MAIN LIBRARY AGRIC. DEPT.
Ube IRural TTert^oofe Series
Edited by L. H. BAILEY
SOILS
THEIR PROPERTIES AND MANAGEMENT
ftfje l&ural EexMSoofc Series
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, Fippin and Backman, 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.
Widtsoc, Principles of Irrigation Prac-
SOILS
THEIR PROPERTIES AND MANAGEMENT
BY
T. LYTTLETON LYON, Ph.D.
PROFESSOR OF SOIL TECHNOLOGY, CORNELL UNIVEBSITY
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
Nefo gork
THE MACMILLAN COMPANY
1916
All rights reserved
36^
^
si?*:
COPYRIGHT, 1909 AND 1915,
By THE MACM1LLAN COMPANY.
Set up and electrotyped. Published September, 1915. Reprinted
September, 1915; January, August, 1916.
TSTortoooti $wss
J. S. Cushing 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 wisli 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.
3402 LO
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.
V
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 — Hydration, 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
The 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, 36 — The extension of the ice sheet,
37 — The ice as a soil builder, 38 — Glacial till soils, 39
viii TABLE OF CONTENTS
PAGES
— Composition of glacial soils, 40 — Humus of glacial
soils, 41 — Glacial lakes, 42 — Lacustrine soils — glacial
lake, 43 — Lacustrine soils — recent lake, 44 — iEolian
soils, 45 — Loess soils, 46 — Distribution of loess, 47 —
Adobe soils, 48 — Sand dunes, 49 — Volcanic dust, 50.
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 —
N^ Soil and subsoil of humid regions, 61 — Soil and subsoil
of arid regions, 62.
CHAPTER VI
The 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
Some Physical Properties of the Soil .... 108-125
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 of the original material on
the resultant humus, 100 — Effects of organic matter on
soil, 101 — Maintenance of soil organic matter, 102.
126-152
CHAPTER IX
The Colloidal Matter of Soils .....
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.
153-169
CHAPTER X
Soil 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,
X TABLE OF CONTENTS
PAGES
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
The Forms or Soil Water and their Movement . . 198-242
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.
TABLE OF CONTENTS xi
CHAPTER XII
paqss
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, 175 — 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 of 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 — Resume" of mulch
control, 196 — Water saved by a mulch, 197 — Effect of
mulehes 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, 220 — Heat supply and its effects, 227
— Control of soil temperature, 228.
CHAPTER XV
\J
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 xiii
CHAPTER XVI
PAGES
The Absorptive Properties of Soils .... 349-374
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, 251) — 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, 275 — Drainage rec-
ords at Rothamsted, 276 — Drainage records at Brom-
berg, 277 — Losses of nitrogen and calcium, 278 —
Composition of surface water, 279.
CHAPTER XVII
Acid or Sour Soils 376-390
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 nourish 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
XIV
TABLE OF CONTENTS
indicators of acidity, 298 — Quantitative determinations
of acidity, 299 — Potassium nitrate method, 300 — Lime-
water method, 301 — Resume, 302.
CHAPTER XVIII
Alkali Soils .........
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.
391-403
CHAPTER XIX
Absorption of Nutritive Salts by Agricultural Plants
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, 331 — 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.
404-420
TABLE OF CONTENTS XV
v
CHAPTER XX
PAGES
Organisms in the Soil 421-442
Macrooryanisms.
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 —
Distribution of bacteria, 351 — Numbers of bacteria, 352
— Numbers as influenced by season, 353 — Conditions
affecting growth, 351 — Oxygen, 355 — Moisture, 356 —
Temperature, 357 — Organic matter, 358 — Soil acidity,
359 — Functions of soil bacteria, 300 — 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
The Nitrogen Cycle 443-474
Decay and putrefaction, 365 — Ammonification, 366 —
Bacteria and substances concerned in ammonification,
367 — Nitrification, 3(58 — 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
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.
•
475-488
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 xvil
PAGES
— 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 534-545
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 —
xviii TABLE OF CONTENTS
PAGES
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 577-618
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
TABLE OF CONTENTS xix
PAGES
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 — Resume^ 521.
CHAPTER XXVII
Green Manures 619-626
Effects of green-manuring, 522 — Quantities of plant
constituents added by green-manuring, 52:} — Decay of
green manure, 624 — 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.
CHAPTER XXVIII
Land Drainage 627-662
Extent of drainage needed in humid regions, 531 —
History of drainage, 532 — 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, 539 —
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, 550 — Silt basins, 557 — Surface intakes,
558 — 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\sum£,
564.
CHAPTER XXIX
Tillage
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, 673 — Depth and
width of furrow, 574 — Plow sole, 575 — Hillside plow,
576 — Covering rubbish, 677 — 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.
663-681
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, 014 — 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/ : -SOJLS; .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,
1915.
4 SOILS: PBOPERTIES AND MANAGEMENT
Elements obtained from Elements coming directly from l
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 (C0 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, while 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
partially or wholly decayed organic matter present therein.
It is utilized 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
(Ca3(P04)2). 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 (P0 4 ) 2 ).
The potassium of the soil exists largely in feldspar
(K 2 . A1 2 C>3 . 6 Si0 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 1 estimates the composition
of the lithosphere, which makes up 93 per cent of the
known terrestrial matter, as follows : —
1 Clarke, F. W. Data of Geochemistry. U. S. Geol. Survey,
Bui. 491, p. 33. 1911.
SOILS: PROPERTIES AND MANAGEMENT
Oxygen
Silicon
Aluminium
Iron . .
Calcium .
Magnesium
47.17
28.00
7.84
4.44
3.42
2.27
Sodium
Potassium
Hydrogen
Carbon
Sulfur
Phosphorus
2.43
2.49
.23
.19
.11
.11
i.*7
The briefest scrutiny of this table reveals the fact that
the lighter elements are the more abundant in 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 7
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-
taining 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
(Fe203), calcium carbonate (CaCOs), or silica (SiQa).
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 . A1 2 3 . 6 Si0 2
3. Plagioclase Na 2 . A1 2 3 . 6 Si0 2 , CaO . A1 2 3 . 2 Si0 2
or combinations
4. Hornblende Chiefly Ca(MgFe) 3 SiA 2 with
Na 2 Al 2 Si 4 Oi 2 and (MgFe) 2 . (AlFe) 2 .
Si 2 0i 2
5. Augite . . Chiefly CaMgSi 2 06 with
(AlFe) 2 Si 2 6
6. Muscovite 2 H 2 . K 2 . 3 A1 2 3 . 6 Si0 2
(MgFe)
7. Biotite . .
8. Olivine .
9. Serpentine
10. Epidote .
11. Apatite
12. Zircon . .
13. Chlorite .
14. Calcite
15. Dolomite .
16. Gypsum
17. Talc . .
18. Hematite .
19. Siderite
20. Limonite .
21. Kaolinite .
22. Zeolites .
(Si0 4 );
6SiO s
(HK), (MgFe), (AlFe),
2 (MgFe)O . Si0 2
3 MgO . 2 Si0 2 . 2 H 2
H 2 0.4Ca0.3(AlFe) 2 3
3 Ca 3 P 2 8 + (CaFl 2 ) or (CaCl 2 ) or
combinations
Zr0 2 . Si0 2
H 40 (FeMg) 23 Ali4Sii 3 O90
CaC0 3
CaC0 3 . MgC0 3
CaS0 4 . 2 H 2
H 2 0.3Mg0.4Si0 2
Fe 2 3
FeC0 3
2 Fe 2 3 . 3 H 2
2 H 2 . A1 2 3 . 2 Si0 2
Complex hydrated aluminium silicates of
Ca, K, and Na as Philolite (CaK 2 N 2 )
Al 2 Sii O 24 . 5 H 2
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 more 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 are 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 transpor-
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 over 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 the 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
SOTL-FOBMING 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
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
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 = 2FeO + 4 H 2 S0 4
4 FeO + 2 = 2 Fe 2 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 the influence of
20 SOILS: PROPERTIES AND MANAGEMENT
carbon dioxide (C0 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 + 2 C0 2 = 2 FeC0 3 + 4 H 2 S0 4 or
2 NaOH + C0 2 = Na 2 C0 3 + H 2
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 3 - 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 imdergone.
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
hydration 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 3 (red) + 3 H 2 = 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 : —
KAlSisOg + HOH = HAlSi 3 8 + KOH
2 KOH + C0 2 = K 2 C0 3 + H 2
HAlSi 3 8 - 2 Si0 2 = HAlSi0 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 and 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 transportive 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 grain, 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 Miiller, 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.
Experimentale, pp. 27 and 252. Paris, 1847.
r
SOIL-FORMjrfG 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 Si0 2 Per cent of Si0 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 8
Hornblende . Ca(M gF e) 2 (Si0 3 ) with {^ggSj
Olivine . . (MgFe) 2 Si0 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." *
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 x 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
Si0 2 . .
Al 2 3 Fe 2 03
CaO . .
MgO . .
K 2 . •.
Na 2 .
Phonolite
Diabase
21.64
10.85
12.60
15.65
1.07
3.09
.40
2.20
.28
1.21
5.45
.50
41.44
33.50
Granite
9.49
8.36
.60
.71
1.68
1.23
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
Si0 2 .
A1 2 3 .
Fe 2 3 .
CaO .
MgO.
K 2 .
Na 2 0.
P 2 5 .
Ignition
Rock
60.69
16.89
9.06
4.44
1.06
4.25
2.82
.25
.62
Clay
45.31
26.55
12.18
00.00
.40
1.10
.22
.47
13.75
Percentage
Lost
52.45
00.00
14.35
100.00
74.70
83.52
95.03
00.00
Gain
Virginia Limestone and its Residual Clay 2
Si0 2
A1 2 3
Fe 2 3
CaO
MgO
K 2
Na 2
p 2 o 5
co 2
H 2
Rock
7.41
1.91
.98
28.29
18.17
1.08
.09
.03
41.57
.57
Clay
57.57
20.44
7.93
.51
1.21
4.91
.23
.10
.38
6.69
Percentage
Lost
27.30
00.00
24.89
99.83
99.38
57.49
76.04
68.78
99.15
Gain
1 Merrill, G. P. Bui. Geol. Soc. Araer., Vol. 8, p. 160. 1879.
2 Diller, 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.
iS
»,
\
Fe*C
N
/.
fa
\
\
Afr
/
\
1 g O
\
L
\
H
o
\
\
I
/
\
\
/
a
\
J
(ajFresf) roc/(
\
(
\>
1*
(b)G
lay
s
Fig. 1. — Diagrammatic representation of the chemical composition of
fresh granite and its residual clay. See analyses above.
SOIL-FORMING PROCESSES
29
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
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, due 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
80 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 : —
Residual
Sedentary
Cumulose
Transported
Gravity — Colluvial
[ Alluvial
Water I Marine
I Lacustrine
Ice — Glacial
Wind — iEolian
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
'-:'■ :/r\ '^~- : '«!<',- ■* '" "■ ' : *..*•""- ^
;?*•■*:
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 nonsjjc&tified 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, l'rnn^n^ 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 33
Arkansas Limestone and its Residual Clay l
Si0 2
A1 2 3
Fe 2 0.-
MnO
CaQL
MgO
K 2
Na 2
C0 2
Fresh Rock
Clay
4.13
33.69
4.19
30.30
2.35
1.99
4.33
14.98
44.79
3.91
.30
.26
.35
.96
.16
.61
34.10
.00
Percentage
Lost
.00
11.35
. 89.56
57.59
98.93
89.38
66.36
53.26
100.00
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 l>e 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
1 Penrose, 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 Marbot, C. F., and others. U. S.
D. A., Bur. Soils, Bui. 96. 1913.
V,
ils, 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 algse, moss, reeds, flags, grass, and the like.
These plants thrive, die, and fall down only to be covered
by the water in w T hich 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
Mineral matter
Organic matter
Nitrogen
P2O5 ....
K 2 . . . .
Moisture . .
1
2
31.60
24.79
68.40
67.63
2.63
2.03
.20
.19
.17
.15
—
7.58
80.40
15.77
.15
.65
3.83
1. Muck — Pickel, G. M. Muck : • Composition and Utili-
zation. Fla. Exp. Sta., Bui. 13. 1891.
2. Muck — Rept. Can. Exp. Farms, 1910. Rept. of chem-
ist, p. 160.
3. Marsh mud — Rept. Can. Exp. Farms, 1910. Rept. 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 * show the general
character of this soil : —
Si0 2 . .
A1 2 3 . Fe 2 3
CaO . .
MgO . .
K 2 . .
Na 2 . .
P 2 5 . .
C0 2 . .
Ignition
25.28
5.65
3.02
3.30
37.52
48.51
.12
1.96
.22
.23
.25
.30
.40
Trace
29.02
39.80
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,
!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.
46
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 iEolian
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
SOILS: 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.
•0 <o
:^ s Ms,
<>&&k
«i
o
n
( ( ' r'
(ttf£WJT/fi
ICE
{Jh/VAobb?
< 1 1 /C£ '/>
n
m>
^Z^ j^
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
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 ftvater 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
westward 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. jEolian 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 iEolian 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 1 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. Geo!. 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
Si0 2
A1 2 3
Fe 2 3
MgO
CaO
Na 2
K 2
P 2 5
C0 2
H 2
A
B
c
72.68
60.69
74.46
12.03
7.95
12.26
3.53
2.61
3.25
1.11
4.56
1.12
1.59
8.96
1.69
1.68
1.17
1.43
2.13
1.08
1.83
.23
.13
.09
.39
9.64
.49
2.50
1.14
2.70
67.10
10.26
2.52
1.24
5.88
1.42
2.68
.11
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 * 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 : —
Si0 2 . . .
A1 2 3 . . .
Fe 2 3 . . .
CaO . . .
MgO . . .
K 2 ...
Na^O . . .
C0 2 ...
P 2 O s . . .
Organic matter
66.69
44.64
14.16
13.19
4.38
5.12
2.49
13.91
1.28
2.96
1.21
1.71
.67
.59
.77
8.55
. .29
.94
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. iEolian
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.
f 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
Si0 2
A1 2 3
Fe 2 3
P 2 5
CaO
C0 2
MgO
Na 2
K 2
Light Sandy
Loam from
Maryland
Average of
5 Samples 1
Corn and
Wheat Clay
Loam Soil
Average of
3 Samples 1
Residual
Soil from
Virginia
Gneiss 2
92.30
80.55
45.31
3.20
8.82
26.55
.91
2.67
12.18
.05
.42
.47
.41
.47
trace
.08
.05
trace
.35
.29
.40
.50
.49
.22
.70
1.22
1.10
Residual
Soil from
Virginia
Limestone 3
57.57
20.44
7.93
.10
.51
.38
1.20
.23
4.91
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 61
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. Rept., U. S.
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 \Vis< oxsin
Residual
Glacial
1
2
3
4
SiO :
A1 2 3
Fe 2 3
MgO
CaO
Na 2
K 2
P 2 O s
C0 2
H 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
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 GEOCHEMICAL 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 l 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 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: PHOPERTIES AND MANAGES! EST
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 ! bring out the points already dealt
with in their fullest significance: —
Percentage of P2O5, CaO, MgO, and K 2 in Soils of Dif-
ferent Provinces
Soils
PiOs
CaO
MgO
KiO
Total
7 Coastal plain ....
3 Residual (crystalline) .
10 Glacial
.07
.25
.22
.14
.67
1.36
.16
.75
.79
.70
2.08
2.08
1.07
3.75
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 the
driftless area of Wisconsin with the glaciated parts/
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 GEOCIIEMICAL 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-
4ain_jriQre ol the i mportant -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 op
696 Samples 1
75.87
88.21
7.21
3.66
5.48
3.88
.16
.12
1.43
.13
1.27
.29
.35
.14
.67
.21
5.15
4.40
1.13
1.22
Average
Composition of
l.ithosphere 1
Insoluble residue and soluble
Si0 2
A1 2 3
Fe 2 3
P2O5
CaO
MgO
Na 2
K 2
Water and ignition . . .
Humus
59.36 (Si0 2 )
14.81
6.34
.29
4.78
3.74
3.35
2.98
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 l 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 : —
WH/TE
0LACX
BROWM/SH
R5D
Fig. 9. — A triangular representation of the three primary soil colors
and their mixtures.
CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 75
A soil low in humus, and with the iron either absent or
unoxidized, 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 matterj
fairly good aeration must be provided. If such a coi/
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 1 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 Avould cause the formation of hematite
1 Crosby, W. O. Colors of Soils. Proc. Boston Soc. 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 3 Red
Turgite .... 2 Fe 2 3 . H 2
Goethite .... Fe 2 3 . H 2
Limonite .... 2 F 2 3 . 3 H 2
Xanthosiderite . . Fe 2 3 . 2 H 2
Limnite .... Fe 2 3 . 3 H 2 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 op 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 Hilgard, E. W. Soils, pp. 283-285. New York. 1906.
80
SOILS: PROPERTIES 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
(b), 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 GEOCHEMICAL 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 l
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.
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 grind ing-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.
8 Atterberg, A. Die Mechanische Bodenanalyse und die
Klassifikation der Mineralboden Schwedens. Internat. 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. Internat. 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
(Used to separate sands in practically all
j) methods)
3. Water
In motion
At rest
2. Air (Cushman's air elutriator)
Gravity (Schone's elutriator and
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 1 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
87
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. Schone'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
Europe, 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
des Naturalistes de Moscow, 40, Part 1, p. 324. 1867. Uber
Schlammanalyse und einen neuen Schlammapparat. Berlin,
1867. Also see Wiley, H. W. Agricultural Analysis, Vol. I,
pp. 231-241. Easton, Pa. 1906.
88
SOILS: PROPERTIES AND MANAGEMENT
f
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
,/jL sample previous to the determi-
nation. The tendency toward
granulation during the process
of separation itself is fatal to
JL^Y_— j! 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
base of a cylindrical tube (see
Fig. 12) in which is placed a
rapidly revolving stirrer. This
is designed to counteract convex
currents and to prevent the
Fig. 12. — Hilgard's churn
elutriator for mechanical
soil analysis of particles
above .01 mm. in diameter,
(e), Intake; (p), stirrer;
(c), screen; (a), separating
chamber ; (o), outlet tube.
1 Hilgard, E. W. Methods of Physical and Chemical Soil
Analysis. Ann. Rept. 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 x
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. — Separatory bottle of Yoder's centrifugal elutriator. (B), Bottle ;
(e), intake ; (a), tube for conducting liquid to bottom of separatory
bottle; (o), outlet; (C), centrifuge ; (u>), 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
beaker 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.
Rept. Connecticut Agr. Exp. Sta., 1886, pp. 141-158; 1887, pp.
144-162; 1888, pp. 154-157.
THE SOIL PARTICLE 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
decanted 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 1 silt cylinder (Fig.
14). — This method 2 is similar to the beaker method in
1 Appiani, G. Ueber einen Schlammapparat fur die Analyse
der Boden- und Thonarten. Forsch. a. d. Gebiete d. Agri-
Physik, Band 17, Seite 291-297. 1894.
2 Atterberg, A. Die Mechanische 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-
tion is complete. The top of this
siphon is opposite the 5-centimeter
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. Easton,
Pa. 1906.
Fig. 14. — Atterberg's
silt cylinder for the
mechanical analysis
of soil by subsidence.
THE 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 l 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. S4.
1912.
94
SOIL S : PROPER TIES 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
weighed. 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
soil 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
is simple, 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
because of the uses which the investigators wished to
make of such analyses. Some of the best-known group-
ings are given below : —
96
SOILS: PROPEPTIES AND MANAGEMENT
Various Textural Classifications used in the Mechani-
cal Analyses of Soils. Expressed in Diameter of
Particles in Millimeters
Separate
Osborne '
HlLGARD*
Bureau op
Soils 3
English 4
Atterberg 6
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. Rept. 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. Rept. 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.
5 Atterberg, A. Die Mechanische Bodenanalyse und die
Klassiflkation der Mineralboden Schwedens. Internat. Mitt,
f. Bodenkunde, Band II, Heft 4, Seite 312-342. 1912.
THE SOIL PARTICLE
97
4 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 l of a Dunkirk Fine Sandy Loam
and a Dunkirk Clay
Separate
Fine gravel .
Coarse sand .
Medium sand
Fine sand
Very fine sand
Silt . . .
Clay . . .
Size in
Millimeters
2-1
1-.5
.5-.25
.25-. 10
.10-.05
.05-.005
below .005
Fine Sandy
Loam
%
1
2
3
22
35
27
10
%
1
2
2
6
7
39
43
1 Soil Survey Field Book, pp. 152, 154. U. S. D. A., Bur.
Soils. 1906.
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 aFe 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
Glacial and loessial . .
Marine
Arid
12
6
4
3
15%
12%
5%
37%
21%
15% .
8%
42%
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,
glagial, 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
Soils
Num-
BEROF
Sam-
ples
Percentage of
P2O5 IN
Percentage of
K2O IN
Percentage of
CaO in
Sand
Silt
Clay
Sand
Silt
Clay
Sand
Silt
Clay
Crystalline residual
Limestone residual
Coastal plain
Glacial and loessial
Arid soils
3
3
7
10
2
.07
.28
.03
.15
.19
.22
.23
.10
.23
.24
.70
.37
.34
.86
.45
1.60
1.46
.37
1.72
3.05
2.37
1.83
1.33
2.30
4.15
2.86
2.62
1.62
3.07
5.06
.50
12.26
.07
1.28
4.09
.82
10.96
.19
1.30
9.22
.94
9.92
.55
2.69
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.
i02
SOILS: FROPERTJES 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
*SiOt
AW,
FejOj
CaO
MgO
K 2
PjOi
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 Niihrstoffen 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 l 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.
iHall, 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 x
|
a
g
o
>
go
GO
5
1
g§
a
S
<
GO
£
Si
m
S
I
1
S
i
3
£ .
PS z
6
a
GO
7
3
Coarse sands . . .
135
12
31
19
20
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 " of 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.
J 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
ci^r
to zo jo 40 so eo 70 80 so 1009b
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 x 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
Barley
Potato
Hop
Fruit
(9 samples)
(9 samples)
(8 samples)
(7 samples)
(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.
Tew 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 ; (b) , 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
Orthoclase . . 2.6
Plagioclase . . 2.7
Mica .... 3.0
Olivine .... 3.4
Calcite .... 2.7
Dolomite ... 2.9
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
Apatite
. 3.2
Gypsum .
. 2.3
Hematite
. 5.2
Limonite .
. 4.0
Serpentine
. 2.6
Chlorite .
. . 2.2
Talc . .
. 2.7
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 l 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
matfer present. As the specific gravity of the soil humus
usually ranges from 1.2 to 1.7, the more humus there 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
1.5 to 2.0, according to the amount 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
mqre or less pore space, depending on textural and struc-
1 Whitney, M. Some Physical Properties of Soils. U. S.
D. A., Weather 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 * 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
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 of pore space = 100 — . — ~ — — X — —
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 * as follows : —
Weight per
Percentage of
Cubic Foot
Pore Space
First foot
79.0
52.2
Second foot
92.6
44.0
Third foot
104.6
36.8
Fourth foot
106.2
35.8
Fifth*foot
111.0
32.9
Sixth foot
111.1
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.
by the author, Madison, Wisconsin. 1910.
111. Published
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 tt Z) 3 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 : —
I'd g
Approximate
Separates
Limits
Number op Parti-
cles to the Gram
£ , 2 H
Number of
Particles in one
of Each Separate
^S to
Gram of Sandy
Loam
Fine grav-
el . .
2-1 mm.
209
1
2
Coarse
sand .
1-.5 mm.
1,670
4
67
Medium
sand .
.5-.2S 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
CLA8S
Coarse sands .
Sands ....
Fine sands . .
Sandy loams
Fine sandy loams
Loams ....
Silt loams . . .
Sandy clays . .
Clay loams . .
Silty clay loams
Clays . . . .
Approximate Number op
Particles
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 = it 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 be more 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
op 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
79
213
222
294
307
417
430
458
653
286
280
248
671
699
926
967
1313
1354
1442
2057
23,055
22,549
Fine sands . .
Sandy loams .
Fine sandy loams
Loams ....
20,014
53,965
56,180
74,410
Silt loams .
77,700
Sandy clays . .
Clay loams . .
Silty clay loams .
Clays ....
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 l decided that we were
in need of a single term which not only would give an
indication regarding the size of grain, 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. Rept.,
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 x 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,
Fig. 20. — King's aspirator for £ing f oun( J fa res ults fairlv
the determination of the rate j 1 1 1 .
of air movement through accurate, and showed that
soils. (G), Pressure gauge; tne calculated and the ob-
(<S), soil column ; (L), water; in* 1 1
U), aspirator; (HO, weight. . served flow ot water through
1 King, F. H. Principles and Conditions of the Movements
of Ground Water. U. S. Geol. Survey, 19th Ann. Rept., 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
sands 1 agreed rather closely (see Fig. 20). The effective
diameter of the particles of some of our common soils, to-
gether with the effective surface exposed, is given
below : 2 —
Soil
Coarse sandy soil
Sandy soil .
Sandy loam .
Loam . . .
Loamy clay soil
Fine clay son* .
Very fine clay
Effective
Diameter
.1432 mm.
.0755 mm.
.0303 mm.
.0219 mm.
.0140 mm.
.008(1 mm.
.0049 mm.
Percentage
of Pore
Space
34.9
34.4
38.8
44.1
45.3
48.0
52.9
Effective Sur-
face Exposed in
One Cubic Foot
of Soil
8,318 sq. ft.
15,870 sq. ft.
36,880 sq. ft.
46,510 sq. ft.
71,316 sq.ft.
110,500 sq. ft.
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.
mid., d. 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
l
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.
2 Hilgard, E. W. Soils, p. 130. New York. 1906.
3 Wollny, E. Die Zersetzung der Organischen Stoffe, p. 194.
Heidelberg. 1897.
128 SOILS: PROPERTIES AMD MANAGEMENT
The carbohydrates, having the general formula of
C x (H 2 0),„ include such compounds as glucose, starch.
cellulose, dextrose, cane sugar, and the like. The fats
and oils may be represented in plants by such glyceridea
as butyrin, stearin, olein, palmitin, and the like. The
proteins arc l>y 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 [END PRODUCTS
UNDECOMPOSED 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
Jbeen 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 (C40H14O12) gave humic
acid (C40H12O12), which in turn yielded geic acid
(C40H19O14), followed by apocrenic acid (C48H12O24),
and finally by crenic acid (C 2 4Hi 2 0i6)- 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 1 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, w T hile 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 138
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. Diamine- acids
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 2 X
Ci8H3 6 6 4 Histidine, C 6 H 9 2 N 3
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
Monohydroxy stearic acid,
C18H36U3
Agroceric acid, C21H42O3
Agrosteral, C22H22O . H 2
Paraffinic acid, C24H48O2
Lignoceric acid, C24H48O2
Phytosterol, C 26 H 4 40 . H 2
Pentosan, C5H3O4
Oxalic acid, C2H2O4
Succinic acid, C4H 6 4
Sacharaic acid, CeHsOio
Acrylic acid, C 3 H 4 2
Mannite, C 6 Hi 4 06
Rhamnose, CeHsOio
Salicylic aldehyde,
C 6 H 4 OHCOH
Arginine, C6H14O2X4
Cytosine, C4H 5 OX 3 . H 2
Xanthine, C 5 H 4 02X T 4
Hypoxan thine, C5H4ON4
Tysine, C6H 14 2 N2
Adenine, C 5 H 5 N 5
Choline, C 5 H 15 2 N
Trimethylamine, C 3 H 9 N
Quanine, CH 5 N 3
Creatinine, C 4 H 7 ON 3
Creatine, C 4 H 9 2 X 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 of
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: PROPERTIES AND MANAGEMEST
histidine and creatinine l may be mentioned. Here ib
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 weli 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, dihydroxy stearic
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
>xic 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-
boxy lie 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
cumarine, 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 insanitary 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 Schreiner, O., and Skinner, J. J. Harmful Effects of
Aldehydes in Soils. U. S. D. A., Bui. 108 (Professional Paper).
1914.
3 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 Schreiner, 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 Schreiner
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 of 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 * to be a measure of bacterial activity.
With 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
Soil air (average of 19 analyses)
Atmospheric air
A sandy soil
A sandy soil plus manure
18.33
20.96
19.72
10.35
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 (N0 2 ) and ultimately in nitrates (N0 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,),
carbon disulfide (CS2), 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 l found that when humus was extracted with an
alkali and then precipitated with an acid, it yielded from
five to twenty-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. Ekp. Sta., Bui. 53, pp. 29-30. 1897.
THE 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 1
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 listec? 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
lignification 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 Carbonized 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 wdth 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 ORGANIC 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 <*f 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 drawn 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 1.724,1
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 Ililgard 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 alldwing 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 l 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 3 3.12
A1 2 3 3.48
K 2 7.50
Na 2 8.13
CaO 09
MgO 36
P 2 5 . . . . 12.37
S0 3 98
C0 2 1.64
The relatively high percentage of phosphoric acid is
immediately noticeable in this analysis. This indicates
1 Snvder, Harry. Soils. Minnesota Agr. Exp. Sta., Bui. 41,
p. 30. " 1895.
L
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 .
Northeastern States
South Central States .
Southeastern States
Semiarid States . . .
Arid States ....
1.84
1.66
1.16
.93
.99
.89
.76
.60
.55
.41
.62
.64
3.06
3.73
1.80
1.53
2.64
1.05
1.07
1.35
.65
.73
1.11
.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
3
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
41 Arid uplands soils .
15 Subirrigated arid soils
24 Humid soils . . .
Humus in
Soil
(Percentage)
.91
1.06
4.58
Nitrogen in
Humus
(Percentage)
15.23
8.38
4.23
Nitrogen in
Soil
(Percentage)
.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, E. W. Soils, pp. 136-137. New York.
2 Ibid., p. 139.
1911.
148 SOILS: PROPERTIES AND MANAGEMENT
The Humus of a Russian Alluvial
Soil
Depth in Feet
Percentage or
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
8.76
.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 !
in the study of Nebraska Soils : —
Composition op a Nance County, Nebraska, Soil near
Genoa
Depth
Percent-
age op
Nitrogen
Percent-
age op
Carbon
Percent-
age op
Humus
Percent-
age op
Ash IN
Humus
Ratio op
in Feet
C
N
H
N
c
H
1
1
3
4
5
6
.255
.102
.056
.042
.034
.027
2.61
.85
.31
.24
.17
.14
2.47
1.00
.40
.30
.19
.16
1.61
.90
.52
.64
.33
.36
10.2
8.3
5.5
5.7
5.0
5.2
9.7
9.8
7.1
7.1
5.6
5.9
1.0
.9
.8
.8
.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. Rept., 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 * 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
49.28
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-under 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. As 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 two 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
materials 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
either 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 ^ancLimportant 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.
Niklas, H. Die Kolloidchemie und ihre Bedeutung fiir
Bodenkunde, Geologie, und Mineralogie. Internat. Mitt,
fur 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-
303. 1911.
Konig, J., Hasenbaumer, J., und Hassler, C. Bestimmung
der Kolloide im Ackerboden. Landw. Ver. Stat., Band 76,
Heft 5/6, Seite 377-441. 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., Band 60, Heft 1, Seite 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 par ticles. 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 I 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 range 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
*SU<5PE/yS/Ort3
ySOt-UT/Ort$
f %%VER.SIBLC
■/0./ZC VE/Z -SfBt-C
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 will 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, witli 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 flocculation. 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 flocculation 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 thSir generation. 1 —
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
1 Van Bemmelen, J. M. Die Absorption. Seite 114-115.
Dresden, 1910. Also, Die Absorptionsverbindungen und das
Absorptionsvermogen 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,
2d ser., Vol. 6, pp. 1-19. 1868.
Cushman, A. S. The Colloid Theory of Plasticity. Trans.
Amer. 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 8 + 2 H 2 + C0 2 = H4Al 2 Si 2 9 + 4 Si0 2 +
K 2 C0 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 C0 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
semipermeable 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-
nated 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-
tlVe 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 Jm^T^^lJ^lj^P^^^ matte r increases plasticit y ; 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-
,ti on, and p revents 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 th^-cofitc Ql of colloidal conditions. Since this control
and utilization of colloid3~lTrfktences 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-
cal in nature. These important methods l are three in
number: Van Bemmelen's, Ashley's, and Mitscherlich's.
Van 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 Ehrenfcerg and Pick, 2 who claim that the drying over
1 Rodewald, IT., 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 B6den.
Fuhling's Landw. Zeit., Band 61, Heft 3, Seite 73-83. 1912.
THE COLLOIDAL MATTER OF SOILS 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 wate r 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 to geth er of the small part icles 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 w T hich 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. Internat.
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. 16, 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 CoUoid 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 soil
is bound together 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 Bodenkunde, by E. A. Mitschertich, 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 118-157. 1878. Also, Wissenschaftlich
praktische Untersuchungen auf deni Gebiete des Pflanzenbaues,
Band I, Seite 22. 1875.
176 SOILS: PROPERTIES AND MANAGEMENT
Puchner * 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 (W). (C) is a coun-
terpoise.
. ! 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.
Internat. Mitt. f. Bodenkunde, Band II, Heft 2-3, Seite 149-
189. 1912.
3 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 ' 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 fiber die
Kokareszenz verschiedener Bodenarten. Internat. Mitt, fur
Bodenkunde, Band III, Heft 2-3, Seite 141-239. 1913.
N
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
grani^lation, 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 soil, by hastening and increasing granulation, will
tend to lower cohesion at every moisture content ranging
from a dry to a saturated condition. The following
data, taken from Puchner, 1 bring out the points just
discussed : —
1 Puchner, H. Untersuchungen iiber 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
40 *
per cent
20
per cent
per cent
Clay
2 clay + 1 quartz
1 clay + 2 quartz
Quartz ....
2 'clay + 1 humus
1 clay + 2 humus
Humus ....
Pulverized loam .
Granulated loam
(granules .5-9 mm
diam.) . . . "
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
ZCOOOprasTK
J
(6 "
(4- -
cz>jy/
tZ '
fO
a
Q/Z^f/P.T-Zy
<?
4-
2
HI/MUX
2
Fig. 26
so
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
*
fn
SO
JO
20
c/.jy
c
/(
£
7 J
4-
o <?e>
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
cohesion is developed principally by the water film. As
this thins, its pulling power increases £nd the curve
ascends; 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
\COHES/Ott
PLASTICITY
\C. &/
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, sucfy 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 x
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. E. 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 power 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 in 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 SOILS: 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 l 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
Pippin, 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 1()0.0
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 many, 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 *
Percentage
of penetration
100
82
73
58
50
1. Clay
2. Clay plus 5 per cent of muck
3. Clay plus 15 per cent of muck
4. Clay plus 25 per cent of muck
5. Clay plus 50 per cent of muck
1 Fippin, E. O. Some Causes of Soil Granulation. Trans.
Amer. Soc. Agron., Vol. 2, pp. 106-121. 1910.
192 SOILS: PROPEUTIES AND MANAGEMENT
Puddled Clay plus Muck Extract i
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.
1 Fippin, 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 ; * the soil
was dried once and the untreated soil was used as 100
per cent : —
• » 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 CaC0 3 98
6. Clay plus 10 per cent CaC0 3 Ill
7. Clay plus 25 per cent CaC0 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 ! 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
realized 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-
row 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 A.Xh MANAGEMENT
force, the share is sloping and is set to deliver a slanting
cut, and the mold board 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 onnrlitinn of any soil are found to be p lasticity 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- 172S) 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 •*■ 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: PROPEliTIES 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 beeanse 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 201
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 water 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 : —
HYGROSCOPIC! CAPILLARY | FREE
Fig. 30. — Diagram representing the three forms of water that may
be present in a soil.
£1.33. Hygroscopic water. — The hygroscopic water
a soil has been spoken of as the water of condensation,
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 becomes
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 x 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
19.76
20.41
Kaolin
.47
3.55
Limestone
.29
.32
Quartz
.07
.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 oh 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-
tionsvermogen und die Hygroskopizitat der Bodenkonstitu-
enten. 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 exposm*. Such water cannot be expelled by ordi-
nary drying, but the material must be subjected to a high
heat in order to drive oft' even a part of the water so held.
The qUMtiotl 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 hninic 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., Rept. 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 200°
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 . .
7 clay loams
9 loams . .
4 sandy loams «
4 sands . .
31.97
17.15
12.06
7.39
2.93
10.45
6.06
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 *
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,
them^goyern the hvgroscopicity of most soflsT"^
135. Nature oTthe^Im.^^Ke" 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 bacteria. 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.
Quartz
Humus
30
Per cent
.045
4.055
50
Per cent
.053
7.765
70
Per cent
.076
10.589
90
Per cent
.119
15.676
100
Per cent
.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. Untersuchungen uber 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 ] find that as the temperature is increased
the hygroscopicity becomes lowered, thus following the
general laws of adsorption. Hilgard, however, obtained
opposite 1 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 Mitselierlich * 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.
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 vexnjg^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 per
Kilo of Dry Soil
Coarse quartz
150
Podunk fine sandy loam
Norfolk sand
200
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-
differentiated. The capillary water differs
Fig. 31. — Diagram
showing the rela-
tionship of the
hygroscopic and
capillary water
films surrounding
a soil particle,
(s) , particle;
(0, zone of influ-
ence of particle ;
(w), outer edge of
hygroscopic zone ;
(c), capillary film.
fore clearly
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
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 he maintaining its maximum
thickness of capillary film. It is also quite evident that
as the capillary Him 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 earning a capil-
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 outw 7 ard
pull. Under the stress of this pull developed by the
surface tension acting in this film of very great curvature,
Fit;. 32. Diagram showing the coalescence
aw) 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.
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 eause a
change in the opposite direction. This theory lias 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 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 he 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
most potent in its influence on capillary capacity.
144. Texture and the amount of capillary water. —
The finer the texture of a soil, the greater is 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 Wollny, E. Untersuchungen iiber die Wasserkopacitat der
Bodenarten. Forsch. a. d. Gebiete der Agri.-Physik, Band 9,
Seite 361-378. 1886.
THE FORMS OF SOIL WATER
215
coarse soil. Due to these two conditions, a soil of 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
assume a greater curvature and thus
support this extra weight thrown on
them. This curvature must be sufficient
to balance the curvature pressure of the
particles below plus the weight of the
water in the connecting films. The par-
ticles 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
Fig. 33. — Diagram
showing the ad-
justment of the
capillary film in a
long column and
the appearance of
free or gravita-
tional water if the
weight is too
great for the sup-
porting films.
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
>u
44 \
''"•'..
CLAY
3
8S
1
\
\
\
1
30 \
SANDY '•-
1
>•
\
20
^LOAM
Hi
SA/VO
_____
\-
-
P
A
r *
z
S J
OC/otYATCR.
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. 34) from Buckingham l 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 l in his study of
loose and compact soils. The following curves repre-
sent the general trend of his results : —
30 \,
V^
1
I
|
pa
\
10
SS<: ^ a ^
\
V
JO °7bWAr£G
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 swelf 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
Storer, 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
storer, 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 SOIL WATER
221
Soil
Per-
cent-
age OF
Or-
ganic
MATT3R
Per-
cent-
age
of
Sands
Per-
centage
of Silt
Per-
centage
OF
Clay
Mois-
ture
Equiva-
lent
1. Norfolk coarse sand .
2. Norfolk fine sandy loam
3. Yazoo loam ....
4. Waverly silt laom . .
5. Houston clay loam .
6. Houston clay . . .
.9
1.3
1.3
2.0
3.7
1.4
87.9
73.4
25.8
14.9
30.9
10.0
7.3
18.1
64.1
62.9
42.5
56.6
4.8
8.5
10.1
22.2
26.6
33.4
4.6
6.8
18.9
24.4
32.4
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.
A small 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.
2 This text, paragraph 181.
New York. 1911,
222 SOILS: PROPERTIES AND MANAGEMENT
Fig. 36. — Diagram show-
ing the mechanics of the
capillary movement of
water in soil. The read-
justment takes place in
the direction of (.4) due
to the high tension devel-
oped by the sharp film
curvature at this point.
to become equal, due to the pulling force developed by
♦he 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
contain unequal amounts of water
in the angles of curvature (see
Fig. 36). In this case the greater
pull would exist at A, since the
angle here is more acute. Conse-
quently water must move through
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-
Fig. 37. — Diagram for the ex-
planation of the effect of
thickness of water fiJm about
soil particles upon ease of
capillary movement.
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.
THE FORMS OF SOIL WATER 225
face of the former column. King 1 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 op Column in Inches
Evaporation at Surface
in Inches a Day
6
12
18
24
30
.114
.111
.080
.034
.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 tips Movements
of Ground Water. U. S. Geol. Sur., 19th Ar^Rept., 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
its maximum capillary moisture should be raised, some
of the water would be lost as free water, since the pulling
power of 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 x 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 liber die Kapillare Leitung
des Wassers. Forsch. a. d. Gebiete d. Agri.-Phvsik, 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
Sand . . .
Clay . . .
Silt ....
Inches
3.5
.5
2.5
Inches
5.0
5.7
14.5
Inches
5.9
8.9
20.6
Inches
6.8
10.9
24.2
Inches
6.8
12.2
26.2
Inches
6.9
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
merits 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
Very fine sand
Silt . . .
Clay . . .
.5 -.25
.25 -.10
.10 -.05
.05 -.005
.005-
.98
1.78
4.05
9.99
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 1 (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, while 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
spaces. 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
234 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. His
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. Untersuehungen iiber die Permeabilitat des
Bodens fur Wasser. Forsch. 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. Rept., 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 liber 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. Rept., 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 * 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 : —
« „ . ,1 [percentage of pore space
Percentage of air space when M _ (percentage of h 2
capillarily saturated w \
: J I X ap. sp. gr.)
Percentage of free water pos- 1 _ [p ercentage of air space
sible 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 FORMS 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
G>rac/e
Fig. 38. — Cross section of a lysimeter at the Rothamsted Experiment
Station, England, in'), soil column under study; (p), outlet for
collected drainage water.
240 SOILS: PROPERTIES 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
immediatelv trustworthv.
O 6~Sewer7?/e ' ' \J
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 half 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 l 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.
1 Warington, 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, rinding 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 * 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. Soe. 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
Ind., Bui. 285. 1913.
246 SOILS: PROPERTIES AND MANAGEMENT
Water Requirements of Plants by Different
Investigators
Crop
- ' -
te w z
5 o •<
1 « J
Barley .
Beans
Buckwheat
Clover .
Maize
MiUet .
Oats
Peas
Potatoes
Rape
Rye . .
Wheat .
H
258
209
269
259
247
[m B'-l
774
646
233
447
665
416
912
w O
310
282
363
310
376
273
353
338
464
576
271
503
477
385
M
3
468
337
469
563
544
< SO 05
534
736
578
797
368
310
597
788
636
441
685
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, however.
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., 11th 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
the rainfall, the higher is the humidity and the lower is
the relative transpiration. This accounts for the high
figures obtained by Widtsoe l 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 l 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
loam soil : —
Lincoln, Nebraska, with corn grown in a
Effect of Soil Moisture on Transpiration
Barley — Hellriegel
Corn — Montgomery
Soil Moisture Per-
centage of Total
Capacity
Transpiration
Ratio
Soil Moisture Per-
centage of Total
Capacity
Transpiration
Ratio
80
60
40
30
20
10
277
240
216
223
168
180
100
80
60
45
35
290
262
239
229
252
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 x
Units 2 of Ca(N0s>2
Applied
Dry Matter Produced
per Pot (Grams)
Transpiration Ratio .
4
8
12
16
20
1,111
8,479
13,936
18,288
23,026
25,504
724
399
347
345
302
292
1 Hellriegel, H. Beitrage zu den Naturwissenschaftlichen
Grundlage des Ackerbaus, p. 629. Braunschweig. 1883.
2 A unit of Ca(N0 3 ) 2 equals 1 mg. -equivalent. A mg.-
equivalent of Ca(N0 3 ) 2 equals 82.1 mg.
WATER OF SOIL IN ITS RELATION TO PLANTS 251
Relative Water Requirements of Corn on Different
Types of Nebraska Soils, 1911. Montgomery 1
Soil
Dry Weight of Plants
in Grams per Pot
Transpiration Ratio
Manured
Unmanured
Manured
Unmanured
Poor (15 bushels) . .
Medium (30 bushels) .
Fertile (50 bushels) . .
376
413
472
113
184
270
350
341
346
549
479
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. Rept., 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
WATER 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 of Dry Matter between Grain and Straw
with Varying Amounts of Water
Inches op Water
Grain in Percentage op
Total Dry Matter
5
44.4
7|
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 Freekmann, W. Der
Einfluss des Wassergehaltes des Bodens auf die Ernten und
die Ausbilding Verschiedener Getriedevarietaten. Jour. f.
Landw., Band 51, Seite 253-269. 1903.
WATER 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
1
I*
r
"ca/2/v
~~W/i£AT
/
rs
so eo 30 4o so 6om. fi a O
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.
1 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 W T asser in dem Versehiedenem 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
Dry
Inches
Dry
Inches
Dry
Inches
Matter
OF
Matter
op
Matter
op Water
Wheat
Water
Corn
Water
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 x
Soil
Wilting Point
Percentage op
Hygroscopic Water
Coarse sandy soil
Sandy garden soil
Fine humous sand
Sandy loam . .
Calcareous soil .
Peat ....
1.5
4.6
6.2
7.8
9.8
49.7
1.15
3.00
3.98
5.74
5.20
42.30
1 Heinrich, R. Ueber das Vermogen der Pflanzen den Boden
an Wasser zu erschopfen. Jahresbericht der Agri.-chem.,
Band 18, Seite 368-372. 1875.
258 SOILS: PROPERTIES AND MANAGEMENT
Relation of the Wilting Point to the Hygroscopic Co-
efficient. BltlGGS AND SHANTZ l
Soil
Hygroscopic
Coefficient
Wilting Point
Coarse sand
Fine sand
Fine sand
Sandy loam
Sandy loam ....
Fine sandy loam . . .
Loam
Loam
Clay loam
.5
1.5
2.3
3.5
4.4
6.5
7.8
9.8
11.4
.9
2.6
3.3
4.8
6.3
9.7
10.3
13.9
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, where 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 formula? 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 (em)r 2 Q per cent)
2. Wilting point = Hygroscopic coefficient (em)r 71 per oent)
.68
3. Wilting point
= Water-holding capacity (Hilgard method) ~ 21 (orror 8 3 per cent )
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 of 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
5.50
3.33
Fine sand
6.74
3.70
Sandy loam
Sandy loam
Sandy loam
Loam . .
9.70
14.50
18.60
23.80
4.80
9.60
8.84
12.40
Loam . .
25.00
13.90
Clay loam
Clay loam
27.40
29.30
14.50
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 sand + .12 silt + .57 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 P0INT LENTOC/iPILLARtTY MAX/MUM WATER
COEEE/C/ENTk I POINT CONTENT
mW05C0P/(>[\ ^OPTIMUM WATER CONTENT \ \EREE«JUPEfm/VIBLE
AVAILABLE MOISTURE
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 1 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 x
placed 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 Einfluss kleinerer oder Grosserer
Mengen 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.
O- A )r~/yo/f
S Fi/aporation'
/77m
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,
leaching, 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-
nomic 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 soil in a humid region more
than equals that withdrawn by the crops. The results
from the Rothamsted drain gauges x 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 : —
Drainage through
Proportion of
Rainfall Drained
through Soil
Rain-
fall
Depth in Inches
Per Cent
20
40
60
20
40
60
January ....
2.32
1.82
2.05
1.96
78.5
88.4
84.5
February .
1.97
1.42
1.57
1.48
72.2
80.0
75.2
March . .
1.83
0.87
1.02
0.95
47.6
55.6
52.0
April . .
1.89
0.50
0.57
0.53
26.5
30.0
28.0
May . .
2.11
0.49
0.55
0.50
23.2
26.1
23.6
June . .
2.36
0.63
0.65
0.62
24.0
27.6
26.3
July . .
2.73
0.69
0.70
0.65
25.3
25.6
23.8
August . .
2.67
0.62
0.62
0.58
23.2
23.2 21.7
September
2.52
0.88
0.83
0.76
35.0
32.8 30.0
October
3.20
1.85
1.84
1.68
57.8
57.5, 52.5
November .
2.86
2.11
2.18
2.04
76.7
76.3
72.4
December .
ear
2.52
2.02
2.15
2.04
80.3
85.4
81.0
Mean total per j
28.98
13.90
14.73
13.79
48.2
51.0
48.0
Winter, October to
March ....
14.70
10.09
10.81
10.15
68.6
72.8
69.0
Summer, April to
September . . .
14.28
3.81
3.92
3.64
26.6
27.4 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.
pJ 23. London.
The Book of the Rothamsted Experiments,
1905.
THE CONTROL OF SOIL MOISTURE
267
K
esf/A/F
4LL^
*s
\
\
/
S
/
\
\
J7/P4//
1AG£
/
/
\
\
^
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, rthen, is first, to have a loose,
open structure of soil in order to facilitate ready entrance
of the water jj 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
Records from Rothamsted (1870-1878) x
271
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 controlled 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 n^ulch 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- CONTROL OF SOIL MOISTUBE 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 well 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 1 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 : —
/O /S ZO 0*Y<5 £J.4P&£0
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.
1 Buckingham, E. Studies on the Movement of Soil Mois-
ture. U. S. D. A., Bur. Soils, Bui. 38, pp. 18-24. 1907.
27.8 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 easilv maintained as a mulch than clav soil ;
THE CONTROL OF SOIL MOISTURE
279
(4) from two to three inches is ordinarily the most effec-
tive depth; (5) after a heavy rain, the soil mulch must
be 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
16.8
16.4
13.2
10.1
9.6
10.8
Second foot
9.4
Third foot
9.5
Fourth foot
8.9
Fifth foot
8.5
Average :
13.2
9.4
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 * 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 Cates, J. Si, 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 *
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 : —
Check plot
Weeds removed, but not cultivated .
Mulched with straw
Check plot
No cultivation ; weeds allowed to grow
One cultivation ; weeds allowed to
grow
Check plot
Yields Calcu-
lated to Basis
of 100 ON
Check Plots
Soil Moisture
during August
Per Cent
1 Cates, 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 CONTROL OF SOIL MOISTURE 283
crop 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 well 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 where 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 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 ! 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 com-
1 King, F. IL, The Soil, p. 189. New York. 1906.
THE 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, which is one reason for the smaller
growth of crops near trees. Bearing on the efficiency of
windbreaks, results by King * 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 Stewart 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. H. 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: PROPERTIES 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.
f
J
fO
0CG/OD OF OaS£ZVAT/OM
IS 20 2S- JO 3
40
*S 0*
V s
i
<iO
^>
f.
T -^\
-\
V
/'"
S-
J 1
*
V.
r\
*
',
6
"-■■■
\
V
\
\
}
\
\
;
J
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.
Run-off and leaching are reduced by increased capillary
capacity, a structural relationship. Evaporation is
checked by a soil mulch, which depends for its effective-
ness 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
clearly 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 a soil in which active growth does not take place almost
immediately, since certain molds and fungi, active at
low 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,
G. J. An Investigation of Soil Temperature and Some of the
Most Important Factors Influencing It. Michigan Agr. Exp.
Sta., Technical Bui. 17, pp. 194-196. 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 l 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 op
Certain Seeds (in Degrees Fahrenheit)
Minimum
Optimum
Maximum
Corn
Scarlet bean
Pumpkins
Wheat
Barley
49
49
52
41
41
93
93
93
84
84
115
115
115
108
99
The Relation of Temperature to the Growth of Certain
Plants (in Degrees Fahrenheit)
Wheat .
Barley .
Corn
Peas . .
Buckwheat
Melon .
Pumpkin
Minimum
32-40
32-40
40-51
32-40
32-40
60-65
51-60
Optimum
77-88
77-88
88-98
77-88
77-88
88-98
98-111
Maximum
88-98
88-98
98-111
88-98
98^-111
111-122
111-122
1 Haberlandt, F. Die Oberen und Unteren Temperatur-
grenze fur 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
germinate 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 tw6 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, 
