry matter produced as the moisture is increased.
. ! Widtsoe, J. A. The Production of Dry Matter with Dif-
ferent Quantities of Irrigation Water. Utah Agr. Exp. Sta.,
Bui. 116, pp. 19-25. 1912.
2 Seelhorst, C. von, und Krzymowski, Dr. Versuch iiber
den Einfluss, welchen das Wasser in dem Verschiedenem Vegeta-
tionsstadien des Hafers auf sein Wachstum ausiibt. Jour. f.
Landw., Band 53, Seite 357-370. 1905. Also, Atterberg, A.
Die Variationen der Nahrstoffgehalte bei dem Hafer. Jour. f.
Landw., Band 49, Seite 97-113. 1901.
256 SOILS: PROPERTIES AND MANAGEMENT
CROP YIELD IN POUNDS TO THE ACRE AS INFLUENCED BY
DIFFERENT AMOUNTS OF WATER. WIDTSOE
INCHES
OF WATER
DRY
MATTER
WHEAT
INCHES
OF
WATER
DRY
MATTER
CORN
INCHES
OF
WATER
DRY
MATTER
POTATO
18.74
4,969
13.04
10,757
11.17
2310
21.24
5,545
15.54
12,762
13.67
2730
' 23.74
5,684
20.54
13,092
16.17
2925
28.74
6,279
25.54
13,856
21.17
3405
38.74
6,672
30.54
14,606
26.17
4005
48.74
7,229
35.54
15,294
36.17
3660
63.74
7,999
60.54
12,637
51.17
3797
177. Availability of the water in the soil. From the
discussion already presented regarding the forms of water
in the soil, the ways in which they are held, and their
movements, it is evident that all the moisture present
in a soil is not available for plant growth. Three divisions
of the soil water may be made on this basis : unavailable,
available, and super-available.
178. Unavailable soil water. As has been shown in
a previous paragraph, free or capillary water may become
of little use to a plant through distance, since capillarity
is unable to pump the water fast enough to supply ordinary
crop needs. Water near at hand or in the immediate
zone of the rootlet may also become unavailable through
the obstruction of capillarity, friction instead of distance
being the cause in this case. As the rootlet thins the
interstitial film at any point, capillarity occurs and
water moves toward the absorbing surface. This move-
ment is rapid enough for plant needs until the film chan-
nels on the particles become thin. (See Fig. 37.) As the
zone of hygroscopic influence of the particle is approached
WATER OF SOIL IN ITS RELATION TO PLANTS 257
the viscosity increases very rapidly and cuts down the
capillarity to such a point that the needs of the plant
are unsatisfied. Wilting therefore occurs simply be-
cause the soil is unable to move the water rapidly
enough for crop needs. As the viscosity of the water
increases very rapidly after the point of lento-capillarity
is reached, the wilting coefficient is a figure somewhat
less than the percentage representing the lento-capillarity ;
also, it is greater than the hygroscopic coefficient, since
wilting due to viscosity occurs before it is possible
for the film to become thinned to the zone of hygro-
scopicity. Not only all the hygroscopic water is unavail-
able, then, but also a certain small quantity of the
capillary water lying between the point of wilting and
the hygroscopic film. This relationship is shown by
data from the work of Heinrich and of Briggs and Shantz ;
men working at widely different times and under entirely
different conditions.
RELATION OF THE WILTING POINT TO THE HYGROSCOPIC CO-
EFFICIENT. HEINRICH *
SOIL
WILTING POINT
PERCENTAGE OP
HYGROSCOPIC WATER
Coarse sandy soil . . .
Sandy garden soil . . .
Fine humous sand . . .
Sandy loam
1.5
4.6
6.2
7 8
1.15
3.00
3.98
5 74
Calcareous soil ....
Peat
9.8
49 7
5.20
42 30
1 Ilcinrich, R. Ueber das Vermogen der Pflanzen den Boden
an Wasser zu erschopfen. Jahresbericht der Agri.-chein.,
Band 18, Seite 368-372. 1875.
258 SOILS: PROPERTIES AND MANAGEMENT
RELATION OF THE WILTING POINT TO THE HYGROSCOPIC CO-
EFFICIENT. BRIGGS AND SHANTZ 1
SOIL
HYGROSCOPIC
COEFFICIENT
WILTING POINT
Coarse sand
5
9
Fine sand
1.5
26
Fine sand
Sandy loam ....
2.3
3.5
3.3
48
Sandy loam ....
Fine sandy loam . . .
Loam
Loam .
4.4
6.5
7.8
98
6.3
9.7
10.3
13 9
Clay loam
11.4
16.3
179. The wilting coefficient of plants. It has been
known for many years that the common plants possess
different capacities for resisting drought. This has
usually been ascribed to one or more of three causes:
(1) difference in root extensions; (2) difference in ability
to become adjusted to a slow intake of water ; and (3) dif-
ference in pulling power against the viscosity of the water
film. The last two capabilities would argue for different
wilting coefficients for different crops on the same soil.
The most extended work on this subject has been by
Briggs and Shantz, 2 who found that the permanent wilting
point in a saturated atmosphere is practically the same
for all plants. Later Caldwell 3 demonstrated that this
1 Briggs, L. J., and Shantz, H. L. The Wilting Coefficient
for Different Plants and its Indirect Determination. U. S.
D. A., Bur. Plant Indus., Bui. 230, p. 65. 1912.
2 Briggs, L. J., and Shantz, H. L. The Wilting Coefficient
for Different Plants and its Indirect Determination. U. S.
D. A., Bur. Plant Indus., Bui. 230. 1912.
3 Caldwell, J. S. The Relation of Environmental Condi-
tions to the Phenomenon of Permanent Wilting in Plants.
Physiological Researches, Station N, Baltimore. U. S. D. A.,
Vol. I, No. 1. July, 1913.
WATER OF SOIL IN ITS RELATION TO PLANTS 259
relationship of the physical constants of the soil to the
wilting point depends on the rate at which the plant
loses water, showing that the soil factors are not entirely
dominant in this respect. This work seemed, neverthe-
less, to indicate that the conclusions of Briggs and Shantz
were correct for plants of humid regions, w r here the wilt-
ing occurred in a saturated atmosphere. If such is the
case, it can be accounted for only by the fact that the
soil forces in their effect on the wilting point are so power-
ful as to override any distinguishing characteristics that
the plant itself may possess, or at least reduce such an
influence within the error of actual experimentation.
180. Determination of the wilting point. Briggs and
Shantz, 1 in their investigations, devised a very accurate
method for making determinations of the wilting point.
Glass tumblers holding about 250 cubic centimeters of
soil in an optimum condition were used. The seeds were
placed in this soil, after which soft paraffin was poured
over the surface in order to stop evaporation, thus re-
moving this disturbing factor in the capillary equilibrium
of the moisture. The seedlings on germination were
able to push through this paraffin. While the plants
were developing, the tumblers were kept standing in a
constant-temperature vat of water in order to prevent
condensation of moisture on the inside of the glass. The
vegetative room was under temperature control. When
definite wilting occurred, as determined in a saturated
atmosphere, a moisture test was made on the soil. The
resulting figure, within experimental error, represents
the wilting point for the soil used.
1 Briggs, L. J., and Shantz, H. L. The Wilting Coefficient
for Different Plants and its Indirect Determination. U. S.
D. A., Bur. Plant Indus., Bui. 230, pp. 10-14. 1912.
260 SOILS: PROPERTIES AND MANAGEMENT
181. Calculation of the wilting point. In studying
the correlation of this wilting coefficient to soil conditions,
Briggs and Shantz 1 advanced the following relationships.
Expressed as formulae they represent methods of at
least approximating the wilting point from other soil
factors. These formulae are arranged in the order of
their reliability, based on the data obtained by the
authors :
1. Wilting point = Moisture equivalent (error 2 Q per cent)
1.84
2. Wilting point = Hygroscopic coefficient (error 7l per ^
.68
3. Wilting point
= Water-holding capacity (Hilgard method) -21 (orror g 3 per cent )
2.9
182. Relation of texture to the wilting point. From
the data already quoted 2 from Heinrich and from Briggs
and Shantz regarding the hygroscopic coefficient and the
wilting point, it is evident that a very close relationship
exists between the texture of the soil and the percentage
of moisture at which plants wilt. The finer the soil
texture, the higher is the wilting point. The following
figures, from Briggs and Shantz, 3 bring out the point very
clearly :
1 Briggs, L. J., and Shantz, H. L. The Wilting Coefficient
for Different Plants and its Indirect Determination. U. S. D.
A., Bur. Plant Indus., Bui. 230, pp. 56-77. 1912.
2 This text, paragraph 178.
3 Briggs, L. J., and Shantz, H. L. The Wilting Coefficient
for Different Plants and its Indirect Determination. U. S. D.
A., Bur. Plant Indus., Bui. 230, pp. 26-33. 1912.
WATER OF SOIL IN ITS RELATION TO PLANTS 261
RELATION OP TEXTURE TO THE WILTING POINT OF KUBANKA
WHEAT
SOIL
MOISTURE
EQUIVALENT
WILTING POINT OF
KUBANKA WHEAT
Sand
1.55
.86
Fine sand
4.66
2.60
Fine sand
Fine sand
5.50
6 74
3.33
3.70
Sandy loam
9.70
4.80
Sandy loam
Sandy loam
14.50
18.60
9.60
8.84
Loam . ....
23.80
12.40
Loam
25.00
13.90
Clay loam
27.40
14.50
Clay loam
29.30
17.10
Briggs and Shantz have attempted to express this
correlation by a formula which, while very inaccurate,
shows in general the relationships already expressed.
The correlation in this case is made between the wilting
point and the mechanical composition of the soil :
Wilting point = .01 of sand + .12 of silt + .57 of clay
(error 10 per cent)
183. Available and super-available water. Advanc-
ing from the wilting, or critical, moisture content of a
soil, all the remaining capillary water is found to be avail-
able for normal plant use. However, when free water
begins to appear, a condition adverse to plant growth is
established, and as the saturation point is approached
this condition becomes more adverse. This free water
is designated as the super-available water, since it is
beyond the available and its presence generates condi-
tions unfavorable to plant growth. The upper limit of
262 SOILS: PROPERTIES AND MANAGEMENT
the capillary water is called the maximum water content
for plant growth. The bad effects of free water on the
plant arise largely from the poor aeration that results
from its presence. Not only are the roots deprived of
their oxygen, but toxic materials tend to accumulate.
Favorable bacterial activities, such as nitrification and
ammonification, are much retarded also.
The various forms of water in the soil and their avail-
ability to the plant are illustrated diagrammatically in
the following figure.
HYGROSCOPIC W n/NT LENTOCflPILLfiFUTY MAXIMUM VWTR
COEFFICIENT^. \ POINT CONTENT
fir(tf05COP/(^\\ ^OPTIMUM WATER CONTENT 1 \r~fESUKfWll/l8LE
AVAIL ABLE. MO/SW/ff
FIG. 41. Diagram showing the forms of water in the soil and their
relationship to the plant.
184. Optimum moisture for plant growth. It is
very evident that there must be some moisture condition
of a soil which is best for plant development. This is
usually designated as the optimum content. It is not
to be assumed, however, that the total range of the
available soil water represents this condition for optimum
plant growth. Nor is this optimum water content in
any particular soil to be designated by a definite per-
centage. In reality the moisture in a soil may undergo
considerable fluctuation and yet allow the plant to develop
normally. This is because the physical condition of the
soil changes with varying water content and the plant is
able to accommodate itself to such a fluctuation without
a disturbance in its normal development occurring.
Wollny l has shown that the optimum moisture for com-
1 Wollny, E. Untersuchung iiber den Einfluss der Wachs-
thumsfaktoren auf des Produktionsvermogen der Kultur-
WATER OF SOIL IN ITS RELATION TO PLANTS 263
mon field crops in general covers a range from 60 to 80
per cent of the water capacity of the soil. Mayer 1
plum I the optimum moisture content of wheat at 80
per cent of the water capacity of the soil, rye at 75 per
cent, barley at 75 per cent, and oats at from 85 to 90
per cent. Such estimates not only emphasize the range
of optimum moisture conditions, but at the same time
show the relatively high percentage of moisture necessary
for maximum crop growth.
Granulation has considerable influence on the range of
optimum moisture conditions, since the better the granu-
lation, the better is the soil able to accommodate itself
to changes in water content without disturbance of
plant growth. The poorer the tilth of any soil, the
narrower does this fluctuating in optimum moisture be-
come. In moisture conservation and control a granular
soil is one of the first improvements to be aimed at.
Drainage, liming, addition of organic matter, and tillage,
by leading up to such a condition, increase the effective-
ness and economy of utilization of soil moisture.
pflanzen. Forsch. a. d. Gebiete d. Agri.-Physik, Band 20,
Seite 53-109. 1897.
1 Mayer, A. Uber den P^influss kleinerer oder Grosserer
Mi'iigen von Wasser auf die Entwickelung einiger Kultur-
pflanzen. Jour. f. Landw., Band 46, Seite 167-184, 1898.
CHAPTER XIII
THE CONTROL OF SOIL MOISTURE
IN the discussion of the water requirements of plants,
it was apparent that for a normal yield of any crop the
amount used by the plant alone was very great, varying
from five to ten acre inches according to conditions. Were
this the only loss of water, the question of raising crops
with given amounts of rainfall would be a simple one.
Three further sources of water loss, however, are usually
found functioning in the soil and tending to lower the
water that would go toward transpiration, a loss absolutely
necessary for proper plant growth. The various ways by
which water finds an exit from a soil are (1) transpiration,
(2) run-off over the surface, (3) percolation, and (4) evap-
oration. The following diagram makes clear their re-
lationships.
Transpiration
Transpiration^
<y
oft
mm
\- >
W^--^
' ' .-*." .-. ' i ''.'' ; 1 : '* Percolation ^ /: -f v '
. .. . .'. *-'J:.'\/ r J^,. <. ' ' V/ *
-^U-^ ' ^^ ' ^
t==l-v-
FIG. 42. Diagram illustrating the ways by which water may be lost
from a soil.
264
THE CONTROL OF SOIL MOISTURE 265
It is immediately obvious that as the losses by run-off,
Iraching, and evaporation increase, the amount of water
left for crop utilization decreases. In arid and semiarid
regions this is fatal to plant growth, while in humid regions
it may be such a factor in periods of drought as to se-
riously reduce the harvest. Control of moisture is there-
fore necessary in all regions, and this really consists in
so adjusting run-off, leaching, and evaporation as to
maintain optimum moisture conditions in the soil at all
times. This control should result in a proper and eco-
nomical utilization of the soil water by the plant.
185. Run-off losses. In regions of heavy rainfall
or in areas where the land is sloping or rather impervious
to water, a considerable amount of the moisture received
as rain is likely to be lost by running away over the sur-
face. Under such conditions two considerations are im-
portant : (1) by not entering the soil the water is lost for
plant use ; and (2) washing of the soil may occur, which
if allowed to proceed may entirely ruin the land. The
amount of run-off varies with the rainfall, the slope, and
the character of the soil. In some regions it may rise
as high as 50 per cent of the rainfall, while in arid regions
it is of course very nearly zero. As a general thing, this
loss is estimated with the losses by leaching, the two being
expressed as one figure.
186. Percolation losses. When at any time the
amount of rainfall entering a soil becomes greater than
the water-holding capacity of the soil, losses by percola-
tion will result. The losses will depend largely on the
amount and distribution of the rainfall and the capa-
bility of the soil to hold moisture. The bad effects of
excessive percolation are twofold : (1) the actual loss of
water, and (2) the leaching-out of salts that may function
266 SOILS: PROPERTIES AND MANAGEMENT
as plant-food. The quantity of nutrient elements lost
annually from the average bare soil in a humid region
more than equals that withdrawn by a crop. The results
from the Rothamsted drain gauges 1 from 1871 to 1904
on a clay loam soil of three different depths are inter-
esting as to the light that they afford regarding actual
drainage losses :
RAIN-
FALL
DRAINAGE THROUGH
SOIL
PROPORTION OF
RAINFALL DRAINED
THROUGH SOIL
Depth in Inches
Per Cent
20
40
60
20
40
60
January ....
February ....
March
2.32
1.97
1.83
1.89
2.11
2.36
2.73
2.67
2.52
3.20
2.86
2.52
1.82
1.42
0.87
0.50
0.49
0.63
0.69
0.62
0.88
1.85
2.11
2.02
2.05
1.57
1.02
0.57
0.55
0.65
0.70
0.62
0.83
1.84
2.18
2.15
1.96
1.48
0.95
0.53
0.50
0.62
0.65
0.58
0.76
1.68
2.04
2.04
78.5
72.2
47.6
26.5
23.2
24.0
25.3
23.2
35.0
57.8
76.7
80.3
88.4
80.0
55.6
30.0
26.1
27.6
25.6
23.2
32.8
57.5
76.3
85.4
84.5
75.2
52.0
28.0
23.6
26.3
23.8
21.7
30.0
52.5
72.4
81.0
April
May
June
July
August
September . . .
October ....
November ....
December ....
Mean total per year
Winter, October to
March ....
Summer, April to
September . . .
28.98
14.70
14.28
13.90
10.09
3.81
14.73
10.81
3.92
13.79
10.15
3.64
48.2
68.6
26.6
51.0
72.8
27.4
48.0
69.0
25.4
The rainfall and relative loss through the 40-inch depth
of soil is shown graphically in the following diagram :
1 Hall, A. D. The Book of the Rothamsted Experiments,
23. London. 1905.
TIIK CONTROL OF SOIL MOISTURE 267
\
\
\
\
FIG. 43. Rainfall and percolation losses through a 40-inch soil column.
Lysimeter records, Rothamsted Experiment Station, England.
It appears from these figures that about 50 per cent
of the rainfall in such a climate as that of England is
lost by percolation alone. It appears also that the loss
is much lower in summer than in winter, the ratio being
about one to three. Also, the longer the soil column,
the less is the percolation, due to the greater water-
holding capacity possessed by the longer column.
187. Methods of checking loss by run-off and leaching.
- It must not be inferred that the soil is never in such
a condition that percolation, and even run-off, are not
advantageous. Often in winter the excess water may
be drained over the surface with no damage whatsoever.
Also, when the soil becomes filled with free water, either
in winter or in the growing season, drainage must
take place in order to establish optimum soil conditions.
The control of the free water of the soil may be brought
268 SOILS: PROPERTIES AND MANAGEMENT
about by drainage operations or by methods that will
increase the water-holding capacity of the soil. The
former is really a matter of engineering technique and
will be treated in a separate chapter. The latter is a
function of the soil itself and must be specifically con-
sidered at this point.
The necessity of giving attention to losses due to
run-off and leaching varies with climatic conditions.
In very humid regions these losses are of grave importance,
while in arid regions they are insignificant as compared
with losses by evaporation. For example, in England
the losses by percolation and run-off in many cases are
as high as 60 per cent of the rainfall. In the Mississippi
River basin the loss is 50 per cent, in the Missouri it is
about 20, while in the Great Basin it drops to zero. This
does not indicate that drainage is not practiced in the
last-named region, however, for, owing to over-irrigation,
seepage, and other conditions, drainage operations often
become as important as in humid climates.
The quantity of water entering a soil is determined
almost entirely by the physical condition of the soil.
If the soil is loose and open, the water enters readily and
little is lost over the surface as run-off. If, on the other
hand, the soil is compact, impervious, and hard, most
of the rainfall runs away, and not only is there a serious
loss of water, but considerable erosion may also result.
The first step in checking run-off losses, therefore, is
strictly physical in nature. As the water that has entered
the soil moves downward it is continually being changed
to capillary water in its passage. If the capillary capacity
of the soil is high, a greater percentage of this rain water
becomes capillary and a less percentage is left to be carried
away as gravitational water. The secret in the control
THE CONTROL OF SOIL MOISTURE 269
of run-off and percolation, then, is first, to have a loose,
open structure of soil in order to facilitate ready entrance
of the water; and secondly, to promote and encourage
a physical condition of soil which provides high capillary
capacity. Drainage, lime, humus, and good tillage en-
courage granulation, which has so much to do with the
proper entrance of water into the soil and its proper
handling and utilization therein. The benefits of drain-
age are felt only when free water, superavailable to
plants, becomes present. Its quick removal, therefore,
not only betters the physical condition of the soil, but
also aids in the maintenance of the optimum moisture
conditions for the plants.
Fall and early spring plowing is always recommended
as a means of increasing the moisture capacity of the
soil, particularly where organic matter is well supplied.
It provides a deep soil, and should establish the best
conditions for the storage of moisture, as well as food,
for the plant. If organic matter is not supplied, deep
plowing is not advisable on light sandy soil ; but on
clay soil it is beneficial because of the loosening and granu-
lating effect. Fall plowing in particular is to be recom-
mended for such soil, as the loose condition produced
facilitates the entrance of surface water while the granu-
lation that the soil undergoes during the winter increases
its water-holding power. A soil in excellent physical
condition may contain considerably more water than
the soil of the same texture but in poor tilth, and yet
present better conditions for crop growth. Where fall
plowing cannot be done, early spring plowing is the next
best procedure.
188. Evaporation losses. Evaporation of soil water
takes place almost entirely at the surface, exceptions
270 SOILS: PROPERTIES AND MANAGEMENT
being where deep, large cracks occur, which allow thermal
loss directly from the subsoil. This loss of water by
direct evaporation from the soil may be excessive and
may result in direct reduction of the crop yield a type
of loss so familiar that examples hardly need be cited.
In the results with the Rothamsted rain gauges about
50 per cent of the annual rainfall was regained in the
drainage water. Since the gauges bore no crop, the
remaining 50 per cent must have been lost by evapora-
tion. It should be noted that in the summer months
under warm temperature this loss was greatest, amount-
ing to 75 per cent of the rainfall. Correspondingly, in
the semiarid and arid sections of the country, where
there is little or no drainage, the rainfall is all lost by
evaporation. Investigations indicate that about 70 per
cent of the precipitation on the land surface is derived
from evaporation from land surface. Even in humid
regions, where the annual rainfall is ample for maximum
crop production, the crops are frequently reduced below
the profit point by prolonged periods of dry weather in
the growing season, during which the loss of water from
the plants, coupled with the loss from the soil, exhausts
the moisture supply.
While run-off and percolation are directly proportional
to the rainfall, loss by evaporation does not vary to such
a degree. The loss by percolation depends almost
directly upon the amount of rainfall above the retentive
power of the soil. In years of heavy precipitation,
losses by percolation must increase. Evaporation from
the soil depends largely upon the time that the soil
surface is moist, and this will not vary markedly from
year to year. The following figures from the Rothamsted
drain gauges may be quoted in this regard :
THE CONTROL OF SOIL MOISTURE
271
RECORDS FROM ROTHAMSTED (1870-1878) 1
RAINFALL INCHES
EVAPORATION INCHES
PERCOLATION INCHES
22.9
17.3
5.6
26.3
18.4
7.9
29.3
18.1
11.2
30.8
18.3
12.5
31.6
16.6
15.0
32.6
18.0
14.6
34.2
18.0
16.2
35.8
18.3
17.5
42.7
17.2
25.5
A rough calculation may be made which will show the
apportionment of the yearly rainfall in a humid region of
the temperate zone between the three forms of losses
run-off and percolation, evaporation, and transpiration.
The percolation under a rainfall, say, of 28 inches, as
shown by the Rothamsted work, is roughly 14 inches, or
50 per cent. The water requirement of an ordinary
crop is about 7 inches. This leaves a loss of 7 inches
to be credited to evaporation. In other words, one-
half the rainfall goes as run-off and percolation, while
the other half is divided about equally between the plant
and loss by evaporation. While run-off and percolation
may be checked to some extent, not enough conservation
can occur in this direction to tide a crop over a period
of drought. Paramount attention should therefore be
directed toward the checking of losses by evaporation,
since moisture thus saved means just that amount added
to the water available for crop use. It should be remem-
bered that over a large proportion of cultivated lands
1 Warington, R. Physical Properties of the Soil, p. 109.
1900.
272 SOILS: PROPERTIES AND MANAGEMENT
the crop yields are curtailed more directly by lack than
by excess of water. It is a common observation that
soils which ordinarily give a low yield in seasons of nor-
mal or low rainfall give good yields in a wet season,
indicating how dominant is this influence of moisture on
soil fertility.
189. Methods of checking evaporation losses. All
methods for the reduction or elimination of evaporation
losses depend on one or both of two functions : (1) the
actual control of evaporation as it occurs at the surface ;
and (2) the prevention of the movement of capillary
water upward to take the place of the moisture already
lost. It has been shown that as water is lost at the sur-
face of a soil, movement is induced and capillarity is
set up. Such action, if allowed to continue, must ulti-
mately bring about great losses. The obstruction of
capillarity would obviously lower these losses to a marked
degree. As it is difficult and often impracticable to en-
tirely eliminate evaporation, the most successful methods
of water control usually include a change in the structural
condition of the soil which tends toward a lower capil-
larity, especially at the surface. Of all the methods of
moisture conservation, the use of a mulch has been found
most satisfactory. The consideration of mulches is
therefore one of the most important phases in the study
of moisture control.
190. Mulches for moisture control. Any material
applied to the surface of a soil primarily to prevent loss
by evaporation may be designated as a mulch. It may
at the same time fulfill other useful functions, such as
the keeping down of weeds and the maintaining of a
uniform soil temperature. By the conservation of the
moisture, more water remains in the soil for the solution
THE CONTEOL OF SOIL MOISTURE 273
of the essential elements, and bacterial activity is en-
couraged. As a general rule, more soluble plant-food is
likely to be found under a mulched soil, other conditions
being equal, than under a soil not so treated.
191. Kinds of mulches. Mulches are of two general
sorts, artificial and natural. In the former case, foreign
material is merely spread over the soil surface and evapora-
tion is obstructed thereby. Manure, straw, leaves, and
the like, may be used successfully. Such mulches, while
very effective, are not generally applicable to field crops
where intertillage is practiced, since they would make
cultivation absolutely impossible by cumbering the soil
surface with a large amount of inert material. Their
use is therefore limited to intensive crops such as are
found in trucking operations. Leaves, including pine
needles, and sawdust are very effective as a mulch, but
some precautions should be observed in their application.
For example, the oak is rich in tannic acid, which may
be washed out of the mulch into the soil and by its effect
on the growing plant may cause a lowering of productivity.
In some European countries, as \vell as in a few localities
in America, stones have been drawn on the soil to serve
as a mulch, particularly in orchard and vineyard culture,
with markedly beneficial effects. Particularly is this
true on such lands as are too steep to permit cultivation.
As further evidence of the utility of this practice, it has
been observed in the fruit-growing section of the Ozark
Mountains, and doubtless in other regions, that the
removal of stones from the land not only results in the
soil's becoming harder, but also reduces crop yield by
increasing loss of moisture. It is therefore necessary
for the farmer to decide whether the inconvenience
to tillage or other operations due to the presence of
274 SOILS: PROPERTIES AND MANAGEMENT
stones may not be more than offset by their beneficial
effects.
The materials for mulching mentioned above are all
strictly artificial, and their application is greatly limited,
due to the lack of material and the expense involved.
They are therefore used only under special conditions.
The second type of mulch is almost universal in its prac-
tical availability.
By proper cultivation almost any soil surface may be
brought into such a condition that evaporation of mois-
ture is more rapid than the upward capillary movement.
This is because surface tillage produces a loose, open
structure, which, while increasing the rate of thermal
movement of the water, at the same time obstructs
capillary action. The surface layer, therefore, quickly
becomes air-dry and is in a condition designated as a
soil mulch. As it differs from the soil below only in
structure, it has numerous advantages over artificial
mulches, at the same time performing successfully all
the functions of the latter. Since not only the water in
the mulch is sacrificed but also a small quantity pumped
upward by capillarity during the operation, speed in
formation is of importance. The tillage implements
that give the maximum looseness and granulation will
prove the most successful. A spike-tooth harrow or a
weeder is the instrument ordinarily employed.
192. The functions of a mulch. A soil mulch depends
for its effectiveness on two functions (1) the shutting-
off of evaporation, and (2) the checking of capillary move-
ment upward. It has already been shown that thermal
movement of water through dry soil layers is practically
nil; 1 therefore, as long as the soil is dry, evaporation is
1 This text, paragraph 166.
THE CONTROL OF SOIL MOISTURE 275
very low. Moreover, any layer of air-dry soil resists
wetting, principally because of the resins and oils that
become deposited on the surface of the soil particles.
This material, called " agricere," has a low surface tension
and the capillary water film is not easily resumed under
such conditions. Again, if the soil is well granulated
it is able to assume a looser and more open structure.
The interstitial angles, which afford spaces for capillary
surfaces, are cut down, and the capillary pulling power
of the layer is much reduced even if it should assume a
film of water. It is evident that looseness and dryness
are the essentials in the efficiency of a soil mulch. As
long as a mulch is dry, texture is not a very important
factor in efficiency, a dry sand being about as effective
as a dry clay. Texture is important, however, in the
length of time that a mulch will remain effective. Due
to the fact that the capillary power of a clay is so great,
it will become moist from below after a few days ; while
a sand mulch, if there is no rain, will remain dry for an
indefinite period. On a heavy clay soil in fine tilth a
mulch may be destroyed by moist, foggy weather, or
by a number of days of very humid atmosphere ; such a
condition, by causing condensation of moisture on the
clay, hastens the reestablishment of capillarity with the
subsoil, thus allowing moisture to be pumped up and
lost.
193. The soil mulch versus the dust mulch. A few
words will not be amiss at this point regarding the term
" dust mulch," which is observed so commonly in soil
literature. This term would indicate that the mulch is
in a very fine condition, its granulation having been
broken down. Such a condition would not be conducive
to efficiency, as it would encourage capillarity, while at
276 SOILS: PROPERTIES AND MANAGEMENT
the same time it would become puddled on wetting
certainly a very undesirable condition. As a matter of
fact, efficient mulches are not in a dust form, but are
granulated and much looser than could be obtained were
they finely divided. It is evident that the term " dust
mulch " is incorrect and should be superseded by " soil
mulch," a figure of speech which more exactly expresses
the true field conditions.
194. Formation of a mulch. It has already been
stated that a mulch should be formed as quickly as
possible. This would not be such a factor were the
mulch adjusted only once in a season. It is necessary,
however, especially in humid regions, to re-form a mulch
every week or ten days. The cutting-down of formation
losses therefore becomes important. In general the
mulch should be made just as soon after a rain as it is
possible to work the land, since the most rapid evapora-
tion occurs during the few hours immediately after a
rain, when the soil is very moist. Even after light showers
the soil should be quickly cultivated, since the rain may
have established a capillary communication with the
surface and thus provided for a rapid loss of the water
already conserved by previous work. Under arid con-
ditions, where the atmosphere is dry and hot and in free
circulation, the surface soil is quickly dried out after a
rain. This drying takes place so rapidly that the capil-
lary films quickly become so thin that movement is
stopped and no more water is brought to the surface.
The soil may be ever so hard and compact, but as long
as it is kept dry it very effectively conserves the moisture
below. The more rapid the loss, the more quickly will
the mulch condition be created, and therefore the less the
total loss of water is likely to be. This has been demon-
THE CONTROL OF SOIL MOISTURE
277
strated by Buckingham 1 in some experiments in which
arid climate conditions were created at the surface of a
capillary column forty-six inches in height. The soil
was a fine sandy loam. At first the loss of water under
the arid conditions was very rapid and exceeded that
under the humid conditions; but the rate of loss soon
dropped considerably below that of the humid column,
and continued to fall behind during the twenty days
of the experiment. The differences in this case were
due to self-mulching, a very common phenomenon of
arid land soils, particularly those of a loamy character.
This self-mulching is often seen in sands in humid
regions. The under layers of a sand pile are always
moist, due to the self-mulching tendencies of the sur-
face. The results of Buckingham are shown in the fol-
lowing curves :
HUMID
FIG. 44. Evaporation curves on a sandy loam under humid and arid
conditions. Self-mulching has occurred under the arid conditions
and a reduction in evaporation has resulted.
ARID
1 Buckingham, E. Studies on the Movement of Soil Mois-
ture. U. S. D. A., Bur. Soils, Bui. 38, pp. 18-24. 1907.
278 SOILS: PROPERTIES AND MANAGEMENT
195. Depth of a mulch. The depth of a mulch is an
important question in humid regions. Not only must
all the water in the layer be sacrificed in order to make
the mulch effective, but the plant-food of that layer
is temporarily, withdrawn from use. In humid areas,
where the surface soil is usually not over eight or ten
inches in depth, the latter consideration is vital, since
the fertility of the soil would be greatly depressed by a
deep soil mulch. Another factor to be considered here
is the possible root pruning that may occur while the
mulch is being formed. While not of importance early
in the season, it is worthy of considerable attention when
the intertilled crop attains some age. It has been shown,
with such crops as corn, that considerable depression in
yield may result from the maintenance of a mulch at too
great a depth, some of the feeding roots being cut off
thereby. For such reasons the average depth of mulch
for humid regions and in dry-farming operations has
become regulated to about three inches, although in the
late cultivation of corn a less depth than this is advocated.
In irrigated regions where little rainfall occurs and where
the soil is very deep and uniformly fertile, mulches as
deep as ten or twelve inches are sometimes found, es-
pecially in orchards. As rainfall occurs but few times
during the season, such a mulch often needs no attention
except for its original formation. With crops having
shallow roots a thinner mulch layer must of course be used.
196. Resume of mulch control. To summarize briefly,
the cardinal points in mulch control are : (1) mulches are
more effective and more easily maintained in an arid than
in a humid climate; (2) their efficiency depends directly
on their dryness, looseness, and granulation; (3) sandy
soil is more easily maintained as a mulch than clay soil ;
THE CONTROL OF SOIL MOISTURE
279
(4) from two to three inches is ordinarily the most effec-
tive iK-pth; (5) after a heavy rain, the soil mulch must
he renewed by tillage, and this is much more urgent on
clay than on sandy soil ; even without rain, a clay mulch
may become inefficient ; (6) tillage for mulch purposes
must ordinarily be more frequent in spring or during
periods of heavy rain, than at other times of the year;
(7) the use of foreign materials as mulches may be justi-
fied under special circumstances.
197. Water saved by a mulch. It is very difficult
to quote data regarding the capacity of a mulch to con-
serve moisture, since conditions vary to such a degree
from one region to another. Again, water may not be the
limiting factor in crop growth, and even if moisture were
saved there might be but little influence on crop yields.
As a general rule, mulches are most easily maintained
and most effective in arid and semiarid regions. Since
there is no doubt that moisture, under such conditions, is
the limiting factor in plant growth, data from such regions
should be especially significant.
MOISTURE CONTENT OF MULCHED AND UNMULCHED EASTERN
MONTANA SOILS. AVERAGE OF THREE YEARS. 1 OCTOBER 6
MULCHED
UNMULCHED
First foot
168
108
Second foot
Third foot
I' 1 ! Mirth foot
16.4
13.2
10 1
9.4
9.5
89
Fifth foot
96
85
Average
13 2
94
1 Buckman, H. O. Moisture and Nitrates in Dry Land
Agriculture. Proc. Amer. Soc. Agron., Vol. 2, p. 131. 1910.
280 SOILS: PROPERTIES AND MANAGEMENT
If the wilting point of this soil is 6 per cent, the mulched
area contains more than twice as much available moisture.
This 3.8 per cent of available moisture by which the
mulched soil excels the unmulched is equivalent in a five-
foot depth to about 250 tons of water, enough to increase
the crop by a ton of dry matter certainly not an insig-
nificant saving where crop yield and moisture are so very
closely correlated.
A considerable amount of experimentation 1 is available
which seems to indicate that mulching a soil does not in-
crease its yield over a soil not so treated. One reason for
this, as already suggested, may be in the fact that water
may not have been the limiting factor, the rainfall having
been just right in amount and distribution. Again, the
roots may have so intercepted the capillary water as to
have allowed no more evaporation from the unmulched
soil than from the mulched. In some soils hard layers
often form which act in repelling capillary movement.
Such a condition would function as successfully in check-
ing losses as if a true mulch were present. In the study
of mulches and their value in increasing a crop, decided
opinions should not be advanced until every phase has
been thoroughly investigated regarding the exact factors
dominant in the determination of yield. The extended
use of soil mulches in the Corn-Belt and in dry-farming
operations argues for their benefits.
198. Effect of mulches other than on moisture. That
mulching a soil has other effects besides the conserving of
moisture is universally evident. In general the physical
condition of the soil is always better after a crop that has
1 Gates, J. S., and Cox, H. R. The Weed Factor in the
Cultivation of Corn. U. S. D. A., Bur. Plant Indus., Bui.
257. 1912.
THE CONTROL OF SOIL MOISTURE
281
been intertilled. Not only has the surface been kept
well granulated, but the presence of optimum moisture
below has allowed the granulating agents to become
more active. The following of potatoes by corn is, at
least partially, an attempt to take advantage of the
better tilth of the soil with a crop that is particularly
benefited thereby. Again, a mulch not only tends to
allow a ready entrance of water into the soil, but at the
same time increases the water-holding capacity factors
already emphasized in the discussion of control of losses
by percolation and run-off. By keeping down weeds l
another saving is effected, not only in moisture but also
in plant-food. Some results from an experiment 2 con-
ducted at, Cornell University serve to illustrate the re-
lation of mulches and weeds to soil moisture and crop
production in a humid region in a season of good rainfall.
The crop grown was maize. Every third plot was a
check and was given normal treatment :
YIELDS CALCU-
LATED TO BASIS
OF 100 ON
CHECK PLOTS
SOIL MOISTUKE
DURING AUGUST
PER CENT
Check plot
100
96
121
100
31
98
100
21.1
18.2
25.0
18.2
9.8
17.0
17.7
Weeds removed, but not cultivated .
Mulched with straw
Check plot
No cultivation ;
One cultivation
Grow
weeds allowed to grow
; weeds allowed to
Check plot
1 Gates, J. S., and Cox, H. R. The Weed Factor in the Culti-
vation of Corn. U. S. D. A., Bur. Plant Indus., Bui. 257. 1912.
2 Craig, C. E. The Cause of Injury to Maize by Weeds.
Presented as a thesis for the degree of M. S. A., Cornell Uni-
versity. Unpublished. June, 1908.
282 SOILS: PROPERTIES AND MANAGEMENT
The application of a soil mulch is not confined to
intertilled crops such as maize, potatoes, vineyards,
fallow, and the like. Under some conditions it may be
applied to grain fields with good results. In those sections
of the country where dry farming is practiced, it is not
uncommon to drag the grainfield with a sharp-tooth
harrow, the teeth pointing backward. This is begun when
the plants are small, and may be continued until they
attain a considerable size or until they sufficiently shade
the ground to greatly reduce surface evaporation.
199. General usefulness of a mulch. While a soil
mulch is used primarily in order to conserve moisture,
its relationships are different in different regions accord-
ing to climatic and cropping peculiarities. In dry-
farming regions a mulch is maintained as nearly as possible
the year round, since moisture must be carried from the
previous summer and winter to the growing season in
order to supplement the rainfall occurring at that time.
In irrigated regions a mulch is useful in two ways by
conserving the rainfall and by checking the loss of irriga-
tion water; after the latter is once in the soil less addi-
tional water need be applied and the consequent cost of
irrigation is much less. Again, in arid regions where
there is an excess of soluble salts, rapid evaporation is
detrimental since these salts tend to concentrate near
the surface and become harmful to plants. The pre-
vention of the rise of alkali is therefore a very important
function of the soil mulch in such regions.
In humid regions the utilization of a soil mulch is
much less intense, since the conservation of moisture
over long periods is unnecessary, due to the rainfall.
However, during the growing season periods of drought
occur, when if available water is lacking in the soil, the
THE L'OXTHOL OF SOIL MOISTURE 283
cTop suffers. The amount of moisture conserved by a
mulch will usually keep the plant growing normally
through such periods, while crops on soils not so treated
may suffer greatly. The tiding of crops over short periods
of light rainfall is the chief function of mulches in humid
climates.
200. Other practices affecting evaporation losses. -
Although the control of water by mulches is such an
important consideration, other means of checking losses
are available. These may be grouped under five heads :
(1) fall and early spring plowing, (2) rolling, (3) shelters,
(4) level cultivation, (5) plants.
201. Fall and early spring plowing. Fall and early
spring plowing owe much of their efficiency to the con-
servation of moisture effected through the creation of a
mulch over the surface. Fall plowing may be practiced
for a number of reasons, but in regions of deficient rain-
fall, particularly in winter, the conservation of the mois-
ture in the soil at the close of the growing season is an
important consideration. This practice is w r ell adapted
to those soils in semiarid sections that do not blow too
badly when fall-plowed, and where the winter rain is
not sufficient to saturate the soil. If the soil is left in
the bare, hard condition resulting from the removal of
a crop of maize, wheat, or barley, a large quantity of
water may be lost by evaporation during the fall months.
For the average farmer in humid regions wltere the
winter rainfall is sufficient to saturate the soil, early
spring plowing, coupled with tillage, is very important.
Not only may moisture be conserved, but the soil is
worked at the stage when it yields most readily to pul-
verization. Fallow r land, and bare stubble land of fine-
textured soil, are most benefited, since they become
284 SOILS: PROPERTIES AND MANAGEMENT
compact to the very surface as a result of the winter
rain and snow, and are therefore in condition for the
most rapid loss of water. They should be plowed as
early as practicable without injury to their structure.
At the Wisconsin Experiment Station 1 two adjacent
pieces of land very uniform in character were plowed
seven days apart. At the time when the second plot
was plowed, it was found to have lost 1.75 inches of
water from the surface four feet in the previous seven
days ; while the piece plowed earlier had actually gained,
doubtless by increased capillarity, a slight amount of water
over what it had contained when plowed. There was a
conservation of nearly two inches of water in the root zone
as a result of plowing one week earlier enough to
produce 1500 pounds of dry matter in maize to the acre,
if properly utilized.
202. Rolling. Very often in the spring, when the
seed bed is very loose, rolling is resorted to, in order to
bring about a compaction of the soil. At the same
time capillarity is established with the firmer earth
beneath, and as the moisture moves upward a rapid
germination of the seed is induced. Care must be taken
that this capillarity be checked once it has performed
this office, as great losses from evaporation may occur
at the surface and the crop be robbed of much available
water. It is an economic procedure in such cases to
follow the roller after a few days with a harrow, in order
that a mulch may be established and this loss checked.
203. Shelters. Shelters of any kind, whether natural
or artificial, tend to break the wind velocity and thereby
check losses by evaporation. Strips of timber are coin-
* King, F. II. , The Soil, p. 189. New York. 1906.
CONTROL OF SOIL MOISTURE 285
monly grown or retained for this purpose. Wooden fences
and walls of one sort or another have a similar effect.
Windbreaks composed of growing plants have the dis-
advantage that for a considerable distance beyond the
spread of their branches their roots penetrate the soil
and use the moisture, w r hich is one reason for the smaller
growth of crops near trees. Bearing on the efficiency of
windbreaks, results by King 1 show that when the rate
of evaporation at twenty, forty, and sixty feet to the
leeward of a black oak grove fifteen to twenty feet high
was 11.5, 11.6, and 11.9 cubic centimeters, respectively,
from a wet surface of twenty-seven square inches, the
evaporation was 14.5, 14.2, and 14.7 cubic centimeters,
at two hundred and eighty, three hundred, and three
hundred and twenty feet distant or 24 per cent greater
at the outer stations than at the inner ones. A scanty
hedgerow reduced evaporation 30 per cent at twenty
feet and 7 per cent at one hundred and fifty feet, below
the evaporation at three hundred feet from the hedge.
Very often tent shelters are used in the growing of
tobacco. The commonest form of the tent is a frame
eight or nine feet high, over which is spread a loosely
woven cloth. Investigations by Stew r art 2 in Connecticut
showed : (1) That the tent greatly reduced the velocity
of the wind. This reduction amounted to 93 per cent
when the outside velocity was seven miles an hour, and
85 per cent when the outside velocity was twenty miles
an hour, there being a small regular decrease in relative
efficiency with increased velocity of the wind. (2) The
relative humidity under the tent was higher than outside,
1 King, F. IT. The Soil, p. 205. New York. 1906.
2 Stewart, J. B. Effects of Shading on Soil Conditions.
U. S. D. A., Bur. Soils, Bui. 39. 1907.
286 SOILS: PROPEBTIES AND MANAGEMENT
and during a good part of the time attained a difference
of 10 per cent. The effect of this was to reduce evapora-
tion by from 53 to 63 per cent on different days in July,
in spite of a higher temperature inside the tent. (3) The
direct effect of this was to increase the moisture content
in the soil in spite of a larger crop growth under the tent.
These differences are shown by the following curves (see
Fig. 45), which represent the percentage of water in the
soil to a depth of nine inches from June 13 to August 1.
PEG/OD OF O3SVAT/OM
" \
FIG. 45. Curves showing the percentage of moisture in a sandy soil to
the depth of nine inches inside and outside of a loosely woven tent
over a period of about fifty days. Heavy line, moisture inside of
tent ; broken line, moisture curve of soil outside of tent.
Not only was the tent effective in preventing evapora-
tion and thereby increasing the average moisture content
of the soil, but the soil was able to maintain a more uni-
form content, due to the freer movement and adjustment
of the capillary water under the tent conditions more
conducive to rapid crop growth.
204. Level cultivation. The velocity of the wind
next to the ground may be checked by ridging the soil.
It is doubtful whether this practice conserves moisture,
THE CONTROL OF SOIL MOISTURE 287
because a greater amount of surface is exposed over
which evaporation may take place. On the other hand,
wide experience, as well as investigation, indicates that,
for the conservation of water, level culture is better than
ridged culture. This principle has led to the gradual
abandonment of the practice of " laying by " corn and
potatoes with a high ridge. In all regions of deficient
rainfall, the best practice prescribes level tillage and a
fine, dry mulch, both of which are attained by the frequent
use of shallow-running small-tooth cultivators. Many
experiments have demonstrated the larger crop yields
to be obtained, on the average, from this practice.
205. Plants. Plants growing on the soil tend to
check evaporation from two causes (1) their shading
effects, and (2) the tendency of the roots to intercept
capillary water as it moves upward and to appropriate
it for plant growth. Plants, however, tend to intercept
a certain amount of rain and prevent its ever reaching
the soil. The amount of water wasted in this way by
forests ranges from 15 to 30 per cent. In general this
tendency just about offsets the saving that occurs from
shading.
206. Summary of moisture control. It is clearly seen
from the discussion of moisture control that the structural
condition of the soil is the secret of successful operation.
Uiiii-oit' and leaching are reduced by increased capillary
capacity, a structural relationship. Evaporation is
checked by a soil mulch, which depends for its effective-
IM-SS on its physical condition. Drainage, lime, addition
of organic matter, and tillage in perfecting granulation
function in increasing the ease and effectiveness with
which soil moisture may be controlled. It must be
dearly kept in mind that all such control is directed
288 SOILS: PROPERTIES AND MANAGEMENT
toward the regulation of the soil moisture in such a way
that an optimum water supply may be held constantly
in the soil during the growing season. If this can be
accomplished, the largest crop yields may be expected that
are possible under the existing fertility conditions of any
soil.
CHAPTER XIV
SOIL HEAT 1
NORMAL plant growth is practically suspended below a
temperature of about 40 F., while proper germination of
seeds does not proceed much below that temperature.
As a rule it is not desirable to place either seeds or plants
in ;i soil in which active growth does not take place almost
immediately, since certain molds and fungi, active at
lo\v temperature, may sap their vitality and ultimately
cause their destruction. The desirable chemical reactions
in the soil are checked to a certain extent by lack of heat,
while the important biological activities are greatly im-
peded, if not brought entirely to a standstill, when the
soil temperature approaches 32 F. Such functions as
the decay and putrefaction of organic matter, the forma-
tion of ammonia from simple humic bodies, the building-up
of this ammonia into the nitrate form, and the fixation
of the free nitrogen either by free-fixing or symbiotic
bacteria, depend on an optimum soil temperature.
A knowledge of the functions of heat, therefore, es-
pecially as to its relationship to plant growth and bac-
terial activities, becomes important; for the farmer can
to a certain extent control soil temperature. He is able
1 For bibliography of the literature of soil heat, see Bouyoucos,
(!. ). An Investigation of Soil Temperature and Some of the
Most Important Factors [nfliiciicin^ It. Michigan Agr. Exp.
Sta., 'lYrlinical IJul. 17, pp. 194-190. 1913.
U 289
290 SOILS: PROPERTIES AND MANAGEMENT
also to govern the time when his sowing and planting are
performed in such a way that the soil will be fitted, at
least as far as heat is concerned, for proper seed germina-
tion and plant growth.
207. Relation of heat to germination and growth. -
In order to show the exact relationship of heat to ger-
mination of seeds and to the growth of plants, the follow-
ing data from Haberlandt 1 are given. While these
tables are not exact, they show clearly the necessity of
careful control of temperature in the propagation of
plants :
THE RELATION OF TEMPERATURE TO THE GERMINATION OF
CERTAIN SEEDS (IN DEGREES FAHRENHEIT)
MINIMUM
OPTIMUM
MAXIMUM
Corn
49
93
115
Scarlet bean
49
93
115
Pumpkins
52
93
115
Wheat
41
84
108
Barley
41
84
99
THE RELATION OF TEMPERATURE TO THE GROWTH OF CERTAIN
PLANTS (IN DEGREES FAHRENHEIT)
MINIMUM
OPTIMUM
MAXIMUM
Wheat
32-40
77-88
88-98
Barley
Corn . . ....
32-40
40-51
77-88
88-98
88-98
98-111
Peas
Buckwheat
32-40
32-40
77-88
77-88
88-98
98-111
Melon
Pumpkin
60-65
51-60
88-98
98-111
111-122
111-122
1 Haberlandt, F. Die Oberen und Unteren Temperatur-
grenze fiir die Keimung der Wichtigeren Landwirthschaftlichen
Samereien. Landw. Versuchs. Stat., Band 17, Seite 104-116.
1874.
SOIL HEAT 291
It is noticeable that there are here three groups of
plants as far as temperature conditions for optimum
growth are concerned. Wheat represents the crops that
uvrminute and grow at a relatively low temperature.
Corn requires a medium high temperature for proper
growth, while melons and pumpkins represent crops the
temperature requirements of which are very high. These
needs must be supplied for a proper development of such
plants, and must of course be considered in crop adapta-
tion as well as in soil management in general.
208. Chemical and physical changes due to heat. -
In the soil a certain amount of chemical action is going on,
no matter what the temperature may be ; but it is with-
out doubt true that this activity is greatly accelerated by
an increase in soil heat. This arises from two causes :
(1) because heat increases the solubility of the soil con-
stituents; and (2) because the activity of the soil or-
ganisms is stimulated to such an extent as to in
turn influence chemical reaction. The increased pro-
duction of carbon dioxide is a good example of this re-
lationship. The warming of .the soil in spring and
summer, therefore, by stimulating the amount of solu-
tion, increases to a marked extent the constituents avail-
able for plant growth.
The effect of temperature is less marked in a direct
way on the structure of the soil than on its chemical or
biological nature unless the freezing point is reached.
At this point, if moisture is present, the soil mass is dis-
rupted and may become rather granulated if the freezing
process is often repeated. The practice of fall-plowing
in order to better the tilth of the soil is really taking advan-
tage of this natural phenomenon. A change in tem-
perature also causes the expansion or contraction of the
292 SOILS: PROPERTIES AND MANAGEMENT
soil gases and may greatly facilitate their movement.
This is essentially a physical relationship. It must be
kept in mind, however, that with heat as with other soil
factors, no clear-cut and distinct discussion of its effects
in one direction may be made without considering the
indirect influences that are continually opening up avenues
which lead to phases more or less foreign to the one under
discussion. This serves to emphasize the close correla-
tion of the various factors and conditions that must be
dealt with in a study of soils.
209. Sources of soil heat. The soil may receive
heat directly or indirectly from three general sources:
(1) from the sun, (2) from the stars, and (3) by
conduction from the heated interior of the earth.
The two last-named sources are so unimportant as
to warrant no further discussion, since the amount of
heat received by the soil therefrom is so small as to be
negligible.
The sun, then, either directly or indirectly supplies
all the heat and energy that make it possible for soils to
support vegetation. This heat is derived in various ways,
as follows :
(1) By direct radiation of rays, both of light and of
invisible heat. These rays when absorbed tend to raise
the temperature of the absorbing medium. This source
of heat is by far the most important and may be desig-
nated as the direct method of heat induction.
(2) A considerable amount of heat may be derived by
radiation and conduction from the atmosphere surround-
ing the earth. This heat has of course been originally
obtained from the sun and is passed on to the soil, the
length of the waves being somewhat changed in the transi-
tion. Clouds may sometimes serve as a blanket and shut
SOIL HEAT 293
in around the earth heat that would otherwise be entirely
lost so far as the soil is concerned.
(3) A certain amount of heat may be brought to the
soil by precipitation. A warm spring rain, by falling
on the earth and percolating into its subsoil, may be a
determining factor in crop growth. Although the
aggregate amount of heat added in this way is small,
the opportuneness of its application is of no small
importance. A warm rain often imparts an impetus
to plant growth which may be noticeable for many weeks
afterward.
(4) A large amount of heat is annually entrapped by
growing plants. This energy is stored up and may ulti-
mately be liberated by the decay of the tissue. If such
oxidation takes place in the soil, as it very largely should
under good farm management, a certain amount of heat
is liberated in the soil. How important this is it is
difficult to say, for such energy is given off so grad-
ually as to be rendered difficult of measurement. Bac-
terial activity is very closely allied to the utilization
of such heat. Except under exceptional conditions, as
in hotbeds or very heavily manured lands, such heat
has no appreciable effect in altering the temperature of
a normal soil.
210. Factors affecting soil temperature. The tem-
perature that the soil of any given locality may attain
is dependent on a certain group of factors so closely re-
lated as to make their separate consideration sometimes
rather difficult. For convenience these factors may be
listed as follows, the actual temperatures and their
probable fluctuations under field conditions being re-
served until the various intrinsic and external factors of
soil heat have been discussed :
294 SOILS: PROPERTIES AND MANAGEMENT
1. Specific heat
2. Absorption
3. Radiation
4. Conductivity and convection
5. Evaporation of moisture
6. Organic decay
7. Slope
8. Heat supply and its effects
211. Specific heat. The specific heat of any material
may be defined as its thermal capacity as compared with
that of water. It is the ratio of the quantity of heat
required to raise the temperature of a given weight of
the substance one degree Centigrade to the quantity
needed to change an equal weight of water from 19.5
to 20.5 Centigrade. A knowledge of the specific heat
of soil is important because of the general light it sheds
on the warming-up of a soil in spring and on its rate of
cooling in autumn. The data from a number of investi-
gations, in the order of their priority, is here quoted,
the calculations being based on dry soil :
WEIGHT SPECIFIC HEAT OF SOILS
Pfaundler 1 (1866) Liebenberg 2 (1878)
Fine sand . . . .1923 Coarse sand . . .1920
Alluvial soil . . .2507 Diluvial loam . .2250
Granite soil . . .3489 Fine loam . . . .2770
Humous soil . . .4143 Humous loam . .3290
Peat 5069 Granite soil . . .3880
1 Pfaundler, L. Ueber die Warme Capacitat Verschiedener
Bodenarten und deren Einfluss auf die Pflanze. Ann. d.
Physik u. Chemie, Band 2Q5, Seite 102-135. Leipzig, 1866.
2 Liebenberg, R. von. See Lang, C. Ueber Warme Capa-
citat der Bodenconstituenten. Forsch. a. d. Gebiete d. Agri.-
Physik, Band I, Siete 118. 1878.
-SOIL HEAT 295
Lang* (1S78) Patten 2 (1909)
Coarse sand . . .1980 Norfolk sand . . .1848
Limestone soil . .2490 Podunk fine sandy
Humous soil . . .2570 loam 1828
Garden soil . . .2670 Hagerstown loam .1914
Peat 4770 Leonardtown loam .1944
Galveston clay . .2097
Bouyoucos 3 (1913)
Sand 1929
Gravel 2045
Clay 2059
Loam 2154
Peat 2525
212. Variations of specific heat. These figures show
a considerable amount of variation, part of which is of
course due (1) to inaccuracies in the designation of the
materials used, (2) to differences in methods, and (3)
to differences in technique. Allowing for these probable
errors, there still seem to be other factors involved. One
of these might be texture, since, according to the earlier
investigators, the finer mineral soils seem to possess a
higher specific heat. The data of Bouyoucos and Patten,
however, seem to controvert this assumption. An in-
vestigation more to the point is that of Ulrich. 4 In work-
1 Lang, C. Ueber Warme Capacitat der Bodenconstitu-
enten. Forsch. a. d. Gebiete d. Agri.-Physik, Band I, Seite
109-147. 1878.
2 Patten, H. E. Heat Transference in Soils. U. S. D. A.,
Bur. Soils, Bui. 59, p. 34. 1909.
3 Bouyoucos, G. J. An Investigation of Soil Temperature.
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 12. 1913.
4 Ulrich, R. Untersuchungen iiber die Warmekapazitat
der Bodenkonstituenten. Forsch. a. d. Geb. d. Agri.-Physik,
Band 17, Seite 1-31. 1894.
296 SOILS: PROPERTIES AND MANAGEMENT
ing with various grades of quartz sand he obtained
practically identical specific heats with the various sepa-
rates :
SPECIFIC HEAT OP VARIOUS GRADES OF SAND AS FOUND BY
ULRICH
Diameter of Sands in Millimeters Specific Heat
2- 1 .1912
1-.5 .1908
.5-.25 .1922
.25-171 .1919
.171-.114 .1919
.114-.071 .1904
.071-.010 .1890
It is evident, therefore, not only that texture has no
very great direct effect on specific heat, but also that
the controlling factor in the data already quoted is the
composition of the soil. The predominate minerals
found in soils possess a specific heat of from .180 to .220. *
This rather narrow range would normally be still further
lessened, since an average soil is a complex of the
different minerals. Humus, then, possessing a -specific
heat of about .5 must, when added to any soil, in-
crease markedly its thermal capacity and would un-
doubtedly be the determining factor in weight specific
heat of the mixture.
213. Specific heat based on volume of soil. Under
normal conditions, however, the soil contains a consider-
able amount of pore space, and different soils would
1 Ulrich, R. Untersuchungen liber die Warmekapazitat
der Bodenkonstitenten. Forsch. a. d. Geb. d. Agri.-Physik,
Band 17, Seite 1-31. 1894.
SOIL HEAT
297
therefore show different weights to the cubic foot. A
specific heat comparison based on weight, therefore, does
not yield a fair idea of the heat capacities of two soils.
The multiplication of the weight specific heat by the
apparent specific gravity of the soil in question will
obviously yield a volume specific heat, which is a
much more rational basis for comparison. A quota-
tion from Ulrich l makes clear the value of such a com-
putation :
SPECIFIC HEAT OP SOIL EXPRESSED BY WEIGHT AND BY VOL-
UME OF SOIL
APPARENT
SPECIFIC
GRAVITY
SPECIFIC HEAT
BY WEIGHT
SPECIFIC HEAT
BY VOLUME
Sand
1 52
1909
2901
Clay
1 04
2243
2333
Humus ....
.37
.4431
.1639
It is evident that in the first case the specific heat is
governed by the organic content of the soils in question;
the greater the amount of organic material present, the
higher is the thermal capacity. Such is not the case when
the specific heat of the soil is calculated on a volume basis.
In an expression of the thermal capacity on this rational
basis, namely, that of volume, the apparent specific grav-
ity, or volume weight, is the dominant factor. The ad-
dition of humus when this method of expression is em-
ployed merely serves to lower the volume weight, and
1 Ulrich, R. Untersuchungen iiber die Warmekapazitat der
Bodenkonstituenten. Forsch. a. d. Geb. d. Agri.-Physik,
Band 17, Seite 1-31. 1894.
SOILS: PROPERTIES AND MANAGEMENT
a reduction of specific heat thereby occurs. Under such
conditions more heat is necessary to raise the temperature
of the sand than is the case with the weight expression.
This is because of its high apparent specific gravity. The
clay shows very little change, as its apparent specific
gravity is about one; but the humus exhibits a marked
falling-off, due to its exceedingly low volume weight.
The factor that tends to vary the specific heat of dry
soil under natural conditions, therefore, is the apparent
specific gravity, or the volume weight. By deep and
efficient plowing the farmer may encourage the warm-
ing of his soil, due to a lowered thermal capacity. By
increasing its humus content he may attain the
same result, since the volume weight is depressed to
a markedly greater extent than the specific heat is in-
creased by the addition of organic matter. In fact, any
operation on or any addition to the soil that will vary
its apparent specific gravity will in turn affect the specific
heat.
214. Effect of water on specific heat. One other
factor, much more potent than the two already men-
tioned, is yet to be discussed. This factor is water, so
universally present in soils and of the greatest importance
in all natural soil phenomena. As the specific heat of
water is very high compared with the thermal capacity
of the soil constituents, any addition of it must naturally
raise the specific heat of a normal soil. That moisture,
not apparent specific gravity nor organic content, is the
controlling factor is demonstrated from the following
data, calculated by Ulrich 1 on a volume basis : -
1 Ulrich, R. Untersuchungen tiber die Warmekapazitat der
Bodenkonstituenten. Forsch. a. d. Geb. d. Agri.-Physik, Band
17, Seite 27. 1894.
SOIL HEAT
299
THE EFFECT OF SOIL MOISTURE ON THE VOLUME SPECIFIC
Hi; AT OF SOIL, THE MOISTURE BEING EXPRESSED AS A PER-
CENTAGE OF THE TOTAL WATER CAPACITY
t ~
figg
IM
.6755
.8458
.9449
Quartz 1.2919
Kaolin
Humus .1647
.3300
.L'<)ir>
.2427
.3682
.3558
.3207
.4063
.4170
.3987
.4445
.4783
.4767
.4826
.5395
.5548
.5208
.6008
.6328
.5589
.6620
.7108
.5972
.7233
.7888
.6353
.7812
.8669
It is at once evident, from these data and the accom-
panying curves (see Fig. 46), that moisture, in its effect
on the specific heat of an average soil, is so potent as to
1US
To go
30 4.0
60 70
60
FIG. 40. C'virvti.s showing the effect of moisture on the volume specific
heat of soils of different texture and humus content.
300 SOILS: PROPERTIES AND MANAGEMENT
entirely obscure in most cases the variations due directly to
such factors as apparent specific gravity and humus con-
tent. Organic matter, because of its high water capacity,
usually accentuates the dominance of moisture in this
respect. While a humous soil of low volume weight may
warm up most easily when dry, its high water content may
so increase its thermal capacity as to markedly retard
its temperature changes. This is exemplified by Petit l
and Bouyoucos 2 in their study of frost penetration in
peat. This soil was the last to freeze in winter and,
conversely, the last to thaw in spring. The advantage
of removing excess water by drainage is of importance
from this standpoint, as a wet soil is necessarily a colder
soil in spring than one that is well drained. This at least
partially accounts for the fact that a sandy soil is usually
an early one, and is therefore of particular value in truck-
ing operations.
215. Absorptive power of soils for heat. The greater
proportion of the heat received by the soil is obtained
by direct radiation from the sun. This radiant heat is
propelled by free wave action in the ether, the space
intervening between the sun and the earth being but
little affected by the transfer. Were the total amount
of heat received from a vertical sun by any unit surface
wholly absorbed by a layer of soil twelve inches thick,
the temperature of the soil would rise thirty degrees
Fahrenheit an hour. Such is not the case under normal
1 Petit, A. Untersuchungen iiber den Einfluss des Frostes
auf die Temperaturverhaltnisse der Boden von Verschiedener
Physikolischer Beschaffenheit. Forsch. a. d. Geb. d. Agri.-
Physik, Band 16, Seite 285-310. 1893.
2 Bouyoucos, G. J. An Investigation of Soil Temperature.
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 214. 1913.
SOIL HEAT 301
conditions, however, as the atmosphere continuously
refracts, reflects, and absorbs a certain amount of this
radiant energy. More important still are certain inher-
ent qualities of the soil itself which function materially
in the modification of the amount of heat absorbed. These
intrinsic factors are color, reflection, texture, and structure.
216. Effect of color on absorption of heat. (See Fig.
47.) In a natural soil it is very difficult to effect a change
in soil color without changing the texture, structure, and
more particularly the constitution, of the particles. In
order to eliminate these disturbing factors in a study of
heat, a quartz sand colored with various dyes was used
by Bouyoucos. 1 The following data, taken at Lansing,
Michigan, on a clear, warm day in August, illustrate
the general effects of color on absorption :
EFFECT OF DIFFERENT COLORS ON HEAT ABSORPTION BY
QUARTZ SAND, AUGUST 5, 1.30 P.M.
Color Temperature
(Degrees Centigrade)
Black 37.6
Blue 36.7
Red 35.9
Green 34.7
Yellow 32.6
White 31.7
It is quite evident that the darker the soil, the greater
is its absorptive power. This is because of differences
in reflection, a light-colored soil reflecting more of the
heat rays than one of a darker color. There might be a
question here as to the difference in radiation arising from
1 Bouyoucos, G. J. An Investigation of Soil Temperature.
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 31. 1913.
302 SOILS: PROPERTIES AND MANAGEMENT
color, the white soils radiating more heat than the black
ones. The following data from Bouyoucos, 1 substantiating
those of Lang, 2 are a conclusive negative answer to such
a query :
RADIATION OF DIFFERENT-COLORED SANDS, WHITE BEING
TAKEN AS 1.00
White 1.000
Black 991
Blue 981
Green 981
Red " .991
Yellow 989
The addition of organic matter, provided its decay has
been of the proper sort, will consequently always raise
the soil temperature, other factors of course being equal.
Wollny, 3 in experimentation with soils covered with thin
layers of different-colored material, found marked dif-
ferences under field conditions. The black soil not only
exhibited the -highest temperature, but also showed a
greater amount of fluctuation. The minimum tempera-
tures of the different-colored soils were almost the same.
The temperature differences of course decreased with
depth. Some typical data obtained on a clear day, as
quoted from Wollny's work, are as follows :
1 Bouyoucos, G. J. An Investigation of Soil Temperature.
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 30. 1913.
2 Lang, C. tiber Warme-absorption und Emission des
Boden. Forsch. a. d. Gebiete d. Agri.-Physik, Band I, Seite
379-407. 1878.
3 Wollny, E. Untersuchungen iiber den Einfluss der Farbe
des Bodens auf dessen Erwarmung. Forsch. a. d. Geb. d.
Agri.-Physik, Band I, Seite 43-69. 1878. Also, Untersuch-
ungen iiber den Einfluss der Farbe des Bodens auf dessen Erwar-
mung. Forsch. a. d. Geb. d. Agri.-Physik, Band IV, Seite
327-365. 1881.
SOIL HEAT
303
: NATURES OF DiFFERENT-COLORED SOILS AT A DEPTH OF
. TAKEN' Jl.VE 23, 1876, AT MUNICH (iN DEGREES
. i'KiUADE)
TIME
AIR
BLACK
WHITE
Midnight ....
2 \ M
9.6
100
13.8
124
13.8
124
4
76
107
108
6
16.0
9.6
9.6
8
10 . .
19.8
230
10.4
157
10.9
13 8
Noon
2 P M
25.4
254
22.1
268
17.6
21 2
4
24.8
294
23 6
6
8
10
22.6
19.4
16.1
28.8
27.2
24.0
24.0
23.6
21.6
WV/TE
J
FIG. 47. Curves showing the temperature variation of different-colored
soils at a four inch depth compared with air temperature. Munich,
June 23, 1876.
304 SOILS: PROPERTIES AND MANAGEMENT
Besides the quite obvious effect of the dark color on
the rate of heat absorption, two other points are worthy
of notice. The first is the tendency of the soil tempera-
ture to lag behind the temperature of the air, and the
second is the almost equal minimum reached by the two
soils. The latter point would seem to indicate also that
color had little differential effect on the heat lost from
the soil by radiation into the air.
217. Effects of texture and structure on heat absorp-
tion. Ordinarily the texture and the structure of a
soil, other conditions being equal, have little direct
influence on rate of absorption. Wollny 1 found with
dry and moist soil that the coarser the particles, the higher
is the temperature during warm weather. A loose, open
structure was always more favorable for high tempera-
tures than one more finely pulverized. Wollny's tem-
perature differences, however, were very small, and it is
probable that the experimental error, particularly due
to lack of moisture control, was greater than the observed
differences. Under normal conditions the practical effects
arising from the influence of texture and structure on
rate of absorption are probably entirely eliminated by
other factors. The importance of texture and structure,
as will be shown later, is in the direction of the control
of soil heat through their influence on soil moisture.
Moisture in turn is a potent factor in the ultimate soil
temperature, as it influences specific heat, radiation, and
evaporation to such an extent.
218. Radiation of heat by soil. The principal loss
1 Wollny, E. Untersuchungen tiber den Einfluss der Struk-
tur des Bodens auf dessen Feuchtigkeits- und Temperatur-
verhaltnisse. Forscli. a. d. Geb. d. Agri.-Physik, Band V,
Seite 145-209. 1882.
SOIL HEAT
305
of heat by the soil is through radiation, this radiation
being controlled by certain factors of which moisture
content, soil mulches, artificial coverings, shelters, and
clouds are the most important. Color as a factor in
radiation has already been eliminated by the work of
Bouycucos and Lang. The effects of texture and struc-
ture have also been investigated by these authors, as
well as by other physicists. The general results seem
to indicate that unless a dry soil is dealt with these factors
may be eliminated from consideration as far as their
direct practical effect on radiation is concerned. Of
course, indirectly through their influence on such factors
as moisture, they are of extreme importance.
An increase in the moisture of a soil has the general
effect of heightening the radiation ratio. This, together
with the effects of evaporation and of increased specific
heat, accounts for the fact that an undrained soil in spring
is a cold soil. Bouyoucos 1 found the following relation-
ships between moist and dry soils : -
EFFECT OF MOISTURE ON RADIATION
SOIL
PERCENTAGE OF
MOISTURE
RADIATION OF
MOIST SOIL
RADIATION OP
DRY SOIL
Gravel
Sand
Clay
Loam
4.7
5.3
17.2
258
100
100
100
100
92.4
93.1
91.9
909
Peat
849
100
86 1
Mulches, either natural or artificial, tend to check the
loss of soil heat through their covering effect and their
1 Bouyoucos, G. J. An Investigation of Soil Temperature.
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 34. 1913.
306 SOILS: PROPERTIES AND MANAGEMENT
influence on radiation. As a mulch is usually dry, its
radiant power is lower than that of the moist soil beneath.
Shelters decrease radiation by checking air movement.
The vegetation growing on soil also lowers radiation
through its covering effect, although the temperature of
soils covered with vegetation is usually low in summer
due to the obstruction of the sun's rays. Clouds, by
shutting in the heat, tend to check radiation and in many
cases prevent a frost that would otherwise occur. The
protecting effect of snow is well illustrated from the fol-
lowing data, taken from Boussingoult :
EFFECT OF SNOW ON SOIL TEMPERATURE. 1 (TEMPERATURE IN
DEGREES CENTIGRADE)
DATE AND HOUR
AIR
ON
SNOW
UNDER
SNOW
Feb
11
f>
P M
4- 2.5
1 5
00
Feb.
Feb
12,
13
7
7
A.M.
AM. ...
- 3.0
3 8
- 12.0
8.2
- 3.5
- 2.0
Feb.
13,
5
30 P.M
+ 4.5
- 1.0
0.0
One of the important features of soil heat radiation is
its effect on air temperature. As the radiant energy from
the sun passes through the atmosphere, very little of the
heat is appropriated, due to the wave lengths. But,
as this energy is radiated from the soil, the heat waves
have become shortened and are readily taken up by the
atmosphere, particularly if the latter is moist. How-
ever, as the air is always in motion its heat is not con-
trolled by the soil radiation of any particular locality.
1 Warington, R. Lectures on Some of the Physical Proper-
ties of Soils, p. 159. Oxford. 1900.
SOIL HEAT 307
In fact, the soil may be warmed by conduction of heat
from air to soil. This probably occurs to some extent
in spring, when the air is growing warmer, due to low
specific heat and its movement. The changes in air
temperatures are always more rapid and usually greater
in range, due to the factors cited above.
219. Conductivity and convection of heat in soils. -
While radiation has to do with the transfer of heat by
ether waves, conductivity is a term used in relation to
molecular transmission of energy through the body under
investigation. It may be defined as the amount of heat
in calories that will pass across a cube of unit edge (1
centimeter), in unit time (1 second) under a temperature
gradient of 1 degree Centigrade. Convection refers to
the transmission of heat by actual apparent and visible
movements of matter. It is to these two modes of trans-
fer that we owe the possibility of the soil's warming below
a surface that receives most of its heat as radiant energy.
It must be remembered that in studying the soil we are
dealing with a material made up of mineral and organic
compounds and always containing, under normal condi-
tions, a certain quantity of water. Air likewise is always
present, which, while a poor conductor of heat, may carry
energy by convection. Besides these varying substances,
often in loose contact and usually containing air capable
of considerable movement, there is bound to occur a
certain amount of transfer resistance which is the heat
resistance found at the boundary of two substances in
contact. The study of heat movement downward through
a soil is difficult to analyze, since it is almost impossible
to control the factors concerned while varying any one.
In a normal soil this heat movement occurs through both
the agency of conduction and that of convection, depend-
308 SOILS: PROPERTIES AND MANAGEMENT
ing on the texture and structure of the soil and the amount
of moisture present.
220. Measurement of conductivity. Ordinarily the
conductivity of a soil is measured by applying a constant
source of heat as quickly as possible and measuring the
change in temperature by means of thermometers set
in the soil at regular intervals. (See Fig. 48.) The soil in
question should be homogeneous in composition and of
uniform compaction, and should contain a definite mois-
ture content. It should of course be at a temperature
equilibrium before the heat is applied. Ordinarily radia-
tion and convection currents are diminished somewhat
by inclosing the soil in an insulated compartment. The
study of heat movement downward instead of laterally
is to be recommended, in order that unnecessary air
circulation may be avoided to some extent.
FIG. 48. Longitudinal section of apparatus for the study of heat con-
ductivity of soil. (C), water at constant temperature; (0, ther-
mometer; (P), copper plates; (F), screw clamp for pressing soil
firmly against source of heat ; (r) , skids for soil box.
221. Effect of texture on conductivity of heat. The
conductivity of a soil is affected by a number of factors
SOIL TIE AT 309
which may or may not lend themselves to modification
in the field. From the fact that type is of primary im-
portance in choosing a soil, texture in its relation to con-
ductivity might be considered first. From the work of
\YaicntT 1 and Potts 2 it is clearly established that the
coarser the texture of a soil, the faster the rate of conduc-
tion of heat will be, other factors remaining constant.
1 >ata quoted from the findings of Bouyoucos 3 substantiate
these results :
CONDUCTIVITY OF VARIOUS SOILS AS MEASURED BY THE TIME
i IRED FOR A THERMOMETER 7 INCHES FROM THE SOURCE
OF HEAT TO SHOW A RISE IN TEMPERATURE
.j Relative Rate of
Conductivity
Sand 1.00
Loam 1.81
Clay 1.77
Peat 4.61
Such results as these are only comparative and qualita-
tive. The difficulties of quantitative determinations are
so beset by error that only one investigator has as yet
made any consistent attempt along this line. Patten, 4
who has prosecuted such an investigation, finds that such
work may be vitiated by thermometer spacing, size of
thermometer, error in readings, moisture control, and
1 Wagner, F. Untersuchungen iiber das Relative Warme-
leitungsvermogen Verschiedner Bodenarten. Forsch. a. d.
Oeb. d. Agri.-Physik, Band VI, Seite 1-51. 1885.
2 Potts, E. Untersuchungen Betreffend die Fortpflan-
zung der Warme in Boden durch Leitung. Landw. Ver. Stat.,
Band XX, Seite 273-355. 1877.
3 Bouyoucos, G. J. An Investigation of Soil Temperature.
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 20. 1913.
4 Patten, II. K. Heat Transfer in Soils. U. S. D. A., Bur.
Soils, Bui. 51). 1909.
310 SOILS: PROPERTIES AND MANAGEMENT
the necessity of taking time-temperature curves in the
unsteady state. His results, expressed as metric K (the
heat conductivity coefficient in C. G. S. units), show the
same general comparisons as already presented :
HEAT CONDUCTIVITY OF DIFFERENT SOILS
K in C.G.S. units
Soil (See Definition of
Conductivity)
Coarse quartz 000917
Leonardtown loam . . . .000882
Podunk fine sandy loam . . .000792
Hagerstown loam 000699
Galveston clay 000577
Muck 000349
222. Effects of humus and structure on conductivity. -
A disturbing factor always present when soils are used
in the determination of the effect of texture on conduc-
tivity, is humus. It is evident, in dry soil at least, that
an increase in the organic content of a soil means a lower-
ing in conductivity. Humus, therefore, must be listed
as a second factor tending to vary the movement of heat
through soils. A third factor is the structural condition
of the soil under examination. Wagner 1 has shown in
this regard that the more compact a soil, the faster is
the conduction of heat. This is probably due to the more
intimate contact of the soil grains, and a consequent
cutting-down of the insulation factors and diminution
of the transfer resistance.
223. Influence of moisture on heat conductivity in soil.
The greatest single factor to be considered in conduc-
1 Wagner, F. Untersuchungen iiber das Relative Warme-
leitungsvermogen Verschiedner Bodenarten. Forsch. a. d.
Geb. d. Agri.-Physik, Band VI, Seite 1-51. 1885.
SOIL HEAT
311
tivity study, however, is the moisture content of the soil.
The following curve for quartz powder, taken from Pat-
ten's * work, illustrates its effect and shows how its influ-
ence may heavily override the factors already mentioned.
FIG. 49. Effect of moisture upon the apparent specific volume, heat
conductivity, and diffusivity of coarse quartz powder.
1 Patten, H. E. Heat Transfer in Soils. U. S. D. A.,
Bur, Soils, Bui. 59, p. 30. 1909.
312 SOILS: PROPERTIES AND MANAGEMENT
At first glance it appears peculiar that the heat move-
ment through a soil, the mineral constituents of which
possess a conductivity coefficient of about .01066, should
be raised by the addition of a liquid possessing a value of
K of about .00149, a conductivity about one-seventh of
the soil minerals. The explanation of this as given by
Patten is a lowering of the transfer resistance. He has
calculated that heat will pass from soil to water approxi-
mately one hundred and fifty times more easily than
from soil to air. This being true, it is evident that as
the water is increased in any soil and the air decreased,
the conductivity coefficient increases. It must be kept
in mind, however, that as the moisture increases, the
total amount of heat necessary to raise this soil to a given
temperature must also be increased. The necessity for
the maintenance of a medium moisture content in any
soil becomes apparent, although the conductivity may
not thereby be at its maximum. The curves in question
show that not only is there a change of volume weight,
but also there is a decrease in diffusivity with high water
percentages another reason for avoiding excessive
moisture contents in a field soil.
As has already been noted, the warming-up of a soil
becomes less and less rapid as the subsoil is penetrated.
This is not due to lessened conductivity, but rather to
a lessened heat supply. Bouyoucos 1 has shown that
under natural conditions the tendency of heat is to travel
downward more rapidly than laterally, due to a higher
moisture in the lower depths of the average field soil.
The time-temperature curves and the temperature gradi-
1 Bouyoucos, G. J. An Investigation of Soil Temperature,
Michigan Agr. Exp. Sta., Tech. Bui. 17, p. 25. 1913.
SOIL HEAT
313
ent for quartz powder as drawn by Patten l (see Figs. 50
and 51) illustrate the effect of distance on temperature
rise, the conductivity coefficient remaining constant,
50 lime inmtn.
FIG. 50. Temperature time curves for quartz powder at various dis-
tances from the source of heat.
From this brief discussion of conductivity it may be
established that such a movement is of importance to
plants in carrying heat downward into the soil. While
it is affected directly by tex-
ture, structure, and humus
to a certain extent, moisture
is the dominant factor. Under
natural conditions it is neces-
sary to maintain a medium
moisture content, although
the conductivity of heat is
not then at its maximum.
&
60
40
Zo
\
\
\
\
\
X
^
* l_
/ 2 3 4- s 6 jc/rr
U/sia.nce from source or neat
FIG. 51. Temperature gradi-
However, it must always be -cnt for air-dry coarse quartz
remembered that convection
is active under such conditions and may do much in
facilitating heat distribution. Good tilth and increased
organic content of any soil, by raising the optimum
1 Patten, H. E. Heat Transfer in Soils, U. S. D. A.,
Bur. Soils, Bui. 59, pp. 23-24. 1909.
314 SOILS: PROPERTIES AND MANAGEMENT
moisture content for plant growth, will place the soil in
the best possible condition, consistent with plant devel-
opment, for good heat movement.
224. Effect of evaporation of water on soil temperature.
- There is perhaps no factor, besides the loss of heat by
direct radiation, which exerts such an effect on soil tem-
perature as does evaporation. The fact that water does
not allow the long rays received by direct radiation to pass
readily through it accounts for its rapid vaporization.
This evaporation, caused by an increased molecular
activity, requires a certain expenditure of heat, result-
ing in a cooling effect on the water and consequently on
any material in close contact* with it. To evaporate a
pound of water requires the withdrawal of about 966
heat units. 1 This is sufficient to lower the temperature
of a cubic foot of saturated clay soil about 10 Fahrenheit.
The difference in temperature exhibited by wet and dry-
bulb thermometers measures the cooling effect of
evaporation.
Any condition that increases the rate of evaporation
lowers the temperature of the surface concerned. The
amount of water present is undoubtedly the controlling fac-
tor in this regard. King 2 found, in his study of a drained
and an undrained soil in April, that the- former maintained
a temperature ranging from 2.5 to 12.5 Fahrenheit
higher than the latter. Parks 3 records the same general
1 An English heat unit is the amount of energy necessary to
raise one pound of pure water from 32 to 33 F. It is equal to
about 778 foot-pounds.
2 King, F. H. Physics of Agriculture, p. 220. Published
by the author, Madison, Wisconsin. 1910.
3 Parks, J. On the Influence of Water on the Temperature
of Soils. Jour. Roy. Agri. Soc. Eng., Vol. 5, pp. 119-146.
1845.
SOIL HEAT 315
results in England. Wollny l finds a wet soil to be the
cooler in the daytime, the difference being roughly pro-
portional to the amount of water present. The effect
of the amount of water on the rate of evaporation is of
course influenced to a certain extent by texture, struc-
ture, and humus, since these factors exert such a marked
influence on water capacity and capillary movement.
The practical importance of a study of the effect of
evaporation on soil temperature lies in the fact that evap-
oration can be controlled to a certain extent under field
conditions. This is not so true, unfortunately, of radia-
tion and conduction. Thorough underdrainage is the
dominant operation in the prevention of cooling by
evaporation. By this removal of excess water the specific
heat is lowered, radiation is slightly retarded, and con-
vection is facilitated. This means a faster warming of
the soil, tending toward an optimum temperature rela-
tion as far as the plant is concerned. Optimum moisture
encourages optimum heat conditions, as well as other
favorable relations whether chemical, physical, or biologi-
cal. Drainage, lime, humus, and tillage figure in heat
control as well as in other phases of soil improvement.
225. Effect of organic decay on soil temperature. -
Besides the effect of organic matter on color and its conse-
quent influence on the absorption of heat, it may function
in another direction, namely, in producing heat of fer-
mentation. How far this liberation of heat under field
1 Wollny, !". Untersuchungen iiber den Einfluss des
Wassers auf die Boden. Forsch. a. d. Geb. d. Agri.-Physik,
Band IV, Soil.- 147-190. 1881. Also, Untersuchungen iiber
den Kin Muss der Oberflachlichen Abtrochnung des Boden auf
deasen Temperatur- und Feuchligkeitsverhaltnisse. Forsch. a. d.
Geb. d. Agri.-Physik, Band III, Seite 325-348. 1880.
316 SOILS: PROPERTIES AND MANAGEMENT
conditions is effective in bringing about any important
modification of soil temperature it is often difficult to
decide. In greenhouses and hotbeds perceptible increases
are obtained by the use of large quantities of fresh manure,
as high an increase as 7.5 degrees Centigrade has been
observed under such conditions. In the field, however,
where the absorption and radiation of heat are very large,
where the organic matter makes up only a fraction of the
soil's components, and where the applications of barnyard
manure are relatively small compared to the bulk of the
soil, it is doubtful whether any important increase of
soil heat actually occurs. Georgeson, 1 working in Japan
with varying quantities of manure, obtained during the
first twenty days an excess over the check of only 3.4
degrees Fahrenheit from an application of eighty tons an
acre. With twenty tons the increase was 1.7 degrees.
Wagner 2 obtained similar results, finding an average
excess of 1 degree Fahrenheit from the use of twenty
tons of barnyard manure. Bouyoucos 3 has obtained the
latest data on this subject. Under controlled laboratory
conditions he found that unless excessive amounts of
manure were applied no appreciable effects were observed.
With an application of ten tons the highest rise was one-
half degree Centigrade; after one hundred and three
days the manured soil was only one-fourth degree higher
than the untreated. Such results show that the heat of
fermentation has little important practical influence
1 Georgeson, C. C. Influence of Manure on Soil Tempera-
ture. Agr. Sci., Vol. I, pp. 25-52. 1887.
2 Wagner, F. Uber den Einfluss der Dungung mit Organ-
ischen Substance auf die Bodenteniperatur. Forsch. a. d.
Geb. d. Agri.-Physik, Band V, Seite 373-405. 1882.
3 Bouyoucos, G. J. An Investigation of Soil Temperature,
Michigan Agr. Exp. Sta., Tech. Bui. 17, pp. 180-190. 1913.
JSQZL HEAT 317
on soil temperature, so far as the total bulk is concerned.
There are without doubt certain localized influences,
both chemical and biological, but how important they
may be it is rather difficult to say. From what is known
at the present time it seems that organic matter exerts
its greatest temperature effects through the darkening of
the color and the increase in moisture capacity of the
soil.
226. Relation of slope to soil temperature. The rela-
tion of exposure to soil heat is the last phase to be con-
sidered, with the exception of meteorological factors,
which are external in their relationships rather than intrin-
sic as have been most of the phases already discussed.
The slope of a surface varies the amount of heat
absorbed from the sun, without affecting, of course, the
absorptive power of the surface involved. The greater
the inclination of a soil from a right-angle interception
of the heat rays, the less rapid will be its rise of tempera-
ture in a given unit of time, the source of heat remaining
constant. This is because the greater the inclination,
the greater is the amount of surface a given amount of
heat must serve. It is evident that a less amount of
heat will reach each unit of soil surface, and a conse-
quent slower rise in temperature of the soil so situated
will result. Under normal conditions, therefore, any
inclination that will cause a surface to approach a right-
angle interception of the sun's rays will not only increase
its rate of temperature rise but at the same time will
increase its average seasonal temperature. In the North
Temperate Zone this of course is a southerly inclination.
The following diagram, illustrating the conditions on the
42d parallel at noon on June 21, makes clear this rela-
tionship :
318 SOILS: PROPERTIES AND MANAGEMENT
FIG. 52. Diagram showing the proportional amount of heat received to
the unit area by different slopes on June 21, at the 42d parallel north.
It is seen that in this case a southerly slope of 20
receives to a unit area the greatest amount of heat, the
level soils and the soils having northerly inclination of
20 differing in the order named. The following table
shows the proportional amount of heat received by each
one of these soils per unit area at midday with such an
inclination of the sun's rays :
PROPORTIONAL AMOUNT OF HEAT RECEIVED PER UNIT AREA
BY DIFFERENT SLOPES ON JUNE 21, AT THE 42o PARALLEL
NORTH LATITUDE
20 Southerly slope = 106
Level = 100
20 Northerly slope = 81
SOIL HEAT 319
These figures show not only that the slope itself is impor-
tant, but also that the direction of the inclination must
play a part in the selection of land with its probable
temperature relationships borne in mind. The investi-
gations of Wollny, 1 which have since been corroborated
by King 2 and others, may be cited at this point as typi-
cal : -
AVF.RAGE TEMPERATURE AT 6 INCHES OP A HUMOUS SANDY
LOAM FROM APRIL TO OCTOBER, 1877, MUNICH, GERMANY
Temperature
in Degrees
Centigrade
South 14.46
Southeast 14.42
Southwest 14.42
East 13.99
West 13.98
Northwest 13.64
Northeast 13.56
North 13.52
1 Wollny, E. Untersuchungen liber den Einfluss der
Exposition auf die Erwarmung des Bodens. Forsch. a. d.
(Ich. d. Agri.-Physik, Band I, Seite 263-294, 1878; Unter-
suchungen iiber die Feuchtigkeits- und Temperaturverhalt-
nisse des Bodens bei Verschiedener Neigung des Terrains gegen
den Horizont. Forsch. a. d. Geb. d. Agri.-Physik, Band IX,
S-i to 1-70, 1886; Untersuchungen iiber die Feuchtigkeits-
u ml Temperaturverhaltnisse des Bodens bei Verschiedener
-'img des Terrains gegen die Htimmelsrichtung und gegen
den Horizont. Forsch. a. d. Geb. d. Agri.-Physik, Band X,
Scite 1-54, 1887; Untersuchungen iiber die Temperaturver-
hallnisso des Bodens bei Verschiedener Neigung des Terrains
gegen die Hummelsriohtung und gegenden Horizont. Forsch.
a. d. Geb. d. Agri.-Physik, Band X, Seite 345-364. 1887.
2 King, F. H. Physics of Agriculture, p. 218. Published
by the author, Madison, Wisconsin. 1910.
320 SOILS: PROPERTIES AND MANAGEMENT
Wollny found also that the soil temperature on the
southward slopes varied according to the time of year.
For example, the southeasterly inclination was highest
in the early season, the southerly slope during mid-season,
and the southwesterly slope during the fall. A southeast-
erly slope is usually preferred by gardeners. Orchardists
also pay strict attention to aspect, as it often is a factor
in susceptibility to sun scald and other diseases.
King, in comparing a red clay with a southerly slope
of 18 to that on a level on July 21, obtained the follow-
ing results :
TEMPERATURE IN DEGREES FAHRENHEIT OF RED CLAY AS
INFLUENCED BY SLOPE
FIKST FOOT
SECOND FOOT
THIRD FOOT
Southerly slope . . .
Level
70.3
672
68.1
654
66.4
63 6
3.1
2.7
2.8
It is apparent immediately that the influence of slope
is not confined to the surface, but, owing to conduction
and convection, is felt to a considerable depth. Slope,
therefore, together with moisture control, becomes a
dominant factor in the heat relations of a soil. This is
particularly true with specialized crops, with which the
early warming of the soil is important. A normally early
soil may become late because of exposure, or a naturally
late soil may become earlier due to an inclination south-
ward. Slope many times is a dominant factor in the
adaptation of crop to soil.
227. Heat supply and its effects. The direct heat
supply is without doubt the controlling factor in soil
SOIL HEAT
321
temperature, influenced, of course, by the conditions
already discussed. The effect of this heat supply is re-
flected in the seasonal, monthly, and daily soil tempera-
tures at the surface and at varying depths below. The
following data illustrate the differences that may ordi-
narily be expected to take place from season to season on
an average soil :
AVERAGE TEMPERATURE READINGS TAKEN AT BRESLAU, GER-
MANY. 1 AVKUAGB OF TEN YEARS, 1901-1910 (IN DEGREES
FAHRENHEIT)
AIR
1 INCH
DEEP
8
INCHES
DEEP
16
INCHES
DEEP
28
INCHES
DEEP
40
INCHES
DEEP
52
INCHES
DEEP
Winter . . .
29.4
28.2
33.3
34.9
37.1
38.7
40.6
Spring
Summer . . .
45.5
63.3
44.9
62.8
45.3
63.4
45.4
63.4
44.5
61.6
43.7
59.3
43.5
57.5
Autumn . . .
44.8
43.7
48.6
50.5
52.1
52.6
53.3
AVERAGE TEMPERATURE READINGS TAKEN AT LINCOLN, NE-
BRASKA. 2 AVERAGE OF TWELVE YEARS, 1890-1902 (IN
DEGREES FAHRENHEIT)
AIR
1 INCH
DEEP
3
INCHES
DEEP
6
INCHES
DEEP
12
INCHES
DEEP
24
INCHES
DEEP
36
INCHES
DEEP
Winter . . .
25.9
28.8
28.8
29.5
32.2
36.3
39.1
Spring . . .
Summer .
49.9
73.8
54.8
83.0
53.6
80.9
51.6
79.1
48.5
73.8
45.7
69.0
44.3
66.2
Autumn . . .
53.9
56.4
57.6
57.1
57.5
59.3
60.3
1 Schulze, B., and Burmester, H. Beobochtungen uber
Temperaturverhaltnisse der Bodenoberfliiche und verschiedener
Bodentiefen. Internat. Mitt, fur Bodenkunde, Band II, Heft
2-3, Seite 133-148. 1912.
2 Swezey, G. D. Soil Temperatures at Lincoln, Nebraska.
Nebraska Agr. Exp. Sta., 16th Ann. Rept., pp. 95-102. 1903.
322 SOILS: PROPERTIES AND MANAGEMENT
These average readings, taken at different points, are
supported by the data of other observers. 1 It is apparent
that seasonal variation of soil temperature is considerable,
even at the lower depths. The surface layers of soil seem
to vary nearly in accord with the air temperature, and
therefore exhibit a greater fluctuation than the subsoil.
In general the surface soil is warmer in spring and sum-
mer than the lower layers, but cooler in fall and winter.
The following data taken at Lincoln, Nebraska, may be
of interest :
AVERAGE MONTHLY TEMPERATURE READINGS 2 TAKEN AT LIN-
COLN, NEBRASKA. AVERAGE OP TWELVE YEARS.
AIR
1 INCH
3
INCHES
6
INCHES
9
INCHES
12
INCHES
24
INCHES
36
INCHES
DEEP
DEEP
DEEP
DEEP
DEEP
DEEP
January . .
25.2
27.3
27.8
28.6
30.0
31.2
35.4
38.5
February
24.2
27.7
27.3
27.8
28.3
30.2
33.5
35.5
March . .
35.8
38.2
37.2
36.6
35.6
35.4
35.4
35.8
April . . .
52.1
57.5
56.0
53.3
50.6
49.3
45.6
43.8
May . . .
61.9
68.7
67.5
65.1
63.3
60.7
56.2
53.3
June . . .
71.0
78.1
78.0
75.7
73.8
69.9
64.6
61.3
July . . .
76.0
85.1
83.6
81.6
79.3
75.7
70.2
67.4
August
74.5
82.9
81.3
80.1
78.5
75.7
72.2
69.8
September .
67.6
73.8
73.4
72.0
70.7
69.2
68.7
67.6
October .
55.5
56.7
58.4
57.8
58.3
57.8
60.0
61.3
November .
38.7
38.7
40.9
41.5
43.3
44.7
49.2
52.2
December
28.3
31.6
31.4
32.0
33.4
35.2
40.1
43.3
Average . .
50.9
55.5
55.3
54.6
53.8
52.9
52.6
52.5
Range . .
51.8
57.8
56.3
53.8
51.0
45.5
38.7
34.3
1 Ebermayer, E. Untersuchungen iiber das Verlialten
Verschiedener Bodenarten gegen Warme. Forsch. a. d. Geb.
d. Agri-Physik, Band 14, Seite 195-253. 1891.
2 Swezey, G. D. Soil Temperatures at Lincoln, Nebraska.
Nebraska Agr. Exp. Sta., 16th Ann. Rept., pp. 95-102. 1903.
SOIL HEAT
323
The upper soil layers vary in accordance with the air
temperature, the maximum and the minimum occurring
in the same month. A lagging (see Fig. 53) is apparent
in the subsoil, due to the slow response of this area to the
heat penetrating from above. These figures also show
the surface soil to be warmer in spring and summer, and
cooler in winter and fall, than the lower depths. The
surface soil not only never falls as low in temperature as
the air, but reaches a higher point in summer. This is
shown in the range of the air and soil temperatures. The
range for the air is 51.8, while that for the soil is 57.8,
56.3, 53.8, 51.0, 45.5, 38.7, and 34.3, respectively,
for the depths ranging from 1 inch to 36 inches. While
this range of soil temperature is greater in the aggregate
than that of the air, the changes are much slower and
often extend over a number of days, while the air may vary
many degrees in an hour.
36*
'. Curves showing the average monthly temperature readings at
various soil depths. Average of t\relve years, Lincoln, Nebraska.
324 SOILS: PROPERTIES AND MANAGEMENT
The daily and hourly temperature of the air and the
soil may be fairly constant or rather variable, according
to conditions. On days of sunshine, however, consis-
tent changes may be expected. The air temperature rises
from morning until about two o'clock, when the maxi-
mum is reached. It then falls rapidly. The soil, how-
ever, does not reach its maximum temperature until
later in the afternoon, due to the lagging so apparent in
soil temperature changes. This lagging is greater in
the lower layers than at the surface. The following data, 1
taken on a bright day on May 26 in Germany, illustrate
the ordinary differences that may be expected in soil
and air temperatures :
HOURLY TEMPERATURES TAKEN IN GERMANY ON MAY 26, 1884,
ON A LOAM SOIL AT 4-lNCH DEPTH (IN DEGREES FAHRENHEIT)
HOUR
AIR
BARE SOIL
JVlidnight
55.4
63.5
2 A.M
54.3
60.4
4 . .
52.7
58.5
6
67.6
57.0
8
76.4
58.4
10
82.0
63.3
Noon
83.5
69.8
2 p M
85.6
74.8
4 . .
84.2
77.9
6
78.1
77.7
8 . .
68.7
73.9
10
65.1
69.8
1 Wollny, E. Untersuchungen iiber den Einfluss der
Pflanzendecke und der Beschattimg auf die Physikolischen
Eigenschaften des Bodens. Forsch. a. d. Geb. d. Agri.-Physik.
Band 6, Seite 197-256. 1885.
SOIL HEAT
325
FIG. 54. Curves showing the hourly temperatures of bare soil at a
depth of four inches and of the air above the soil. , May 26, Germany.
The temperature of the soil at the surface may often
exceed that of the air, and the amount of daily fluctua-
tion may be greater ; but for the lower depths the tem-
perature curve flattens out. The subsoil shows but
little daily, and even monthly, variation, and is affected
only by seasonal changes.
228. Control of soil temperature. The means of
practical control and modification of soil temperature
are those commonly in vogue in good soil management.
'The most important factor is, of course, soil moisture.
Good drainage, proper tilth developed by deep plowing,
plenty of lime, and sufficient organic matter, favor opti-
mum moisture conditions. Such moisture regulation
means a lowered specific heat and good conductivity.
The use of a soil mulch or an artificial covering not only
will check evaporation but will markedly retard loss of
heat by radiation. Anv farmer who so controls his soil
326 SOILS: PROPERTIES AND MANAGEMENT
moisture that optimum conditions as far as the plant is
concerned may be obtained, should have no fear of a
poor utilization of heat.
The increase of soil humus, of course, may act directly
in heat control by darkening the color and increasing
absorption. A soil mulch, being dry, not only checks
evaporation but lowers radiation while increasing absorp-
tion. Any methods of handling the land which tend to
better the physical condition of the soil and increase its
tilth tend also toward a proper heat control at the same
time. The whole question may be summarized by say-
ing that if a farmer adopts a proper system of moisture
control and at the same time employs methods that tend
always toward a better physical condition of the soil,
the problem of control of soil heat will be automatically
solved. He will then have brought about the best condi-
tions for heat absorption and will have facilitated conduc-
tion and convection, while at the same time retarding
losses by evaporation and radiation.
CHAPTER XV
Al'AILABILITY OF PLANT NUTRIENTS AS
DETERMINED BY CHEMICAL ANALYSIS
FORTUNATELY for mankind, only an exceedingly minute
proportion of the soil is at any one time soluble in water
or in the aqueous solutions with which it is in contact.
It is this great degree of insolubility that gives the soil
its permanence, for in humid regions, without this property,
it would be rapidly carried away in the drainage water.
The portion of the soil that is soluble in the various natural
solvents with which it comes in contact furnishes mineral-
food materials for plants. The great mass of soil, which
is relatively insoluble, is constantly subjected to natural
processes which very slowly bring its constituents into
solution. The agents that are concerned in the decom-
position of rock also act on the soil to bring about its
further disintegration, and thereby render it more soluble ;
while added to these are the operations of tillage, which
contribute to the same end.
Only the surfaces of the soil particles come into contact
with the decomposing agents, and hence it is the surface
matter of the particles that gradually goes into solution.
The factors that determine how rapidly solution shall
proceed are : (1) the amount of surface exposed, which,
as has been seen, varies with the size of the particles ; (2)
the composition of the particles; (3) the strength of the
decomposing and solvent agencies. Were it not for this
827
328 SOILS: PROPERTIES AND MANAGEMENT
process, there would soon be no mineral food available
to plants, as drainage water and the growth of crops take
up relatively large quantities of these substances each
year; but in spite of this loss the soil is able to provide
at least some plant-food material for each crop, when
called upon by the plant.
229. Solubility of the soil in various solvents. For
purposes of analyses that are intended to show the amounts
of mineral plant-food materials in a soil, any one of sev-
eral different solvents may be used. These solvents differ
in strength, and consequently the percentages of the
various constituents obtained from samples of the same
soil are different for each solvent. A chemical analysis
of a soil is a determination of the quantities of the con-
stituents that have been dissolved in the solvent used.
Therefore it will readily be seen that the interpretation
of a chemical analysis must depend largely on the nature
of the solvent, and, unless the solvent is equivalent in
its action to some process or processes in nature, the results
must be entirely arbitrary.
The methods that have been used for obtaining solu-
tions of the soil for analysis may be grouped as follows :
1. Complete solution of the soil.
2. Partial solution with strong acids.
3. Partial solution with weak acids.
4. Extraction with water.
230. Complete solution of the soil. By the use of
hydrofluoric and sulfuric acids and by fusion with alkalies,
the entire soil mass may be decomposed and all its inor-
ganic constituents determined. 1 Such an analysis shows
1 Wiley, Harvey W. Principles and Practices of Agri-
cultural Chemical Analysis, Vol. 1, pp. 398-399. 1906.
AVAILABILITY OF PLANT NUTRIENTS 329
the total quantity of the plant-food materials except
nitrogen, which is never determined in any of the acid
solutions but by a separate process. 1 A deficiency of
any particular substance may be discovered in this way,
but nothing can be learned as to the ability of the plant
to obtain nutriment from the soil. A rock may show as
much mineral plant-food material as a rich soil. This
method of analysis is used only to ascertain the ultimate
limitations of a soil or its possible deficiency in any essen-
tial constituent. Results of such analyses are to be found
in paragraphs 46, 48, 52, 53 of this text.
231. Partial solution with strong acids. While sul-
furic, nitric, and hydrochloric acids have all been used as
solvents, 2 the one most commonly employed is hydrochloric
acid of 1.115 specific gravity. 3 It has been used to such
an extent that it may be considered the standard solvent,
and a statement of a chemical analysis of a soil in this
country may be considered as based on this solvent unless
otherwise stated.
1 Official and Provisional Methods of Analysis. U. S. D. A.,
Bur. Chem., Bui. 107 (revised), p. 19. 1908.
2 Analyses using concentrated mineral acids on the same soil.
From Snyder, Harry. Soils. Minnesota Agr. Exp. Sta., Bui.
41, p. 66. 1895.
HYDRO-
CHLORIC
NITRIC
SuiiPURIC
Total insoluble percentage .
Potash percentage
Lime percentage
81.20
0.42
55
83.45
0.30
30
80.45
0.52
53
Magnesia percentage ....
Phosphoric acid percentage . . .
Sulfuric acid percentage ....
0.40
0.23
0.08
o!s2
0.23
0.08
0.52
0.26
0.10
3 Official and Provisional Methods of Analysis. U. S. D. A.,
Bur. Chem., Bui. 107 (revised), pp. 14-18. 1908.
330 SOILS: PROPERTIES AND MANAGEMENT
An analysis by this method is supposed to show the
proportion of plant-food materials in a soil that are in a
condition to be ultimately used by plants at the time
when the analysis is made, and the plant-food materials
that are not dissolved by treatment with hydrochloric
acid are assumed to be in a condition in which plants
cannot use them. The difficulty with this assumption is
that, while treatment with hydrochloric acid of a given
strength marks a definite point in the solubility of the
compounds in the soil, it does not bear a uniform rela-
tion to the natural processes by which these compounds
become available to the plant.
In the case of most soils a large proportion is not de-
composed by treatment with strong hydrochloric acid,
and the portion that is dissolved may contain a larger or
a smaller quantity of the agriculturally important ele-
ments, depending on the character of the soil. Thus if
calcium is present as a phosphate, a larger proportion
will be dissolved by the acid than if it is in the form of
silicate. The form in which potassium occurs also in-
fluences greatly the amount shown by analysis.
Snyder l has analyzed a number of soils by means of
digestion with strong hydrochloric acid, and has then
decomposed the acid-insoluble residue by fusion and
determined its composition. Veitch 2 has analyzed soils
by the hydrochloric acid method and by means of com-
plete solution. A few examples are given below to show
how soils may vary in the solubility of their constituents
in strong hydrochloric acid :
1 Snyder, Harry. Soils. Minnesota Agr. Exp. Sta., Bui. 41,
p. 35. 1895.
2 Veitch, F. P. The Chemical Composition of Maryland
Soils. Maryland Agr. Exp. Sta., Bui. 70, p. 103. 1901.
AVAILABILITY OF PLANT NUTRIENTS 331
PERCENTAGE OF CONSTITUENTS NOT SOLUBLE IN HCL,
1.115 SP. GR.
SOIL FBOM MINNESOTA
SOIL PROM MARYLAND
Fair
Haven
Holden
Experi-
ment
Station
Colum-
bia
Chesa-
peake
Hudson
River
Shale
Potash . . .
94
81
83
95
67
73
Lime . . .
25
61
41
90
82
37
Magnesia . .
58
76
36
34
29
28
Phosphoric
anhydride .
40
45
18
66
15
Sulfuric anhy-
dride . .
74
90
20
~
~
~
232. Significance of a strong hydrochloric acid analysis.
- This method of analysis was originally thought to
give some indication of both the permanent fertility and
the immediate manurial needs of a soil ; but for each
question the accuracy of the deductions is limited by a
number of conditions that make it impossible invariably
to predict from an analysis how productive a soil may be
or what particular manure may be profitably applied.
It is very apparent that the chemical composition of a
soil is only one of the many factors affecting its produc-
tiveness. Unfortunately, not all the factors are under-
stood, and consequently these unknown ones cannot be
determined either qualitatively or quantitatively. If
it ever becomes possible to determine quantitatively all
the factors entering into soil productiveness in the field
condition, the problem will be solved.
233. Relation of texture to solubility. The ratio of
sand to clay in a soil, and the distribution of the fer-
tilizing materials in these constituents, will affect the mini-
332 SOILS: PROPERTIES AND MANAGEMENT
mum quantity of any constituent required to produce
a good crop. Hilgard has shown that the addition of
four or five volumes of quartz sand to one volume of a
heavy, but highly productive, black clay soil greatly
increased the productiveness, while diluting the potash
content of the mixture to 0.12 per cent and the phosphoric
acid to 0.03 per cent. It is evident that in this soil the
plant-food materials were in a condition to be easily
taken up by the plant when the physical condition of the
soil was suitable.
If these small quantities of food elements had been
distributed in the sand particles as well as in the original
clay, the result would doubtless have been different.
Suppose, for example, that fifty per cent of the potash
and phosphoric acid had been in the sand particles and
the remainder in the clay; in that case the former, in a
soil exposing much the less surface to dissolving liquids,
would be proportionately less soluble, and as the minimum
quantity is approached, as shown by the more dilute soil's
yielding less than the other, the effect would doubtless
have been to decrease the production. In some soils,
particularly those of arid regions, the larger particles
may carry much of the mineral nutrients, in which case
it is quite evident that a higher percentage of fertility
is required than in soils carrying the plant-food material
largely in the small particles.
234. Nature of the subsoil. The nature and com-
position of the subsoil is naturally a factor in determining
soil productiveness, and must be considered as well as
the top soil. An impervious subsoil, or a very loose
sandy one, will confine the productive zone largely to
the topsoil and hence require a greater proportionate
amount of fertility in that part of the soil.
AVAILABILITY OF PLANT NUTRIENTS 333
235. Calcium carbonate. A determination of the
amount of calcium present as a carbonate is important
as an aid to the interpretation of an analysis of the soil.
Lime not so combined is generally in the form of a sili-
cate, or possibly a phosphate. If there is a large quantity
of calcium carbonate in a soil, the potash, phosphoric
acid, and nitrogen are likely to be more readily soluble,
and smaller quantities are sufficient for crop growth, than
if the calcium is not found in this form. The effect of
the carbonate of lime on the nitrogen l compounds is to
furnish a base for the acids produced in the formation of
nitrates, and its presence promotes this process. It
probably replaces potassium in certain compounds where
otherwise it would be secured with more difficulty. It
insures the presence of some phosphates of lime, in which
form phosphorus is more soluble than when combined
with iron. The form of the manures to be used on the
soil will also depend in large measure on the presence
or the absence of calcium carbonate. For example,
where calcium carbonate is deficient, steamed bone or
Thomas slag are likely to be more profitable than super-
phosphate, and nitrate of soda than sulfate of ammonium.
Finally, the absence of calcium carbonate indicates the
need of liming, and if the analyses show a considerable
quantity of potash and phosphoric acid, but practice
shows these materials to be somewhat deficient, it is
probable that liming will be very beneficial, and that
manures carrying these substances will not be so essential
as the chemical analysis would indicate. It must be
stated, however, that there are cases for which these de-
ductions do not hold, owing to the intervention of other
factors.
1 Not determined in the hydrochloric acid extract.
334 SOILS: PROPERTIES AND MANAGEMENT
236. Deficiency of ingredients and manurial needs. -
Many standards have been set for the minimum quantity
of each of the important soil constituents that must be
present in order to insure a productive soil. Experi-
ence has shown, however, that no definite standards hold
for all soils. By comparing analyses of soils of known
productivity with that of a soil under investigation it is
an easy matter to ascertain whether the soil contains a
large quantity of each agriculturally important ingredi-
ent; but when the quantity of any constituent is low,
it becomes a difficult matter to tell how this will affect
the agricultural value of the soil. Some soils will be
productive with 0.05 per cent of phosphoric anhydride,
while others are unproductive when all the plant nutrients
are present in ample quantity.
The fact that the degree of productiveness of a soil
cannot always be gauged by its analysis gives rise to a
similar uncertainty with regard to its manurial needs.
A soil may contain potassium in very large quantities,
sufficient to produce crops for hundreds of years, as indi-
cated by a strong hydrochloric acid analysis, and yet a
potassium salt may be used with profit. On the other
hand, it is evident that as the content of any constituent
becomes less, the probable need for its application be-
comes greater, and a knowledge of the composition of
the soil thus suggests a practice without assuring its
success. An analysis of the hydrochloric acid extract,
therefore, cannot be taken as an infallible guide to the
fertilizer needs of a soil, and of itself should not be relied
upon; but in connection with other knowledge, particu-
larly that derived from fertilizer tests, it may be useful.
237. Partial solution with weak acids. The difficulty
in judging of the properties of a soil from the results of
AVAILABILITY OF PLANT NUTRIENTS 335
a strong hydrochloric acid analysis has led to the use of
weak acids for obtaining the solution. These weak acids
dissolve much less of the soil constituents than do the
strong acids, and the portion so dissolved is supposed to
represent more nearly the amount that the plant can make
use of. Both dilute organic acids and dilute mineral
acids have been used. Among the former are citric,
acetic, oxalic, and tartaric acids. The assumption on
which the use of the organic acids is based is that they
correspond to the solvent agents in the soil combined with
the solvent action that the plant is supposed to possess,
and thus dissolve from the soil the quantities of nutrients
that the plant could take up if it came in contact with
all the soil particles to a depth represented by the sample
analyzed.
238. Advantages in the use of dilute acids. The ac-
tion of each of these dilute acids on the same soil does
not give equal quantities of the various constituents in
solution. The dilute acids naturally dissolve a much
smaller amount of material from the soil than does strong
hydrochloric acid. The dilute acids permit the detection
of smaller quantities of easily soluble phosphoric acid and
potash than does the latter, larger quantities of soil being
used. For example, a chemical analysis of the strong
hydrochloric acid solution is very likely not to show any
increase in the phosphorus or potassium in a soil that may
have been abundantly manured with these fertilizers
and its productiveness greatly increased thereby. This
is because the amount of plant-food material added is so
small in comparison with the weight of the area of soil
nine inches deep over which it is spread that the increase
in percentage may well come within the limits of analytical
error. An acre of soil nine inches deep weighs about
336 SOILS: PROPERTIES AND MANAGEMENT
2,500,000 pounds. If to this there is added a dressing
of 2500 pounds of phosphoric acid fertilizer containing
400 pounds of phosphoric acid, it would increase the per-
centage of that constituent in the soil only 0.016 per cent
a difference that could not be detected by the analysis
of the hydrochloric acid solution.
239. The one-per-cent citric acid method. This
method was proposed by Dyer 1 and was shown by him
to give results with Rothamsted soils that permitted of
an accurate estimation of their relative productivity.
Dyer adopted the one-per-cent strength as the result of
an investigation in which he determined the acidity of
the juices in the roots of over one hundred species or
varieties of plants representing twenty different natural
orders. The average acidity of the juices of the twenty
orders, calculated to crystallized citric acid, was 0.91
per cent, which led Dyer to adopt a strength of 1 per cent.
It must be said, however, that the different varieties
varied greatly in this respect, some having ten times as
much acidity as others. The implication is that plants pro-
duce a solvent action on a soil in proportion to the acidity
of their juices, but an examination of Dyer's figures does
not show that the size of the crop ordinarily produced by
the plants tested would in many cases correspond to the
acidity of these juices. Thus, of the Cruciferse the horse-
radish has several times the acidity of the Swedish turnip
or of the field cabbage, although the crop produced by the
former is much less than that of the latter.
240. Usefulness of the citric acid method. As shown
by Dyer, the use of a one-per-cent solution of citric
1 Dyer, Bernard. On the Analytical Determination of Prob-
ably Available "Mineral" Plant Food in Soils. Jour. Chem.
Soc., Vol. LXV, pp. 115-167. 1894.
AVAILABILITY OF PLANT NUTRIENTS 337
acid is well adapted to show the amount of easily
soluble phosphoric acid and potash in certain soils, but
for other soils it has failed to give satisfaction in
the hands of a number of analysts. It is doubtless
best suited to soils rich in calcium and low in iron and
aluminium.
The reason urged by Dyer for the superiority of the
citric acid method over the hydrochloric acid extraction
is that soils, shown by experience to need phosphoric
manures, yielded a relatively much greater quantity of
phosphorus to citric acid than to hydrochloric acid when
compared with soils not needing this element.
The application of both the hydrochloric and citric acid
methods to a soil, when used to supplement each other,
may add greatly to a knowledge of the potential and
present productiveness of the soil.
According to Dyer, 1 for cereals and for most other
crops there should be present in a soil at least .01 per cent
of phosphoric acid, soluble in one-per-cent citric acid.
A soil containing less than this quantity is deficient in
phosphoric acid, unless this acid exists largely in the form
of ferric or aluminium phosphate, which is not readily
soluble in citric acid but is fairly available to the plant.
Sod land contains organic compounds of phosphorus that
are readily available to the plant; hence such soil, to
indicate sufficiency, should show by analysis more than
0.01 per cent of phosphoric acid. The quantity of potash
soluble in the same solvent should also be not less than
0.01 per cent in arable land.
1 Dyer, Bernard. A Chemical Study of the Phosphoric
Acid and Potash Contents of the Wheat Soils of Broadbalk
Field, Rothamsted. Philosoph. Trans. Royal Soc. London,
Series B, Vol. 194, pp. 235-290. 1901.
338 SOILS: PROPERTIES AND MANAGEMENT
241. Dilute mineral acids. Of the mineral acids
in a diluted form used for extracting soils, those that
have received the most attention are one-fifth normal
nitric l or hydrochloric acid and one two-hundredth
normal hydrochloric acid. 2 The methods employing
these solvents are admittedly empirical. There is no
natural relation between these solvents and the processes
by which the plant obtains its nutriment from the soil.
The solvent that has received the most attention is
one-fifth normal nitric acid. In ease of manipulation
this is preferable to the one-per-cent citric acid, which is
rather tedious to work with. It has been used nearly
as extensively in this country as the latter has in Great
Britain. Its use has been confined largely to the deter-
mination of the readily available phosphorus and potas-
sium in the soil, as has the citric acid method. It is
obvious that some minerals are more readily soluble than
are others, and for that reason the method will distinguish
between phosphorus and potassium in different forms.
The calcium phosphates are supposed to be entirely soluble
in this solvent. According to Fraps 3 it dissolves iron
and aluminium phosphates to only a slight extent, thus
distinguishing between these forms of phosphorus. Fraps
finds also that no potassium is removed from orthoclase
and microcline, that less than ten per cent is dissolved
1 Official and Provisional Methods of Analysis. U. S. D. A.,
Bur. Chem., Bui. 107 (revised), p. 18. 1908.
2 Wiley, H. W. Principles and Practice of Agricultural
Analysis, pp. 394-396. Easton, Pennsylvania. 1906.
3 Fraps, G. S. Active Phosphoric Acid and Its Relation
to the Needs of the Soil for Phosphoric Acid in Pot Experi-
ments. Texas Agr. Exp. Sta., Bui. 126, pp. 7-72. 1909.
Also, The Active Potash of the Soil and Its Relation to Pot
Experiments. Texas Agr. Exp. Sta., Bui. 145, pp. 5-39. 1912.
AVAILABILITY OF PLANT NUTRIENTS 339
from glauconite and biotite, and that from fifteen to sixty
per cent is dissolved from muscovite, nephelite, leucite,
apophyllite and phillipsite.
There are several factors, however, that make the use
of one-fifth normal nitric acid an uncertain guide to the
available phosphorus and potassium in the soil. When
a soil is treated with the acid some of it is neutralized by
the reactions that result and thus its strength is lessened.
This may have no relation to the quantities of phosphorus
or potassium dissolved. Some analysts correct for the
neutralization and some do not. Again, as with strong
hydrochloric acid, the degree of solubility of the soil con-
stituents in the nitric acid may not correspond with the
ability of the plant to obtain these substances. With
this, as with the other methods discussed, the objection
holds that the result cannot be taken as an infallible guide
to the productiveness of a soil, or to its fertilizer needs;
but each of the methods affords some information in
regard to a soil, and is thus of value.
242. Extraction with an aqueous solution of carbon
dioxide. As carbon dioxide is a universal constituent
of the water of the soil, and without doubt a potent factor
in the decomposition of the mineral matter, it has been
proposed to use a solution of carbon dioxide as a solvent
in soil analysis. The amounts of soil constituents taken
up by this solvent are much less than are taken up by
any of the others heretofore mentioned, but all mineral
substances used by plants are soluble in it to some extent.
The amount of phosphorus is so small as to make its
detection by the gravimetric method difficult. Like
other methods employing very weak solvents, this method
is open to the objection that the extraction fails to remove
a considerable portion of the dissolved matter that is
340 SOILS: PROPERTIES AND MANAGEMENT
retained by adsorption, and as this varies with soils of
different texture a fair comparison of such soils is impos-
sible.
243. Extraction with pure water. When soil is di-
gested with distilled water, all the mineral substances
used by plants are dissolved from it, but in very small
quantities. It has been proposed to use this extract for
soil analysis on the ground that it involves no artificial
solvent the presence or amount of which in the soil is
doubtful, but shows those substances that are undoubtedly
in a condition to be used by plants. By determining the
water content of the soil and using a known quantity of
water for the extraction, the percentage of the various
constituents in the soil water or in the dry soil may be
calculated.
The substances dissolved from the soil by extraction
with distilled water are probably only those contained
in the soil-water solution, including a part of the solutes
held by adsorption. The aqueous extract does not con-
tain the entire quantity of the nutritive salts in solution
in the soil water, and hence is not a measure of the
fertility held in that form. An undetermined quantity
of nutrients is retained in the water, in the very small
spaces and on the surface of the soil particles. It is,
however, a fair comparative measure of the content of
available nutrients.
244. Influence of absorption. The quantity of ex-
tracted material depends on the absorptive properties of
the soil and on the amount of water used in the extrac-
tion, or on the number of extractions. Analyses of the
aqueous extract of a clay and of a sandy soil on the Cornell
University farm serve to illustrate the greater retentive
power of the former for nitrates. Sodium nitrate was
AVAILABILITY OF PLANT NUTRIENTS
341
applied to ti clay soil and to a sandy loam soil at the rate
of 640 pounds to the acre. Analyses of aqueous extracts
some ninety days later showed the following :
KIND OF SOIL
FERTILIZER
NITRATES IN SOIL
(Parts per million)
Clay
Sodium nitrate
7.8
Clay
No fertilizer
1.8
Sandy loam ....
Sodium nitrate
150.0
Sandy loam . . . . .
No fertilizer
29.7
There was apparently a much greater retention of
nitrate by the clay soil, as shown by a comparison of the
fertilized and the unfertilized plats on both soils.
Schulze l extracted a rich soil by slowly leaching 1000
grams with pure water, so that one liter passed through
in twenty-four hours. The extract for each twenty-four
hours was analyzed every day for a period of six days.
The total amounts dissolved during each period were as
follows :
SUCCESSIVE EXTRACTIONS
TOTAL MATTER
DISSOLVED
(Grams)
VOLATILE
(Grams)
INORGANIC
(Grams)
First
Second
Third
0.535
0.120
0261
0.340
0.057
101
0.195
0.063
160
Fourth
Fifth
0.203
0260
0.083
0082
0.120
178
Sixth
0.200
0.077
0.123
1 Schulze, F. Ueber den Phosphorsaure-Gehalt des Wasser-
Auszugs der Ackererde. Landw. Vers. Stat., Band 6, Seite
40&-412. 1864.
342 SOILS: PROPERTIES AND MANAGEMENT
It will be noticed that the dissolved matter, both or-
ganic and inorganic, fell off markedly after the first ex-
traction, which was larger because of the matter in solu-
tion in the soil water. Later extractions were doubtless
supplied largely from the substances held by adsorption,
which gradually diffuse into the water extract as the
tendency to maintain equilibrium of the solution overcomes
the adsorptive action. With the removal of the adsorbed
substances, the equilibrium between the soil particles
and the surrounding solution is disturbed, solvent action
is increased, and more material gradually passes from the
soil into the solution. In this way the uniform and con-
tinuous body of extractives is maintained.
245. Other factors influencing extraction. For pur-
poses of soil analysis, the quantity of water used for extrac-
tion must be placed at some arbitrary figure, and this
is open to the objection that it does not represent accu-
rately the soil- water solution. Analyses of soils of different
types are not comparable, and the water extract cannot
be considered as measuring the concentration, or even
the composition, of the solution existing between the
root hair and the soil particles. However, for studying
some of the changes which go on in the soil and which are
detectable in the soil-water solution, the practice may be
followed to advantage.
246. The soil solution in situ. It has already been
pointed out that the interstitial spaces of any arable soil
contain more or less water all the time; that there is a
constant tendency for this water to assume the capillary
condition owing to the gravitational movement of free
water ; and that the normal evaporation of moisture from
the soil tends to reduce the capillary film to the condition
of hygroscopic water (par. 132). As the movement
AVAILABILITY OF PLANT NUTRIENTS 343
of free water is comparatively rapid and that of capil-
lary water relatively slow, the soil moisture supply is
usually somewhere between the point of lento-capillarity
and free water. In this condition each particle or aggre-
gation of particles is enveloped in a thin moisture film,
and this film water is constantly in motion although the
movement is rather slow.
Soils are more or less soluble in pure water; and in soil
water, charged as it always is with carbon dioxide, they
are still more readily soluble. Consequently the moisture
films constantly tend to approach a state of equilibrium
with respect to the water-soluble matter in the soil parti-
cles. If plants are entirely dependent for their mineral
nutrients on the supply in the soil-water solution, the
strength of this solution becomes an important matter.
The supply of mineral nutrients for higher plants will be
discussed later (par. 339). Even if the plant itself
has no influence on the supply of mineral nutrients that go
into solution, the quantity of food that it finds in the
soil solution already prepared for its use must constitute
an important factor in its growth.
Unfortunately there is no adequate method of ascer-
taining the strength of the solution. Attempts have
been made to remove this solution from the soil, but it
is altogether unlikely that the analyses of the liquid
obtained represent the composition of the soil solution,
because of the very small quantity of the liquid available
for analysis and also because of the uncertainty that the
sample obtained was representative of the soil solution.
247. Devices for obtaining a soil solution. An at-
tempt by Briggs and McLane 1 to sample the soil solution
1 Briggs, Lyman J., and McLane, John W. The Moisture
Equivalent of Soils. U. S. D. A., Bur. Soils, Bui. 45, pp. 5-8. 1907.
344 SOILS: PROPERTIES AND MANAGEMENT
involved the use of centrifugal motion, which developed
a force of two or three thousand times that of gravita-
tion. When the soil contained a rather large quantity
of capillary water, a small amount of it could be removed
in this way.
Another device, by Briggs and McCall, 1 consists of a
close-grained, unglazed, porcelain tube, closed at one end
and provided at the other with a tubulure, by which it
can be connected with an exhausted receiver. This
tube is moistened and buried in the soil. If the moisture
content of the soil is sufficient to reduce the pressure of
the capillary water surface in the soil to less than the dif-
ference between the pressure inside and outside of the
tube, there will be a movement of water inward. This
water may be collected and analyzed.
More recently Van Suchtelen has used another method
to obtain the soil solution. 2 He replaces the soil water
by means of paraffin in a liquid state, at the same time
subjecting the soil to suction on a filter. The displaced
water is considered to represent the soil solution.
248. Composition and concentration of the soil solu-
tion. It has generally been held that because some soils
are more productive than others, and because fertilizers
containing soluble salts frequently increase the yields of
crops, the soil solution in the better-yielding soils is more
concentrated, at least as regards plant nutrients, than is
that in the poorer soils. The argument is, of course,
1 Briggs, L. J., and McCall, A. G. An Artificial Root for
Inducing Capillary Movement of Soil Moisture. Science,
N. S., Vol. 20, pp. 566-569. 1904.
2 Van Suchtelen, F. H. H. Methode zur Gewinnung der
Natiirlichen Bodenlosung. Jour. f. Landw., Band 60, Seite
369-370. 1912.
AVAILABILITY OF PLANT NUTEIENTS 345
based on the assumption that, other things being equal,
plant growth is a function of the concentration of the
plant nutrients in the soil solution. According to this
conception, increased or decreased soil fertility is reflected
in the composition and concentration of the soil solution,
and this in turn in crop yields. The soil solution is there-
fore a variable quantity, and, to some extent at least, within
the control of man. An elaborate explanation for the
responsiveness of the soil solution has been worked out
by Van Bemmelen and his school.
249. Variability in composition and concentration of
the soil solution. The process of rock weathering has,
according to Van Bemmelen, 1 Biltz, 2 and others, resulted
in deep-seated chemical changes in some of the mineral
constituents of the soil, whereby there are formed com-
plex colloidal silicates which, in the form of gels, cover
the surfaces of the soil particles. These colloidal com-
plexes may contain iron, aluminium, calcium, magnesium,
potassium, phosphorus, and other substances, which are
absorbed from the different electrolytes as ions or as
salts and depend in quantity on the concentration of the
solution from which they are absorbed. They therefore
act like solid solutions, whose composition changes
with every change in the concentration of the liquid solu-
tion that comes in contact with them. This relation of
the colloidal complexes to the soil water with which they
come in contact is essentially different from that of the
1 Van Bemmelen, J. M. Beitrage zur Kenntniss der
Verwitterungsprodukte der Silikate in Ton, Vulkanischen, und
Laterit-Boden. Zeit. f. Anorganische Chemie, Band 42,
Seite 265-324. 1904.
2 Biltz, W. Ueber die Gegenseitige Beeinflussung Col-
loidal (Jdoster Stoffe. Ber. deutsch. chem. GeselL, Band
37, Seite 1095-1116. 1904.
346 SOILS: PROPERTIES AND MANAGEMENT
pure minerals, as they are not true chemical combina-
tions. The organic matter in the soil adds another class
of colloidal matter ; so that, in the opinion of Van Bem-
melen, 1 the colloidal silicates and the colloidal humus form,
in various proportions, a mass of colloidal complexes that
control the composition of the soil solution. The col-
loidal condition of this material is readily decomposable
under variations in temperature and concentration of
solutions, and would doubtless be in a state of constant
transition in the soil.
This conception of the soil surface would account for
changes in the concentration of the soil solution due to
the application of soluble fertilizers, and would also
explain the continued effect of such fertilizers on the
theory that they are absorbed by the colloidal complexes
and redissolved as the soil solution tends to become more
dilute.
A somewhat different view has been taken by Whitney
and Cameron, who hold that the composition and con-
centration of the solution in all soils is practically the
same. Their conception, according to a recent paper by
Cameron, 2 appears to differ from that of Van Bemmelen
in assuming that the soil water is in contact with the soil
particles for such a short time that the quantity of matter
that goes into solution is too slight to bear any relation
to the total quantity of soluble matter in the soil. The
soil solution does not come into equilibrium with the
soil mass, nor even approximate such a condition. The
1 Van Bemmelen, J. M. Die Zusammenset7Aing der Acker-
erde. Landw. Vers. Stat., Band 37, Seite 347-373. 1890.
2 Cameron, F. K. Concentration of the Soil Solution.
Original Communications, Eighth International Congress of
Applied Chem., Vol. 15, pp. 43-48. 1912.
AVAILABILITY OF PLANT NUTRIENTS 347
solution being similar in all soils, it follows that the rela-
tive productiveness of different soils bears no relation to
the supply of soluble nutrients, but must be due to other
factors. Hence soluble fertilizers increase plant growth,
not by supplying a greater quantity of plant nutrients,
but through other effects on the soil as, for instance,
their favorable influence on tilth, or through the de-
struction of toxic matter.
250. Discussion of the theories regarding soil solu-
tions. The difficulty in securing a true sample of the
soil solution as it exists in situ complicates any attempt
to ascertain how these theories comport with the actual
condition of the soil solution. A number of attempts
have been made to throw light on this subject, but none
of the data obtained is of a nature to definitely prove the
correctness of either theory. The evidence, so far as it
goes, indicates that the water extract of soils differs in
concentration in different soils, and is increased, under
some conditions, by large and continued applications
of soluble fertilizers. There can be no doubt, more-
over, that plant growth in properly balanced nutrient
solutions increases with the concentration of the solu-
tion up to several thousand parts to the million, as has
been demonstrated by many experiments.
One rather convincing experiment may be quoted.
Hall, Brenchley, and Underwood 1 analyzed the water
extract from certain plats on the Rothamsted Experi-
ment Station farm, the fertilizer treatment and the
yields of which had been recorded for a long term of years.
1 Hall, A. D., Brenchley, W. E., and Underwood, T. M.
The Soil Solution and the Mineral Constituents of the Soil.
Philosoph. Trans. Royal Soc. London, Series B, Vol. 204, pp.
179-200. 1913.
348 SOILS: PROPERTIES AND MANAGEMENT
Complete analyses of the soil from the several plats were
also made :
YIELDS OF CROPS, AND COMPOSITION OF SOIL AND WATER
EXTRACT OF SOIL, ON ROTHAMSTED EXPERIMENT STATION
FARM
COMPLETE ANALYSIS
WATER EXTRACT
YIELD
ACRE
(.Pounds)
P 2 5
(percentage)
K 2
(percentage)
PzOs
(p. p. m.)
K 2
(p. p. m.)
Unmanured . . .
1,276
0.099
0.183
0.525
3.40
N + P 2 6 . . .
3,972
0.173
0.248
3.900
3.88
N + K 2 . . .
2,985
0.102
0.257
0.808
30.33
Complete fertilizer
5,087
0.182
0.326
4.025
24.03
Farm manure . .
6,184
0.176
0.167
4.463
26.45
A similarly treated set of plats, which had been planted
to another crop and analyzed as were these, gave similar
results. It is a very striking example of the effect of
long-continued treatment of the soil with a certain fertilizer
on the composition of the water extract. The subject,
however, must be investigated further, as it is of funda-
mental importance to a knowledge of the properties of
soils.
CHAPTER XVI
THE ABSORPTIVE PROPERTIES OF SOILS
IF the brown water extract from manure is filtered
through a clay soil not containing soluble alkalies, the
filtrate will be nearly colorless. Many solutions of dye-
stuffs are affected in the same way. Solutions of alkali
or alkaline earth salts are more or less modified by this
operation, the bases being retained by the soil to a greater
extent than are the acids. Thus, when a solution of
the nitrate, sulfate, or chloride of any one of these bases
is filtered through the soil, a part of the base is absorbed
by the soil, while most of the acid comes through in the
filtrate. If these bases are in the form of phosphates
or silicates, not only the base is absorbed, but the acid
as well.
251. Substitution of bases. Associated with the
absorption of the base from solution, there is liberation
of some other base from the soil, which combines with
the acid in the solution and appears in the filtrate as a
salt of that acid.
\Ylien absorption takes place from solution, the base
is never entirely removed, no matter how dilute the solu-
tion may be. A dilute solution of potassium chloride
filtered through a soil will produce a filtrate containing
some calcium, magnesium, or sodium chloride, or all
these salts, and some potassium chloride. The more
dilute the solution, the larger will be the proportion re-
349
350 SOILS: PROPERTIES AND MANAGEMENT
tained, but the less the total quantity absorbed. Peters 1
treated 100 grams of soil with 250 cubic centimeters of a
solution of potassium salts, and found that the potassium
of different salts was retained in different proportions,
and that the stronger solutions lost relatively less than
the weaker, while more potassium was removed from the
stronger solutions.
i a u NORMAL
uV NORMAL
Grams K2O absorbed
Grams KaO absorbed
KC1
0.3124
1990
K 2 SO 4
03362
02098
K 2 CO 4
0.5747
03134
The same bases are not always absorbed in the same
proportion by different soils ; one soil may have a greater
absorptive power for potassium, while another may re-
tain relatively more ammonia. They seem to be inter-
changeable, as any absorbed base may be released by
another in solution. The absorptive power of a soil for
certain bases is reflected in the composition of the drainage
water from the soil. The composition of the drainage
water varies with different soils, and a soluble fertilizer
applied to one soil will have a different effect on the com-
position of the drainage water than if applied to a different
soil. This is well illustrated from lysimeter experiments
by Gerlach 2 at Bromberg. Several soils were used,
1 Peters, E. Ueber die Absorption von Kali durch Acker-
erde. Landw. Vers. Stat., Band 2, Seite 113-151. 1860.
2 Gerlach, Dr. Uber die durch sickerwasser dem Boden
Entzogenen Menge Wasser und Nahrstoffe. 111. Landw. Zei-
tung, 30 Jahrgange, Heft 95, Seite 871-881. 1910.
THE ABSORPTIVE PROPERTIES OF SOILS 351
one of each being fertilized and one unfertilized. The
lysimeters were 1.2 meters deep and contained 4 cubic
meters of soil. The drainage water was caught and
analyzed for four years. The first year there was no
crop, the second year potatoes were grown, the third
oats, and the fourth rye. The following results were
shown :
A v KB AGE COMPOSITION OF DRAINAGE WATER IN PARTS PER
MILLION
SOIL
TREATMENT
TOTAL
NITROGEN
NITRIC
NITROGEN
ORGANIC
NITROGEN
K 2 O
CaO
405
507
Moor soil . .
| Fertilized
[ Untreated
32.7
65.0
30.0
60.3
2.7
4.7
32.2
26.2
Loamy sand
low in humus
[ Fertilized
{ Untreated
25.5
20.9
25.1
20.4
0.4
0.5
25.1
8.5
92
90
Sandy loam
high in humus
f Fertilized
I Untreated
67.8
69.5
64.6
66.1
3.1
3.4
70.2
47.4
399
414
Absorption will not proceed to an unlimited extent.
A soil will cease to absorb any particular substance after
a certain quantity has been taken up. This quantity
will vary with every soil. Clay and loam soils have
greater absorptive power than sandy soils. This differ-
ence, both as to amount and as to rate of absorption, is
well shown by the following curves adapted from Schreiner
and Failyer. 1
1 Schreiner, O., and Failyer, G. H. The Absorption of
Phosphates and Potassium by Soils. U. S. D. A., Bur. Soils,
Bui. 32. 1906. See also Cameron, F. K., and Bell, J. M.
The Mineral Constituents of the Soil Solution. U. S. D. A.,
Bur. Soils, Bui. 30, pp. 42-66. 1905. Patten, H. E., and Wag-
gaman, W. H. Absorption by Soils. U. S. D. A., Bur. Soils,
Bui. 52. 1908.
352 SOILS: PROPERTIES AND MANAGEMENT
5O/L
FIG. 55. Curves showing the absorption of PC>4 in parts to a million
by various soils from a solution of monocalcium phosphate, contain-
ing 200 parts to a million of PC>4. The volume of the percolate is
used as the abscissas.
Note. The law which appears to govern absorption of phos-
phates and potash by the soil may be expressed mathematically
as follows :
in which K is a constant, A the maximum quantity possible
for the soil to absorb, and y the quantity actually fixed when v,
volume of the solution, has percolated through.
A short discussion of the mathematics of this law may be
found in the following publication : Schreiner, O., and Failyer,
G. H. The Absorption of Phosphates and Potassium by Soils.
U. S. D. A., Bur. Soils, Bui. 32, pp. 23-24, 37-39. 1906.
THE AH so UPTIME PROPERTIES OF SOILS 353
SANDYSOtL
FIG. 56. Curves showing the absorption of K in parts to a million by
various soils from a solution containing 200 parts to a million of K.
The volume of the percolate is used as the abscissas.
252. Time required for absorption. The amount of
absorption depends on the time of contact between the
soil and the solution. While a large part of the dissolved
base is taken up in a short time after being placed in
contact with the soil, the maximum absorption is effected
only after a considerable period. Ammonia, according
to Way, reaches its maximum absorption in half an hour ;
while Henneberg and Stohmann 1 found that phosphorus
required twenty-four hours to reach the same degree of
absorption.
1 Henneberg, W., and Stohmann, F. Ueber das Verhalten der
Ackererde gegen Losungon von Ammoniak und Ammoniaksalzen.
Jour. f. Landw., None Folge, Band 3 (Der ganze Reihe siebenter
Jahrgang), Seite 25-47. 1859.
2A
354 SOILS: PROPERTIES AND MANAGEMENT
This, however, has no significance so far as danger
from loss of a soluble fertilizer constituent is concerned,
since water, even after a heavy rain, would not pass so
quickly through the soil that absorption would not take
place, except possibly in the case of soil of a very coarse
texture. The depth through which the substance is
distributed in the soil may, however, be influenced by
the time required for its absorption. Ordinarily ferti-
lizers do not penetrate very far into the soil. Demolon
and Bronet 1 have investigated the rate and distance of
penetration of certain soluble salts in soils, and find that
a total rainfall of ten inches is not sufficient to carry down
sodium nitrate in a sandy soil to a depth of eight inches.
253. Insolubility of certain absorbed substances. -
Although bases once absorbed may be easily displaced
by other bases, it is difficult to dissolve them from the
soil with pure water. Peters 2 treated 100 grams of soil
with 250 cubic centimeters of water containing potassium
chloride, of which 0.2114 gram of K 2 O was absorbed.
The soil was then leached with distilled water, using 125
cubic centimeters of water daily for ten days. At the
end of that time 0.0875 gram of K 2 O had been removed,
or at the rate of 28,100 parts of water to one part of
K 2 O dissolved from the soil. Henneberg and Stoh-
mann 3 found that it required 10,000 parts of water
1 Demolon, A., and Bronet, G. Sur la Penetration des
Engrais Solubles dans les Sols. Ann. Agron., Tome 28, pp.
401-418. 1911.
2 Peters, E. Ueber die Absorption von Kali durch Acker-
erde. Landw. Vers. Stat., Band 2, Seite 113-151. 1860.
3 Henneberg, W., and Stohmann, F. Ueber das Verhalten
der Ackererde gegen Losungen von Ammoniak und Ammoniak-
salzen. Jour. f. Landw., Neue Folge, Band 3 (Der ganze Reihe
siebenter Jahrgang), Seite 25-47. 1859.
THE ABSORPTIVE PROPERTIES OF SOILS 355
to dissolve one part of absorbed ammonia from the
soil.
254. Influence of size of particles. The surface area
of the soil particles determines to some extent the amount
of substance absorbed. For this and other reasons, a
fine-grained soil absorbs a greater quantity of material
than a coarse-grained soil. In fact, it was early shown by
Way 1 that the phenomenon of absorption is largely a
function of the silt, clay, and humus of the soil.
255. Causes of absorption. A number of causes
have been assigned for the absorption of substances by
soils, and there can be no doubt that the phenomenon is
not due to any one process. Several distinct causes are
now very generally recognized, while others that have
been suggested may have a part in the result.
256. Zeolites. As the result of his extended researches
on absorption of soils, Way concluded that the property
of absorption, or fixation of bases, rests largely with the
hydrated silicates of aluminium, containing calcium or
magnesium and one of the alkali metals, these compounds
being known as zeolites. He prepared artificially a
hydrated silicate of aluminium and sodium, and found
that by treating this with a solution of a calcium salt
he could replace most of the sodium, obtaining thereby
a silicate of aluminium, calcium, and part of the sodium
that was originally contained in the silicate. The re-
mainder of the sodium could be replaced by potassium
from solution and, likewise, by magnesium and ammonium.
1 Way, J. T. On the Power of Soils to Absorb Manure.
Jour. Royal Agr. Soc. England, Vol. 11, pp. 313-379. 1850.
Also, On the Power of Soils to Absorb Manure. Jour. Royal Agr.
Soc. England, Vol. 13, pp. 123-143. 1852. Also, On the
Influence of Lime on the " Absorptive Properties " of Soils. Jour,
Royal Agr. Soc. England, Vol. 15, pp. 491-515. 1854.
356 SOILS: PROPERTIES AND MANAGEMENT
Way found further that exposure to a strong heat de-
stroyed the absorptive properties of these substances, as
did also treatment with strong hydrochloric acid. In all
these respects the absorptive properties of the soil and
of the zeolites coincide.
257. Chabazite. Eichhorn 1 experimented with the
natural zeolite chabazite, and found that he could produce
substitutions by means of the proper salt solutions. In
column I of the table below is given the composition of
chabazite used for the experiment ; in column II is stated
its composition after treatment with a solution of sodium
chloride; and in column III the composition after the
zeolite is further treated with a solution of ammonium
chloride :
COMPOSITION OF CHABAZITE ORIGINALLY AND AFTER TREAT-
MENT WITH SODIUM CHLORIDE AND AFTERWARDS WITH
AMMONIUM CHLORIDE
COLUMN
i
II
in
Si0 2
A1 2 O 3
47.4
20.7
48.3
21.0
51.3
22.2
CaO
104
67
42
K 2 O
0.7
0.6
( f
Na 2
HoO
0.4
202
5.4
18.3
| 0.6
14.9
(NH 4 ) 2 ....
0.0
0.0
6.9
The substitutions were evidently made at the expense
of calcium in the compound, both when treated with
sodium and when treated with ammonium salts in chemi-
cally equivalent quantities. These and subsequent ex-
1 Eichhorn, H. Ueber die Einwirkung Verdiinuter Salz-
losungen auf Ackererde. Landw. Centrlb. f. Deutschland,
6 Jahrgang, Band 2, Seite 169-175. 1858.
THE ABSORPTIVE PROPERTIES OF SOILS 357
periments by numerous investigators have been rather
widely accepted as indicating that the zeolites are at
least partly responsible for the absorptive properties of
soils. It has been shown further that the absorptive
power of a soil is more or less proportional to the quantities
of acid-soluble silicates it contains. The zeolites being
rather easily soluble in strong mineral acids, it is held
that the bases so combined are more readily available
to plants than in most combinations found in the soil,
and yet are not easily leached out of it.
258. Presence of zeolites questioned. On the other
hand, zeolites have never been definitely proved to be
present in soils. Merrill l has attempted to show that
they cannot be of wide occurrence in soils, but neither
their absence nor their presence has been demonstrated.
Since the time when Way first published his researches
in 1850, the zeolite constituents of the soil have generally
been held to be largely responsible for its absorptive
power for bases.
259. Absorption of phosphoric acid. It has already
been said that although hydrochloric, sulfuric, and nitric
acids are not absorbed by soils, except in small quantities,
phosphoric acid is absorbed and retained in an almost
insoluble condition so far as extraction with water is
concerned. That this absorption cannot be due to
zeolites is generally conceded, and has recently been
demonstrated, for permutite at least, by Rostworowski
and Wiegner, 2 who in a carefully conducted experiment
1 Merrill, G. P. Rocks, Rock Weathering, and Soils, pp.
362-367. New York. 1906.
2 Rostworowski, S., and Wiegner, G. Die Absorption der
Phosphorsaure durch "Zeolithe" Permutite. Jour. f. Landw.,
Band 60, Seite 223-235. 1912.
358 SOILS: PROPERTIES AND MANAGEMENT
with this zeolite which is an amorphous gel containing
potassium, calcium, aluminium and silicic acid found
that there was no absorption of phosphoric acid from a
neutralized solution of monocalcium phosphate or from
a solution of dicalcium phosphate at various degrees of
concentration.
260. Formation of insoluble phosphates. The reten-
tion of soluble phosphoric acid in soils may be easily ac-
counted for by the fact that there are present in all soils
hydrated ferric oxide and hydrated silicates of alumina,
and frequently calcium carbonate, with which substances
phosphoric acid in solution would naturally form com-
pounds insoluble in water. Iron and aluminium phos-
phates are practically insoluble in water containing carbon
dioxide or weak organic acids such as might be present
in soil water. Calcium carbonate forms with a soluble
phosphate fertilizer some dicalcium phosphate, the
solubility of which in soil water is much greater than
the iron and aluminium phosphates. This is one of the
advantages of keeping a soil well supplied with lime if
a superphosphate fertilizer is to be used. Even the
tricalcic phosphate, although less soluble than the dicalcic,
is more readily soluble than the iron and aluminium
phosphates. As lime has a tendency to move downward
in soil, and as phosphoric acid is retained in the plowed
depth when added as a fertilizer, it is important that the
applications of lime be sufficiently frequent to keep this
part of the soil in a condition to form the lime phosphates.
Cameron l has suggested that the absorption of phos-
phoric acid is probably due to the formation with lime
or ferric oxide of a solid solution, which might account
1 Cameron, F. K. The Soil Solution, p. 59. Easton, Penn-
sylvania. 1911.
THE ABSORPTIVE PROPERTIES OF SOILS 359
for the availability of phosphorus in soils to which a
superphosphate fertilizer had been applied many months
previously. It might explain also the availability of
a superphosphate on soils devoid of calcium carbonate.
Although such availability is always less than where
this carbonate exists, it is greater than would be ac-
counted for by the solubility of ordinary iron phosphate.
261. Adsorption. There is a physical fixation, termed
adsorption, due to the concentration of the soil solution
in immediate contact with the surface of the particles.
The phenomenon is familiarly exemplified in the clarify-
ing effect of the charcoal filter. This process results in
the retention, in fine-grained soils, of considerable soluble
material that would otherwise be washed out. In the
case of nitrates, which are not retained by the zeolites,
adsorption is an important factor (par. 244). If a
solution of a known quantity of nitrate of soda is added
to a clay soil, and an attempt is then made to extract
the nitrate from the soil with distilled water, it will be
found impossible to recover a very appreciable propor-
tion of the amount added. While adsorption probably
does not account for all the nitrates retained, there can
be no doubt that it plays an important part. Nutritive
salts held in this way are readily available to the plant,
whose root hairs come in contact with the soil particles.
It is not impossible that other fertilizer constituents are
held by the soil in this manner.
262. Absorption by colloids. According to Van Bem-
melen, 1 who has made a very exhaustive study of this
1 Van Bemmelen, J. M. Die Absorptionsverbindungen und
das Absorptionsvermogen der Ackererde. Landw. Vers. Stat.,
Band 35, Seite 69-136. 1888. Also, Die Absorption, Seite
548. Dresden, 1910.
360 SOILS: PROPERTIES AND MANAGEMENT
subject, absorption by soils is, without doubt, largely
due to the presence of colloidal matter which exerts an
absorbent action for water, gases, solutes, and solids in
suspension. The colloidal matters in soils that contrib-
ute to their absorptive properties are the following :
(1) remains of plant and animal tissues ;
(2) humous substances ;
(3) colloidal iron oxide ;
(4) colloidal silicic acid ;
(5) amorphous colloidal silicates that have been formed
through weathering.
Van Bemmelen also credits crystalline silicates with
absorbent properties, although he does not consider that
their action is very important. Absorption is brought
about also by true chemical combination of soil com-
pounds with substances in solution, by which certain of
the cations or anions in solution are chemically combined
and remain in the soil in a very difficultly soluble condition.
263. Absorptive properties of colloidal matter. -
Among the products of rock weathering there have been
found in soils amorphous substances that are of the nature
of colloidal gels. These, with the other colloidal matter,
are contained in the very small particles that remain for
a long time in suspension when soil is stirred up in water.
These colloids are coagulated by many acids, and by
some bases and salts. This is especially true of the
material that is dialyzable. Some of these again go into
solution on being treated with water, while others remain
insoluble until they undergo molecular change. Many
colloids form hydrogels with soil water. These hydrogels
are not ordinary chemical compounds. Gels dry very
slowly. They adsorb water in varying quantities, not
THE ABSORPTIVE PROPERTIES OF SOILS 361
in certain definite proportion as do crystalloids in the
process of crystallization. The more water adsorbed by
colloids, the less firmly is it held in combination. There-
fore it is easier to evaporate the water when a large quan-
tity has been taken up, and as the amount decreases it
becomes more difficult to drive it off.
Another property of colloidal matter is that when it
is separated from solution it carries down with it other
substances in the solution from which it is precipitated.
If, on the other hand, the colloidal matter has been pre-
cipitated in a pure state, it absorbs substances from
solutions with which it remains in contact for some time.
The substances taken up in this way are not chemically
combined, but substances that unite chemically may be
absorbed.
The combinations produced by absorption are weak
and it is possible to leach out the combined substances,
which are generally held in the water of the gels. The
following example of one kind of absorption is given
by Van Bemmelen : 1 ten grams of a hydrogel having the
composition SiO 2 . 4.2 H 2 O, shaken with 100 cubic cen-
timeter solution of 20 molecular equivalent KC1, will
absorb 0.8 to 1.1 molecular equivalent of the dissolved
substance. The absorption in this case was as if the
solution had been diluted with 4.2 to 5.8 cubic centimeters
of water. As the amount of gel water in 10 grams of
hydrogel of SiO 2 is about 5 cubic centimeters, the as-
sumption may be made that the dissolved substance is
taken up in equal concentration by the gel water. Ten
grams of hydrogel of SiO 2 shaken with 100 cubic centi-
1 Van Bemmelen, J. M. Die Absorptionsverbindungen und
das Absorptions vermogen der A.ckererde. Landw. Vers. Stat.,
Band 35, Seite 75. 1888.
3(52 SOILS: PROPERTIES AND MANAGEMENT
meter solution of 50 molecular equivalent KC1 that
is, 2^ times the concentration of the former solution
absorbs 2f times as much, or 2.1 to 2.5 molecular equiva-
lent. This applies also to concentrations five times
stronger than the first mentioned above, but beyond that
the relation is not so simple. It serves, however, to
illustrate the manner in which the absorption takes place
from dilute solutions.
264. Selective absorption. A selective absorption is
very common, especially from solutions of salts having
weak acids, a greater fixation of the bases taking place
than of the acids. Dissociation of the salts takes place
in the solution, the bases being absorbed, in consequence
of which further dissociation occurs; and this proceeds
until an equilibrium is established between the absorbing
and combining power of the colloidal material and the
reverse action of the water and resulting acids. In this
way the absorptive power decreases as the amount ab-
sorbed becomes greater.
The colloidal silicates possess the property of absorbing
a certain base when presented to it in solution, and con-
tributing in return a chemically equivalent quantity of
some other base. Potassium is most firmly combined
in the soil and most strongly withdrawn from solution,
with an exchange of a chemically equivalent quantity of
calcium, sodium, and magnesium, which passes into the
solution. If a soil is treated with a solution of potassium,
magnesium, sodium, or calcium salts of equal concentra-
tion, the concentration of the solution in the end is less
for the potassium than for the magnesium, and less for
the magnesium than for the sodium and the calcium,
because the potassium is most strongly bound in the
colloidal material, while the calcium and sodium are least
THE ABSORPTIVE PROPERTIES OF SOILS 863
so. In other words, the action of a calcium salt in solu-
tion on the absorbed potassium combination is less than
the action of a dissolved potassium salt on the absorbed
calcium combination. Thus it comes about that under
similar conditions of temperature, volume, and concen-
tration of the solution, the quantity of calcium or of
sodium or of magnesium that goes into solution when
colloidal silicates are treated with a solution of a potassium
salt is greater than the quantity of potassium that would
go into solution if the same silicates were treated with a
solution containing the salts of any of these other bases.
265. Absorptive power of colloidal silicates. The
quantity of a substance that a certain weight of a colloidal
silicate can absorb increases with the strength of the
solution of the substance presented for absorption, be-
cause the final solution can remain stronger and conse-
quently its solvent power for that particular substance
is less. The point of equilibrium between the fixing
power of the colloid and the solvent action of the solvent
therefore varies with the strength of the solution.
The nature of the acid with which a base is combined
likewise has an influence on the quantity of the base
absorbed. A base combined with a weak acid is ab-
sorbed in greater amount than the same base combined
with a strong acid. This is presumably because the
stronger acid remaining in solution has a greater solvent
action.
266. Absorption by colloids versus absorption by
zeolites. The early conception of the phenomenon of
fixation in soils was naturally a chemical one and was
founded on the chemical knowledge of that day. The
fact that the substitution of bases in the solutions passed
through the soil was in chemically equivalent quantities,
364 SOILS: PROPERTIES AND MANAGEMENT
placed it in line with what was known regarding chemical
reactions. Zeolites were found to possess absorbent
properties of a similar nature toward salts in solution, a
characteristic of which is the substitution of bases and
the appearance in solution of the released base in com-
bination with the acid of the original salt. It was a
natural conclusion that true mineral zeolites exist in
soil and that the absorptive properties of soil are due to
their action.
Many years later, when the principles of physical
chemistry had been applied to the study of colloids, it
was shown that absorptive properties are possessed by
certain colloids similar to those characteristic of soils.
Zeolites have never actually been isolated from any soil.
This fact has always occasioned some doubt as to the
hypothesis to which their properties have given rise.
Colloids, on the other hand, are well known to occur in soils,
but the exact nature of soil colloidal matter is not well
understood ; consequently there is considerable indefinite-
ness about the extent of their absorptive function, and
even Van Bemmelen grants the crystalloids a part in this
phenomenon.
The zeolite hypothesis furnished an explanation for
the form in which the available plant-food materials of
the soil are held. On it is largely based the idea that
the solution of a soil in strong hydrochloric acid repre-
sents the nutrients that are available to plants. The
silicates that go into solution are held to be the zeolitic
silicic acid and the bases with which it is united. .The
fact that such treatment largely destroys the absorptive
properties of a soil is taken as a proof of this. It would,
however, answer equally well as an argument in favor
of colloidal absorption, as the colloidal condition of the
THE ABSORPTIVE PROPERTIES OF SOILS 365
silicates would be destroyed by the same treatment.
On the whole, the evidence appears to be in favor of
the dominance of colloidal absorption rather than
crystalloidal absorption by soils, with its important
function in conserving soluble fertilizers and retaining
a supply of plant nutrients in a more or less readily
available condition.
267. Absorption by organic matter. The partially
decomposed organic matter in soils, especially that part
which has undergone such transformations as to form
humus (par. 90), has an absorptive power. Soils rich
in humus, without doubt, owe much of their fertility
to the retention by that constituent of a large supply of
readily available plant-food material. Many prairie soils
that have been reduced in productiveness under culti-
vation respond to the application of organic matter in a
remarkable manner. Humus in these soils seems to be the
chief conserver of readily available plant-food materials.
Van Bemmelen, 1 who has studied these compounds,
states that soils hold colloidal humous compounds con-
taining ammonia, potassium, sodium, and other sub-
stances, as well as iron oxide. A part is soluble, or forms
soluble compounds with alkalies, but the principal part
is insoluble. Some of these latter compounds are of a
colloidal nature and of changing composition. The soluble
matter is easily precipitated by a salt solution and carries
down with it bases from the solution. Absorption of bases
also takes place from solution, with substitution of one
base for another. Potassium is more strongly held in
combination than is calcium or magnesium. Bases are
removed, however, only from salts of the weaker acids.
1 Van Bemmelen, J. M. Die Absorption, Seite 135-141.
Dresden, 1910.
366 SOILS: PROPERTIES AND MANAGEMENT
268. Absorption of water vapor and of gases by soils.
Hygroscopic water in soils has already been discussed (pars.
133, 134, 135). It need merely be remarked here that
there is a close relation between the absorptive power of a
soil for water vapor and for bases. Soils having a high
content of humus and composed of very fine material are
likely to have great absorptive properties for both vapors
and solutes.
In a similar way soils absorb gases. The deodorizing
property of soil is well known. Decomposing organic
matter is rendered inoffensive by covering it with soil.
Gases produced in the processes of decomposition are
largely absorbed by the soil. The fertility of the soil
may be increased by the absorption of certain gases.
269. Absorption of ammonia. Ammonia, which exists
in minute quantities in the air, is absorbed by soils, and
also when given off by decomposing organic matter in the
soil. As all nitrogeneous organic matter may eventually
form ammonia when decomposed, the ability of the soil
to absorb it is very important. Quartz alone will absorb
only a very small quantity of ammonia, while a clay soil
will hold practically all that is likely to be produced by
the decomposition of the organic matter incorporated in it.
270. Absorption of carbon dioxide. Carbon dioxide
is absorbed by soils to a very considerable extent, and
this also adds to the productiveness of soils, since it aids
in their decomposition. The supply of carbon dioxide
comes from decomposing organic matter and from plant
roots. As will be explained later, the soil air always
contains a considerable supply of this gas, and its con-
densation and absorption is constantly going on. It
forms soluble bicarbonates with the alkalies and bases
of soils, producing a readily available plant-food material.
THE ABSORPTIVE PEOPERTIES OF SOILS 367
271. Absorption of nitrogen and oxygen. Nitrogen
is absorbed by soils to a greater degree than is oxygen.
The latter probably is of greater importance to soil fer-
tility, as its absorption is accompanied by oxidation of
other absorbed gases. Because of their absorptive prop-
erties and their great surface area, soils have strong
oxidizing power.
The absorption of gases by soils is largely an absorp-
tion phenomenon, the gases being condensed on the
surface of the particles. Von Dobeneck 1 has shown
that the absorption is greater, the finer the particles
of soil; but this increase is not directly proportional
to the increase in surface, large particles apparently
having a greater adsorptive power than their surface
ami would indicate.
272. Relation of temperature to gas absorption. -
The temperature of the soil influences its absorptive
properties for vapors. As the temperature increases the
absorption becomes less. Hilgard 2 does not find this
to be the case (par. 136). He exposed soils to a moisture-
saturated atmosphere and found that they absorbed
more moisture at high than at low temperatures. In
his conclusions, however, he is doubtless in error. All
the work previous to his gave a directly contrary result,
and a more recent investigation by Patten and Gallagher 3
confirmed the work of the earlier investigators.
1 Von Dobeneck, A. F. Untersuchungen iiber das Adsorp-
tionsvermogon und die Hygroskopizitat der Bodenkonstit-
uenten. Forsch. a. d. Agri.-Physik., Band 15. Seite 163-228.
1892.
2 Hilgard, E. W. Soils, pp. 196-198. New York, 1906.
3 Patten, H. E., and Gallagher, F. E. Absorption of Gases
and Vapors by Soils. U. S. D. A., Bur. Soils, Bui. 51, pp. 31-
35. 1908.
368 SOILS: PROPERTIES AND MANAGEMENT
273. Relation of absorptive capacity to productiveness.
The absorptive capacity of a soil is not so much a
measure of its immediate as of its permanent productive-
ness. It is well known that a very sandy soil responds
quickly to the application of soluble manures, but that
the effect is confined mainly to one season ; while a clay
soil, although not so quickly responsive to fertilization,
shows the effect of the application much more markedly
the second or the third year than does the sandy soil.
Adsorption, which is largely shown in sandy soil, holds
the nutritive material in a very readily available con-
dition, while absorption by amorphous compounds
renders these substances somewhat less readily available.
There are also other reasons why the sandy soil is more
responsive. King, 1 in working with eight types of soil
from different parts of the United States, found that
those soils removing the most potassium from solution
gave the largest yield of crops. It would not be per-
missible, however, to adopt this test as a method for
determining productiveness in soil.
274. Absorption as related to drainage. The drainage
water from cultivated fields in humid regions, and to a
less extent in semiarid and arid regions, except where
irrigation is practiced, carries off very considerable quan-
tities of plant-food material. The loss of this material
is due to the operation of the various natural disintegrating
agents on the soil mass, and to the application of fertilizing
materials in a soluble form. The various absorptive prop-
erties stand between the natural solubility of the soil and
the tendency to loss in drainage, and hold, in a condition
1 King, F. H. Influence of Farm Yard Manure upon Yield
and upon the Water-Soluble Salts of Soils, p. 25. Madison,
Wisconsin. 1904.
TI1E ABSORPTIVE PROPERTIES OF SOILS 369
in which they may readily be used by the plant, these
materials which would otherwise be lost.
275. Substances usually carried in drainage water. -
However, some material is always lost in drainage water,
of which, among the bases of the soil, those most likely to
be found are calcium, sodium, magnesium, and potassium;
and of the acids, carbonic, nitric, sulfuric, and hydrochloric.
Nitric acid and lime undergo the most serious losses.
The former may be curtailed to a great extent by keeping
crops growing on the soil during all the time that nitri-
fication is going on, and if the crop does not mature, or
if for any other reason it is not desired to harvest the
crop, it should be plowed under, to return the nitrogen in
the form of organic matter. A crop used for this purpose
is called a catch crop. Rye is used rather commonly as
a catch crop, as it continues growth until late in the fall
and resumes growth early in the spring, conserving ni-
trates whenever nitrification is likely to occur, and
it may then be plowed under to prepare the land for
another crop. Rye also has the advantage of small
cost for seed.
The loss of calcium cannot well be prevented, and
the use of commercial fertilizers always greatly increases
such loss. The only remedy is the application of some
form of calcium to the soil.
276. Drainage records at Rothamsted. Drainage
water from a series of plats at the Rothamsted Experi-
ment Station, which have been manured in various
ways and planted to wheat each year since 1852, have
been analyzed at certain times, and the results of these
analyses, as compiled by Hall, 1 give some idea of the loss
1 Hall, A. D. The Book of the Rothamsted experiments,
pp. 1237-239. New York, 1905.
23
370 SOILS: PROPERTIES AND MANAGEMENT
of salts from cultivated soils. The drainage water was
obtained from the tile drains, a line of which extended
under each plat from one end to the other and opened
into a ditch, so that the water could be collected when
desired. The analyses are shown in the table on page
371.
Ammoniacal nitrogen in the drainage water is very small
in quantity, but nitrate nitrogen is present in quantities
sufficient to make the loss of some concern. The use of
sodium nitrate occasioned the greatest loss of nitrogen,
while ammonium salts and farm manure contributed
nearly as much. From forty to fifty pounds of nitrogen to
the acre may be lost annually in this way ; this amount
would have a commercial value of eight or nine dollars.
277. Drainage records at Bromberg. It is not
always the case that a manured soil loses more fertilizing
material than an unfertilized one. Gerlach l reports
experiments in soil tanks at the Bromberg Institute of
Agriculture, as the result of which five soils, when ration-
ally fertilized, yielded larger crops and lost in the main
less nitrogen and lime in the drainage water than the
same soils unmanured. The loss of potash was slightly
greater*from the manured than from the unmanured soils.
Apparently the stimulation that the plants received
from the fertilizer enabled them to make such a good
growth that they absorbed more soluble nitrogen and
lime in excess of the unfertilized plants than was added
in the fertilizer, and nearly as much potash.
1 Gerlach, M. Ueber die durch Sickerwasser dem Boden
Entzogenen Menge Wasser und Nahrstoffe. Illus. Landw.
Zeitung, 30 Jahrgang, Heft 95, Seite 871-881. 1910. Also,
Untersuchungen iiber die Menge und Zusammensetzung der
Sickerwasser. Mitt. K. W. Inst. f. Landw. in Bromberg, Band
3, Seite 351-381. 1910.
THE ABSORPTIVE PROPERTIES OF SOILS 371
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COMPOSITION OF DRAINAGE WATER FR
MANURES APPLIED, RATE TO THE ACRE
Plat
2 Farm manure, 14 tons
3 and 4 No manure
5 Minerals only
6 Minerals + 200 pounds ammonium salts . .
7 Minerals -f- 400 pounds ammonium salts . .
8 Minerals + GOO pounds ammonium salts . .
9 Minerals + 550 pounds nitrate of soda . . .
10 400 pounds ammonium salts alone ....
11 400 pounds ammonium salts + superphosphate
12 400 pounds ammonium salts + superphos-
phates + sulphate soda
13 400 pounds ammonium salts + superphosphates
+ sulphate potash
14 400 pounds ammonium salts + superphosphates
+ sulphate magnesia t . . .
16 Minerals + 1,100 pounds nitrate of soda . .
372 SOILS: PROPERTIES AND MANAGEMENT
278. Losses of nitrogen and calcium. The most
serious losses of plant nutrients in the drainage water of
soils are those of nitrogen and calcium, and both are to
an extent unavoidable. Potassium and phosphorus,
which must also be purchased in manures, are lost only
at the rate of a few pounds to the acre. Nitrogen and
calcium may be conserved by maintaining a crop on the
soil continually. A large removal of nitrogen in the
drainage water is usually accompanied by a large re-
moval of calcium ; for nitrogen is leached from the soil
mainly in the form of nitric acid, which of course com-
bines with a base, and calcium being the base finally
liberated it is carried off in drainage water. While most
of the calcium in drainage water is in the form of bicar-
bonate, the quantity is greatly increased by nitric acid.
The relation of nitric acid to calcium in drainage water
is shown by experiments with soil in large tanks from which
drainage water was collected. Plants were grown in the
soil of certain tanks, while others had none, other conditions
being similar. Analyses of the drainage water at Ithaca,
New York, as reported by Lyon and Bizzell 1 show a greatly
increased loss of calcium from the unplanted tanks, from
which the loss of nitrate nitrogen was also much greater :
NITROGEN AND CALCIUM REMOVED IN DRAINAGE WATER BE-
TWEEN MAY 23, 1910, AND MAY 1, 1911. CALCULATED TO
POUNDS TO THE ACRE
CROP GROWN
NITRATE NITROGEN
CALCIUM
None
119.6
406.7
Maize
108
1580
Oats
12.5
173.4
1 Lyon, T. L., and Bizzell, J. A. Composition of the Drain-
age Water of a Soil with and without Vegetation. Jour. Indus,
and Eng. Chem., Vol. 3, pp. 742-743. 1911.
THE ABSORPTIVE PROPERTIES OF SOILS 373
Where crops were present to absorb the nitric acid,
calcium was greatly conserved. The quantities of ma-
terial carried off in drainage water was doubtless ab-
normally high in this case, as the soil had recently been
placed in the tanks.
279. Composition of surface water. Another method
proposed for obtaining these data is to analyze and
measure the water draining from a known area of land.
Norton * has done this in the valley of Richland Creek,
Arkansas, and has calculated the loss of a number of the
soil constituents. A comparison of the figures obtained
by Norton with those obtained by Lyon and Bizzell in
the experiments just quoted will give some idea of the
quantities of mineral matter removed from soils by
drainage water. The Arkansas soil had presumably
received little manure. The soil in the Cornell Uni-
versity tanks had previously received fifteen tons of
stable manure. The Arkansas drainage doubtless in-
cluded some surface water that had never passed through
the soil and was therefore poor in mineral matter; the
large quantity of volatile matter indicates its surface
nature, as water that passes through a soil contains little
organic matter.
There is little similarity in the results of these analyses.
They serve, however, to bring out the differences between
the composition of the run-off and the drainage water of
soils, in so far as that may be judged from widely dif-
ferent soils and climatic conditions, including the
rainfall.
1 Norton, J. H. Quantity and Composition of Drainage
Water and a Comparison of Temperature, Evaporation, and
RainfaU. Journal Am. Chem. Soo., Vol. 30, pp. 1186-1190.
1908.
374 SOILS: PROPERTIES AND MANAGEMENT
SUBSTANCES REMOVED IN DRAINAGE WATER FROM ONE ACRE
OF LAND. POUNDS IN ONE YEAR
LYON ANI
) BjZZELL
Planted Soil
Bare Soil
Total solids ....
Organic matter . . .
Nitrogen
Potash
794.0
134.0
4.0
50
800
11
6
2584
119
11
Phosphoric acid .
Lime
0.1
81
Trace
158
Trace
407
It will be seen that the total solids in the drainage
water from the Arkansas land and from the planted
tank were not greatly different in amount, but that
some of the constituents differed greatly. This was
notably the case with organic matter and with lime.
The former was doubtless carried largely in the run-off
and not in the leachings. The latter was probably more
abundant in the glaciated soil used in the tank than
in the residual soil of Arkansas.
CHAPTER XVII
ACID, OR SOUR, SOILS
SOME soils are known as acid, or sour, soils. The
property of acidity is of practical significance because
some plants do not grow so well on sour soils as they do
on soils that are neutral or alkaline ; on the other hand,
some crops prefer an acid soil. Sour soils are rarely met
with in arid regions, but in humid sections of the United
States they are commonly found.
280. Nature of soil acidity. Soils may be acid, or
sour, so far as their relation to plant growth is concerned,
(1) when free acids are present, (2) when no soluble free
acid exists, but when there is a deficiency of basic material
in the soil. Decomposition of organic matter in certain
soils under an inadequate supply of oxygen often results
in the formation of considerable quantities of organic
acids, as has already been explained (par. 93).
281. Positive acidity. The formation of organic
acids under conditions of insufficient oxygen supply is
frequently seen in muck and other soils high in organic
matter that are saturated with water and that are also
deficient in lime. In such cases an acid condition is very
likely to be found, but when the land is drained the
acidity usually disappears because of the better aeration
resulting. When a large quantity of green vegetation
is plowed under, as is done in green-manuring land, a
sour condition sometimes appears after the material has
375
376 SOILS: PROPERTIES AND MANAGEMENT
had time partially to decay. The acidity of soil that
arises from the presence of free acids has been termed
positive acidity.
It is to be presumed that soils in which free acids exist
are rather deficient in basic material, and that the bases
are held so firmly combined that some of the relatively
weak organic acid present is not capable of forming salts
with them. Plummer 1 has shown that dihydroxystearic
acid when added to an acid soil had a distinctly toxic
effect on wheat plants, but when added to the same soil
previously treated with lime there was no toxic effect,
indicating that this substance retained its acid properties
in the unlimed soil.
282. Negative acidity. A soil deficient in basic
material but containing no soluble free acids may be
sour as regards its relation to plant growth. At least
such a soil may be greatly benefited by liming, although
it shows no acidity to most of the ordinary indicators of
acidity when these are used in the customary way. This
condition has been termed negative acidity and is really
not acidity according to a correct use of the word. Such
acidity does not have a direct effect on the plant, but
an indirect one arising from a lack of bases. Soils that
are acid in this sense always have a large capacity for
absorbing lime or other bases, before exhibiting an al-
kaline reaction. Calcium being, as has already been
seen (par. 264), the base most liberally released to solution,
there is a tendency toward the formation of calcium
carbonate in any soil dependent on the equilibrium be-
1 Plummer, J. K. The Isolation of Dihydroxystearic Acid
from Volusia Silt Loam. Thesis presented in partial fulfillment
of the requirements for the degree of Master of Science. Cornell
University Library (not published). 1911.
ACID, OR SOUR, SOILS 377
twcen the basic material and the absorptive substances
in the soil. Thus, a soil containing large quantities of
clay, and other absorbent substances requires more basic
material for the formation of calcium carbonate than
docs a soil having less absorptive material. Further-
more, with the same original content of basic material,
the former soil requires a greater addition of lime to
overcome its sourness than does the latter. For this
reason a heavy soil usually requires a larger dressing of
lime to correct its acidity than does a light one.
Even if a soil does not have its absorptive capacity for
bases satisfied, there is some formation of calcium car-
bonate constantly taking place, as is evidenced by the
removal of the bicarbonate of calcium in the drainage
water of soils that are distinctly acid. The benefits that
soils derive from the presence of calcium carbonate will
be mentioned later (pars. 454-457). It need only be said
here that its presence in insufficient quantity constitutes
a form of so-called acidity, or sourness, in soils. The
formation of calcium carbonate in a given soil increases
with the mass of base. The effect of an application
of lime, therefore, is to increase the quantity of car-
bonate formed, even when the absorptive capacity of
the soil is not satisfied. This is why even relatively
small applications of lime are beneficial to soils having
great absorptive capacity.
283. Production of sour soils. Soils in a humid
ion tend to become acid. This may be due to any one
or more of several causes: (1) removal of calcium and
other bases in drainage water; (2) removal of bases by
plants; (3) formation of salts of the bases with organic
matter incorporated with soil; (4) accumulation of acid
residues of fertilizers.
378 SOILS: PROPERTIES AND MANAGEMENT
284. Removal of bases by drainage as a cause for
acidity. The most potent cause of acid soils is doubt-
less the removal of bases in drainage water. The quan-
tities of basic material that may be lost from an acre of
soil are shown elsewhere (pars. 278, 279). These bases are
removed largely as bicarbonates, being obtained from
the hydrated aluminium silicates and other colloidal
matter. When the soil is uncropped a considerable loss
of lime occurs in the form of nitrate. As the decom-
position of the organic matter of the soil always results
in the formation of carbon dioxide and nitric acid, and
as decomposition is continually going on except when the
temperature of the soil becomes too low to admit of it,
the drain of bases from the soil is almost continuous.
Formation of carbon dioxide and of nitric acid occurs
largely in the surface soil; consequently the removal
of bases begins there. The result is that soils are
likely to contain less calcium in the surface layers than
at lower depths. Ames and Gaither 1 have shown from
a large number of analyses of Ohio soils that those con-
taining calcium carbonate in appreciable quantities have
more calcium in the subsoil than in the surface six
inches. In other soils this was not uniformly the case.
Leaching is, of course, greater in amount where con-
siderable quantities of calcium carbonate are present
than where it is lacking.
285. Removal of bases by plants. Plants always
remove more bases than acids from soils in the process
of their growth. The table in paragraph 339 showing the
composition of the ash of some crops indicates that the
calcium, potassium, and magnesium removed from
1 Ames, J. W., and Gaither, E. W. Soil Investigations.
Ohio Agr. Exp. Sta., Bui. 261. 1913.
ACID, OR SOUK, SOILS 379
the soil in this way is very considerable. When the
vegetation on the land is returned to it after life
ceases and its organic material is again incorporated
with the soil, there is no loss in this way, but in ordi-
nary agricultural practices most of the above-ground
portion of the crops is removed from the land. The
manure of growing animals returns to the soil only a
small proportion of the calcium that was originally in
the plants.
Breazeale and LeClerc l found that the selective action
of plants in absorbing more bases than acids from a
nutrient solution caused the solution to become toxic to
wheat seedlings because of its acidity.
286. Effect of green manures on acidity. Although
the return of vegetation to the land on which it grew
does not result in any actual loss of basic material to
the soil, it generally results in the formation and libera-
tion of organic acids that unite with the basic material
and thus render it neutral. In soils deficient in lime
the incorporation of green-manure crops has been con-
sidered to temporarily produce an acid condition.
Coville 2 determined the acidity of some green-ma-
nure crops, on the basis of which he has estimated
the acidity, in terms of ground limestone required to
neutralize it, when the lime contained in the crop is de-
ducted from the total lime required. This is given in the
table on the next page.
1 Breazeale, J. F., and LeClerc, J. A. The Growth of Wheat
Seedlings as Affected by Acid or Alkaline Conditions. U. S.
D. A., Bur. Chem., Bui. 149. 1912.
2 Coville, J. V. The Agricultural Utilization of Acid Lands
by Means of Acid-Tolerant Crops. U. S. D. A., Bui. No. 6,
p. 5. 1913.
380 SOILS: PROPERTIES AND MANAGEMENT
WEIGHT, LIME CONTENT, AND ACIDITY OF GREEN MANURES
TO THE ACRE
CROP
i WEIGHT
(tons)
LIME
CONTENT
(pounds)
ACIDITY, EX-
PRESSED AS LIME
REQUIREMENT
(pounds)
Alfalfa
2
139
267
Red clover ....
Cowpea
Rye . ....
2
2J
2
131
92
11
142
200
178
Broom sedge ....
1
4
89
As decomposition proceeds the acids are oxidized, and
finally basic material is held largely in combination with
so-called humus of the soil. This is doubtless in the
form of a colloidal complex, not a definite chemical com-
pound. Analyses by Snyder 1 of purified humous ash
from eight productive prairie soils have been averaged
and are presented in tabular form in paragraph 97.
The quantity of basic material ordinarily held by the
organic matter of the soil is small compared with the
total soil content. The bases contained in humus are
principally potassium and sodium not calcium, as
might be expected in the salt of an organic acid formed
in the soil. Humus in the soil tends to overcome acidity
and functions as an alkali. In respect to its composition
and properties, much of it resembles a colloidal com-
plex rather than a chemical combination of soil bases
with organic acids.
It has often been observed that land from which forest
has been cleared will yield good crops of red clover for
1 Snyder, Harry. Soils. Minnesota Agr. Exp. Sta., Bui.
41. 1895.
ACID, OR SOUR, SOILS 381
several decades, after which it becomes more and more
difficult to obtain a crop until the attempt must finally
be abandoned. The change from forest to tillage has
opened the way for an acid condition of soil, through the
loss of bases carried off in the crops and the destruction
of humus by tillage. The dissipation of humus is doubt-
U-ss the more serious source of loss. Instances may be
cited in which a farm has been so managed as to main-
tain the humus supply and the ability of the soil to pro-
duce red clover, although surrounding farms, on which
humus has been depleted, have completely failed to grow
this crop.
Apparently humus holds the basic constituents of the
soil in a form in which they function as rather easily
soluble salts, instead of locking them up as insoluble
silicates. A given quantity of base in a soil is therefore
more effective in preventing acidity by combining with
weak acids, and possibly in forming carbonates, if the soil
is well supplied with humus than if it is lacking in that
constituent.
287. Effect of fertilizers on soil acidity. That the
continued use of ammonium sulfate on land may result
in producing a sour condition has been shown by a num-
ber of investigators. The absorption and nitrification
of the ammonia of that salt, and its final utilization by
plants, leaves sulfuric acid, which combines with calcium
and escapes in the drainage water. This may occur even
when this fertilizer is used in quantities not excessive,
but continued for many years, as has been shown by
Gardner and Brown 1 at the Pennsylvania Experiment
1 Gardner, F. D., and Brown, B. E. The Lime Require-
ment of the General Fertilizer Plats as Determined Periodically.
Kept. Pennsylvania Agr. Exp. Sta., 1910-1911, pp. 25-60.
382 SOILS: PROPERTIES AND MANAGEMENT
Station. Other fertilizers leaving an acid radicle in the
soil also act in this way. It is conceivable that potassium
chloride and potassium sulfate might have a tendency
to produce an acid condition, but the bases in these salts
do not disappear from the soil so quickly as would am-
monia, and consequently their action is slower.
The use of free sulfur on the land as a means of com-
bating certain fungous diseases may lead to the formation
of a sour soil through the oxidation of the sulfur with
formation of sulfuric acid. Lint l has found that a
soil in which sulfur was used at the rate of 600 pounds
to the acre for prevention of potato scab, changed in ks
lime requirement from 2431 pounds to 4177 pounds as a
result of the one treatment.
288. Acidity in relation to climate and to formation of
soil. In an arid or a semiarid climate soils are not likely
to become sour. The great source of lime removal,
leaching, operates to only. a slight extent, or not at all,
in a dry climate. The removal of bases in crops is ap-
parently offset by the upward movement of bicarbonates
in the capillary water. Experience shows that acidity is
not a problem in soils of dry countries.
Soils that are derived from limestone or that have been
mixed with limestone soils in the process of their forma-
tion are, under similar climatic conditions, less likely
to become acid than are soils that originally contained
less lime. The fact that a soil is derived from limestone,
however, does not insure that it may not be benefited by
an application of lime.
289. Weeds that flourish on sour soils. The acidity
or the basicity of soils influences very greatly the growth
1 Lint, H. Clay. The Influence of Sulfur on Soil Acidity.
Jour. Indus, and Eng. Chem., Vol. 6, pp. 747-748. 1914.
ACID, OR SOUR, SOILS 383
of vegetation and determines to a large degree its nature.
The flora undergoes a considerable variation as a soil
changes from a basic to a sour condition. This is because
some plants are injured to a greater extent than are others
by the conditions that accompany an acid reaction of the
soil. Some higher plants really grow better on a sour
soil than they do on an alkaline one, but these form only
a minority of the plants of agricultural importance.
\\Yeds that abound and appear to flourish on acid soils
may do so either because they grow better on sour soil
than on basic, or because other vegetation growing on
the soil does not thrive and therefore the dominant weeds
of the region have less competition than they otherwise
would have. There are certain weeds that may be taken
to indicate a sour soil when present in large numbers.
Some of these are found in one part of the country and
some in another :
WEEDS THAT FLOURISH ON SOUR SOILS
Common name Botanical name
Sheep sorrel 1 . . . Rumex acetosella
Paintbrush .... Hieracium aurantiacum
Daisy . , . . . Bellis perennis
Horsetail rush 2 . . Equisetum arvense
Corn spurry 2 . . . Spergula arvensis
Wood horsetail 2 . . Equisetum syhaticum
Plantain l . . . . Plantago major
Goose grass 3 . . . Polygonum aviculare
1 Knisely, A. L. Acid Soils. Oregon Agr. Exp. Sta., Bui.
90, p. 23. 1906.
2 Whitson, A. R., and Weir, W. W. Soil Acidity and Liming.
Wisconsin Agr. Exp. Sta., Bui. 230, pp. 7-11. 1913.
3 Voelcker, J. A. The Woburn Field Experiments. Jour,
Royal Agr. Soc. England, Vol. 69, pp. 337-357. 1908.
384 SOILS: PROPERTIES AND MANAGEMENT
290. Crops adapted to sour soils. There are some
useful agricultural plants that grow better on sour soils
than on alkaline soils, while other plants are apparently
indifferent to the condition of the soil in this respect. As
acid soils are of very common occurrence, and as the
correction of this difficulty may not always be financially
profitable or otherwise desirable, it is important to know
what plants will thrive and how agricultural practice
may be maintained on such soils. A list of these plants,
based on different authorities, is herewith given :
CROPS ADAPTED TO SOUR SOILS
Blueberry 1 Hairy vetch 1
Cranberry 2 Crimson clover 1
Strawberry 1 Potato 2
Blackberry 2 Sweet potato 1
Raspberry 2 Rye 2
Blackcap 2 Millet 2
Watermelon 2 Buckwheat 1
Turnip * Carrot 1
Red top 2 Lupine 2
Rhode Island bent-grass 2 Serradella 2
Cowpea 1 Radish 2
Soybean l Velvet bean 2
Castor bean 2
The very considerable number of these plants, and
especially the inclusion among them of legumes that
may be grown for soil improvement, suggest the possi-
1 Coville, F. W. The Agricultural Utilization of Acid Lands
by Means of Acid-Tolerant Crops. U. S. D. A., Bui. No. 6,
pp. 7-12. 1913.
2 Wheeler, H. J. The Liming of Soils. U. S. D. A., Farmers'
Bui. 77. 1905.
AC'TD, OR SOUR, SOILS 385
bility of a successful agricultural practice on acid soils
where the important money crop to be grown, or some
other condition, would make it undesirable to correct
the soil acidity. There are certain crops, such as blue-
berries and cranberries, that require an acid soil ; there
are others, such as potatoes, that may suft'er less from
disease if the soil is sour. These crops are sometimes the
ones that are of greatest financial importance in a region,
and it therefore becomes desirable to maintain an acid
condition of soil.
291. Crops that are injured by acid soils. There
are many plants that are injured by a sour condition of
the soil, and these include some of the most important
farm crops. It should therefore be borne in mind that
for most farm practice an acid soil is very undesirable.
One notable reason for this is that such crops as red
clover and alfalfa, which are of great value both as a
means of improving soil and for hay, can be grown only
with great uncertainty or not at all on acid soils.
CROPS THAT ARE INJURED BY SOUR SOILS 1
Alfalfa Salsify Cauliflower
Red clover Squash Cabbage
Saltbush Spinach Cucumber
Timothy Red beet Lettuce
Kentucky blue-grass Sorghum Onion
Maize Barley Okra
Oats Sugar beet Peanut
Pepper Currant Tobacco
Parsnip Mangel-wurzel Kohlrabi
Pumpkin Celery Eggplant
1 Wheeler, H. J. The Laming of Soils. U. S. D. A., Farmers'
Bui. 77 (revised). 1905.
2c
386 SOILS: PROPERTIES AND MANAGEMENT
While soils may be either sour or alkaline, there are
also degrees of sourness. Thus a soil may be so sour as
to completely prevent the growth of one kind of plant
and yet produce excellent crops of another plant which
would have perished if the soil had been more acid. For
example, red clover will grow fairly well on soil that is
too sour to raise alfalfa.
292. Qualitative tests for acidity. A simple test to
indicate an acid condition of soil is not so easy of execu-
tion nor so infallible in its prediction as might be desired.
The object of such a test is to ascertain whether a soil is
not well adapted to the growth of certain plants and
whether the application of lime would benefit it in this
respect. A number of tests have been proposed which
will be outlined and briefly discussed.
293. Litmus paper test. Blue litmus paper is brought
into contact with the wet soil. A rapid and decided
change to red is taken to indicate an acid condition of
the soil. Carbonic acid, which is always present in soils,
is supposed to give only a faint pink color to the litmus
paper. Various ways of bringing the paper into contact
with the soil have been recommended, among others the
interposing of filter paper between the soil and the litmus
paper. 1 It is also generally pointed out that the acid
perspiration on the fingers may lead to delusion.
A criticism of the test has been made by Cameron, 2
who states that the absorbent action of soils for bases is
greater than is that of paper, while for acids the reverse
1 Kellerman, K. F., and Robinson, T. R. Legume Inoculation
and the Litmus Reaction of Soils. U. S. D. A., Bur. Plant Indus.,
Circ. 71, pp. 3-11. 1910.
2 Cameron, F. K. The Soil Solution, pp. 65-66. Easton,
Pennsylvania. 1911.
ACID, OB SOUK, SOILS 387
is the case. Consequently the base that had produced
the blue color is absorbed from the litmus, leaving the
acid compound, which is red. Cameron concludes that
the test is unreliable, and proposes to extract the soil
with water, boil it in order to expel carbon dioxide, and
then test the reaction of the solution.
Much litmus paper that is sold is of very poor quality ;
but when good paper is used and the test is carefully
made, the general experience has been that it is a fairly
good, although not an infallible, guide to the need of a
soil for lime. Red coloration due to absorptive action is
probably an advantage rather than a source of error in
the test, as a soil strongly absorptive of bases is likely
to need lime. This coloration does not necessarily in-
dicate the presence of free acid, but merely need of lime.
294. Ammonia test. In this test the soil is stirred
with a dilute solution of ammonia hydroxide. After
settling, if the supernatant liquid on standing takes on
a dark chocolate or a black color it is said to be acid.
This method, which has been proposed by Muntz, 1 is
not of general application and would not always be re-
liable in the case of soils of arid regions. The depth of
color is not a guide to the degree of acidity, since many
acid soils are low in organic matter.
295. Zinc sulfide test. A test recently proposed by
Truog 2 consists in mixing the soil to be tested with a
small quantity of calcium chloride and a very little zinc
sulfide. Water is added and the mixture is heated to
1 Wheeler, H. J., Hartwell, B. L., and Sargent, C. L. Chemi-
cal Methods for Ascertaining the Lime Requirements of Soils.
Rhode Island Agr. Exp. Sta., Bui. 62, pp. 65-88. 1899.
2 Truog, E. A. New Method for the Determination of Soil
Acidity. Science, N. S., Vol. 40, pp. 246-248. 1914.
388 SOILS: PROPERTIES AND MANAGEMENT
boiling. A strip of moistened lead acetate paper is held
over the mouth of the flask for three minutes while the
boiling proceeds. If the soil is acid, the paper will be
darkened on the underside; if the soil is not acid, no
darkening will occur.
This method is evidently designed to test the need of
the soil for lime as well as actual acidity, for the absorp-
tion of calcium from the dissociated chloride would leave
free hydrochloric acid. The action of this acid on zinc
sulfide would generate hydrogen sulfide, thus blackening
the lead acetate paper.
A somewhat similar principle is involved in the proposal
to use a solution of potassium nitrate in the litmus paper
test.
296. Litmus paper and potassium nitrate. This is
performed in the same manner as the former litmus
paper test, except for the substitution of a saturated
solution of potassium nitrate instead of distilled water
for moistening the soil.
297. Acid test for carbonates. In this test a dry
sample of the soil is treated with a few drops of dilute
hydrochloric acid. Effervescence indicates the presence
of carbonates or bicarbonates in sufficient quantities to
insure an alkaline soil, although sometimes lime may
still be beneficial.
Whitson and Weir 1 have objected to this method on
the ground that the displacement of air in the pore spaces
of the soil by the dilute acid may be mistaken for evolu-
tion of carbon dioxide. In the hands of an experienced
and careful operator this would not necessarily invalidate
the method.
1 Whitson, A. R., and Weir, W. W. Soil Acidity and Lim-
ing. Wisconsin Agr. Exp. Sta., Bui. 230, pp. 7-11. 1913.
ACID, OR HOUR, SOILS 389
298. Plants as indicators of acidity. In addition
to these chemical tests for acidity there may also be
mentioned what is perhaps the most reliable indication
of the need of lime, namely, the failure of a soil to produce
red clover, and the presence of those weeds that have
previously been shown to thrive on sour soil (par. 289).
When a soil bears this relation to the plant growth it may
safely be assumed that those plants included in the list of
crops that are injured by sour soils \vill yield better if the
soil is limed than if it is not so treated. The crops adapted
to sour soils may not be injured.
299. Quantitative determinations of acidity. A num-
ber of quantitative methods for determining the degree
of acidity or the lime requirements of soils have been
devised. Only a few of these need be mentioned.
300. Potassium nitrate method. 1 The soil is shaken
with a normal solution of potassium nitrate for three
hours, and then allowed to stand overnight. An aliquot
portion of the supernatant liquid is boiled in order to
expel carbon dioxide, and when cool it is titrated with
a standard solution of sodium hydroxide.
This method does not estimate either the free acid
or the lime requirement of the soil. What it does is
to give the absorptive power of the soil for potassium
when in equilibrium with a solution containing the
acid with which the potassium was originally in com-
bination. There is a substitution of bases during the
contact of the nitrate solution with the soil, and a
partial decomposition of these salts during the titration
with alkali.
1 Official and Provisional Methods of Analysis. Association
of Official Agricultural Chemists. U. S. D. A., Bur. Chem.,
Bui. 107 (revised), p. 20. 1908.
390 SOILS: PROPERTIES AND MANAGEMENT
301. Limewater method. A measured quantity of
a standard solution of limewater is brought into contact
with the soil and absorption is accomplished by evapora-
tion, after which water is added and the filtrate is tested
with phenolphthalein. Failure to produce a pink color
shows that the lime requirement of the soil has not been
reached; an alkaline reaction shows that an excess of
lime has been added. A number of tests must be made
in order to reach a point below which the indicator shows
no color and above which it does. The lime requirement
may thus be indicated. This determination was devised
by Veitch, 1 and is a useful method since it indicates to
within a few hundred pounds the quantity of lime re-
quired to satisfy the absorptive power of a soil.
302. Resume. In conclusion, a few facts regarding
so-called acid soils may be restated : (1) acidity is not
always due to free acids, but often to the lack of an abun-
dance of bases ; (2) it is not injurious to all plants, but is
likely to depress the yields of the majority of agricultu-
rally important crops, while some valuable ones are bene-
fited by it; (3) it may be overcome sometimes by aera-
tion of the soil, and always by the application of lime or
wood ashes. The correction of acidity by means of lime
will be discussed in a later chapter, as will also the rela-
tion of certain bacteria to acidity.
1 Veitch, F. P. The Estimation of Soil Acidity and the Lime
Requirements of Soils. Jour. Am. Chem. Soc., Vol. 24, pp.
1120-1128. 1902.
CHAPTER XVIII
ALKALI SOILS
IT has already been shown that soils are acted upon by
a great variety of weathering agents which gradually
render soluble a portion of the most susceptible constitu-
ents. This soluble material becomes a part of the soil
solution and may come in contact with the roots of any
crop growing on the soil. In humid regions, where a
large quantity of water percolates through the soil, this
soluble matter has little opportunity to collect. In arid
regions, however, where loss by drainage is slight, these
salts may often collect in large amounts. During periods
of drought they are carried upward by the capillary rise
of the soil water, while during periods of rainfall they may
move downward again in proportion to the leaching action.
At one time the lower soil may contain considerably more
soluble salt than the upper ; at another time the condition
may be reversed, in which case the solution in contact
with plant roots may contain so much soluble matter
that vegetation is injured or destroyed. This excess of
soluble salts usually has a marked alkaline reaction, but
in any case it produces what is termed an alkali soil.
303. Composition of alkali salts. The materials dis-
solved in the soil water consist of all the substances found
in the soil, but as the rates of solubility of these substances
vary greatly there is accumulated a much larger quantity
of some substances than of others. Carbonates, sulfates,
391
392 SOILS: PROPERTIES AND MANAGEMENT
and chlorides of sodium, potassium, calcium, and mag-
nesium occur in the largest amounts. Sodium may be
present as carbonate, sulfate, chloride, phosphate, and
nitrate. Potassium may be similarly combined. Mag-
nesium is likely to appear as a sulfate or a chloride, and
calcium as a sulfate, a chloride, or a carbonate. One
salt will predominate in some soils, and other salts in other
soils. A base may be present in combination with several
different acids. The nature of the prevailing salt greatly
influences the effect on vegetation. The table on page
393 gives the composition of the soluble salts from a
number of alkali soils.
A few years ago Headden l called attention to large
accumulations of nitrates in certain localities in Colorado.
These salts dissolve in the soil water and are frequently
present in such large quantities as to be injurious to
vegetation.
304. White and black alkali. Sulfates and chlorides
of the alkalies, when concentrated on the surface of the
soil, produce a white incrustation, which is very common
in alkali regions during a dry period as a result of evapora-
tion of moisture. Incrustations of this character are
called white alkali.
Carbonates of the alkalies, particularly sodium car-
bonate, dissolve organic matter from the soil, thus giving
a dark color to the solution and to the incrustation. For
this reason alkali containing large quantities of these
salts is called black alkali. Black or brown alkali may
also be produced by calcium chloride or by an excess of
sodium nitrate.
1 Headden, W. P. ' Deterioration in the Quality of Sugar
Beets due to Nitrates Formed in the Soil. Colorado Agr.
Exp. Sta., Bui. 183. 1912.
ALKALI SSLTS
393
PERCENTAGE COMPOSITION OF ALKALI SALTS IN SOILS
COLORADO l
EXP. STA.
CALIFORNIA, 2
(TULARE)
EXP. STA.
YAKIMA,
WASHINGTON, 8
12-24 INCHES
BILLINGS,
MONTANA 4
YUMA,
ARIZONA B
1
O
!*
uO,a
3<-i o
02
1
O
0-72 In-
ches
KC1
K 2 S0 4 . . .
K 2 CO 3 . .
Na 2 SO 4 . .
NaNO 3 . .
Na 2 CO 3 . .
NaCl . . .
Na 3 HPO 4 . .
MgS0 4 . .
MgCl 2 . . .
CaCl 2 . . .
NaHC0 3 . .
CaSO 4 . .
Ca(HCO 3 ) 2 .
Mg(HC0 3 ) 2 .
(NH 4 ) 2 C0 3 .
1.64
5.61
1.60
4.00
22.10
3.95
21.41
9.73
25.28
19.78
32.58
14.75
2.25
85.57
35.12
33.07
13.86
7.28
6.61
0.55
81.15
13.77
8.90
4.06
6.88
3.98
12.71
17.29
7.71
0.25
0.28
6.61
36.72
1.87
16.48
15.73
0.67
2.71
22.06
10.07
21.02
32.25
21.48
1.41
1 Headden, W. P. The Fixation of Nitrogen. Colorado
Agr. Exp. Sta., Bui. 155, p. 10. 1910.
2 Hilgard, E. W. Soils, p. 442. New York, 1906.
3 Dorsey, C. W. Alkali Soils of the United States. U. S.
D. A., Bur. Soils, Bui. 35, p. 79. 1906.
4 Ibid., p. 103. 5 Ibid. t p. 109.
394 SOILS: PROPERTIES AND MANAGEMENT
Black alkali is muck more destructive to vegetation
than is white. A quantity of white alkali that would
not seriously interfere with the growth of most crops
might completely prevent the development of useful
plants if the alkali were black.
305. Effect of alkali on crops. The presence of
relatively large amounts of salts dissolved in water and
brought into contact with a plant cell has been shown to
cause a shrinkage of the protoplasmic lining of the cell,
the shrinking increasing with the concentration of the
solution. This causes the plant to wilt, to cease growth,
and finally to die. The nature of the salt, and the species
and even the individuality of the plant, determine the
point of concentration at which the plant succumbs.
The directly injurious effect of the chlorides, sulfates,
nitrates, and other salts of the alkalies and alkali earths
is due to this action on the cell contents of the plants.
The carbonates of the alkalies have, in addition, a cor-
roding effect on the plant tissues, dissolving the parts
of the plant with which they come in contact. Indirectly
alkali salts may injure plants by their influence on the
soil tilth, soil organisms, and fungous and bacterial
diseases.
306. Effect on different plants. The factors that
determine the tolerance of plants toward alkali are:
(1) the physiological constitution of the plant; (2) the
rooting habit. The first is not well understood, but
resistance varies with species, and even with individuals
of the same species. So far as the rooting habit influences
tolerance of alkali, the advantage is with the deep-rooted
plants such as alfalfa and sugar beets, probably because
a part of the root is in a less strongly impregnated part
of the soil.
ALKALI SALTS
395
Of the cereals, barley and oats are the most tolerant,
these being able in some cases to produce good crops on
soil containing two-tenths per cent of white alkali. Of the
forage crops, a number of valuable grasses are able to
grow on soil containing considerably more than two-tenths
per cent of alkali. Timothy, smooth brome, and alfalfa
are the cultivated forage plants most tolerant of alkali,
although they do not equal the native grasses in this
respect. Cotton also tolerates a considerable amount of
alkali.
Lough ridge, 1 after experiments and observation for a
number of years, has obtained data regarding the resist-
ance of various crops to the several alkali salts. His
results are given in part below, expressed in pounds to
an acre to a depth of four feet :
CROP
NazSO*
Na 2 CO 3
NaCl
TOTAL ALKALI
Grapes . . .
40,800
7,550
9,640
45,760
Oranges . . .
18,600
3,840
3,360
21,840
Pears . . .
17,800
1,760
1,360
20,920
Apples . . .
14,240
640
1,240
16,120
Peaches . . .
9,600
680
1,000
11,280
Rye ....
9,800
960
1,720
12,480
Barley . . .
12,020
12,170
5,100
25,520
Sugar beets
52,640
4,000
5,440
59,840
Sorghum . .
61,840
9,840
9,680
81,360
Alfalfa . . .
102,480
2,360
5,760
110,320
Saltbush . .
125,640
18,560
12,520
156,720
1 Loughridge, R. H. Tolerance of Alkali by Various Cul-
fcures. California Agr. Exp. Sta., Bui. 133. 1901. See also
Kearney, T. II., and Harter, L. L. Comparative Tolerance of
Various Plants for the Salts Common in Alkali Soils. U. S. D. A..
Bur. Plant Indus., Bui. 113. 1907.
396 SOILS: PROPERTIES AND MANAGEMENT
Although in general the results as to the resistance to
alkali of the various crops are so conflicting, the Bureau
of Soils, 1 in its alkali mapping, has been able to make a
rough classification as follows :
PERCENTAGE OF TOTAL
SALTS
PERCENTAGE OF
BLACK ALKALI
CROPS
to 0.20
0.20 to 0.40
Less than 0.05
0.05 to 0.10
All crops grow
All but most sensitive
0.40 to 0.60
0.60 to 1.00
1.00 to 3.00
0.10 to 0.20
0.20 to 0.30
0.30 and above
Old alfalfa, sugar beet,
barley, and sorghum
Only most resistant plants
No plants
307. Other conditions that influence the action of
alkali. The higher the water content of the soil, the
less is the injury to plants from alkali; but should
the same soil become dry, the previous large quantity of
water would, by bringing into solution a larger amount
of alkali, render the solution stronger than it would
otherwise have been, and thus cause greater injury (see
Fig. 57).
The distribution of the alkali at different depths may
have an important bearing on its effect on plants.
Young plants and shallow-rooted crops may be entirely
destroyed by the concentration of alkali at the surface,
while the same quantity evenly distributed through the
soil, or carried by moisture to a lower depth, would have
caused no injury. A loam soil, by reason of its greater
water-holding capacity and adsorptive power, will carry
more alkali without injury to plants than will a sandy
1 Dorsey, C. W. Alkali Soils of the United States. U. S.
D. A., Bur. Soils, Bui. 35, pp. 23-25. 1906.
ALKALI SALTS
397
soil. Certain of the alkali salts exert a deflocculating
action on clay soils and effect an indirect injury in that
way.
/H 100 OF jotL
7
* -MKALI H.\f>OfHH
FIG. 57. Diagram showing the amount and composition of alkali salts
at various depths. Tulare, California.
308. Accumulation of alkali. The alkali salts, being
readily soluble, are carried by the soil water where there
is any lateral movement, as is often the case where land
slopes to some one point. Low-lying lands adjacent to
such slopes are thus likely to contain considerable alkali,
and the " alkali spots " of semiarid regions and the
large accumulations of alkali in many of the valley lands
of arid regions are traceable to this cause.
309. Irrigation and alkali. In irrigated regions, the
injurious effect of alkali is in many cases discovered only
398 SOILS: PROPERTIES AND MANAGEMENT
after irrigation has been practiced for a few years. This
is due to what is known as a " rise of alkali," and comes
about through the accumulation, near the surface of the
soil, of salts that were formerly distributed throughout
a depth of perhaps many feet. Before the land was
irrigated, the rainfall penetrated only a slight depth into
the soil, and when evaporation took place, salts were
drawn to the surface from only a small volume of soil.
When, however, irrigation water is turned on the land,
the soil becomes wet to a depth of perhaps fifteen or
twenty feet. During the portion of the year in which
the soil is allowed to dry, large quantities of salts are
carried toward the surface by the upward-moving capil-
lary water. Although these salts are in part carried
down again by the next irrigation, the upward movement
constantly exceeds the downward one. This is because
the descending water passes largely through the non-
capillary interstitial spaces, while the ascending water
passes entirely through the capillary spaces. The smaller
spaces, therefore, contain a considerable quantity of
soluble salts after the downward movement ceases and
the upward movement begins. In other words, the
volume of water carrying the salts downward in the
capillary spaces is less than that carrying them upward
through these spaces. Surface tension causes the salts
to accumulate largely in the capillary spaces, and it is
therefore the direction of the principal movement through
these spaces that determines the point of accumulation
of the alkali.
There are large areas of land in Egypt, in India, and
even in France and Italy, as well as in this country, that
have suffered in this way, and not infrequently they have
reverted to a desert state.
ALKALI SALTS 399
310. The handling of alkali lands. 1 Ordinarily there
are two general ways in which alkali lands may be handled
in order to avoid the injurious effect of soluble salts.
The first of these is eradication, the second may be
designated as control. In the former case, an at-
tempt is made to actually eliminate by various means
some of the alkali. In the latter, methods of soil
management are employed which will keep the salts
well distributed throughout the soil. In many cases
soils would grow excellent crops if the alkali could
only be kept well distributed through the soil layers
so that no toxic action could occur, at least within
the root zone. In general, steps should always be taken
toward the control of alkali, whether eradication is at-
tempted or not. Under irrigation, careful control is
always wise.
311. Eradication of alkali. Of methods designed to
at least partially free the soil of alkali, the commonest
are: (1) leaching with underdrainage, (2) correction with
gypsum, (3) scraping, and (4) flushing.
312. Leaching with underdrainage. Of the various
methods for removing an excess of soluble salts, the use
of tile drains is the most thorough and satisfactory.
When this method is used in an irrigated region, heavy
and repeated applications of water must be made, to
leach out the alkali from the soil and drain it off through
the tile. When used for the amelioration of alkali spots
1 Dorsey, C. W. Reclamation of Alkali Soils. U. S. D. A.,
Bur. Soils, Bui. 34. 1906. Also, Hilgard, E. W. Utilization
and Reclamation of Alkali Lands, Soils. New York, 1911.
Also, Brown, C. F., and Hart, R. A. Reclamation of Seeped
and Alkali Lands. Utah Agr. Exp. Sta., Bui. 111. 1910.
Also, Dorsey, C. W. Reclamation of Alkali Soils at Billings,
Montana. U. S. D. A., Bur. Soils, Bui. 44. 1907.
400 SOILS: PROPERTIES AND MANAGEMENT
in a semiarid region, the natural rainfall will in time
effect the removal.
In laying tiles it is necessary to have them at such a
depth that soluble salt in the soil beneath them will not
readily rise to the surface. This will depend on those
properties of the soil governing the capillary move-
ment of water. Three or four feet in depth is usually
sufficient, but the capillary movement should first be
estimated.
After the drains have been placed, the land is flooded
with water to a depth of several inches. The water is
allowed to soak into the soil and to pass off through the
drains, leaching out part of the alkali in the process.
Before the soil has time to become very dry the flooding
is repeated, and the operation is kept up until the land
is brought into a satisfactory condition.
Crops that will stand flooding may be grown during
this treatment, and they will serve to keep the soil
from puddling, as it is likely to do if allowed to become
dry on the surface. If crops are not grown, the soil
should be harrowed between floodings. The operation
should not be carried to a point where the soluble
salts are reduced below the needs of the crop, or so
low that, they lose entirely their effect on the retention
of moisture.
313. Correction with gypsum. The use of gypsum
on black alkali land has sometimes been practiced for
the purpose of converting the alkali carbonates into
sulfates, thus ameliorating the injurious properties of
the alkali without decreasing the amount. The quantity
of gypsum required may be calculated from the amount
and composition of the alkali. The soil must be kept
moist, in order to bring about the reaction, and the
ALKALI SALTS 401
gypsum should be harrowed into the surface, not plowed
under. The reaction is as follows :
Na 2 CO 3 + CaSO 4 = CaCO 3 + Na 2 SO 4
When soil containing black alkali is to be tile-drained,
it is recommended that the land shall first be treated
with gypsum, as the substitution of alkali sulfates for
carbonates causes the soil to assume a much less compact
condition and thus facilitates drainage. It also prevents
the loss of organic matter dissolved by the carbonate of
soda and the soluble phosphates, both of which are pre-
cipitated by the change.
314. Scraping. Removal of the alkali incrustation
that has accumulated at the surface is sometimes re-
sorted to. Very often the rise of alkali is encouraged
by applications of irrigation water, which is allowed
to evaporate unretarded. The salts are thus carried
upward by the capillary movement of the soil water.
This method of alkali eradication is never very effi-
cient, and is often dangerous, as it encourages the
presence of very large amounts of alkali salts in the sur-
face soil.
315. Flushing. Often alkali accumulations may be
washed from the soil surface by turning on a rapidly
moving stream of water. The texture of the soil, as well
as the slope of the land, must be just right for such a
procedure. Generally so much water enters the soil
that the land remains heavily impregnated with alkali
salts. Both this method and the previous one, even
if successful, are only temporary. Moreover, lands
carrying so much alkali as to admit of either one of these
procedures may be so heavily charged as never to yield
to any form of either eradication or control.
2D
402 SOILS: PROPERTIES AND MANAGEMENT
316. Control of alkali. Where excessive amounts oi
soluble salts do not exist in a soil, the control of the alkali
with a view of keeping it well distributed in the soil
column is the best practice. The retardation of evapora-
tion is, of course, the main object in this procedure. The
intensive use of the soil mulch is therefore to be advocated,
especially in all irrigation operations where alkali concen-
trations are likely to occur. Such a method of soil
management not only saves moisture, but also prevents
the excessive translocation of soluble salts into the root
zone. This method of control is the most economical,
the cheapest, and the one to be advocated on all occasions,
no matter what may have been the previous means of
dealing with the alkali situation.
317. Cropping with tolerant plants. Certain soils
that are strongly impregnated with alkali may be grad-
ually improved by cropping with sugar beets and other
crops that are tolerant of alkali and that remove large
quantities of salts. This is more likely to be efficacious
where irrigation is not practiced. Certain crops, more-
over, while somewhat seriously injured while young, are
very resistant once their root systems are developed.
A good example is alfalfa, the young plants being very
tender while the more mature ones are extremely resistant.
Temporary eradication of alkali may allow such a crop to
be established. It will then maintain itself in spite of the
concentrations that may later occur.
318. Alkali spots. In semiarid regions small areas of
alkali are often found, varying from a few square yards
to several acres in size. The quantities of alkali in these
are usually not sufficient to prevent the growth of plants
in years of good rainfall, but in periods of drought the
concentration of the salts and the compact condition that
ALKALI SALTS 403
they tend to produce combine to injure the crop. The
methods already mentioned for treating alkali land are
of service on these small areas, and, in addition, the
plowing-under of fresh farm manure has been found to
improve their productiveness. This, with surface drain-
age, deep tillage, and good cultivation in order to prevent
the soil from drying out, will usually remedy the difficulty.
In many cases these spots become highly productive under
proper treatment.
CHAPTER XIX
ABSORPTION OF NUTRITIVE SALTS BY
AGRICULTURAL PLANTS
ALL the salts taken up by the roots of agricultural
plants are in solution when absorbed. The movement
into the root thus depends on the presence of moisture,
which is the medium of transfer. The root-hairs are the
great absorbing organs of the plant, and through the
cells of their delicate tissues the solutions of the various
salts are passed.
319. How plants absorb nutrients. The nature and
quantity of material absorbed by a plant is determined
by the law of diffusion. From the cells of the root-hairs
the dissolved salts are transferred to other parts of the
plant, where they undergo the metabolic processes that
determine which constituents shall be retained in the
tissues of the plant. The unused ions that remain in
the plant juices prevent by their presence the further
absorption of those particular substances from the soil
water. It thus happens that the composition of the
ash of a plant may be very different from that of the
substances presented to it in solution. For example,
aluminium, although always present in the soil in a very
slightly soluble form, is present in mere traces in the ash
of most plants. On the other hand, iodine, although
present in sea water only in the most minute quantities,
is present in large quantities in the ash of certain marine
algae.
404
ABSORPTION OF NUTRITIVE SALTS 405
A plant will, in general, take up more of a nutritive
substance if it is presented in large amount, as compared
with the other soluble substances in the nutrient solution,
than if it is presented in small amount. Thus, the per-
centage of nitrogen in maize, oats, and wheat may be
increased by increasing the ratio of nitrogen to other
nutritive substances in the nutrient media. This is
also true of potassium and phosphorus, respectively.
This fact is accounted for by the maintenance of the
diffusive equilibrium at a higher level for a particular
ion which is relatively abundant in the nutrient solution,
thus preventing the return of the excess from the plant.
320. Relation between root-hairs and soil particles. -
In a rich, moist soil the number of root-hairs is very
great, while in a poor or a very dry soil or in a saturated
soil there are comparatively few root-hairs. The con-
nection between the root-hairs and the soil particles is
extremely intimate. When in contact with a particle
of soil, a root-hair in many cases almost incloses it, and
by means of its mucilaginous wall forms a contact so close
as practically to make the solution between the particle
and the cell wall distinct from that between the soil
particles themselves.
There has been considerable difference of opinion as to
how a plant can obtain its mineral nutrients from a sub-
stance so difficultly soluble as the soil. This has arisen
because of the conflicting nature and the inadequate
character of the data available.
321. Liebig and Sachs on solvent action of plant
roots. Liebig l called attention to the fact that a plant
may obtain one hundred times as much phosphorus and
1 Liebig, J. Die Chemie in Ihrer Anwendung auf Agrikultur.
1802.
406 SOILS: PROPERTIES AND MANAGEMENT
nitrogen and fifty times as much potassium as can be
extracted from the same volume of soil with pure water
or with water containing carbon dioxide. It has, of
course, been recognized that the soil water is aided in its
solvent action by a variety of substances that may be
normally present in solution, beginning with the gases
taken up by rain in its descent through the atmosphere,
and further added to by the carbon dioxide and the or-
ganic and mineral substances obtained from the soil. It
has been held that the plant roots aid solution of mineral
matter by excretion of acids, which act effectively as
solvents. The well-known root tracings on limestone
and marble have been taken as proof of the excretion of
such acids. Sachs, 1 and later other investigators, grew
plants of various kinds in soil and other media in which
was placed a slab of polished marble or dolomite or cal-
cium phosphate, covered with a layer of washed sand.
After the plants had made sufficient growth the slabs were
removed, and on the surfaces were found corroded trac-
ings, corresponding to the lines of contact between the
rootlets and the minerals.
322. Czapek's experiments. In order to test this
theory, Czapek 2 repeated the experiments of Sachs,
using plates of gypsum mixed with the ground mineral
that he wished to test, and this mixture he spread over a
glass plate. Using these plates in the same manner as
previously described, Czapek found that, while plates of
calcium carbonate and of calcium phosphate were cor-
roded by the plant roots, plates of aluminium phosphate
1 Sachs, J. Auflosung des Marmors durch Mais-Wurzeln
Bot. Zeitung, 18 Jahrgang, Seite 117-119. I860.
2 Czapek, J. Zur Lehre von den Wurzelausscheidung. Jahrb.
f. Wiss. Bot., Band 29, Seite 321-390. 1896.
ABSORPTION OF NUTRITIVE SALTS 407
were not. He concludes that if the tracings are due to
a rids excreted by the plant roots, the acids so excreted
must be those that have no solvent action on aluminium
phosphate. This would limit the excreted acids to car-
bonic, acetic, propionic, and butyric. Czapek also re-
plies to the argument that the acids producing the tracings
must be non-volatile ones because of the definite lines
made in the mineral, by stating that the excretion of
carbon dioxide alone would be sufficient to account for
the observations since it dissolves in water to form car-
bonic acid, and that carbonic acid is always present in
the cell w r alls of the root epidermis. By means of micro-
chemical analyses of the exudations of root-hairs grown
in a water-saturated atmosphere, Czapek found potassium,
magnesium, calcium, phosphorus, and chlorine in the
exudate. He concludes that the solvent action of plant
roots is due to acid salts of mineral acids, particularly
acid potassium phosphate. He has not proved, however,
that the exudations were not from dead root-hairs nor
from the dead cells of the rootcap. In either case they
would have some solvent action, but whether sufficient
to make them of importance is doubtful.
323. Secretion of an oxidizing enzyme by plant roots. -
Molisch l found that root-hairs secrete a substance having
properties corresponding to those of an oxidizing enzyme.
His work has been repeated by others who have failed to
obtain similar results, but lately Schreiner and Reed 2 have
1 Molisch, H. Ueber Wurzelausscheidungen und deren
Kinwirkung auf Organische Substanzen. Sitzungsber. Akad.
Wiss. Wien-Math. Nat., Band 96, Seite 84-109. 1888. Ab-
slract in Chem. Centrlb., Band 18, Seite 1513, 1888, and in
Centrlb. f. Agr. Chem., Band 17, Seite 428, 1888.
2 Schreiner, Oswald, and Reed, H. S. Studies on the Oxidiz-
ing Powers of Roots. Bot. Gazette, Vol. 47, p. 355. 1909.
408 SOILS: PROPERTIES AND MANAGEMENT
demonstrated an oxidizing action of roots that is appar-
ently due to a peroxidase. Oxidation alone, however,
would hardly suffice to account for the solvent action
accompanying the development of plant roots, although
it is doubtless an important function and useful in other
ways.
324. Importance of carbon dioxide as a solvent. -
Stoklasa and Ernst 1 have contributed much to this sub-
ject during the last decade. Stoklasa's earlier experiments,
conducted by maintaining the plant roots in a saturated
atmosphere, gave only carbon dioxide in the exudate.
In this he is in agreement with most of the recent inves-
tigators of this subject. Stoklasa emphasizes the im-
portance of carbon dioxide as a solvent by showing the
quantity produced by plants and by microorganisms.
He estimates that in one acre of soil to a depth of sixteen
inches there are sixty-eight pounds of carbon dioxide
produced by bacterial respiration in two hundred days,
and fifty-four pounds of carbon dioxide excreted by plant
roots in one hundred days; these periods he considers
as representing the year's activity of bacteria and higher
plants.
In later experiments, Stoklasa and Ernst 2 found that
when plants do not have a sufficient supply of oxygen in
the air surrounding their roots, they secrete acetic and
formic acids from the root-hairs. These investigators
believe that these acids are toxic rather than beneficial,
1 Stoklasa, J., and Ernst, A. Ueber den Ursprung die Menge
und die Bedeutimg des Kohlendioxyds im Boden. Centrlb. f.
Bakt., II, Band 14, Seite 723-736. *1905.
2 Stoklasa, J., and Ernst, A. Beitrage zur Losung der Frage
der Chemischen Natur des Wurzelsekretes. Jahrb. f. Wiss.
Bot., Band 46, Seite 55-102. 1908-1909.
ABSORPTION OF NUTRITIVE SALTS 40 ( J
and that they are responsible, in large measure, for the
injurious effect on plants of a very compact condition of
soil. In the same communication these authors report
an experiment in which it was found that the kinds of
plants that excrete the largest quantity of carbon dioxide
from their roots are the ones that absorb the greatest
quantities of phosphorus from gneiss and from basalt.
This, however, does not necessarily connote any conse-
quential relation between these physiological functions.
Barakov l drew air through planted soils contained in
large tanks. He found that the maximum production
of carbon dioxide occurred at the time when the plants
were blossoming, whether the plants blossomed early
or late in the season. This he considered to indicate
that the plant assists most vigorously in the solution of
nutrient materials at the time when it is most active in
absorbing them.
325. Insufficiency of carbon dioxide. Pfeiffer and
Blanck 2 passed carbon dioxide through soil contained in
vessels in which plants were growing. The soil in some ves-
sels contained a difficultly soluble tricalcic phosphate, that
in other vessels the more easily soluble dicalcic phosphate,
and that in still other vessels was unfertilized. Another
set of vessels having the same fertilizer treatment received
no carbon dioxide. The soil receiving carbon dioxide
produced larger yields of dry matter and phosphorus in
Barakov, F. The Carbon Dioxide Content of Soils at
Different Periods of Plant Growth. Jour. Exp. Agr. (Russian),
Vol. 11, pp. 321-342. 1910. The authors are indebted to
Dr. J. Davidson for a translation of this paper.
2 Pfeiffer, Th., and Blanck, E. Die Saureausscheidung
der Wurzeln und die Loslichkeit der Bodennahrstoffe in Koh-
lensaurehaltigem Wasser. Landw. Vers. Stat., Band 77, Seite
217-268. 1912.
410 SOILS: PROPERTIES AND MANAGEMENT
the crop on the soil to which dicalcic phosphate had been
applied than did the soil not receiving carbon dioxide;
but the soil to which no phosphate was added yielded
equally well whether it received carbon dioxide or not.
The plants used were oats, peas, and lupines. These
investigators conclude that carbon dioxide is not a suffi-
cient solvent to account for the mineral nutrients obtained
from soils by plants.
326. The present status of the question. The avail-
able evidence on excretion of acids other than carbonic
by the roots of plants does not admit of any very satis-
factory conclusion as to their relative importance in the
acquisition of plant-food materials. There can be no
doubt, however, that carbon dioxide resulting from root
exudation and from decomposition of organic matter in
the soil plays a very prominent part in this operation.
The very large quantity of carbon dioxide in the soil,
amounting in some cases to from .5 to nearly 10 per cent
of the soil air, or several hundred times that of the at-
mospheric air, must aid greatly in dissolving the soil
particles.
Whatever may be the concentration of the soil water,
it seems probable that the liquid which is found where
the root-hair comes in contact with the soil particle, and
which is separated, in part at least, from the remainder
of the soil water, must have a density much greater than
that found elsewhere in the soil. That portion of the
soil water immediately in contact with the soil grain is a
much stronger solution than the water farther from the
soil surfaces, because of the adsorptive action of the
particles.
Many plants grown in solutions of nutritive salts have
few or no root-hairs, but absorb through the epidermal
ABSORPTION OF NUTRITIVE SALTS 411
tissue of the roots. If the plant depended wholly on the
prepared solution in the soil water, a similar structure
would doubtless suffice. The special modification by
which the root-hairs come in intimate contact with the
soil particle and almost surround it, indicates a direct
relation between the soil particles and the plant, and
not merely between the soil solution and the plant.
\e\v root-hairs are constantly being formed, and the
old ones become inactive and disappear. The contact
of a root-hair with a soil particle is not long-continued.
\Vhether the period of contact is determined by the
ability of the root to absorb nutriment from the particle
is not known. Certain it is that only a small portion of
the particle is removed.
327. Possible root action on colloidal complexes. -
It has already been stated that there is some evidence
to lead to the belief that the surfaces of soil particles are
covered to a large extent with colloidal complexes, com-
posed of both organic and inorganic matter having vigor-
ous absorptive properties and holding the bases and phos-
phorus in an absorbed condition. Roots of growing
plants have been found to cause coagulation of at least
some colloids, possibly by leaving an acid residue in the
nutrient solution by reason of the selective absorption of
bases and rejection of the acids of the dissolved salts.
It is conceivable that the root-hair, by removing bases
from the solution existing between the cell wall and the
colloidal covering of the soil particle, may cause coagula-
tion of the colloidal matter and thus liberate the plant-
food materials held by absorption. The liberated ma-
terial, being of a readily soluble nature, would be taken
up by the solution between the rootlet and the soil particle,
from which the root-hair could readily absorb it. Such
412 SOILS: PROPERTIES AND MANAGEMENT
an hypothesis would account for the ability of plants
to obtain a quantity of nutrient materials far in excess of
what can be accounted for by the solvent action of pure
water, and even beyond what many investigators are
willing to attribute to the solvent action of water charged
with carbon dioxide.
328. Why crops vary in their absorptive powers. -
As has already been pointed out (pars. 331-336), crops of
different kinds vary greatly in their ability to draw
nourishment from the soil. The difference between the
nitrogen, phosphorus, and potassium taken up by a corn
crop of average size and a wheat crop of average size is
very striking. In the table on page 419 it is seen that
two tons of red clover contain three times as much potash,
nearly ten times as much lime, and somewhat more phos-
phoric acid than does a crop of thirty bushels of wheat
including the straw.
The difference in absorbing power may be due to either
one or both of two causes : (1) a larger absorbing system;
(2) a more active absorbing system. The former is deter-
mined by the extent of the root-hair surfaces; the latter
by the intensity of the absorbing action.
329. Extent of absorbing system. Plants with large
root systems may be expected to absorb the larger amounts
of nutrients from the soil. Such is usually the case,
although the extent of the root system is not necessarily
proportional to the total area of the absorbing surfaces
of the root-hairs.
330. Absorptive Activity. The absorptive activity of
a plant under any given condition of soil and climate de-
pends on : (l) the rapidity and completeness with which
the plant elaborates the substances taken from the soil
into plant substance, or otherwise removes them from
ABSORPTION OF NUTRITIVE SALTS 413
solution ; (2) the extent to which the exudations from the
root-hairs whether these be carbon dioxide, salts of
mineral acids, or organic acids act on the soil particles.
The first of these is a function of the vital energy of the
plant and its ability to utilize sunshine and carbon dioxide
to produce organic matter. It may be compared to the
property which enables one animal to do more work than
another animal of the same weight on a similar ration.
The removal from the ascending water current in the
plant of substances derived from the soil is accomplished
in the leaves. By the dissociation of these substances,
ions are constantly furnished for metabolism into materials
that may be built into the tissues of the plant. The re-
maining ions are kept in the solution. There is a con-
stant tendency to bring the composition and density of
the solution into equilibrium, by diffusion and diosmosis,
with the solution between the soil particle and the root-
hair. The rapidity with which the metabolic process
removes a substance from the solution in the plant, there-
fore, determines the rate at which it is removed from a
solution of given composition and density in the soil.
Plants making a rapid growth remove more nutrients in
a given time than those making a slower growth, when
the nutrient solution is of a given composition and density.
Another factor that affects the rate of absorption of
salts from the soil is the solvent influence of exudates from
the root-hairs. This subject has already been treated (pars.
321-326), and it only remains to be said that this action
apparently varies with different kinds of plants, and
probably accounts in no small measure for the difference
in the ability of different plants to withdraw salts from
the soil.
These several factors, which, when combined, deter-
414 SOILS: PROPERTIES AND MANAGEMENT
mine the so-called " feeding power " of the plant, are
recognized by the popular terms " weak feeder " and
" strong feeder," applied, on the one hand, to such
crops as wheat or onions, which require very careful soil
preparation and manuring, and, on the other hand, to
maize, oats, or cabbage, which demand relatively less
care. In the manuring and rotating of crops, this differ-
ence in absorptive power must be considered, in order not
only to secure the maximum effect on the crop manured,
but also to get the greatest residual effect of the manure
on succeeding crops.
331. The absorptive power of cereals. Cereals have
the power of utilizing the potassium and phosphorus of
the soil to a considerable degree, but they generally re-
quire fertilization with nitrogen salts. Most of the cereals,
such as wheat, rye, oats, and barley, take up the principal
part of their nitrogen early in the season, before the nitri-
fication processes have been sufficiently operative to fur-
nish a large supply of nitrogen; hence nitrogen is the
fertilizer constituent that usually gives the best results,
and should be added in a soluble form. Wheat, in partic-
ular, needs a large amount of soluble nitrogen early in
its spring growth. Since it is a " delicate feeder," it does
best after a cultivated crop or a fallow, by which the
nitrogen has been converted into a soluble form. Oats
can make better use of the soil fertility and do not require
so much manuring. Maize is a very coarse " feeder,"
and, while it removes a large quantity of plant-food from
the soil, it does not require that this shall be added in a
soluble form. Farm manure and other slowly acting
manures may well be applied for the maize crop. The
long growing period required by the maize plant gives it
opportunity to utilize the nitrogen as it becomes avail-
ABSORPTION OF NUTRITIVE SALTS 415
able during the summer, when armnonification and nitri-
fication are active. Phosphorus is the substance usually
most needed by maize.
332. The feeding of grass crops. Grasses, when in
meadow or in pasture, are greatly benefited by manures.
They are less vigorous " feeders " than the cereals, have
shorter roots, and, when left down for more than one
year, the lack of aeration in the soil causes decomposition
to decrease. There is usually a more active fixation of
nitrogen in grass lands than in cultivated lands, but this
becomes available very slowly.
Different soils and different climatic conditions neces-
sitate different methods of manuring for grass. Farm
manures may well be applied to meadows in all situations,
while the use of nitrogen is generally profitable.
333. Leguminous crops. Most of the leguminous
crops are deep-rooted and are vigorous " feeders." Their
ability to take nitrogen from the air makes the use of that
fertilizer constituent unnecessary except in a few in-
stances, such as young alfalfa on poor soil, where a small
application of nitrate of soda is usually beneficial. Lime
and potassium are the substances most beneficial to leg-
umes on the majority of soils.
334. Root crops. Many root crops will utilize very
large quantities of plant-food if it is in a form in which
they can use it. Phosphates and nitrogen are the sub-
stances generally required, the latter especially by beets
and carrots.
335. Vegetables. In growing vegetables, the object
is to produce a rapid growth of leaves and stalks rather
than seeds, and often this growth is made very early in
the season. As a consequence, a soluble form of nitrogen
is very desirable. Farm manure should also have a promi-
416 SOILS: PROPERTIES AND MANAGEMENT
nent part in the treatment, as it keeps the soil in a mechan-
ical condition favorable to retention of moisture, which
vegetables require in large amounts, and it also supplies
needed fertility. The very intensive method of culture
employed in the production of vegetables necessitates the
use of much greater quantities of manures than are used
for field crops, and the great value of the product justifies
the practice.
336. Fruits. In manuring fruits, with the exception
of some of the small, rapidly-growing ones, it is the aim
to maintain a continuous supply of nutrients available
to the plant, but not sufficient for stimulation except
during the early life of the tree, when rapid growth of
wood is desired. An acre of apple trees in bearing re-
moves as much plant-food material from the soil in a
season as does an acre of wheat. Farm manure and a
complete fertilizer may be used, of which the constituents
should be in a fairly available form, as a constant supply
is necessary. A young growing orchard requires con-
siderably more nitrogen than does an old orchard. Some
nitrate of soda in early spring is desirable.
337. Mineral substances absorbed by plants. The
plant, in its process of growth, withdraws from the soil
certain mineral substances that are presented to its roots
in a dissolved condition. As the salts in solution are
rather numerous, and since the diffusion by which the
absorption is accomplished does not admit of the entire
exclusion of any ion capable of diosmosis, there are to be
found in the plant most of the mineral constituents of the
soil. Some of these are concerned in the vital processes
of the plant and are essential to its growth ; others seem
to have no specific function, but are generally present.
The substances commonly met with in the ash of plants
ABSORPTION OF NUTRITIVE SALTS 417
are potassium, sodium, calcium, magnesium, iron, man-
ganese, aluminium, phosphorus, sulfur, silicon, and chlo-
rine. In addition to these, nitrogen is absorbed from the
soil in the form of soluble salts. Of these the substances
known to be absolutely essential to the normal growth of
plants to maturity, are potassium, calcium, magnesium,
iron, phosphorus, sulfur, and nitrogen, while the others
are probably beneficial to the plant in some way not yet
discovered.
Of the substances acting as plant nutrients, each must
be present in an amount sufficient to make possible the
maximum growth consistent with other conditions, or
the yield of the crop will be curtailed by its deficiency.
To some extent certain essential substances may be re-
placed by others, as, for instance, potassium by sodium;
but such substitution is probably possible only in some
physiological role other than that of an elemental con-
stituent of an organic compound. The substances that are
likely to be so deficient in an available form in any soil
as to curtail the yield of crops, are potassium, phosphorus,
nitrogen, and possibly sulfur; while the addition of cer-
tain forms of calcium is likely to be beneficial because of
its relation to other constituents and properties of the soil.
It is for the purpose of supplying these substances, and
to some extent to improve the mechanical condition of
the soil, that mineral manures are used.
338. Relation of plant growth to concentration of nu-
trient solution. It has already been stated that the
addition of soluble salts to a soil has been found by some
experimenters to apparently increase the concentration
of the soil solution (par. 250). It has also been found
that plant growth, as measured by weight of plants, in-
creases with the concentration of the nutrient solution in
2 E
418 SOILS: PROPERTIES AND MANAGEMENT
which the plants are grown. 1 This is the way in which
it is generally believed that soluble fertilizer salts benefit
plant growth. Insoluble plant-food materials have a
similar, but less active, result because they do not increase
the concentration of the soil solution to as great an extent.
339. Quantities of plant-food material removed by
crops. The utilization of mineral substances by crops
is a source of loss of fertility to agricultural soils. In a
state of nature, the loss in this way is comparatively small,
as the native vegetation falls on the ground, and in the
process of decomposition the ash is almost entirely returned
to the soil. Under natural conditions, soil usually in-
creases in fertility; for, while there is some loss through
drainage and other sources, this is more than counter-
balanced by the action of the natural agencies of disin-
tegration and decomposition, and the fixation of atmos-
pheric nitrogen affords a constant, though small, supply
of that important soil ingredient.
When land is put under cultivation, a very different
condition is presented. Crops are removed from the
land, and only partially returned to it in manure or straw.
This withdraws annually a certain small proportion of
the total quantity of mineral substances, but, what is of
more immediate importance, it withdraws all of this in a
readily available form.
The following table, computed by Warington, 2 shows
1 Hall, A. D., Brenchley, W. E., and Underwood, L. M.
The Soil Solution and the Mineral Constituents of the Soil.
Philosoph. Trans. Royal Soc. London, Series B, Vol. 204, pp.
179-200. 1913. Also, Lyon, T. L., and Bizzell, J. A. The
Plant as an Indicator of the Relative Density of Soil Solutions.
Proc. Am. Soc. Agron., Vol. 4, pp. 35-49. 1912.
2 Warington, R. Chemistry of the Farm, pp. 64-65. Lon-
don. 1894.
ABSORPTION OF NUTRITIVE SALTS
419
the quantities of nitrogen, potassium, phosphorus, and
lime removed from an acre of soil by some of the common
crops. The entire harvested crop is included :
TOTAL
NlTRO-
POTASH
LIME
PHOS-
PHORIC
CROP
YIELD
ASH
N
K 2 O
CaO
ACID
PzOs
(Pounds)
(Pounds)
(Pounds)
(Pounds)
(Pounds)
Wheat ....
30 bushels
172
48 .
28.8
9.2
21.1
Barley ....
40 bushels
157
48
35.7
9.2
20.7
Oats ....
45 bushels
191
55
46.1
11.6
19.4
Maize ....
30 bushels
121
43
36.3
18.0
Meadow hay . .
1| tons
203
49
50.9
32.1
12.3
Red clover
2 tons
258
102
83.4
90.1
24.9
Potatoes . . .
6 tons
127
47
76.5
3.4
21.5
Turnips . . .
17 tons
364
192
148.8
74.0
33.1
340. Quantities of plant-food materials contained in
soils. Comparing the figures given above with those
showing the percentages of the fertilizing constituents in
certain soils, it is evident that there is a supply in most
arable soils that will afford nutriment for average crops
for a very long period of time. (See pars. 46, 48, 52, 53.)
341. Possible exhaustion of mineral nutrients. On
the other hand, when it is considered that the soil must
be depended upon to furnish food for humanity and
domestic animals as long as they shall continue to in-
habit the earth, at least so far as is now known, the very
apparent possibility of exhausting, even in a period of
several hundred years, the supply of plant nutrients be-
comes a matter of grave concern. The visible sources of
supply, to replace or supplement those in the soils now
cultivated, are, for the mineral substances, the subsoil
and the natural deposits of phosphates, potash salts, and
420 SOILS: PROPERTIES AND MANAGEMENT
limestone; and for nitrogen, deposits of nitrates, the by-
product of coal distillation, and the nitrogen of the
atmosphere. The last of these is inexhaustible, and the
exhaustion of the nitrogen supply, which a few years ago
was thought to be a matter of less than half a century,
has now ceased to cause any apprehension. The conser-
vation or extension of the suppty of mineral nutrients is
now of supreme importance. The utilization of city refuse
and the discovery of new mineral deposits are develop-
ments well within the range of possibility, but neither of
these promises to afford more than partial relief. The
utilization of the subsoil through the gradual removal by
natural agencies of the topsoil will, without doubt, tend
to constantly renew the supply. The removal of topsoil
by wind and erosion is, even on level land, a very con-
siderable factor. The large amount of sediment carried
in streams immediately after a rain, especially in summer,
gives some idea of the extent of this shifting. This affects
chiefly the surface soil, and thereby brings the subsoil
into the range of root action.
There is little doubt that a moderate supply of plant-
food materials will always be available in most soils, but
for progressive agriculture manures must be used
CHAPTER XX
ORGANISMS IN THE SOIL
A VAST number of organisms, animal and vegetable, live
in the soil. By far the greater part of these belong to
plant life, and these comprise the forms of greatest influ-
ence in producing the changes in structure and composition
that contribute to soil productiveness. Most of the
organisms are so minute as to be seen only by the aid of
the microscope, while a much smaller proportion range
from these to the size of the larger rodents. They may
thus be classed as microorganisms and macroorganisms.
The latter class will be considered first.
MACROORGANISMS OF THE SOIL
Of the macroorganisms in the soil the animal forms
belong chiefly to (1) rodents, (2) worms, and (3) insects;
and the plant forms to (1) the large fungi and (2) plant
roots.
342. Rodents. The burrowing habits of rodents
of which the ground squirrel, the mole, the gopher, and the
prairie dog are familiar examples result in the pulveri-
zation and transfer of very considerable quantities of soil.
While the activities of these animals are often not favor-
able to agriculture, the effect on the character of the soil
is rather beneficial and is analogous to that of good tillage.
Their burrows also serve to aerate and drain the soil, and
421
422 SOILS: PROPERTIES AND MANAGEMENT
in permanent pastures and meadows are of much value
in this way.
343. Worms. The common earthworm is the most
conspicuous example of the benefit that may accrue from
this form of life. Darwin, as the result of careful measure-
ments, states that the quantity of soil passed through
these creatures may, in a favorable soil in a humid climate,
amount to ten tons of dry earth per acre annually. The
earthworm obtains its nourishment from the organic
matter of the soil, but takes into its alimentary canal the
inorganic matter as well, expelling the latter in the form
of casts after it has passed entirely through the body.
The ejected material is to some extent disintegrated, and
is in a flocculated condition. The holes left in the soil
serve to increase aeration and drainage, and the move-
ments of the worms bring about a notable transportation
of lower soil to the surface, which aids still more in effect-
ing aeration. Darwin's studies led him to state that from
one-tenth to two-tenths of an inch of soil is yearly brought
to the surface of land in which earthworms exist in normal
numbers.
Instances are on record of land flooded for a consider-
able period so that the worms were destroyed, and the
productiveness of the soil was seriously impaired until it
was restocked with earthworms.
Wollny conducted experiments with soil, the soil in one
case containing earthworms and in another case not con-
taining them. Although there was much variation in his
results, they were in every case in favor of the soil con-
taining the worms, and in a number of the tests the yield
on rich soil was several times as great as where no worms
were present.
Earthworms naturally seek a heavy, compact soil, and
ORGANISMS IN THE SOIL 423
it is in soil of this character that they are most needed
because of the stirring and aeration they accomplish.
Sandy soil and the soils of arid regions, in which are
found few or no earthworms, are not usually in need of
their activities.
344. Insects. There is a less definite, and probably
less effective, action of a similar kind produced by insects.
Ants, beetles, and the myriads of other burrowing insects
and their larvse effect a considerable movement of soil
particles, with a consequent aeration of the soil. At the
same time they incorporate into the soil a considerable
quantity of organic matter.
345. Large fungi. The larger fungi are chiefly con-
cerned in bringing about the first stages in the decom-
position of woody matter, which is disintegrated through
the growth in its tissues of the root mycelia of the fungi.
These break down the structure, and thus greatly facili-
tate the work of the decay bacteria. Action of this kind
is largely confined to the forest and is not of great im-
portance in cultivated soil.
Another function of the large fungi is exercised in the
intimate, and possibly symbiotic, relation of the fungal
hyphse to the roots of many forest trees, in soil where
nitrification proceeds very slowly, if at all, for nitrates are
apparently not abundant in forest soils. This envelop-
ing system of hyphse, which may consist of masses in a
definite zone of the cortex with occasional filaments pass-
ing outward into the soil, or which may surround the root
with a dense mass of interwoven hyphse, is called mycor-
rhiza.
The cereal, cruciferous, leguminous, and solanaceous
plants are not associated with mycorrhiza. Mycotrophic
plants are usually those that live in a humous soil filled
424 SOILS: PROPERTIES AND MANAGEMENT
with the mycclia of fungi. It is thought that the mycor-
rhiza aid the higher plants to obtain nutriment that they
must strive for in competition with the fungi.
Mycotrophic plants are also able to grow with a very
small transpiration of moisture, as is well known to be
the case with many conifers; and this restricted tran-
spiration would doubtless result in lack of nutriment were
it not for the assistance of the mycorrhiza.
346. Plant roots. The roots of plants assist in pro-
moting productiveness of the soil both by contributing
organic matter and by leaving, on their decay, openings
which render the soil more permeable to water and which
also facilitate drainage and aeration. The dense mass of
rootlets, with their minute hairs that are left in the soil
after every harvest, furnish a well-distributed supply of
organic manure, which is not confined to the furrow slice,
as is artificially incorporated manure. The drainage and
aeration of the lower soil, due to the openings left by the
decomposed roots, are of the greatest importance in heavy
soil, and the beneficial effects of clover and other deep-
rooted plants are due in no small measure to this function.
MICROORGANISMS OF THE SOIL
Of the microorganisms commonly exist-
ing in soils, the greater part belong to
plant rather than to animal life. Of the
latter, the only organisms of well-known
economical importance are the nematodes
(Fig. 58), whose injurious effect on plant
growth is accomplished through the for-
mation of galls on the roots, in which the
FIG. 58. Nema- young are hatched and live to sexual
todes entering '
a plant root. maturity.
ORGANISMS IN THE SOIL 425
347. Plant microorganisms. The microscopic plants
of the soil may be classed as slime molds, bacteria, fungi,
and algse.
348. Plant microorganisms injurious to higher plants.
-Injurious plant microorganisms are confined mostly
to fungi and bacteria. They may be entirely parasitic in
their habits, or only partially so. They injure plants by
attacking the roots. Those that attack other parts of
plants may live in the soil during their spore stage, but
they are 'not strictly microorganisms of the soil. Some
of the more common diseases produced by soil organisms
are : wilt of cotton, cowpeas, watermelon, flax, tobacco,
tomatoes, and other plants; damping-off of a large num-
ber of plants ; root-rot ; galls.
These fungi or bacteria may live for long periods, prob-
ably indefinitely, in the soil, if the conditions necessary
for their growth are maintained. Some of them will die
within a few years if their host plants are not grown on
the soil, but others are able to maintain existence on
almost any organic substance. Once a soil is infected,
it is likely to remain so for a long time, or indeed indefi-
nitely. Infection is easily carried. Soil from infected
fields may be carried on implements, plants, or rubbish
of any kind, in soil used for inoculation of leguminous
crops, or even in stable manure containing infected plants
or in the feces resulting from the feeding of infected plants.
Flooding of land by which soil is washed from one field
to another may be a means of infection.
Prevention is the best defense from diseases produced
by these soil organisms. Once disease has procured a
foothold, it is practically impossible to eradicate all its
organisms. Rotation of crops is effective for some dis-
eases, but entire absence of the host crop is oftener neces-
426 SOILS: PROPERTIES AND MANAGEMENT
sary. The use of lime is beneficial in the case of certain
diseases. Chemicals of various kinds have been tried
with little success. Steam sterilization is a practical
method of treating greenhouse soils for a number of dis-
eases. The breeding of plants immune to the disease af-
fecting its particular species has been successfully carried
out in the case of the cowpea and cotton plants, and can
doubtless be accomplished with others.
In regions in which farming is confined largely to one
crop or to a limited number of cereals, it is the common
experience that yields decrease greatly in the course of a
score of years after the virgin soil is broken. The cause
for this is attributed by Bolley 1 in large measure to a
diseased condition of the plants due to the growth of
various fungi that inhabit the soil and attack the crops
grown on it. He reports that he has experimented with
pure cultures taken from wheat grains, straw, and roots,
and has demonstrated that certain strains or species of
Fusarium, Helminthosporium, Alternaria, Macrosporium,
Colletotrichum, and Cephalothecium are directly capable
of attacking and destroying growing plants of wheat, oats,
barley, brome grass, and quack grass, and that within
limits the disease may be transferred from one type of
crop to another.
349. Plant microorganisms not injurious to higher
plants. The vegetable microorganisms of the soil all
take an active part in removing dead plants and animals
from the surface of the soil, and in bringing about the other
operations that are necessary for the production of plants.
The first step in the preparation for plant growth is to
remove the remains of plants and animals that would
1 Bolley, H. L. Wheat. North Dakota Agr. Exp. Sta.,
Bui. 107. 1913.
ORGANISMS IN THE SOIL 427
otherwise accumulate to the exclusion of other plants.
These are decomposed through the action of organisms
of various kinds, the intermediate and final products of
decomposition assisting plant production by contributing
nitrogen and certain mineral compounds that are a
directly available source of plant nutriment, and also by
the effect of certain of the decomposition products on the
mineral substances of the soil, by which they are rendered
soluble and hence available to the plant.
Through these operations the supply of carbon and
nitrogen required for the production of organic matter is
kept in circulation. The complex organic compounds
in the bodies of dead plants or animals, in which condi-
tion plants cannot use them, are, under the action of
microorganisms, converted by a number of stages into
the very simple compounds used by plants. In the course
of this process a part of the nitrogen is sometimes lost
into the air by conversion into free nitrogen, but fortu-
nately this may be recovered and even more nitrogen
taken from the air by certain other organisms of the soil.
The slime molds, bacteria, fungi, and algse all play a
part in these processes, but none of them so actively
during every stage of the processes as do the bacteria.
Molds and fungi are particularly active in the early stages
of decomposition of both nitrogenous and non-nitrogenous
organic matter. Molds are also capable of ammonifying
proteins, and even re-forming the complex protein bodies
from the nitrogen of ammonium salts. Certain of the
molds and of the algae are apparently able to fix atmos-
pheric nitrogen, and contribute a supply of carbohydrates
required for the use of the nitrogen-fixing bacteria. Among
these are Aspergillus niger and Penicillium glaucum.
It also seems probable that the fungi associated with
428 SOILS: PROPERTIES AND MANAGEMENT
the roots of many forest trees and known as mycorrhizal
fungi have the ability to fix atmospheric nitrogen, and that
in some way the trees obtain a part, at least, of the nitro-
gen so fixed. The growth of forests on poor, sandy soil
containing practically no nitrogen has been urged as an
example of this process.
350. Bacteria. Of the several forms of microorgan-
isms found in the soil, bacteria are the most important.
In fact, the abundant and continued growth of plants on
the soil is absolutely dependent on the presence of bacteria,
for through their action chemical changes are brought
about which result in making soluble both organic and
inorganic material necessary for the life of higher plants,
and which, in part at least, would not otherwise occur.
Bacteria are thus trans-
formers, not producers, of
fertility in the soil, although,
as will be seen later, certain
kinds of bacteria take nitro-
gen from the air and leave it
in the soil. With this excep-
tion, however, they add no
plant-food to the soil. It is
their action in rendering
available to the plant ma-
terial already present in the
soil that constitutes their
greatest present value in crop
production. It is to their
activity in conveying nitro-
gen from the air to the soil
that we are indebted for most of our supply of nitrogen
in virgin soils (see Fig. 59).
FIG. 59. Some types of soil mi-
croorganisms highly magnified,
(a) , nitrate formers ; (&) , ni-
trite formers; (c), B. graveo-
lens ; (ct) , B. fusiformis ; (e) , B.
subtilis ; (/) , Clostridium pas-
teurianum.
ORGANISMS IN THE SOIL 429
It is not usually the entire absence of bacteria from the
soil that is to be avoided in practice, for all arable soils con-
tain bacteria, although sometimes not all of the desirable
forms ; but, as great bacterial activity is required for the
large production of crops, the practical problem is to main-
tain a condition of soil most favorable to such activity.
351. Distribution of bacteria. Bacteria are found
almost universally in soils, although they are much more
numerous in some soils than in others. A number of in-
vestigators have stated that in soils from different locali-
ties and of different types that they have examined, the
numbers of bacteria were proportional to the productive-
ness of the soils. The number of bacteria present has in
some cases been shown to be proportional to the amount
of humus contained in the soil. It is natural to expect
that within certain limits both these findings will hold. The
conditions obtaining in a productive soil are those favorable
to the development of certain forms of bacteria, and these
kinds constitute a very large proportion of those generally
found in soils. However, there is evidence that compara-
tively unproductive soils may contain a large number of
bacteria that are presumably not favorable to plant growth.
Samples of soil taken from certain productive and rela-
tively unproductive parts of a field on the Cornell Uni-
versity farm contained a larger number of bacteria in the
poor soil, although the two soils were equally well drained
and the good soil had slightly more organic matter.
They had also received practically the same treatment
during the preceding few years :
Character of Number of bacteria
soil per gram of dry soil
Good 1,200,000
Poor . . . 1,600,000
430 SOILS: PROPERTIES AND MANAGEMENT
After wheat had been growing for two months on these
soils in the greenhouse, the soils being maintained at the
same moisture content, the samples showed the following
count :
Character of Number of bacteria
soil to a gram of dry soil
Good 760,000
Poor 1,120,000
Another reason why this relation between the number
of bacteria and soil productiveness does not hold is that
the bacteria having the same functions in relation to plant-
food do not always have the same physiological efficiency.
In other words, they do not have the same virulence, a
small number in some cases being able to bring about the
same changes that in other cases require a much greater
number.
Bacteria are found chiefly in the upper layers of soil,
although not in large numbers at the immediate surface
of the ground. In humid regions the layer between the
first inch and the sixth or the seventh inch contains, in
most soils, the great bulk of bacteria present. In arid or
semiarid regions, bacteria are found at greater depths
and the densest population is located at lower levels than
in humid regions. This is largely because of the deeper
penetration of the air and the conditions that accom-
pany it.
352. Numbers of bacteria. The number of bacteria in
a soil will naturally vary with the conditions that favor or
discourage their growth. In very sandy soils, forest soils,
desert soils, water-logged soils, and soils low in humus,
the bacteria are either absent or comparatively few in
numbers. In soils very rich in organic matter, especially
ORGANISMS IN THE SOIL 431
where animal manure has been applied or where a carcass
has been buried, the number becomes very large, as
many as 100,000,000 to a gram of soil having been found ;
while in soil of ordinary fertility and tilth the numbers
range from 1,000,000 to 5,000,000 to a gram. The extreme
rapidity with which reproduction occurs makes it possible
for the number to increase enormously when conditions
are favorable for their growth.
The table on page 432 shows the number of bacteria
to a gram of soil found in different parts of the United
States during some portion of the growing season.
The figures showing the number of bacteria in each
gram of soil that are presented in this table cannot be
used for a comparison of the relative numbers of bacteria
in soils of different regions of this country, because dif-
ferent methods were used by the experimenters in making
the estimations. They are, however, an indication of
what may be considered the ordinary range in arable
soils.
353. Numbers as influenced by season. It might be
supposed that, like most plants, bacteria would develop
most rapidly in summer months and that they would be
found in largest numbers at that season, at least in regions
of low temperatures during the winter months. That
this is not always the case has been shown by Conn, 1
who found as the result of periodical enumeration of bac-
teria throughout a term of two years that the highest
counts were obtained during the winter months, when
the soil was frozen. This does not mean that all classes
of bacteria are present in largest numbers at that season,
but, as explained by Conn, it seems likely that certain
1 Conn, H. J. Bacteria in Frozen Soils II. Centrlb. f.
Bakt., II, Band 32, Seite 70-97. 1912.
432 SOILS: PROPERTIES AND MANAGEMENT
NUMBER OF BACTERIA TO A GRAM OF SOIL DURING SOME
PERIOD OF THE GROWING SEASON
STATE
SOIL
DEPTH
CROP
INVESTI-
GATOR
NUMBER
OF
BACTERIA
Delaware .
Stiff clay
3
Orchard in high
Chester 1
2,200,000
inches
state of culti-
vation. In
cover crops
Delaware .
Adjoining soil above
3
Meadow for
Chester 1
450,000
and of same char-
inches
twelve years
acter
Delaware .
Of same type as
3
Vegetables and
Chester *
1,800,000
above
inches
heavily ma-
nured
Delaware .
Scarlet clover
Chester l
3,360,000
plowed under
and alter-
nated with
maize for ten
years
Kansas
Loam, rich in humus
30
Alfalfa, five
Mayo and
33,931,808
inches
years
Kinsley 2
Kansas
Loam, richer in hu-
30
Alfalfa
Mayo and
53,596,060
mus than soil above
inches
Kinsley 2
Kansas
Thin soil, gumbo
30
Mixed grasses
Mayo and
78,534
subsoil
inches
Kinsley 2
Kansas
Loam, low in humus
30
Oats and wheat
Mayo and
8,543,006
inches
Kinsley 2
Iowa . .
Marshall loam, no
top
Oats
Brown 3
1,930,000
lime applied
soil
Iowa . .
Marshall loam, 1,000
Oats
Brown 3
2,342,000
pounds lime per
top
acre
soil
Iowa . .
Marshall loam, 2,000
Oats
Brown '
2,787,000
pounds lime per
top
acre
soil
Iowa . .
Marshall loam, 6,000
Oats
Brown 3
3,766,000
pounds lime per
top
acre
soil
1 Chester, F. D. The Bacteriological Analysis of Soils.
Delaware Agr. Exp. Sta., Bui. 65. 1904.
2 Mayo, U. S., and Kinsley, A. F. Bacteria of the Soil.
Kansas Agr. Exp. Sta., Bui. 117. 1903.
3 Brown, P. E. Some Bacterial Effects of Liming. Iowa
Agr. Exp. Sta., Research Bui. 2. 1911.
ORGANISMS IN THE SOIL
433
forms predominate in summer and others in winter (see
1-V 60).
%,
S09
,9,0
j>/tf
/*//
P'at
IB
X^fr
^V-i_
A*y.
^>
"\
,'
/
\
/
\
\
/
/\
/\
\
/
,'
L
\ \
,-
N
_,-'
t/7
F
r>
N ^
V
into.
_^'
-of
/
\
J
\
/
^\
^
^
->.
/
\J
\i
^<
G,t .
^
x/
/
. .'
- '
-
FIG. 60. Periodical enumeration of bacteria in soil of two plats during
two years, expressed in millions to a gram of dry soil.
Brown and Smith l obtained results that in the main
confirmed Conn's work, and they advanced the theory
that the concentration of the soil solution immediately
surrounding the soil particles, together with the high
surface tension exerted by the soil particles, prevents the
freezing of the surface film and that this water forms a
suitable medium for the development of bacteria.
354. Conditions affecting growth. Many conditions
of the soil affect the growth of bacteria. Among the most
important of these are the supply of oxygen and moisture,
the temperature, the presence of organic matter, and the
acidity or the basicity of the soil.
355. Oxygen. All soil bacteria require for their
growth a certain amount of oxygen. Some bacteria, how-
ever, can continue their activities with much less oxygen
than can others. Those requiring an abundant supply
of oxygen have been called aerobic bacteria, while those
preferring little or no air are designated as anaerobic
1 Brown, P. E., and Smith, R. E. Bacterial Activities in
Frozen Soil. Iowa Agr. Exp. Sta., Research Bui. 4. 19)2
2r
434 SOILS: PROPERTIES AND MANAGEMENT
bacteria. This is an important distinction, because those
bacteria that are of the greatest benefit to the soil are, in
the main, aerobes, and those that are injurious in their
action are chiefly anaerobes. However, it seems likely
that an aerobic bacterium may gradually accommodate
itself within certain limits to an environment containing
less oxygen, and an anaerobic bacterium may accommo-
date itself to the presence of a larger amount of oxygen.
Thus a bacterium may be most active in the presence of
an abundant supply of oxygen, but, when subjected to
conditions in which the supply is small, growth continues
but with lessened vigor. The term facultative bacteria
has been used to designate those bacteria that are able
to adapt themselves to considerable variation in oxygen
supply. The structure, tilth, and drainage of the soil
consequently determine largely whether aerobic or an-
aerobic bacteria shall be more active.
356. Moisture. Bacteria require some moisture for
their growth. A notable decrease in the moisture con-
tent of the soil may temporarily decrease the number of
bacteria by limiting their development to the films of
moisture surrounding the particles. With a decrease in
the moisture content of a soil, there occurs an increase
in the oxygen in the interstitial spaces. Those bacteria
that thrive in the presence of oxygen are thereby favored,
and the character of the bacterial flora is correspondingly
changed. When the soil remains saturated, or nearly
so, for any considerable period, the anaerobic forms
assert themselves, and the usually beneficial activities
of the aerobic bacteria are temporarily suspended.
The most favorable moisture condition for the activity
of the most desirable bacteria is that found in a well-
drained soil.
ORGANISMS IN THE SOIL 435
357. Temperature. Soil bacteria, like other plants,
continue life and growth under a considerable range of
temperature. Freezing, while rendering bacteria dor-
mant, does not kill them, and growth begins slightly
above that point. It has been shown that nitrification
goes on at temperatures as low as from 37 to 39 F. It
is not, however, until the temperature is considerably
higher that their functions are pronounced. From 70
to 110 F. their activity is greatest, and it diminishes
perceptibly below or above those points. The thermal
death point of most forms of bacteria is found at some
point between 110 and 160 F., but the spore forms even
resist boiling. Only in some desert soils does the natural
temperature reach a point sufficiently high to actually
destroy bacteria, and there only near the surface. In fact,
it is seldom that soil temperatures become sufficiently high
to curtail bacterial activity.
358. Organic matter. The presence of a certain
amount of organic matter is essential to the growth of
most, but not all, forms of soil bacteria. The organic
matter of the soil, consisting as it does of the remains of
a large variety of substances, furnishes a suitable food
supply for a very great number of forms of organisms.
The action of one set of bacteria on the cellular matter of
plants embodied in the soil produces compounds suited to
other forms, and so from one stage of decomposition to
another this constantly changing material affords sus-
tenance to a bacterial flora the extent and variety of which
it is difficult to conceive. Not only do bacteria affect the
organic matter of the soil, but, in the case of certain
forms, their activities produce changes in the inorganic
matter that cause it to become more soluble and more
easily available to the plant.
436 SOILS: PROPERTIES AND MANAGEMENT
A soil low in organic matter usually has a lower bac-
terial content than one containing a larger amount, and,
under favorable conditions, the beneficial action, to a cer-
tain point at least, increases with the content of organic
substance; but, as the products of bacterial life are
generally injurious to the organisms producing them,
such factors as the rate of aeration and the basicity of
the soil must determine the effectiveness of the organic
matter.
359. Soil acidity. A soil having an acid reaction
makes a poor medium for the growth of certain bacteria.
A neutral or a slightly alkaline soil furnishes the most
favorable condition for the development of the forms of
bacteria most beneficial to arable land. The activities of
many soil bacteria result in the formation of acids which
are injurious to the bacteria themselves, and, unless there
is present some basic substance with which these can
combine, bacterial development is inhibited by their own
products. This is one of the reasons why lime is so often of
great benefit when applied to soils, and especially to those
on which alfalfa and red clover are growing. For the
same reason, the presence of lime hastens decay of or-
ganic matter in certain soils, and the conversion of nitrog-
enous material with a minimum loss into compounds
available to the plants. As showing the value of lime
in the process of nitrate formation, it has been pointed
out that in the presence of an adequate supply of lime
the availability of ammonium salts is almost as high
as that of nitrate salts, but where the supply of lime
is insufficient the value of ammonium salts is relatively
rather low.
360. Functions of soil bacteria. Bacteria have a part
in many of the processes of the soil which greatly affects
ORGANISMS IN THE SOIL 437
its productiveness. It has become customary to refer to
the changes produced by certain forms of bacteria as their
function in contributing to soil productiveness.
361. Decomposition of mineral matter. Certain bac-
teria decompose some of the mineral matter of the
soil and render it more easily available to the plant.
While the nature of the processes and their extent
are not known, there is sufficient evidence to justify
the above statement. It is well known that several
forms of bacteria are instrumental in decomposing rock,
and that sulfur and iron compounds are acted upon by
other forms.
To what extent the very difficultly soluble forms of
phosphorus, as tricalcium phosphate for example, are
rendered soluble and available to agricultural plants by
microorganisms, is a matter of great importance. The
extent to which the subject has been investigated is
rather limited, but, in the main, there is indicated a con-
siderable action of both bacteria and fungi on tricalcium
phosphate.
362. Influence of certain bacteria and molds on the
solubility of phosphates. Some very significant experi-
ments were performed by Stoklasa, Duchacek, and Pitra, 1
who found that bone meal, when brought into contact
with pure cultures of certain bacteria, was apparently
rendered soluble, the extent to which the solubility pro-
gressed varying with the different forms of bacteria
brought into contact with it. The percentage of the total
phosphorus in the meal that was rendered soluble was as
follows :
1 Stoklasa, J., Duchacek, F., and Pitra, J. Uber den Einfluss
der Bakteriun auf die Knochenzersetzimg. Centrlb. f. Bakt.,
II, Band 6, Seite 526-535, 554-558. 1900.
438 SOILS: PROPERTIES AND MANAGEMENT
Per cent
Not inoculated 3.83
B. megatherium 21.56
E. fluorescens 9.19
B. proteus vulgaris 14.79
B. butyricus Hueppe 15.55
B. mycoides 23.03
B. mesentericus 20.60
Lohnis l quotes Grazia e Cerza to have found that
Aspergillus niger, Penicillium glaucum, and P. brevicaule,
isolated from garden soil, when placed in nutrient solu-
tion with tricalcium phosphate, assimilated one-fifth to
one-third of the phosphorus in sixty days.
There is some difference of opinion whether the solvent
action arising from bacterial growth is due entirely to the
acids that are produced by the bacteria exerting such
action, or whether there is also some other influence exer-
cised by bacteria. Stoklasa accounts for the solvent
action of the bacteria in his experiments by the bacterial
secretion of proteolytic and diastatic enzymes acting on
the bone meal. In opposition to this idea, Krober 2
maintains that the solvent action depends on the kind of
fermentation that the organic matter undergoes, acid fer-
mentation rendering the phosphates more soluble, while
ammoniacal fermentation results in no solvent action on
tricalcium phosphate and, in the presence of sufficient
basic material, may render the monocalcium and dical-
1 Lohnis, F. Handbuch d. Landw. Bakteriologie, Seite 700.
Berlin. 1910.
2 Krober, E. Uber das Loslichwerden der Phosphorsaure
aus Wasserunloslichen Verbindungen unter der Einwirkimg von
Bakterien und Hefen. Jour. f. Landw., Band 57, Seite 5-80.
1909-1910.
ORGANISMS IN THE SOIL 439
cium phosphates insoluble. He would limit the solvent
action of bacteria to the effect of the acids they produce.
Sackett, Patten, and Brown x have in a measure repeated
Stoklasa's experiments and obtained somewhat similar
results, which lead them to conclude that there is a
solvent agent other than the acids produced by the
bacteria.
It would appear from these experiments that bacteria,
and possibly fungi, commonly found in soils act on tri-
calcium phosphate in such a manner as to render a part
of it soluble. Nevertheless, experiments that have been
conducted for the purpose of ascertaining whether tri-
calcium phosphate in soils is rendered more readily avail-
able to plants when large quantities of decomposing organic
matter are present than when this is not the case, have
not, in the main, indicated that the decomposing organic
matter increases availability of the phosphorus (par. 439) .
An explanation of this may possibly be found in the
occurrence of a reverse biological process which results
in the transformation of soluble phosphates into insoluble
ones, the occurrence of such a process having been found
by Stoklasa 2 and others.
The carbon dioxide produced by bacteria is a solvent
for many of the silicates of the soil, and may free calcium
and potassium from hornblende and feldspar.
Various groups of sulfur bacteria, through the produc-
tion of H 2 S and H 2 SO 4 , act on iron in the soil and con-
vert it into sulfide and sulfate. Carbon dioxide also
^ackett, W. G., Patten, A. J., and Brown, C. W. The
Solvent Action of Soil Bacteria upon the Insoluble Phosphates
of Raw Bone Meal and Natural Rock Phosphate. Michigan
Agr. Exp. Sta., Special Bui. 43. 1908.
2 Stoklasa, J. Biochemischer Kreislauf des Phosphat-Ions
im Boden. Centrlb. f. Bakt., II, Band 29, Seite 385-519. 1913.
440 SOILS: PROPERTIES AND MANAGEMENT
plays a part in the solution of iron. The lower fungi and
the algse precipitate iron from solution as iron oxide.
363. Decomposition of non-nitrogenous organic matter.
The organic matter commonly decomposed in soils
contains a large proportion of compounds containing no
nitrogen. Many non-nitrogenous substances decompose
rather rapidly, and the organic nitrogen disappears less
rapidly than the carbon, hydrogen, and oxygen of organic
bodies.
Humus always contains a higher percentage of nitrogen
than do the plants from which it is formed.
The non-nitrogenous substances consist of cellulose and
allied compounds forming the cell walls of plants, and the
carbohydrates, organic acids, fats, and the like, contained
in them. The dissolution of cellulose is brought about
by the action of the enzyme cytase secreted by a number
of fungi, and is also probably accomplished by the Bacillus
amylobacter, but whether through the secretion of an
enzyme is not known. Other bacteria have been reported
to secrete a cytase that acts on certain constituents of the
cell wall. It is probable that numerous organisms capa-
ble of fermenting cellulose and allied substances exist in
the soil, accomplishing this decomposition through the
production of cytase.
The effect of cytase on cellulose and other fiber is to
hydrolyze it with the formation of sugar, as glucose,
mannose, zylose, arabinose, and the like.
Starch is converted into glucose by a ferment (diastase)
either present in the plant itself or possibly secreted by
fungi or bacteria. All the sugars are finally converted
into organic acids which may combine with mineral bases.
Distinct organisms have been isolated that can utilize
for their development formates, acetates, propionates,
ORGANISMS IN THE SOIL 441
butyrates, and the like, the final product being carbon
dioxide and water. Thus, step by step, the non-nitroge-
nous matter incorporated with the soil is carried by
one and another form of organism from the most com-
plex to the simplest combinations.
The final product of the decomposition of carbonaceous
matter being carbon dioxide, there is a return to the air
of the compound from which the carbon of the decompos-
ing substance was originally derived. In the plant, un-
less it is saprophytic, the carbon of the tissues comes
largely from the carbon dioxide of the air, from which
more complex carbon-bearing compounds are produced and
utilized in its functions or in its tissues. A portion of the
carbon is returned to the air by the plant in the form of
carbon dioxide; the remainder is retained by the plant,
and may be returned by the process of decay or may be
consumed by an animal, and, as the result of its physio-
logical processes, either exhaled as carbon dioxide or
deposited in the tissues to be later decomposed and con-
verted into carbon dioxide. The soil is thus the scene of
at least a part of the varied transformations through
which carbon is continually passing as it is utilized by
higher plants, animals, bacteria, and fungi.
The non-nitrogenous organic substances in their various
stages furnish food for a large number of bacteria, among
which are those concerned in the decomposition of mineral
matter and in the processes of nitrification and nitrogen
fixation. There are, therefore, two ways in which these
substances are of great importance in soil fertility : (1) as
a source of carbon dioxide and of organic acids ; (2) as
a food supply for useful soil bacteria.
364. Decomposition of nitrogenous organic matter.
The decomposition of nitrogenous organic matter is ac-
442 SOILS: PROPERTIES AND MANAGEMENT
complished by a series of changes from one compound to
another, as was seen to be the case with the non-nitroge-
nous materials. The final products are carbon dioxide,
water, usually some hydrocarbon gases resulting from
the carbon and hydrogen of the organic matter, and also
some hydrogen sulfide or other gas containing sulfur
or a final oxidation of the sulfur of the proteids into sul-
fates; while the nitrogen is ultimately converted into
nitrates, or into free nitrogen, although a portion of the
original nitrogen sometimes escapes into the air in the
intermediate stage, ammonia.
The processes will be discussed under the following
heads, which represent certain more or less definite stages
in the decomposition: 1, decay and putrefaction; 2, am-
monification ; 3, nitrification ; 4, denitrification ; 5, fixa-
tion of atmospheric nitrogen. These various processes
form what has been termed the nitrogen cycle.
CHAPTER XXI
THE NITROGEN CYCLE
OF the various elements composing the nutrients used
by plants, nitrogen has the highest commercial value.
It is, moreover, absorbed in large quantities by agricul-
tural plants and the supply is constantly liable to loss in
drainage water and in the gaseous form. Its importance
to agriculture has led to much study of its occurrence,
combinations, reactions, and movements in the soil.
When it is recalled that the nitrogen gas of the* atmos-
phere is the one primitive source of the world's supply of
nitrogen, it becomes apparent that the agencies that have
been instrumental in its transfer from one condition to
another have been extremely active. The movement of
nitrogen from air to soil, from soil to plant, from plant
back to soil or to animal, and from animal back to soil,
with a return to air at various stages, involves many
forces, many factors, many organisms, and many re-
actions.
365. Decay and putrefaction. Decomposition of the
nitrogenous organic matter of the soil, consisting largely
of the proteins, begins with either one of two processes
decay or putrefaction. Decay is produced by aerobic
bacteria, and naturally occurs when the conditions are
most favorable for their development. When the condi-
tions are otherwise, the growth of these bacteria is checked,
and then further decomposition would be extremely slow
443
444 SOILS: PROPERTIES AND MANAGEMENT
were it not for the other process putrefaction. Putre-
faction is produced by anaerobic bacteria. In the same
body, and consequently in the same soil, decay and putre-
faction may be in progress simultaneously, decay taking
place on the outside and on the surfaces of other parts
exposed to the air, while putrefaction occurs on the in-
terior, where the supply of oxygen is limited. By means
of the two processes, decomposition is greatly facilitated.
Decay (see Fig. 61) produces a very rapid and complete
decomposition of the substance in which it operates, most
of the carbon and hydrogen being quickly converted into
carbon dioxide and water, and the nitrogen into ammonia
and probably some free nitrogen. The latter is possibly
due to the oxidation of ammonia, thus
4 NH 3 + 3 O 2 = 6 H 2 O + 2 N 2
The sulfur of the proteins finally appears in the form of
sulfates.
What the intermediate products are has not been deter-
mined, but in the decay of meat, in which there was an
abundant supply of oxygen, succinic, palmytic, oleic, and
phenyl-propionic acids have been found.
Putrefaction results in a large number of complex inter-
mediate compounds and proceeds much more slowly.
Many of the substances thus produced are highly poison-
ous, and most of them have a very offensive odor. They
may be further broken down by decay when the condi-
tions are suitable, or by a continuation of the process of
putrefaction. In either case, the poisonous properties
and the odor are removed.
In the process of decomposition of organic matter two
classes of substances are produced : (1) those that have
been excreted or secreted by the bacterium, and therefore
THE NITROGEN CYCLE 445
have passed through the metabolic processes of the organ-
ism ; (2) those that have been formed because of the
removal of certain atoms by bacteria or enzymes from
compounds, thus necessitating a readjustment of the
remaining atoms and the consequent formation of a new
compound.
Putrefaction is carried on by a large number of forms
of bacteria, the resulting product depending on the sub-
stance in process of decomposition and on the bacteria
involved. Some of the characteristic, although not con-
stant, products formed in the putrefaction of albumin
and proteins are albumoses, peptones, and amino acids,
followed by the formation of cadaverine, putrescine, ska-
tol, and indol. Where an abundant supply of oxygen is
present, or where a sufficient supply of carbohydrates
exist, these substances are not formed. There are many
other products of putrefaction, including a number of
gases, as carbon dioxide, hydrogen sulfide, marsh gas,
phosphine, hydrogen, nitrogen, and the like.
It will be noticed that these changes, like those occur-
ring in the non-nitrogenous organic matter, involve a
breaking-down of the more complex compounds and the
formation of simpler ones ; and that a very large number
of bacteria are concerned in the various steps, while even
the same substances may be decomposed and the same
resulting compounds formed by a number of different
species of bacteria.
Present-day knowledge of the subject does not make
it possible to present a list of the bacteria concerned in
each step, or to name all the intermediate products
formed ; but for the student of the soil the principal
consideration is a knowledge of the circumstances under
which the nitrogen is made available to plants, and the
446 SOILS: PROPERTIES AND MANAGEMENT
conditions that are likely to result in its loss from the
soil.
366. Ammonification. Decay and putrefaction may
be considered as the beginning of the process of ammoni-
fication. Ammonification (see Fig. 61), as its name
implies, is that stage of the process during which am-
monia is formed from the intermediate products.
Like the other processes of decomposition, there are
many species of bacteria capable of forming ammonia
from nitrogenous organic substances. Different forms
display different abilities in converting nitrogen of the
same organic material into ammonia, some acting more
rapidly or more thoroughly than others. In tests by
certain investigators in which the same bacteria are used
on different substances, the order of their efficiency is
changed with the change of substance. It seems likely,
therefore, that certain forms are most efficient when
acting on certain organic compounds; that, in other
words, each species is best adapted to the decom-
position of certain substances, while capable of attack-
ing others although less effectively. This characteristic
preference of a class of bacteria for the decomposition of
certain substances is made evident by the experiments
of Sackett, 1 who found that in some soils dried blood was
ammonified more rapidly than was cottonseed meal, while
in other soils the reverse was true.
367. Bacteria and substances concerned in ammoni-
fication. Among the bacteria producing ammonifica-
tion are B. mycoides, E. subtilis, B. mesentcricus vul-
gatus, B. janthinus, and B. proteus mdgaris. Of these,
B. mycoides has been very carefully studied, and the
1 Sackett, W. G. The Ammonifying Efficiency of Certain
Colorado Soils. Colorado Agr. Exp. Sta., Bui. 184. 1912.
THE NITROGEN CYCLE 447
findings of Marchal 1 may be taken as representative
of the process of ammonification. He found that when
this bacterium was seeded on a neutral solution of albumin,
ammonia and carbon dioxide were produced, together
with small amounts of peptone, leucine, tyrosine, and
formic, butyric, and propionic acids. He concludes that
in the process, atmospheric oxygen is used, and that
the carbon of the albumin is converted into carbon dioxide,
the sulfur into sulfuric acid, and the hydrogen partly
into water, and partly into ammonia by combining
with the nitrogen of the organic substance. He suggests
that a complete decomposition of the albumin occurs
according to the following reaction :
C72H 112 Ni 8 S022 + 77 O 2
= 29 H 2 + 72 C0 2 + SO 3 + 18 NH 3
The greatest activity occurred at a temperature of 86
F., and as low as 68 F. action was rather strong. Access
of an increased amount of air, produced by increasing the
surface of the liquid, increased the rate of ammonification.
A slightly acid reaction in the liquid produced the maxi-
mum activity, but in a neutral or even slightly acid me-
dium the process was continued, although much less
actively.
Marchal found that B. mycoides was also capable of
ammonifying casein, fibrin, legumin, glutin, myosin,
serin, peptones, creatine, leucine, tyrosine, and asparagine,
but not urea.
368. Nitrification. Some agricultural plants can util-
ize ammonium salts as a source of nitrogen. This has
1 Marchal, E. Sur la Production de I'Ammoniaque dans
le Sol par les Microbes. Bulletins de 1'Acad. Royale de Belg.,
3 series, F. 25, pp. 727-776, 1893.
448 SOILS: PROPERTIES AND MANAGEMENT
been determined for maize, rice, peas, barley, and po-
tatoes. Other plants, such as beets, show a decided
preference for nitrogen in the form of nitrates. Whether
any of the common crops can thrive as well on ammo-
nium salts as on nitrates has not been finally demon-
strated. In most arable soils the transformation of nitro-
gen does not stop with its conversion into ammonia, but
goes on by an oxidation process to the formation of first
nitrous, and then nitric, acids (see Fig. 61). This may be
considered to proceed according to the following equa-
tions :
2 NH 8 + 3 O 2 = 2 HNO 2 + 2 H 2 O
2 HNO 2 + O 2 = 2 HNO 3
The acid in either case combines with one of the bases
of the soil, usually calcium, so that calcium nitrate
results.
Each of these steps is brought about by a distinct
bacterium, but the bacteria are closely related. Collec-
tively they are called nitrobacteria. Nitrosomonas and
Nitrosococcus are the bacteria concerned in the conver-
sion of ammonia into nitrous acid or nitrites. The former
are supposed to be characteristic of European, and the
latter of American, soils. They are sometimes referred
to as nitrous ferments.
Nitrobacter are those bacteria that convert nitrites
into nitrates. They are also designated nitric ferments.
There seem to be some differences in bacteria from dif-
ferent soils, but the differences are slight and the condi-
tions favoring the actions of the bacteria are similar. It
is also true that the conditions favoring the action of
Nitrosomonas and Nitrobacter are similar, and they
are generally found in the same soils, although some
THE NITROGEN CYCLE
449
experiments show that in the same soil nitrites may
sometimes accumulate, indicating conditions more favor-
able to the development of the Nitrosomonas bacteria.
To tzntmaf
; Free wrrope/? ~ a . a
^rt/TG/F/flAT/ON
\. ... -..-.-. xr*^--:- : r
.-/.. : .>>>^
FIG. 61. Diagrammatic representation of the movements of nitrogen
between soil, plant, animal, and atmosphere. This has been termed
the nitrogen cycle.
The formation of nitrates usually follows closely on the
production of nitrites, so that there is rarely more than
a trace of the latter to be found in soils. A soil favorable
to the process of nitrification is usually well adapted to
all the processes of nitrogen transformation.
Marked differences have been found in the nitrifying
power of bacteria from different soils. Highly productive
soils have generally been found to contain bacteria having
greater nitrifying efficiency than those from less produc-
tive soils, but this may not always be the case, as other
factors may limit the productiveness.
369. Effect of organic matter on nitrification. A
peculiarity in the artificial culture of nitrifying bacteria
2G
450 SOILS: PROPERTIES AND MANAGEMENT
is that they cannot be grown in artificial media containing
organic matter. This property for a long time prevented
the isolation and identification of these organisms, as it
was hardly conceivable that organisms living in the dark,
where energy cannot be obtained from sunlight, could
exist without using the energy stored by organic matter.
It has been suggested, in explanation of this, that the
energy produced by the oxidation involved in the process
of nitrification makes possible the growth of the organisms
under these apparently impossible conditions. Some
experimenters report having grown nitrobacteria in or-
ganic media, but it is generally believed at present that
this is not possible and that there has been some error in
the work of these experimenters.
The presence of peptone in the proportion of 500
parts per million completely prevents the development
of nitrobacteria, and one-half that quantity checks it;
while 150 parts of ammonia to the million has a similar
effect. In a normal soil the quantity of soluble ammo-
nium salts is well below this amount, as must also be that
of soluble organic matter. In confirmation of the inhibit-
ing effect of organic matter on the nitrobacteria, cases
have been reported of soils very rich in organic matter
in which no bacteria of this type exist.
It has also been stated that very heavy manuring
with organic manures results in decreased nitrification
in the soil. While this may be true where farm manure
is used in the quantities sometimes applied in gardening
operations, it is not likely to be the case in soils on which
ordinary field crops are grown. The principle is well
illustrated by the dry-earth closet. Manure mixed with
earth in relatively small proportions and kept aerated
by occasional mixing undergoes a very thorough decora-
THE NITROGEN CYCLE
451
position of the manure but without any corresponding
increase in nitrates. On the other hand, under field con-
ditions, manure used in relatively small amounts does
not undergo this serious loss.
The application of twenty tons of farm manure to the
acre to sod on a clay loam soil for three consecutive years,
at Cornell University, resulted in a larger production
of nitrates on the manured soil than on a contiguous plat
of similar soil left unmanured. This was true during the
third year of the applications, when the land was in sod,
and also during the fourth year, when no manure was
applied to either plat and when both plats were planted
to corn, as may be seen from the following table :
NITRATES PRODUCED ON HEAVILY MANURED AND ON UN-
MANURED SOIL
NO 3 IN PARTS
DRY
TO A MILLION,
SOIL
Unmanured
soil
Twenty tons
manure to the
acre for three
years
Land in timothy
April 23
82
21
May 3 ...
4 1
46
May 14
3.3
4.5
May 30
20
40
June 1 ...
24
20
June 13
0.8
1 l
June 20
1 3
30
July 24
22
28
August 14
1.8
3
Land in maize
May 19
175
20 1
June 22
July 6
42.8
50
79.3
105
July 28
1950
3040
August 10
151.0
184.0
452 SOILS: PROPERTIES AND MANAGEMENT
370. Effect of soil aeration on nitrification. Probably
the most potent factor governing nitrification in the soil
is the supply of air. In clay, and even in loam soils, the
tendency to compactness is such as to prevent the pres-
ence of sufficient air to enable nitrification to proceed
as rapidly as desirable unless the soil is well tilled. Col-
umns of soil eight inches in diameter and eight inches in
depth were removed from a field of clay loam on the Cor-
nell University farm, and carried to the greenhouse with-
out disturbing the structure of the soil as it existed in
the field. At the same time, vessels of similar size were
filled with soil dug from a spot near by. These may be
termed unaerated and aerated soils. Both were kept
at the same temperature and moisture content in the
greenhouse, but no plants were grown on them. The
production of nitrates was as follows :
NITRATES IN D
TO THE
RY SOIL, PARTS
VllLLION
Unaerated soil
Aerated soil
When taken from field
3.2
3.2
After standing one month ....
After standing two months ....
4.2
9.0
17.6
45.6
371. Effect of sod on nitrification. Nitrification
proceeds slowly on sod land, especially if the soil is heavy.
On the same type of soil as that used in the experiment
last described, the average quantities of nitrates for each
month of the growing season in the surface eight inches
of sod land, as compared with maize land under the same
manuring, were as follows :
THE NITROGEN CYCLE
453
MONTH
NlTBATES IN D
TO THE
BY SOIL, PABTS
VIlLLION
Sod land
Maize land
April . ...
8.9
May
3.0
17.1
June .
24
403
July . . ....
4.0
1940
August
54
1867
The amount of nitrogen removed by the maize crop
was greater than that removed by the timothy; conse-
quently the greater amount in the former soil cannot be
due to the effect of the crop.
So far as the conservation of nitrogen is concerned,
sod is an ideal crop, for nitrates are formed very little
faster than they are used, and are not carried off in large
quantities by the drainage water.
In the corn land as much as 175 pounds of nitrate
nitrogen was present in the first twelve inches of one
acre, or fully three times as much as was used by the
crop.
372. Depths at which nitrification takes place. War-
ington 1 concluded from his experiments that nitrification
takes place only in the surface six feet of soil. Hall 2
has pointed to the fact that no more nitrates were leached
from the 60-inch lysimeter at Rothamsted than from the
one 40 inches deep ; which is very good evidence that in
1 Warington, R. On the Distribution of the Nitrifying
Organism in the Soil. Trans. Chem. Soc., Vol. 51, p. 118.
1887.
2 Hall, A. D. The Book of the Rothamsted Experiments,
p. 230. New York, 1905.
454 SOILS: PROPERTIES AND MANAGEMENT
that particular soil nitrification does not take place below
40 inches from the surface. In more porous soils, how-
ever, nitrification probably extends deeper, especially
in the rich and porous subsoils of arid and semiarid regions.
In all probability, nitrification is largely confined to
the furrow slice, where the opening-up of the soil by til-
lage has provided the necessary air, and where the tem-
perature rises to a point more favorable to the action
of nitrifying bacteria. The results from the aerated and
unaerated soils as shown above represent the differences
that doubtless exist between the furrow slice and the sub-
soil so far as nitrification is concerned.
373. Loss of nitrates from the soil. Nitrogen hav-
ing been converted into the form of nitric acid, it im-
mediately combines with available bases in the soil,
forming salts, all of which are very easily soluble and
which are carried in solution by the soil water. In a
region of much rainfall, the removal of nitrates in the
drainage water is very rapid. Hall 1 states that nitrates
formed during the summer or the autumn of one year are
practically all removed from the soil of the Rothamsted
fields before the crops of the following year have advanced
sufficiently to utilize them. It was formerly customary
to fertilize with ammonium salts in autumn, but the
drainage water showed on analysis such a large quantity
of nitrates during the months intervening between the
time of fertilizing and the opening of the growing season
that the practice was discontinued.
In regions of less rainfall or of greater surface evapora-
tion, the loss in this way is less, reaching a minimum in an
arid region when irrigation is not practiced. Under
i Hall, A. D. The Soil, p. 176. New York, 1903.
THE NITROGEN CYCLE 455
such conditions, there is a return of nitrates to the upper
soil as capillary water moves upward to replace evapo-
rated water. In fact, wherever evaporation takes place
to any considerable extent there is some movement of
this kind. The need for catch crops to take up and pre-
serve nitrogen is therefore greater in a humid region than
in an arid or a semiarid one. A system of cropping that
allows the land to stand idle for some time, or a crop that
requires intertillage, as does maize, fails to utilize all the
nitrates produced, and promotes the loss of nitrogen in
drainage water.
374. Nitrate reduction. The nitrogen-transforming
bacteria thus far studied have been those that cause
the oxidation of nitrogen as the result of their activi-
ties. A number of forms of bacteria that accomplish a
reverse action may now be considered. The several
processes involved are commonly designated by the
general term denitrification, and comprise the follow-
ing : 1, reduction of nitrates to nitrites and ammonia;
2, reduction of nitrates to nitrites, and of these to ele-
mentary nitrogen.
The number of organisms that possess the ability to
accomplish one or more of these processes is very large
in fact, greater than the number involved in the oxida-
tion processes ; but, in spite of their numbers, permanent
loss of nitrogen in ordinary arable soils is unimportant
in amount, although in heaps of barnyard manure it
may be a very serious cause of loss.
Some of the specific bacteria reported as bringing about
nitrate reduction are : B. ramosm and B. pestifer, which
reduce nitrates ; B. mycoides, B. subtilis, B. mesentericus
vulgatus, and many other ammonification bacteria which
are capable of converting nitrates into ammonia.
456 SOILS: PROPERTIES AND MANAGEMENT
B. denitrificans alpha and B. denitrificans beta reduce
nitrates with the evolution of gaseous nitrogen.
375. Nitrate-assimilating organisms. In addition to
the nitrate-reducing bacteria already mentioned, there are
other bacteria which also utilize nitrates ; but, like higher
plants, these convert the nitrogen into organic nitrog-
enous substances. However, as they operate in the
dark and cannot obtain energy from sunlight, they must
have organic acids or carbohydrates as a source of energy.
While these bacteria cannot be considered as nitrate
reducers, they help to deplete the supply of nitrates when
conditions are favorable for their development. What
these conditions are is not well understood, nor can any
estimate be made as to the extent of their operations.
376. Denitrification. The term denitrification may
be used to include both the process of nitrate reduction
and that of nitrate assimilation (see Fig. 61).
Most of the denitrifying bacteria perform their func-
tions only under a limited amount of oxygen, while others
can operate in the presence of a more liberal supply;
but, in general, thorough aeration of the soil practically
prevents denitrification. Straw apparently carries an
abundant supply of denitrifying organisms, and also
furnishes a supply of carbohydrates which favor their
action; so that stable manure is very likely to undergo
denitrification, and straw or coarse stable manure are
conducive to the growth of denitrifying bacteria in the soil.
Under ordinary farm conditions, denitrification is
of no significance in the soil where proper drainage and
good tillage are practiced. Warington 1 showed that if
1 Warington, R. Investigations at Rothamsted Experi-
mental Station. U. S. D. A., Office of Exp. Sta., Bui. 8, p. 64.
1892.
THE NITROGEN CYCLE 457
an arable soil is kept saturated with water to the exclu-
sion of air, nitrates added to the soil are decomposed,
with the evolution of nitrogen gas. As lack of drainage
is usually most pronounced in early spring, when the soil
is likely to be depleted of nitrates, it is not likely that
much loss arises in this way unless a nitrate fertilizer has
been added. Among the many difficulties arising from
poor drainage, denitrification of an expensive fertilizer
may be a very considerable item.
The addition of a nitrate fertilizer to a well-drained soil
receiving stable manure is not likely to result in a loss of ni-
trates unless the dressings of manure have been extremely
heavy. Hall 1 states that at Rothamsted, where large quan-
tities of nitrate of soda are used every year in connection
with annual dressings of farm manure, the nitrate produces
nearly as large an increase when added to the manured
as when added to the unmanured plat. In other words,
there appears to be no loss of nitrate by denitrification.
It is possible to reach a point in manuring at which
denitrification may take place. Market-gardeners some-
times reach this point, when fifty tons or more of farm
manure, in addition to a nitrate fertilizer, are added to
the soil. Plowing under heavy crops of green manure
may produce the same result. In either case, the best
way to overcome the difficulty is to allow the organic
matter to partly decompose before adding the fertilizer.
The removal of the easily decomposable carbohydrates
needed by the denitrifying organisms decreases or pre-
cludes their activity.
377. Nitrogen fixation through symbiosis with higher
plants. It has long been recognized by farmers that
1 Hall, A. D. The Book of the Rothamsted Experiments,
pp. 114-115. New York, 1905.
458 SOILS: PROPERTIES AND MANAGEMENT
certain crops, as clover, alfalfa, peas, beans, and some
others, improve the soil, making it possible to grow larger
crops of cereals after these crops have been on the land.
Within the past century the benefit has been traced to
an increase in the nitrogen content of the soil, and the
specific plants so affecting the soil were found to be, with
a few exceptions, those belonging to the family of legumes.
It has furthermore been demonstrated that under certain
conditions these plants utilize the uncombined nitrogen of
the atmosphere (see Fig. 61), and that they contain,
both in the aerial portions and in the roots, a very high
percentage of nitrogen. In consequence, the decomposi-
tion of even the roots of the plants in the soil leaves a
large amount of nitrogenous matter.
378. Relation of bacteria to nodules on roots. It has
also been shown that the utilization of atmospheric nitro-
gen is accomplished through the aid of certain bacteria
that live in nodules (tubercles) on the roots of the plants.
These bacteria take free nitrogen from the air in the soil,
and the host plant secures it in some form from the bac-
teria or their products. The presence of a certain species
of bacteria is necessary for the formation of tubercles.
Leguminous plants grown in cultures or in soil not con-
taining the necessary bacteria do not form nodules and do
not utilize atmospheric nitrogen, the result being that
the crop produced is less in amount and the percentage
of nitrogen in the crop is less than if nodules were formed.
The nodules are not normally a part of leguminous
plants, but are evidently caused by some irritation of the
root surface, much as a gall is caused to develop on a leaf
or a branch of a tree by an insect. In a culture contain-
ing the proper bacteria, the prick of a needle on the root
surface will cause a nodule to form in the course of a few
THE NITROGEN CYCLE 459
days. The entrance of the organism is effected through
a root-hair which it penetrates, and it may be seen as a
filament extending the entire length of the hair and into
the cells of the cortex of the root, where the growth of
the tubercle starts.
Even where the causative bacteria occur in cultures
or in the soil, a leguminous plant may not secure any
atmospheric nitrogen, or perhaps only a small quantity,
if there is an abundant supply of readily available com-
bined nitrogen on which the plant may draw. The bac-
teria have the ability to utilize combined nitrogen as
well as uncombined nitrogen, and prefer to have it in
the former condition. On soils rich in nitrogen, legumes
may therefore add little or no nitrogen to the soil ; while
in properly inoculated soils deficient in nitrogen an impor-
tant gain of nitrogen results.
While B. radicicola is considered the organism common
to all leguminous plants, it is now known that the organ-
isms from one species of legume are not equally well adapted
to the production of tubercles on each of the other species
of legumes. They show greater activity on some species
than on others, but do not develop so successfully on all
species as on the one from which the organisms were
taken. It was rather generally believed at one time that
the longer any species of legume is in contact with the
organisms from another species, the more active this
species becomes and the greater is the utilization of
atmospheric nitrogen. Considerable doubt has been cast
on this view in recent years, and it is now generally con-
ceded that the bacteria of certain legumes are not capable
of inoculating certain other species of legumes.
379. Transfer of nitrogen to the plant. It has been
shown by several investigators that bacteria from the
460 SOILS: PROPEETIES AND MANAGEMENT
nodules of legumes are able to fix atmospheric nitrogen
even when not associated with leguminous plants. There
would seem to be no doubt, therefore, that the fixation of
nitrogen in the tubercles of legumes is accomplished di-
rectly by this organism, not by the plant itself nor through
any combination of the plant and the organism though
both of these hypotheses have been advanced. The part
played by the plant is doubtless to furnish the carbohydrates
which are required in large quantities by all nitrogen-fixing
organisms and which the legumes are able to supply in
large amounts. The utilization of large quantities of
carbohydrates by the nitrogen-fixing bacteria in the tuber-
cles may also account for the small proportion of non-
nitrogenous organic matter in the plants.
How the plant absorbs this nitrogen after it has been
secured by the bacteria is less well understood. Early
in the growth of the tubercle, a mucilaginous substance
is produced, which permeates the tissues of the plant in
the form of long, slender threads containing the bacteria.
These threads develop by branching or budding, and form
what have been called Y and T forms, known as bac-
teroids, which are peculiar to these bacteria. The threads
finally disappear, and the bacteria diffuse themselves more
or less through the tissues of the root. What part the
bacteroids play in the transfer of nitrogen is not known.
It has been suggested that in this form the nitrogen is
absorbed by the tissues of the plant. It seems quite likely
that the nitrogen compounds produced within the bacteria
cells are diffused through the cell wall and absorbed by
the plant.
380. Soil inoculation for legumes. Immediately fol-
lowing the discovery of the nitrogen-fixing bacteria, the
possibility was conceived of securing a better growth of
THE NITROGEN CYCLE 461
leguminous crops on soils not having previously grown
such crops successfully. Extensive experiments showed
the practicability of inoculating land for a certain legumi-
nous crop by spreading on its surface soil from a field
on which the same crop is successfully growing. It is
manifestly much better to apply the organisms from a
certain species of legumes from a field having grown the
same species, than to attempt to use organisms from an-
other species of legumes. The fact that soil inoculation
by means of soil from other fields may possibly transmit
weed seeds and fungous diseases, and also necessitates
the transportation of a great bulk and weight of material,
has led to numerous efforts to inoculate soil by means
of pure cultures. The pure culture may also make it
possible to bring to the soil bacteria of greater physio-
logical efficiency than those already there.
The first attempt at inoculation by pure cultures
was made in Germany, the cultures being sold under the
name of " nitragin." Careful experiments made with
this material previous to the year 1900 did not show
it to be very efficient ; but in recent years improvements
in the method of manipulating the cultures have resulted
in much greater success. In " nitragin " the medium
used for growing the organisms is gelatin, and before use
this was formerly dissolved in water; but now a solution
of greater density is used in order to prevent a change of
osmotic pressure, which may cause plasmolysis and result
in the destruction of the bacteria.
Within recent years a number of cultures for soil
inoculation have been offered to the public. The first
of these utilized absorbent cotton to transmit the bac-
teria in a dry state from the pure culture in the laboratory
to the user of the culture, who was to prepare therefrom
462 SOILS: PROPERTIES AND MANAGEMENT
another culture to be used for inoculating the soil. Care-
ful investigation of this method showed that its weakness
lay in drying the cultures on the absorbent cotton, which
frequently resulted in the death of the organisms. More
recently, liquid cultures have been placed on the market
in this country, and these have, in the main, proved to
be more successful, notably those sent out by the United
States Department of Agriculture. Another very suc-
cessful culture medium, now being distributed by the
Department of Plant Physiology at Cornell University, is
steamed soil. The process of steaming under a pressure
of two or three atmospheres increases greatly the solu-
bility of both organic and inorganic matter, and produces
a medium highly favorable to the development of the
organisms isolated from the nodules of legumes.
Liquid cultures for legume inoculation have now been
prepared and distributed by the United States Depart-
ment of Agriculture for seven years, and during this time
a record has been kept of the results so far as it has been
possible to do this. These are summarized by Keller-
man 1 as follows : average percentage of success, 76 ;
average percentage of failure, 24. If, however, the doubt-
ful reports are included with the failures, the percentage
of success is reduced to 38. Kellerman states as his
opinion that inoculation with pure liquid cultures is as
certain a means of infection as is inoculation with soil
from fields on which legumes have been successfully grown
for extended periods, if the soil to be infected is one well
adapted to the leguminous crop; but on soils not well
suited to legumes, the use of soil from old fields is a much
more satisfactory medium with which to attempt inocula-
1 Kellerman, K. F. The Present Status of Soil Inoculation.
Centrlb. f. Bakt., II, Band 34, Seite 42-50. 1912.
////<; NITROGEN CYCLE 463
turn. It is only a question of time until a successful
im-thud of inoculating soil from artificial cultures will be
found. In the meantime, inoculation by means of in-
fc>ted soil is the most practical method.
381. Nitrogen fixation without symbiosis with higher
plants. If a soil is allowed to stand idle, either without
vegetation or in grass, it will, under favorable moisture
conditions in the northern states, accumulate in one or two
years an appreciable amount of nitrogen not present at the
beginning of the period. At the Rothamsted Experiment
Station, one of the fields in volunteer plants, consisting
mainly of grass without legumes, gained in the course of
twenty years about twenty-five pounds of nitrogen per
acre annually. 1 According to Hall, the nitrogen brought
down by rain would account for about five pounds to the
acre per annum, and dust, bird droppings, and the like, for
a little more.
382. Nitrogen-fixing organisms. Direct experiment
has shown that certain bacteria have the ability to utilize
atmospheric nitrogen and to leave it in the soil in a com-
bined form (see Fig. 61). An anaerobic bacillus Clos-
tridium pasteurianum was first found to produce this
result. Later, a commercial culture called " alinit " was
placed on the market in Germany, claimed to contain
l>ii<'trrium ellenbachensis, with which the soil was to be
inoculated, and it was claimed that a large fixation of
atmospheric nitrogen would result. A number of tests of
this material failed to show that it caused any marked
fixation of atmospheric nitrogen.
A number of other nitrogen-fixing organisms have
since been discovered. There are: (1) several members
1 Hall, A. D. On the Accumulation of Fertility by Land
Allowed to Run Wild. Jour. Agr. Sci., Vol. 1, p. 241. 1905.
464 SOILS: PROPERTIES AND MANAGEMENT
of the group designated Azotobacter, which are aerobic
bacteria, and which some investigators hold to be capable of
fixing atmospheric nitrogen when grown in pure cultures,
while others believe them to be able to do so, at least in
large amounts, only in the presence of certain other
organisms; (2) members of the Granulobacter group,
which are large spore-bearing bacilli of anaerobic habits;
(3) Bacillus radiobacter, which appear to be closely related
to or identical with the B. radicicola of legume tubercles.
The last-named has been shown to be able to fix atmospheric
nitrogen even when not growing in symbiosis with leg-
umes.
There are doubtless many other nitrogen-fixing or-
ganisms still to be discovered.
A peculiarity of these nitrogen-fixing organisms is
their use of carbohydrates, which they decompose in
the process of nitrogen fixation. They secure more
atmospheric nitrogen when in a nitrogen-free medium.
The presence of soluble lime or magnesium salts, especially
carbonates, is necessary for the best performance of the
nitrogen-fixing function, as is also the presence of a some-
what easily soluble form of phosphorus. The organisms
are exceedingly sensitive to an acid condition of the soil.
383. Mixed cultures of nitrogen-fixing organisms. -
Mixed cultures of the various organisms mentioned fix
larger amounts of nitrogen than do the pure cultures
of any one of them, while some forms are incapable o
fixing nitrogen in pure cultures. Certain algae, particularly
the blue-green algse, aid greatly in promoting growth and
nitrogen fixation by these organisms. This they probably
do by producing carbohydrates, which are used by the
bacteria as a source of energy for nitrogen fixation, the
bacteria furnishing the algae with nitrogenous compounds.
THE NITROGEN CYCLE 465
To what extent the relation is symbiotic is not known at
present, but it seems probable that a relation may exist
similar to that between leguminous plants and the nitrogen-
gathering baeteria in their nodules.
384. Nitrogen fixation and denitrification antagonistic.
- Nitrogen fixation and denitrification are reverse pro-
es. The former is, for most bacteria, favored by
an a.l) i ind a nt supply of air and a moderately high tempera-
ture. Thus, at 75 F. fixation was rapid, at 59 F. it was
decreased, and at 44 F. there was no fixation. Denitri-
fication is favored by a somewhat limited supply of
oxygen.
There is no reason to believe that the practical impor-
tance of nitrogen fixation without legumes is equal, under
the most favorable conditions, to that with legumes.
A further knowledge of the organisms effecting fixation
and of their habits will doubtless make possible a greater
utilization of their powers to supplement the use of leg-
umes as a source of combined nitrogen in the soil.
TREATMENT OF SOILS WITH VOLATILE ANTISEPTICS AND
WITH HEAT
Attention was first drawn to the effects of carbon bisul-
fide on the soil in a paper by Girard 1 and one by Oberlin 2
which appeared in 1894. Girard noticed that soil treated
with carbon disulfide for the purpose of combating a para-
sitic disease of sugar-beet was more productive than it
1 ( ! irard, A. Recherches sur I'Augmentation des Recoltes
par 1' Injection dans le Sol du Sulfure de Carbone a Doses Mas-
sives. Bui. Soc. Nationale d'Agric., Tome 54, p. 356. 1894.
2 Oberlin. Bodenmiidigkeit und Schwefelkohlenstoff. Mainz,
1894.
2n
466 SOILS: PROPERTIES AND MANAGEMENT
was before such treatment. The beneficial effect of the
treatment extended to the second year.
Oberlin found a somewhat similar condition where
the soil of vineyards treated with carbon bisulfide to kill
phylloxera showed greatly increased productiveness after
the treatment. The effect of carbon bisulfide on the
vineyard soil was to make it possible to raise grapes con-
tinually on the same land, whereas it had previously been
necessary to rest the land by growing a succession of
other crops at intervals of several years. It was noticed,
however, that immediately after treatment the plants did
not grow so well as under normal conditions. Systematic
investigations of the subject then began, and as early as
1895 Pagnoul 1 reported that when carbon bisulfide is
applied to soils nitrification is temporarily depressed.
Investigation of the effect of heat on soil had begun
somewhat earlier, when Frank 2 showed in 1888 that it
increases the quantities of soluble matter, both organic
and inorganic, as well as causing the soil to be more pro-
ductive.
The subject has been investigated by a large number of
persons, and in addition to carbon bisulfide a considerable
number of other volatile antiseptics, including ether,
chloroform, and toluene, have been found to influence the
productiveness of soils. The effect of heat, particularly
in steam, at various temperatures from slightly above
normal to more than 200 C., has also been studied, whiL
1 Pagnoul, M. Nouvelles Recherches sur les Transforma-
tions que Subit 1'Azote dans le Sol. Annales Agronomique,
Tome 21, pp. 497-501. 1895.
2 Frank, B. Ueber den Einfluss welchen das Sterilisiren
des Erdbodens auf die Pflanzen Entwickehmg ausiibt. Ber.
d. Deut. Bot. Gesell. (Generalversainmlungs Heft) Band 4,
Seite 87-97. 1888.
THE NITROGEN CYCLE 467
it has been found that the mere drying of soils effects
important changes in their solubility and in the bacterial
processes that occur in them. As the result of the in-
vestigations, certain well-established facts have been
worked out in connection with certain treatments when
applied to most soils.
385. Effects of carbon bisulfide and heat on properties
of soils. Volatile antiseptics usually increase the pro-
ductiveness of soils, although there may be at first a slight
temporary retardation of plant growth. It is of course
customary to permit the antiseptic to volatilize from
the soil before seed is planted. For this purpose the soil
is spread out in a thin layer, in which condition it is al-
lowed to remain until the odor of the antiseptic has dis-
appeared. The soil is then placed in vessels and moistened
and the seeds are planted in it.
Other characteristic effects of treatment with volatile
antiseptics reported by different investigators are : (1)
an initial decrease in the numbers of bacteria, followed by
a long-continued increase ; (2) a disturbance of the equi-
librium of the flora, by which certain bacteria multiply
more rapidly than others; (3) a slight initial increase
in ammonia content, followed by a considerable increase
in the rate of production of ammonia ; (4) depression of
the process by which ammonia is converted into nitric
acid, and a very slow recovery in the activity of the bac-
teria concerned, as a result of which ammonia accumulates
in the soil ; (5) an increase in the rate at which oxidation
takes j;lace in soils; (6) destruction of protozoa.
386. Hypotheses to account for effects of carbon
bisulfide and of heat. A number of hypotheses have
been formulated by which to account for the increased
plant growth and for changes induced in soils by treat-
468 SOILS: PROPERTIES AND MANAGEMENT
ment with heat and volatile antiseptics. A number of
these theories will be mentioned, but it should be remem-
bered that much important work on the subject has been
done by investigators who have not advanced any hy-
potheses.
387. Koch's theory. Koch J was the first to offer
any explanation. In 1899 he stated it as his opinion that
carbon bisulfide has a directly stimulating action on the
plants themselves. He later 2 found ether to have a
similar action, and continued his experiments with carbon
bisulfide. He found that soil sterilized with heat pro-
duced better crops when treated with carbon bisulfide
than when not so treated, and concludes that the effect
of the antiseptic, therefore, cannot be due to the effect
of the antiseptic on bacteria. He also experimented with
field soils, and showed that the size of the crop on treated
soils is not proportional to the quantity of nitrogen
contained.
The theory of Koch has been supported by Fred, 3 \vho
fertilized soils with an abundant supply of sodium nitrate
and found that in every case in which carbon bisulfide
was added the growth and yield of crop were much su-
perior to those in the corresponding pots not treated with
that substance. He concludes that as there was no lack
of plant-food and other conditions favorable to plant
1 Koch, A. Untersuchungen uber die Ursachen der Ruben-
miidigkeit mit Besonderes Beriicksicktigung der Schwefel-
kohlenstoffbehandhmg. Arb. Deut. Landw. Gesell., Heft 40,
Seite 7-38. 1899.
2 Koch, A. Ueber die Wirkung von Aether Schwefelkoh-
lenstoff auf Hohere und Niedere Pflanzen. Centrlb. f. Bakt.,
II, Band 31, Seite 175-185. 1911-1912.
3 Fred, ft. B. Effect of Fresh and Well-rotted Manure
on Plant Growth. Virginia Poly. Inst. Agr. Exp. Sta., Ann.
Kept. 1909-1910, pp. 142-159.
THE NITROGEN CYCLIC 469
growth, the effect of the antiseptic must have been directly
on the plants.
388. Hiltner and Stormer's theory. According to
Hiltner and Stormer, 1 the effect of treatment with carbon
bisulfide is to cause a disturbance in the equilibrium of
the different forms of soil bacteria. These investigators
compared the numbers in three groups of bacteria that
developed on gelatin plates inoculated from soil infu-
sions. The groups were Streptothrix, liquefiers, and
non-liquefiers. The normal relation of these in the soil
with which they worked was 20 per cent Streptothrix,
5 per cent liquefiers, and 70 per cent non-liquefiers. After
treatment with carbon bisulfide the relative proportions
were 5 per cent, 10 per cent, and 85 per cent, respectively.
From 70 to 75 per cent of the whole number of bacteria
were destroyed by the treatment, but the numbers rapidly
increased after treatment, rising in a few weeks to 50,-
000,000 to a gram in a soil that contained 10,000,000 to a
gram before treatment. This increase is due largely to
the development of the non-liquefiers, the Streptothrix
remaining at about the same actual number.
The fact that the equilibrium of the bacterial flora
was so greatly disturbed by the treatment with carbon
bisulfide led Hiltner and Stormer to believe that the greater
productiveness of the soil after treatment is due to the
greater effectiveness of the surviving and rapidly develop-
ing forms in rendering available the supply of plant
1 Hiltner, L., and Stormer, K. Studien iiber die Bakteri-
enflora des Ackerbodens, mit besonderer Beriicksichtigung
ihres Verhaltens nach eincr Hdiandlung mit Schwefelkohlenstoff
und nach Brache. Arb. Biol. Abt. f. Land- u. Forstwirtschaft
am Kaiserl. Ges. Amt., Band III. Heft 5. Berlin, 1903. Ab-
stract in Centrlb. f. Agrikultur Chemio, 33 Jahrg., Seite 361-
374. 1904.
470 SOILS: PROPERTIES AND MANAGEMENT
nutrients in the soil, and to a decrease in the number of
denitrifying bacteria, which obviates loss of available
nitrogen through their action.
Heinze, 1 working with soils treated with carbon bisul-
fide, and Pfeiffer, Frank, Friedlander, and Ehrenberg, 2
working with steamed soils, found that there was a large
fixation of nitrogen following these treatments. They
conclude that this is at least partly responsible for the
greater productiveness of the soils after the treatments
mentioned.
389. Russell and Hutchinson's theory. The next
comprehensive theory to be brought forward was one by
Russell and Hutchinson, who account for the increased
productiveness of soils partially sterilized, either by heat
or by volatile antiseptics, as due to the use by plants of
the ammonia, which, as had been shown by previous
investigators, accumulated in soils so treated by reason
of the stimulation given to the process of ammonification
and the depression of nitrification. They hold, further-
more, that the stimulation of ammonification is brought
about by the greatly increased numbers of bacteria in
the soil following the destruction of some larger organisms,
probably protozoa or allied forms, that normally interfere
with the activities of the ammonifying bacteria. Care-
ful experiments by these investigators have shown that
there is a much larger quantity of nitrogen in the combined
forms of ammonia and nitrates in partially sterilized
1 Heinze, B. Eine Weitere Mitteilungen liber den Schwefel-
kohlenstoff und die CS 2 -Behandhmg des Bodens. Centrlb.
f. Bakt., II, Band 18, Seite 56-74, 246-264, 462-470, 624-634,
790-798. 1907.
2 Pfeiffer, Th., Frank, L., Friedlander, K., and Ehrenberg,
P. Der Stickstoffhaushalt des Ackerbodens. Mitt. d. Landw.
Inst. d. Konigl. Univ. Breslau, Band 4, Seite 715-851. 1909.
THE NITROGEN CYCLE 471
soils than in untreated soils. There can be no doubt,
therefore, that, at least for some higher plants, the quan-
tity of available nitrogen is greater in the treated soils.
The relation of protozoa to the ammonifying bacteria
is somewhat more difficult of demonstration. Methods
for the enumeration of protozoa in the soil are not suffi-
ciently well worked out to admit of an entirely satisfactory
study of their relation to the ammonifying bacteria.
However, Russell and Hutchinson do not hold that pro-
tozoa are necessarily the limiting factor in ammonia
production in normal soils, but grant that some other
organism of comparatively large size may be responsible
for this. They intimate also that not only the available
nitrogen, but also the quantities of other plant nutrients,
are limited by organisms destroyed by partial steriliza-
tion ; otherwise increased productiveness induced by
partial sterilization would be confined to soils in which
nitrogen is normally the limiting factor. The theory
docs imply, however, that plant-food is the limiting factor
in all soils benefited by partial sterilization under the
conditions of the experiment. 1
1 Russell, E. J., and Darbishire, F. V. Oxidation in soils
and its relation to productiveness. Part 2. The influence
of partial sterilization. Jour. Agr. Sci., Vol. 2, pp. 305-326.
1907.
Russell, E. J., and Hutchinson, H. B. The effect of partial
sterilization of soil on the production of plant food. Jour.
Agr. Sci., Vol. 3, pp. 111-144. 1909.
Kussell, E. J., and Hutchinson, H. B. The effect of partial
sterilization of soil on the production of plant food. Part 2.
Russell, E. J., and Hutchinson, H. B. The limitation of bac-
terial numbers in normal soils and its consequences. Jour. Agr.
Sci., Vol. r,, pp. 1.72-221. 1903.
Buddin. W. Partial sterilization of soil by volatile and
non-volatile antiseptics. Jour. Agr. Sci., Vol. 6, pp. 417-451.
1914.
472 SOILS: PROPERTIES AND MANAGEMENT
Some typical results of investigations by Russell and
Hutchinson on the effect of partial sterilization on bac-
teria numbers, ammonia production, and presence of
protozoa are given below :
BACTERIA
AFTER SIXTY-
EIGHT DAYS,
TO A GRAM OP
DRY SOIL
NITROGEN
AS NH~3 AND
NOs AFTER
SIXTY-EIGHT
DAYS, PARTS
TO A MILLION
OF DRY SOIL
DETRI-
MENTAL
FACTOR
PROTOZOA
FOUND
Untreated soil . .
11,100,000
13.0
Present
f Ciliates
< Amoeba
( Monads
Soil heated to 40 C.
for three hours
7,500,000
14.4
Present
f Ciliates
< Amoeba
[Monads
Soil heated to 56 C.j
for three hours J
37,500,000
36.7
Killed
All killed
390. Greig-Smith's theory. An entirely different
explanation of the effect of partial sterilization on soils
has been advanced by Greig-Smith. 1 He states that
when disinfectants are applied to the soil their action is
a double one. They kill the less resistant bacteria, and
dissolve from the surfaces of the soil particles a waxy
covering, to which he has given the name " agricere."
The surviving bacteria, among which are the beneficial
ones, are able to develop more rapidly because of the
greater accessibility of the food supply which the re-
moval of the " agricere " has exposed.
Greig-Smith holds that heat destroys substances toxic
1 Greig-Smith, R. The Bacteriotoxins and the "Agricere"
of Soils. Centrlb. f. Bakt., II, Band 30, Seite 154-15G. 1911.
THE NITROGEN CYCLE
473
to bacteria, and also certain of the less resistant bacteria,
thus permitting the more resistant species to multiply
VITV rapidly owing to the absence of the bacteriotoxins.
In order to ascertain whether chloroform has any effect
other than the destruction of protozoa, Greig-Smith
applied it to soil previously heated to 62 C. (which he
had found was sufficient to kill all protozoa), and then
determined the number of bacteria in untreated soil, in
heated soil, and in soil heated and treated with chloro-
form. The counts to a gram of soil were made- at inter-
vals, and are shown below : 1
AT
START
4 DAYS
12 DAYS
25 DAYS
39 DAYS
Untreated soil
52
680,000
2,700,000
4,300,000
5,400,000
Soil heated
at 62 C.
16
15,800,000
11,800,000
9,000,000
8,000,000
Soil heated
at 62 C.
and treated
with chlo-
roform
13
24,600,000
45,400,000
41,600,000
90,000,000
Greig-Smith concludes that as the bacteria developed
more rapidly in the soil treated with chloroform after
heating than in the soil which was only heated and in
which the protozoa were presumably dead, the chloro-
form must have exerted some beneficial effect other than
the destruction of protozoa, and assumes that this is due
to the removal of " agricere."
Partial or complete sterilization of soils has been prac-
1 Greig-Smith, R. Contributions to our Knowledge of Soil
Fertility. Proc. Linnaean Soc. New South Wales, 1912, Part II,
I)]). 238-243.
474 SOILS: PROPERTIES AND MANAGEMENT
ticed in greenhouses for a long time, principally for the
purpose of combating plant diseases. Its value in in-
creasing productiveness has been .a consideration since
this phase of the subject has been emphasized by investi-
gations, and the treatment of " sewage-sick " soils has
been shown by Russell and Golding l to be a practical
matter. It is as a means of studying the principles of
soil fertility, however, that the investigation of the sub-
ject of partial sterilization of the soil is of greatest im-
portance.
1 Russell, E. J., and Golding, J. Investigations on "sick-
ness" in soil. Part 1. "Sewage sickness." Jour. Agr. Sci.,
Vol. 5, pp. 27-47. 1912.
Russell, E. J., and Golding, J. Investigations on "sickness"
in soil. Part 2. "Sickness" in glasshouse soils. Jour. Agr.
Sci., Vol. 5, pp. 86-111. 1912.
CHAPTER XXII
THE SOIL AIR
THE air of the soil is merely a continuation of the
atmospheric air into the interstitial spaces of the soil,
when these are not filled with water. As it is more or
less inclosed by the soil, movement does not take place
so readily as it does above the surface of the ground and
hence the soil air is more greatly influenced by its sur-
roundings than is atmospheric air. This leads to impor-
tant differences in composition between the atmospheric
air and soil air, the composition of the latter depending
on a variety of conditions in which physical, chemical,
and biological properties play a part.
FACTORS THAT DETERMINE VOLUME
The amount of air that soils contain varies with their
properties, and in any one soil the air content varies with
certain changes to which the soil is subject from time to
time. The factors that influence the volume of air in
soils are: (1) texture; (2) structure; (3) organic matter;
(4) moisture content.
391. Texture. The size of the soil particles affects
the air capacity of the soil in exactly the same way
as it does the pore space, since in dry soil they are
identical. A fine-textured soil in a dry condition would
therefore contain as large a volume of air as would a
476
476 SOILS: PROPERTIES AND MANAGEMENT
coarse-textured soil, provided the particles were spherical
and all of the same size. Under the conditions actually
existing in the field, the soils composed of small particles
generally possess the larger amount of air space.
392. Structure. The volume of air in a water-free
soil being identical with the pore space, the formation
of aggregates of particles is favorable to a large air volume.
The volume of air in any soil, therefore, changes from
time to time; and particularly is this true of a fine-
grained soil, in which the changes in structure are greater
than in a soil with large particles. A change in soil
structure may greatly alter the volume of air contained
by changing the pore space, thereby influencing the pro-
ductiveness. Clay is affected to the greatest extent in
this way.
393. Organic matter. Since organic matter is more
porous than mineral particles of any size or arrangement,
the effect of that constituent is always to increase the
volume of air. While this is generally beneficial in a
humid region, it is often very injurious in an arid region.
Unless sufficient water falls on the soil to wash the soil
particles around the organic matter and to maintain a
supply sufficient to promote decomposition, the presence
of vegetable matter leaves the soil so open that the capil-
lary rise of moisture is interfered with, and the consider-
able movement of air keeps the soil dry, with the result
that the portion of the soil layer mixed with and lying
above the organic matter is too dry to germinate seeds
or to support plant growth.
394. Moisture content. It is quite evident that the
larger the proportion of the interstitial space filled with
water, the smaller will be the quantity of air contained.
This does not necessarily mean that the higher the per-
TIJK SOIL AIR 477
centage of water in the soil, the smaller will be the volume
of air, since the amount of pore space determines both the
water and the air capacity. A soil with 30 per cent
moisture may contain more air than one with a water
content of 20 per cent, because of the tendency of mois-
ture to move the soil particles farther apart.
In soils in the field, the average diameter of the cross
section of the pore space is the most potent factor in
determining the volume of air. Small spaces are likely
to hold water, while larger spaces, not retaining water
against gravity, are filled with air.
In a clay soil the volume of air is increased, other
things being equal, by the formation of granules, and is
decreased by deflocculation or compaction. The volume
of air in any soil may be calculated from the following
formula :
% air space = % pore space - (% H 2 O X ap. sp. gr.)
COMPOSITION OF SOIL AIR
The air of the soil differs from that of the outside
atmosphere in that it contains more water vapor, a much
larger proportion of carbon dioxide, a correspondingly
smaller amount of oxygen, and slightly larger quantities
of other gases, including ammonia, methane, hydrogen
sulfide, and the like, formed by the decomposition of
organic matter.
395. Analyses of soil air. The composition of the
air of several soils, as determined by Boussingault and
Lewy, is quoted by Johnson 1 in the table following : -
1 Johnson, S. W. How Crops Feed, p. 219. New York, 1891.
478 SOILS: PROPERTIES AND MANAGEMENT
CHARACTER
OP SOIL
VOLUME IN ONE
ACRE OF SOIL TO
DEPTH OF 14
INCHES
COMPOSITION OF 100 PARTS
OF SOIL AIR BY VOLUME
Air
(Cu. ft.)
Carbon
Dioxide
(Cu. ft.)
Carbon
Dioxide
Oxygen
Nitrogen
Sandy subsoil of forest
Loamy subsoil of forest .
Surface soil of forest .
Clay soil
4,416
3,530
5,891
10,310
11,182
11,182
11,783
11,783
21,049
14
28
57
71
86
172
257
1,144
772
0.24
0.79
0.87
0.66
0.74
1.54
2.21
9.74
3.64
19.66
19.61
19.99
19.02
18.80
79.55
79.52
79.35
80.24
79.66
Soil of asparagus bed not
manured for one year .
Soil of asparagus bed
freshly manured . .
Sandy soil, six days after
manuri ng
Sandy soil, ten days after
manuring (three days
of rain)
10.35
16.45
79.91
79.91
Vegetable mold compost
There are several factors that influence the composi-
tion of the soil air, those of greatest importance being
the production and escape of carbon dioxide.
396. Sources of carbon dioxide in soil air. The
presence of carbon dioxide in soils is due in small part
to infiltration from the atmospheric air, there being a
tendency for the carbon dioxide, which is heavier than
oxygen and nitrogen, to settle out. It may also have a
purely chemical origin. But in much greater measure
is the carbon dioxide a product of biological processes
that occur in the soil. At one time it was believed that
the formation of carbon dioxide in soils was a purely
chemical process of oxidation, and possibly a part of the
gas is formed in that way. It has already been seen that
there is a condensation of gases in the manifold pores
////; SOIL AIR 479
of the soil (see par. 268), the organic portion of which is
especially capable of condensing gases. Oxygen con-
densed on the surface of this organic matter would, in
the words of Johnson, 1 " spend itself in chemical action,"
of which carbon dioxide would be the result.
There is now no doubt, however, that biological pro-
cesses are largely responsible for the occurrence of the
large quantity of carbon dioxide in the soil air. There
are two distinct processes involved : (1) the physiological
action of bacteria by which they absorb oxygen and give
off carbon dioxide, and (2) the excretion of carbon dioxide
by plant roots. The extent to which carbon dioxide is
produced in normal soils in these two ways has been es-
timated by Stoklasa, 2 who has done much work on the
subject. lie concludes that the microorganisms in
an acre of soil to a depth of four feet may produce between
sixty-five and seventy pounds of carbon dioxide a day
for two hundred days in the year, and that during the
growing period the roots of oats or wheat would give
off nearly as much to an acre.
397. Production of carbon dioxide as affecting com-
position. Although the formation of carbon dioxide
in the soil depends on the decomposition of organic
matter, it is not always proportional to the quantity
of organic matter present. The rate of decomposition
varies greatly, and where this is depressed, as is sometimes
i in muck or forest soils, the content of carbon dioxide
is relatively low. A high percentage of organic matter
^Johnson, S. W. How Crops Feed, p. 218. New York,
1891.
2 Stoklasa, J. Ueber den Ursprung die Menge und die
Bedeutung des Kohlendioxids im Boden. Centrlb. f. Bakt., II,
Band 14, Seite 723-73G. 190 ">.
480 SOILS: PROPERTIES AND MANAGEMENT
is in itself likely to prevent a proportional formation of
carbon dioxide, since the accumulation of the gas may
inhibit further activity of the decomposing organisms.
Ramann 1 states that the percentage of carbon dioxide
in the soil air has the following relations :
1. The carbon dioxide increases with the depth.
2. In general the percentage of carbon dioxide rises
and falls with the temperature, being higher in the warm
months and lower in the cold months.
3. Changes in temperature and air pressure change the
percentage of carbon dioxide.
4. In the same soil the content of carbon dioxide varies
greatly from year to year.
5. An increase of moisture in the soil increases the per-
centage of carbon dioxide.
6. The amount of carbon dioxide varies in different
parts of the soil.
The movement of carbon dioxide from the soil depends
chiefly on diffusion into the outside atmosphere. The
conditions governing diffusion, which will be discussed
elsewhere (par. 400), therefore largely determine the
rate of loss of carbon dioxide from the soil.
FUNCTIONS OF THE SOIL AIR
Both oxygen and carbon dioxide, as they exist in the
air of the soil, have important relations to the processes
by which the soil is maintained in a habitable condition
for the roots of plants. Deprived of these gases, the soil
would soon become sterile.
398. Oxygen. An all-important process in the soil
is that of oxidation, because by it the organic matter
1 Ramann, E. Bodenkunde. Seite 301. Berlin, 1905.
'/'///<; 80 JL AIR 481
that would soon accumulate to the exclusion of higher
plant life is disposed of, and the plant-food materials
are brought into a condition in which they may be ab-
sorbed by plant roots. The presence of oxygen is essen-
tial to the life of the decomposing organisms and to the
complete decay of organic matter. Through this pro-
cess, roots of past crops, as well as other organic matter
that has been plowed under, are removed from the soil.
The process of decay gives rise to products, chiefly car-
bon dioxide, that are solvents of mineral matter, and leaves
the nitrogen and ash constituents more or less available
for plant use.
Oxygen is also necessary for the germination of seeds
and the growth of plant roots. These phenomena, al-
though not involving the removal of large quantities
of oxygen, are yet entirely dependent on its presence in
considerable amounts.
399. Carbon dioxide. The solvent action of carbon
dioxide is its most important function in the soil. By
this action it prepares for absorption by plant roots most
of the mineral substances found in the soil. Although
a weak acid when dissolved in water, its universal pres-
ence and continuous formation during the growing season
results in a large total effect.
Carbonic acid dissolves from the soil more or less of
all the nutrients required by plants. The amounts so
dissolved are appreciably greater than those dissolved
in pure water. The constant formation of carbon dioxide
by decomposition of organic matter keeps this solvent
continually in contact with the soil.
Carbon dioxide serves a useful purpose in combining
with certain bases to form compounds beneficial to the
soil. Particularly is this the case with calcium carbonate,
2i
482 SOILS: PROPERTIES AND MANAGEMENT
which is of the greatest benefit to the soil in maintaining
a slight alkalinity very favorable to the development of
many beneficial bacteria and to the maintenance of good
tilth.
Stoklasa l has correlated the carbon dioxide production
with the quantity of phosphates found in the drainage
water from certain soils. Some of his results are given
below :
PzOs IN DRAINAGE
WATER
(Kilograms to a hectare)
RELATIVE PRODUC-
TION OP COz
(Milligrams to a kilo-
gram soil in 24 hours)
Loam
5 2
24
Clay
3 5
15
Lime soil
Humous soil
5.8
8.4
36
56
Stoklasa considers that the production of carbon dioxide
is a measure of the intensity of bacterial action in the
soil, and that in consequence of this activity the phos-
phorus is rendered soluble.
When carbon dioxide is combined as sodium carbonate
or potassium carbonate in considerable quantity, as in
certain alkali soils, a very injurious action on plant roots
and on soil structure results. On plants the carbonate
acts as a direct poison (see par. 305). The effect on
soil structure is to deflocculate the particles producing
the separate grain or the compact arrangement (see par.
420).
1 Stoklasa, J. Methoden zur Bestimmung der Atmungs-
intensitat der Bakterien im Boden. Zeit. f. d. Landw. Versuchs-
wesen in Oesterreich, Band 14, Seite 1243-79. 1911.
THE SOIL AIR 483
MOVEMENT OF SOIL AIR
There is a constant movement of the air in the inter-
stitial spaces of the soil and an exchange of gases between
the soil atmosphere and the outside atmosphere, as well
as a more general, but probably less effective, movement
of the air out of or into the soil, as the controlling condi-
tions may determine. The movement may be produced
by any one or more of the following phenomena : (1)
diffusion of gases; (2) movement of water; (3) changes
in atmospheric pressure; (4) changes of temperature in
atmosphere or in soil ; (5) suction produced by wind.
400. Diffusion of gases. The wide difference in the
composition of soil and atmospheric air gives rise to a
movement of gases due to a tendency for the external
and the internal gases to come into equilibrium. Accord-
ing to Buckingham, 1 the interchange of atmospheric and
soil air is due in large measure to diffusion.
The rate of movement of the soil air due to diffusion
is dependent on the aggregate volume of the interstitial
spaces, not on their average size. Thus, it is the porosity
of the soil that influences most largely the diffusion of
the air from it. Consequently the size of the particles
is not a factor, but good tilth permits diffusion to take
place more rapidly than does a compact condition of soil,
as the volume of the pore space is thereby increased.
Compacting the soil in any way, as by rolling or trampling,
has the opposite effect.
401. Movement of water. As water, when present
in a soil, fills certain of the interstitial spaces, it decreases
the air space when it enters the soil and increases it when
1 Buckingham, E. Contributions to Our Knowledge of
the Aeration of Soils. U. S. D. A., Bur. Soils, Bui. 25. 1904.
484 SOILS: PROPERTIES AND MANAGEMENT
it leaves. The downward movement of rain water pro-
duces a movement of soil air by forcing it out through
the drainage channel below, while at the same time a
fresh supply of air is drawn in behind the wave of satura-
tion as the water passes down from the surface. The
movement thus occasioned extends to a depth where
the soil becomes permanently saturated with water.
Twenty-five per cent of the air in a soil may be driven out
by a normal change in the moisture content of the soil.
402. Changes in atmospheric pressure. Waves or
high or of low atmospheric pressure, frequently involving
a change of 0.5 inch on the mercury gauge, cross the con-
tinent alternately every few days. The presence of a
low pressure allows the soil air to expand and issue from
the soil, while a high pressure following causes the out-
side air to enter in order to equalize the pressure. An
appreciable, but not important, movement of soil air is
produced in this way.
The size of the interstitial spaces is more potent than
their volume in effecting soil ventilation by this and the
following methods.
403. Changes of temperature in atmosphere or in soil.
A movement of soil air may be induced by a change of
temperature in the atmosphere or in the soil itself. Changes
in atmospheric temperature act in the same way as do
changes in atmospheric pressure; in fact, it is the effect
of temperature on air pressure that causes the movement.
Like the movement due to atmospheric pressure, it is
not great ; but where the soil immediately at the surface
of the ground attains a temperature of 120 F. at midday,
as is the case in the Corn Belt, the movement must be
appreciable.
The diurnal change in soil temperature decreases
THE SOIL AIR 485
rapidly from the surface downward, due to the absorp-
tion and slow conduction of heat (see par. 227). At
the Nebraska Experiment Station 1 the average diurnal
range for the month of August, 1891, was as follows :
DIURNAL RANGE OF AIR AND SOIL TEMPERATURES
Degrees Fahrenheit
Air 5 feet above ground 14.4
Soil 1 inch below surface 17.9
Soil 3 inches below surface 14.8
Soil 6 inches below surface 9.2
Soil 9 inches below surface 6.6
Soil 12 inches below surface 4.3
Soil 24 inches below surface 0.5
Soil 36 inches below surface 0.0
This soil contains about fifty per cent of pore space, in
the upper foot of which forty per cent is normally filled
with water during the summer months. This leaves 518
cubic inches of air in the upper cubic foot of soil. With
an increase in temperature, the air expands -j-^j- in volume
for each degree Fahrenheit. The average increase of
temperature is, in this case, about 11 degrees Fahrenheit
for the first foot. The air exhaled or inhaled by each
cubic foot of soil would then be
518 X 11 i
= 11.6 cubic inches
491
As this is slightly over two per cent of the air contained
in the upper foot of soil, and as the movement below
that depth is negligible, the change in composition at any
1 Swezey, G. D. Soil Temperatures at Lincoln, Nebraska.
Neb. Agr. Exp. Sta., 10th Ann. Kept., pp. 95-102. 1903.
486 SOILS: PROPERTIES AND MANAGEMENT
one time is not great ; but this pumping effect is kept up
day after day, although less energetically in the cooler
seasons of the year. In proportion as poor drainage
equalizes the temperature it would prevent this type of
circulation. The total effect, assisted by diffusion, is to
aid materially in ventilating the soil. Owing to diffusion
of air in the interstitial spaces, the air expelled is different
in composition from that inhaled.
404. Suction produced by wind. The movement of
wind, being almost always in gusts, alternately increases
and decreases the atmospheric pressure at the surface
of the soil. There is a tendency, -therefore, for the soil
air to escape and for atmospheric air to penetrate the
soil with each change in pressure. The effect presumably
influences only the superficial air spaces, but it must be
very frequent in its action. No measurements have
been made and no definite estimate of its effect can be
stated.
METHODS FOR MODIFYING THE VOLUME AND THE MOVE-
MENT OF SOIL AIR
The conditions that influence the ventilation of soils
are : (1) volume and size of the interstitial spaces ; (2)
moisture content; (3) daily and annual range in tem-
perature.
Although the size of the interstitial spaces does not
appear to greatly influence the diffusion of gases from
a soil, it has a marked effect on certain of the other pro-
cesses by which air enters and leaves the soil. A sandy
soil, a soil in good tilth, and, particularly, a soil composed
of clods, permit of more rapid movement of air than does
a compact soil.
THE SOIL AIR 487
While a certain movement of air through the soil is
desirable, and indeed necessary, for the reasons already
stated a very considerable movement is injurious unless
there is an abundant rainfall. The effect of air move-
ment through the soil is to remove soil moisture. In a
region of light rainfall and low atmospheric humidity, this
may be disastrous if the soil is not kept compact by care-
ful tillage. On the other hand, in a humid region and
in clay soil there is likely to be too small a supply of oxygen
for the use of crops and lower plant life unless the soil
is well stirred.
405. Tillage. The ordinary operations of tillage
greatly influence the ventilation of the soil. When a soil
is plowed, the soil at the bottom of the furrow is exposed
directly to the air at the surface, and, by the separation
of adhering particles and aggregates of particles, air
is brought into contact with particles that may previously
have been completely shut off from air. It is partly
because of its effect on soil ventilation that plowing is
beneficial, and the necessity for its practice is greater
in a humid region and on a heavy soil than in a region
of light rainfall and on a light soil. The practice of list-
ing corn, by which the soil is sometimes left unplowed
for a number of years, although in semiarid regions pro-
ductive of crops of sufficient yield to make them profitable,
would fail utterly on the heavy soils of a humid region.
Subsoiling, by loosening the subsoil, increases the
ventilation to a greater depth. Rolling and subsurface
packing both diminish the volume and the movement of
air. Their essential difference is in their effect on mois-
ture rather than on air. Harrowing and cultivation have
the opposite effect, and both increase the production of
nitrates in the soil by promoting aeration.
488 SOILS: PROPERTIES AND MANAGEMENT
406. Manures. Farm manures, lime, and those
amendments that improve the structure of the soil, have
for that reason a beneficial action on soil aeration. By
their effect on the physical condition of the soil they
increase its permeability, and by their action in con-
tributing to the production of carbon dioxide they stimu-
late diffusion.
It is chiefly through its effect in increasing the volume
of air space in soils that farm manure is injurious in light
soils of semiarid regions. It may thus be injurious in-
stead of beneficial, if used under certain conditions.
407. Underdrainage. By lowering the water table,
underdrainage by means of tiles removes from the soil
the water from all but the small capillary spaces, and
leaves free to the air the remainder of the interstitial
spaces. There is also a very considerable movement
of air through the drains, and a movement of air upward
from the drains to the surface of the soil, which serves
to aerate to some extent this intervening layer. The
aeration of the soil brought about by underdrainage
is one of its beneficial features.
408. Irrigation. The influence of irrigation on the
soil is much like that of rainfall. The alternate filling
and emptying of the interstitial spaces with water and
air causes a very considerable change of air.
409. Cropping. The roots of plants left in the soil
after a crop has been harvested decay and leave channels
in the soil through which air penetrates. Below the fur-
row slice, where the soil is not stirred and where it is
usually more dense than at the surface, this affords an
important means of aeration. The absorption of moisture
from the soil by roots also causes the air to penetrate, in
order to replace the water withdrawn.
CHAPTER XXIII
COMMERCIAL FERTILIZERS
As treated in this volume, manures include all those
uihstances, with the exception of water (the function and
Application of which is discussed in par. 167), that are
added to soils to make them more productive. There are
several ways in which manures applied to soils may in-
crease plant growth : (1) by addition of the nutrient mate-
rials utilized by plants, which is the chief function of
most of the so-called commercial fertilizers; (2) by im-
provement of the physical condition of a soil, which
usually results from the application of lime and the in-
corporation of organic matter ; (3) by favoring the action
of useful bacteria, which is one of the beneficial results
of farm manure and also of lime; (4) by counteracting
the effects of toxic substances as, for instance, the
conversion of sodium carbonate into sulfate by gypsum,
or the neutralization of acidity, or possibly the destruc-
tion of toxic organic substances by certain salts; (5) by
catalytic action, either on chemical processes in the soil
or by its influence on those bacteria that exert a favorable
influence on soil fertility or by direct stimulation of the
plant.
410. Early ideas of the function of manures.
Manures were at one time supposed to pulverize the soil,
and the French word manceuvrer, from which the word
manure comes, implies to work with the hand. This
489
490 SOILS: PROPERTIES AND MANAGEMENT
idea probably originated through the observation that
farm manure, which was the only manure in use at that
time, made the soil less cloddy.
It has been argued, notably by Jethro Tull, 1 that since
tillage pulverizes the soil it may be used as a substitute
for manures. There are, however, conditions aside from
tilth that are influenced by manures, and good tilth alone
will not suffice to maintain a permanently intensive agri-
culture. It is true in the United States, as it is in Europe,
that a large consumption of manures goes hand in hand
with a highly developed and intensive system of farming.
411. Development of the idea of the nutrient function
of manures. While the use of animal excrement on cul-
tivated soils was practiced as far back as systematic agri-
culture can be definitely traced, the earliest record of
the use of mineral salts for increasing the yield of crops
was published in 1669 by Sir Kenelm Digby. 2 He says :
" By the help of plain salt petre, diluted in water, and
mingled with some other fit earthly substance, that may
familiarize it a little with the corn into which I endeavored
to introduce it, I have made the barrenest ground far
outgo the richest in giving a prodigiously plentiful har-
vest." His dissertation does not, however, show any
true conception of the reason for the increase in the crop
through the use of this fertilizer. In fact, the want of
any real knowledge at that time of the composition of
the plant would have made this impossible.
In 1804, Theodore de Saussure 3 published his chemical
1 Tull, Jethro. Horse-Hoeing Husbandry. London. 1829.
2 Digby, Kenelm. A Discourse Concerning the Vegetation
of Plants. London. 1669.
3 Saussure, Theodore de. Recherches Chimiques sur la
Vegetation. Paris. 1804.
COMMERCIAL FERTILIZERS 491
researches on plants, in which he, for the first time,
called attention to the significance of the ash ingredients
of plants, and pointed out that without them plant life
is impossible and, further, that only the ash of the plant
tissue is derived from the soil.
Justus von Liebig, 1 in his writings published about
the middle of the nineteenth century, emphasized still
more strongly the importance of mineral matter in the
plant and the extraction of this matter from the soil.
He refuted the theory, at that time popular, that plants
absorb their carbon from humus, but he made the mis-
take of attaching little importance to the presence of
humus in the soil. He showed the importance of potas-
sium and phosphorus in manures, but in his later expres-
sions he failed to appreciate the value of nitrogenous
manures, holding that a sufficient amount is washed
from the atmosphere in the form of ammonia.
A true conception of the necessity for a supply of
combined nitrogen in the soil was even at that time enter-
tained by Boussingault and by Sir John Lawes, although
the elaborate experiments conducted by Lawes, Gilbert,
and Pugh 2 in 1857 were required to fully demonstrate
the fact. Their care in conducting the experiments
resulted in their sterilizing the soil with which they ex-
perimented, and hence their failure to discover the utiliza-
tion of free atmospheric nitrogen by legumes.
1 Liebig, J. Justus von. Principles of Agricultural Chemistry
with Special Reference to the Late Researches Made in England.
London. 1855. Also, Chemistry in its Applications to Agri-
culture and Physiology. New York. 1556.
2 Lawes, J. B., Gilbert, J. IT., and Pugh, E. On the Sources
of the Nitrogen of Vegetation, with Special Reference to the
Question whether Plants Assimilate Free or Uncombined Nitro-
gen. Rothamsted Memoirs, Vol. 1, No. 1. 1862.
492 SOILS: PROPERTIES AND MANAGEMENT
Between 1840 and 1850, Sir John Lawes began the
manufacture of bone superphosphate, and about the
same time Peruvian guano and nitrate of soda were intro-
duced into Europe. The commercial fertilizer industry
thus dates from that time.
412. Classes of manures. While manures are very
numerous as to kind and while a certain manure may have
a number of distinct functions, they may yet be roughly
divided into classes. They will accordingly be treated
here under the following heads : (1) commercial fertilizers ;
(2) soil amendments; (3) farm manures; (4) green
manures.
413. Commercial fertilizers. Although the commer-
cial fertilizer industry is little more than half a century
old, the sale of fertilizers in this country amounts to more
than $110,000,000 annually. Animal refuse and phos-
phate fertilizers are exported, while nitrate of soda and
potassium salts are imported.
Of the fertilizers sold in the United States in 1909,
about fifty per cent was consumed in the South Atlantic
States, in an area lying within three hundred miles of
the seaboard. Nearly one-half of the remainder was pur-
chased in the Middle Atlantic and New England States.
Only five per cent was purchased west of the Mississippi
River. 1
Primarily the function of commercial fertilizers is to
add plant nutrients to the soil, usually in a form more
readily soluble than those already present in large quan-
tity. While other beneficial effects may be produced by
certain fertilizers, these are usually of secondary impor-
tance as compared with the addition of the plant nutrients.
1 Statistics from Thirteenth Census of the United States.
Abstract of the Census, p. 372. Washington. 1913.
COMMERCIAL FERTILIZERS 493
414. Fertilizer constituents. Prepared fertilizers, as
found on the market, are usually composed of a number
of ingredients. Since these are the carriers of the fertiliz-
ing material, and since it is on their composition and solu-
bility that the value of a fertilizer depends, a knowledge
of the properties of these constituents is of interest to
every one who uses fertilizers and is a valuable aid in their
purchase.
FERTILIZERS USED FOR THEIR NITROGEN
Nitrogen is the most expensive constituent of manures
and is of great importance, since it is very likely to be
deficient in soils. A commercial fertilizer may have its
nitrogen in the form of soluble inorganic salt, or combined
as organic material. On the form of combination de-
pends to a certain extent the value of the nitrogen, as
the soluble inorganic salts are very readily available to
the plant, while the organic forms must pass through the
various processes leading to nitrification before the
plant can use the nitrogen so contained. The inorganic
nitrogen fertilizers are sodium nitrate, ammonium sulfate,
calcium nitrate, and calcium cyanamide.
415. Forms in which nitrogen exists in soils. There
are several forms in which nitrogen exists in soils. The
uncombined nitrogen of the soil air constitutes the largest
supply because of its diffusibility with the atmospheric
air. Next in quantity is the nitrogen of organic com-
pounds, ranging from 0.05 to 0.3 per cent in ordinary
arable land and slightly, but appreciably, soluble in soil
water. In upland cultivated soils the nitrogen of nitrate
salts forms the next largest supply, but rarely exceeds
20 per cent of the total combined nitrogen of the soil.
494 SOILS: PROPERTIES AND MANAGEMENT
In swamp and inundated soils the nitrogen of ammonium
salts and nitrites forms a larger proportion of the soil
nitrogen than does the nitrate nitrogen, but in well
aerated soils these compounds exist in very small quan-
tities.
416. Forms in which nitrogen is absorbed by plants. -
The utilization of atmospheric nitrogen by leguminous
plants and by a few others that have nodule-bearing roots
has been established beyond question; but the extent
to which this form of nitrogen may be utilized by other
plants, or the identity of the plants that participate in
its use, are subjects on which opinions differ, and which
are still being investigated.
417. Use of nitrates by plants. Boussingault first
demonstrated the importance of nitrates for higher
plants. Previous to that time ammonia had been con-
sidered the chief source of nitrogen, and at a still earlier
time humus had been considered the source. Liebig
gave the weight of his influence in favor of ammonia
as the supply. He was unaware, of course, of the trans-
formation of ammonia nitrogen into nitrates in the soil.
Since the publication of the experiments by Boussin-
gault and the later work on nitrification, there has
been a tendency to consider nitrate nitrogen as the
only available supply of nitrogen for agricultural plants.
While this is an extreme view of the matter, the
fact remains that all the higher plants, including the
legumes, appear to be able to absorb nitrates, and this
form of nitrogen has frequently proved of greater benefit
to plants than other forms of nitrogen tested at the
same time.
418. Ammonia as a plant-food. That rice plants on
swamps use ammonia nitrogen rather than other forms
COMMERCIAL FERTILIZERS 495
has been demonstrated by Kellner l and later by Kelley. 2
On upland soils, however, it is presumable that rice plants
utilize nitrate nitrogen, which would indicate that some
plants, at least, may adapt themselves to the use of the
more abundant form of nitrogen.
Hutchinson and Miller 3 found that peas obtained
nitrogen from ammonium salts as readily as from sodium
nitrate, but that wheat plants, although able to obtain
nitrogen directly from ammonium salts, grew much better
in a solution containing nitrates. One feature brought
out by the numerous experiments with ammonium salts
is the difference between plants of various kinds in respect
to their ability to absorb nitrogen in this form.
419. Utilization of humus compounds by plants. -
One of the early beliefs in regard to plant nutrition w T as
that organic matter as such is directly absorbed by higher
plants. This opinion was afterwards entirely replaced
by the mineral theory propounded by Liebig ; and still
later the discovery of the nitrifying process almost dis-
posed completely of the belief that organic matter is a
food for higher plants. It is quite certain, however, that
some organic nitrogenous compounds furnish suitable
nutrient material for some higher plants without under-
going bacterial change.
Hutchinson and Miller, in the paper just referred to,
give the following list of the organic substances used in
1 Kellner, O. Agrikulturchemische Studien iiber die Reis-
kiiltur. Landw. Vers. Stat., Band 30, Seite 18-41. 1884.
2 Kelley, W. P. The Assimilation of Nitrogen by Rice.
Hawaii Agr. Exp. Sta., Bui. 24, pp. 5-20. 1911.
3 Hutchinson, H. B., and Miller, N. H. J. The Direct
Assimilation of Inorganic and Organic Forms of Nitrogen by
Higher Plants. Centrlb. f. Bakt., II, Band 30, Seite 513-547.
1911.
496 SOILS: PROPERTIES AND MANAGEMENT
experiments by various investigators, and their avail-
ability for the nutriment of higher plants : -
READILY ASSIMILATED
Ammonium salts
Acetamide CH 3 . CO . NH 2
Urea CO<*J
Barbituric acid (with calcium carbonate)
/NH . CO\
^ . co/
Alloxan CO^SS'r^XX)
MNI 11 . lAK
Hurnates
ASSIMILATED
Formamide H . CO . NH 2
Glycine NH 2 . CH 2 . COOH
Aminopropionic acid CH 3 . CH(NH 2 ) . COOH
/NH 2
Guanidine hydrochloride [C = NH HC1
\NH 2 _
/"VTT C*
Cyanuric acid
CO . NH 2
Oxamide
CO . NH 2
CH(NH 2 )COOH
Sodium aspartate
Clia . COOH
Peptone
COMMERCIAL FERTILIZERS 497
DOUBTFUL
Trimethylamine
,,/XH . NH
para-H-Urazme CO^V-TT
Hexamethylenetetramine
NOT ASSIMILATED
Ethyl nitrate Hydroxylamine hydrochloride
Propionitrile Methyl carbonate
Toxic
Tetranitromethane
This list comprises only those substances that have been
used in experiments with peas. Many other substances
remain to be tested, and those already tested may act
differently with other plants.
One of the organic compounds isolated from soils by
Shorey, 1 called creatinine, has been shown by Skinner 2
to be used directly by plants as a source of nitrogen,
and to have produced a better growth of wheat seedlings
than did an equivalent quantity of nitrogen in the form
of sodium nitrate. Histidine, arginine, and creatine
have also been found in soils and shown to be a direct
source of nitrogen for wheat seedlings (par. 92) .
These and numerous other investigations of this subject
show that amine as well as amide nitrogen is assimilated
by at least some agricultural plants, but to w r hat extent
most of these compounds may successfully replace the
1 Shorey, E. C. I. The Isolation of Creatinine from Soils.
U. S. D. A., Bur. Soils, Bui. 83, pp. 11-22. 1911.
2 Skinner, J. J. III. Effects of Creatinine on Plant Growth
U. S. D. A., Bur. Soils, Bui. 83, pp. 33-44. 1911.
2K
498 SOILS: PROPERTIES AND MANAGEMENT
inorganic forms of nitrogen has not been definitely worked
out. Certain organic nitrogenous fertilizers as, for ex-
ample, dried blood have a high commercial value, the
nitrogen in this form selling for more a pound than the nitro-
gen in any of the inorganic salts. Many crops, especially
among garden vegetables, are most successfully grown only
when supplied with organic nitrogenous material. Some
nitrate nitrogen is always present under natural soil con-
ditions, so that crops are never limited to organic nitro-
gen alone ; and it may be that the latter form of nitrogen
is most useful when it supplements the nitrate nitrogen.
420. Sodium nitrate. This now constitutes the prin-
cipal source of inorganic nitrogen in commercial fertilizers.
The salt exists in the crude condition in northern Chili.
The crude salt is purified by crystallization, and as put
on the market it contains about 96 per cent sodium
nitrate, or about 16 per cent of nitrogen, 2 per cent of
water, and small amounts of chlorides, sulfates, and in-
soluble matter. The cost of nitrogen in this form is
from fifteen to eighteen cents a pound.
Because of its easy availability, sodium nitrate acts
quickly in inducing growth. For this reason it is used
much by market gardeners, and for other purposes when
a rapid growth is desired. It is the most active form of
nitrogen. A light dressing on meadowland in early
spring assists greatly in hastening growth by furnishing
available nitrogen before the conditions are favorable
for the process of nitrification. On small grain a similarly
useful purpose is served where the soil is not rich.
Owing to the fact that nitrate is not absorbed by the
soil in large quantities, it is easily lost in the drainage
water ; for this reason it should be applied only when crops
are growing on the soil, and then only in moderate quantity.
COMMERCIAL FERTILIZERS 499
The continued and abundant use of sodium nitrate
on the soil may result, through its deflocculating action,
in breaking down aggregates of soil particles, thus com-
pacting and injuring the structure. This effect is attrib-
uted to the accumulation of sodium salts, particularly
the carbonate, as the sodium is not utilized by the plant
to the same extent as is the nitrogen.
421. Ammonium sulfate. When coal is distilled, a
portion of the nitrogen is liberated as ammonia and is
collected by passing the products of distillation through
water in which the ammonia is soluble, forming the am-
moniacal liquor. The ammonia thus held is distilled into
sulfuric acid, with the formation of ammonium sulfate
and the removal of impure gases.
Commercial ammonium sulfate contains about twenty
per cent of nitrogen. It is the most concentrated form in
which nitrogen can be purchased as a fertilizer, having
from sixty to eighty pounds more of nitrogen to a ton
than sodium nitrate. It is therefore economical to
handle. Its effect on crops is not so rapid as that of sodium
nitrate, but it is not so quickly carried from the soil by
drainage water, as the ammonium salts are readily ab-
sorbed by the soil. A pound of nitrogen in the form of
ammonium sulfate has about the same agricultural value
as the same amount in the form of nitrate if the soil on
which it is used is abundantly supplied with lime; but
on an acid soil ammonium sulfate has less value.
The long and extensive use of ammonium sulfate on a
soil has a tendency to produce an acid condition, through
the accumulation of sulfates which are not largely taken
up by plants.
Ammonium sulfate, like sodium nitrate, should not be
applied in autumn, as the ammonia is converted into
500 SOILS: PROPERTIES AND MANAGEMENT
nitrates and leached from the soil in sufficient quantities to
entail a very decided loss of nitrogen. There is not likely
to be so large a loss of nitrogen from ammonium salts as
from nitrates, and, as would naturally be expected, there is
greater loss of nitrogen when these salts are used alone than
when they are combined with other fertilizing ingredients.
Hall 1 has estimated the loss of nitrogen from certain
drained plats at the Rothamsted Experiment Station.
This estimate is based on the concentration of the drain-
age from the different plats, of which there was no record
of total flow, but for which the measurements of flow from
the lysimeter draining 60 inches of soil were taken and the
total loss of nitrates was calculated on this basis. Esti-
mated in this way the effects of several different methods
of manuring are shown in the accompanying table :
POUNDS TO THE ACRE OF NITRIC NITROGEN IN DRAINAGE WATER
TREATMENT
1879-80
1880-81
Spring
sowing
to
harvest
Harvest
to
spring
sowing
Spring
sowing
to
harvest
Harvest
to
spring
sowing
Umnanured < ....
1.7
1.6
18.3
45.0
9.6
42.9
19.0
10.8
13.3
12.6
15.6
59.9
14.3
16.4
0.6
0.7
4.3
15.0
3.4
7.4
3.7
17.1
17.7
21.4
41.0
74.9
35.2
25.3
Mineral fertilizers only
Minerals + 400 pounds ammonium
salts
Minerals + 550 pounds nitrate of
soda
Minerals 4- 400 pounds ammonium
salts applied in autumn ....
400 pounds ammonium salts alone .
400 pounds ammonium salts + sul-
phate of potash
Estimated drainage in inches .
11.1
4.7
1.8
18.8
1 Hall, A. D. The Book of the Rothamsted Experiments,
p. 235. New York, 1905.
COMMERCIAL FERTILIZERS 501
This table, in addition to confirming the statements
already made in regard to the loss of nitrogen in drainage
water, also shows how closely the supply of available
nitrogen was used by the crops on those plats, which were
evidently in need of nitrogen fertilization as the plats
lost very little nitrogen during the growing season, while
during the remainder of the year they lost nearly as
much as did some of the nitrogen-manured plats. The
table also indicates that the loss when nitrate is used is
greater than when ammonium salts are applied, as the
amount of nitrogen in the 550 pounds of nitrate is really
eight pounds to the acre more than in the 400 pounds of
ammonium sulfate, which is not sufficient to account for
the difference in the loss. However, half of the nitrate-
treated plat received no other manure and produced only
a small crop, which would naturally result in a greater
loss by drainage.
422. Fertilizers containing atmospheric nitrogen.
The vast store of atmospheric nitrogen, chemically un-
combined but very inert, will furnish an inexhaustible
supply of this highly valuable fertilizing element, when it
can with reasonable economy be combined in some manner
resulting in a product that will be commercially trans-
portable and that will, when placed in the soil, be or be-
come soluble without liberating substances toxic to plants.
The importance of the nitrogen supply for agriculture may
be appreciated when it is considered that nitrates are
being carried off in the drainage water of all cultivated
soils at the rate of twenty-five to fifty pounds, and even
more, to the acre annually, and that nearly as much
more is removed in crops.
The exhaustion of the supply of nitrogen in most soils
may be accomplished within one or two generations of
502 SOILS : PROPERTIES AND MANAGEMENT
men, unless a renewal of the supply is brought about in
some way. Natural processes provide for an annual ac-
cretion through the washing-down of ammonia and
nitrates by rain water from the atmosphere, and through
the fixation of free atmospheric nitrogen by bacteria ; but
without the frequent use of leguminous crops, the supply
could not be maintained. Farm practice of the present
day requires the application of nitrogen in some form of
manure, and, as the end of the commercial supply of com-
bined nitrogen is easily in sight, there is urgent need of
discovering a new source. This has been done by com-
bining calcium with atmospheric nitrogen in the forms of
calcium cyanamide and calcium nitrate.
423. Cyanamid. The trade name for calcium cyana-
mide is " cyanamid " and that name is therefore used
in this volume. One process for the production of cyana-
mid consists in passing nitrogen into closed retorts con-
taining powdered calcium carbide heated to a high tem-
perature; the product being calcium cyanamide and free
carbon :
CaC 2 + 2 N = CaCN 2 + C
The free carbon remains distributed in the cyanamide
and gives the fertilizer a black color. The nitrogen re-
quired for the process is obtained either by passing air
over heated copper, or by the fractional distillation of
liquid air.
The fertilizer, as placed on the market, is a heavy,
black powder or granulated material with a somewhat dis-
agreeable odor.
424. Composition of cyanamid. 1 Cyanamid as manu-
1 Cyanamid is a trade name ; the chemical compound is
spelled cyanamide.
COMMERCIAL FERTILIZERS 503
factured in this country has about the following composi-
tion : l - Per cent
( 'alcium cyanamide .... CaCX 2 45.92
Calcium carbonate .... CaCOa 4.04
Calcium sulfide CaS 1.73
Calcium phosphide .... CasP 2 0.04
Calcium hydroxide .... Ca(OH) 2 26.60
Free carbon C 13.14
Iron and alumina R 2 O3 1.98
Silica SiO 2 1.62
Magnesia MgO 0.15
Combined moisture .... 3.12
Free moisture H 2 O 0.35
Undetermined 1.31
100.00
According to this composition the material would con-
tain 16 per cent of nitrogen. Lime in the forms of carbo-
nate and hydroxide would add somewhat to its value,
and the residue of the calcium cyanamide, which upon
decomposition is also calcium hydroxide, is likewise ben-
eficial to the soil.
425. Changes of calcium cyanamide in the soil. -
Calcium cyanamide must be decomposed in the soil be-
fore its nitrogen becomes available to plants. There
are several steps in the decomposition process by which
the nitrogen finally emerges in the form of ammonia.
These, according to Pranke in the work just cited, con-
sist first of hydrolysis, by which acid calcium cyanamide
and calcium hydroxide are formed :
2 CN . XCa + 2 H 2 O = (CN . NH) 2 Ca + Ca(OH) 2
calcium water acid calcium calcium
cyanamide cyanamide hydroxide
1 Pranke, E. J. Cyanamid, p. 8. Eastern, Pennsylvania. 1913.
504 SOILS: PROPERTIES AND MANAGEMENT
The acid calcium cyanamide quickly loses its calcium,
leaving free cyanamide. Investigators differ as to the
process involved in this change, but the ultimate condi-
tion of the calcium is carbonate. The three explanations
of the process may be represented by the following re-
actions :
1. (CN . NH) 2 Ca-fC0 2 +H 2 = 2 CN . NH 2 + CaCO 3
In this reaction the carbon dioxide of the soil water is
supposed to cause precipitation of the calcium.
2. (CN . NH) 2 Ca + 2 H 2 O = 2 CN . NH 2 + Ca(OH) 2
In this case hydrolysis occasions the reaction. The
hydroxide would, of course, be converted into carbonate
in the soil.
3. (CN . NH) 2 Ca + CO 2 = CN . NH 2 + CaCN 2 CO 2
acid calcium carbon free calcium
cyanamide dioxide cyanamide cyanamide
carbonate
CaCN 2 CO 2 + H 2 O = CN . NH 2 + CaCO 3
free calcium
cyanamide carbonate
By this reaction calcium cyanamide carbonate is an in-
termediate product, but is at once hydrolyzed and free
cyanamide produced.
The next step in the process is the formation of urea by
hydrolysis of the free cyanamide :
CN . NH 2 + H 2 = CO(NH 2 ) 2
free cyanamide water urea
The changes up to the production of urea are independent
of bacterial action. The urea is converted through bac-
terial action into ammonium carbonate :
COMMERCIAL FERTILIZERS 505
CO(NH 2 ) 2 + 2 H 2 = (NH 4 ) 2 CO 3
urea ammonium
carbonate
Phis may be converted into nitrates in the usual manner.
426. The use of cyanamid. The changes as here
described are those that proceed under favorable condi-
tions in the soil. When conditions are not favorable
as, for example, when a soil is saturated with water or
when it is acid some more or less injurious products
may be formed. For this reason cyanamid is not likely
to be so satisfactory on soils of this nature as on better
soils. To very sandy soils it is not well suited. Ordi-
narily its fertilizing value is not greatly below that of
sodium nitrate, and is about equal to that of ammonium
sulfate when not used in heavy applications.
It should be incorporated with the soil at least a week
before planting, as it may injure the young plants if de-
composition has not proceeded far enough to remove its
somewhat toxic properties. As it must undergo this
decomposition before its nitrogen becomes available to
the young plants, there is an added reason for this pre-
caution. It does not give its best results as a top-dressing
because it requires incorporation with the soil for its
proper decomposition.
427. Calcium nitrate. The other process for com-
bining atmospheric nitrogen is of more recent invention
than that for the manufacture of calcium cyanamid but
is not conducted on a commercial scale in this country;
however, with the vast opportunities for developing elec-
tric power which are offered in certain localities, factories
for the manufacture of calcium nitrate will some day be
established.
The process employs an electric arc to produce nitric
506 SOILS: PROPERTIES AND MANAGEMENT
oxide by the combustion of atmospheric nitrogen, accord-
ing to the simple equation : -
N 2 + 2 =2NO
NO + O = NO 2
A very high power is required for this synthesis, in-
volving a temperature of 2500 to 3000 C., and the
expense of the operation is determined almost entirely by
the cost of the electricity.
The nitric oxide gas is passed through milk of lime,
giving basic calcium nitrate :
Ca(OH) 2 + 2 HNO 3 = Ca(NO 3 ) 2 + 2 H 2 O
The calcium nitrate resulting from this process has a
yellowish white color, and is easily soluble in water but
deliquesces very rapidly in the air. This last property
can be overcome by adding an excess of lime in the manu-
facture, thus producing a basic calcium nitrate which
contains only 8.9 per cent of nitrogen. Another way of
avoiding the difficulties involved by the deliquescent
property of the nitrate is practiced by the factory at
Nottoden, Norway. This consists in first melting the
product, then grinding it fine and packing it in air-tight
casks. The fertilizer thus prepared contains from 11 to
13 per cent of nitrogen.
Calcium nitrate contains its nitrogen in a form directly
available to plants. It resembles sodium nitrate in its
solubility, availability, and lack of absorption by the soil.
It may be spread on the surface of the ground, as it exerts
no poisonous action and does not tend to form a crust, as
does sodium nitrate.
The relative values of the different soluble nitrogen
fertilizers vary with a great many conditions and can be
COMMERCIAL FERTILIZERS 507
accurately judged only by a large number of tests. At
present, both calcium nitrate and cyanamid are being
produced at less cost per pound of nitrogen than is sodium
nitrate, when laid down in the neighborhood of the fac-
tories in Europe. It seems fairly certain that, when the
processes have been further improved, the result will be
to greatly reduce the cost of available nitrogen.
428. Organic nitrogen in fertilizers. The commercial
fertilizers containing organic nitrogen include cottonseed
meal, which contains 7 per cent of nitrogen when free
from hulls; linseed meal, with 5.5 per cent of nitrogen;
castor pomace, with 6 per cent of nitrogen; and a num-
ber of refuse products from packing houses, among which
are red dried blood and black dried blood, the former
having about 13 per cent of nitrogen and the latter from
6 to 12 per cent ; dried meat and hoof meal, with 12 to
13 per cent of nitrogen; ground fish, with 8 per cent of
nitrogen ; and tankage, of which the concentrated product
has a nitrogen content of from 10 to 12 per cent and the
crushed tankage from 4 to 9 per cent ; also leather meal
and wool-and-hair waste, but these, because of their
mechanical condition, are of very little value.
The meals made from seeds are primarily stock foods
but are sometimes used as manures. They decompose
rather slowly in the soil, owing to their high oil content,
and are much more profitably fed to live stock than ap-
plied as farm manure. They contain some phosphorus
and potash as well as nitrogen.
Guano consists of the excrement and carcasses of sea
fowl. The composition of guano depends on the climate
of the region in which it is found. Guano from an arid
region contains nitrogen, phosphorus, and potassium, while
that from a region where rains occur contains only phos-
508 SOILS: PROPERTIES AND MANAGEMENT
phorus the nitrogen and potassium having been largely
leached out. In a dry guano the nitrogen exists as uric
acid, urates, and, in small quantities, ammonium salts.
A damp guano contains more ammonia. The phosphorus
is present as calcium phosphate, ammonium phosphate,
and the phosphates of other alkalies. A portion of
the phosphate is readily soluble in water. Thus all
the plant-food either is directly soluble or becomes so
soon after admixture with the soil. The composition
is extremely variable. The best Peruvian guano con-
tains from 10 to 12 per cent of nitrogen, from 12 to 15
per cent of phosphoric acid, and from 3 to 4 per cent of
potash.
Guano was formerly a very important fertilizing ma-
terial, but the supply has become so nearly exhausted
that it is relatively unimportant at the present time.
Of the abattoir products, dried blood is the most readily
decomposed, and therefore has its nitrogen in the most
available form. In fact, it produces results more quickly
than any other form of organic nitrogen. It requires a
condition of soil favorable to decomposition and nitrifica-
tion, which prevents its exerting a strong action in early
spring. It should be applied to the soil before the crop
is planted. The black dried blood contains from 2 to 4
per cent of phosphoric acid.
Dried meat contains a high percentage of nitrogen, but
does not decompose so easily as dried blood, and is not so
desirable a form of nitrogen. It can be fed to hogs or
poultry to advantage, and the resulting manure is very
high in nitrogen.
Hoof meal, while high in nitrogen, decomposes slowly,
being less active than dried blood. It is of use in increas-
ing the store of nitrogen in a depleted soil.
COMMERCIAL FERTILIZE!!* 509
Ground fish is an excellent form of nitrogen, and is as
readily available as blood but has a lower nitrogen content.
Tankage is highly variable in composition, and the con-
centrated tankage, being more finely ground, undergoes
more readily the decomposition necessary for the utiliza-
tion of the nitrogen. Crushed tankage contains from 3 to
12 per cent of phosphoric acid, in addition to its nitrogen.
Leather meal and wool-and-hair waste when untreated
are in such a tough and undecomposable condition that
they may remain in the soil for years without losing their
structure. They are not to be recommended as manures.
429. Availability of organic nitrogenous fertilizers. -
The forms in which combined nitrogen is available to
most agricultural plants has already been stated to be
nitrates, ammonium salts, and certain organic compounds.
Of the latter the simple compounds, as urea, appear to be
most readily taken up by plants. Decomposition is there-
fore a necessary process for most of these fertilizers, and
their usefulness is, in general, proportional to the readi-
ness with which aerobic decomposition proceeds, or to the
proportion of available compounds that they contain in
their original condition. Guano, for instance apparently,
contains much nitrogen that is available without further
decomposition. Dried blood quickly decomposes and
soon forms available substances, consisting of the simpler
organic nitrogenous compounds, ammonia and nitrates.
The decomposition process is a biological one, arising from
the action of microorganisms that first break down the
complicated organic compounds, forming simpler ones,
and finally carry the nitrogen into the form of ammonia,
then to nitrous acid, and at last to nitric acid.
Numerous attempts have been made to determine the
relative availability of the nitrogen in various organic
510 SOILS: PEOPERTIES AND MANAGEMENT
nitrogenous fertilizers. A few such tests, in which nitrate
of soda and ammonium sulfate are used as a basis for com-
parison, are given in the table below, the statement being
in terms of percentage availability when nitrate of soda
is taken as one hundred. The experiments quoted were
conducted by Wagner and Dorsch, 1 by Johnson, Jenkins,
and Britton, 2 and by Voorhees and Lipman. 3
PERCENTAGE AVAILABILITY OF FERTILIZER NITROGEN
WAGNER
AND
DORSCH
JOHNSON
AND
OTHERS
VOORHEES
AND
LIPMAN
Nitrate of soda
100
100
100
Sulfate of ammonia
Dried blood
90
70
73
70
64
Bone meal
60
17
Stable manure
45
53
Tankage
49
Horn and hoof meal
Linseed meal
70
68
69
Cottonseed meal
Castor pomace
65
65
Wool waste
30
Leather meal
20
Dry ground fish
64
One difficulty in drawing conclusions from these experi-
ments is that the substances grouped under the same
name are not always identical in the method of their
1 Wagner, P., and Dorsch, F. Die Stickstoffdiingung der
Landw. Kulturpflanzen, Erstes Teil. Berlin. 1892.
2 Johnson, S. W., Jenkins, E. H., and Britton, W. E. Experi-
ments on the Availability of Fertilizer-Nitrogen. Connecticut
Agr. Exp. Sta., 21st Annual Kept., Part 4, pp. 257-277. 1897.
3 Voorhees, E. B., and Lipman, J. G. Investigations Rela-
tive to the Use of Nitrogenous Materials, 1898-1907. New
Jersey Agr. .Exp. Sta., Bui. 221. 1909.
COMMERCIAL FERTILIZERS 511
preparation or in their composition. Another discrep-
ancy arises from the fact that all soils do not respond in
the same relative degree to any one fertilizer. Thus,
Sackett l found that in some soils dried blood was am-
monified more rapidly than was cottonseed meal, while
in other soils the reverse was true; and that a similar
difference obtained in soils with respect to the ammoni-
fication of alfalfa meal and flaxseed meal. It would
therefore appear to be impossible to make any close dis-
tinctions in the relative availability of the nitrogen in
various organic nitrogenous fertilizers. A considerable
number of these experiments are, in the aggregate, useful
in pointing out the probable relative availabilities of the
more widely differing nitrogen-bearing substances.
FERTILIZERS USED FOR THEIR PHOSPHORUS
Phosphorus is generally present in combination with
lime, iron, or alumina. Some of the phosphates contain
also organic matter, in which case they generally carry
some nitrogen. Phosphates associated with organic
matter decompose more quickly in the soil than do un-
treated mineral phosphates.
430. Bone phosphate. Formerly bones were used
entirely in the raw condition, ground or unground. When
ground they act as a fertilizer more quickly than when
unground. Raw bones contain about 22 per cent of phos-
phoric acid and 4 per cent of nitrogen. The phosphorus
is in the form of tricalcic phosphate (CaaCPO^).
Most of the bone now on the market is first boiled or
1 Sackett, W. G. The Ammonifying Efficiency of Certain
Colorado Soils. Colorado Agr. Exp. Sta., Bui. 184, pp. 3-23.
1912.
512 SOILS: PROPERTIES AND MANAGEMENT
steamed. This frees it from fat and nitrogenous matter,
both of which are used in other ways. Steamed bone is
more valuable as a fertilizer than raw bone, because the
fat in the latter retards decomposition and also because
steamed bone is in a better mechanical condition. The
form of the phosphoric acid is the same as in raw bone
and constitutes from 28 to 30 per cent of the product,
while the nitrogen is reduced to Ij per cent.
Bone tankage, which has already been spoken of as a
nitrogenous fertilizer, contains from 7 to 9 per cent of
phosphoric acid, largely in the form of tricalcium phos-
phate. All these bone phosphates are slow-acting ma-
nures, and should be used in a finely ground form and for
the permanent benefit of the soil rather than as an imme-
diate source of nitrogen or phosphorus.
431. Mineral phosphates. There are many natural
deposits of mineral phosphates in different parts of the
world, some of the most important of which are in North
America. The phosphorus in all these is in the form of
tricalcium phosphate, but the materials associated with
it vary greatly.
Apatite is found in large quantities in the provinces of
Ontario and Quebec, Canada. It exists chiefly in crys-
talline form. The tricalcium phosphate of which it is
composed is in one form associated with calcium fluoride
and in the other with calcium chloride. The Canadian
apatite contains about 40 per cent of phosphoric acid,
being richer than that found elsewhere. Phosphorite is
another name for apatite, but is chiefly applied to the
impure amorphous form.
Coprolites are concretionary nodules found in the
chalk or other deposits in the south of England and in
France. They contain from 25 to 30 per cent of phos-
COMMERCIAL FERTILIZERS 513
phoric acid, the other constituents being calcium carbonate
iiul silica.
South Carolina phosphate contains from 26 to 28 per
cent of phosphoric acid and a very small amount of iron
and alumina. As these substances interfere with the
manufacture of superphosphate from rock, their presence
is very undesirable rock containing more than from
3 to 6 per cent being unsuitable for that purpose.
Florida phosphates exist in the form of soft phosphate,
pebble phosphate, and bowlder phosphate. Soft phos-
phate contains from 18 to 30 per cent of phosphoric acid,
and because of its being more easily ground than most
of these rocks it is often applied to the land without being
first converted into a superphosphate. The other two
forms, pebble phosphate and bowlder phosphate, are
highly variable in composition, ranging from 20 to 40 per
cent in phosphoric acid content. Tennessee phosphate
contains from 30 to 35 per cent of phosphoric acid.
Basic slag, or, as it is also called, phosphate slag or
Thomas phosphate, is a by-product in the manufacture
of steel from pig-iron rich in phosphorus. The phos-
phorus present is usually considered to be in the form of
tetracalcium phosphate, (CaO) 4 P 2 O5, or possibly a double
silicate and phosphate of lime having the composition
(CaC^frPtOeSiOs. It contains also calcium, magnesium,
aluminium, iron, manganese silica, and sulfur. Because
of the presence of iron and aluminium, and because its
phosphorus is more readily soluble than tricalcium phos-
phate, the ground slag is applied directly to the soil with-
out treatment with acid.
The degree of fineness to which the slag is ground is
supposed to be an important factor in determining its
solubility in the soil. It is much more soluble in water
2L
514 SOILS: PROPERTIES AND MANAGEMENT
charged with carbon dioxide than in pure water, a property
that greatly increases its value because of the fact that
soil water always contains more or less of this gas. It is
also readily acted upon by organic acids. For this reason
it is particularly effective in a peat soil, and likewise in
most soils deficient in lime. As it contains a considerable
quantity of free lime it has another beneficial effect on
such soils.
432. Superphosphate fertilizers. In order to render
more readily available to plants the phosphorus contained
in bone and mineral phosphates, the raw material, purified
by being washed and finely ground, is treated with sulfuric
acid. This results in a replacement of phosphoric acid by
sulfuric acid, with the formation of monocalcium phos-
phate and calcium sulfate, and a smaller amount of dical-
cium phosphate, according to the reactions :
Ca 3 (PO 4 ) 2 + 2 H 2 SO 4 = CaH 4 (PO 4 ) 2 + 2 CaSO 4
Ca 3 (PO 4 ) 2 + H 2 SO 4 = Ca 2 H 2 (PO 4 ) 2 + CaSO 4
The tricalcium phosphate being in excess of the sul-
furic acid used, some of it remains unchanged.
In the treatment of phosphate rock some of the sul-
furic acid is consumed in acting on the impurities present,
which usually consist of calcium and magnesium carbo-
nates, iron and aluminium phosphates, and calcium chlo-
ride or fluoride, converting the bases into sulfates and
freeing carbon dioxide, water, hydrochloric acid, and
hydrofluoric acid. The resulting superphosphate is there-
fore a mixture of monocalcium phosphate, dicalcium phos-
phate, tricalcium phosphate, calcium sulfate, and iron and
aluminium sulfates.
In the superphosphates made from bone, the iron and
aluminium sulfates do not exist in any considerable
COMMERCIAL FERTILIZERS 515
quantities. However, as long as the phosphorus remains
in the form of monocalcium phosphate, the value of a
pound of available phosphorus in the two kinds of fertilizer
is the same ; but the remaining tricalcium phosphate has
a greater value in the bone than in the rock superphosphate.
The superphosphates made from animal bone contain
about 12 per cent of available phosphoric acid and from
3 to 4 per cent of insoluble phosphoric acid. They also
contain some nitrogen. Bone ash and bone black super-
phosphates contain practically all their phosphorus in an
available form, but they contain little or no nitrogen.
South Carolina rock superphosphate contains from 12 to
1-4 per cent of available phosphoric acid, including from
1 to 3 per cent of reverted phosphoric acid. The best
Florida rock superphosphates contain from 17 per cent
downward of available phosphoric acid, some of which is
reverted. The Tennessee superphosphates contain from
1-4 to 18 per cent of available phosphoric acid.
Double superphosphates. In making superphosphates
a material rich in phosphorus must be used, not less than
60 per cent of tricalcium phosphate being necessary for
their profitable production. The poorer materials are
sometimes used in making what is known as double super-
phosphates. For this purpose they are treated with an
excess of dilute sulfuric acid ; the dissolved phosphorus
and the excess of sulfuric acid are separated from the mass
by filtering, and are then used for treating phosphates
rich in tricalcium phosphate and thus forming superphos-
phates. The superphosphates so formed contain more
than twice as much phosphorus as those made in the
ordinary way.
433. Reverted phosphoric acid. A change sometimes
occurs in superphosphates on standing by which some of
516 SOILS: PROPERTIES AND MANAGEMENT
the phosphoric acid becomes less easily soluble, and to
that extent the value of the fertilizer is decreased. This
change, known as reversion, is much more likely to occur
in superphosphates made from rock than in those derived
from bone. It will also vary in different samples, a well-
made article usually undergoing little change even after
long standing. It is supposed to be caused by the presence
of undecomposed tricalcium phosphate and of iron and
aluminium sulfates.
434. Relative availability of phosphate fertilizers. -
Superphosphates and double superphosphates contain
their phosphorus in a form in which it can be taken up
by the plant at once. They are therefore best applied
at the time when the crop is planted, or shortly before,
or they may be applied when the crop is growing. Crude
phosphates, on the other hand, become available only
through the natural processes in the soil. They should
be applied in quantity sufficient to meet the needs of the
crops for a number of years.
Reverted phosphorus, although not soluble in water,
is readily soluble in dilute acids. It is now generally
believed that in this form an available supply of phos-
phorus is furnished to the plant. In a statement of fer-
tilizer analyses reverted phosphorus is termed citrate-
soluble, and this and the water-soluble are termed available.
The degree of fineness to which the material is ground
makes a great difference in the availability of the less
soluble phosphate fertilizers, especially in the ground-rock
phosphates and in ground bone. This material should be
ground fine enough to pass through a sieve having meshes
at least one-fiftieth of an inch in diameter.
435. Changes that occur when superphosphate is added
to soils. When incorporated with soils superphosphate
F /;/,' 77L/Z /;//* 517
undergoes changes, the nature of which depends more or
less on the properties of the particular soil with which it
is mixed. Xo matter how readily soluble the phosphorus
may he in the fertilizer, it soon becomes insoluble in the
soil, only a fractional proportion of it being recoverable in
water extracts. Absorption by colloidal complexes is the
fate of a part of the phosphorus, in which condition it is
still available to plants, especially when the colloidal
matter becomes coagulated. The excess phosphorus en-
ters into combination with the calcium of the soil, form-
ing tricalcium phosphate and some dicalcium phosphate,
and with the iron or the aluminium, forming phosphates of
those metals. The latter compounds are less readily soluble
than the former, and probably do not serve as a direct
source of phosphorus for plants ; while tricalcium phosphate,
although acted upon by plant roots, is not so readily avail-
able as is the phosphorus held by the colloidal matter.
It is desirable that there should be an abundant supply
of calcium in a soil to which a superphosphate is added, be-
cause the phosphorus not absorbed by the colloidal matter
of the soil w r ill, under such circumstances, form more cal-
cium phosphate than if only a small supply of lime is pres-
ent, according to the law of mass action. The great loss
of availability through the conversion of phosphorus into
iron and aluminium phosphates may thus be mitigated.
436. Other factors influencing the availability of tri-
calcium phosphate. As this is the form in which phos-
phorus is probably most extensively held in the ordinary
soil, and as it is also a cheap form of phosphorus in manures,
it is a matter of some importance to know the most favor-
able conditions for its utilization by agricultural plants.
Experimentation by numerous investigators has estab-
lished at least four factors that influence the availability
518 SOILS: PROPERTIES AND MANAGEMENT
of this substance : (1) kind of plant grown ; (2) degree of
basicity of soil; (3) fermentation of organic matter;
(4) character of the accompanying salts.
437. Effect of plants on the availability of tricalcium
phosphate. It is to be expected that the various kinds
of plants should not all exert an equal influence on the
availability of the phosphorus of tricalcium phosphate.
Prianischnikov 1 found that lupines, mustard, peas,
buckwheat, and vetch responded to fertilization with raw
rock phosphate in the order named, while the cereals did
not respond at all. He did not include maize in his
experiments, but that crop is said to respond well to diffi-
cultly soluble phosphates. It is generally considered
that those plants which have a long growing season are
better able to utilize tricalcium phosphate than are more
rapidly growing plants. An explanation for the ability
of some plants to utilize the phosphorus of difficultly
soluble phosphates more successfully than do other plants
has been sought in the rate of excretion of carbon dioxide
by plant roots. It has already been stated (par. 324) that
Stoklasa and Ernst found that the capacity of a plant to
absorb phosphorus from difficultly soluble phosphates is
proportional to the rate at which carbon dioxide is given
off by the roots, but that the experiments of Kossowitch
and Barakoif failed to confirm these results. This ques-
tion is bound up with the larger one involving the solvent
action of plant roots, regarding which little is now known.
438. Effect of basicity on the availability of tricalcium
phosphate. It is recognized that raw rock phosphate is
more available to the same plant in some soils than in
others, and a number of persons have stated, as the result
1 Prianischnikov, D. Bericht fiber Verschiedene Versuche
mit Rohphosphaten unter Reduction. Moscow. 1910.
COMMERCIAL FERTILIZERS 519
of experimentation, that the availability is greater in acid
soils than in those strongly basic. If acidity of the soil
is due to the presence of free acid (positive acidity), it is
conceivable that the availability may be due to the sol-
vent action of the soil acid on the calcium of the trical-
cium phosphate, producing the dicalcium salt which ap-
pears to be fairly readily available to plants. When,
however, soil acidity is due to a lack of basicity (apparent
acidity), the case is different. Gedroiz 1 explains this
on the basis of the absorptive properties of the apparently
acid soil. He regards rock phosphate, not as a chemical
compound, but as a solid solution of dicalcium phosphate
with lime. It is this excessive basicity of the phosphate
which is responsible for its unavailability. Absorption of
the excess calcium would leave the phosphate in a more
readily available condition by forming the dicalcium salt,
and this is brought about in an apparently acid soil.
Gedroiz experimented with a highly basic soil that did
not respond to fertilization with rock phosphate. He
subjected this soil to repeated washings with distilled
water charged with carbon dioxide. After such treatment
the soil gave a marked increase in crop with rock phos-
phate as compared with the same soil untreated. Accord-
ing to Gedroiz the greater availability of the phosphate
after treatment with carbonic acid was due to the removal
of bases and the greater absorptive power of the soil
brought about thereby. This was further corroborated
by the fact that the treated soil responded to a test for
unsaturation while the untreated soil did not. Without
1 Gedroiz, K. K. Soils to which Rock Phosphates may
be Applied with Advantage. Jour. Exp. Agronomy (Russian),
Vol. 12, pp. 529-539, 811-816. 1911. The authors are in-
debted to Dr. J. Davidson for the translation.
520 SOILS: PROPERTIES AND MANAGEMENT
necessarily accepting all of Gedroiz's explanation of the
phenomenon, there can be little doubt that lack of basicity
is a factor in the availability of raw rock phosphates in
some soils.
439. Influence of fermenting organic matter. There
has been great difference of opinion among investigators
as to the effect of fermentation of organic matter on the
availability of the phosphorus of tricalcium phosphate.
The contention that the availability is increased probably
originated with Stoklasa, 1 the results of whose experi-
ments with bone meal indicated that the availability is
increased by fermentation. A large number of experi-
ments have been conducted with raw rock phosphate
composted with stable manure, among which may be
mentioned those by Hartwell and Pember 2 and also by
Tottingham and Hoffman 3 who in carefully conducted
experiments failed to find that the availability of the raw
phosphate was increased by fermentation with stable
manure. Opposing results have also been obtained, how-
ever, and the evidence is somewhat conflicting. Krober, 4
who thinks that the action of bacteria is due to the acids
they produce, explains the contradictions in the various
1 Stoklasa, J., Duchacek, F., and Pitra, J. Ueber den Ein-
fluss der Bakterien auf die Knochenzersetzung. Centrlb. f.
Bakt., II, Band 6, Seite 526-535, 554-558. 1900.
2 Hartwell, B. L., and Pember, F. R. The effect of cow dung
on the availability of rock phosphate. Rhode Island Agr.
Exp. Sta., Bui. 151. 1912.
3 Tottingham, W. E., and Hoffman, C. The Nature of the
Changes in Solubility and Availability of Phosphorus in Fer-
menting Mixtures. Wisconsin Agr. Exp. Sta., Research Bui.
29. 1913.
4 Krober, E. Ueber das Loslichwerden der Phosphorsaure
aus Wasserunloslichen Verbindungen unter der Einwirkung
von Bakterien und Hefen. Jour. f. Landw., Band 57, Seite
5-80. 1909-1910.
COMMERCIAL FERTILIZERS 521
experiments as arising from the different kinds of fer-
mentation that the organic matter undergoes. He thinks
that acid fermentation renders the phosphate more readily
soluble, while fermentation that does not give rise to acids
leaves it in an insoluble condition.
Parallel with the biological process that results in the
transformation of insoluble phosphates into soluble, there
is, according to Stoklasa and others, a reverse biological
process resulting in the transformation of soluble phos-
phates into insoluble.
Whatever may be the conditions under which raw rock
phosphate is rendered more readily soluble or available
by fermentation of organic matter, it does not appear
that composting with stable manure produces this change,
at least from results of numerous experiments including
those mentioned above. These have been mainly opposed
to any such conclusion.
440. Influence of other salts. The presence of cer-
tain salts has been found to influence the availability of
difficultly soluble phosphates. The subject has been in-
vestigated by a large number of experimenters and it will
be possible to summarize their results only in part and
very briefly. It has been found, for instance, that cal-
cium carbonate decreases the availability of raw rock
phosphate and bone-meal. Sodium nitrate reduces the
availability of the tricalcium phosphates, while the am-
monium salts increase their availability. Iron salts
decrease availability. The influence of other salts has not
been so well worked out. Prianischnikov, 1 as the result
of his extended experiments on the subject, holds that
1 Prianischnikov, D. Ueber den Einfluss von Kohlensauren
Kalk auf die Wirkung von Verschiedenen Phosphaten. Landw
Vers. Stat., Band 75, Seite 357-376. 1911.
522 SOILS: PROPERTIES AND MANAGEMENT
salts from which plants absorb acid in larger amounts
than they do bases decrease availability, or at least do not
affect it, while salts from which plants absorb the bases in
greater quantity than the acids have a tendency to render
the phosphate more available, because of the solvent
action of the acid.
FERTILIZERS USED FOR THEIR POTASSIUM
The production of potassium fertilizers is largely con-
fined to Germany, where there are extensive beds varying
from 50 to 150 feet in thickness, lying under a region of
country extending from the Harz Mountains to the Elbe
River and known as the Stassfurt deposits. Deposits
have lately been discovered in other parts of Germany.
441. Stassfurt salts. The Stassfurt salts contain
their potassium either as a chloride or as a sulfate. The
chloride has the advantage of being more diffusible in the
soil, but in most respects the sulfate is preferable. Potas-
sium chloride in large applications has an injurious effect
on certain crops, among which are tobacco, sugar beets,
and potatoes. On cereals, legumes, and grasses, the
muriate appears to have no injurious effect.
The mineral produced in largest quantities by the
Stassfurt mines is kainit. Chemically it consists of mag-
nesium and potassium sulfate and magnesium chloride,
or of magnesium sulfate and potassium chloride. Kainit
has the same effect on plants as has potassium chloride.
It contains from 12 to 20 per cent of potash and from 25
to 45 per cent of sodium chloride, with some chloride and
sulfate of magnesium.
Kainit should be applied to the soil a considerable
time before the crop for which it is intended is planted.
COMMERCIAL FERTILIZERS 523
It should not be drilled in with the seed, as the action of
the chlorides in direct contact with the seed may injure
its viability. In addition to the potassium added to the
soil by kainit, there are also in this fertilizer magnesium
and sodium. The magnesium may be objectionable if
there is much already present in the soil (see par. 458).
Sodium may to some extent replace potassium in the soil
economy, and in that way may be beneficial.
Silvinit contains its potassium both as chloride and as
sulfate. It also contains sodium and magnesium chlorides.
Potash constitutes about 16 per cent of the material.
Owing to the presence of chlorides, it has the same effect
on plants as has kainit.
The commercial form of potassium chloride generally
contains about 80 per cent of potassium chloride or 50
per cent of potash. The impurities are largely sodium
chloride and insoluble mineral matter. The possible
injury to certain crops from the use of the chloride has
already been mentioned. For crops not so affected, potas-
sium chloride is a quickly acting and effective carrier of
potassium, and one of the cheapest forms.
High-grade sulfate of potassium contains from 48 to
50 per cent of potash. Unlike the muriate it is not in-
jurious to crops, but is more expensive.
There are a number of other Stassfurt salts, consisting
of mixtures of potassium, sodium, and magnesium in the
form of chlorides and sulfates. They are not so widely
used for fertilizers as are those mentioned above.
442. Wood ashes. For some time after the use of
fertilizers became an important farm practice, wood
ashes constituted a large proportion of the source of supply
of potassium. They also contain a considerable quantity
of lime and a small amount of phosphorus. The product
524 SOILS: PROPERTIES AND MANAGEMENT
known as unleached wood ashes contains from 5 to 6 per
cent of potash, 2 per cent of phosphoric acid, and 30 per
cent of lime. Leached wood ashes contain about 1 per cent
of potash, lj per cent of phosphoric acid, and from 28 to 29
per cent of lime. They contain the potassium in the form
of a carbonate, which is alkaline in its reaction and in large
amount may be injurious to seeds. They are beneficial
to acid soils through the action of both the potassium and
calcium salts. The lime is valuable for the other effects it
has on the properties of the soil. (See pars. 454-457.)
443. Insoluble potassium fertilizers. Insoluble forms
of potassium, existing in many rocks usually in the form
of a silicate, are not regarded as having any manurial
value. Experiments with finely ground feldspar have been
conducted by a number of investigators, but have, in the
main, given little encouragement for the successful use of
this material. An insoluble form of potassium is not
given any value in the rating of a fertilizer based on the
results of its analysis.
SULFUR AND SULFATES AS FERTILIZERS
The use of these substances as a means of increasing
plant growth when applied to soils has recently received
revived attention. The use of free sulfur has been in-
vestigated to some extent in France and Germany. There
have been suggested three ways in which it may be bene-
ficial to plants (1) as a direct stimulant; (2) by its in-
fluence on the activities of microorganisms; (3) as a
source of plant-food, which might otherwise be deficient.
444. The use of free sulfur. Boullanger l added
flowers of sulfur to a soil at the rate of 23 parts to a million
1 Boullanger, E. Action du soufre en fleur sur la vegetation.
Compt. Rend. Acad. Sci. Paris, T. 154, pp. 369-370. 1912.
COMMERCIAL FERTILIZERS 525
of soil. He obtained increased growth in all treated soils
on which carrots, beans, celery, lettuce, sorrel, chicory,
potatoes, onions, and spinach were grown, the weight of
the crops on the treated soil being from 10 per cent to 40
per cent greater than those on the untreated soil. On
soils that had been sterilized before applying sulfur the
effect was much less, from which he concludes that the
beneficial effects were due to the influence of the sulfur
on the microorganisms of the soil. There may be some
question, however, whether this conclusion is justifiable.
Sulfur was found by Boullanger and Dugardin 1 to favor
ammonification in soils. Beneficial effects from the use
of free sulfur have also been obtained by Demelon, 2 and
by Bernhard 3 among others, while von Feilitzen 4 found
it to be ineffective as a fertilizer.
That free sulfur may, under some conditions, exert a
beneficial influence on plant growth must undoubtedly be
conceded, but how the action is brought about remains to
be conclusively demonstrated. Free sulfur is insoluble and
cannot be absorbed by plant roots. However it is readily
oxidized in soils 5 eventually producing sulf ates with bases in
the soil and in this form may readily be taken up by plants.
1 Boullanger, E., and Dugardin, M. Mecanisme de Faction
fertilisante du soufre. Compt. Rend. Acad. Sci. Paris, T. 155,
pp. 327-329. 1912.
2 Demelon, A. Sur Faction fertilisante du soufre. Compt.
Rend. Acad. Sci. Paris, T. 154, pp. 524-526. 1912.
3 Bernhard, A. Versuche tiber die Wirkung des Schwefels als
Dung im Jahre 1911. Deutsche Landw. Presse. Band 39, p. 275.
1912.
4 von Feilitzen, H. Ueber die Verwendung der Schwefel-
blute zur Bekampfung des Kartoffelschorfes und als indirektes
Dungemittel. Fuhling's Landw. Zeit. Band 62, Seite 7. 1913.
6 Mares, M. N. Des transformations que subit le soufre
en poudre quand il es reponds sur le sol. Compt. Rend. Acad.
Sci. Paris, T. 69, pp. 974-979. 1869.
526 SOILS: PROPERTIES AND MANAGEMENT
445. Sulfur as sulfate. There is less experimental
evidence regarding the effect of sulfur in the form of
sulfate on plant growth than there is for the free sulfur.
The fact that the bases with which .the sulfate is com-
bined are likely to have an effect on plant growth, makes
the accumulation of proof by experimentation a somewhat
more difficult matter. That there may be a possible de-
ficiency of sulfur in arable soils has been pointed out by
several investigators, including Hart and Peterson l in
this country. They point out that crops remove more
sulfur from the soil than was shown by the early deter-
minations of sulfur in plant ash, from which a large part
of the sulfur was volatilized during the process. They
then proceed to calculate the sulfur removed by a num-
ber of crops on the basis of their own methods and
compare this with the phosphorus in similar crops.
POUNDS SULFUR TRIOXIDE AND PHOSPHORUS PENTOXIDE
REMOVED TO THE ACRE BY AVERAGE CROPS
CONTENT IN POUNDS TO THE
CROP AND YIELD TO THE ACRE
ACRE
SO 3
P 2 O 6
Wheat (30 bu.)
15.7
21.1
Barley (40 bu )
14.3
20.7
Oats (45 bu )
19.7
19.7
Corn (30 bu.)
12.0
18.0
Alfalfa (9000 Ib. dry wt.) ....
64.8
39.9
Turnips (4657 Ib. dry wt.) . . . .
92.2
33.1
Cabbage (4800 Ib. dry wt.) . . . .
98.0
61.0
Potatoes (3360 Ib. dry wt.) . . . .
11.5
21.5
Meadow hay (2822 Ib. dry wt.) . .
11.3
12.3
1 Hart, E. B., and Peterson, W. H. Sulphur requirements
of farm crops in relation to the soil and air supply. Wise.
Agr. Exp. Sta., Research Bui. No. 14. 1911.
COMMERCIAL FERTILIZERS
527
They then call attention to the quantities of sulfur
trioxide contained in average soils which, as shown by
Hilgard, are less than the quantities of phosphorus pen-
toxide.
CONTENT IN POUNDS TO THE
ACRE
S03
P 2 O 5
Sandy soils
Clay soils
1650
2250
2610
4230
To ascertain whether the supply of sulfur in the soil
is really depleted by cropping, the same authors made
parallel determinations of sulfur in five virgin soils and
in five soils of the same respective types that had been
cropped for sixty years. In each type the cropped soil
contained less sulfur than the virgin soil, the average for
the cropped soils being .053 per cent SO 3 and for the
virgin soils .085 per cent SO 3 .
There is no doubt that the quantity of sulfur carried
down by rain and snow is much less than that removed
in drainage water. There can be no question therefore
that most soils, and especially cultivated soils, are losing
more sulfur than they receive by natural processes.
It has been customary to add to soils manures of one
kind or another that contain more or less sulfur. Among
these are farm manure and other animal or bird excre-
ments, residues of crops, animal offal, gypsum or land
plaster, superphosphate, ammonium sulfate, potassium
sulfate, kainit, and the like, all of which contain conse-
quential quantities of sulfur. It seems probable that
528 SOILS: PROPERTIES AND MANAGEMENT
any system of soil management that does not include
one or more of these substances would probably, on some
soils at least, be improved by making provision for the
application of sulfur in some form.
CATALYTIC FERTILIZERS
The term catalytic fertilizers has been used rather
loosely to designate a class of substances that, when added
to a soil, increase plant growth by apparently accelerating
the processes that normally take place in soils. They
do not function as fertilizers because their value does not
lie in the nutrients that they possess, but they may
properly be classed as soil amendments. However,
substances not classed as catalyzers, such as lime, have
such action, and in all probability most of the fertilizers
do also, so that it is difficult to draw any definite distinc-
tion and the term will doubtless be used only temporarily.
446. Nature of catalytic action. The term catalysis
is employed in a chemical sense to mean a change brought
about in a compound by an agent that itself remains
stable. As an example of this may be cited the part that
hydrochloric acid plays in the inversion of cane sugar,
the acid not entering into the reaction but by its presence
greatly accelerating it. When an attempt is made to
study these phenomena in soils, it becomes difficult,
owing to the multiplicity of factors and reactions, to
determine whether the agent is acting in a purely cata-
lytic manner.
447. Catalytic action of soils. Most soils themselves
act as catalyzers in so far as they hasten the decomposition
of hydrogen peroxide. Many substances, both organic
and inorganic, have this property, and it is not necessarily
COMMERCIAL FERTILIZERS 529
entirely lost to the soil after the organic matter has been
destroyed by ignition. It is therefore not due to an
enzyme, as stated by Konig, Hasenbaumer, and Coppen-
rath, 1 who first investigated the subject, nor entirely to
organic substances in the soil. Doubtless there are several,
or perhaps many, activating substances any of which have
this property. It is altogether likely that other catalyzers
exist in soils, and that they affect various reactions that are
concerned in plant production. Among these substances,
as pointed out by Konig, Hasenbaumer, and Coppenrath, 2
are manganese and iron oxides, which are well known to
exert catalytic action on certain reactions. While soils
naturally possess certain catalytic powers, it seems possible
to still further activate some soils by proper applications
of so-called catalytic fertilizers.
Organic matter is doubtless concerned in the catalytic
properties of soils, and the investigators just mentioned
found that in six soils the catalytic action stood in almost
direct relation to the humus content ; Sullivan and Reid, 3
however, did not find this correlation to hold. Both
organic and inorganic substances are involved in this
property of soils, but the forms in which they operate
are not well understood. In the main productive soils
have a strong catalytic effect and very poor soils are weak
in this respect, but this correlation also is not constant.
1 Konig, J., Hasenbaumer, J., and Coppenrath, E. Einige
Neue Eigenschaften des Ackerbodens. Landw. Vers. Stat.,
Band 63, Seite 471-478. 1905-1906.
2 Konig, J., Hasenbaumer, J., and Coppenrath, E. Bezieh-
ungen zwischen den Eigenschaften des Bodens und der Nahr-
stoffaufnahme durch die pflanzen. Landw. Vers. Stat., Band
66, Seite 401-461. 1907.
3 Sullivan, M. X., and Reid, F. R. Studies in Soil Catalysis,
U. S. D. A., Bur. Soils, Bui. 86. 1912.
2M
530 SOILS: PROPERTIES AND MANAGEMENT
448. Substances used as catalytic fertilizers. A
large number of substances have been found to act as
catalytic fertilizers. Among these are various salts of
manganese, iron, aluminium, zinc, lead, copper, nickel,
cobalt, uranium, boron, cerium, lanthanum, and the like.
These substances stimulate plant growth when used in
small quantities, and are toxic in large amounts. In
water cultures a much less quantity of any of them is
required to produce an injurious action on plant growth
than when applied to an equal volume of soil. The
absorptive properties of the soil and the less ready diffusi-
bility serve to mitigate the toxic action.
Different kinds of plants respond differently to the same
concentration of any of these substances. For instance,
Montemartini 1 found that uranium, copper, zinc, alumin-
ium, and cadmium oxides retard the germination of beans
and accelerate the germination of maize when used in equal
concentrations.
Of the various plant stimulants mentioned, manganese
is the only one that gives promise, at the present time,
of usefulness on a commercial basis, and it is the only one
that will receive separate treatment in this book.
449. Manganese. It seems probable that all soils
contain manganese, but the quantity present in some
soils is very small, often being less than 0.01 per cent;
in other soils, however, more than 1 per cent is found,
and Kelly 2 reports an Hawaiian soil containing 9.74 per
1 Montemartini, L. Quoted with other experiments on this
subject by N. H. J. Miller, in Annual Reports on the Progress
of Chemistry, Vol. 10, pp. 229-230. 1914.
2 Kelly, M. P. The Influence of Manganese on the Growth
of Pineapples. Jour. Indus, and Eng. Chem., Vol. 1, p. 533.
1909.
COMMERCIAL FERTILIZERS 531
cent of Mn 3 O 4 . Sullivan and Robinson l examined
twenty-six American soils and found the content of MnO
to vary from 0.01 to 0.51 per cent, the average being
0.071 per cent.
Manganese is a universal constituent not only of soils,
but likewise of plants grown under natural conditions;
in plants the quantities present vary much more than in
soils, and range from a few tenths of one per cent to nearly
one-half of the total ash. However, plants may be pro-
duced in water cultures or other media in which apparently
no manganese is present and a normal growth and fructi-
fication will follow. It is evident, therefore, that any
benefit to plant growth that may accrue through the
addition of manganese to the soil is not due to its function
as a nutrient material in the sense in which nitrogen,
potassium, and phosphorus act in that capacity.
450. Physiological role of manganese. It was the
discovery by Bertrand 2 of the existence of manganese in
the oxidizing enzymes of plants and of its function in
stimulating the oxygen-carrying power of these catalytic
agents that suggested its use as a stimulating agent in
crop production. In water cultures a very dilute solution
of manganese salts increases plant growth, but beyond a
very low concentration its effect is toxic. Plants differ
widely in their response to manganese, with respect both
to stimulation and to injury. A certain concentration
may be stimulating to one plant and toxic to another.
Experiments in the application of manganese salts
1 Sullivan, M. X., and Robinson, W. O. Manganese as a
Fertilizer, U. S. D. A., Bur. Soils, Circ. 75. 1912.
2 Bertrand, (!. Sur 1' Action Oxydante des Sels Manganeux
et sur la Constitution Chemique des Oxydases. Compt. Rend,
Acad. Sci. Paris, Tome 124, pp. 135.3-1358. 1897.
532 SOILS: PROPERTIES AND MANAGEMENT
to soils have not afforded as satisfactory results as have
the trials with water cultures. Applications of a certain
salt of manganese, when applied at the same rates to
different soils, have in some cases produced increased
growth, have in other cases had no apparent effect, and
have in still other cases proved injurious to plants. The
reason for this is doubtless to be found in the inherent
properties of the particular soil to which the application
is made.
451. Action of manganese as a fertilizer. The fact
that manganese stimulates plant growth in water cultures
is very good evidence that it has at least a direct action
on the plant. Whether it has a further influence through
reactions brought about in the soil is less evident, although
it seems likely that such is the case. Thus, Skinner and
Sullivan 1 conclude from some of their experiments that
oxidation in some soils is increased by the application of
manganese salts. It also seems probable that manganese
may have some influence on the activity of the microor-
ganisms of the soil, but this has not been definitely demon-
strated.
452. Forms of manganese and response of soils. -
The manganese salts that have been found to be effective
as fertilizers are the sulfate, the chloride, the nitrate, the
carbonate, and the dioxide. Of these the first has been
most generally used, and in quantities up to 50 pounds
an acre it has in most cases not been toxic. On acid
soils it is not supposed to exercise any beneficial action,
and on very productive soils Skinner and Sullivan, in
the experiments cited above, found it to be ineffective;
1 Skinner, J. J., and Sullivan, M. X. The Action of Man-
ganese in Soils. U. S. D. A., Bui. 42. 1914.
COMMERCIAL FERTILIZERS 533
while they obtained appreciable benefit from its use on
poor soils. They argue that since very productive soils
have great oxidative power the use of manganese is un-
necessary, but since poor soils undergo insufficient oxida-
tion the stimulation that this process receives by the
application of manganese is productive of much good.
Accordingly manganese is most profitably used on poor
soils not deficient in lime.
CHAPTER XXIV
SOIL AMENDMENTS
CERTAIN substances are sometimes added to soils for
the purpose of increasing productiveness through their
influence on the physical structure of the soil, and thereby
on the chemical and bacteriological properties. These
substances are called soil amendments. It is true that
they may add essential plant ingredients to the soil, but
tha^ function is of minor importance.
453. Salts of calcium. Calcium, although essential
to plant growth, seldom needs to be added to the soil to
supply the plant directly ; but because of its effect on the
soil properties, its use is beneficial to a great number of
soils.
454. Effect on tilth and bacterial action. On clay
soils the effect ol lime is to bring the fine particles into
aggregates which are loosely cemented by calcium carbon-
ate. The effect of this structure on tilth has already been
explained (par. 120). On sandy soils the carbonate of
calcium serves to bind some of the particles together,
making the structure somewhat firmer and increasing the
water-holding power. It should be used only in small
quantities on sandy soils.
There is a tendency for most cultivated soils to become
acid, as has already been explained (par. 283). Acidity
may reach a point where it becomes directly injurious to
certain plants, but it becomes indirectly injurious before
534
SOIL AMENDMENTS 535
that point is readied. One way in which this occurs is
by curtailing the quantity of calcium carbonate in the
soil. An easily available base to combine with the
organic acids affords the most favorable condition for the
decomposition processes due to bacterial action, and
hence the best results cannot be obtained where carbonate
of lime is not present. Its action in improving tilth also
facilitates desirable forms of bacteriological activity by
increasing the permeability of the soil for air.
455. Liberation of plant-food materials. It has been
stated that the alkalies and the alkaline earths are more
or less interchangeable in certain compounds in the soil.
The addition of lime may in this way liberate potassium,
when otherwise it would be difficult for crops to obtain
a sufficient supply from a particular soil. The substitu-
tion of bases has been discussed (par. 251) and the
liberation of potassium is in accord with these phenomena.
Magnesium, although rarely deficient, may also be made
available in this way. The use of calcium salts may,
under some soil conditions, render phosphorus more use-
ful, probably by supplying a base more soluble than iron
or alumina, with which, in soils deficient in calcium, the
phosphorus might otherwise be combined. Experiments
by Prianischnikov, 1 in which plants were grown in washed
sand containing Hellriegel's nutrient solution to which
mono-, di-, and tri-calcium phosphate respectively were
added, both with and without calcium carbonate, showed
a decreased availability of the tricalcium phosphate due
to the presence of the carbonate, but neither a reduced
nor an increased availability of the other forms of phos-
1 Prianischnikov, D. Ueber den Einfluss von Kohlensauren
Kalk auf die Wirkung von Verschiedenen Phosphaten. Landw.
Vers. Stat., Band 75, Seite 357-376. 1911.
536 SOILS: PROPERTIES AND MANAGEMENT
phorus arising from the presence of carbonate. Neither
did the availability of iron or aluminium phosphate
appear to be influenced by calcium carbonate.
These and recent experiments by Simmermaeher 1 and
others tend to discredit the earlier conclusions as quoted
above and as set forth by Deherain 2 regarding the favor-
able influence of lime on the availability of phosphorus.
However, the preponderating evidence is still with the
earlier experimenters. The principles that underlie the
effect of lime on availability of phosphorus are discussed
in paragraphs 259 and 260.
456. Influence of lime on the formation of nitrates in
soil. It has already been remarked that nitrification
proceeds very slowly in acid soils. A soluble base
must be present with which the nitric acid may com-
bine, otherwise the process will be inhibited by the toxic
effect of the acid on the bacteria concerned in the forma-
tion of the acid. The addition of lime is the most
economical method of providing the base. This is
often a matter of great moment for crops that respond
readily to nitrate nitrogen, and is one of the important
reasons for applying lime to sour soils. The fact that
some plants grow better in some soils than in strongly
basic ones is also an indication that such plants absorb
a considerable part of their nitrogen in forms other than
nitrates.
Many investigators have found that the presence of
calcium carbonate promotes the ammonifying and nitrify-
ing process. The addition of calcium carbonate to a
1 Simmermacher, W. Einwirkung der Kohlensauren Kalkes
bei der Dungung von Haferkulturen init Mono- und Dicalcium
Phosphat. Landw. Vers. Stat., Band 77, Seite 441-471. 1912.
2 Deherain, P. P. Traite de Chemie Agricole, p. 525. 1892.
SOIL AMENDMENTS 537
sandy loam soil was found by Kellerman and Robinson l
to favor the formation of nitrates up to an application
of 2 per cent, which is much more than would ever be
applied in practice. It must be kept in mind, however,
that this limit does not apply to all soils, as the absorp-
tive properties of the soil for lime will determine the
maximum application that may profitably be made.
Kellerman and Robinson found also that the application
of magnesium carbonate in excess of 0.25 per cent in-
hibited the formation of nitrates. Kelly 2 also has
recently reported that the addition of magnesium car-
bonate to the soils with which he experimented resulted
in a marked depression of both ammonification and nitri-
fication, and that the addition of calcium carbonate did
not overcome this depressing influence.
457. Effect on toxic substances and plant diseases. -
Free acids are toxic to most agricultural plants. Some
plants are much more sensitive than others. Alfalfa,
for example, should have a slightly alkaline medium for
its best growth, and any acid is very injurious. Calcium
salts, in neutralizing acidity, remove this toxic condition.
A liberal application of lime is therefore a precaution
against injury of this kind.
The. presence of soluble calcium, with its effects on the
soil, retards the development of certain plant diseases,
such as the " finger and toe " disease of the Cruciferse.
On the other hand, it may promote some diseases, as, for
example, potato scab.
1 Kellerman, K. F., and Robinson, F. R. Lime and Legume
Inoculation. Science, n. s., Vol. 32, pp. 159-160. 1910.
2 Kelly, W. P. The Effect of Calcium and Magnesium Car-
bonates on Some Biological Transformations of Nitrogen in
Soils. Univ. of Calif. Pub., Agr. Sci., Vol. 1, No. 3, pp. 39-49.
1912.
538 SOILS: PROPERTIES AND MANAGEMENT
458. The lime-magnesia ratio. The physiological
balancing of magnesium by calcium was first worked out
by Loew, 1 and the ratio in which these two cations should
exist in nutrient solutions in order to secure the best
growth of certain agricultural plants has been very satis-
factorily demonstrated by the experiments of many
investigators. The optimum ratio varies with different
kinds of plants, and in general the calcium must exceed
the magnesium in amount, but there is a limit beyond
which it should not be present. If calcium alone is
present, it acts as a toxic agent on the plant, and mag-
nesium acts in a similar way. It is only when the ratio
between these cations falls within certain limits that
they exert no toxic action. This ratio varies between
one part of calcium oxide to one part of magnesium
oxide, and seven parts of calcium oxide to one part of
magnesium oxide.
In the soil the relations of calcium and magnesium to
plant growth are not so simple. It is impossible to
determine the actual or the relative quantities of these
cations that are available for absorption by the plant.
This is mainly because of the absorptive properties of
soils, by which they remove the bases from solution and
hold them in a somewhat difficultly soluble form.. The
ratio of calcium to magnesium is not likely to disturb
crop yields in soils unless the quantity of magnesium
present happens to be very large. Gile and Ageton 2
have found ordinarily fertile soils having ratios as high
1 Loew, O. The Physiological Role of the Mineral Nutrients
of Plants. U. S. D. A., Bur. Plant Indus., Bui. 1, p. 53. 1901.
2 Gile, P. P., and Ageton, C. U. The Significance of the
Lime-Magnesia Ratio in Soil Analyses. Journ. Indus, and
Eng. Chem., Vol. 5, pp. 33-35. 1913.
SOIL AMENDMENTS 539
as 500 CaO to 1 MgO by weight. On the other hand,
excessive applications of magnesium compounds have
been found to be injurious on some soils. Even on a
very heavy clay soil, at Cornell University, an applica-
tion of 1333 pounds to the acre of magnesite markedly
decreased the yields of sorghum and oats. The soil
originally contained about equal parts of calcium and
magnesium.
459. Forms of calcium. Calcium is used on the
soil in the form of calcium oxide, or quicklime (CaO),
water-slaked lime (Ca(OH) 2 ), air-slaked lime (CaCO 3 ),
ground limestone, marl (also a carbonate), and calcium
sulfate, or gypsum (CaSO4 . 2 H 2 0). The application of
any of these is usually called liming the soil, although
gypsum does not serve exactly the same purpose as do
the other forms. Owing to differences in the molecular
weights of these compounds of calcium, it requires more
of some forms than of others to furnish the same amount
of calcium. Approximately equivalent quantities of some
of the common forms when fairly pure are :
Quicklime 56 pounds
Water-slaked lime 74 pounds
Air-slaked lime, marl, and ground limestone 100 pounds
Quicklime, and the hydrate, when added to the soil, even-
tually assume some of the more insoluble forms of com-
bination or remain as the carbonate, never being present
as the oxide. It is always desirable to have present in the
soil at least a small amount of calcium carbonate.
460. Caustic limes. Quicklime and water-slaked lime
have a markedly alkaline reaction, and hence neutralize
quickly any active acidity that may exist in the soil.
They act quickly also in liberating plant-food, particularly
540 SOILS: PROPERTIES AND MANAGEMENT
nitrogen. Some soils respond more rapidly to quicklime
or water-slaked lime than to carbonate of lime, especially
when the carbonate is in the form of marl or ground lime-
stone, these substances never being in such a finely pul-
verized condition as is caustic lime. The use of the
caustic forms of lime has been said to result in the loss
of nitrogen by the too rapid decomposition of organic
compounds.
On clays the granulating effect of caustic lime is more
marked than that of the carbonate, and for this reason
the former has a distinct advantage for use on heavy clay.
For the same reason an occasional moderate dressing is
better than a heavy dressing given less frequently.
461. Carbonate of lime. Air-slaked lime has the
advantage of being in a finely divided condition, and
does not produce the injurious action on organic matter
that is sometimes attributed to caustic lime. Its effect
on the granulation of clay soils is probably less pro-
nounced than that of caustic lime.
Marl (par. 27) differs from air-slaked lime principally
in its property of being in a less finely pulverized condi-
tion. It acts less quickly than does caustic lime. Owing
to the fact that marl deposits differ greatly in the com-
position of their products, it is well to know the quality
of the material before buying it. The carbonate of lime
in marl may vary from 5 or 10 to 90 or 95 per cent in
different samples.
Ground limestone has been used extensively in recent
years. It is very important that it be finely ground, as
on the comminution of the material much of its efficiency
depends. However, it is doubtful whether there is any
advantage in making it finer than is required to pass
through a sieve with 50 meshes to the inch.
SOIL AMENDMENTS 541
462. Relative effectiveness of caustic lime and car-
bonate. In order to test the value of ground limestone
and other forms of calcium carbonate, experiments in
which it was compared with caustic lime have been con-
ducted at some of the experiment stations. Reports of
tests at the Pennsylvania Experiment Station, 1 in which
plats treated with slaked lime at the rate of two tons per
acre once in four years were compared with plats treated
with ground limestone at the rate of two tons to the acre
every two years, show that at the end of twenty years,
in every case, the total yields were greater on the plats
receiving ground limestone. After the treatment on
these plats had been continued for sixteen years, a de-
termination of nitrogen showed the upper nine inches of
soil on the limestone-treated plats to contain 2979 pounds
of nitrogen to the acre, and the slaked-lime plats to con-
tain 2604 pounds. It may be inferred from these figures
that the slaked lime caused a slightly .greater destruction
of organic matter than did the limestone.
Patterson 2 also conducted experiments for eleven
years with caustic lime produced by burning both stone
and shells, and the carbonate of lime in ground shells and
shell marl. The average crops of maize, wheat, and hay
were all larger on the plats treated with carbonate of
lime.
. While these experiments show, at first glance, results
1 Waters, H. J., and Hess, E. H. General Fertilizer Experi-
ments. Pennsylvania State College, Kept. 1894, Part 2, pp.
258-281. Also, Hunt, T. F. Soil Fertility. Pennsylvania
Agr. Exp. Sta., Bui. 90. 1909.
2 Patterson, H. J. Lime, Sources and Relation to Agri-
culture. Maryland Agr. Exp. Sta., Bui. 66, pp. 127-130. 1900.
Also, Investigations on the Liming of Soils. Maryland Agr. Exp.
Sta., Bui. 110, pp. 16-21. 1906.
542 SOILS: PROPERTIES AND MANAGEMENT
rather favorable to the use of carbonate of lime, a careful
analysis of them by Wheeler l raises some doubt as to
the legitimacy of this interpretation. He points out, for
instance, that in the Pennsylvania experiments excessive
quantities of lime were used, and that no farm manure
nor commercial fertilizers were applied to the plats be-
tween which comparisons were made.
There is, unfortunately, a paucity of definite and con-
clusive data that may be applied to the solution of the
question as to the relative values of these different forms
of lime for use as soil amendments, but some information
has accumulated through experience and practice that
may be taken as a fairly safe guide in their use. It is
well known, for instance, that burned lime has a more
pronounced effect on soil granulation than has the car-
bonate, and may therefore be expected to be more bene-
ficial to heavy clay soils. On the other hand, burned
lime is not so desirable a form to apply to very sandy
soils, especially when they are likely to be dry, as there
is danger that organic matter will be destroyed.
463. Sulfate of calcium. Gypsum, in which form
calcium sulfate is usually applied to soils, has been used
for many years and was a popular soil amendment in
this country before the common commercial fertilizers
were used to any great extent. It frequently went by
the name of land plaster, and, as it was rather widely
distributed in nature and not difficult to obtain, it was
ground and largely used in many localities throughout
the eastern states. Its popularity has waned in recent
1 Wheeler, H. J. Is the Recommendation that Only Ground
Limestone Should be Used for Agricultural Purposes a Sound
and Rational One? National Lime Manufacturers' Assoc.,
Bui. 4. 1912.
SOIL AMENDMENTS 543
years, and its effectiveness has apparently decreased as
the soils on which it was used have been longer under
cultivation. Possibly this is due to the tendency of these
soils to become more acid, which has caused the gypsum
to be less effective in liberating potassium a property
with which it has generally been credited. At present
gypsum is not very generally used on soils. It must be re-
membered, however, that superphosphates always contain
a considerable proportion of this material, and it may add
appreciably to the beneficial effects of that fertilizer.
Aside from its action in liberating potassium (the actual
extent of which has never been very clearly demon-
strated), gypsum serves to supply sulfur to the soil. The
sulfur, while it may be needed in some soils, has the dis-
advantage of being present as an acid ; and if the acid
is added in larger quantity than is removed by plants,
there is a resulting loss of basic material in the drainage
water and a tendency for the soil to become sour.
The action of gypsum in improving tilth is less marked
than that of caustic lime or of the carbonate. As a source
of calcium it is of no moment, as, if applied in such quan-
tities as those in which the other forms are used, the
sulfate would be very injurious. Ordinarily it is applied
at the rate of only a few hundred pounds to the acre at the
most. On the whole, gypsum is not an adequate substi-
tute for, nor so desirable a form of, calcium as the oxide,
the hydroxide, or the carbonate.
464. Common salt. Sodium chloride has a marked
effect on some soils, but wherein its effectiveness lies
is not well understood. The addition of sodium and of
chlorine as plant constituents is clearly not the reason,
as these substances are always present in soils in avail-
able form far in excess of their requirements.
544 SOILS: PROPERTIES AND MANAGEMENT
The effect of sodium chloride on clay-bearing soils
is to liberate certain plant nutrients, among which are
calcium, magnesium, potassium, and phosphorus. This
action, although limited in amount, is probably, in some
cases at least, partly responsible for the beneficial action
of common salt.
The structure of the soil is improved by the applica-
tion of sodium chloride, just as it is by lime, although
usually not to the same extent.
Another effect of salt is to conserve and distribute
soil moisture. Its conserving action is probably due to
an increase in the density of the soil-water solution, thus
retarding transpiration. The film movement of water is
likewise increased by the presence of salt in the solution,
and in this way the upward movement of bottom water
is facilitated and the supply within reach of the roots
maintained in time of drought.
It has been seen that sodium is not one of the substances
essential to the growth of plants. But that sodium may
be substituted, in part, for the potassium absorbed by
agricultural plants in their normal growth, has been
shown in this country by the experiments of Wheeler
and Adams ; l and the more ready availability of the
sodium applied as a chloride than of the potassium in
its natural condition in some soils probably accounts in
part for the beneficial effects of this salt.
It is not all soils, however, that are benefited by salt,
its usefulness not being of such wide application as that
of lime. Certain crops, as previously mentioned, are
injured by the presence of chlorine.
1 Wheeler, H. J., and Adams, G. E. Concerning the Agri-
cultural Value of Sodium Salts. Rhode Island Agr. Exp. Sta.,
Bui. 100. 1905.
SOIL AMENDMENTS 545
465. Muck. The effect of muck (par. 72) is to
change the structure of soils, making a heavy clay soil
lighter and more porous, and binding together the par-
ticles of a sandy soil. Both classes of soils, but particu-
larly the sandy type, have a greater water-holding
capacity after treatment with muck, owing to its great
absorptive power which amounts to 70 per cent or more
of its own weight. It is to its content of organic matter
that the physical effects of muck are due.
Muck contains 1 to 2 per cent of organic nitrogen,
calculated to dry matter, which does not readily undergo
ammonification. The addition of farm manure (which
ferments readily) and of lime serves to hasten ammoni-
fication. Its use as an absorbent in the stable fits it well
for use on the land.
Very large applications of muck are necessary when
it is used to improve the structure of the soil. From
ten to forty or fifty tons per acre are frequently applied.
Muck has been used successfully as a carrier of Bacillus
radicicola; for this it is eminently adapted by its absorbent
qualities, which prevent it from drying out and thus caus-
ing injury to the bacteria. At the rate of thirty pounds
to the acre it has served as a highly effective medium for
inoculating soil for alfalfa. 1
Muck is also used as a filler in certain commercial
fertilizers.
1 Lyon, T. L., and Bizzell, J. A. Some Experiments in Top-
Dressing Timothy and Alfalfa. Cornell Univ. Agr. Exp. Sin.,
Bui. :*:). 1913.
2N
CHAPTER XXV
FERTILIZER PRACTICE
THE purchase and use of commercial fertilizers in an
economical way requires not only specific technical
knowledge of the various materials, as already set forth,
but also a certain amount of general knowledge both
practical and theoretical. There are at present so many
fertilizing materials on the market under various trade
names, that the question as to the best one to buy for a
certain crop growing under definite soil and climatic
conditions becomes a difficult one. The greater the
general knowledge, therefore, that a person possesses
as to the effects of the different elements on plant growth,
as to fertilizer inspection and control, as to methods of
buying, as to home mixing, as to methods and time of
application, and as to mixtures for special crops, the
better he is able to utilize fertilizers that will result in
financial gain. That a fertilizer shall be profitable is
the ultimate desideratum. Moreover, as all fertilizers
exert, either directly on indirectly, a residual effect, the
problem necessarily broadens into a study of the systems
of applying fertilizers to a series of crops or to a rotation,
rather than a study of the effects of one particular ferti-
lizer application on one particular crop.
Note. For discussions of fertilizer practice see Halligan, J. E.,
Soil Fertility and Fertilizers, Chapters 13-17. Easton, Penn-
sylvania. 1912. Also, Van Slyke, L. L. Fertilizers and Crops,
Chapters 21-25, and 27-35. New York, 1912. Also, Fraps,
G. S. Principles of Agricultural Chemistry, Chapter 16. Eas-
ton, Pennsylvania. 1913.
546
FERTILIZER PRACTICE 547
466. Effects of nitrogen on plant growth. 1 Of the
three primary elements of a fertilizer, nitrogen 2 seems
to have the quickest and most pronounced effect, not
only when present in excess of the other constituents,
but also when moderately used. It tends primarily to
encourage aboveground vegetative growth and to impart
to the leaves a deep green color, a lack of which is usually
due to insufficient nitrogen. It tends in cereals to in-
crease the plumpness of the grain, and with all plants it
is a regulator in that it governs to a certain extent the
utilization of potash and phosphoric acid. Its application
tends to produce succulence, a quality particularly de-
sirable in certain crops. In its general effects it is very
similar to moisture, especially when supplied in excessive
quantities.
The peculiarity of nitrogen lies not only in its absolute
necessity for plant growth, its stimulation of the vegeta-
tive parts, and its close relationship to the general tone
and vigor of the crop, but also in the fact that it was not
one of the original elements of the earth's crust. During
the formation of the soil it slowly and gradually became
present, brought down by rains and fixed naturally in the
soil itself mostly through the agency of bacterial action.
Even now it exists largely locked up in complex nitrog-
enous compounds of the humus and the less decayed
organic matter, and becomes slowly available to plants
1 Discussions of the effects of the various elements on plants
may be found as follows : Russell, E. J. Soil Conditions and
Plant Growth, Chapter II, pp. 19-50. London, 1912. Also,
Hall, A. D. Fertilizers and Manures, Chapters III, V, and VI.
New York, 1910.
2 For a discussion of nitrogen in relation to crop yield, see
Hunt, T. F. The Importance of Nitrogen in the Growth of
Plants. Cornell Univ. Agr. P]xp. Sta., Bui. 247. 1907.
548 SOILS: PROPERTIES AND MANAGEMENT
largely through bacterial activity. It may be stated with
certainty that one of the possible limiting factors to
crop growth is a lack of water-soluble nitrogen at critical
periods in amounts necessary for normal crop develop-
ment. Since soluble nitrogen may be very readily lost
from the soil by leaching, the problem of proper plant
nutrition becomes a serious one. Not only must the
farmer be able to so regulate its addition in fertilizers
as to obtain the highest efficiency, but he must understand
the control and encouragement of the natural fixation
as well. The emphasis placed on all phases of the nitrogen
problem serves to reveal its great importance in fertility
practices.
Because of the immediately visible effect from the ap-
plication of soluble nitrogen, the average farmer is prone
to ascribe too much importance to its influence in proper
crop development. This attitude is unfortunate, since
nitrogen is the highest-priced constituent of ordinary
fertilizers. Moreover, of the three primary elements
it is the only one which added in excess will result in
harmful after effects on the crop. Its general influences,
besides its functions in the metabolic and synthetic
processes of plant development, may be listed briefly as
follows :
1. Nitrogen tends to increase the growth of the above-
ground parts.
2. It delays maturity by encouraging vegetative growth.
This oftentimes endangers the crop to frost, or
may cause trees to winter badly.
3. It increases the ratio of straw to grain in cereals, and
the ratio of leaves to underground parts in root
crops.
FKRTTLIZER PRACTICE 549
4. It weakens the strain and causes lodging in grain. This
is due to an extreme lengthening of the internodes,
and as the head fills the stem is no longer able to
support the increased weight.
5. It lowers quality. This is especially noticeable in
certain grains and fruits, as barley and peaches.
The shipping qualities of fruit and vegetables are
also impaired.
6. It increases the percentage of nitrogen in the crop,
particularly in the straw of cereals and in timothy
hay.
7. It decreases resistance to disease. This is probably due
to a change in the physiological resistance to disease
within the plant, and also to a thinning of the cell
wall, allowing a more ready infection from without.
While certain plants, as the grasses, lettuce, radishes,
and the like, depend for their usefulness on plenty of
nitrogen, for the average crop it is generally better to
limit the amount of nitrogen so that growth may be
normal. This results in a better utilization of the nitro-
gen and in a marked reduction of the fertilizer cost for a
unit of crop growth. This is a vital factor in all fertil-
izer practice, and shows immediately whether fertilization
is or is not an economic success.
467. Effects of phosphorus on plant growth. It is
difficult to determine exactly the functions of phosphoric
acid in the economy of even the simplest plants. Neither
cell division nor the formation of fat and albumen go on
to a sufficient extent without it. Starch may be pro-
duced when it is lacking, but will not change to sugar.
As grain does not form without its presence, it very
probably is concerned in the production of nucleoproteid
550 SOILS: PROPERTIES AND MANAGEMENT
materials. Its close relationship to cell division may
account for its presence in seeds. Its general effects on
plant growth may be listed as follows :
1. Phosphorus hastens maturity by its effect on rate of
ripening. This makes phosphorus especially valu-
able in wet years, and in cold climates where the
season is short.
2. It increases root development, especially of lateral and
fibrous rootlets. This renders it valuable with
such soils as do not encourage root extension and
to such crops as naturally have a restricted root
development. Phosphorus is therefore valuable in
fall-sown crops, in years of drought, and for farm-
ing on arid land.
3. It decreases the ratio of straw to grain by hastening the
filling of the grain and by promoting maturity.
4. It strengthens the straw, due to its balancing effect on
the nitrogen.
5. It improves the quality of the crop. This has been
recognized in the handling of pastures in England
and France. The effect on vegetables is also
marked.
6. It increases percentage of phosphorus in the crop.
With cereals this is particularly noticeable in the
straw.
7. It increases resistance to disease, due probably to
more normal cell development.
Excessive phosphorus ordinarily has no bad effect,
as it does not stimulate any part excessively as does ni-
trogen, nor does it lead to a development which is detri-
mental. Its lack is not quickly apparent, as in the case
FERTILIZER PRACTICE 551
of nitrogen, and as a consequence phosphorus starvation
may occur without any suspicion thereof being enter-
tained by the farmer.
One of the most important phases to be noted from this
comparison of the effects of nitrogen and phosphorus is
the balancing powers of the latter on the unfavorable in-
fluences generated by the presence of an undue quantity
of the former. This is a vital factor in fertilizer practice,
since normal fertilizer stimulation always results in the
most economic gains. Such a normal increase is obtained
only when the plant functions of the several fertilizer
constituents are in proper accord.
468. Effects of potassium on plant growth. The
effects of potash are more localized than those of nitrogen
and phosphorus. Potash is essential to starch formation,
either in photosynthesis or in translocation, and is a
necessary component of chlorophyll. It is important
in grain formation, giving plump, heavy kernels. In
general it tends to impart tone and vigor to a plant. In
increasing resistance to disease it tends to counteract
the ill effects of too much nitrogen, Avhile in delaying
maturity it works against the ripening influences of
phosphoric acid. In a general way it exerts a balancing
effect on both nitrogen and phosphate fertilizer materials,
and consequently is necessary in a mixed fertilizer, es-
pecially if the potash of the soil is lacking or unavail-
able. As with phosphorus, it may be present in large
quantities in the soil and yet exert no harmful effect on
the crop.
469. Law of the minimum. In connection with the
obvious importance of utilizing, for any particular soil
and crop, a fertilizer well balanced as to the three primary
elements, two queries naturally arise. These are
552 SOILS: PROPERTIES AND MANAGEMENT
(1) What are the right proportions of nitrogen, phos-
phorus, and potash to apply under given conditions?
(2) What would be the effect if any one of these should
not be present in such a quantity as to make it equal in
function to the others? The first query cannot be dis-
posed of until the question of fertilizer mixtures has been
considered. The second, however, is not affected by so
many factors, and is more clearly a question of the func-
tion of the elements concerned.
Any element that exists in relatively small amounts as
compared with the other important constituents natu-
rally becomes the controlling factor in crop development.
Any reduction or increase in this element will cause a
corresponding reduction or increase in the crop yield.
This element, then, is said to be " in the minimum."
In fertilizer practice, ideal conditions would exist if no
constituent functioned as a decided minimum and the
entire influence of each single element were fully utilized.
In other words, the fertilizer would be balanced as to its
relationship to normal plant growth. That such a con-
dition is more or less ideal and theoretical is obvious,
from the fact that the various fertilizer carriers undergo
more or less radical changes after being applied to the
soil. The composition of the soil itself is also a disturb-
ing factor. Nevertheless, the nearer an approach can be
made to such conditions, the greater will be the economy
of fertilizer practice.
Numerous persons have investigated the question as
to what effect an increase of an element in the minimum
may have on crop yield, and various ideas have been
advanced thereon. The idea of a definite law governing
the increase of plant growth according as the element
in the minimum is increased, was first suggested by
FERTILIZER PRACTICE 553
Liebig. Wagner 1 later stated definitely that up to a
certain point the increase yield was proportional to the
increase in the application. This, however, evidently
cannot apply except over a very limited field, since it is
a matter of common observation that increased crop
yield becomes lower as the lacking element is supplied.
Recently Mitscherlieh 2 has formulated a law r3 which is a
logarithmic, rather than a direct, function of the increase
in the element occupying the position of the minimum.
Mitscherlich's law may be stated concisely as follows:
the increased grow r th produced by a unit increase of the
element in the minimum is proportional to the decre-
ment from the maximum. The following curve (see
Fig. 62) constructed from data obtained by Mitscherlieh, 4
shows the trend of the increased growth curve as governed
by increased applications of an element in the minimum,
other factors being, of course, under control. This curve
is maintained by Mitscherlich to approximate a theoretical
curve of a definite mathematical formula.
1 Wagner, II. Beitrage zur Dungerlehre. Landw. Jahr.j
Band 12, Seite 691 ff. 1883.
2 Mitscherlich, A. E. Das Gesetz des Minimums und das
Gesetz des Abnehmen den Bodenertrages. Landw. Jahr.,
Band 38, Seite 537-552. 1909.
Also, Ein Beitrage zur Erforschung der Ausnutzung des
im Minimum Vorhandenen Nahrstoffes durch die Pflanze.
Landw. Jahr., Band 39, Seite 133-156. 1910.
-^ = (a y)k. Integrating, log (a y) = c kx.
dx
y = total yield from any number of increments.
x = amount of any particular fertilizer constituent utilized.
a = maximum yield and is a constant.
k = a constant depending on y and x, variables.
4 Mitscherlich, A. E. Uber das Gesetz des Minimums und
die sieli aus dicsom Krgobonden Schlussfolgerungen. Landw.
Ver. Stat., Band 75, Seite 231-263. 1911.
554 SOILS: PROPERTIES AND MANAGEMENT
FIG. 62. Curve showing the increased growth of oats under the in-
fluence of constantly increasing amounts of phosphorus, that ele-
ment being in the minimum.
The formula as proposed by Mitscherlich has been
questioned by several investigators, 1 who have shown
that a number of conditions, such as light, heat, and
1 Pfeiffer, Th., Blanck, E., and Flugel, M. Wasser und Licht
als Vegetationsfaktoren und ihre Beziekungen zum Gesetze vom
Minimum. Landw. Ver. Stat., Band 76, Seite 211-223. 1912.
Also, Maze, P. Reckerckes sur les Relations de la Plante
FERTILIZER PRACTICE 555
moisture, tend to disturb the application of such a law.
The fact that crop yield is the summation of so many
varying factors seems to argue in favor of no hard and
fast rule regarding the increased growth due to the added
increments of an element in the minimum. It is enough,
in the practical utilization of fertilizers, to remember that
this curve in general approximates the one already cited,
and that in order to obtain the best results from a com-
plete fertilizer a mixture should be used that is approxi-
mately balanced so far as the effects of the elements are
concerned, the crop as well as the chemical constitution
of the soil being considered.
470. Fertilizer brands. In an attempt to meet the
demands for well-balanced fertilizers suited to various
crops and soils, manufacturers have placed on the market
numberless brands of materials containing usually at
least two of the important elements, and nearly always
the three; the former being designated as incomplete
fertilizers, while the latter are spoken of as complete
fertilizers. These various brands usually have some
catchy name, such as " The Ureka Corn Special/' " Far-
mers' Potato and Corn Fertilizer," "The Golden Har-
vest," or " The Empire State Sure Crop Phosphate."
Such a name frequently implies the usefulness of the
material for some particular crop, but oftener it has no
relation either to crop or to soil. Ordinarily the name
should be ignored in the purchase of fertilizers.
A brand of fertilizer is usually made up of a number
of materials containing the important ingredients. These
materials, already described, are called carriers. The
making-up of a commercial fertilizer consists, then, in
avec les Elements Nutritifs du Sol. Compt. Rend., Vol. 154,
pp. 1711-1714. 1912.
556 SOILS: PROPERTIES AND MANAGEMENT
merely mixing the various carriers together so that the
required percentages of nitrogen, potash, and phosphoric
acid are obtained, care being taken that no detrimental
reaction shall occur and that a physical condition con-
sistent with easy distribution shall be maintained. v If
the substances used are difficultly soluble, the fertilizer
is not so valuable as one composed of easily soluble con-
stituents. The general solubility of the various in-
gredients should be known by a prospective purchaser.
The various brands on the market, besides being
complete or incomplete, may be designated as high-grade
or low-grade. These terms may be used in two ways
high-grade or low-grade as to availability, or high-grade
or low-grade as to amount of plant-food constituents
carried. A low-grade fertilizer in the percentages of
nitrogen, phosphoric acid, and potash is always encum-
bered with a large amount of inert material, \vhich adds
to the cost of mixing, transportation, and handling. It
is thus usually a more expensive fertilizer to a unit of
plant-food obtained than one of higher grade. Except
for special purposes, a low-grade fertilizer as to avail-
ability should be bought sparingly or not at all.
471. Fertilizer inspection and control. With the
many different materials available for mixing commercial
fertilizers, and from the fact that so many opportunities
are open for fraud either as to availability or as to guaran-
tee, laws have been found necessary for controlling the
sale of fertilizers. Most states have such a law, the
western laws generally being superior to those in force
in eastern states, where the fertilizer sale is heavier.
This is because the western regulations are more recent
and the legislators have had the advantage of the ex-
perience gained where fertilizers have long been used.
FERTILIZER PRACTICE 557
Moreover, the legislators in such states have not been so
strongly confronted with fertilizer lobbying, and have
consequently l>een free to enact stricter laws than were
-ible where fertilizers are such an important com-
nuT?ial commodity.
Usually certain provisions are common to all fertilizer
laws. In general, all fertilizers selling for a certain price
or over (usually $5 a ton) must pay a state license fee and
print the following data on the bag or an authorized tag :
1. Number of net pounds of fertilizer to a package.
2. Name, brand, or trade-mark.
3. Name and address of manufacturer.
4. Chemical composition or guarantee.
The composition of a commercial fertilizer is ordinarily
expressed simply; for example, as a 3-6-10, meaning 3
per cent of nitrogen, 6 per cent of phosphoric acid, and
10 per cent of potash. This, however, is too brief for a
guaranteed analysis on goods exposed for sale, as it gives
no idea whatsoever regarding the solubility of the ma-
terials. As might be expected, there is a wide range in
the character of the guarantee required by the various
states. For example, some states insist on the statement
of the percentage of both nitrogen and ammonia, while
others insist only on the percentage of nitrogen. Some
require the soluble, the reverted, and the total phosphoric
acid, while others require only the soluble and the re-
verted. As to potash, in some cases the soluble must be
stated, while in other cases the total must be given. In
general, a guarantee should show not only the amount
of the various constituents, but also their form or avail-
ability. The guarantee required by North Dakota is
excellent in this respect :
558 SOILS: PROPERTIES AND MANAGEMENT
Guarantee required by the State of North Dakota
Percentage of N in nitrates Percentage of P 2 O 5 soluble
Percentage of N as ammonia in water
Percentage of N total Percentage of P2O 5 re-
verted
Percentage of P2O 5 in-
Percentage of K 2 O soluble soluble
Percentage of K 2 O as chloride Percentage of P 2 O 5 total
Since a fertilizer law is designed primarily to protect
not only the purchasers but also the manufacturers, a
certain amount of variation is allowed below a guarantee.
This is a matter of extreme variation in the different states.
Ordinarily, also, the offering for sale of any leather matter
or its products, either separately or in mixtures, is pro-
hibited, unless so stated specifically on the package.
For the enforcement of such laws, the states usually
provide adequate machinery. The inspection and analyses
may be in the hands of the state department of agricul-
ture, of the director of the state agricultural experiment
station, of a state chemist, or under the control of any
two of these. In any case, a corps of inspectors is pro-
vided, the members of which take samples of the fertilizers
on the market throughout the state. These samples are
analyzed in laboratories provided for the purpose, in
order to ascertain whether the mixture is up to its guar-
antee. If the fertilizer falls below the guarantee, allow-
ing, of course, for the variation permitted by law, the
manufacturer is subject to prosecution.
A more effective check on fraudulent guarantees, how-
ever, is found in publicity. The state law usually pro-
vides for the publication each year of the guaranteed and
found analyses of all brands inspected. Not only has
FERTILIZER PRACTICE 559
this proved effective in preventing fraud, but it is really
a great advantage to the honest manufacturer.
The expenses for the inspection and control of fertilizers
are usually defrayed by the license fees, which average
for the different states from ten to twenty dollars a year
for each brand selling for $5 or more a ton. In the
eastern states this fee produces a net return greatly in
excess of the expenses incurred by the fertilizer inspection
and control, and consequently has become the source of
a handsome income for these states.
472. Trade values of fertilizers. It has become cus-
tomary for the authorities charged with fertilizer inspec-
tion and control in the various states to adopt each year
a schedule of the trade values of the various elements as
they appear on the market in unmixed lots. These
values are obtained by averaging all the wholesale prices
of a ton for the various unmixed supplies for the six
months preceding March 1, to which is added 20 per cent
of the price to cover cost of handling. The trade values
for 1912 were as follows : l
TRADE VALUES OF PLANT-FOOD ELEMENTS IN RAW MA-
TERIALS AND CHEMICALS
Cents a pound
Nitrogen in ammonia salts 18|
Nitrogen in nitrates 18|
Organic nitrogen in dry and fine fish, meat, and blood 20
Organic nitrogen in fine bone, tankage, and mixed
fertilizer 19
Organic nitrogen in coarse bone and tankage . . 15
Organic nitrogen in castor pomace and cottonseed
meal 20
1 New York (Geneva), Agr. Exp. Sta., Bui. 371, p. 434. 1913.
560 SOILS: PROPERTIES AND MANAGEMENT
Cents a pound
Phosphoric acid, water-soluble 4|
Phosphoric acid, citrate-soluble (reverted) ... 4
Phosphoric acid, in fine bone, fish, and tankage . 4
Phosphoric acid, in cottonseed meal and castor
pomace 4
Phosphoric acid, in coarse fish, bone, tankage, and
ashes 3|
Phosphoric acid in mixed fertilizers, insoluble in
water or ammonium citrate 2
Potash as high-grade sulfate, in forms free from chlo-
rides, in ashes, etc. 5J
Potash as muriate 4J
Potash as castor pomace and cottonseed meal . . 5
It must be remembered that these prices are seaboard
evaluations, and represent the cost to the manufacturer
of the elements as they exist in the unmixed carrier?.
This is called the commercial evaluation of a fertilizer,
and is the first of a number of items that enter into the
total cost, or the price the farmer must pay on the retail
market. The items that make up this ultimate price
may be listed as follows : (1) wholesale cash cost, or com-
mercial evaluation; (2) cost of mixing; (3) profit of
manufacturers; (4) transportation; (5) storage, com-
mission to agents, bad debts, and so forth ; and (6) profit
of retailer. These additional charges are often sufficient
to double the original commercial value of the fertilize
constituents.
It is evident that by knowing the composition of a fer-
tilizer, and the carriers of the various constituents, the
commercial evaluation of the mixture may be easily cal-
culated. However, what the farmer must pay depends
FERTILIZER PRACTICE 561
to a large extent on the additional charges already listed.
Thus, a fertilizer evaluated at $22 a ton on the New York
market may cost the farmer $35, or even $45, after having
passed through the hands of the manufacturer and the
retail merchant. This commercial evaluation, however,
must not be confused with the agricultural evaluation,
which is the value of the increased crop produced by the
application of the fertilizer. It is evident that the agri-
cultural value will vary with the soil, the crop, or the
season, and may be above or below the total cost accord-
ing to circumstances. In good fertilizer practice, the
excess of the agricultural value over the total cost of the
fertilizer, all costs incidental with the growing, harvest-
ing, and marketing of the increase being first deducted,
should be sufficient to give a handsome profit on the
investment.
473. The buying of mixed goods. The successful
buying of mixed fertilizers on the retail market depends
on two things : (1) the selection of a suitable composi-
tion, with carriers of known value ; and (2) the purchase
of high-grade goods. The farmer who observes these
two points will have at least purchased successfully.
Whether he obtains a profit from the use of the fertilizer
depends on the balancing of a number of factors more or
less variable from season to season.
The selection of a suitable fertilizer, as to carriers and
composition for any particular crop or soil, entails first
of all a study of the guarantee. Should the guarantee
be such as that already cited, a large amount of informa-
tion is at hand concerning the forms of the carriers and
the availability of the important constituents. This
knowledge, properly correlated with the probable needs
of the crop and the soil, will determine whether that
2o
562 SOILS: PROPERTIES AND MANAGEMENT
particular brand should be purchased or not. The real
question here is not the actual quantities of the elements
in a ton of the fertilizer, but their balance among them-
selves. The actual pounds of nitrogen, phosphoric acid,
or potash applied per acre can be governed by the rate
at which the mixture is applied.
The purchase of high-grade goods is the second impor-
tant point to be considered. Data collected from practi-
cally every state show that the higher the grade of the
fertilizer, both as to availability and as to the percentage
of the constituents carried, the greater is the amount of
plant-food obtained for every dollar expended. The
following data, taken from Vermont * for 1909, are the
average of one hundred and thirty brands and are typical
data in this regard :
PS
K
r< a a
W P5
FERTILIZER
tii
i -
15
s
H >
'UTTING $
FERTILIZ
OF FARM
COST (IN CENTS)
OF ONE POUND OF
PI
hi
if
ill
>w
w o
gSjS
s g
ii
W PH O
OQ OH
II
Si 8
N
P 2 5
K 2
^11
(cents)
Low grade
*$13.52
$27.10
$13.58
$1.00
38
7.6
8.5
50.0
Medium
grade .
18.22
30.00
11.78
0.65
31
6.3
7.0
60.6
High grade
26.30
38.93
12.63
0.48
28
5.7
6.3
67.6
It is noticeable at once that the lower the grade of the
fertilizer, the higher is the proportional cost of placing
the goods on the market. In other words, it costs just
1 Hills, J. L., Jones, C. H., and Miner, H. L. Commercial
Fertilizers. Vermont Agr. Col., Bui. 143, pp. 147-149. 1909.
FERTILIZER PRACTICE 563
as much per ton to market a low-grade material as a
high-grade one. This accounts for the fact that the ele-
ments are cheaper per pound in a high-grade mixture,
and that the value of plant-food received for every dollar
expended is greater.
474. Home-mixing fertilizers. In comparing the above
commercial evaluations with the prices actually paid
by the farmer on the retail market, it is found that the
latter shows an increase ranging from 48 to 100 per cent.
This is due to the charges for mixing, transportation, han-
dling, storage, commission, interest on capital, profit,
and other items, made during the passage of the material
from the wholesale dealer to the user. In order to escape
these costs, many farmers have begun the practice of
buying the separate carriers, thus avoiding these charges
except, of course, that of transportation. In many
cases the mixing on the farm costs nothing, as it can be
done in winter when the farm work is not pressing. Even
if the farmer must charge himself with this mixing, it
seldom amounts to more than fifty cents a ton.
As might be expected, this practice has met with much
opposition from manufacturers. In general it is claimed
that the factory goods are more finely ground than those
mixed by the farmer, and . consequently the ready-mixed
goods are not only more uniform but also in better physi-
cal condition. Also, the manufacturer is able to treat
certain materials with acids, and thus increase their
availability. While these reasons are more or less valid,
good results may be expected from a fertilizer even though
it may not be quite uniform, as the soil tends to equalize
this deficiency. Moreover, by screening and by using
a proper filler, a farmer can obtain a physical condition
which will in no way interfere with the drilling of the ma-
564 SOILS: PROPERTIES AND MANAGEMENT
terial. While, obviously, one farmer alone cannot afford
to buy direct from the wholesale dealer because of the
high freight charges on small lots, this objection is being
met by clubs and various organizations whereby the
single carriers may be bought in carload lots.
It is evident that when a farmer mixes his own fertilizer
he is able to obtain not only pure goods, but high-grade-
goods as well, thus reducing freight. Moreover, as a gen-
eral thing home mixing is cheaper than buying the ready-
mixed goods. A quotation from Connecticut l for 1906
illustrates about what this saving may be :
PLANT-FOOD PURCHASED FOR $30
POUNDS
N
POUNDS
PzOs
POUNDS
K*O
TOTAL
Nitrogenous superphosphates
Best quality
73
188
111
372
Least valuable
23
279
53
355
Special manures
Best Quality
69
170
143
382
Lowest quality
32
174
66
272
Home mixtures
Average of all
77
200
168
445
A third point, and by some considered to be more im-
portant than those already discussed, is the educational
value of home mixing. No farmer can mix his own fertil-
izer without becoming familiar with the carriers, their
availability, and their effects. He is forced to study their
influence on the crops more closely, and thus is placed
1 Jenkins, K. H., and Winton, A. L. Fertilizer Report.
Conn. (New Haven) Agr. Expt. Sta., Kept. 1906, Part I, pp.
1-106.
FERTILIZER PE ACT ICE 565
in a position to make changes that will tend to a higher
efficiency of the constituents. The chances are that he
will alter his fertilizer mixture as his rotation progresses
and his soil changes in fertility.
Such arguments do not always mean, however, that
it pays to mix at home. As a matter of fact, in many
cases it does not pay, especially where only a small amount
of fertilizer is needed and it is impossible to cooperate
with other farmers. As a general rule, fertilizers should
be bought by the method that will give the greatest value
for every dollar expended. Farmers often can avail
themselves of the advantage of both systems by asking
for bids from various manufacturers on carload lots of
mixed goods having a certain designated composition.
The farmers in this case designate the carriers as well.
All the advantages of machinery mixing may thus be
gained, with the lower cost which has made home mixing
so popular.
475. Fertilizers not to be mixed. Every farmer who
practices home mixing should keep in mind that there
are certain fertilizers which should not be mixed. This
is due to the fact that a number of materials carry lime
in the oxide, the hydrate, or the carbonate form. This
lime, particularly the caustic forms, may react in three
directions, depending on the fertilizer with which it is in
contact : (1) in setting free ammonia, (2) in causing re-
version of acid phosphate, and (3) in producing a bad
physical condition, especially when in contact with ma-
terials more or less deliquescent. Van Slyke 1 may be
quoted in this regard as follows :
Slyke, L. L. Fertilizers and Crops, pp. 485-486.
New York, 1912.
566 SOILS: PROPERTIES AND MANAGEMENT
Calcium oxide
Calcium hydrate
- Wood ashes
Basic slag
Calcium cyanamid
Basic calcium
nitrate
Calcium oxide
Calcium hydrate
2 Calcium carbonate
Wood ashes
Basic calcium
nitrate
Calcium oxide
Calcium hydrate
Basic calcium
nitrate
ammonium sul-
fate
animal manures,
should not be] as tankage,
mixed with blood, and the
like
nitrogenous
guanos
i, u i. u [soluble phos-
should not be
. i .,, \ phates
mixed with - , . ,
[of any kind
should not be
mixed with
(unless applied
immediately)
sodium nitrate
potassium chlo-
ride
kainit, and the
like
Neither is it wise to allow moist acid phosphate to lie
in contact with large quantities of sodium nitrate, as
nitric acid may be slowly liberated by free sulfuric or
phosphoric acid. Also, large quantities of calcium cyana-
mid should not be mixed with acid phosphate because
of the lime contained in the former. If, however, the
ratio is not greater than one to ten, the results are bene-
ficial, since the reaction, without causing serious rever-
sion of the phosphate, generates enough heat to quickly
season the mixture. The fine and dry condition of the
cyanamid is also conducive to a good mechanical condi-
tion, and accounts for the fact that this material is in
such favor with manufacturers of mixed goods.
FERTILIZER PRACTICE 567
476. How to mix fertilizers. As the various carriers
are bought under guarantee, the percentages of nitrogen,
phosphoric acid, and potash in the ingredients to be mixed
are accurately known. The calculation of the amounts
of each carrier and of the filler necessary to make up a
ton of a fertilizer having a certain formula, then becomes
a matter of simple arithmetic. The mixing is an equally
simple operation. The implements needed in home mixing
are as follows: (1) a tight floor, (2) platform scales,
(3) a sand screen with from three to six meshes to an inch,
(4) a tamper or a grinder, (5) shovels, a rake, and like
tools.
First, the various ingredients, after being crushed and
screened if lumpy, are weighed out in amounts sufficient
for the unit of fertilizer to be mixed at any one time.
The bulkiest material is spread on the floor first and leveled
uniformly by raking. The remaining ingredients are
then spread in thin layers above the first, in the order
of their bulk. Beginning at one side, the material is
next shoveled over, care being taken that the shovel
reaches the bottom of the pile each time. The pile is
then again leveled, and the process is repeated a sufficient
number of times to insure thorough mixing. Sometimes
a mixing machine may be used for this operation. For
storage and general convenience, the fertilizer may be
weighed into sacks of from 100 to 150 pounds capacity
and put in a dry place until needed for use.
A word of caution should be inserted here regarding the
concentration of the mixture. Some farmers, in order
to lessen the work of mixing and application in the field,
raise the percentage of the elements exceedingly high
a condition very likely to occur when high-grade materials
are used. This is bad practice, in that it may interfere
568 SOILS: PROPERTIES AND MANAGEMENT
with germination and may also injure the young plants.
Also, it is likely to result not only in a poor physical condi-
tion but also in uneven distribution, which will bring about
a lowered efficiency of the fertilizer.- The use of sufficient
dry, finely divided filler will obviate such dangers.
477. Factors affecting the efficiency of fertilizers. -
The agricultural value of a fertilizer is necessarily a vari-
able quantity, since, in applying fertilizers, a material
subject to change is placed in contact with two wide
variables, the soil and the crop. The general factors
that govern the effect of fertilizers may be listed as
follows :
1 . Seed, crop, and adaptation of crop to soil. It is quite
evident that different crops will respond differ-
ently to the same fertilizer elements. Also, the
strength of the seed, the management of the crop,
and the adaptation of crop to soil, will be potent
factors in variation.
2. Temperature, sunshine, and rainfall. These factors
are meteorological and, of course, are dominant
in the growth of the plant. Rainfall especially
is important, as an optimum moisture content
is conducive to good plant development. In
general, as shown by experiments in Ohio and
Pennsylvania, the higher the rainfall, the greater
is the efficiency of the fertilizer used.
3. Drainage. This is of great importance in ferti-
lizer practice, since it places the soil in a better
condition from all standpoints for plant growth.
In other words, the better the normal soil condi-
tions, the better should be the reaction from ferti-
lizer application.
FERTILIZER PRACTICE 569
4. Physical condition of the soil. The addition of lime
and organic matter, the utilization of drainage,
tillage, and the like, all are conducive to higher
crop returns through the indirect effect on fertilizer
efficiency.
5. Lime. Lime, by improving physical conditions,
by setting plant-food free, by correcting acidity,
by stimulating bacterial action, and by tending
to eliminate toxic materials either directly or
indirectly, is of great importance in fertilizer
practice. In fact, certain fertilizers, such as
ammonium sulfate and acid phosphate, do not
reach their full efficiency unless plenty of lime is
present.
6. Organic matter. Besides the effect of organic matter
on physical conditions and chemical reactions
which indirectly influence fertilizer action, an im-
portant action is set up by organic matter in the
encouragement of bacterial functions. As the
favorable changes of fertilizers, especially those
carrying nitrogen, is due to biological activity,
the presence of organic materials becomes doubly
important.
7. Chemical composition of the soil. Since the full
return from a fertilizer is derived when the ele-
ments are well balanced, the actual constitution
of the soil becomes a factor, especially when ready
availability is obtainable. Therefore, in choosing
a fertilizer and deciding on the amounts to apply,
the chemical condition of the soil is no mean factor.
While the conditions affecting fertilizer efficiency have
thus been so briefly disposed of, it is evident that a more
570 SOILS: PROPERTIES AND MANAGEMENT
detailed consideration of the question would be not only
interesting but also profitable, would space permit. One
point of broader scope, however, than the addition of a
well-balanced food stimulation, stands out clearly in this
consideration. The necessity of putting a soil in any
given climate into the best possible condition for plant
growth is paramount. This means that drainage, lime,
humus, and tillage, in the order named, must be raised
to their highest perfection. Under such improvements
the further use of commercial fertilizers may or may not
be a paying investment.
478. Method and time of applying fertilizers. The
distribution of the fertilizer by means of machinery is
much more satisfactory than is broadcasting by hand,
as the former method gives a more uniform distribution.
Cereals and other crops are now usually planted with a
drill or a planter provided with an attachment for dropping
the fertilizer at the same time that the seed is sown, the
fertilizer being by this method placed under the surface
of the soil. Broadcasting machines are also used, which
leave the fertilizer uniformly distributed on the surface
of the ground, thus permitting it to be harrowed in suffi-
ciently before the seed is planted, and preventing injury
to the seed by the chemical activity of the fertilizing
material.
Corn planters with fertilizer attachments deposit
the fertilizer beneath the seed, thus avoiding a possible
detrimental contact. Grain drills do not do this, and,
where the amount of fertilizer used exceeds 300 or 400
pounds an acre, it is better to apply it before seeding.
Grass and other small seeds should be planted only after
the fertilizer has been mixed with the soil for several
days. For crops to which large quantities of fertilizers
FERTILIZER PRACTICE 571
are to be added, especially potatoes and garden crops,
it is desirable to drop only a portion of the fertilizer with
the seed, the remainder having been broadcasted by ma-
chinery and harrowed in earlier.
479. Fertilizing crops. Three primary considerations
must be observed in the actual utilization of fertilizers:
(1) the percentage of nitrogen, phosphorus, and potash
suited to the crop and the soil ; (2) the availability of the
carriers; and (3) the amounts to be applied. It is evi-
dent, due to so many factors that are difficult to control,
that fertilizer formulas for different crops on particular
soils are difficult to determine. In fact, such data can
never be more than merely suggestive. Further, the
best quantity of a mixture to apply, even though it is
perfectly balanced, is a figure that can only be approxi-
mated. Probably the largest percentage of the fertilizer
waste that occurs annually can be charged to this factor.
Many farmers make the mistake of applying too much
fertilizer. As a consequence, any information along such
lines can only be suggestive, rather than literal, it being
understood that the general formulas suitable to various
crops, and the quantities ordinarily applied, are subject
to wide variations.
The fact that there are so many mixtures on the market
in this country for the same crops would be rather amus-
ing, did it not so strikingly expose the ignorance of the
manufacturer as well as the gullibility of the public.
Recognizing the need of standard formulas subject to
change according to local conditions, Van Slyke 1 has
offered the following for general use :
Slyke, L. L. Fertilizers and Crops, p. 528. New
York. 1912.
572 SOILS: PROPERTIES AND MANAGEMENT
FERTILIZER FORMULAS FOR GENERAL APPLICATION
CROPS
PERCENTAGE
OP N
PERCENTAGE
OF P 2 Oi
PERCENTAGE
OF K:O
Leguminous
1
8
10
Cereal
3
8
5
Garden . ....
4
8
10
3
6
9
Orchard
2
5
10
Root
3
8
7
While it is recognized that these formulas are probably
far from correct in their application to such groups as
the garden crops, where so many entirely different plants
are concerned, it is felt that they furnish the basis, as
far as our knowledge now extends, for a more economic
fertilization. The variation of such mixtures to suit
specific needs is a part of fertilizer practice.
The carriers largely used for such readily available
mixtures are sodium nitrate, acid phosphate, and potassium
chloride or sulfate. Tankage or blood is often substituted
for sodium nitrate where humus is desirable, while am-
monium sulfate and calcium cyanamid are growing in
popularity. Raw rock phosphate and basic slag are used
rather largely in separate applications, the amounts
being usually larger than with the ordinary fertilizer
materials.
The other phase of fertilizer practice is In the amount
to be applied. With all the groups considered above
except garden and root crops, the applications are rela-
tively light, ranging from 150 to 300 pounds to an acre.
Where excessive vegetative growth is required, as in silage,
the rate may be increased to 500 pounds. In the top-
dressings of meadows or grains, the rate varies from 75
FERTILIZER PRACTICE 573
to 150 pounds an acre. ^ Very often this dressing is sodium
nitrate alone. With g'arden and root crops the amount
of fertilizer applied is very large, ranging from 800 to
sometimes as high as 2000 pounds. The cropping here
is intensive, and the expenditure for fertilization may be
large and yet yield handsome profits.
It must always be remembered that in fertilizer prac-
tice the very high yields obtained under fertilizer stimu-
lation are not always the ones that give the best returns
on the money invested. In other words, the law of
diminishing returns is a factor in the influence of ferti-
lization on crop yield. This relationship is clearly shown
by the curve illustrating the law of the minimum (par.
469), in which the return for each increment of fertilizer
becomes less and less as the total quantity added becomes
greater. It is evident, therefore, that with an excessive
application of any mixture, the returns to an increment
will at last become so small that the increased crop fails
entirely to pay for even the fertilizer, not to mention such
charges as cost of application, harvesting of increased
crop, storage, and the like. The application of moderate
amounts of fertilizer is to be urged for all soils until the
maximum paying dose that may be applied to any given
crop is ascertained by careful experimentation. Over-
fertilization probably accounts for the fact that such a
large proportion of the fertilizers sold to the farmers each
year not only is entirely wasted, but probably in some
cases even becomes detrimental to crop yield.
480. Systems of fertilization. During the evolution
of fertilizer practice, particularly since the early part
of the nineteenth century, a number of systems of apply-
ing fertilizer have been advocated or have been in actual
use. These may be listed as follows :
574 SOILS : PROPERTIES AND MANAGEMENT
1. Single-element system. This^ was one of the first
to be suggested, and was advocated because each
particular crop was supposed at that time to
respond largely to one element. Thus, nitrogen
was supposed to dominate wheat, rye, and oats;
phosphoric acid, to dominate corn, turnips, and
sorghum; and potash to dominate potatoes,
clover, and beans. Present knowledge of the
balancing effects of fertilizers shows this idea to
be fallacious.
2. Abundant supply of minerals. This system had
its origin from the fact that potash and phosphoric
acid are relatively cheap and are slowly leached
from the soil, while nitrogen is expensive and easily
lost. Such a plan, therefore, provides always plenty
of potash and phosphorus, which is to be balanced
each season with sufficient nitrogen to give paying
yields.
3. A system based on the plant-food taken out by the
crop. According to this plan, as much plant-
food is added each year as will probably be taken
out by the plant, this being determined by chemi-
cal analyses. This system overlooks the fact
not only that different plants feed differently on
the same soil, but that the same crop exhibits
marked variability with change of season and
change of soil. Moreover, no allowance is made
for losses by leaching, which are known to equal
at times the losses due to plant growth.
4. Irrational system. This is the plan followed by
many farmers where fertilizers are an important fac-
tor in soil management. The formula is changed
from year to year, in a vain attempt to strike a
FERTILIZER PRACTICE 575
high point in production. The same continual shift
is found in the quantities applied. Too often
the specific brand used is determined by the trade
name that it carries or by the recommendation of
the retail merchant, rather than from a careful
consideration of the guarantee or of the carriers
for each important element. The educational
phase of home mixing should do much to eliminate
this system.
5. Fertilization of the money crop. In trucking or in
general farming operations one crop is usually a
money crop. Naturally its stimulation by heavy
fertilization will pay better than applications to
crops that bring less on the market. The general
plan in this system is to allow the crops following
the money crop to utilize the residuum. When
this residual influence works out, the system is
likely to be a profitable one ; but when the follow-
ing crops fail to respond, the method becomes
wasteful in the extreme.
In the selection of a system that will result in an ef-
fective utilization of fertilizers, only two of the plans de-
scribed above need be considered. In any fertilizer,
phosphoric acid and potash should always be present in
amounts sufficient to more than balance the nitrogen,
since the activity of nitrogen is so pronounced. There-
fore a scheme that calls for an abundance of minerals is
a sound one. This, coupled with the heavy fertilization
of the money crop, does not, however, constitute what
might be considered a rational system, since the crops
that follow may or may not be adequately supplied with
plant-food. Unwise fertilization often leaves the soil,
576 SOILS: PROPERTIES AND MANAGEMENT
as far as its balance is concerned, less able to yield a
paying crop than before. The careful fertilization of the
rotation, then, with special attention to the money crop,
is the only rational system that can ordinarily be employed,
since it not only cares for the crop on the land but also
looks to those that are to succeed. The attention that
must necessarily be paid to the fertility of the soil in such
a system insures the establishment of a soil management
which will ultimately result in a great conservation of
fertility, while at the same time raising the yields and
increasing the prosperity of the farming class.
CHAPTER XXVI
FARM MANURES
OF all the by-products of the farm, barnyard manure
is probably the most important, since it affords a means
whereby the unused portion of the crop, the residue of
the finished farm product, may again be returned to the
soil. This country is now entering on an era in which
the prevention of all waste is becoming more and more
necessary and a nearer approach to a self-sustaining sys-
tem of agriculture far more essential. A clear under-
standing of the composition of farm manure, the changes
it undergoes, and its avenues of loss, and also of methods
for its practical handling, and a realization of its effects
both on soil and on crop, are of vital importance. This
need appeals not only to the practical man but to the
theoretical and technical man as well, for here is a field
in which theory and practice not only meet but widely
overlap.
481. General character and function of farm manures.
- The term farm manure may be employed in reference to
the refuse from all animals of the farm, although, as a
general rule, the bulk of the ordinary manure which ul-
timately finds its way back to the land is produced by
cattle and horses. This arises not only because these
animals consume the greater part of the grain and rough-
age on the average farm, but also because the methods
of handling them make it easier and more practicable to
2p 677
578 SOILS: PROPERTIES AND MANAGEMENT
conserve their excreta. Yard manure generally refers
to mixed manures. The mixing usually occurs during
storage, either for convenience in handling or for the pur-
pose of checking losses and facilitating fermentation.
Thus, horse and cow manures are commonly mixed, since
the too rapid fermentation and probable loss of ammonia
in the former is checked, while at the same time a more
rapid and much more complete decay is encouraged in
the latter.
The ordinary manure consists of two original compo-
nents, the solid and the liquid portion. As these con-
stituents differ greatly, not only in composition but also
in physical properties, their proportions must appreciably
affect the quality of the excreta and its agricultural value.
Litter added for bedding or for adsorptive purposes is
almost always an important factor, for while it prevents
losses of the soluble constituents it may at the same time
lower the value of the product for a unit amount.
Farm manure ordinarily fulfills two functions which
are usually not so simultaneously yet clearly developed
in any other material that of a direct and that of an
indirect fertilizer. Consisting of 73 per cent of water
and only 27 per cent of dry matter, the percentages of
plant-food are necessarily low. As mixed farm manure
contains on the average 1 0.50 per cent of nitrogen, 0.25
per cent of phosphoric acid, and 0.60 per cent of potash,
considerable quantities of plant-food elements are added
in an ordinary application. Ten tons of average manure,
even if only one-half of the nitrogen, one-sixth of the
phosphorus, and one-half of the potash are readily avail-
able, is equivalent to 300 pounds of sodium nitrate, 60
!See Analyses, Storer F. H. Agriculture, pp. 237-248.
New York. 1910.
FARM MANURES 579
pounds of acid phosphate, and 125 pounds of potassium
chloride. This is equivalent to the addition of 485
pounds of an approximately 10-2-12 ready-mixed ferti-
lizer. Moreover, from the fact that so large an amount
of the plant-food carried is not readily available, it acts
as a residuum, which is slowly given up to the succeeding
crops. It has been shown in England * that paying in-
creased returns may be obtained from manure four years
after its application. At Rothamsted, England, 2 a
residual impetus was noticeable on crops forty years after
the soil was manured. This, however, is an exceptional
case.
Farm manure may act as an indirect fertilizer in its
tendency toward improved physical relations. The addi-
tion of organic matter is the vital factor here. Better
tilth, better aeration, improved drainage, and increased
water capacity are the necessary accessories to a rise in
humus content. The influence of manure on the avail-
ability of the mineral constituents of the soil is not the
least of its indirect effects. Even the increased adsorp-
tive power of the soil should be mentioned, in its tendency
toward the counteraction of toxic principles.
Another general characteristic of average farm manure
is that, while it is a fertilizer, it is an unbalanced one.
Proportional very roughly to a 10-2-12 commercial mix-
ture, any one acquainted with general fertilizer practice
can see that it is too high in nitrogen and too low in avail-
able phosphoric acid. The elimination of such a condi-
1 Voelcker, J. A., and Hall, A. D. The Valuation of Unex-
hausted Manure Obtained by the Consumption of Foods by
Stock. London. 1903.
2 Hall, A. D. Fertilizers and Manures, p. 213. New York.
1910.
580 SOILS: PROPERTIES AND MANAGEMENT
tion and a balancing thereby of the plant ration is one
of the many problems that present themselves in the
economic handling and utilization of animal residues.
482. Variable composition of manures. The manure
produced on an average farm is rather likely to vary
markedly in composition and character from time to
time. It may even change radically from one day to
another. There are five general factors that are usually
listed as being responsible for this variation: (1) litter;
(2) class of animal ; (3) individuality, condition, and age of
animal ; (4) food of animal ; and (5) handling of manure.
483. Litter. Perhaps under ordinary circumstances
the amount and character of the litter has as much to
do with the variation in manurial composition as has
any other one factor, if not more. By an increase in
the amount of bedding, the product becomes bulky,
light in weight, and difficult to handle. This is likely
to be the case with manure from livery stables, where
the litter is used to keep the horses clean and not for
purposes of plant-food conservation. That bedding must
also exert a marked effect on chemical composition is
evident from the following analyses :
COMPOSITION OP LITTER
N
P 2 Os
K 2
Sawdust shavings ....
Oat straw
0.10
0.62
0.20
0.20
0.40
1.04
Peat
2.63
0.20
0.17
L6av6s ....
0.65
0.15
0.30
Sawdust and shavings add little of value to the manure
and really act as a diluent. While they are good absorb-
FARM MANURES
581
ents they decompose so slowly as to make them somewhat
objectionable on light soils. Leaves decompose readily,
but add little fertility. Oat straw carries no more nitro-
gen than does average manure, and this nitrogen, like
that of peat or muck, is not readily available as plant-
food. Litter, however, is of such extreme importance as
an adsorbent that the resistant qualities of even such
materials as shavings can be to a degree ignored. Be-
cause of the influence of the bedding on composition,
manure should never be bought unless this phase has
been carefully looked into.
484. Class of animal. The second factor causing
radical variation in the composition of farm manure is
the class of animal by which it is produced. The following
figures, 1 compiled from Ohio, Connecticut, and New York
(at Cornell University), illustrate this point clearly:
PERCENTAGE OP
H 2 O
N
P
K
Horse manure with straw
Cow manure with straw-
62.80
78.00
0.57
0.46
0.12
0.13
0.54
0.36
A working horse on maintenance ration will return in
the manure almost all the nitrogen and minerals taken
as food. In other words, the building-up and the break-
ing-down, or elimination, processes are about equal.
A young fattening pig, on the other hand, will return only
about 85 per cent of the nitrogen received as food and 96
per cent of the mineral material, and a milking cow 75
per cent and 89 per cent, respectively.
1 Thorne, C. E. Farm Manures, p. 89. New York. 1914.
582 SOILS: PROPERTIES AND MANAGEMENT
485. Individuality, condition, and age of animal. -
Various animals differ in capacity, some retaining much
more of the elements contained in the food than do others,
and consequently producing a poorer manure. The
service to which the animal is subjected is also a factor.
A milch cow will certainly utilize more nutriments than
an animal not in that condition. Age is perhaps more
accountable for variation in farm manure than either of
the other two causes. A young animal gaining in muscle
and bone is storing away large quantities of nitrogen,
phosphorus, and potash, and producing a manure corre-
spondingly poorer in these ingredients.
486. Food of animal. Since the animal will retain
only a certain quantity of the food elements, it is reason-
able to suppose that the richer the food, the richer will
be the corresponding excrement, both liquid and solid.
Such has proved to be the case. Wheeler, 1 in studying
the rations of chickens, found the following difference
in the manure produced : -
PERCENTAGE OP
T> f\1X
H 2
N
P
K
Fresh hen manure (nitrog-
enous ration) . . .
59.7
0.80
0.41
0.27
Fresh hen manure (car-
bonaceous ration) . .
55.3
0.66
0.32
0.21
From Ohio, 2 where the production of manure has been
most thoroughly investigated, the following data may be
quoted :
1 Wheeler, W. P. Poultry Feeding Experiments. Kept.
New York (Geneva) Agr. Exp. Sta., No. 8, p. 62. 1889.
2 Thome, C. E., and others. The Maintenance of Fertility.
Ohio Agr. Exp. Sta., Bui. 183. 1907.
FARM MANURES
583
EFFECT OF RATION ON MANURIAL COMPOSITION
PERCENTAGE OF
N
P
K
Corn and mixed hay . . .
1.49
0.23
1.11
Corn, oil meal, and hay
1.55
0.24
1.02
Corn, oil meal, and clover
1.68
0.26
1.04
487. Handling manure. In dealing with a product
of which almost one-half is liquid, there is great danger
that a considerable amount of valuable plant-food will be
lost by leaching. The modification and consequent
lowering of the plant-food value of farm manure is a
vital question in the economic handling of this product.
Next to the litter, lack of care is perhaps the most im-
portant single factor concerned in altering the chemical
composition of manures in general. The influence of
handling is so clearly brought out by the following figures
from Schutt, 1 on mixed horse and cow manure, that further
discussion seems unnecessary. The protected manure in
this case was in a bin under a shed. The exposed sample
was in a similar bin but unprotected from the weather :
Loss AT END OF
Loss AT END OF
Six MONTHS
TWELVE MONTHS
(Percentage)
(Percentage)
Protected
Exposed
Protected
Exposed
Loss of
organic matter
58
65
60
69
Loss of
nitrogen . . .
19
30
23
40
Loss of
phosphoric acid
12
4
16
Loss of
potash . . .
3
29
3
36
1 Schutt, M. A. Barnyard Manure. Canadian Dept. Agr.,
Centr. Exp. Farm, Bui. 31. 1898.
584 SOILS: PROPERTIES AND MANAGEMENT
488. Composition and character of farm manures. -
Although the probable composition of farm manures is so
difficult to state in exact figures, compilations of the
available data have yielded percentages which, while
they demand a most liberal interpretation, afford con-
siderable light regarding the differences that normally
exist between the excrement of various animals. Of
these compilations, Van Slyke's is perhaps the best.
THE COMPOSITION OF FRESH MANURE l
EXCREMENT
PERCENTAGE OF
H 2 O
N
P 2 5
K 2
Solid 80 per cent
75
0.55
0.30
0.40
Horse
Liquid 20 per cent
90
1.35
Trace
1.25
Whole manure
78
0.70
0.25
0.55
Cow
' Solid 70 per cent
Liquid 30 per cent
85
92
0.40
1.00
0.20
Trace
0.10
1.35
Whole manure
86
0.60
0.15
0.45
Solid 67 per cent
60
0.75
0.50
0.45
Sheep
Liquid 33 per cent
85
1.35
0.05
2.10
Whole manure
68
0.95
0.35
1.00
Solid 60 per cent
80
0.55
0.50
0.40
Swine
Liquid 40 per cent
97
0.40
0.10
0.45
Whole manure
87
0.50
0.35
0.40
Since the horse does not ruminate its food, the manure
is likely to be of an open character. It is also a fairly
dry manure, as is that from sheep, the liquid in these two
manures making up 20 and 33 per cent, respectively,
of the whole product. The complete manure from these
two animals contains 78 and 68 per cent, respectively,
1 Van Slyke, L. L. Fertilizers and Crops, p. 291. New
York. 1912.
FARM MANURES 585
of water a considerable contrast to the 86 and 87 per
cent presented by the cattle and swine excrements.
Cattle and swine manures, being very wet, are rather
solid and compact. The air, therefore, is likely to be
excluded to a large degree and decomposition is relatively
slow. They are usually spoken of as cold, inert manures
as compared with the dry, open, rapidly heating excre-
ments obtained from the horse and the sheep.
In every case except that of swine the liquid portion
of the various excrements is much the richer in nitrogen,
containing on the average more than twice as much when
compared on the percentage basis. The liquid is also
richer in potash than the solid, averaging for the four
classes of animals 1.36 per cent as compared to 0.34 per
cent contained in the solid manure. Most of the phos-
phoric acid, however, is contained in the solid excrement,
only traces being found in the urine except in the case
of the swine. It is therefore evident that the liquid
manure, pound for pound, is more valuable in so far as
the plant-food elements are concerned. The advantage
leans heavily toward the urine also in that the constit-
uents therein contained are immediately available; this
cannot be said of the solid manure.
489. Actual plant-food in liquid and solid excrement. -
While the liquid manure carries more nutriments to an
equal weight than the solid, it yet remains to be seen
which actually carries more of the primary food elements.
In general, more solid manure is excreted than liquid,
tending to throw the advantage toward the former in so
far as total food elements are concerned. The following
table, adopted from Van Slyke, 1 bears on this point :
Slyke, L. L. Fertilizers and Crops, p. 295. New
York. 1912.
586 SOILS: PROPERTIES AND MANAGEMENT
DISTRIBUTION OF PLANT-FOOD CONSTITUENTS BETWEEN THE
LIQUID AND THE SOLID OF WHOLE MANURE
EXCREMENT
PERCENTAGE
OF TOTAL
NITROGEN
PERCENTAGE
OF TOTAL
PHOSPHORIC
ACID
PERCENTAGE
OF TOTAL
POTASH
Solid
Liquid
Solid
Liquid
Solid
Liquid
Horse
62
49
52
67
38
51
48
33
100
100
95
88
5
12
56
15
30
57
44
85
70
43
Cow
Sheep
Swine
Average
Average for horse and
cow
57
55
43
45
95
100
5
40
35
60
65
It is seen here that a little more than one-half the
nitrogen, almost all the phosphoric acid, and about
two-fifths of the potash, are found in the solid manure.
Nevertheless this apparent advantage of the solid manure
is balanced by the ready availability of the constituents
carried by the urine, giving it in total about an equal
commercial and agricultural value with the solid excre-
ment. Such figures are suggestive of the care that should
be taken of the liquid manure. Its ready loss of nitrogen
by fermentation, and the ease with which all its valuable
constituents may escape by leaching, should make it
an object of especial regard in handling.
490. Production of manure. A well-fed, moderately
worked horse will produce daily from 45 to 55 pounds
of manure, of which from 10 to 11 pounds is urine. A
cow, on the other hand, having a greater food capacity,
will excrete from 70 to 90 pounds during the same period,
of which from 20 to 30 pounds is liquid. Swine and
FARM MANURES
587
sheep, varying so greatly in weight, will excrete such
widely different quantities that it is difficult and mis-
leading to express the amount based on the individual.
A clearer method of comparison is that employed below,
in which a thousand pounds in weight of animal is made
the basis of the calculation :
MANURE EXCRETED BY VARIOUS FARM ANIMALS TO THE 1000
POUNDS LIVE WEIGHT
ANIMAL
POUNDS
A DAY
TONS A
YEAR
Horse 1
50
9 1
Cow 2 . .
70
12 7
Steer 3
40
7.3
Swine 4
85
155
Sheep 5
34
62
It is quite evident that, for the weight of animal, the
swine and the cow give the heaviest production of manure
on the farm, but it should be remembered also that they
consume the greatest amount of food. Whether these
animals are the most economical in production of manure
must depend largely on age and individuality.
1 Roberts, I. P., and Wing, H. H. On the Deterioration of
Farmyard Manure by Leaching and Fermentation. Cornell
Univ. Agr. Exp. Sta., Bui. 13. 1889. Also, Roberts, I. P.
The Production and Care of Farm Manure. Cornell Univ.
Agr. Exp. Sta., Bui. 27. 1891. Also, Watson, G. C. The
Production of Manure. Cornell Univ. Agr. Exp. Sta., Bui. 56.
1893.
2 Thome, C. E. Farm Manures, p. 97. New York. 1914.
3 Thome, C. E., and others. The Maintenance of Fertility.
Ohio Agr. Exp. Sta., Bui. 183. 1907.
4 Watson, G. C. The Production of Manure. Cornell Univ.
Agr. Exp. Sta., Bui. 56. 1893.
5 Van Slyke, L. L. Fertilizers and Crops, p. 294. New
York. 1912.
588 SOILS: PROPERTIES AND MANAGEMENT
491. Heiden's formulas. Perhaps a better and more
nearly accurate means of calculating the probable pro-
duction of manure is from the food consumed, rather
than from the combined weight of animals kept. Formu-
las have been devised from experimental data in Ger-
many and are designated as Heiden's formulas. 1 From
the amount of absolute dry matter fed and the excrement
produced, Heiden was able to determine certain definite
relationships of the latter to the former. These, of course,
varied for different animals, being 2.10 for the horse,- 3.80
for the cow, and 1.80 for sheep. For example, if a horse
received 20 pounds of dry matter daily, the manurial
production would be 42 pounds. Such formulas are of
particular value on English farms, where the incoming
renter must pay the preceding tenant for the manure
produced on the farm during previous years.
492. Poultry manure. The excrement from poultry
is extremely variable, due to causes that have already
been discussed. In general, this manure is much richer
than that from other farm animals. Storer 2 cites the
following analysis :
COMPOSITION OF POULTRY MANURE
Per cent
Water 0.56
Nitrogen 1.60
Phosphoric acid 1.75
Potash 0.90
Lime 2.25
1 Henry, W. A. Feeds and Feeding, p. 265. Madison
Wisconsin. 1904.
2 Storer, F. H. Agriculture, Vol. 1, p. 613. New York.
1910. Also, Vorhees, E. B. Ground Bone and Miscellaneous
Samples. New Jersey Agr. Exp. Sta., Bui. 84. 1891. Also.
FARM MANURES 589
It is evident that poultry excrement is the most valu-
able manure produced on the farm. It dries readily
and the loss of nitrogen by fermentation is not great.
Because of its great strength farmers are very careful
regarding its application, as injurious effects on the crop
may result. Notwithstanding its great value it probably
receives less care than any other manure produced on the
farm.
493. Commercial and agricultural evaluation of
manures. For purposes of comparison, experimenta-
tion, and sale, farm manures are often evaluated in a way
similar to that used with commercial fertilizers. The
great difficulty here lies in arriving at prices for the im-
portant constituents which are at all comparable with the
value of the manure in the field. The following figures
are calculated from the preceding tables, and show not
only the comparative value of the fresh excrement from
different sources but also what might be considered as
fair prices a ton for the manures. The value of the nitro-
gen is here placed at ten cents a pound, the phosphoric
acid at two and one-half cents, and the potash at four
cents :
Value of
manure
a ton
Swine manure $1.50
Cow manure 1.64
Horse manure . 1.97
Sheep manure 2.87
Poultry manure 4.80
Average of cow manure and horse manure mixed . 1.80
Goessman, C. A. Massachusetts State Exp. Sta., Bui. 37,
1890, and Bui. 63, 1S96.
590 SOILS: PROPERTIES AND MANAGEMENT
This commercial evaluation, of course, must be applied
with care because of the many factors tending to vary
the composition of the excrement. Litter, particularly,
will exert a great influence in this direction. Perhaps a
safe figure as regards the commercial value of manure
as it is likely to be handled on the average farm is about
$1.50 a ton. This approaches more nearly the price that a
market gardener, for example, may pay for such a product.
This commercial evaluation must never become con-
fused with what is known as the agricultural value of a
manure. The former is based on composition, while
the latter arises from the effects as measured in crop
growth. A manure of high commercial value may, when
placed on the soil, yield only a low to medium agricultural
return. This latter evaluation is, of course, the one of
greatest significance in agricultural practice. A very
good example of this might be cited from the Ohio experi-
ments l with manure. In this case both treated and
untreated manures were evaluated commercially and
were then applied to the land. The value of the increased
crops in a three-years' rotation was then calculated in
terms of return to a ton of manure applied :
COMMERCIAL AND AGRICULTURAL EVALUATION OF MANURES
MANURE
COMMER-
CIAL VALUE
AGRICUL-
TURAL
VALUE
Yard in.ciiru.r6 untreated
$1 41
$2 15
Yard manure plus floats
2.04
3.31
Yard manure plus acid phosphate
Yard manure plus kainit
1.65
1.45
3.67
2.79
Yard manure plus gypsum
1 48
2 76
1 Thome, C. E., and others. The Maintenance of Fertility.
Ohio Agr. Exp. Sta., Bui. 183, pp. 206-209. 1907.
FARM MANURES 591
In practice, then, it is this agricultural evaluation which
must be especially watched. Its expression should be
not only in net yield to the acre, but also in net return
to a ton of manure applied.
494. The fermentation of manure. 1 During the
processes of digestion the food of animals becomes more
or less decomposed and decayed. This condition comes
about partly because of the digestive processes and
partly from the bacterial action that takes place, largely
in the lower intestines. Of these two influences within
the animal, bacterial activities are probably of the greater
importance as far as the breaking-up of the complicated
foodstuffs is concerned. The fresh excrement, then, as
it comes from the stable, consists of decayed or partially
decayed plant materials, with a certain amount of broken-
down animal tissue and mucus. This is more or less
intimately mixed with litter and the whole mass is wetted,
or moistened, with the liquid excrement, carrying, as it
does, considerable quantities of soluble nitrogen and
potash. This mass of material, ranging from the most
complex compounds to the most simple, is teeming with
bacteria, especially those that function in decay and putre-
faction. The number very often runs into billions to a
gram of excrement. In such an environment it is of
little wonder that biological changes go on so rapidly.
Although so many different groups of organisms live
and function in manure, and although so many products,
both simple and complex, are continually being split
off, for convenience and simplicity the bacteria may be
1 Good discussions may be found as follows : Lipman, J. G.
Bacteria in Relation to Country Life, pp. 303-356. New York.
1911. Hall, A. D. Manures and Fertilizers, pp. 184-210.
New York. 1910.
592 SOILS: PROPERTIES AND MANAGEMENT
grouped under two heads, aerobic and anaerobic. The
former work in the presence of oxygen, the latter when
air is either lacking or only very slightly present. This
grouping is not a distinct one by any means, as many
organisms may function not only in air but also when
oxygen is lacking. The products, however, are as dif-
ferent under these two conditions as if they arose from
distinct organisms.
495. Aerobic action. When manure is first produced
it is likely to be rather loose, and if allowed to dry at
once it becomes well aerated. The first bacterial action
is therefore likely to be rather largely aerobic in nature.
Transformations are very rapid and are accompanied by
considerable heat, ranging from 100 to 150 F. and some-
times higher. This action falls largely on the simple
nitrogenous compounds. Urea is principally affected,
and will very quickly disappear from well-aerated manure.
The reaction is as follows :
CON 2 H 4 + 2 H 2 = (NH 4 ) 2 CO 3
The ammonium carbonate is a volatile compound, and
on the least exposure and evaporation of the manurial
liquids it changes into ammonia and carbon dioxide.
Thus nitrogen may be rapidly lost from manure by the
unwise allowing of excessive aerobic decay and decom-
position to proceed.
This complex group of aerobic putrefactive organisms
also attack to a certain extent the more complicated ni-
trogenous compounds, as well as some of the simpler car-
bohydrates contained in the solid and the liquid portions
of the manure. More carbon dioxide therefore results,
as well as certain simplified products which ultimately
may be reduced to such a form as to be available as plant-
FARM MANURES 593
food. In other words, the whole mass of the manure
tends to simpler forms. The mass becomes decayed,
humus is produced, and available plant-food is evolved.
496. Anaerobic action. As the manure becomes
compacted, especially if it is left moist, oxygen is grad-
ually excluded within the heap and its place is taken by
carbon dioxide, which is given off during the process of
any form of bacterial activity. The fermentation now
changes from aerobic to anaerobic, it becomes slower, and
the temperature falls to as low as 80 or 90 F. New
organisms may now function, and even some of the same
ones that were active under aerobic conditions may con-
tinue to be effective. The process is now a deep-seated
one and the products become changed to a considerable
degree. Carbon dioxide, of course, continues to be evolved,
but instead of ammonia being formed the nitrogenous
matter is converted into the usual putrefactive products,
such as indol, skatol, and the like. The carbonaceous
matter is resolved into numerous hydrocarbons, of which
methane (CH 4 ) is prominent; and as a by-product of
the breaking-down of the proteins, hydrogen sulfide
(H 2 S) and sulfur dioxide (SO 2 ) are evolved. The com-
plex nitrogenous and carbohydrate bodies are attacked
with the splitting-off, not only of simpler materials, but
often of those more complex. Such compounds may be
listed in general as organic acids and humic bodies.
They of course ultimately succumb to simplification.
497. Fermentation in general. In any process of
fermentation, acids tend to form which if not neutralized
will render the mass acid and impede bacterial activity.
This occurs when the solid excrement decomposes alone.
The liquid manure, however, is alkaline and will tend
to correct any acidity due to fermentation. The advan-
2 Q
594 SOILS: PROPERTIES AND MANAGEMENT
tage of either handling the liquid and the solid together,
or pumping the liquid over the solid at intervals, is there-
fore apparent.
The general changes in any manure pile can readily
be recapitulated. First is the aerobic action, with escape
of ammonia and carbon dioxide. Next the manure is
wetted, it compacts, and the slow, deep-seated decay
sets in with a simplification of some compounds, with
the production of acids, and with a gradual formation
of humic materials. As the manure becomes alternately
wet and dry, the two general processes may follow each
other in rapid succession, the anaerobic bacteria attack-
ing the complex materials, the aerobic affecting both the
complex and the simpler compounds. Carbon dioxide
is given off continuously during the process.
498. Gases from manure. The changes in the
composition of the gases drawn from wet and compact
manure, as compared with those from the same pile dry
and open, are well shown from results by Deherain. 1
The pile in this experiment was about eight feet high :
COMPOSITION OF GASES FKOM DRY AND MOIST MANURE
PERCENTAGE OP
MAN
CO 2
O 2
CHi
N
[Top
7.2
7.0
0.0
85.8
Dry manure
Middle
14.5
4.7
1.3
79.5
I Bottom
50.8
0.0
49.2
0.0
Wet and
[Top
42.7
1.1
52.4
9.8
compact
Middle
49.8
0.0
48.3
2.2
manure
[ Bottom
47.8
0.0
51.2
1.0
1 Hall, A. D. Fertilizers and Manures, p. 188. New York. 1910.
FARM MANURES
595
'It is noticeable that nitrogen ceases to be lost under
anaerobic conditions, but the production of methane is
much increased. Carbon dioxide is present at all times.
499. Change of bulk and composition of rotting manure.
- Because of the great loss of carbon dioxide during the
fermentation processes, there is a considerable change in
bulk of the manure. Fresh excrement loses 20 per cent
in bulk by partial rotting, 40 per cent by more thorough
rotting, and 60 per cent by becoming completely decom-
posed. This means that 1000 pounds of fresh manure
may be reduced to 800, 600, or 400 pounds, according
to the degree of change it has undergone.
Although considerable loss of nitrogen may have oc-
curred through aerobic bacterial action, and although
both nitrogen and the minerals may have been consider-
ably leached away, the loss of carbon dioxide is so much
greater that generally there is an actual percentage in-
crease of the former constituents in the well-rotted prod-
uct. This relationship is well shown by figures from
Wolff, 1 in which the samples were compared on the basis
of equal amounts of dry matter :
COMPOSITION OF FRESH AND DECOMPOSED MANURE
FRESH
(Per cent)
ROTTED
(Per cent)
Ash
381
476
Nitrogen
039
049
Potash
0.45
0.56
Lime
049
061
Magnesia
12
15
Phosphoric acid
18
023
Sulfuric acid
10
13
1 Aikraan, C. M. Manures and Manuring, p. 288.
burgh and London. 1910.
Edin-
596 SOILS: PROPERTIES AND MANAGEMENT
It must be remembered, however, that this is only a
general case and holds good only when the manure has
had fairly careful attention. When the manure has been
improperly handled, the soluble constituents may be lost
as soon as formed and a rotted product may result which
is very low in nitrogen, potassium, and phosphorus. It is
therefore evident that the handling of the fresh manure is
a controlling factor in the ultimate value of the product.
A further insight into the condition of rotted manure
is given by Voelcker, 1 the data being calculated to a dry-
weight basis :
FRESH
MANURE
(Per cent)
ROTTED
MANURE
(Per cent)
Soluble organic matter
7 33
15 09
Soluble inorganic matter
Insoluble organic matter
4.55
76.14
5.98
51.34
Insoluble inorganic matter
11 98
27 59
These figures show the increased soluble matter in
well decomposed manure and emphasize the value of
rotting. The great loss of organic matter through the
giving-off of carbon dioxide is also evident.
500. Fire-fanging of manure. A change of a fermenta-
tive nature which sometimes takes place in loose and well-
dried manure is fire-fanging. Many farmers consider
this to be due to actual combustion, as the manure is very
light in weight and has every appearance of being burned.
This condition, however, is produced by fungi instead
of bacteria, and the dry and dusty appearance of the
1 Halligan, J. E.
ton, Pennsjdvania.
Soil Fertility and Fertilizers, p. 67.
1912.
Eas-
FARM MANURES
597
manure is due to the mycelium, which penetrates in all
directions and uses up the valuable constituents. Manure
thus affected is of little value either as plant-food or as a
soil amendment.
501. Waste of farm manures. Any system of agri-
culture, in order to be permanent, must arrange for the
addition of as much plant-food as is removed in the crop
and the drainage water combined. Even if all of the
crop were returned to the soil, a permanent system of
agriculture would fall far short of being established, since
at least as much plant-food is removed by leaching as by
cropping. As a matter of fact, it is not even possible to
return to the land as farm manure all the constituents
taken off in the crop, due to the ease with which loss occurs.
These losses may be grouped under two general heads :
(1) those that occur as the food passes through the animal ;
and (2) those that are due to leaching and fermentation.
502. Losses due to digestion. A certain quantity of
material is necessarily taken from the original food as it
passes through the animal. This loss falls most heavily
on the organic matter and only slightly on the mineral
constituents. Wolff * presents the following figures aver-
aged from all classes of animals :
PERCENTAGE OF ORIGINAL FOOD CONSTITUENTS RECOVERED
IN FRESH MANURE
SOLID
MANURE
LIQUID
MANURE
TOTAL
Organic matter .
Nitrogen
42.5
40 1
3.4
472
45.9
87 3
Minerals
59.7
39.0
98.7
1 Aikman, C. M. Manures and Manuring, pp. 228 and 232.
Edinburgh and London. 1910.
598 SOILS: PROPERTIES AND MANAGEMENT
It is to be noted that the organic matter of the food
has sustained an average loss of about 55 per cent, while
the loss of nitrogen and of minerals has been 13 per cent
and 2 per cent, respectively. The loss of the organic
matter is especially serious, although it can be replaced by
using green manures and the practicing of a proper rotation.
The loss of nitrogen can be replaced only by the growing
of legumes or by the addition of a nitrogenous fertilizer.
503. Losses due to leaching and fermentation. As
about one-half of the nitrogen and two-thirds of the potash
in farm manures is in a soluble condition, the possibility
of loss by leaching is very great, especially where the
manure is exposed to heavy rainfall. The loss of phos-
phorus is also of some consequence. In addition, the
fermentation, especially that of an aerobic nature, will
cause the formation of ammonia, which may be lost in
large quantities if steps are not taken to control such
action. It is evident that losses by leaching may be
checked considerably by protecting the manures from
excessive rainfall and by providing tight floors in the
stable or an impervious bottom in the manure pit or
under the manure pile. Packing and moistening the
manure will change the aerobic fermentation to anaerobic,
thus reducing very markedly the production of ammonia
while allowing a simplification of the manurial compounds
to proceed steadily. All wise methods of handling and
storing manures provide against these losses through
leaching and fermentation by protecting the manure from
rain and by controlling fermentation through moisten-
ing and compacting.
It is very difficult, in quoting figures for waste of
manure, to separate the losses due to leaching from those
due to fermentation. The two processes go on simul-
FARM MANURES
599
taneously, and the loss from one source is dependent, to
a certain extent, on the other. It is only the nitrogen,
however, that may be lost by both fermentation and
leaching, the minerals being wasted only through the
latter avenue. A few figures regarding the losses to
manures when exposed to atmospheric conditions may
not be amiss at this point :
LOSSES FROM MANURE THROUGH LEACHING AND
FERMENTATION
NEW
YORK 1
NEW
YORK 1
CANADA 2
NEW
YORK 1
NEW
JERSEY 3
(Aver-
age for
eight
years)
OHIO*
Kind of Manure
Horse
Horse
Horse
Cow
Cow
Steer
Time exposed (days)
Loss of nitrogen (per-
centage) ....
Loss of phosphoric acid
(percentage) . . .
Loss of potash (percent-
age)
183
36
50
60
183
60
47
76
274
40
16
34
183
41
19
8
77
31
19
43
91
30
23
58
It seems evident that when manure is exposed to at-
mospheric agencies, even under the best conditions, the
losses of nitrogen, phosphoric acid, and potash will be
on the average 45, 30, and 50 per cent, respectively.
1 Roberts, I. P., and Wing, H. H. On the Deterioration
of Farmyard Manure by Leaching and Fermentation. Cornell
Univ. Agr. Exp. Sta., Bui. 13. 1889.
2 Schutt, M. A. Barnyard Manure. Canadian Dept. Agr.,
Centr. Exp. Farms, Bui. 31. 1898.
3 Thorne, C. E. Farm Manures, p. 146. New York. 1914.
4 Thorne, C. E., and others. The Maintenance of Fertility.
Ohio Agr. Exp. Sta., Bui. 183. 1907.
600 SOILS: PROPERTIES AND MANAGEMENT
Under conditions on the average farm such losses may
easily rise to 50 per cent of all the constituents, and prob-
ably very much higher as regards nitrogen and potash.
From one-half to three-fourths of the important elements
contained in the original food fails to again reach the
land. Hall, 1 quoting from Woods' experiments at Cam-
bridge, shows that about 10 per cent of the nitrogen in
the food consumed is retained by the animal. He also
shows that 15 per cent of nitrogen is lost during the making,
and from 10 to 25 per cent during the storage, of the ma-
nure, even under the best conditions. This gives a total
loss of nitrogen amounting to from 35 to 50 per cent. If
this is the loss under the best conditions, it can readily
be seen that the loss on an average farm must approach
65 or 75 per cent.
Some idea as to separate losses from fermentation and
leaching may be gained from data drawn from Canada. 2
In this experiment a mixture of horse dung and cow dung
was divided. One-half was placed in a bin under a shed ;
the other half was exposed to the weather, outside in a
similar bin. After a year the two portions were analyzed
and the losses computed :
LOSSES FROM MANURE AFTER TWELVE MONTHS
PROTECTED
(Per cent)
UNPROTECTED
(Per cent)
Loss of organic matter
Loss of nitrogen
60
23
69
40
Loss of phosphoric acid
Loss of potash
4
3
16
36
1 Hall, A. D. Fertilizers and Manures, p. 198. New York.
1910.
2 Schutt, M. A. Barnyard Manure. Canadian Dept. Agr.,
Centr. Exp. Farm, Bui. 31. 1898.
FAHM MANURES 601
Evidently the losses by fermentation are very consider-
ably augmented by exposure, especially if the rainfall
is high. This waste not only is very considerable as
regards the nitrogen, but is especially high as far as the
organic matter is concerned. Such figures serve also
to emphasize again the importance of shielding manure
in storage from excessive rainfall. Some water is, of
course, necessary, but too much serves only to carry
away the materials already soluble or rendered soluble
by fermentation.
504. Increased value of protected manure. From
the previous discussion it is evident that a well-protected
and carefully preserved manure will be higher in plant-
food constituents than one not so handled. Moreover,
the agricultural value of such manure will be higher.
This is shown by actual tests from Ohio. 1 Over a period
of fourteen years, in a three-years' rotation of corn, wheat,
and hay, a stall manure gave a yield 30 per cent higher
than that with a yard manure, the quantities applied in
each case being equal. In New Jersey, in comparing
fresh manure with leached manure the former showed a
gain in crop yield 53 per cent higher than the latter over
a period of three years immediately following the appli-
cation. Such figures are worthy of careful considera-
tion by the average farmer.
505. The money waste of manure. To make the
seriousness of the question of waste in manures more
striking, the probable losses may be calculated in money
value for the United States. The entire live stock of all
kinds in this country may be roughly calculated as equiv-
1 Thome, C. E., and others. Plans and Summary Tables
of the Experiments of the Central Farm. Ohio Agr. Exp. Sta.,
Circ. 120. 1912.
602 SOILS: PROPERTIES AND MANAGEMENT
alent in manure producing capacity to about 100,000,000
cattle, each weighing 1000 pounds. Assuming that
each animal will produce manure to the value of $21
a year and that the cattle are yarded for four months,
the total value of excrement produced during the yarding
period would be, in round numbers, $700,000,000. If
only one-third of the value of the manure is lost by mis-
handling, an annual waste of $233,000,000 would occur.
This is a very conservative estimate regarding the losses
of farm manure throughout the United States. The an-
nual sale of commercial fertilizers in this country, prob-
ably amounting to over $100,000,000, is entirely inade-
quate to replace this loss.
506. Handling of manures. l The ultimate considera-
tion in a study of farm manures comprises the best
methods of economic handling, both as to labor and as to
the saving of the constituents carried by the product. The
greater the amount of plant-food that can be saved in
the manure and returned to the land, the less will be the
necessity of commercial sources of these elements. Many
methods present themselves as being more or less effica-
cious, but none are absolutely perfect, as losses by fer-
mentation are bound to occur even though leaching is
entirely prevented. Methods of handling are usually
chosen because of their adaptability to particular cir-
cumstances, rather than because of the exact amount
of valuable constituents that they will conserve.
1 Good discussions of handling farm manure are as follows :
Hart, E. B. Getting the Most Profit from Farm Manure.
Wisconsin Agr. Exp. Sta., Bui. 221. 1912. Beal, W. H. Barn-
yard Manure. U. S. D. A., Farmers' Bui. 192. 1904. Roberts,
I. P. The Fertility of the Land, Chapter IX, pp. 188-213.
New York. 1904.
FARM MANURES 603
507. Care of manure in the stalls. Considerable loss
to manure occurs in the stable, due to fermentation and
leaching. Before the litter can absorb the liquid, it
is likely to ferment and to leach away in exceptional
amounts. Therefore the first care is as to bedding, which
should be chosen for its absorptive properties, its cost,
and its cleanliness. The following table 1 expresses the
absorptive capacity of some common litters :
ABSORPTIVE POWER OF BEDDING FOR WATER
Per cent
Wheat straw 220
Oak leaves 162
Peat 600
Sawdust 435
Spent tan 450
Air-dry humous soil 50
Dry peat moss 1300
Muck 200
The amount of litter to be used is determined by the
character of the food. If the food is watery, the bedding
should be increased. In general, the litter may amount
to about one-third of the dry matter of the food consumed.
Sheep require about a pound of bedding a head, cattle
from eight to ten pounds, and horses from six to seven
pounds. Xo more litter than is necessary to keep the
animal clean and to absorb the liquid manure should be
used, as the excrement is thus diluted unnecessarily with
material which often does not carry large quantities of
fertilizing ingredients.
1 Deal, W. H. Barnyard Manure. U. S. D. A., Farmers'
Bui. 192. 1904.
604 SOILS: PROPERTIES AND MANAGEMENT
The next care is that floors shall be tight, so that free
liquids cannot drain away but will be held in contact
with the absorbing materials. The preserving of manures
in stalls with tight floors has been for years, a common
method of handling dung in England. The trampling
of the animals, and the continued addition of litter with
the liquid and solid excrement, explain the reason for
the success of the method. The following data, from
Ohio, 1 show the relative recovery of food elements in
manure produced on a cement floor and on an earth floor,
respectively. The experiment was conducted with steers
over a period of six months.
RECOVERY OF FOOD ELEMENTS IN MANURE PRODUCED
ON CEMENT FLOOR; ON EARTH FLOOR
PER CENT
PER CENT
Nitrogen
Phosphorus
74.7
775
62.4
789
Potash
87.8
78.4
508. Hauling directly to the field. Where it is pos-
sible to haul directly to the field, this practice is to be
advised, since opportunities for excessive losses by leach-
ing and fermentation are thereby prevented. Manure
may even be spread on frozen ground or on the top of
snow, provided the land is fairly level and the snow is
not too deep. This system saves time and labor, and
when leaching does occur the soluble portions of the ma-
nure are carried directly into the soil.
509. Cement pit. Very often it is not convenient
1 Thorne, C. E. The Maintenance of Fertility. Ohio
Agr. Exp. Sta., Bui. 183, p. 199. 1907.
FARM MANURES 605
nor possible, especially in certain parts of the year, to
haul manure directly to the field. Means of storage
must therefore be provided. Some farmers, if the amount
of manure produced on their lands is large, find it prof-
itable to construct manure pits of concrete. These
storage pits are usually rectangular in shape, with a shed
covering, and with open ends so that a team may drive
in at one end and out at the other. In such a pit leaching
is prevented by the covering and by the solid bottom.
By keeping the manure carefully spread and well mois-
tened, fermentation may proceed with a minimum loss
of nitrogen. Some dairymen even go so far as to utilize
a cistern, into which is shoveled both the liquid and the
solid manure. Later, when fermentation has proceeded
sufficiently, the material is pumped out and applied to
the land. This method is not to be advocated in this
country except under particular conditions.
510. Covered barnyard. Another method of storage
is by means of a covered barnyard. Such a yard must
have an impervious bottom. The manure is spread out
in the yard, and if animals are allowed to exercise here
the manure is kept thoroughly packed as well as damp.
The storage of manure in deep stalls, a favorite method
in England, is similar to this system and has been shown
to be very economical. It also affords an opportunity
for the mixing of the manure from different classes of
animals. The desirability of this has already been shown
regarding horse and cow excrements. The advantages
of trampling, as far as the keeping qualities of manure
are concerned, are clearly shown by the following figures
taken from the work of Frear : 1 -
1 Frear, W. Losses of Manure. Pennsylvania Agr. Exp
Sta., Bui. 63. 1903.
606 SOILS: PROPERTIES AND MANAGEMENT
Loss OF MANURE IN COVERED SHEDS
PERCENTAGE OF
N
K 2
PjOs
Covered
Covered
and trampled . . .
and untrampled .
5.7
34.1
5.5
19.8
8.5
14.2
Throwing manure in heaps under a shed and allowing
hogs to work the mass over, is an economical practice so
far as food utilization is concerned. It interferes, how-
ever, with proper and economical packing of the manure.
The question to be decided is whether the added food
value of the manure overbalances the extra losses by
fermentation incurred by the rooting of the swine.
511. Piles outside. Very often it is necessary to
store manure outside, fully exposed to the weather.
When this is the case, certain precautions must be ob-
served. In the first place, the pile should be located on
level ground far enough from any building so that it
receives no extra water therefrom in times of storm. The
earth under the pile should be slightly dished in order
to prevent loss of excess water. If possible, the soil
of the depression should be puddled, or, better, lined with
cement.
The sides of the heap should be perpendicular, so as
to shed water readily. The manure must be kept moist
in dry weather in order to decrease aerobic action. Each
addition of manure should be packed in place, the fresh
on and above the older. This allows the carbon dioxide
from the well-rotted dung to pervade the fresher and
looser portions, thus quickly establishing the aerobic
FARM MANURES 607
conditions so essential to economic and favorable fer-
mentation.
Placing fresh manure in small heaps in the field to be
spread later, is, in the first place, poor economy of labor.
Moreover, it encourages loss by fermentation, while at
the same time the soluble portions of the pile escape into
the soil immediately underneath. There is thus a poor
distribution of the essential elements of the dung, and
when the manure is finally spread, an overfeeding of
plants at one point and an underfeeding at another results.
A low efficiency of the manure is thus realized. This
method of handling manure is not to be recommended.
512. Distribution of manure in the field. In the
actual application of manure to the land, certain general
principles should always be kept in mind. In the first
place, evenness of distribution is to be desired, since it
tends to raise the efficiency of the manure by encouraging
a more uniform plant growth. This evenness of spread-
ing is much aided by fineness of division. Moreover, it
is generally better, especially in diversified farming on
medium to heavy soils, to decrease the amounts at each
spreading and apply oftener. Thus, instead of adding
20 tons to the acre, 10 tons would be applied and twice
as much area covered. The applications would then
be made oftener. A larger and quicker return in net
crop yield per ton of manure applied would be realized.
This has been strikingly shown by the Ohio experiments 1
over a test for eighteen years in a three-years rotation of
wheat, clover, and potatoes, the manure being placed on
the wheat and affecting the clover and the potatoes as a
1 Thorne, C. E., and others. Plans and Summary Tables
of the Experiments at the Central Farm. Ohio Agr. Exp.
Sta., Circ. 120, p. 108. 1912.
608 SOILS: PROPERTIES AND MANAGEMENT
residuum. The results are expressed in yield per ton of
manure applied :
YIELD TO THE TON OF MANURE WHEN APPLIED IN
DIFFERENT AMOUNTS
WHEAT
(Bushels)
CLOVER
(Pounds)
POTATOES
(Bushels)
4 tons to
the acre ....
8.0
177
37.3
8 tons to
the acre . . .
4.1
150
19.4
16 tons to
the acre ....
2.4
99
11.6
Not only is the increased efficiency from lower appli-
cations apparent, but a great recovery of the manurial
fertility in the crops also results, The Ohio experiments
have shown that in the first rotation after the manure
is applied, a recovery may be expected from a treatment
of 8 tons 25 to 30 per cent higher than from one of
16 tons.
Evenness of application and fineness of division are
greatly facilitated by the use of a manure spreader. This
also makes possible the uniform application of small
amounts of manure, even as low as five or six tons to the
acre. It is impossible to spread so small an amount by
hand and obtain an even distribution. Moreover, a
spreader lessens the labor and more than doubles the
amount of manure one man can apply a day. When
any quantity of manure is to be handled, a manure spreader
will pay for itself in a season or two at the most.
Whether manure should be plowed under or not depends
largely on the crop on which it is used. On timothy it
is spread as a top-dressing. Ordinarily, however, it is
plowed under. This is particularly necessary if the
FARM MANURES 609
manure is long, coarse, and not well rotted. It should
not be turned under so deep, however, as to prevent ready
decay. If manure is fine and well decomposed, it may
be harrowed into the surface soil. The method employed
depends on the crop, the soil, and the condition of the
manure. The amount to be applied varies consider-
ably. Eight tons to the acre would be a light dressing,
15 tons a medium dressing, and 25 tons heavy for an
ordinary soil. On trucking lands, however, as high as
50 or 100 tons is often used.
513. Reinforcement of manure. The reinforcement
of farm manures is designed to accomplish two things in
the handling of this product : (1) checking loss by leaching
and fermentation, and (2) balancing the manure and
rendering its agricultural value higher. Four chemicals
may be used in this reinforcement : gypsum (CaSO*),
kainit (KC1, mostly), acid phosphate
CaSO 4 ), and floats (raw rock phosphate,
Gypsum is supposed to act on the ammonia, changing
it to ammonium sulfate, a stable compound. It is rather
insoluble, however, so that its action is slow. It may be
applied in the stable or on the manure pile. The rate
is about 100 pounds to the ton of manure. It has no
balancing effect.
Kainit is added to react with any ammonia that may
be produced and also to increase the potash in the manure.
It is soluble, and because of its caustic tendencies it must
not come into contact with the feet of the animals. It
must not be spread on the manure, therefore, until the
stock has been removed. Since manure is unbalanced
as to phosphorus, the agricultural value of this reinforce-
ment is likely to be slight. Kainit is usually added at
the rate of 50 pounds to the ton of manure.
