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LIBRARY

FACULTY OF FORESTRY
DIVERSITY OF TORONTO

Cfje teal Cext=2£ook Series

Edited by L. H. Bailey

THE PRINCIPLES OF SOIL
MANAGEMENT

THE MACMILLAN COMPANY

NEW YORK • BOSTON ■ CHICAGO
ATLANTA ■ SAN FRANCISCO

MACMILLAN & CO., Limited

LONDON • BOMBAY ■ CALCUTTA
MELBOURNE

THE MACMILLAN CO. OF CANADA, Ltd.

TORONTO

Plowing— the most fundamental and far-reaching operation
in soil management.

THE PRINCIPLES

OF

SOIL MANAGEMENT

BY

T. LYTTLETON LYON, Ph.D.

AND

ELMER 0. FIPPIN, B.S.A.

Professors of Soil Technology, in the New York State College
of Agriculture at Cornell University

LIBRARY

FACULTY OF FORESTRY

SECOND EDITION

UNIVERSITY OF TORONTO

THE MACMILLAN COMPANY

LONDON: MACMILLAN & CO.. Ltd.

1910

£11 rights reserved . X

A

\

Copyright, 1909
By THE MACMILLAN COMPANY

Set up and electro typed.

Published, December, 1909

Reprinted, June, 1910

5

Lib
\O\l0

SEEN BY
PRESERVATION

SERVICES

DATE

M^tutW «•***

.IBount Wtltasant T&ttut

J. Horace McFarland Company
Harrisburg, Pennsylvania

PREFACE TO THE RURAL
TEXT -BOOK SERIES

In 1895 the preface was written for the Rural Science
Series. It set forth the purpose of the Series to be the
desire to place in readable form the best results of
scientific thought and discovery relating to agriculture
and country life, in order that the general public might
be made aware of the progress, and that farmers might
be led more effectively to apply the information in their
daily work. It was the hope that the Series, under the
present writer's direction or another's, might gradually
extend itself to the whole range of agricultural scientific
literature. The books now included in The Rural Science
Series are about two dozen, making nearly two volumes,
on the average, for each year. The number of writers on
agricultural topics is increasing, the knowledge on all
subjects is rapidly accumulating, and the reading-public
is gradually enlarging; there is every reason to expect,
therefore, that the Series will extend itself still more
rapidly in the years to come.

It was considered to be an auspicious circumstance
that the Rural Science Series began with a book on the
soil, for this grounded the enterprise. The scientific and

(v)

VI PREFACE TO THE RURAL TEXT-BOOK SERIES

literary character of this first volume also won a good
hearing for the undertaking.

The time has come when special texts on agri-
cultural and rural subjects are needed in educational
institutions; and I now, therefore, project another line
of rural books, to be known as The Rural Text-Book
Series. This Series is to be coordinate with the other
Series, the former designed primarily for popular read-
ing and for general use, this one for class-room work
and for special use in consultation and reference. It is
planned that the Rural Text-Book Series shall cover
the entire range of public-school and college texts.

I consider it to be significant that I am able to begin
this new Series, also, with a book on the soil. These
two soil books well illustrate the two methods of treat-
ment of a subject; and this later one impels us anew not
to forget, in all our new discussions, and especially amid
the social and economic speculations on which we are
now entering, that a well-maintained soil is the first
essential, not only to agricultural progress but to human
prosperity. The soil is the greatest natural resource.
We must never, in our philosophy, get away from the
land.

Attention is called to the analysis of the subject-
matter of this volume as outlined in the table of contents
and expanded in the text. The educational value of
any subject or volume lies not so much in the information

PREFACE TO THE RURAL TEXT-BOOK SERIES Vll

that is presented as in the organization of the information
into a systematic treatment, whereby a philosophy of
the subject is developed. A college text should be a
unity, rounding up the subject so completely as to give
the student a grasp of the material as one problem,
and at the same time expounding the reasons on which
the treatment rests. When the student has completed
any text, he should have a clear mental topography of
the subject that it treats. So may the agricultural
subjects be made the agencies in developing clear think-
ing, sound argument, constructive imagination, and
effective application to the needs of life.

L. H. BAILEY.

Ithaca, N. Y.
October 1, 1909

AUTHORS' PREFACE

In teaching introductory courses in soil technology
to agricultural students, the authors feel that the use of
a text book enables the student to get a more thorough
mental discipline and a better grasp of the details of the
subject than can result from a course of lectures. The
present book is the outgrowth of their experience in
teaching soil technology through a period of several
years. It has been their endeavor to present the appli-
cation of science to soil problems from the standpoint
of crop-production rather than that of any one of the
underlying sciences of geology, chemistry, physics or
bacteriology. This has necessitated drawing from a wide
range of literature, and arranging the material in a form
which it is thought adequately represents all phases of
the subject. The sources of such data have been freely
drawn upon, and the authors take this opportunity to
express their obligations for the aid they have received
from a very large number of papers and books dealing
with soils, and which it has not been found practicable to
credit specifically in the text, as has been done in many
instances.

It may happen that some teachers will not wish to

(ix)

X AUTHORS1 PREFACE

follow the entire text, in which event we think it will be
found possible to omit certain sections and yet have a
connected treatment of the subject. On the other hand,
very little attempt has been made to supply illustrations
of the principles which are explained. Such illustrations
and amplifications are left to be added by the teacher as
local conditions and interests may dictate.

The book, as its title implies, deals largely with the
Principles of Soil Technology, and applications of these
to local practice should constitute a part of the instruc-
tion.

Attention is called to the outline of contents, which
shows the method of treatment and the relation of the
several parts of the subject. As an elementary treatise, it
has been the aim to properly balance the discussion of
all phases of the subject, which may be followed in
greater detail in advanced courses.

In the illustrations, endeavor has been made to
include cuts of all of the more common types of soil-
working implements. We are indebted to the United
States Bureau of Soils for several illustrations, and to
Pfeffer's 'Pfiangenphysiology' for three cuts which, by
mistake, were not credited in the text.

THE AUTHORS

Cornell University,

Ithaca, N. Y.

October 18, 1909.

OUTLINE AND TABLE
OF CONTENTS

PAGE

A.. The Soil as a Medium for Root-Development 1

1. The rock and its products 2

I. The elements of plant-food 3

a. Elements essential to plant-growth (1).*

b. General abundance of plant-food elements (2).

II. Important soil-forming minerals 4

a. Soil-forming minerals, their composition and

properties (3).

b. Relative abundance of common minerals (4).

III. Important soil-forming rocks .' 9

a. Igneous, Aqueous, iEolian and Metamorphic
rocks (5).

IV. Chemical and physical agencies of rock decay. ... 14

a. Atmosphere (6).

b. Heat and cold (7).

c. Water (8).

d. Ice — Glaciers (9).

e. Plants and animals (10).

V. Geological classification and chemical composition

of soils 30

a. Sedentary soils (11).

(1) Residual (12).

(2) Cumulose (13).

b. Transported soils (14).

(1) Gravity or Colluvial (15)
f (2) Water (16).
j (a) Marine soils (17).

(b) Lacustrine soils (18).

(c) Alluvial soils (19).

(3) Ice— Glacial soils (20).

(4) Wind— iEolian soils (21).
♦Number in parenthesis refers to section

(xi)

Xli OUTLINE AND TABLE OF CONTENTS

PAGE

VI. Humid and arid soils 64

VII. Resume1 of scheme of classification and general char-
acteristics of the groups 66

2. The soil mass. Physical properties of the soil and their

modification 68

a. Soil and subsoil (22).

I. Inorganic constituents 69

a. Texture (23).

(a) Textural classification (24).

(1) Textural groups (25).

(2) Agricultural classes based on texture (26).

(6) Some physical properties of arid and humid
soils (27).

(c) Some properties of soil separates and classes

(28).

(1) Number of particles (29).

(2) Surface area of particles (30).

(3) Chemical composition of soil separates (31).

(d) Modification of soil texture (32).
5. Structure (33).

(a) Some aspects of soil structure (34).

(1) Ideal arrangement (35).

(2) Porosity (36).

(3) Weight (37).

(4) Plasticity (38).

(5) Cementing materials (39).

(6) Color (40).

(7) Physical absorption (41).

(b) Conditions affecting structure (42).

(c) Means of modifying structure (43).

(1) Variation in moisture content (44).

(2) Formation of ice crystals (45).

(3) Tillage (46).

(4) Growth of plant roots (47).

(5) Organic matter (48).

(6) Soluble salts (49).

(7) Animal life (50).

(8) Rainfall (51),

OUTLINE AND TABLE OF CONTENTS Xlll

PAGE

II. Organic constituents of the soil . 119

a. Sources, derivation and forms (52).

b. Chemical composition (53).

c. Amounts present (54).

d. Some physical properties (55).

(1) Solubility (56).

(2) Weight (57).

(3) Absorptive properties (58).

(4) Volume changes (59).

(5) Plasticity (60).

e. Effects of organic matter (61).

(1) Physical effects (62).

(2) Chemical effects (63).

/. Maintenance of organic matter (64).

B. The Soil as a Reservoir for Water 133

I. Functions in plant-growth 133

II. Amount of water in the soil 135

Determined by

a. The supply (65).

b. Retentive capacity (66).

1. Statement of water-content (67).

2. Forms and availability (68).

3. Amounts of each form (69).

(a) Hygroscopic water (70).

(b) Capillary water (71).
Determined by

(1) Texture (72).

(2) Structure (73).

(3) Content of organic matter (74).

(a') Volume of water held by different soils
— maximum, minimum and opti-
mum water-content (75).

(b') Available water in some field soils (76).

(e') Relation of surface tension to capil-
larity (77).

(c) Gravitational water (78).

c. Amount and rate of loss (79).

XIV OUTLINE AND TABLE OF CONTENTS

PAGE

III. Movement of soil-water 165

a. Gravitational movement (80).

b. Capillary or film movement (81).

1. Principles governing capillary movement (82).

2. Extent, rate and importance of capillary move-

ment (83).
Determined by

(a) Texture (84).

(b) Dampness of soil particles (85).

(c) Structure (86).

(d) Surface tension (87).

(e) Condition of surfaces of particles (88).

3. Examples of amount of water moved (89).

c. Thermal movement (90).

IV. Control of soil-water 190

a. Means of increasing water-content of the soil (91).

1. Decreasing loss (92).

(a) Percolation (93).
(6) Evaporation (94).

(1) Mulches (95).

(a') Mulching plow land (96).
(b') Fall and spring plowing (97).

(2) Other surface treatments (98)

2. Increasing the water capacity (99).

3. Irrigation (100).

(a) Factors affecting the duty of water (101).
(6) Methods of applying water (102).

(1) Floodings (103).

(2) Furrows (104).

(3) Overhead sprays (105).

(4) Sub-irrigation (106).

b. Means of decreasing the water-content of the soil (107).

1. Drainage by ditches (108).
(a) Effects of drainage (109).

(1) Firms the soil (110).

(2) Improves the structure (111).

(3) Increases the available water (112).

OUTLINE AND TABLE OF CONTENTS XV

PAGE

(4) Improves the aeration (113).

(5) Raises the average temperature (114).

(6) Influences the growth of organisms (115).

(7) Increases the food-supply (116).

(8) Enlarges the root-zone (117).

(9) Reduces "heaving" (118).

(10) Removes injurious salts from alkali- soils

(119).

(11) Reduces erosion (120).

(12) Increases crop-yields and improves sani-

tary conditions (121).

(b) Principles of drainage (122).

(1) Open or surface drains (123).

(2) Covered or under-drains (124).

2. Other types of drainage (125).

3. Surface culture (126).

C. Plant Nutrients of the Soil 267

I. Solubility of the soil through natural processes 267

II. Solubility of the soil in various solvents 268

a. Complete solution of the soil (127).

b. Digestion with strong hydrochloric acid (128).

1. Interpretation of results of analysis of hydro-
chloric acid solution (129).

(a) Permanent fertility and manurial needs (130)

(b) Relation of texture to solubility (131).

(c) Nature of subsoil (132).

(d) Calcium carbonate (133).

(e) Estimation of deficiency of ingredients (134).
(J) Conclusions (135).

c. Extraction with dilute organic acids (136).

1. Advantages in showing manurial needs (137).

2. Usefulness of citric acid (138).

d. Extraction with aqueous solution of carbon dioxid

(139).

e. Extraction with pure water (140).

1. Influence of absorption (141).

2. Other factors (142).

xvi OUTLINE AND TABLE OF CONTENTS

PAGE

III. Mineral substances absorbed by plants 279

a. Substances found in ash of plants (143).

b. Amounts of plant-food material removed by crops(144).

c. Amounts of plant-food material contained in soils(145).

d. Possible exhaustion of mineral nutrients (146).

IV. Acquisition of nutritive salts by agricultural plants. . . . 286

a. Selective absorption (147).

b. Relation between root-hairs and soil-particles (148).

c. Absorptive power of different crops (149).

1. Extent of absorbing system (150).

2. Osmotic activity (151).

3. Cereal crops (152).

4. Grass crops (153).

5. Leguminous crops (154).

6. Root crops (155).

7. Vegetables (156).

8. Fruits (157).

V. Absorption by the soil of substances in solution 297

a. Substitution of bases (158).

b. Time required for absorption (159).

c. Insolubility of certain absorbed substances (160).

d. Influence of size of particles (161).

e. Causes of absorption (162).

1. Zeolites (163).

2. Other absorbents (164).
/. Adsorption (165).

g. Occlusion (166).

h. Adsorption as related to drainage (167).

1. Substances usually carried in drainage water (168).

2. Drainage records at Rothamsted (169).

i. Relation of absorptive capacity to productiveness(170).

VI. Alkali soils 307

a. Composition of alkali salts (171).

b. White and black alkali (174).

c. Effect of alkali on crops (173).

1. Direct effect (174).

2. Indirect effect (175).

OUTLINE AND TABLE OF CONTENTS xvil

PAGE

3. Effect upon different crops (176).

4. Other conditions influencing the action of alkali

(177).
d. Reclamation of alkali land (178).

1. Irrigation and alkali (179).

2. Under-drainage (180).

3. Correction of black alkali (181).

4. Retarding evaporation (182).

5. Cropping with tolerant plants (183).

6. Other methods (184).

7. Alkali spots (185).

VII. Manures 319

a. Early ideas of the function of manures (186).

b. Development of the idea of the nutrient function of

manures (187).

c. Classes of manures (188).

1. Commercial fertilizers (189).

(a) Function of commercial fertilizers (190).

(b) Fertilizer constituents (191).

(c) Fertilizers used for their nitrogen (192).

(1) Sodium nitrate (193).

(2) Ammonium sulfate (194).

(3) Calcium cyanamid (195).

(4) Calcium nitrate (196).

(5) Organic nitrogen in fertilizers (197).

(d) Fertilizers used for their phosphorus (198).

(1) Bone phosphate (199).

(2) Mineral phosphates (200).

(3) Superphosphate fertilizers (201).

(4) Reverted phosphoric acid (202).

(5) Double superphosphates (203).

(6) Relative availability of phosphate fer-

tilizers (204).

(e) Fertilizers used for their potassium (205).

(1) Stassfurt salts (206).

(2) Wood-ashes (207).

(3) Insoluble potassium fertilizers (208).

XVlil OUTLINE AND TABLE OF CONTENTS

PAGE

2. Fertilizer practice (209).

(a) Brands of fertilizers (210).

(b) Fertilizer inspection (211).

(c) Trade values of fertilizers (212).

(d) Computation of the commercial value of a

fertilizer (213).

(e) Mixing fertilizers on the farm (214).
(/) Methods of applying fertilizers (215).

3. Soil amendments (216).

(a) Salts of calcium (217).

(1) Effect on tilth (218).

(2) Liberation of plant-food materials (219).

(3) Effect on toxic substances and plant

diseases (220).'

(4) Forms of calcium (221).
(a') Caustic lime (222).

(V) Carbonate of lime (223).

(c') Sulfate of lime (224).
(6) Common salt (225).
(c) Muck (226).

4. Factors affecting the efficiency of fertilizers (227).

(a) Soil-moisture content (228).
(6) Soil-acidity (229).

(c) Organic matter (230).

(d) Structure or tilth of the soil (231).

(e) Cumulative need for fertilizer (232).

5. Farm manures (233).

(a) Solid excreta (234).

(b) Urine (235).

(c) Litter (236).

(d) Manures produced by different animals (237).

(1) Horse manure (238).

(2) Cow manure (239).

(3) Swine manure (240).

(4) Sheep manure (241).

(5) Relative values of animal manures (242).

(6) Poultry manure (243).

OUTLINE AND TABLE OF CONTENTS XIX

PAGE

(e) Factors affecting the values of manures (244).

(1) Age of animal (245).

(2) Food of animal (246).

(3) Use of animal (247).

(/) Deterioration of farm manure (248).

(1) Fermentations (249).

(2) Leaching (250).

(g) Methods of handling (251).
(h) Place in rotation (252).
(i) Functions (253).
6. Green manures (254).

(a) Leguminous crops (255).

(b) Cereal crops (256).

D. Organisms in the Soil 388

I. Macro-organisms of the soil 388

a. Rodents (257).

b. Worms (258).

c. Insects (259).

d. Large fungi (260).

e. Plant-roots (261).

II. Micro-organisms of the soil 391

a. Plant micro-organisms (262).

1. Plant micro-organisms injurious to higher plants

(263).

2. Plant micro-organisms not injurious to higher(264).

3. Bacteria (265).

(a) Distribution (266).

(b) Numbers (267).

(c) Conditions affecting growth (268).

(1) Oxygen (269).

(2) Moisture (270).

(3) Temperature (271).

(4) Organic matter (272).

(5) Soil acidity (273).

(d) Functions of soil bacteria (274).

(1) Decomposition of mineral matter (275).

XX OUTLINE AND TABLE OF CONTENTS

PAGE

(2) Decomposition of non-nitrogenous organic

matter (276).

(3) Decomposition of nitrogenous organic

matter (277).

(a') Decay and putrefaction (278).
(b') Ammonification (279).
(O Nitrification (280).

(1') Effect of organic matter on nitri-
fication (281).

(2') Effect of soil-aeration on nitrifica-
tion (282).

(3') Effect of sod on nitrification (283).

(4') Depth at which nitrification takes

place (284).
(5') Loss of nitrates from the soil (285).

(4) Denitrification (286).

(5) Nitrogen fixation through symbiosis with

higher plants (287).
(a') Relation of bacteria to nodules on

roots (288).
(b') Transfer of nitrogen to the plant (289).
(c') Soil-inoculation for legumes (290).

(6) Nitrogen fixation without symbiosis with

higher plants (291).
(a') Nitrogen-fixing organisms (292).

(6') Mixed cultures of nitrogen-fixing

organisms (293).
(c') Nitrogen-fixation and denitrification '

antagonistic (294).

E. The Soil-Air 432

I. Factors determining volume 432

a. Texture (295).

b. Structure (296).

c. Organic matter (297).

d. Moisture content (298).

II. Composition of soil-air 434

a. Analysis of soil-air (299).

b. Production of carbon dioxid as affecting composition

(300).

OUTLINE AND TABLE OF CONTENTS xxi

PAGE

c. Escape of carbon dioxid as affecting composition (301).

d. Effect of roots upon composition (302).

III. Functions of the soil-air 437

a. Oxygen (303).

b. Carbon dioxid (304).

IV. Movement of soil-air 439

a. Diffusion of gases (305).

b. Movement of water (306).

c. Changes in atmospheric pressure (307).

d. Changes in temperature (308).

e. Suction produced by wind (309).

V. Methods for modifying the volume and movement of soil- 443
air

a. Tillage (310).

b. Manures (311).

c. Under-drainage (312).

d. Irrigation (313).

e. Cropping (314).

F. Heat of the Soil 448

I. Function of the heat of the soil in its relation to plant-
growth 448

a. Biological (315).

1. Germination (316).

2. Growth and vegetation (317).

3. Activity of the soil-organisms (318).

b. Chemical changes (319).

c. Physical changes (320).

II. Sources of the heat of the soil 451

a. Solar radiation (321).

b. Conduction (322).

c. Organic decay (323).

III. Temperature of the soil 453

a. Heat supply (324).

b. Specific gravity and specific heat (325).

c. Color of the soil (326).

d. Slope of the soil (327).

xxii OUTLINE AND TABLE OF CONTENTS

PAGE

e. Conductivity (328).
/. Circulation of air (329).
g. Water content (330).

IV. Means of modifying the soil temperature 463

G. External Factors in Soil-Management 465

I. Means of modifying the soil 465

a. Summary of practices (331).
II. Tillage 466

a. Objects of tillage (332).

b. Implements of tillage (333).

1. Effect on the soil (334).

2. Mode of action (335).

(a) Plows (336).

(1) Pulverization (337).

(2) Covering rubbish (338).

(b) Cultivators (339).

(1) Cultivators proper (340).

(2) Leveler and harrow type of cultivator(341).

(3) Seeder cultivators (342).

(c) Packers and crushers (343).

(1) Rollers (344).

(2) Clod crushers (345).

III. Other phases of tillage operations 489

a. Weeds in their relation to crop-production (346).

1. Objectionable qualities of weeds (347).

2. Control of weeds (348).

b. Erosion (349).

1. By water (350).

2. By wind (351).

IV. Adaptation of crops to soil 497

a. Philosophy of crop-adaptation (352).

b. Factors in crop-adaptation (353).

1. Physiological requirements of the plant (354).

2. Requirements for growth supplied by the soil (355).

OUTLINE AND TABLE OF CONTENTS XX111

PAGE

Relation of soil-productiveness to crop-rotations 503

a. Principles underlying crop-rotation (356).

1. Nutrients removed from the soil by different crops

(357).

2. Root systems of different crops (358).

3. Some crops or crop treatments prepare food for

other crops (359).

4. Crops differ in their effect upon soil structure (360).

5. Certain crops check certain weeds (361).

6. Plant diseases and insects checked by removal of

hosts (362).

7. Loss of plant food from unused soil (363).

8. Accumulation of toxic substances (364).

CLASSIFIED LIST OF
ILLUSTRATIONS

PAGE

1. "Plowing" Frontispiece

2. Map of United States, normal annual precipitation. Fig. 41 ... . 137

3. Map of western United States, irrigated and irrigable land.

Fig. 72 227

4. Map of United States, showing relative use of fertilizers. Fig.

108 323

5. Map of United States, sunshine received in different sections.

Fig. 121 452

6. Rock section. Granite. Fig. 1 8

7. Rock section. Diorite. Fig. 2 10

8. Rock section. Basalt. Fig. 3 11

9. Rock section. Fossiliferous limestone. Fig. 4 12

10. Rock section. Chert. Fig. 5 13

11. Rock section. Sandstone. Fig. 6 13

12. Photo-micrograph. Fine sand. Fig. 17 70

13. Photo-micrograph. Silt. Fig. 18 71

14. Influence of water film on soil granulation. Fig. 30 105

15. Weathered Laramie sandstone. Fig. 7 15

16. Types of weathering. Fig. 8 19

17. Weathered limestone in quarry. Fig. 9 22

18. "Pot-holes" in shale rock. Fig. 10 . . 25

19. Lichen on granite rock. Fig. 11 28

20. Tree roots spliting boulder. Fig. 12 29

21. Section of residual soil from limestone. Fig. 13 38

22. Section of muck underlain by marl. Fig. 14 42

23. Section of sedimentary soil. Fig. 15 48

24. Section of glacial soil. Fig. 16 58

25. Proportion of separates in sandy soil. Fig. 21 75

26. Proportion of separates in silt soil. Fig. 22 75

27. Proportion of separates in clay soil. Fig. 23 76

28. Undesirable soil structure. Fig. 27 90

29. Ideal soil structure. Fig. 28 91

30. Excessive checking of clay soil. Fig. 29 99

31. Ice crystals in field soil. Fig. 31 108

32. Honey-comb ice crystals. Fig. 32 109

(xxv)

XXVI CLASSIFIED LIST OF ILLUSTRATIONS

PAGE

33. Ice crystals and soil granulation. Fig. 33 110

34. Clay soil plowed wet. Fig. 35 112

35. Soil in good tilth. Fig. 38 126

36. A mulch of stone. Fig. 63 201

37. Example of clean cultivation. Fig. 65 206

38. Flume for measuring miner's inches. Fig. 73 227

39. Canvas dam. Fig. 74 232

40. Poorly drained clay land. Fig. 76 238

41. Section of tile drain in clay soil. Fig. 77 241

42. Soil granulated by drainage. Fig. 79 246

43. System of surface drains. Fig. 80 249

44. Construction of ditch for tile drain. Fig. 81 251

45. Laying tile by use of tile hook. Fig. 82 252

46. Laying tile by hand. Fig. 83 253

47. Types of drain tile. Fig. 88 257

48. Ditching machine in operation. Fig. 91 260

49. Ditch cut by machine. Fig. 92 261

50. Poorly constructed outlet of drain. Fig. 93 262

51. Result of poorly constructed outlet. Fig. 94 264

52. Well-constructed outlet. Fig. 95 265

53. Filling ditch by use of team. Fig. 96 266

54. Waste of manure by leaching. Fig. 101 302

55. Alkali spot. Fig. 102 308

56. Bromus inermis on alkali soil. Fig. 104 317

57. Wates of manure. Fig. 106 364

58. Manure piled in the field. Fig. 107 382

59. Alfalfa root tubercles. Fig. 114 424

60. Heavy sod freshly broken. Fig. 120 445

61. Erosion on gravelly hillside. Fig. 150 491

62. Terraces used in southern farming. Fig. 151 492

63. Side-hill ditches to prevent erosion. Fig. 152 493

64. Plant roots and erosion. Fig. 153 495

65. Celery and lettuce on muck soil. Fig. 154 498

66. Farm scene on light sand soil. Fig. 155 500

67. Farm scene on limestone loam soil. Fig. 156 502

68. Influence of crop rotation on growth of corn. Fig. 157 507

69. Diagram. Relative size of textural groups. Fig. 19 72

70. Diagram. Ideal arrangement of soil particles. Fig. 26 89

71. Diagram. Forms and proportion of soil water. Fig. 43 141

72. Diagram. Distribution of soil water. Fig. 45 147

73. Diagram. Adjustment of capillary soil water. Fig. 52 171

74. Diagram. Relation of root-hairs to soil water. Fig. 53 174

75. Diagram. Structure of mulched and unmulched soil. Fig. 62. . 199

76. Diagram. Water table in tile-drained land. Fig. 78 245

CLASSIFIED LIST OF ILLUSTRATIONS XXV11

PAGE

77. Diagram. Gridiron system of arranging drains. Fig. 84 254

78. Diagram. System of arranging drains. Fig. 85 255

79. Diagram. System of arranging drains. Fig. 86 255

80. Diagram. Natural system of arranging drains. Fig. 87 256

81. Diagram. Sectional view of ditching machine. Fig. 90 259

82. Diagram. Relation of root-hairs to soil particles. Fig. 99. ... 287

83. Diagram. Effect of deep and shallow tillage on roots. Fig. 100. 294

84. Diagram. Effect of alkali salts on plant cells. Fig. 103 312

85. Diagram. Nematodes entering a root. Fig. 108 392

86. Diagram. Types of soil bacteria. Fig. 109 395

87. Diagram. Influence of surface slope on sunshine received.

Fig. 125 458

88. Curves. Relative size of textural groups. Fig. 20 74

89. Curves. Average analysis of common classes of soil. Fig. 24 . 78

90. Curves. Relation of texture to crop adapation. Fig. 25 79

91. Curves. Relation of texture to water capacity. Fig. 44 145

92. Curves. Distribution of water in columns of soil. Fig. 46. ... 148

93. Curves. Relation of texture to capillary water capacity. Fig. 47. 150

94. Curves. Relation of structure to water capacity. Fig. 48 ... . 152

95. Curves. Water capacity of sandy soil in field. Fig. 49 156

96. Curves. Water capacity of clay soil in field. Fig. 50 157

97. Curves. Water capacity of silt soil in field. Fig. 51 157

98. Curves. Relation of capillary rise to texture. Fig. 54 175

99. Curves. Relation of capillary rise to texture. Fig. 55 177

100. Curves. Relation of capillary rise to texture. Fig. 56 179

101. Curves. Relation of capillary rise to texture. Fig. 57 179

102. Curves. Lateral capillary movement. Fig. 58 184

103. Curves. Annual precipitation and percolation, England. Fig. 61. 193

104. Curves. Relation of evaporation to mulch formation. Fig. 64 . 204

105. Curves. Relation of soil moisture to yield of dry matter. Effect

of weeds. Fig. 66 209

106. Curves. Influence of cloth tent on soil moisture. Fig. 68 ... . 215

107. Curves. Relation of soil condition to the formation of nitrates.

Fig. 112 414

108. Curves. Daily range of soil temperature. Fig. 122 454

109. Curves. Mean annual range of air and soil temperature. Ne-

braska. Fig. 123 455

110. Curves. Effect of color on soil temperature. Fig. 124 457

111. Moldboard plow with shares. Fig. 34 Ill

112. Middlebreaker plow. Fig. 75 236

113. Hillside plow. Fig. 118 441

114. Plow with parts named. Fig. 127 466

115. Heel-plate of plow. Fig. 128 467

116. Sulky moldboard plow. Fig. 129 468

XXVlii CLASSIFIED LIST OF ILLUSTRATIONS

PAGE

117. Sulky disc plow. Fig. 135 476

118. Six gang plow. Fig. 132 473

119. Subsoil plow. Fig. 69 218

120. Subsoil plow. Fig. 70 219

121. One-horse toothed cultivator. Fig. 60 191

122. Weeder. Fig. 59 187

123. Large-shovel cultivator. Fig. Ill 407

124. Small-shovel cultivator. Fig. 113 419

125. Spring-toothed cultivator. Fig. 114 402

126. Blade cultivator. Fig. 115 433

127. Disc cultivator. Fig. 116 436

128. Hand cultivator. Fig. 117 438

129. "Sweep" cultivator. Fig. 137 479

130. Hand tillage implements. Fig. 98 281

131. Spring-toothed harrow. Fig. 36 114

132. Spike-toothed harrow. Fig. 42 140

133. Solid-disc harrow. Fig. 39 130

134. Meeker harrow. Fig. 140 487

135. Meeker harrow. Near view. Fig. 37 118

136. Cut-out-disc harrow. Fig. 97 276

137. Spading-disc harrow. Fig. 126 463

138. Extension disc harrow. Fig. 136 477

139. Acme harrow. Fig. 119 444

140. Clod crusher. Fig. 71 220

141. Scotch chain harrow. Fig. 146 486

142. Solid or barrel roller. Fig. 40 135

143. Bar roller. Fig. 147 487

144. "Float" or plank smoother. Fig. 149 489

145. Campbell sub-surface packer. Fig. 67 212

146. Broadcast seeder. Fig. 138 480

147. Grain drill. Fig. 144 485

148. Sulky lister. Fig. 130 -470

149. One-horse grain drill. Fig. 139 481

150. Stubble digger. Fig. 143 484

151. Garden seeder. Fig. 133 474

152. Berry hoe. Fig. 134 475

153. Beet loosener. Fig. 141 482

154. Cotton-and-corn planter. Fig. 142 483

155. Corn planter. Fig. 145 486

156. Potato digger. Fig. 148 488

157. Hand drainage tools. Fig. 89 258

158. Types of coulters. Fig. 131 471

INTRODUCTION

By L. H. BAILEY

The exposed surface of the crust of the earth tends
always to pass into a loose and disintegrated layer.
In this layer many organisms live, and out of it many of
them derive an essential part of their nourishment.
The organisms die and their remains return to the place
whence they came. In every successive epoch of the
earth's history, this layer has tended to become more
differentiated and complex in each epoch supporting
a higher type of plant, and in each succeeding age main-
taining a more advanced kind of activity. Thus the soil
has been formed, and the evolution of it and of the plant
tribes that grow out of it have been reciprocal, one con-
tributing to the other. If the soil is essential to the
growing of plants, so have the plants been essential to
the formation of soil.

This marvelously thin layer of a few inches or a very
few feet that the farmer knows as "the soil," supports
all plants and all men, and makes it possible for the globe
to sustain a highly developed life. Beyond all calculation
and all comprehension are the powers and the mysteries
of this soft outer covering of tho earth. We do not know

(xxix)

XXX INTRODUCTION

that any vital forces pulsate from the great interior bulk
of the earth. For all we know, the stupendous mass of
materials of which the planet is composed is wholly
dead; and only on the veriest surface does any nerve of
life quicken it into a living sphere. And yet, from this
attenuated layer have come numberless generations
of giants of forests and of beasts, perhaps greater in
their combined bulk than all the soil from which they
have come; and back into this soil they go, until the great
life principle catches up their disorganized units and
builds them again into beings as complex as themselves.

The general evolution of this soil is toward greater
powers; and yet, so nicely balanced are these powers that
within his lifetime a man may ruin any part of it that
society allows him to hold; and in despair he throws it
back to nature to reinvigorate and to heal. We are ac-
customed to think of the power of man in gaining domin-
ion over the forces of nature, — he bends to his use the
expansive powers of steam, the energy of electric cur-
rents, and he ranges through space in the light that he
concentrates in his telescope; but while he is doing all
this he sets at naught the powers in the soil beneath his
feet, wastes them, and deprives himself of vast sources
of energy. Man will never gain dominion until he learns
from nature how to maintain the augmenting powers
of the disintegrating crust of the earth.

There are three great kinds of natural resources, —

INTRODUCTION XXXI

the earth itself, the atmosphere that envelopes it and which
may be considered an outer layer of it, and the sunshine.
From these three, and all the materials and forces that
are in them contained, we derive the conditions of our
existence and express our outlook to destiny. We can
do little to control or modify the atmosphere or the
sunlight; but the surface of the earth is ours to do with
it much as we will. It is the one great resource over which
we have dominion. Within this crust are great stores of
minerals and of metals and of other materials that we
can use for our comfort; these materials we can save
and we may use them with economy, but we cannot
cause them to increase. But the soil may be made better
as well as worse, more as well as less; and to save the
producing powers of it is far and away the most import-
ant consideration in the conservation of natural resources.
The man who owns and tills the soil, therefore, owes
an obligation to his fellowmen for the use that he makes
of his land; and his fellowmen owe an equal obligation to
him to see that his lot in society is such that he will
not be obliged to rob the earth in order to maintain his
life. The natural resources of the earth are the heritage
and the property of every one and all of us. We shall
reach the time when we shall not allow a man to till the
earth unless he is able to leave it at least as fertile as he
found it. A man has no moral right to skin the earth,
unless he is forced to do it in sheer self-defence and to

XXXli INTRODUCTION

enable him to live in some epoch of an unequally devel-
oped society; and if there are or have been such social
epochs, then is society itself directly responsible for the
waste of the common heritage.

On every side, therefore, it is important that we study
the soil. Beyond all mere technical agricultural practice,
the principles of soil management must be compre-
hended and taught. There is no good sociology that does
not recognize this fact.

We tend always to discuss great subjects from one
point of view. So has the soil usually been treated from
the chemical point of view, from the geological, from the
agricultural. In this book, the authors have attempted
to discuss the soil in all its relations to plant production,
developing the inter-dependence of geological, chemical,
bacteriological, physical and industrial relationships in
such a way as to give the student a grasp, albeit a brief
one, of the entire subject in its many bearings. In its
treatment, the book considers, first, the soil as a medium
for root development ; second, as a reservoir for water ;
third, as a source of nutrients; fourth, as a realm of
organisms; fifth, in its relation to air; sixth, its relation
to heat; and the relation of man to the soil follows as a
consequence and conclusion.

The past few years constitute a period of great activity
in the study of the soil, so much so that many of our most
established opinions have been challenged. Perhaps it

INTRODUCTION XXXlii

is yet too early to rationalize all the new discussions
into a clear course of practice, but we are surely getting
nearer to the fundamental problems, and we shall evolve
a better system of agricultural procedure. The stimula-
tion of inquiry and imagination cannot fail to produce
great results.

So am I glad of every new effort that puts men ration-
ally on their feet on the soil. It will be a great thing when
the soil is known in schools. I wait for good politics and
good institutions to grow out of the soil. I wait for the
time, also, when we shall have good poetry and good
artistic literature developing from subjects associated
with the soil; for we want good literature to appeal to
all men.

THE PRINCIPLES OF SOIL
MANAGEMENT

A. THE SOIL AS A MEDIUM FOR ROOT
DEVELOPMENT

The soil is a medium for the development of plants.
In the main, the plants which are of agricultural impor-
tance are differentiated into root and top, and the
former penetrates the soil in order to obtain food and
moisture, and to afford a firm support for the aerial
portion. Every plant has definite requirements for
its best development. The character of the mature
plant is the result of two sets of forces. The first of
these is the inherent capacity of the seed to develop
and produce a normal individual of its kind. The second
set of forces constitute the environment in which the
plant grows, and of which the soil is one part, the other
component being climate. Every plant is an expression
of the combination and interaction of these three
groups of forces — the seed, the climate, and the soil.

The external factors in plant growth may be further
differentiated into the following: (1) Food, (2) moisture,
(3) heat, (4) light, (5) air, (6) mechanical support,
and (7) freedom from biological enemies, such as fungous
disease and animal attack. With the exception of light,
every one of these factors is partially or wholly deter-

A (1)

2 THE PRINCIPLES OF SOIL MANAGEMENT

mined by the character and condition of the soil. It
is the source of the majority of the nutritive elements,
it contains the water necessary for the plant and in
which is carried its food, it holds air in its pores, and
it absorbs and transmits the necessary heat. Enemies
of one plant may or may not be present; but, if present,
they may exercise a controlling influence. All the parts
of the soil mechanism — for such it must be considered —
are closely related to each of these essential factors,
and it is from this point of view of the growing plant
that the following treatment is developed.

The characteristics of the soil may be viewed from
both the origin of the material and its properties. The
first of these may be termed "The Rock and Its Prod-
uct," and, second, — in so far as they pertain to physical
properties,— "The Soil Mass."*

1. The Rock and Its Products

Since all soil material forms a part of the structure
of the earth, its origin and derivation constitute a part
of the field of geology. The following discussion of
the rock and its products deals primarily with these
facts and processes. But the discussion is not taken
up because of its geological interest, great as that is,
but because of the fundamental connection these
have to the physical, chemical and biological proper-
ties of the soil which determine its ability to grow plants.
The kinds of minerals and rocks in which the essential
elements of plant-food originally occur, and the changes

ELEMENTS OF PLANT-FOOD 3

which they may have undergone in their transition to
the present combinations in the soil, as well as the fact
that the physical properties of the soil are primarily
determined by its derivation, render their study of
fundamental concern in order to understand the soil'
as a medium for plant-growth. The classification and
detailed study of the soil is inseparably linked with its
derivation, because determined by it. On one side, it
supplies certain elements of food whose relative abun-
dance is determined by their distribution in the original
rocks and their concentration or dissipation through
geological changes, and, on the other side, it affords
the physical medium for the development of the plant.

I. THE ELEMENTS OF PLANT-FOOD

The plant must have certain food elements for its
growth and development. These elements are affected
by the changes to which the rock is subjected, and in
the end will reflect the character of these changes.

1. Elements essential to plant-growth. — The. essen-
tial elements of plant-food are ten in number, to which
may be added three others which seem to be useful
under certain conditions. The essential elements may be
divided into two groups, on the basis of their origin:
(1) The elements derived entirely and only from the
solid portion of the soil. These are calcium, magnesium,
potassium, phosphorus, iron and sulfur. (2) The ele-
ments derived either directly or indirectly from air
and water. These are carbon, hydrogen, oxygen and
nitrogen.

4 THE PRINCIPLES OF SOIL MANAGEMENT

2. General abundance of the plant-food elements. —

Having now in mind the essential food elements it is
of interest to know their general abundance in the earth's
cjust. The following table is given by Clark:

Oxygen 47.02 Phosphorus 0.09

Silicon 28.06 Manganese 07

Aluminum 8.16 Sulfur 07

Iron 4.64 Barium 05

Calcium 3.50 Strontium 02

Magnesium 2.62 Chromium 01

Sodium 2.63 Nickel 01

Potassium 2.32 Lithium 01

Titanium 41 Chlorin 01

Hydrogen 17 Fluorine 01

Carbon 12

100.00

The first eight elements form 98.8 per cent of the
earth's crust. In this list are found all of the food ele-
ments except nitrogen, which forms four-fifths of the
atmosphere. All of the food elements except nitrogen
appear among the first thirteen, and in amounts of not
less than .07 per cent. This gives assurance that none
of the food elements are rare. It will appear later that
they are all very generally distributed. The ultimate
source of the elements of the first or so-called incom-
bustible groups is the minerals of the earth's crust.

II. IMPORTANT SOIL-FORMING MINERALS

Minerals are the units of which soils and rocks are
primarily composed. A mineral is a compound occurring
in nature having approximately a definite chemical

SOIL-FORMING MINERALS 5

composition, usually a distinct crystalline form and
definite physical properties. A very large number of
species of minerals which differ greatly from each other
in composition and physical properties have been recog-
nized. It is these differences which renders necessary
a study of those important species which are found in
the soil, in order to gain a thorough knowledge of the
relations which they bear to plant nutrition and the phys-
ical and chemical characteristics of the soil mass. By their
chemical and physical weakness or resistance, they
modify the supply of food elements and determine the
physical make-up of the soil, with all the attendant
physical conditions of heat, moisture, air, etc., which
this limits.

While the number of minerals known is very great,
only a comparatively small number occur in the soil
in important amounts; but these are thoroughly repre-
sentative. All minerals may be divided into two groups:
(1) The original or primary constituents which were
formed at the first consolidation. (2) The secondary
constituents which result from changes in the minerals
subsequent to their first consolidation, and which are due
in large part to the chemical action of percolating water.

3. Soil-forming minerals; their composition and
properties. — The soil is composed of a great variety
of minerals and probably almost every recognized
species could be found in some soil. But the number
of minerals which make up the bulk of soil is rela-
tively small. The following table includes the most
important soil-forming minerals and their leading
properties:

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THE PRINCIPLES OF SOIL MANAGEMENT

4. Relative abundance of the common minerals. —

Hall quotes D'Orbigny as saying that in the earth's
crust the chief minerals are present in the following
proportions:

Feldspars 48

Quartz 35

Micas 8

Talc 5

Carbonate of lime and magnesium 1

Hornblend, augite, etc 1

All other minerals and weathered products 2

100
These general relations agree with the statements
of Chamberlin and Salisbury, who give the following
summary of the salient facts relating to the composition

of minerals: " (1)
Out of the sev-
enty-odd chemical
elements in the
earth, eight form
the chief part of
it. (2) One of
these elements
uniting with the
rest forms nine
leading oxides. (3)
One of these ox-
ides acts as an
acid, and the rest
as bases. (4) By

.biG. 1. bection of granite, magnified. The . .

crystals are orthoclase, microline, plagioclase, their Combination

quartz, black mica or biotite, white mica or mus- . .

covite. (Merrill.) they form a series

SOIL-FORMING ROCKS 9

of silicates, of which a few are easily chief. (5) These
silicates crystallize into a multitude of minerals, of
which again a few are chief. (6) These minerals are
aggregated in various ways to form rocks."

Hundreds of analyses of rocks have been made in
this country and abroad and from these Clark finds the
mineralogical composition of igneous rocks of the earth's
crust to be as follows:

Feldspars 59.5

Hornblend and pyroxine 16.8

Quartz 12.0

Biotite mica 3.8

Titanium minerals 1 .5

Apatite 0.6

94.2
This leaves 5.8 per cent to be distributed among the
more rare minerals.

III. IMPORTANT SOIL-FORMING ROCKS J THEIR PROPER-
TIES AND OCCURRENCE

A rock is an aggregate of minerals. Moreover, it
usually exhibits a considerable degree of consolidation,
and forms an essential portion of the earth's structure.
Very few minerals occur in nature in large pure masses.
They are usually grouped together in different combi-
nations, and, while it is essential to trace the changes
of each mineral, it is also necessary to give attention to
the groups of minerals — rocks — since the association
of minerals determines very largely the processes by
which rocks are transformed into soil and the character-
istics of the resulting soil.

10

THE PRINCIPLES OF SOIL MANAGEMENT

These aggregates of minerals, or rocks, are essentially
without order or arrangement. The minerals are in
irregular crystals or fragments of greatly differing sizes

closely packed to-
gether. The great
variety of miner-
als, as well as the
different physical
forms of the same
mineral, is pro-
ductive of an infi-
nite variety of
rocks. While in-
dividuals may dif-
fer greatly, there
is an easy and
gradual transition
from one form to
another which
renders it impos-
sible to draw hard
and well-defined lines separating each species of rock
from every other species. They blend one into the
other, not only in structure and crystalline form but
also in chemical composition.

The classification of rocks is based upon these facts,
and they are grouped broadly under four main heads,
the distinctions being their origin and structure. Each
oi the main divisions is again divided into groups and
families, the distinctions being those of mineral and
chemical composition, structure and mode of occurrence.

Fig. 2. Photomicrograph of diorite rock. Com-
pare with Figs. 3, 5, and 6, which have a different
mineral composition, crystalline form and struc-
ture. These differences determine the type and
rate of their weathering. (Lord.)

STRUCTURE OF IGNEOUS ROCKS

11

The main divisions are: Igneous rocks — sometimes
called eruptive — which have been brought up from below
in a molten condition from which they have cooled and
solidified. They usually have two or more essential
minerals, and are massive, crystalline, glassy, or, in
certain altered forms, colloidal in structure. Aqueous
rocks have been formed mainly through the agency of
water, as (a) chemical precipitates, or as (b) sedimentary
deposits. They are usually fragmental, but may be
crystalline or colloidal, but never glassy. They have a
laminated or bedded structure, and usually have
many constituent minerals. vEolian rocks are formed
from wind-drifted
material. They
are fragmental in
character and ir-
regularly bedded
in structure. Met-
amorphic rocks
embrace those of
any of the fore-
going divisions
which have been
changed from
their original con-
dition through the
agencies of dyna-
mic and chemical
forces so that they exhibit new properties. They may
have one or many constituent minerals, and in structure
they are usually crystalline and bedded or foliated.

Fig. 3. Photomicrograph of basalt (trap) rock.
(Lord.)

12

THE PRINCIPLES OF SOIL MANAGEMENT

5. Igneous, aqueous, aeolean and metamorphic
rocks. — The igneous rocks are parent to all the other
forms. They may be arranged according to the amount
of silica they contain, those that are rich in that com-
pound being termed acid, and those that are lean,
basic. In this order, some of the most abundant rock

types are granite,
quartz, syenites,
diorites, gabbro,
diabase and ba-
salts.

Of the aqueous
rocks the chemical
precipitates are
relatively of small
importance. They
seldom form ex-
tensive rock
masses and are
usually intimately
mingled with
other types of
rock, especially those of the sedimentary group. The
most important ones agriculturally are the sulfates,
represented by gypsum beds. Certain phosphatic deposits
and some chlorides also belong in this group.

The aqueous sedimentary rocks are the most import-
ant agriculturally of any of the groups of rock, and
especially of the aqueous rocks, because of their large
surface distribution and their physiography. They are
composed of the fragments derived from the degenera-

Fig. 4. Photomicrograph of fossiliferous lime-
stone. (Lord.)

STRUCTURE OF SEDIMENTARY ROCKS

13

tion of all the
older rocks and
from the inorganic
remains of plant
and animal life.
These comprise
clay and shale (ar-
gillaceous), sand-
stone, conglom-
erate and breccia
(aren aceous) ;
limestone and dol-
omite (calcare-
ous) , together with
minor rocks of vol-
canic, phosphatic
and carbonaceous
character.

The sand-
stones, shales,
limestone and dol-
omite are easily
the most promi-
nent of this group,
and, in fact, of all
the types of rock,
in their present
agricultural im-
portance. They
compose immense
strata of rock, and

Fig. 5. Photomicrograph of chert. (Lord.)

Fig. 6. Photomicrograph of sandstone. (Lord.)

14 THE PRINCIPLES OF SOIL MANAGEMENT

are usually arranged in alternating layers of variable
thickness and extent, and have given rise to important
areas of soil.

^Eolian rocks are relatively insignificant, and are
generally of a sandy or clayey character.

Metamorphic rocks are correlated with all the other
types of rock, and have resulted from pronounced
alterations in other rocks. Their individual properties
are therefore similar to the rock from which they were
formed. Often their resistance to decay is increased
by the process, as in quartzite and slate.

IV. CHEMICAL AND PHYSICAL AGENCIES OF
ROCK-DECAY

There are five chief agencies of rock-decay. They
are, (a) the atmosphere, (b) heat and cold, (c) water,
(d) ice, and (c) plants and animals. The operations of
each of these agencies are of two sorts: (1) chemical;
(2) mechanical. The products of these two types of
force are distinctly different in their relaiton to the
plant. The chemical action of the various agencies
results in a changed composition of the minerals.
It results in the breaking down of the mineral com-
pounds, with the possible removal of the elements, as
when feldspar is changed to kaolinite. Here the base —
potash, soda or lime — is replaced by the elements of
water, and may be carried entirely away. The hydrated
residue loses some of its silica, and kaolinite is the result.
In other cases the change may be effected by the addi-
tion of material, as when pyrite is oxidized by the

16 THE PRINCIPLES OF SOIL MANAGEMENT

atmosphere to the sulfate by the direct union of oxygen
with the compound. Whether the process be an addition
or subtraction of material, it usually changes the stabil-
ity of the mineral, and perhaps the stability of the mass
of which the mineral is a part. The chemical action of
one agency often opens the way for the chemical and
mechanical action of other agencies, so that the decay
processes are hastened. This chemical breaking down
of minerals, and thereby of rock masses, is termed
decomposition. The mechanical breaking up of rocks
whereby only the state of division of the material is
changed is termed disintegration. The breaking up of
rocks due to expanison of heat, the freezing of water,
flowing of water, the grinding of glacial ice, and the
expansion of plant roots, are types of disintegration by
which the rock is simply reduced to a finer state of
division. The general tendency is for finer material to
result from decomposition than from simple disin-
tegration.

6. Atmosphere. — The atmosphere is composed of a
mixture of the gases nitrogen and oxygen, in the propor-
tion of four parts of the former to one part of the latter,
together with very minute quantities of carbon dioxide,
nitric oxide, ammonia, and, in even less amounts, other
volatile compounds, and a variable, but usually very
considerable amount of water-vapor, evidenced by
clouds, rain, snow, dew, etc. These gases, dissolved in
the atmospheric moisture, come in contact with rock
masses and change certain of its minerals into com-
pounds more or less soluble than they were originally.
The iron compounds are perhaps the most affected,

SOIL-FORMATION, ATMOSPHERE 17

and the change of the mineral pyrite is typical of the
process.

2Fe S2 + 702 + 2H20 = 2Fe S04 + 2H2S04

Fe S04 + 2H20 = Fe(OH)2 + H2S04

All of these changes of iron compounds under the
action of moist atmosphere are imperfectly understood,
but it is agreed that the above products may result from
the process. Since the sulfate is much more soluble than
the sulfid, the mineral is in this way easily removed.

The purely chemical action of the atmosphere is less
pronounced in its effects than its mechanical action.
As wind, it exerts some pressure upon projecting masses
tending to push them over, but its great work is accom-
blished when the wind carries solid particles of dust and
sand and when it acts on vegetation as a lever. In arid
and semi-arid regions, particularly, the amount of solid
material carried in the atmosphere is very large at
some seasons. There frequently occur dust storms,
when the atmosphere is so filled with wind-driven par-
ticles as to obscure the sun and all objects, at even a
short distance away. In the region of western Nebraska
and Kansas these dust storms are well known, and
on certain soils it is unwise to plow in the fall, because
by spring the soil will have been blown away to the
depth of the furrow, and indeed this sometimes results
from plowing at any other season of the year. Further
west in the mountain region this wind-blown material
is most effective, where the particles may be driven
against the bare rock faces. It then becomes a titanic
sand blast to drill away the rock. It eats into the rock
surface with remarkable rapidity, carving fantastic

18 THE PRINCIPLES OF SOIL MANAGEMENT

forms, as a result of the varying hardness of the rock
and the uneven distribution of the particles. The
abraded particles are born along by the wind and be-
come new tools of destruction.

In humid regions this form of disintegration is less
prominent, but in sandy regions it performs some ef-
fective work. As an example of this effectiveness,
Merrill describes a large sheet of plate-glass, once a
window, in a lighthouse on Cape Cod, — well known for
its sand-dunes. During a severe storm, of not above
forty-eight hours' duration, this became on its exposed
surface so ground by the impact of grains of sand
blown against it as to be no longer transparent, and to
necessitate its removal. He reports that window-panes
in dwelling-houses in the vicinity are frequently drilled
quite through by the same means.

Material blown about by wind is very much rounded
and smoothed by the impacts to which it has heen sub-
ject, a characteristic very much less in evidence in
water-moved material of the same fineness.

Winds also act in conjunction with plants where the
roots have penetrated into a crevice or joint, using the
tops as a lever to push off or further fracture masses of
rock. This process is most effective in rough mountainous
regions where the larger vegetation is just getting a foot-
hold. In passing, attention may be called to this process
of overturning plants as one of nature's cultural methods,
whereby the soil is subjected to very thorough, if long-
drawn-out, tillage.

7. Heat and cold. — In general, heat accelerates all
chemical processes. It greatly increases the solvent

SOIL-FORMATION, HEAT AND COLD

19

power of water for many substances, and renders it a
more destructive agent generally. This action can not
be discussed separately, but must be kept in mind in
the consideration of those other agencies of decompo-

Fig. 8. Two types of rock disintegration. The forms reflect the different
hardness and composition of the rocks

sition. Especially important are alterations of tempera-
ture, by which compounds whose rates of solution are
differently affected by temperature may be successively
acted upon.

Heat acts mechanically in two ways to break up
rocks: (1) Through expanison and contraction due to

20 THE PRINCIPLES OF SOIL MANAGEMENT

changes in temperature. All substances change volume
with changes in temperature. Different minerals expand
at different rates, and the same mineral may have
different rates of expansion along different axes. So
that, when a rock made up of several minerals has its
temperature changed, it expands unequally, and a
strain is set up all through the mass, which, if severe
enough, and repeated often enough, will break it into
small fragments. Further, even if a rock did expand
uniformly in all its parts with changes of temperature,
these changes of temperature are far from uniform.
Heat is conducted slowly into a rock. Since the rock
may have very different temperatures at points a short
distance apart, as a result of this slight conductivity
a great strain may result from expansion due to tem-
perature differences. Merrill quotes Bartlett to the effect
that granite expands .000004852 inch per foot for each
degree of Fahr., marble .000005668 inch, and sandstone
.000009532 inch. While these movements appear
exceedingly small, they are multiplied through many
feet of rock and through many degrees of temperature.
The differences in temperature between day and night
on rock surfaces exposed to the sun is extreme, although
it varies with the color of the rock. (2) When water is
carried below its freezing-point, it may be exceedingly
destructive. In freezing, water expands about one-
eleventh of its volume. It has been determined that
water at a temperature of — 1°C. exerts an expansive
force of 150 tons per square foot, and that to keep it
from becoming ice would require the weight of a column
of granite 1,800 feet high. All rocks are somewhat

SOIL-FORMATION, WATER 21

porous. Soils have a porosity anywhere from 30 to 75
per cent of their volume. Sandstone may have as much
as 25, limestone from .1 to .01. marble .008, and granite
.01 per cent. If this spore space is filled with water, as
is generally the case in nature, and the rock is cooled
below the freezing-point, it is evident that it will be
shattered. As the process is repeated, the fractures
become larger and more numerous.

8. Water. — -The chemical and mechanical action of
water in rock-decay may be discussed separately.

(1) The chemical action may be divided into: (a)
The changes due to pure water, (b) Changes due to
material in solution in the water. Owing to the porosity
of rocks, water is distributed through all the earth's
crust to a depth of many thousand feet.

The first direct result of the presence of water is the
assumption of its elements by many of the minerals.
This is hydration. It may be the direct imbition of
water, as when calcium sulfate in crystallizing takes
into its constitution several molecules of water; or it
may be the substitution of the elements of water for
some elements already in the mineral. The alterations
in the mineral orthoclase feldspar may be taken as the
type of this kind of changes as follows:

K20, A1203, 6Si02 + 6 H20 = 2KOH + H20, A1203, 6Si02, 4H20

Since water is so widely diffused, this process of
hydration is an especially important one. The signifi-
cant chemical effect of hydration is that it alters the
solubility of the mineral, and particularly of the elements
composing the mineral.

22

THE PRINCIPLES OF ZOIL MANAGEMENT

The second direct chemical action of water, and
perhaps the most important of all the chemical changes
involved in soil formation, is that of solution. It is
worth while to remember that no mineral is completely

Fis. 9. Traces of residual soil in a limestone quarry. Note the joints and
partings. Soil of a dark red. silty character

insoluble. They differ greatly in solubility, ranging from
the readily soluble common salt to the exceedingly
insoluble silica or quartz. But all are amenable to the
action of pure water. In the above instance of hydration
of feldspar, we have a type of a large number of changes
in minerals which alter their solubility. And by altering
the solubility of one mineral the other minerals present

SOIL-FORMATION, WATER 23

are opened to attack by any one of many agencies, both
mechanical and chemical. In feldspar, which is very
slightly soluble, hydration and hydrolysis develops
potassium hydrate, a very soluble compound and there-
fore readily removed. Its removal may develop a cavity,
and thus weaken the rock. Agriculturally, the removal
of the base is also significant. It is the basic element,
and therefore largely plant-food elements, or those
which condition soil-productiveness, such as potash,
lime and soda, which are removed by this process.

It is because of the unequal solubility of minerals
that soil results from the process of solution. If all the
minerals of a rock were equally soluble, the rock might
be removed bodily from the exposed surface inward.
Solubility, operating differently for different minerals
in a rock mass, removes one, and leaves the others in a
less coherent mass, which we term soil. It therefore
happens that residual soils comprise the less soluble
portions of the rock from which they were formed.

Materials in solution in water greatly affect its
capacity to dissolve minerals. Carbon dioxid is present
in the air in the pores of rocks and soils in much larger
proportion than in the air above the earth's surface.
It is particularly abundant in the surface layers, where
it is derived from organic decay. It is taken up by the
water as it passes along and becomes a means of solution.
The most striking example of this is in the case of lime
carbonate or limestone. In pure water this mineral is
soluble only to the extent of about one part in twenty-five
thousand, but in carbonated water its solubility is
about one part in one thousand, or twenty-five times as

24 THE PRINCIPLES OF SOIL MANAGEMENT

soluble. It is this solvent action of carbonated water
which has formed the extensive caverns and passages
in every fairly pure limestone formation, and thereby
has given rise to such features as the Mammoth Cave
and the great sinks of Southern Missouri, Kentucky,
Tennessee, Georgia, Florida and many other regions
underlain by limestone.

In the superficial layers of soil, organic acids also
add to the solvent power of the water.

Since water is an universal solvent, in the earth it
contains a large variety of mineral compounds, all of
which affect its solvent power, usually increasing it.

The destructive action of solution is indicated by the
considerable amount of dissolved substances in all
natural waters. The Mississippi river carries in solution
annually sufficient material to cover a square mile of
land ninety feet deep; the Danube, sufficient material
for a depth of eighteen feet; and the Nile, sufficient
material for a depth of thirteen feet.

(2) The mechanical action of water. — The destructive
action of running water transcends all other agencies
of rock degradation in its extent. It has been the most
potent force in carving the earth's surface into its present
form. It is continually at work reducing elevations
and filling depressions.

This destructive action is due largely to the power
of running water to carry material. This transported
material becomes the tool of the water in wearing away
its channel.

The transporting power of water varies as the sixth
power of its velocity of flow. That is to say, if the

SOIL-FORMATION, WATER

25

Fig. 10. "Pot holes" formed in shale rock. The boulders ana pebbles
in the "pot" are set in motion by flowing water and thereby the rock is broken
down with the formation of soil material.

velocity of a stream is doubled, its carrying power will
be increased sixty-four times. But the volume, and
therefore the weight, of a body varies as the cube of its
diameter. Therefore the diameter of the material
carried does not vary directly as the velocity of the
current, but at a less rate.

This power of flowing water to carry rock material

26 THE PRINCIPLES OF SOIL MANAGEMENT

is exemplified in every stream of whatever size. Where
the flow is checked and thereby the carrying power
reduced, some of the coarsest material is deposited.
Where the flow is increased, instead of deposition,
coarser material is picked up. Changing an obstruction
causes extensive regrading of the channel by the current.
Bends in the stream which require a greater velocity on
one side of the channel than on the other cause the same
sort of rearrangement, and this is nicely illustrated
in the meandering of streams. They wind over their
course always cutting away the material on the outer
side of the curves, and depositing it on the inner side
of the curves lower down. Thus the stream is continually
changing its course. It meanders from one side of its
flood-plane to the other. It cuts off large curves and
proceeds to form new ones. All these processes may
be observed in any rivulet, yet they are the exact
counterpart of the things which are taking place in
every large river valley. Careful determinations re-
ported by Bobb, show that the large rivers of the world
remove annually in suspension the following amounts
of material:

Height in feet of Thickness of sedi-

column of sedi- ment, in inches

ment with base if spread over

1 mile square. drainage area.

Mississippi river 241.4 .00223

Potomac 4.0 .00433

Rio Grande 2.8 .00116

Uruguay 10.6 .00085

Rhone 31.1 .1075

Po 59.0 .01139

Danube 93.2 .00354

Nile 38.8 .00042

Mean 76.65 .00614

SOIL-FORMATION, ICE 27

In addition to the material carried in suspension,
a large amount is rolled along the bottom of the channel.

Because of the unequal carrying power of streams
of different velocity, the load of debris is sorted into
groups of somewhat uniform size. In this way have
been formed great areas of clay, silt, sand and gravel
found in all farming sections, and which owe their
peculiar crop-producing properties most. largely to this
sorting action of water.

9. Ice — glaciers. — Masses of ice have exerted a
tremendous influence in the reduction of rocks to soil
material. Their action is chiefly mechanical, but is inti-
mately associated, as a rule, with the action of water.
The chief agency of ice is in the form of glaciers, which
issue from regions of high latitude, or of great elevation,
and in times past have pushed down over much larger
areas of country than they now occupy. A large part
of all of the continents have been overrun by such
masses, which, through their great weight and almost
resistless movement, ground even the hardest rocks to
fine powder and mixed the materials from many sources.
Fragments of rock imbedded in the bottom of the ice
became its tools to scratch and crush the floor upon
which it rested. In this way has come about the scouring
and pulverization of rocks, analogous to the action of
water. The ice appears to have attained a depth of
thousands of feet in some places, and consequently was
able to override even mountainous areas, sweeping away
and grinding to fragments the smaller eminences and
irregularities. Since the access of water was limited,
there was little opportunity for pronounced chemical

28

THE PRINCIPLES OF SOIL MANAGEMENT

change, or the removal of constituents, which fact is
shown in the tables of soil-composition on pages 32-57.
Their influence on surface topography is profound, and
of great importance to the pursuit of agriculture because
of the leveling which results.

Fir. 11. Lichen growing on a granite boulder. These low forms of
plants disintegrate the rock and assist in the decomposition of its constituent
minerals.

10. Plants and animals.— Plants and animals unite
with the other agencies mentioned to effect the breaking
down of minerals and rocks. Like the other processes,
they have both their mechanical and their chemical
side. The development of plant roots in crevices of rock

SOIL-FORMATION, PLANTS AND ANIMALS

2Q

created by other agencies, exerts sufficient pressure to force
them further apart and extend the fractures. Occasional
striking examples of the forcing apart of rock-masses
by plant-growth may be observed. The process is well

Fig. 12. Growing roots of the tree have broken apart and otherwise disinte-
grated the granite boulder, thereby assisting in the formation of soil

illustrated by the lifting of sidewalks and the tipping
over of stone fences, due to the development of trees
near by and even such soft tissues as those of mush-
rooms have been observed pushing up through cement

30 THE PRINCIPLES OF SOIL MANAGEMENT

and brick sidewalks. In mountainous regions wnere
vegetation has gained a foothold in the crevices, the
tops serve the wind as a lever to pry rocks apart. The
overturning of trees is a familiar example of the process.
Animal life also has a part in the mechanical breaking
down of rocks. Burrowing animals are most active.
The gopher, the prairie dog, the badger, the rabbit,
moles, etc., all burrow in the ground and, in the aggre-
gate, move large masses of material. Cray-fish and
earth-worms are even more widespread, and the latter
by their large numbers have a capacity which is likely
to be underrated because it is largely out of sight.
Ants are another very active form of animal life in
effecting soil formation. They burrow into crevices of
rocks and into soil formations, and deposit the material
from the passages at the surface mixed with their acid
saliva. Like the earth-worms, they handle immense
amounts of material.

V. GEOLOGICAL CLASSIFICATION AND CHEMICAL
COMPOSITION OF SOILS

All soil material may be divided into two groups,
depending upon the extent to which it has been moved
in the process of formation. Those materials which have
not been subject to any appreciable transportation
are termed (a) Sedentary. Those which have been
carried to their present position — that is, have been
appreciably moved — are termed (6) Transported. There
are several agencies of transportation, such as gravity,
water, ice and wind. These give rise to subdivisions.

SEDENTARY SOILS 31

11. Sedentary soils. — Sedentary soils are of two
kinds: (1) Residual, or soils consisting of the residue
left behind in rock decomposition. (2) Cumulose, or
soils resulting from the slow accumulation and decay of
plant remains.

12. Residual soils. — There may be as many kinds
of residual soil as there are rocks. Because of similarity
between the species in a group of rocks, a few of these
groups may be considered as types. The most promi-
nent groups are the igneous rocks, the calcareous rocks,
shale or slate and sandstone. Attention will be directed
as far as possible to the relation of the soil composition
to the composition of the original rock and to the char-
acter of the material lost in the transition.

In calculating the relative loss of the different ele-
ments in the transition process, some one element —
usually iron or aluminum, and, in the case of limestone,
silicon — is assumed to have suffered no loss. This
method, adopted from Merrill, is, of course, not strictly
accurate, since every element is subject to losses; but
it serves as a fair comparative basis for the study of the
loss of plant-food elements.

The important areas of residual soil in North America
occur south of the limit of glaciation, which extends
roughly from New York to Cincinnati, thence to St.
Louis and up the Missouri river to the Dakotas, and
west to the Sound region of Washington, where it again
loops well to the south. The residual soils are further
hemmed in by coastal deposits, which have their greatest
extent in the South Atlantic and Gulf Coast region,
where they reach a width of more than a hundred miles.

32

THE PRINCIPLES OF SOIL MANAGEMENT

Table II. — Complete Chemical Composition of Rocks
and Residual Soils.

Granite.

Gneiss.

Diabase.

Basalt.

Crouzet

Haute Loire,

France

District of

Albemarle

Spanish

Columbia

Co., Va.

Guiana

I

II
Re-

Ill

IV
Re-

V

VI

Re-

VII

VIII

Re-

Fresh

sidual

Fresh

sidual

Fresh

sidual

Fresh

sidual

Rock

Sand

Rock

Clay

Rock

Clay

Rock

Soil

1. Silica (SiO,) . .

69.33

65.69

60.69

45.31

49.35

43.38

48.29

37.09

2. Alumina

(A1203) • • • ■

14.33

15.23

16.89

26.55

15.30

18.36

13.25

30.75

3. Ferric iron

(Fe203) ....

4.00

4.39

9.06

12.18

14.25

20.39

17.12

4.31

4. Ferrous iron

(FeO)

5. Sulfur trioxid

(S03)

6. Phosphoric

acid (PA)-

0.10

0.06

0.25

0.47

7. Lime (CaO) . .

3.21

2.63

4.44

t

9.60

2.37

7.37

8.97

8. Carbon Dioxid

(C03)

9. Magnesia

(MgO)

2.44

2.64

1.06

0.40

7.38

3.45

7.03

0.61

10. Soda (Na20).

2.70

2.12

2.82

0.22

1.98

0.14

2.71

1.01

11. Potash (K20).

2.67

2.00

4.25

1.10

0.85

0.59

1.81

0.71

12. Ignition [wa-

ter (HA]...

1.22

99.00

4.70

99.77

0.62

13.75

3.25

11.34

4.92

16.55

In addition to these large areas, many small areas occur
scattered through areas of other kinds of soil.

The most nearly original soil is that formed from
igneous rocks. That is to say, the composition of such
a soil might be expected to approach most nearly to
that of the original rock. The relative composition of
several igneous rocks and the soils derived from them,

CHEMICAL COMPOSITION OF RESIDUAL SOILS 33

Table II. — Complete Chemical Composition of Rock and
Residual Soils, continued.

Soapstone

Igneous
Rock

Sand-
stones

Shales

Limestone

Aver-

Com-

Com-

Com-

age of

posite

posite

posite

Albemarle

about

analy-

analy-

analy-

Carboniferous,

County, Va.

700

ses 253

ses 78

ses 345

Arkansas

sam-

sam-

sam-

sam-

ples

ples

ples

ples

IX

X

Re-

XI

XII

XIII

XIV

XV

XVI

Re-

Fresh

sidual

Fresh

sidual

Rock

Soil

Rock

Clay

1. Silica (Si02) . .

38.85

38.82

59.87

78.66

58.38

5.19

4.13

33.69

2. Alumina

(ALA) ...

12.77

22.61

15.02

4.78

15.47

0.81

4.19

30.30

3. Ferric iron

(Fe203)....

12.86

13.33

2.58

1.08

4.03

0.54

2.35

1.99

4. Ferrous iron

(FeO)

3.40

0.30

2.46

5. Sulfur trioxid

(S03)

0.28

0.07

0.65

0.05

6. Phosphoric

acid(P205).

0.26

0.08

0.17

0.04

3.04

2.54

7. Lime (CaO) . .

6.12

6.13

4.79

5.52

3.12

42.61

44.79

3.91

8. Carbon dioxid

(C02)

0.52

5.04

2.64

41.58

34.10

9. Magnesia

(MgO)

22.58

9.52

4.06

1.17

2.45

7.90

0.30

0.26

10. Soda (NaX>) .

0.11

0.20

3.39

0.45

1.31

0.05 '

0.16

0.61

11. Potash (K,0).

0.19

0.18

2.93

1.32

3.25

0.33

0.35

0.96

12. Ignition [wa-

ter (H20)] .

6.52

9.21

1.86

1.64

5.02
MnO

0.77
0.05

2.26
4.33

10.76
14.98

as given by Merrill, is shown in Table II, numbers I to
VIII. Numbers I and II represent a gray foliated
granite from the District of Columbia, the soil of which
is very sandy. By reference to Column II of Table III,

Q
•A
<

a

H

X
H
O
%
<

O
§

a
o
Ph

fa
O

82

to

o
«

fa

W hH

fa

o

o
o

03

<

I— I

«

Eh
<

►H

as "

■ a; O

O GO

^ O

fa H
O

fa

o
<

H
I?
H
O

OJ
H

Ph

fa

Residual

soil from

soapstone.

Albemarle Co .,

Va.

M

jjooj ajijrja
joj sso[ juao jaj

OS

CO

i—l

5.33
2.66

. o

• <^
r^

• i— i

■co cm

•pco

! OS i-H

CO

y ^uaniijsuoo qoBa
►h 1 jo ssoi luao jaj

43.58
41.48

44.45

76.19

47.05
20.26

Residual
soil from

Basalt.

France

>— <
>

spoj aji}ua
joj ssoj juao jaj

30.34
16.64

3.46

. r-- O i-i
. t>- ^ U5

• CD 1— 1 i-H *

i— i

d

CO

M

>

luanjpsuoo qo^ea
jo ssoj juao jaj

65.56

88.84

47.24

96.38
74.41

83.24

*

Residual
clay from

diabase.
Venezuela

>

jjooj aipua
joj ssoj juao ja<j

20.92
3.27

8.05

. CN <M CO

. ^h oq co

i—i

OS

co

>

luanjrjsuoo qorca
jo ss6[ juao jaj

42.40
21.30

83.23

61.37

95.37

45.88
*

Residual

clay from

gneiss.

Albemarle Co..

Va.

>
I-H

5[ooj ajriua
joj sso[ juao jaj

31.90
1.30

gain
4.44

O 00 iO
O CN CO *

CO
■*

l— 1
hH
H- 1

juaniijsuoo qoraa
jo ss6( juao jaj

52.45
14.35

gain
100.00

74.70
95.03
83.52

*

13.47

Residual
sand from

granite.

District of

Columbia

I— 1
I-H

3{ooj ajriua
joj ssoj juao jaj;

10.50
0.46

0.04
0.81

CD r^ iO CD i
CO (">; 00 i-H |

© © 0(N 1

* 1

t— 1

luanjijsuoo qo^ea
jo ssoj juao ja<j

14.89
3.23

40.00
25.21

1.49

28.62

31.98

*

d

'm

03
o

go
i— i

c

C

t

-

=

<

3. Ferric iron (Fe203) . .

4. Ferrous iron (FeO)

5. Sulfur trioxid (S03) . .

6. Phosphoric acid (P,05)

7. Lime (CaO)

8. Carbon dioxid (C02). .

9. Masnesia CMeO.V .

5

e
Bj

-:

z

X

o

1— I

-

C
Ph

r—
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^ 0>

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(34)

e fl ° ° «

CO"-.-* rt C

— c o-a w
bl« to £ ■£

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to O

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joj ssoj jiiao jaj

juaniijsuoo qoraa
jo ssoj ^uao ja<j

C5 *

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n n h M h lo n oi o a o

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tNCO^iOCONGOddrHtNoj

02

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O

o

S<;

(35)

36 THE PRINCIPLES OF SOIL MANAGEMENT

it will be seen that the total loss from the rock is 13.47
per cent. Column III of Table III represents a gneiss from
Albemarle county, Virginia, under almost the same
climatic conditions as the granite. But the soil is a red
clay of the Cecil series, and represents a loss in transition
from the rock of 44.67 per cent, or three and one-half
times as much as from the granite. The composition of
the two rocks is not greatly different. The differences
in the two soils illustrate the two types of rock-decay.
The granite soil, which is very sandy, probably does
not represent the same advanced stage of decay as the
gneiss soil, and apparently has been subjected most
largely to disintegration, or physical breakdown. On
the other hand, the gneiss soil represents both the disin-
tegration and an advanced stage of chemical change or
decomposition.

In general, the productiveness of a soil depends
even more on its physical characteristics than on its
chemical composition. The physical characteristics of
a residual soil depend quite as much on the stage
and type of decay to which it has been subject as to
its chemical composition. Mechanical processes, such
as abrasion and fracture due to impact, temperature
changes and frost, never produce the same fine texture
which may result from chemical processes, and therefore
such material is usually very sandy. A sand composed of
aluminum silicate minerals in large proportion is increas-
ingly subject to chemical decay, which will reduce it to
a gritty clay of progressive coarseness from the surface
downward. These principles may be summed up in the
statement that the characteristics of a soil are determined

COMPOSITION OF RESIDUAL SOILS 37

by two factors: (1) The original chemical and physical
composition of the rock. (2) The relative prominence of
physical and chemical processes in its formation. These
facts make possible the existence of a full series of soil
from any group of rocks.

The composition of other rocks and soils than those
mentioned above are shown in Columns V to X of Table
II. For comparative purposes, Column XI is also of
great interest, as showing the average composition of
over 700 bulk analyses of igneous rocks as given by Clark.
This gives some idea of the relative abundance of the
several plant-food constituents in the rocks. It will be
noted that the least abundant elements, sulfur and
phosphorus, are present in amounts of several thousand
pounds per acre foot.

Columns XII, XIII and XIV give the analysis of a
composite of many samples of sandstone, shales and
limestones. The first two may be considered as ancient
soils, and their average composition of the mineral
elements should be much the same as modern soils of
the same origin.

Columns XIV to XVI give the composition of lime-
stones, and of a residual soil from such a rock in Ar-
kansas. From a comparison of the first two columns,
it will be found that the rock from which the soil is
derived is far from the average, especially in the amounts
of manganese and phosphorus it carries. A study of
the soil analysis also shows that, while it is derived
from a lime rock, it is not rich in lime, a condition not
uncommon.

Turning now to Table III, there is given the propor-

38

THE PRINCIPLES OF SOIL MANAGEMENT

tion of loss of the different elements calculated to the
amount of the element originally present, and to the
proportion the loss bears to the original rock. This
exhibits some of the reasons for the difference between

"ft? -vx-fc

Fig. 13. Residual soil from limestone. Showing relation to underlying rock

many soils and the rocks from which they were derived.
Assuming that there is any element which is constant
in amount, these figures show that the total loss suffered
by different rocks ranges from 97.64 per cent for the
limestone to 13.47 for the granite. In other words, a
limestone soil represents the supplementary materials
in the original rock, the main constituent having been

LOSSES IN RESIDUAL SOIL FORMATION 39

removed. In this particular sample, 100 feet of rock
would produce only 2.34 feet of soil. It is not uncommon
in limestone soil regions, as Kentucky, Tennessee and
the Ozark region, to find soils forty and more feet in
depth, and, since the average limestone contains nearly
90 per cent of carbonate, these deep layers of soil must
represent some hundreds of feet of rock. This is the re-
sult almost entirely of solution by carbonated waters,
which gradually develop crevices and caverns in the
rock.

Other types of rock, however, do not suffer such a
large amount of loss. The loss, of course, varies with
the character of the processes which are at work, as
has been pointed out in the case of granite and gneiss.
In Columns V and VI, a clay from diabase rock suffered
a loss of 39.51 per cent, and a basalt soil in France rep-
resented a loss of over 60 per cent. The latter are much
more basic than the granite or gneiss, and would there-
fore be more amenable to chemical decay. The soap-
stone, which results from the alteration of pyroxinite
rock, undergoes a loss of 52 per cent in the transition
to soil. In Columns XV to XVIII are given the calcu-
lated loss in changing from the average analysis of
igneous rocks to shale and sandstone respectively. As
was stated above, these latter are ancient soil material,
or potential soil material, and the figures given represent
an attempt to determine the average change which takes
place in the derivation of a shale or sandstone (corre-
sponding to clay or sand soil) from igneous rocks. These
calculations are, of course, less accurate than the pre-
vious figures on such loss, because these rocks have been

40 THE PRINCIPLES OF SOIL MANAGEMENT

subject to mechanical sorting by wind and water, in
addition to the fact that no single element has come
through without loss.

The figures in the first column of each pair show the
proportionate loss of each constituent. The second
column shows what would be expected, viz., that the
elements present in largest amount would be subject
to the largest total loss. But the first column shows
that certain elements are more weak chemically than
others. These elements are lime, magnesium and the
alkalies. While the figures are limited, still phosphoric
acid appears to be subject to a large loss. There is almost
invariably the assumption of water, and frequently of
carbon dioxid, indicating alterations in chemical com-
binations which, while freeing some elements, may render
others more resistant.

The striking change in the physical properties of a
residual soil from the parent rock depends in part upon
this unequal loss of elements. As a rule, unweathered
residual soils are highly colored, usually red or yellow.
This results from the accumulation and alteration of
the iron. Hence, a gray limestone will produce a dark
red clay. Other properties, as the texture, result in the
same way. Any very refractory material, as chert in
limestone or quartz in igneous or secondary rocks,
is likely to persist and remain scattered through the
soil. The cherty hills of Tennessee and the Ozarks are
examples of the former, and the topography of the coun-
try is largely determined by the accumulation of this
material. Some of the stony soils of the Piedmont
regions are examples of the second type of soils. The

CUM U LOSE SOILS 41

occurrences of these refractory materials in layers may
exercise a very unfavorable effect on the agricultural
value of such land.

Further, residual soils are seldom uniform in texture.
The clays are usually gritty, especially when derived
from igneous rocks. It has been suggested that this is
due to the accumulation of silica set free from the silicic
minerals in their loss of alkaline materials. In this state
much of it passes into solution and is removed, which
probably explains some of the large losses of this ele-
ment. But, where the decay is rapid, not all of the
silica can be so removed, and it combines with oxygen,
to form quartz particles.

All of these considerations should be kept in mind
in the study of residual soils, as they assist in under-
standing their characteristics.

13. Cumulose soils. — Cumulose soils consist of
years and even centuries of accumulations of plant
remains. They occur in every section of the coun-
try in areas of from a fraction of an acre to thou-
sands of acres, known as peat bogs and muck swamps.
The one condition which always accompanies these
deposits, and is most largely responsible for their
existence, is poor drainage. Such a condition may
result from a variety of circumstances. In the North
Central states of the glacial section, scattered over the
undulating country, are numerous small depressions
where water accumulates during much of the year,
together with a small amount of sediment from the
surrounding hills. These conditions favor the large
growth of vegetation which, upon its death, is slowly

CUMULOSE SOILS 43

accumulated on the bottom of the depression. The
dead remains are kept saturated with water, which
excludes the air and keeps down the temperature, and
otherwise hinders decay, so that the annual additions
exceed the annual loss by decay. Hence, an accumu-
lation of vegetable remains is inevitable. This is the
genesis of hundreds of the mucky marshes throughout
the country. Old abandoned stream channels are a
common beginning of such accumulations. Very similar
in origin are muck and peat beds, which were formerly
deep lakes. A peculiarity of fresh water deposits of this
sort are beds of marl, or impure lime carbonate, beneath
the vegetable matter.

A slightly different type of these deposits are the
seacoast swamps from Massachusetts to Texas, many of
which are of large extent. These have formed in brackish
water

The chemical composition typical of many of the
cumulose deposits is shown in the accompanying table.
The physical and chemical properties of such soil will
be more fully discussed under the head of physical
properties of organic soils.

Cumulose deposits are characterized chemically by
their large percentage of carbonaceous matter. If the
vegetation suffered no decay and received no mineral
matter, it would be simply a mass of plant tissue; but,
as has been stated, there is every degree of "wash"
mixed with the dead plants. These also have accumu-
lated to all depths from almost nothing to many feet in
thickness. Many areas of soil, such as Miami black clay
and the Clyde soils of the northern states, and the

44

THE PRINCIPLES OF SOIL MANAGEMENT

Table IV
Chemical Composition of Cumulose Deposits

I

II

III

IV

V

VI

VII

a

<o ■

s, a

a a

«s

a
m

i-i a

J a

"C a)

Merrill, p. 315.

Swamp

North River

Carteret Co..

N. Carolina

Illinois Bulletin 123

43

c3 .
ft."
ftl>

• J.

cuO

P

go

o

.5 a

O

a) a
ft— '

(5-

1. Moisture

49.00

30.00
17.00

20.00

6.14
0.10

1.11

2.50

7.70

:3.'70

80.84

1.18
2.69
0.44

0.22
0.02
0.07
0.08

0.06

9.60

87.25

1.52

6.15
0.39
0.36

6.14
0.13
0.06
0.00

2. Volatile matter ....

3. Organic matter ....

4. Insoluble matter . . .

5. Soluble silica (Si02)

6. Insoluble silica (SiO,.

7. Ash

71.80
18.47

84.7
42.3

6.31

0.19

3.4

2

6

8
6
8

86.70
43.35

6.36

0.15
3.36

78.5
39.6

0.36
0.1
3.1

2
2

8. Iron oxide (Fe203). . .

9. Alumnia (A1,03) . . .

10. Lime (CaO)

11. Carbon dioxid (C02)

12. Magnesia (MgO) . . .

13. Soda (Na20)

14. Potash (K,0)

15. Phosphoric acid P205

16. Nitrogen (N)

17. Sulphuric acid (S03) .

2.60
1.38
0.51
1.77
0.10
0.20
0.32
0.32
0.62
1.06

6
2
4

Portsmouth and other soils of the southern states,
represent the very first stages in the formulation of
such cumulose deposits. That is, they are simply
mineral soils with a high content of organic matter.
14. Transported soils. — The four great agencies of
soil-transportation are (1) water, (2) ice, (3) wind, and

TRANSPORTED SOILS 45

(4) gravity. It will be remembered that each of these
agencies was mentioned as active in soil -formation
through the physical and chemical forces brought to
bear on rocks. The material is moved from its original
position and laid down under new conditions which
develop properties entirely different from those pos-
sessed by sedentary soils.

Of the four groups, those soils transported by water
are easily the most extensive, and next to these in area
stand those moved by glacial action. Wind-moved
soils are of much importance in some sections, but
gravity-moved soils are of small extent.

15. Gravity or colluvial soils. — In mountainous or
hilly regions, soil material of all dimensions is moved
down the slope under the pull of gravity. In those
sections of the country where stone fences are common,
the accumulation of soil on the uphill side of the fence,
due to gravity movement, not infrequently reach the
top of the wall. Because of its associations with a hill
(Collis meaning hill), such material is termed colluvial.
The first footings of soil in the niches and at the base
of a rocky ledge are usually of this sort, and in mountain
regions the accumulation of such material is sometimes
large.

16. Water. — -It has been shown how water is able
to transport sand and even boulders several times
heavier than itself, if it be flowing with a sufficient
velocity. (See page 24.) This large transporting power
may be observed in any creek or rivulet, and in every
hilly region it is brought forcibly to the farmer's notice
in the gullies formed by heavy rains. The bed of every

46 THE PRINCIPLES OF SOIL MANAGEMENT

stream is strewn with material which has been dropped
by the water. If the bed of the stream is steep, it is
paved almost entirely with large stones and boulders.
If the bed is very flat and the flow slow, the bottom is
formed by sand or silt. These variations are well illus-
trated by the ripples and quiet pools of almost any
stream, the former being stony, the latter more fine-
textured. This principle of the varying carrying power
of flowing water is of great agricultural importance.
It results in sorting the material which comes into the
water, and the particles of one size are deposited to-
gether. In this way is accumulated a fine pure clay
in one place, a sand at another place, and gravel at
still another. These formations are strikingly different
in their relations to plant-growth because of their dif-
ferent physical and chemical properties, as will be shown
in the further discussion of these matters.

The character of such soil depends upon two factors:

(1) The character of the rocks from which it is derived.

(2) The conditions under which it is deposited.

The soils of this group are by far the most important,
agriculturally, of any which will be discussed, on account
of both their relative area and crop relations. In a
general way, they may be divided into three sub-groups;
but it is impossible to draw any sharp line of distinction
between these groups. These are: (1) Marine soils.
(2) Lake and pond deposits, or lacustrine soils. (3)
Stream-laid, or alluvial soils.

17. Marine soils. — The marine soils occupy large
areas in the United States and many other countries.
They consist of stratified gravels, sands, silts and clays

SOILS DEPOSITED BY WATER 47

deposited in shallow off-shore water, and subsequently
raised above sea-level, where they have been subject
to erosion by the present drainage channels, so that
they are furrowed by a ramifying system of shallow,
steep-sided gorges. These channels reveal the different
sorts of material from coarse to fine, and have exposed
each of them over considerable areas. The material
has not been deposited long enough or buried deep
enough to be much consolidated, although there are
very soft shales and limestones in the Gulf states which
are only partially consolidated.

18. Lacustrine soils. — Closely related to the marine
soils are soils deposited in lakes, such as those fringing
the Great Lakes. These lacustrine soils differ from the
former in the different source of their material and
somewhat different conditions of deposition. Most of
them are fresh-water bodies, but in some instances,
as Great Salt Lake, they are brackish. It is impossible
to draw any definite line of distinction between these
two sub-groups of soils further than in the extent and
character of the waters in which they were deposited,
and for a specific understanding of their characteristics
the respective types must be studied in detail.

19. Alluvial soils. — Along every stream course is a
ribbon of material formed by the deposition from the
water of that stream at either normal or flood time.
Along the steep-bedded streams it is very narrow and
usually coarse, often with a base of stone covered by a
veneer of fine material. As the course becomes less
steep, it widens and is more meandering. The stream
swings from side to side of its valley in large sweeping

48

THE PRINCIPLES OF SOIL MANAGEMENT

curves, which become actually tortuous in very flat
bottoms. Such a crooked channel is much reduced
in capacity over a straight channel, and therefore in
flood season the water is piled up over the bank and

Fis. 15. Shows the stratified arrangement of a gravelly soil

overflows the adjacent land. When the water passes
from the deep channel, its velocity is checked, and some
of the material — the coarsest — is deposited on the
bank. The finer material is carried further out. If there
is a general movement over the whole bottom the
very finest material may not be deposited, and con-

CHEMICAL COMPOSITION OF MARINE SOILS

49

Table V

Chemical Composition of Soils Deposited by Water.
Complete Analyses

Coastal Plain Soils of Maryland

1. Insoluble

2. Silica (Si02)

3. Alumina (A1203)

4. Ferric iron (Fe203) . .

5. Ferrous iron (FeO) . .

6. Sulfur trioxid (S03) .

7. Phosphoric acid (P205)

8. Lime (CaO)

9. Carbon dioxid (C02) .

10. Magnesia (MgO).

11. Soda (Na,0)

12. Potash (K20)

13. Water

14. Organic matter

15. Volatile matter

3.2<a
Hgo
A F a
S o S?

Is

tO~^ *J .
W O B to

j^O cS tu

II

III

c3 rf C to

a -a i £

a>

- " a 3

^ t-. ki

5-2 •-

o S-o ft
0§Cg

Lil

— ' O ■*£

O O

fc£*

Pea*

92.30

83.86

3.20

6.10

0.91

2.63

'6.08

6.12

0.05

0.23

0.41

0.50

0.08

0.06

0.35

0.45

0.50

0.56

0.70

0.92

0.23

1.30

i .13

3.00

80.55
8.82
2.67

0.07
0.42
0.47
0.05
0.29
0.49
1.22
1.28

3.26

IV

t> a o

3 O

•g -a

2 5 »

*Sg>

is o»

0 0 >

~? oj<! •

^> a J?

- c o>

P^ a

.Ph c S

Clay
^ity.
aryla
ree s

.«".a

•Ml* +3

64.26

19.92

5.74

'6.09

0.16

0.44

0.15

0.59

0.58

1.50

0.75

'6.58

V

a a

u .

ca a
pq o

Si

Ph O

s * •

94.32
2.66
1.25

0.02
0.04

6.07

0.11
0.12

1.21

sequently the accumulated soil is a fine, friable loam
rather than a clay. Heavy alluvial clay is seldom found
outside the larger river bottoms and generally in
depressions remote from the channel.

Another source of heavy soil is ponds formed by the
cutting off of bends in the channel. These "ox-bow

D

50

THE PRINCIPLES OF SOIL MANAGEMENT

Table V, continued

Chemical Composition of Soils Deposited by Water.

Complete Analyses

Brick Clays of Southern Plain

1.
2.

3!

4.
5.

6.

7.

9.
10.
11.
12.
13.
14.
15.

Insoluble

Silica (Si02)

Alumina (A1203) . .
Ferric iron (Fe203)
Ferrous iron (FeO)
Sulfur trioxid (SO
Phosphoric acid

(PA)

Lime (CaO) . .
Carbon dioxid CO
Magnesia (MgO)
Soda (Na,0) . .
Potash (K,0) .

Water

Organic matter
Volatile matter

VI

|o

o

Si

a o

■c g

53.75

24.91

7.99

0.70

i .12

2.94

1.03

VII

S3
c 2

OO

'St

o S

o 03

'E s

70.45

17.34

3.16

0.22

I 0.70

0.98

6.63

VIII

0 o
.2 «s

tj „. »
02 gr

90.00
4.60
1.44

0.10

0.10

t

t
3.04

IX

a r

a o
go
o

"S.g

'S E

« B

.2 03 n

o,2

+ silica

44.40

17.90

4.50

9.50
9.55

1.88

t
4.58

X

o

o

£ '

a>'a

•S S-

ft'!

a. 2

<3
02

93.23
sol.
2.36
1.25

0.03
0.12
0.02
0.18
0.07
0.26

2.33

XI

■--3
•-T3

o

ss »

a

03
02

94.46
1.67
0.92
0.32

0.09

0.11

0.07

0.04
0.04
0.19

1.88

bends," " cut-offs," or "bayous," as they are variously
termed, become in succession lakes, ponds and marshes,
where the clay-laden water is gradually evaporated
or filtered away, leaving behind only the very fine
material that may be carried in suspension almost
indefinitely. As a result of these processes much of the

CHEMICAL Ck M POSITION OF GLACIAL LAKE SOILS 51

Table V, continued

Chemical, Composition of Soils Deposited by Water.
Complete Analyses

Glacial and Western Soils

1. Insoluble

2. Silica (Si02)

3. Alumina (A1203)

4. Ferric iron (Fe203)

5. Ferrous iron (FeO) . . .

6. Sulfur trioxid (S03) . ..

7. Phosphoric acid (P205)

8. Lime (CaO)

9. Carbon dioxid (C02) . . .

10. Magnesia (MgO)

11. Soda (Na20)

12. Potash (K20)

13. Water

14. Organic matter

15. Volatile matter

XII

a

o3

hO

03"

55.60

14.80

5.80

0.15
5.70
4.94
2.48
1.07
3.23
5.18

XIII

0J

cs.2
1°

03 terl
CC«

Oj" £
.C OJ

66.69

14.16

4.38

41
j.29
2.49
0.77
1.28
0.67
1.21
4.84
2.00

XIV

o

9)

M
oj

02

Oi
.0

O

T3

19.24
3.26
1.09

0.53

0.23

38.94

29.57

2.75

t

t

1.67
2.96

XV

oj oj

•1 «

03 *

OJ

i-l

56.17
24.25

3.54

2.09

2.57
2.25
4.06
4.69

XVI

oiS

« -

* s

r* 03 ■

C.£P.S

~ c

OJ--. o
•l-l OJ

^ „•, »

i2 £•*
<-> 2 3

40.22
8.47
2.83
0.48
0.13
0.05
15.65
18.76
7.80
0.84
2.36
1.95
0.32

soil formed in stream bottoms is friable and easily tilled,
but they also give rise to some of the heaviest and most
intractable clay soil known. Soils of this sub-group may
be either very uniform or exceedingly variable in fine-
ness. It is evident from what has been said that, the
smaller the stream, the more variable the soil is likely to

52

THE PRINCIPLES OF SOIL MANAGEMENT

Table V, continued

Chemical Composition of Soils Deposited by Water.

Strong Hydrochloric Acid Analyses.

1. Insoluble

2. Silica (SiO,)

3. Alumina (A1X>3). .. .

4. Ferric iron (Fe203) . .

5. Ferrous iron (FeO) .

6. Sulfur trioxid (S03) .

7. Phosphoric acid (P205

8. Lime (CaO)

9. Carbon dioxid(C02).

10. Magnesia (MgO). ..

1 1 . Soda (Na,G)

12. Potash (K,0)

13. Water

14. Organic matter

15. Volatile matter

XVII

XVIII

XIX

XX

XXI

XXII

C

.

>>

O

>

0

O

6
o

>

a

o

"3 c3

»

__

4^

£ S

0

m

j3

O 03

a o

oj m

eo

3 «

■°B

w-a

i- o3
(0

o3 y,
~ 0>

- o3

s *

- <u

• 3

GO

Jn

l^

C3 r ,

0^

EH

^2

o

s

05

■ O

-^HH

03
_0

J4

>i

c
. '51

*H

03

■n

*-•

03

m

a

o3

>

£

m

pq

CB

fa

70.92

44.23

77.05

83.22

84.97
*5.65

81.77
7.68

5.58 1
3.621

{1.58

>4.91
12.66

2.11

1.42

3.33

5.82

2.71

3.07

0.29

0.15

0.02

0.34

0.12

0.18

0.243

0.04

0.05

5.66

23.98

1.03

0.56

0.44

0.32

4.00

18.00

0.81

0.44

1.85

0.94

0.93

1.13

0.16

0.18

0.23

0.25

0.96

0.40

0.48

0.39

0.88

0.22

1.45

0.48

0.80

1.21

3.26

2.42

3.56

1.32

3.69
0.62

2.18
1.54

2.69

7.34

6.52

4.60

XXIII

a

o

-M 03

Ms

O

a -a

o

T) 03

O
o
o

02

79.99
7.76
2.78
3.40

0.15
0.06
0.95

0.21
0.31
0.44
4.10
0.51

* Soluble.

be. It embraces large areas of the most productive soils.
Properly drained, bottom lands are generally regarded
with favor for several of the staple crops. Corn is prob-
ably the most grown. Wheat is important on the heaviest
soils. They are generally rich in organic matter to an
unusual depth because they represent largely the wash

CHEMICAL COMPOSITION OF ALLUVIAL SOILS 53

Table V, continued

Chemical Composition of Soils Deposited by Water. Strong

Hydiocliloric Acid Analyses

XXIV

XXV

XXVI

XXVII

XXVIII

XXIX

o.Z

> o

i- a>

t- i
a a
,£3

>>

03 O
>> ©

.fcs

~"2

0

0

c3 £,

> d

*" o

a .

o c
> a

"^ d"

-3 -
<u C

o?^

.5 o

VI
VI

-2
O r

°r9

to

a u

T3 03

0

o3 03
O »J
— O

J 0

.a

<^ O

is

is

03 £ OJ

» .

3»"

£ o

■* s

"H c

sag

-3>>

W 0J

033

O J3

3

■w 03

s>

i-l- '

k- s

hJ

«

02

1. Insoluble

41.21

39.17

60.21

45.06

51.06

58.57

2. Silica (Si02)

8.37

15.09

9.00

16.43

20.70

5.33

3. Alumina (A1203) . .

10.72

13.61

9.15

10.20

10.54

8.40

4. Ferric iron (Fe203)

3.48

3.98

3.94

4.22

5.82

4.14

5. Ferrous iron (FeO)

. - . .

6. Sulfur trioxid

(SO,)

0.10

0.06

0.11

0.09

0.02

0.15

7. Phosphoric acid

(P20-)

0.19

0.28

0.16

0.27

0.30

0.13

8. Lime (CaO)

7.45

8.10

1.07

8.84

1.35

8.67

9. Carbon dioxid(C02)

14.26

13.27

0.13

7.22

....

7.82

10. Magnesia (MgO) . .

4.48

2.04

0.84

3.02

1.67

2.97

11. Soda (Na20)

0.48

0.40

0.61

0.27

0.33

0.16

12. Potash (K20)

0.25

0.60

0.90

0.81

1.10

1.18

13. Water

0.89

0.81

5.16

....

14. Organic matter . . .

15. Volatile matter. . .

6.22

3.20

14.29

2.61

7.37

3.34

from the surface layer of the upland soils, and they are
not old enough to have lost this supply of organic matter
by decay. Frequently, the supply is replenished by
annual additions.

Table V illustrates the variations in the propor-
tion of the different elements in water deposits of

54 THE PRINCIPLES OF SOIL MANAGEMENT

different physical properties, from different parts of
the United States. Many of these analyses are less
complete with reference to some of the plant-food con-
stituents than is desirable for the purpose here intended.
So far as possible, analyses of the entire soil have been
used, but, where these could not be obtained, analyses
of the strong hydrochloric acid extract are given.

20. Ice — glacial soils. — In many parts of the world
there exist soils which have been formed under the
influence of large bodies of ice.

In earlier times, masses of ice extended far to the
southward over the country now devoted to agricul-
tural purposes. Around the world this mass of ice
appears to have extended down from the north and
south poles to a zig-zag limit. It reached into Asia,
Central Europe and the American Continent as far
south as New York City, Cincinnati, St. Louis, Kansas
City and Omaha, and farther west in the Puget Sound
region it extended south across the Columbia river.
All the country north of this line with the exception
of one or two small areas was covered by an immense
sheet of ice which moved slowly down from the north-
ward. In the southern hemispheres are similar — though
more limited — traces of the same condition.

The depth of the ice was so great that it flowed
over such elevations as Mount Washington in New
Hampshire and over the Adirondacks in New York.
Its general movement in the northern hemisphere was
southward. Its flow was modified by the original topog-
raphy of the country, but its depth was so great it
was able to disregard and override many of the land

GLACIAL SOILS 55

forms. It advanced first through the valleys, and at the
bottom of the mass appears to have been guided in
its flow by these channels. The advance probably
consumed a long period of years, or even centuries,
and the retreat was similarly slow. Along the margin,
as in modern glaciers, there were annual fluctuations
in the position of the ice front which are indicated
by the greater or less accumulation of rock debris, as
undulating piles of earth or terminal moraines. This
ice picked up immense amounts of material along its
way. Most of the original soil overlying the rocks was
swept away. Prominences were torn away or planed
down, and depressions were filled up. Masses of rock
were ground to powder, and boulders were transported
to entirely new surroundings. The advance of the ice
over the country largely disregarded the rock forma-
tions, as it did topographic forms, so that the rocks
and soil materials from many sources were mixed and
ground together. In this way, the granite boulders
strewn over the surface near the southern margin of
the ice extension in the United States were derived
from points hundreds of miles to the northward, even
into northern Canada. The movement was not straight
south, but deflected by broad obstructions in the land,
so that the source of the soil in any region is determined
by the direction of movement in that section. This
movement may often be traced by the kind of rocks
which have been left, and may lead back to the ledges
from which they were derived.

The relation of glacial soils to the underlying rock
depends entirely on the conditions which prevailed

56 THE PRINCIPLES OF SOIL MANAGEMENT

in that region when it was formed. In central Michigan,
the soil bears scarcely any relation to the underlying
rock of the region ; but, in Southern New York and
Northern Pennsylvania, the very shaley character of
the soil may be traced to the broad area of shale rock
which underlies all that section of country, and which
was the main source bf the glacial debris. As one passes
northward through the finger-lake region of New York,
the proportion of limestone and other foreign material
resting on the gray shale increases until the exposures
of ledge limestone are met at Syracuse and Rochester,
portions of which rock had been raked far southward
by the ice-movement. This shifting and mingling
of material must always be kept in mind in examining
glacial soils.

Purely glacial deposits differ in chemical and physical
properties from soils derived from the same formations
by other means. There is a large element of mechanical
grinding without any large amount of chemical change
or solution. The particles have not been subjected to
long-continued leaching, which characterizes residual or
marine soils. Such material is chiefly rock-flour, that is,
pulverized rock. The readily soluble minerals and ele-
ments are therefore present in proportionately larger
amounts than in soil formed by other means. While a
residual soil from limestone may be very poor in lime
carbonate, a glacial soil formed from lime-rock is often
rich in lime, sometimes containing 50 per cent of that
constituent, as has been found in some Dakota soils. As
appears from the tables of analyses, such soils are gen-
erally rich in all of the basic elements.

CHEMICAL COMPOSITION OF GLACIAL SOILS

57

Table VI

Chemical Composition of Glacial Soils
Hydrochloric Acid Analyses

I

II

III

IV

V

VI

0

SO

ej -

°J

-^ to
r2 c
QQ O

0
H

£

T >
>> to

- z

og

+3

DQ

0

2
„0

C m

a 3

->S
o

o

IS
o

£s
OS

u

O

o a

a

_r c

"3 S

CO *

g.ss

0

1. Insoluble

87.85

3.46
3.30

0.04
0.11
0.25

0.39
0.34
0.25

4.09

83.80

4.11

4.72

0.03
0.09
0.18

6.45

0.29
0.22

5.92

83.87

.4.26
3.63

0.10
0.15
0.69

0.62
0.78
0.56

5.64

89.20

3.69
2.26

0.03
0.12
0.13

0.37
0.23
0.21

3.86

73.95
6.85
4.63
3.05

0.04
0.26
0.70
0.36
0.36
0.42
0.40

9.12

74.05

2. Silica (Si02)

3. Alumnia (A1203) ....

4. Ferric iron (Fe203). . .

5. Ferrous iron (FeO)

6. Sulfur trioxide (S03) .

7. Phosphoric acid(P205)

8. Lime (CaO)

9. Carbon dioxid (C02) .

10. Magnesia (MgO)

11. Soda (Na,0)

12. Potash (K20)

13. Water

14. Organic matter

15. Volatile matter

8.46
3.27
5.44

0.12
0.16
0.51
0.09
0.22
0.16
0.22

7.29

The physical properties of glacial soils are also dis-
tinctive. Excepting subsequent modifications due to
water, such deposits show little or no stratification or
sorting. They are heterogeneous in material and arrange-
ment. Much of such material is termed boulder clay,
from the mixture of coarse and fine particles. It is
also to be noted that such soils contain, relatively, a
larger proportion of silt particles, and a smaller amount

58 THE PRINCIPLES OF SOIL MANAGEMENT

'»' 75<j!ghV^ fffo^' -"Jit.-'--* ::■

\&

Fig 16. Section of glacial soil, showing its uneven texture and dense structure.
When unmodified by water action, it usually shows no stratification

of clay, than soil formed by purely chemical process
from the same rock.

Associated with the results of pure ice-action is
much modified glacial till, due to the influence of great
volumes of water. Naturally, the melting of the ice
results in immense volumes of water, which drain
away over, under, or along the ice margin. Temporary
streams of large size and great violence existed

GLACIAL SOILS 59

and there were also ponds and lakes, some of the latter
of very large extent. This water further assisted in
moving the ice debris. Such deposits are called modi-
fied drift, or aqueo-glacial deposits. For this reason,
they have in part been included with glacial soils.
The streams, ponds and lakes associated with the ice
have given rise to much stratified material, and these de-
posits are intimately related in many ways to the purely
ice deposits. Beds of gravel, sand and clay are frequently
found, and so intimate is their relation to the purely
ice deposits that they are sometimes, though incor-
rectly, classed with them. These deposits of modified
till generally rest upon the distinctly ice deposits, and
are of large extent. Around the Great Lakes and in the
large valleys of New York and New England, in the
valley of the Red River of the North, and in many
other places in the Central States, are large areas of
such stratified glacial material, ranging in fineness
from heavy clay to coarse gravel. These materials
constitute some of the most valuable agricultural
lands of the country. The Great Lakes region is notably
productive, and the Red River Valley of the North
is celebrated for its production of small grains.

The thickness of glacial deposits varies greatly.
Pre-glacial valleys may be filled in, and the evidence
of their presence completely obliterated. In general, the
topographic effect of glacial action is to level the surface.
However, in the New England states, where the country
is very mountainous, the rocks very hard and the pre-
glacial soil blanket meager, the present soil covering is
generally thin and very stony.

60 THE PRINCIPLES OF SOIL MANAGEMENT

Further west, where the country is less rugged and
the rocks less refractory, the soil covering is of greater
depth and generally less stony. In the states of the
Mississippi valley, the broad, level areas of excellent
agricultural soil are very largely the result of these
glacial influences.

21. Wind or seolian soils. — Attention has been di-
rected to the transporting power of wind. It is continu-
ally picking up particles, which are deposited in accord
with the same general laws which govern water deposits.
The material thus carried, often to great heights, is
again brought to the surface by gravity. These particles
are frequently accelerated in their fall by rain and
snow. Every particle of fog, of rain and of snow has
for its nucleus a particle of dust around which con-
densation began, and for this reason the atmosphere
is always most clear after precipitation. Large amounts
of material are, in the course of time, brought to earth
in this way.

This continual deposition from the atmosphere is
illustrated by the layer of dust that quickly accumu-
lates in any unoccupied building, however tightly it
may be closed.

Besides this general filtering of dust particles from
the atmosphere, there is the definite drifting of soil
by wind, of which sand-dunes are the most common
illustration. These occur in many parts of the world.
They are likely to be developed wherever dry sand is
exposed to the wind.

Related to these modern wind deposits are immense
areas of soil of great agricultural value, the origin of

,- *

AEOLIAN SOILS, LOESS 61

which is not clearly understood, but which appears to
owe its existence, at least in part, to wind deposition.
This is the so-called loess, a fine, silty soil of remarkable
uniformity in physical and mineralogical composition.
It covers thousands of square miles of country through-
out the Mississippi valley and its tributaries, from Cin-
cinnati to western Nebraska, and from west-central
Wisconsin to southern Mississippi. It lies uncomformably
over formations of many ages, as a mantle of soft earth
of varying thickness. It does not extend over the whole
of the region mentioned, but alternates with other
formations, especially drift. It imparts to the regions
on which it rests a soil character greatly different from
what would exist were it absent.

Neither is it limited to the United States, for it occurs
extensively in central Europe, where it extends from
northern France across Belgium, and up the Rhine,
Oder and Vistula valleys in Germany; and into central
and southern Russia, where it is the basis of the famous
"black earth," or tschernosem. In northern China, von
Richtofen has described it as covering a large part of
the region drained by the Hoang-Ho, where it reaches
a thickness of 1,000 feet.

In thickness it varies greatly. Over much of the
United States it is only a few feet in thickness, generally
thinning toward the outer margin. In the central
areas it may be 150 to 200 feet in thickness, and, simi-
larly, in other countries it is of variable thickness,
reaching the great depth mentioned above for China.

A striking physical character of the loess is its
ability to stand for a long time in vertical cliffs, although

62 THE PRINCIPLES OF SOIL MANAGEMENT

so soft it may be easily carved with a shovel. Another
character common to much of the formation is the
presence of nodules and tubes formed by cementation
by lime carbonate.

The loess is associated in occurrence with the margin
of the glacial deposits, especially in America and Europe,
and possibly in China. Just what this relation is is not
known, but much of the loess seems to be a fine rock-
flour of glacial origin, which has been drifted by the
wind and deposited on both purely glacial deposits
and on residual and water deposits, for it extends from
Illinois southward over the limestone region on to the
coastal plain in Mississippi.

The adobe soils of the arid regions are thought by
some to be related to the loess in mode of formation.
Adobe also has peculiar physical properties, later to be
mentioned, but it exhibits a closer relation to water
deposits with which it has been classed.

In parts of Kansas, Nebraska and other western
states, are soils formed of dust from volcanic vents and
deposited from the atmosphere. Such dust may be so
fine as to be carried long distances and remain in sus-
pension for a long period. Dust from the eruption of
Krokatoa, in the island of Java, was wafted around
the world, and gave a red glow to the sunset for a year
after its discharge.

Table VII shows the chemical composition of the
wind deposits, chiefly loess. Columns I and X are
analyses of the hydrochloric acid solution. All others
are complete analyses. Agriculturally, sand-dunes are
of small value, largely because of their unfavorable

M

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77.75

57.97

7 .96

4.51

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(63)

64 THE PRINCIPLES OP SOIL MANAGEMENT

physical properties. They are also likely to be highly
silicious. But the loess formations are of great agricul-
tural importance, and in this country they constitute
some of the most important soil types. In some sections
its value has been greatly reduced by erosion. Some of
the bluff areas along the Mississippi river are thus
modified, and some of the loess of China is also deeply
eroded.

But the physical properties, as well as the chemical
properties of loess, combine to give it in general a
high agricultural value.

VI. HUMID AND ARID SOILS

In discussing the process by which soil is derived
from rock, attention was directed to the fact that phys-
ical disintegration results in material having different
properties from those derived through chemical decom-
position, and that the relative prominence of these
two processes is dependent largely on climate. Aridity
is one of those phases of climate which markedly alters
the balance between these two processes, giving the
larger ascendancy to the physical. Soils formed under
arid conditions are less fine in texture than those formed
from the same rock in humid regions. A study of soils
in the two regions reveals a much greater prevalence of
the coarser soils — the sandy and loamy soils — in the
arid region.

But chemical processes are not absent, for in every
arid region there is some precipitation which is able to
bring about changes in the minerals, although the

CHEMICAL COMPOSITION OF ARID AND HUMID SOILS 65

products of these chemical changes are likely to accu-
mulate in the soil because of the absence of sufficient
moisture to leach them away. (See page 307.) Their
presence is evidenced by incrustations on the particles
either at the surface or in the mass of the soil. For this
reason, the unfavorable conditions which would tend
to result from the coarser grade of the material is more
than offset by the large amounts of readily soluble
elements present. These differences are well illustrated
by the following table, compiled by Hilgard from the
results of many acid analyses in the two regions. All
soils derived from limestone are excluded.

Table VIII

Chemical Composition of Arid and Humid Soils

Strong Hydrochloric Acid Analyses

I

II

III

Humid regions.
Average of
696 samples

Semi-arid re-
gions. Average
of 178
samples

Arid regions.
Average of
573 samples

1. Insoluble residue

2. Soluble silica (Si02). . .

3. Alumnia (A1203)

4. Ferric iron (Fe,0,). . . .

5. Sulfur trioxid (S03). . .

6. Manganese (Mn02) . . .

7. Phosphoric acid (P205).

8. Lime (CaO)

84.17
4.04
3.66
3.88
0.05
0.13
0.12
0.13
0.29
0.14
0.21
1.22

4.40

75.04
8.46
4.57
2.08
0.02

0.21
0.70
0.47
0.32
0.33
3.24

8.55

69.16
6.71
7.21
5.48
0.06
0.11
0.16
1.43

9. Magnesia (MgO)

10. Soda (Na„0)

11. Potash (K20)

12. Humus

1.27
0.35
0.67
1.13

13. Water and organic
matter

5.15

E

66 THE PRINCIPLES OF SOIL MANAGEMENT

From this table it appears that, in spite of the finer
texture, the humid soils contain 15 per cent less soluble
material and, as compared with the semi-arid region,
9 per cent less soluble material.

VII. RESUME OF SCHEME OF CLASSIFICATION AND GEN-
ERAL CHARACTERISTICS OF THE GROUPS

From the foregoing discussion it appears that each
group of materials may have properties which are
fairly characteristic. Physically, the sedentary materials
differ from the transported material chiefly in arrange-
ment. In the transported soils those laid down by
wind and water are distinctly stratified — that is, ar-
ranged in layers. This is the result of settling or sedi-
mentation from a fluid, and such soils are frequently
spoken of as sedimentary. Wind and water are the
only two media in which sedimentation occurs in nature,
and therefore this arrangement indicates their influence.
Thereby the extent and variation of such deposits may
be largely interpreted.

Upon the basis of these formative differences, it is
possible not only to identify the different soil materials
but to represent their extent upon maps. The broadest
separations represented by sedentary and transported
soils may be termed divisions. Within these divisions
are sub-divisions, according to the agency or material
involved. These are termed provinces, that is, meaning
the region or province where a certain set of conditions
prevailed. For example, in the sedentary division are
residual • soils from igneous rocks and from limestone

SOIL CLASSIFICATION 67

rocks. These latter constitute soil groups, and, simi-
larly, in the transported division there is the sub-division
or province of soils deposited in water, and these are
further sub-divided into those formed in the ocean,
marine; in lakes, lacustrine, and by streams alluvial,
each constituting a soil group. Within the soil group
the first division is the soil series, based upon the fine-
ness of the material, color, drainage and other properties,
and each series is made up of soil types, the material
in each one being practically identical in all respects.
The series and type distinctions will be better under-
stood after a consideration of the physical properties
of soil. Maps of soils based upon such a classification
are constructed by several countries and institutions,
the most extensive being' the United States department
of Agriculture. These maps are constructed upon dif-
ferent scales, but one inch to one mile is the most com-
mon. The maps are accompanied by legends and
reports, for the proper explanation of the conditions
in the area reported upon.

Chemically, there is also a wide variation among soil
materials in the total amount of the elements present.
It might be expected that the repeated and long-con-
tinued mixing of materials from many kinds of rock
would result in a very great uniformity in all soils.
This is true of the number of elements present, for no
important element is absent from any soil. But the
amount may differ greatly. Aside from organic soils
(cumulose), the most striking differences occur in sand
soils. While the average analyses of many sandstones
and sand soils reveals a fair amount of all elements,

OS THE PRINCIPLES OF SOIL MANAGEMENT

there are materials composed almost entirely of the
refractory mineral quartz. Such, for example, is the
barren LaFayette sand of Maryland, which contains
94.4 per cent of silica, and a sandstone occurring in
Utah contains 96.6 per cent of silica. Doubtless, dune
sands as rich in quartz might be found. Not all silica
is in the form of quartz, but it is an indication of the
latter.

Fine-textured soils also exhibit much variation, but
do not go nearly to the extreme in silica content shown
by sand soils.

It is the very exceptional soil of any grade of fineness
which does not contain, in its ultimate analysis, a fair
amount of all of the essential mineral plant-food ele-
ments. Other conditions must also be taken into account
in determining the crop value of such soil — its physical
properties, the climate, the crop, the introduction of
new materials by wind, the movements of water and
the action of plants and animals.

2. The Soil Mass. Physical Properties of the
Soil and Their Modification

The term soil is used to designate that superficial
portion of the earth's surface in which plant roots
distribute themselves. This includes sand, gravel and
boulders, containing practically no available plant-food
material, as well as rich garden soil.

22. Soil and subsoil. — A common and natural dis-
tinction is made of (a) the top soil, which is called
"soil," and which usually extends to the depth of the

THE SOIL AND SUBSOIL 69

furrow slice or a little deeper. It is characterized by
being darker in color, and more friable and porous than
(b) the subsoil, which constitutes the material beneath
the soil in which plant roots are found. (See Fig. 13.)
A distinction is sometimes made between the upper
and the lower subsoil, the former being the layer of
subsoil lying between the top soil and a depth of twenty-
four inches from the surface, the remainder of the sec-
tion being the lower subsoil.

In humid regions the subsoil is usually less productive
than in arid regions, owing to the greater amount of
leaching, and to deficient aeration consequent on the
movement of large quantities of water through the
subsoil. Plowing up the subsoil in the humid region
frequently results in a decreased productiveness, while
in an arid region the soil and subsoil may be freely
mixed without injury, and good crops may be grown
even where the top soil has been entirely removed,
as is sometimes done in preparing land for irrigation.

The soil substance may be conveniently divided
into two groups of constituents which exhibit quite
different properties. These are the inorganic and the

organic.

INORGANIC CONSTITUENTS

The inorganic constituents of the soil are more or
less modified particles of rock, varying in size from
boulders and coarse sand to the finest dust. Each
particle may consist of several minerals, but in those
smaller than coarse sand it is unusual to find them com-
posed of more than one mineral.

70

THE PRINCIPLES OF SOIL MANAGEMENT

23. Texture. — The size of the individual particles
in a soil is a targe determining factor in all of its prop-
erties. The term texture is used to refer to the size of

the individual par-
ticles of which a soil
is composed.

In shape the
particles are very
irregular. Being
minerals or mineral
aggregates, they
tend to have the
characteristic lines
and faces of their
species. Ordinarily,
however, the nu-
merous forces that
have been at work
in the formation of
the soil have rounded or broken the mineral into angu-
lar, jagged or partially smoothed fragments. The relative
number of particles of corresponding sizes varies greatly
in different soils, some being composed largely of coarse
particles while others are made up largely of fine ones.
The relative proportion of these various -sized particles
influences greatly the physical properties of the soil.

24. Textural classification. — -When a soil is divided
into groups of particles of approximately one size, the
process constitutes a mechanical analysis and each
group is a soil separate. The limit in size of each of
these groups is arbitrarily arranged, and is determined

Fig. 17. Fine sand, photomicrograph. Magni-
fied about 110 diameters. Differences in color
indicate differences in mineral composition. Each
particle composed of one mineral.

SOIL TEXTURE

71

by the relative value of the different sizes in determin-
ing the properties of the soil and its crop-producing
power. It is found that the fine groups exert relatively
much more in-
fluence, weight for
weight, than the
coarse ones. There-
fore there are more
divisions made
among the fine than
among the coarse
particles.

25. Textural
groups. — A num-
ber of systems of
grouping have been
devised. The limits
of these groups
have been deter-
mined by the
method of analysis
used by the investigator and by his judgment of the
relative agricultural importance of each group. A
further element which limits the number of groups is
the practicability of recognizing distinctions in the field
based upon them. The following table, from Bulletin 24
of the United States Bureau of Soils, exhibits the most
generally known of these systems of grouping employed
in mechanical analysis. Some of these multiply groups
in the small particles, while others give prominence to
the sand particles.

Fig. 18. Silt soil, photomicrograph. Magnified
about 110 diameters. Stained so that differences
in mineral composition are not so distinguishable
as in Fig. 17. The particles have the same char-
acteristics as those of fine sand. Some of the
smallest particles are of the size of clay.

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TEXTURAL (iROUPS

73

Table VIII

0

Number of

Hilgard

Osborne

U. S. Bureau
of Soils

Hopkins

group

m.m.

m.m.

m.m.

m.m.

1

3.000

3.00

2.000

1.0000

2

1.000

1.00

1.000

0.3200

3

0.500

0.50

0.500

0.1000

4

0.300

0.25

0.250

0.0320

5

0.160

0.05

0.100

0.0100

6

0.120

0.01

0.050

0.0032

7

0.720

0.005

0.0010

8

0.047

9

0.036

10

0.025

11

0.016

12

0.010

13

Of these systems, that of the Bureau of Soils has been
applied to the largest number of samples and is most
widely known. The names which it applies to its dif-
ferent groups or separates are as follows:

1. Fine gravel 2.000-1.000 m.m.

2. Coarse sand 1.000-0.500 m.m.

3. Medium sand 0.500-0.250 m.m.

4. Fine sand '. 0.250-0.100 m.m.

5. Very fine sand 0.100-0.050 m.m.

6. Silt 0.050-0.005 m.m.

7. Clay 0.005-0.000 m.m.

All that material above two millimeters in diameter
is classed as gravel and stone, and in any complete
examination must also be taken into account. The
material resulting from the above analysis is sometimes
termed the fine earth, in distinction from the gravel,
etc. That there are distinctions which should be made
between the grades of gravel is obvious, for small

74

THE PRINCIPLES OF SOIL MANAGEMENT

pebbles constitute a very different condition from large
boulders in all phases of tillage.

The relative dimensions of the particles in the groups
may be illustrated graphically by the following diagram.

Fig. 20. Diagram illustrating the relative size of the groups of particles, made
in mechanical analysis by the Bureau of Soils Classification

26. Agricultural classes based on texture. — Ob-
viously, no natural soil is composed entirely of material like
any one of these groups, but a soil may contain a large
proportion of material of any one size. Thus, a sandy
soil is one containing a large proportion of sand par-
ticles, and the coarser the sand or the larger its propor-
tion the more sandy the soil appears. A clay soil is one
containing a large proportion, but not necessarily a
larger quantity of clay than of material of any other
size. A given amount of fine particles has a larger
effect on the properties of the soil than the same amount
of coarse particles. The presence of silt particles in
addition to clay serves to make a soil more heavy than
if the same quantity of sand were substituted for the silt.

MECHANICAL COMPOSITION OF SOILS

75

,

71

■

1

■

>

■

Fig. 21. Fine sand soil, showing the mechanical composition. Each
vial contains the proportion of particles of given size found in the samples.
Clay on the right; fine gravel on the left. For key to sizes, see Fig. 19 and
page 73.

Fig. 22. Silt loam, showing the mechanical composition. For explanation,

see Fig. 21

76

THE PRINCIPLES OF SOIL MANAGEMENT

A mixture of all the groups without the preponderance
of the properties of any one group constitutes a loam
soil.

For purposes of a soil survey, a classification is made
that permits of finer distinctions. The textures which
have been recognized are given in the table opposite,
together with the limits in mechanical composition

Fig. 23. Heavy clay, showing the mechanical composition. For explanation,
see Fig. 21. Compare with Figs. 21, 22.

which they represent. It is of course, impossible to fix
all of the limits in such a classification, and therefore
only certain groups are specified. This scheme has
been devised by the United States Bureau of Soils in
its soil-survey work.

All those soils having the same general texture,
although they may have been derived in a very different
way, constitute a soil class. Thus there is the sandy
loam class, the silt class, the clay class, etc. The fol-
lowing curves exhibit the average composition of several

AGRICULTURAL CLASSES OF SOIL
Table IX

77

1

Fine
Gravel

2.-1.

m.m.

2

Coarse
Sand

1.-.5
m.m.

3
Me-
dium
Sand

.5-.25
m.m.

4

Fine
Sand

.25 -.10
m.m.

5
•Very
Fine
Sand

.10 -.05
m.m.

6

Silt

.05- .005
in. in.

7

Clay

.005-0
m.m.

Coarse

More than 25
%(l+2)

0-15

0-10

sand

More than 50%

(1+2 + 3)

Less than 20%
(6 + 7)

Medium

Less than 20%
(1+2)

0-15 0-10

sand

More than 20%
(1+2 + 3)

Less than 20%

(6 + 7)

Fine

Less than 20%
(1+2 + 3)

0-15 | 0-10

sand

Less than 20% (6 + 7)

Sandy
loam

More than 20%

(1+2 + 3)

10-35 | 5-15

More than 20% and less
than 50% (6 + 7)

Fine

sandy
loam

Less than 20%

(1+2 + 3)

10-35 | 5-15

More than 20% and less
than 50% (6 + 7)

15-25

Loam

Less than
55% (6)

More than 50% (6 + 7)

Silt loam

More than

55% (6)

Less than
25% (7)

25-55 | 25-35

Clay loam

More than 60% (6 + 7)

Sandy clay

Less than
25% (6)

More than
20% (7)

Less than 60% (6 + 7)

Silt clay

More than

55% (6)

25%-35%
(7)

Clay

More than
35% (7)

More than 60% (6 + 7)

78

THE PRINCIPLES OF SOIL MANAGEMENT

classes, as they are found in the field. The field classi-
fication may not be strictly in accord with the mechani-
cal analysis, for the reason that the same essential
conditions may result from more than one mixture of
groups. By experience much facility in judgment may
be attained.

55
£50

3 45

£40

111

£35

?30

Q.

LLl . >-
CO 40

o'20
SJ15

HI

810

o

3 SANDY y ■ y

1 FINE
GRAVEL

2 COARSE
SAND

3 MEDIUM
SAND

4 FINE
SAND

6 SILT

5 VERY
FINE
SAND

SOIL SEPARATES
Fig. 24. Curves representing the average analysis of seven common
field classes of soil

Taking the soils formed in the same general way,
alluvial for example, they are found, to exhibit all
gradations of fineness from • clay up to the coarsest
gravel and stony loams. All these classes constitute
a soil series. In the same way, there may be a glacial
series or even several of them, lacustrum series, residual
series, etc. The river-bottom soils of the Central states
are chiefly classified by the Bureau of Soils into the
Wabash and Waverly series. Some of the glacial soils
into Miami, Volusia, etc.; coastal plain soils into Norfolk
(yellow), Orangeburg (red), etc., through all the divis-
ions, provinces and groups.

TEXTURE AND CROP RELATIONS

79

This means that, while sandy loams
a class are similar in texture, they may
other properties of importance in plant
complete series is one in which all the
are represented.

Some idea of the relation of these c
crops is given by the following curves,
especially suited to the production of
which they are associated.

or silt loams as
differ in many
production. A

possible classes

lasses of soil to
These soils are
the crops with

7
CLAV

12 3 4 5 6

FINE GRAVEL COARSE SAND MEDIUM SAND FINE SAND VERY FINE SAND SILT

SOIL SEPARATES

Fig. 25. Curves showing the relation of soil texture to crop adaptation

27. Some physical properties of arid and humid
soils. — In discussing the formation of soils, attention
was directed to the effect of climate upon the process,
and it was noted that under arid conditions physical
disintegration is likely to predominate over chemical
decomposition, which results in an average coarser text-
ure of the soil. This appears especially in the greater
proportion of soils of the sandy and loam classes to
those of the silt and clay classes.

Climate also exercises a modifying effect as between

80 THE PRINCIPLES OF SOIL MANAGEMENT

the soil and the subsoil. In humid regions, the large
rainfall and consequent seepage through the soil is
associated with a greater degree of fineness in the sub-
soil than in the soil. On the other hand, in arid regions
where there is not this large rainfall, and consequent
leaching, the subsoil is not finer than the soil, and, in
fact, is inclined to be more coarse.

28. Some properties of soil separates and classes. —
As has been indicated, the justification for a study of
individual soil particles from an agricultural standpoint
is in their fundamental relation to the management of
the soil. Every farmer is well acquainted with the
striking difference in crop relations and tillage properties
of sand and clay. He well knows that they must be
managed differently and are suited to different crops.
He knows sand to be better suited to early maturing
crops, like truck, than to late crops and the grasses.
He knows that one does not withstand dry weather,
while the other will carry a crop through a long
period of drought. The cause traces back to the
size and consequent properties of the soil units. This
will appear more clearly in the discussion of soil
moisture.

29. Number of particles. — Since soil particles run
to very small diameters, the number in any given mass
or volume is very great. This is shown in the following
table, which gives the number of particles in 1 gram
(1 lb. equals 453.6 gr.) of each of the fine earth separates,
considering the particles to be spheres of mean diameter
and of specific gravity 2.65.

If the particles of a soil are assumed to be spheres

NUMBER OF SOIL PARTICLES 81

of uniform diameter and weight, the number in a given
mass of soil may be calculated from the following
formula:

N_ W _ W

^rR3X2.65 7rD3x2.65
6

Where N = Number of particles.
W = Weight of soil used.
R = Mean radius in centimeters.
D = Mean diameter in centimeters.
AttR' = Volume of sphere.

For example, the mean diameter of the medium sand
class is .0375 centimeters, and in 3.5 grams of this
material there would be

3 5 3 5

N = 7^375 ^ X2~65 = ^000737 " 47'500 particleS
6

From the mechanical analysis which gives the weight
of each class of particles in a given amount of soil, the
number of particles of each size may be calculated
by use of the above formula, and the sum of the particles
in each class gives the total number in the sample.

Table X. — Number of Particles in One Gram of Pure Soil
Separate, Supposing that All Particles Are Spherical

Diameter in Number of particles

m.m. in one gram

Fine gravel 2.000-1.000 252

Coarse sand 1.000-0.500 1,723

Medium sand 0.500-0.250 13,500

Fine sand 0.250-0.100 132,000

Very fine sand 0.100-0.050 1,687,000

Silt 0.050-0.005 65,100,000

Clay 0.005-0.000 45,500,000,000

F

82 THE PRINCIPLES OF SOIL MANAGEMENT

Since normal field soils are mixtures in different
proportions of these groups, the number of particles
in unit weight of any class will be different from those
shown above, and will not reach the extreme upper
limits.

The number of particles in one gram oLthe classes
of soil whose analyses are shown by the curves on page
78 is approximately as follows:

Table XI. — Approximate Number of Particles in One

Gram op Soil

Coarse sand 3,276,000,000

Medium sand 3,956,000,000

Sandy loam 6,485,000,000

Fine sandy loam 4,902,000,000

Silt loam 9,639,000,000

Clay loam 16,371,000,000

Clay 19,525,000,000

30. Surface area of soil particles. — The significance of
these large numbers of soil particles in any mass of soil
lies in their relation to the surface area of the particles.
These surfaces of the particles hold on to the moisture
the more, the greater their area. This large surface
also increases the rate of chemical solution, by which
the food constitutes contained in the mineral particles
become available for the plant's use. Another important
property of this immense surface area of soils is to retain
food materials in a semi-available form, as will be exr
plained in discussing the absorptive power of soils.
(See page 299.)

The surface area of a fine-textured soil is greater than
the first thought might indicate. This immense area
exposed by soils is shown by the following table, which

SURFACE AREA OF SOIL PARTICLES

83

gives: (1) The area in square feet of one gram of the
soils represented by the curves on page 78. (2) The
surface area per pound of the same soil. (3) The ap-
proximate weight per cubic foot of the material in the
field. (4) The approximate area of surface in one cubic
foot of these soils as they occur in the field.

The surface area of the particles in a given weight
of soil may be calculated from the formula.

S = ttD2N.
Where S = Surface area in square centimeters.
D = Mean diameter in centimeters.
N = Number of particles in the class or separate.

Thus in the calculation on page 81 there were found
to be approximately 47,500 particles in 3.5 grams of
medium sand. Their surface area, provided the particles
were spherical, would be:

S = tt.03752X 47,500 = 212 sq. cm. =32.8 sq. in.

Table XII. — Internal Surface Area of Field Soils in Square
Feet (Analysis of first seven represented by curves on page 78)

Area per gram
Sq. ft.

Coarse sand

Medium sand . . .

Sandy loam

Fine sandy loam .

Silt loam

Clay loam

Clay

Sand hill

Hobart clay

0.8900
1.0440
1.8000
1.6G00
2.9600
4.0250
4.4130
0.0708
7.2820

II

Area per
pound.
Sq. ft.

405.0

473.0

816.0

756.0

1,340.0

1,825.0

2,000.0

32.2

3,316.0

III

Approximate

weight per

cubic foot.

Pounds

100
96
83
82
77
75
71

110
60

IV

Surface area

per cubic foot

Sq. ft.

40,500
44,500
66,600

62,000
104,000
130,500
142,000
3,540
200:000

84 THE PRINCIPLES OF SOIL MANAGEMENT

From this table it appears that one pound of the
average agricultural soil may have from about 400
square feet, in the case of coarse sand, to 2,000 square
feet internal surface area, in the case of the average
clay. A more reasonable basis of comparison, because
of differences in volume weight, is that of one cubic
foot of the material, as shown by the fourth column,
from which it appears that these soils have from one
to three acres of surface area. These are striking dif-
ferences, particularly those between soils 8 and 9,
which represent extremes in light and heavy soils,
respectively. Number eight is the sand-hill soil of the
Carolinas, and is of exceedingly low agricultural value.
Number nine, Hobart clay, occurs in eastern North
Dakota, and is derived from shale rock. The range in
surface area per cubic foot of these soils is from one-
twelfth of an acre, for the sand, to almost five acres for
the clay. The latter contains 76 per cent of clay in the
subsoil, the former 2 per cent.

31. Chemical composition of the soil separates. —
There is some relation between the soil classes or
separates and their chemical composition. Quartz, for
example, in the original rock resists decay and comes
through largely as sand particles, while the silicate min-
erals undergo much more decay which results in a larger
proportion of clay particles, and this partial difference
in derivation is reflected in the composition of the sepa-
rates. The distribution of plant-food constituents and
the general chemical composition of the classes of a soil
is shown by the following table of results of acid anal-
ysis, obtained by Loughridge as reported by Merrill.

COMPOSITION AND SOLUBILITY OF SOIL CLASSES 85

Table XIII

Conventional name

Clay

Finest silt Fine silt

Medium silt

Coarsest
silt

Per cent present in soil ....

21.64

23.56

12.54

13.67

13.11

Diameter of particles ....

.011-. 000
m.m.

.005- .011
m.m.

.013-.016
m.m.

.022- .027
m.m.

.033- .038
m.m.

Constituents

1 . Insoluble residue.. .

2. Soluble silica (Si02)

3. Aluminum (A1203).

4. Ferric iron (Fe203) .

5. Phosphoric anhy-

drid(PA) ....

6. Sulfur trioxid

(S03)

7. Lime (CaO)

8. Magnesia (MgO) . .

9. Soda (Na,Q)

10. Potash (K20)

1 1 . Volatile matter . . .

Per cent

15.96
33.10
18.19
18.76

0.18

0.06
0.09
1.33

1.47
9.00

Per cent

73.17
9.95
4.32
4.76

0.11

0.02
0.13
0.46
0.24
0.53
5.61

Per cent

87.96
4.27
2.64
2.34

0.03

0.03
0.18
0.26
0.28
0.29
1.72

Per cent

94.13
2.35
1.21
1.03

0.02

0.03
0.09
0.10
0.21
0.12
0.92

100.21

Per cent
96.52

Totals

99.84
75.18

99.30

100.00

Total soluble constitu-
ents

20.52

10.32

5.16

This table illustrates, (1) The much greater solu-
bility of the fine particles in strong hydrochloric acid.
(2) That the absolute amount of food elements dis-
solved is greater in the fine-textured class than in the
coarse-textured class. (3) That the ratio of food ele-
ments dissolved to the aluminum and other refractory
constituents dissolved is narrower in the coarse than in
the fine-textured class.

Failyer's results are summarized in the following
table.

m

fa

H

<!

PS

<

c-

orj

w

i-l

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O

j

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o

Z

(77

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<l

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03

3

H

>>

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CM CN t-- O

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03

CN

■-J i-h c4 -*

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03

S^S o S «

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>> 5? O to >> 52

> £>o Ss > g

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cc

:3 O o 'r
SO rt a

S a

0
^^

•- CO

co co

t^ O CN

I-H

(86)

MODIFICATION OF SOIL TEXTURE 87

These figures, and those published by a number of
other experimenters, clearly show the larger portion of the
phosphorus, calcium, magnesium and potassium in the
fine-textured classes in all kinds of soil. The absolute
amount of the food elements is also greatest in the fine
separates. It is shown that those soils which have
undergone the greatest weathering — the coastal plain
soils — are much the lowest in the food elements through-
out the different classes. On the other hand, glacial
soils are relatively rich in these food elements. There
is also much less difference in composition between
the clay and the sand particles in glacial soils, presumabty
because these soils have been formed largely by mechani-
cal processes, without much weathering or leaching.
The arid soils presented are not fully representative,
but they illustrate the high percentages of the food
elements in all the classes of particles, although the
same concentration in the fine particles is apparent.

It is, therefore, concluded that clay particles are
relatively richer in food elements than sand particles.
But in glacial and arid soils, and to a degree in residual
soils, the sand particles are much richer in food elements
than they are in soils of water-deposition, such as the
coastal plain.

32. Modification of soil texture. — The only feasible
method of changing the texture of a soil is by adding
to it material of a different texture. Thus, the green-
house man considers the requirements of his crops,
and by mixture of fine and coarse material obtains the
texture which is necessary for their best development.
This is entirely practicable where only a small volume

88 THE PRINCIPLES OF SOIL MANAGEMENT

of soil is involved, but under field conditions modifica-
tions of texture artificially are not practicable, because
of the expense involved. The farmer must generally
accept the texture of the soil as he finds it, and make
the best of his conditions by suitable selection of crops
adapted to his soil, and by such modifications of the
structure of the soil as its texture will permit.

33. Structure. — Soil structure deals with the arrange-
ment of the soil particles independently of their size.

34. Some aspects of soil structure. — The arrangement
of the soil particles may be viewed in many different
ways. Upon this arrangement depend several very
important physical properties, which in turn have a
fundamental bearing on chemical and biological prop-
erties.

35. Ideal arrangements. — Taking the simplest case
first, that of spherical particles of one size, these may
be arranged in general forms: (1) In columnar order,
with each particle touching its neighbors at only four
points. (2) In oblique order, with each particle touch-
ing its neighbors at six points. (3) These spheres may
be gathered into larger spheres which rest together
in the second order. In the first the unoccupied or pore
space is 47.64 per cent of the total volume occupied
by the spheres. In the second it is 25.95 per cent. In
the third case, however, where there are spheres within
spheres, the pore space is greatly increased — to 74.05'
per cent. (4) On the other hand, if there are spheres of
several sizes so that the small ones may rest in the
spaces between the large ones, the total pore space will
be reduced below 25.95 per cent, and the spaces may

ARRANGEMENT OF SOIL PARTICLES

89

continue to be filled in by smaller spheres until the
mass is practically solid, without pores. (See Fig. 26.)
It is of course recognized that under field conditions
these ideal arrangements do not pertain, but these
figures illustrate the underlying factors which determine
differences in pore space, and, also, differences in other
physical properties. Soil particles are irregular in shape
and uneven in size. When brought very close together,

Fig. 26. Ideal arrangements of spherical soil particles: (1) Columnar order,
47.64 per cent of pore space. (2) Oblique order, 2.5.95 per cent of pore space.
(3) Compound spheres in oblique order, 74.05 per cent of pore space. (4) Three
sizes of spheres with closest packing, about 5 per cent of pore space.

as occurs in mixing in a wet condition, their molecular
attraction is brought into operation and, especially
when dry, they are held together very securely. In this
way the normal molecular attraction of the soil particles
is increased by the deposition around them of the
material in solution.

Applying these principles to the soil, it is observed
that there may be two general arrangements of the
particles. (1) Each particle may be separate and free
from its neighbors. This is a separate-grain structure.
That is, each particle of soil functions separately. When
by proper manipulation the particles are so packed
together that the small particles quite completely fill in
the spaces between the large ones, so that a very dense

90

THE PRINCIPLES OF SOIL MANAGEMENT

mass is formed (Fig. 26, No. 4), the structure is termed
puddled. The term puddled, in this connection, is re-
lated to the fact that such an arrangement can be

Fig. 27. An example of undesirable structure. A clay soil which had been

puddled by tramping when wet. "Bad tilth." Compare with Fig. 28, showing

'ideal tilth." Note also a type of auger used in examining soils. A common

one and one-half inch wood auger welded to a one-half-inch shank, giving a

total length of about three feet.

obtained only in fine-textured soils when they are mixed
(puddled) in a very wet condition, so that the fine
particles will move into the large spaces.

GOOD AND BAD TILTH

91

On the other hand, the small particles may adhere
to the large ones, or a number of small particles may
adhere together as a group, or granule. When a number
of united particles function together as a single larger
particle or granule, the structure is termed granular.

-«!.'•

*%v

*^PShfr_j

JS /* '-/<

Fig. 28. Ideal tilth of a soil

This arrangement is also termed the crumb structure.
According as these groups are prominent or incon-
spicuous, the soil is said to be well or poorly granulated.
But when the granules reach large size, so that they
interfere with the best functioning of the soil, they are
termed clods. That is, a clod is an unsizable granule.

92 THE PRINCIPLES OF SOIL MANAGEMENT

It is well known that a box of baseballs, or a pile
of boulders, or even a box of sand, does not adhere
together to any appreciable extent. That is, in all the
coarser -textured classes, certainly down to the size
of very fine sand, there is very little tendency to granu-
late. But in the silt, to a small extent, and in the clay,
to a very great extent, granulation is strong.

36. Porosity. — In a mass of particles there is some
unoccupied or pore space. If the particles are fine, then
the intervening spaces are correspondingly small; if
large, the spaces are large. In the discussion of ideal
particles above, it was shown that the pore space is
theoretically independent of the size of the particles,
with any given arrangement. There would be as much
pore space in a cubic foot of buckshot as in one of
marbles. But in the soil this is not true. For, the finer
the particles, the larger the proportion of pore space
is found to be.

A clay has much more total pore space than a sand,
although the individual spaces or openings between
the particles are much smaller in the clay. The approxi-
mate per cent of pore space in a soil may be calculated
by use of the following formula.

v Vw

2.65 Vd

*>- -xioo= ^Exioo

Vs Vs

Where P = Per cent of pore space.

Vs = Volume in c.c. occupied by the soil.
Vw= Weight of water equal to weight of soil in grams.
Vp = Volume in c.c. of pore space in soil.
2.65 = Specific gravity of soil particles.

POROSITY OF SOIL 93

Another and more simple formula which may be
used in the calculation of the pore space is as follows:

P=ioQ-fp-sPgrxioo

Ab. sp. gr.
Where P = Per cent of pore space.
Ap. Sp. = Apparent specific gravity or volume weight.
Ab. Sp. = Absolute specific gravity of soil material.
100% = Total space occupied by soil mass.

This relation between texture and pore space is
exhibited by the following table of figures for soils in
field condition.

Per cent by volume

1. Clean sand 33.50

2. Coarse sand 40.00

3. Medium sand 41.80

4. Fine sand 44.10

5. Sandy loam. 51.00

6. Fine sandy loam 50.00

7. Silt loam 53.00

8. Clay loam 54.00

9. Clay 56.00

10. "Gumbo" clay (Wedgefield) 58.46

11. Heavy clay (Potomac puddled) 47.19

12. Very heavy clay (pipe clay) 65.12

The reason for the greater porosity of the finer soils
appears to be, that the smallest particles are so light
that they do not settle so closely together in proportion
to their size as do the sand particles, because of the
greater friction between their surfaces. When this is
overcome by mixing in water, such material becomes
dense. Treatment greatly affects the structure and
therefore the porosity of the soil. This is well shown

94 THE PRINCIPLES OF SOIL MANAGEMENT

by figures from the Rothamsted fields. The porosity
of the surface nine inches of soil in an old pasture was
56.8 per cent, while in the same depth of a cultivated
field it was 45.5 per cent. Extensive areas of loam soils
in the North Central states have a porosity of from
45 per cent to 49.6 per cent. In many of the heavier
soils it much exceeds 50 per cent, and in well-granu-
lated clays it may reach 70 per cent, or in light sand
it may be less than 40 per cent. In general, it may be
said that about one-half of the volume of ordinary
cultivated soils of intermediate texture is pore space.

The diameter of the individual pore spaces is of
importance, as well as the total volume of pore space,
since these determine the capacity of the soil to retain
and move water and to permit the circulation of gases
in the soil mass, as well as to facilitate the extension of
the plant -roots.

37. Weight. — The weight of soil is the result of two
factors. These are, first, the absolute weight of the indi-
vidual particles, or absolute specific gravity, and second,
the volume of pore space in the mass.

By reference to the table of minerals on page 6,
it will be seen that the minerals entering into the soil
vary greatly in specific gravity — that is, their weight
as compared with an equal volume of water. They
range from about 2.5 to 6 or 8, but the minerals which
make up the great bulk of the soil — quartz, feldspars,
micas, calcite, etc., — all have a specific gravity of from
2.6 to 2.8. (See Table I, pages 6 and 7.) Many deter-
minations of this property have been made. Fineness
does not appear to have any material effect upon it.

WEIGHT OF SOIL

95

Whitney has obtained the following specific gravities of
composite soil separates.

Table

XV

Conventional name

Diameter (m.m.)

Specific gravity

Fine gravel

Coarse sand

2-1

1-.5

.5-25

.25-.10

.1-.05

.050-.005

.005-.000

2.647
2.655

Medium sand

2.648

Fine sand

2.659

Very fine sand

2.680

Silt

Clay

2.698
2.837

There is a very small increase in the specific gravity
of the clay group, probably due to the greater concentra-
tion of the iron compounds here as a result of chemical
processes; but it is not sufficient to materially change
the result. The average specific gravity of soil material
is, therefore, usually taken as 2.65, and this figure is
used in all calculations here given.

Since the pore space enters into the calculation of
the weight of any volume of field soil, this figure is
much more variable for different soils than the one just
given. It is directly related to pore space, and the
larger the volume of pore space, the smaller the unit
weight.

Combining the figures for pore space given above
with that for average specific gravity, the figures in
the following table are obtained.

The weight of a given volume of soil may be deter-
mined from the pore space and specific gravity of the
material, by use of the following formula.

96

THE PRINCIPLES OF SOIL MANAGEMENT

Ws=WwX (2.65 X (100 -P).

Where Ws = Weight of given volume of soil.

Ww = Weight of volume of water equal to volume of soil.
P = Per cent of pore space.
(100 — P)=Per cent of volume occupied by soil.

Or the following formula may be used, and is often

more convenient.

Ws=Ap. Sp.xWw.
Where Ws = Weight of soil.

Ap. Sp.= Apparent specific gravity.

Ww = Weight of volume of water equal to that occupied
by the soil.

Table XVI

1. Clean sand

2. Coarse sand

3. Medium sand

4. Fine sand

5. Sandy loam

6. Fine sandy loam

7. Silt loam

8. Clay loam

9. Clay

10. "Gumbo" clay

11. Puddled heavy clay (Poto-

mac)

12. Heavy pipe clay

13. Old pasture clay loam (Roth-

amsted)

14. Cultivated soil, clay loam

(Rothamsted)

15. Hagerstown loam

16. Janesville loam

Volume weight

Weight per
cubic foot

or apparent

specific gravity

Kilo-
grams

Lbs.

1.76

50.0

110.0

1.60

45.5

100.0

1.54

43.5

96.0

1.48

42.0

93.0

1.30

36.8

81.0

1.32

37.4

82.5

1.24

35.2

77.5

1.22

34.5

76.0

1.17

33.1

72.6

1.10

31.2

68.5

1.39

39.6

87.2

0.93

26.3

58.0

1.14

32.3

71.0

1.43

40.5

89.0

1.44

41.0

90.0

1.33

37.8

83.0

Weight per
acre foot

Lbs.

4,800,000
4,356,000
4,200,000
4,080,000
3,550,000
3,590,000
3,400,000
3,330,000
3,180,000
3,000,000

3,820,000
2,540,000

3,100,000

3,900,000
3,940,000
3,640,000

One kilogram =2.2 pounds.

PLASTICITY OF SOIL 97

This table shows that the finer the soil the lighter
its absolute weight. Clay soils may range from 60 to 90
pounds in weight, according to their fineness and state
of granulation. Sand soils weigh from 90 to 110 pounds.
In practice, soils are spoken of as "light" and "heavy,"
but this use of these terms does not apply to the weight
of the soil. The term light is applied to sandy soil
because the particles move freely. On the other hand,
a clay is termed heavy because of its cohesiveness.

38. Plasticity. — The property of stickiness of soils,
when mixed with water, is termed plasticity. Soils
exhibit it in very different degrees. In general, it may
be said that the finer the soil the greater the plasticity,
and therefore the finest-textured clays generally. exhibit
the greatest degree of plasticity. On the other hand,
plasticity is not absolutely lacking in sandy soil, for,
when moist, this material adheres together and may
support a considerable weight. But, when the water is
removed by drying, the sand will fall apart readily,
and therefore the cohesiveness exhibited was largely
due to the surface tension of the water between the
particles. However, when the clay is dried out, it be-
comes a hard mass, and it has a superior adhesive and
cohesive property when dried from the wet puddled
state.

But while plasticity and great tensile strength appear
to be very closely associated with fine texture, the
fineness does not appear to be entirely responsible for
the property, as is shown by the results of numerous
studies reviewed by Ries. Writers in the past have
dwelt much on the effect of colloidal clay in this connec-

98 THE PRINCIPLES OF SOIL MANAGEMENT

tion. The real significance of colloidal material is some-
what doubtful, and, further, the amount present in
even the most plastic clays is so small as hardly to be
given credit for the effects noted. It seems probable
that plasticity and cohesiveness of the material is due
to several uniting causes, but for all practical purposes
of the farmer it may be identified with fineness of tex-
ture. Associated with plasticity is a certain amount of
shrinkage upon drying, and expansion upon wetting.
The checking of the clay soil is an example of this.
As the water dries out of the soil, the surface film draws
continually closer about the particles, and, if these
are small enough, may move them closer together.
Then, if the whole mass is not drawn together as one
unit, there will be cracks developed as a result of the
shrinkage. The cracks occur where there is a weakness,
from whatever cause, in the structure of the soil. War-
ington reports the results of Schiibler, which show that
a very pure clay, when dried from a thoroughly puddled
condition, contracted 18.3 per cent of its original vol-
ume; a sandy clay contracted 6 per cent, and a sample
of humus, 20 per cent of its volume. Gallagher found
a shrinkage of over 30 per cent in drying out a sample
of muck. These figures illustrate the general fact that
the finer the texture the greater the shrinkage. Con-
versely, on wetting, there is a similar though smaller
degree of expansion.

The checking of soil resulting from this shrinkage
may be very injurious to crops. Where large checks
or cracks are formed, the roots of plants may be injured
or broken. And, further, these cracks greatly hasten

CEMENTING MATERIALS IN SOIL

99

Fig. 29. Excessive checking of a heavy clay soil as a result of drying. Illus-
trates the process of soil granulation.

the drying out of clay soil to a much greater depth than
is possible through surface evaporation. They also
interfere greatly with the advance of roots.

39. Cementing material. — The cohesion of a soil

100 THE PRINCIPLES OF SOIL MANAGEMENT

when dry is due to several causes, one of which is much
the most prominent. This is cementing materials. A
cementing material is any material which binds sur-
faces together. In a gravel or sand pit, masses of the
material are sometimes found united into a conglom-
erate rock. After a protracted dry spell, moist sur-
faces show a white incrustation in the surface layer,
which is due to the deposition of the salts in solution
when the moisture evaporated, and this acts as a bind-
ing material. This is one of the main reasons why a
fully dried soil is usually so much harder than one
slightly moist. The salts of many kinds which were in
solution in the moisture have been deposited.

This composite of dissolved salts is the first of four
common cementing materials which occur in the soil.
It is generally a weak binding material. The second
material is lime. Some soils are very rich in this com-
pound. Particularly is this true of most glacial soils,
and in North Dakota and other sections of the country
extended areas of gravel beds occur, in which the upper
two or three feet are completely bound together by the
deposition of lime between and around the particles.
It has been leached out of the soil above as bicarbonate,
under the influence of carbonated water formed by the
decaying organic matter, but here, in the loose gravel,
by the escape of some of the carbon dioxid it was
deposited. This is the usual history of the process.
Cementation by lime carbonate is a very common and
very general process. The third cementing material
is the various forms of iron — usually oxides — in various
stages of hydration. They have come into solution by

COLOR OF SOILS 101

the assistance of various organic acids, and are again
deposited where there is some change in physical con-
ditions. This form is most common in the unglaciated
section of the country in the older deposits. Some of the
red soils of the coastal plain region exhibit a strong
tendency to "case-harden," — that is, become quite
hard at the surface upon drying, largely due to iron
compounds. The fourth cementing material is silica,
and is less prominent in soil practice than the other
three cementing materials mentioned. It is the binding
material in most sandstones and quartzite rock, as an
advanced stage of silica infiltration.

All these cementing materials except the iron com-
pounds, which are red, yellow or brown, are light-
colored.

40. Color. — A great variety of colors are exhibited
by soils. These are not usually the result of the color
of the individual particles which make up the bulk
of the material. Rather, it is usually the result of
material which adheres to the particles.

There are two chief coloring materials in soil. These
are iron compounds and organic matter. The first gives
rise to red, yellow, blue and gray colors. The latter
gives rise to some shade of black or brown color. When
these are combined, various intermediate tints are
obtained. For example, when a red soil is rich in decayed
organic matter — humus — it becomes of a rich brown
color.

The color of soils, especially as regards iron com-
pounds, is not fully understood, but it is safe to say
that much color is the result of different forms of iron

102 THE PRINCIPLES OF SOIL MANAGEMENT

in the soil. In the boulder clay of the glaciated sections
a bluish color is common, which seems to be due to the
presence of protoxid of iron (FeO), resulting from
the* great deficiency of oxygen. Where this comes
in contact with carbonated water, it may be changed
to the carbonate of iron, which is gray, and consequently
along the line of roots and in the bottom of ponds this
gray color may be found.

Where there is an abundant supply of oxygen, the
iron takes on the sesquioxid (Fe203) form, which
has a deep red color, typified by iron rust. Where the
red soil stands much in contact with water, it may
become yellow by the hydration of the iron (Fe20;j +
H20). In many regions a dark-colored soil is looked
upon as a fertile soil. This relation has developed
because of the association of a dark color with the
presence of organic matter, with all its beneficial effects,
while the light color indicates its absence. This relation
does not hold universally, but it is quite a reliable
guide.

The only instances where the color of the particles
themselves give color to the soil is in some of the clean
quartz sands, where the white color of the dominant
mineral gives color to the mass. In some dark shaley
sands this same principle obtains.

To the experienced person, the color of the soil is a
valuable guide to its condition and productiveness.
Mottled and uneven color, for example, indicates poor
aeration, frequently the result of deficient drainage.

41. Physical absorption. — The soil particles attract
and hold materials upon their surfaces. This physical

MODIFICATION OF SOIL STRUCTURE 103

absorption, or adsorption, as it is sometimes called,
is different from chemical absorption, later to be men-
tioned, with which it is closely associated. As a result
of this property, gases and materials in solution in the
soil moisture are attracted to and, loosely held by the
surface of the soil particles. It varies with the extent of
surface exposed, and is consequently greatest in fine-
textured soil. In clay soil, which has a relatively large
surface area, it is very large, and is an important factor
in the retention of fertilizers.

42. Conditions affecting structure. — The arrange-
ment of the particles in a soil may be modified in many
ways. Some conditions tend to produce the compact
separate-grain structure, while others favor the granular
or crumb structure.

It has been suggested by Whitney, King and others,
that much of the formation of granules in the soil is
due- to the contraction of the moisture film around the
particles, when, for any reason, the moisture content
is reduced. It is known that the soil particles tend to
be drawn together by this reduction in the soil moisture.
Add to this some influence to determine the size of the
granules and a binding material to permanently hold
the granules together, and the essential conditions for
the granular conditions of soil are realized. Several
natural conditions, and the various tillage operations,
probably exert their influence on granulation in this
way. Warington attributes granulation to unequal
expansion and contraction of the soil mass, due to the
unequal imbibition and loss of water. In such a soil,
the cohesive force being different in different parts,

104 THE PRINCIPLES OF SOIL MANAGEMENT

and the internal strains and pressures unequal, a ten-
dency arises for the mass to divide along the lines of
weakness into groups of particles, as the soil moisture
is much reduced below a certain optimum condition.
Tillage operations, development of roots, burrowing of
animals and insects, the presence of humus, and the
development of frost crystals, may assist in further
developing these lines of weakness in the soil mass,
upon 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
the aggregation of particles around certain centers.
The movement of the soil particles is, in every case,
facilitated by the presence of a moderate amount of
moisture.

On the other hand, conditions opposite from the
above, including tillage at inopportune times, the
operation of some natural agencies, as the beating of
rain, erosion, and bad drainage, may not only destroy
the tendency to the granular condition, which is always
strongest in the finest soil, but may induce the opposite
or separate grain structure.

43. Means of modifying structure. — It is apparent
that some of the means of modifying the soil structure
are natural, others are within the control of man. The
following are among the better-known of these factors:
(1) Variation in the water content. (2) Development
of frost crystals. (3) Tillage. (4) Growth of plant
roots. (5) Organic matter. (6) Certain soluble salts.
(7) Earth-worms and other forms of animal life. (8)
Heavy rain storms. Whether a. desirable or an undesir-

WATER CONTENT AND SOIL STRUCTURE

105

able soil structure will result depends upon the combi-
nation of factors in operation. These structural modi-
fications have to do primarily with the finer-textured
soils — the loams, silts and clays, — rather than with
the sandy soils. The structure of the latter can not be
greatly changed.

44. Variation in moisture content. — The alternate
wetting and drying of a clay or a loam soil tends to
produce a granulated structure. It has been suggested
by Whitney that this is due to the contraction of the
moisture film around the
particles, as it is reduced
in drying. The very con-
siderable pressure of the
moisture film and the re-
duced friction due to the
presence of moisture in the
mass, causes the particles
to be drawn together in
small masses. This process
is well illustrated by Fig.
30, which was made from a
micro-slide in which was
mounted a suspension of
fine clay in water. The
water slowly evaporated
from under the cover, and
at last disappeared along
the dark lines which are
formed by the concen-
tration of the particles by

Fig. 30. Photo-micrograph, show-
ing the distribution of soil particles by
a water film. A small quantity of clay
was suspended in water on a micro-
slide and sealed in with balsam. Evap-
oration was permitted to take place very
slowly through a small opening. The
retreat of the water to the dark border
line assembled the soil particles so that
they were left, to form the dark lines
when their mass became too great to be
moved by the surface tensiqn of the
liquid. This illustrates the granulating
influence of a contracting water film,
which is the primary force in operation
during the drying-out of a wet soil.
Note also the uniform curvature of the
film, as indicated by the arrangement of
the soil particles.

10 0 THE PRINCIPLES OF SOIL MANAGEMENT

the moisture film as it contracted. The small particles
are moved into the spaces between the large ones,
thereby reducing the volume, as is shown by the checks.
The checks which result from shrinkage are due to the
unequal contraction. There comes a time when the
general film around the whole mass must rupture. It
breaks, along the line of least resistance, through a large
pore space independently of how this space may have
been formed. If the soil mass is very uniform, there will
be few breaks, and the shrinkage will be, as a whole or
at most, around a relatively few centers. This process
produces clods or overgrown granules. But, if there are
numerous lines of weakness, there will be many centers
of contraction, and consequently a larger number of
small clods or granules will be formed. This is the desir-
able condition, and constitutes good tilth, — that is, the
most favorable physical condition for plant growth.

While once drying produces some checks, — a few
large ones and many small ones, — such a structure
does not constitute good tilth. The process must be
continued further. When the soil is remoistened, it
expands, but usually not to its original wet volume.
Therefore the checks remain as lines of weakness, and,
upon a redrying, are effective in further reducing the
size of the granules. When this process is repeated
a number of times, as occurs under field conditions,
it results in a small and very desirable size of soil granule.
Further, the drying out of the water in the granule
deposits the salts in solution, which binds the particles
together in a somewhat permanent and stable aggre-
gate. The following figures represent the relative force

WETTING AND DRYING, AND STRUCTURE 107

required to sink a knife-edge into a puddled clay soil,
different samples of which were subject to drying and
rewetting a different number of times.

1. Soil dried once 100.00

2. Soil dried twenty times 31.44

3. Soil dried twenty times 30.60

4. Soil dried twenty times 32.05

Average 31.40

*»

From this table it appears that the effect of twenty
times drying is to reduce the force necessary to pene-
trate the soil a given uniform distance to one-third of that
for the untreated sample. This is certainly a large change.

This fact has many practical applications. It should
be observed that the change in structure is not associated
with continual wetness, nor is it any more identified
with a continued dry state. In neither case is the force
necessary to change the structure brought to bear on
the particles. This is exerted in the drying process.
It is a well-known fact that soils which are continually
wet are usually in bad physical condition. In the drain-
age of wet land, it is found that the soil is at first very
refractory; but, when good drainage is established,
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 struc-
ture, 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 ways already noted.

108

THE PRINCIPLES OF SOIL MANAGEMENT

45. Formation of ice crystals. — As will be seen in
the consideration of soil moisture, the water is distrib-
uted in the fine pores in the soil. When it freezes,
it crystallizes in long needle-like crystals. The crystal-
lizing force seems to be considerable. In freezing, the
crystals gradually grow first in the larger spaces. There

is a marked with-
drawal of mois-
ture from the
smallest spaces
to build up the
ice crystals in
the large spaces.
The soil mass is
separated by the
crystal, and the
result of a single
hard freeze of a
wet soil is to
shatter it into
pieces. And the
repetition of this
process by sub-
sequent freezing
further breaks up
the soil, that is,
it creates new
lines of weak-
ness. This weak-

Fig. 31. Ice crystals formed on the surface of • i

a heavy clay soil. These crystals are very effective neSS IS SllOWU

in breaking up the soil and promote the process of i ,i j n •

granulation. by the following

>

■a

£

o

0

o

X

«

M

O

110

THE PRINCIPLES OF SOIL MANAGEMENT

Fio. 33. Effect of freezing on the granulation of clay soil. The lower pan
of soil on the left was not frozen. That on the right was frozen and thawed
once. The upper pan of soil on the left was frozen and thawed three times,
the one on the right five times. Notice the increased number of checks on the
more frequently frozen soil.

table, which represents the effect of repeated freezing
of a uniform sample of wet puddled clay, after which
all were permitted to dry out. The figures are for the
weight necessary to force a knife-edge a uniform dis-
tance into the soil, in each case reduced to basis of 100.

1. Check, unfrozen 100.00

2. Frozen once 30.31

3. Frozen three times ...... 27.33

4. Frozen five times 21.88

TILLAGE AND STRUCTURE

111

This process has several interesting illustrations.
In concrete work, the freezing of the material in the
wet state, before it has an opportunity to harden, is
recognized as decidedly injurious to the strength of
the wall. The development and action of the ice crystals
may be readily observed in the freezing of any thoroughly
wet soil in winter. Such examples are shown in Figs. 31,
32 and 33.

To a much less extent, expansion and contraction
of the soil mass caused by variations in temperature
may contribute to the formation of granules. Any
movement of the particles will tend to produce changes
in the cohesive forces, and when the particles can move
easily they are drawn together by this attraction.

46. Tillage.- — The effect of tillage upon soil structure
is to produce lines of cleavage, and these, when produced
by plowing, are multitudinous, and quite uniformly
distributed. As pointed out by King, plowing when
the moisture content is suitable tends to break the
soil into thin layers, which move one over the other,

Fia. 34. Plow with interchangeable moldboard and share, which adapts
it to different kinds of plowing The plow tends to shear the soil into thin
layers which are very thoroughly broken up.

112

THE PRINCIPLES OF SOIL MANAGEMENT

like the leaves of a book, when the pages are bent. This
disturbance of the existing arrangement of particles
starts in motion two forces: (1) The surface tension
of the water films, which must now readapt themselves

Fig. 35. Clay soil plowed when very wet. Condition indicated by the slickened
soil surfaces and coarse, dense structure

to the new arrangement, and which, by opening larger
spaces, may lose some moisture by evaporation into
the arger interstitial spaces. (2) The cohesive forces
between particles, some of which have been forced
closer together and some farther apart. The strength
of cohesion between small particles, like clay, can be

PLANT ROOTS AND STRUCTURE 113

realized when one considers the tenacity with which
these particles are held together in brick. This cohesive
attraction is inversely proportional to the square of
the distance between the centers of the attracting
bodies. Particles that can be brought so closely to-
gether as can clay particles are thus held with great
firmness. The effect of tillage, when an excess of water
is present, is to force the particles into large masses,
which become clods when dry. These masses are too
large to form granules, and leave the soil in a 'compact
condition, poorly adapted to plant growth. When the
soil is very dry when worked, the particles are not
brought close enough together to cohere, but are pow-
dered, forming the separate -grain structure, which
forms clods when wet. Tillage may thus produce a
granular structure when the moisture is neither excessive
nor deficient, and the separate -grain structure when
either of these conditions exists.

47. Growth of plant roots. — The growth of plant-
roots changes the soil structure by forcing the particles
apart at each growing root point, and possibly by some
action yet to be explained. Crops differ greatly in their
effect upon soil structure. Grass, millet, wheat and
other plants with fine roots are more beneficial to tilth
than coarse or tap-rooted plants as corn, oats and beets.
Grass also affects structure by protecting the surface
of the ground. (See page 119.) It is advisable to prac-
tice a rotation on clay soil, which requires relatively
infrequent plowing, and gives long periods in fine-rooted
grass and grain crops.

48. Organic matter. — Soils rich in humus or decom-

H

114

THE PRINCIPLES OF SOIL MANAGEMENT

posed 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 cul-
tivated soils is well known. For example, in much of
southern New York the hill soils are now recognized
to have a much different relation to crop growth than
they had for a few years after they were cleared. Their
color has changed, and with the decay of the humus

Fig. 36. The spring-toothed harrow. A type of cultivator adapted to all
classes of soil and more efficient than any other in rough and stony ground.

has come a decided physical change in the soil, which
is largely corrected by the restoration of the humus
content. In certain prairie soils the effect of humus
depletion on structure is even more marked. The actions
of humus are many, as will be noted in the more com-
plete discussion of that topic yet to follow; but one
of those actions is on the granular nature of the soil.
(1) As will appear, humus is somewhat plastic, and
tends to hold the soil in a more loose condition than

ORGANIC MATTER AND STRUCTURE 115

would otherwise occur, and the large spaces thus pro-
duced constitute lines of weakness. (2) It is a property
of humus to undergo great change in volume when
dried out. This is another factor akin to the fineness
of the soil, and produces larger shrinkage crack. This
is noticeable in many black clay soils, which check
excessively. (3) The great capacity of humus for
moisture permits a wide range in moisture content,
which produces corresponding physical alteration. (4)
The color of the humus affects the color of the soil,
and thereby increases the rate of change from wet to
the dry state by increased evaporation of moisture.
The relative effects of crude muck, and the ammonia
extract from the same muck, upon the cohesion of the
soil, as indicated by the force required for a uniform
penetration of a knife-edge reduced to a basis of 100,
is shown in the following table. Samples dried and
rewetted twenty times.

Crude Muck

1 . Check 100.00

2. Muck, 5 per cent 82.00

3. Muck. 15 per cent 73.50

4. Muck, 25 per cent 58.48

5. Muck, 50 per cent 50.25

Ammonia Extract of Crude Muck

1. Check 100.00

2. Muck extract, 1 per cent . 85.30

3. Muck extract, 2 per cent 76.40

4. Muck extract, 4 per cent 69.00

This table indicates that the material represented
by the muck extract is the constituent of the muck

116 THE PRINCIPLES OF SOIL MANAGEMENT

which most influences the structure of the soil. All
these processes promote the development of lines of
weakness upon which the water may act. In this way
organic matter promotes granulation.

49. Soluble salts. — In the action of certain salts,
a different process from the previous ones is introduced.
When 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 floccules,
which then rapidly settle to the bottom, so that the
supernatant liquid is left clear or nearly so. This phe-
nomenon is termed flocculation, because of the groups
of particles. It is not an action limited to lime, but in
greater or less degree results from the use of many
substances. Lime is about the most active flocculating
agent, and a very small amount is required. Acids,
especially the mineral acids, are strong flocculating
agents. Many of the common fertilizing materials have
a flocculating power. Some substances, however, pre-
vent or break up flocculation. Such, for example, is the
effect of carbonates of the alkalies. From an agricul-
tural point of view, the various forms of lime are the
most important in this connection because their use
on the soil is practical for the farmer. When introduced
into _ the soil, this flocculating action occurs whereby
granules are formed, the stability of which may be
further increased by other favoring conditions.

The effect of lime on this process and the relative
rapidity of the action for different forms, as shown by
its influence on the cohesion of a puddled soil, is shown

SOLUBLE SALTS AND STRUCTURE 117

by the following table. The force required for a uniform
depth of penetration of the knife-edge is reduced to the
basis of 100 for the check.

1. Check 100.0

2. Calcium carbonate, 5 per cent 98.5

3. Calcium oxid equivalent to CaO in 2 56.5

4. Calcium carbonate, 10 per cent 111.0

5. Calcium oxid equivalent to CaO in 4 ... . 43.5

6. Calcium carbonate, 25 per cent 95.0

7. Calcium oxid equivalent to CaO in 6 33.6

This table indicates that the oxide of lime, or the
hydrate as it would be in the wet soil, is more efficient
in granulating the soil than is the carbonate. It is pos-
sible that this difference is the result of the short time
of contact of the lime with the soil, which was only
a few weeks. In the soil the hydrate will in time change
to the carbonate, owing to the presence of carbon
dioxicl in the soil, so that in the end the form of the
lime would be the same. These figures emphasize
a fact recognized in practice, viz., that a considerable
time is necessary for lime to have its full effect on the
soil, and therefore it should be applied some months
or even a season or two in advance of the crop it is to
benefit.

Warington reports the statement of an English
farmer to the effect that by the use of large amounts
of lime on their heavy clay soil they were 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 which are poor in lime.

118

THE PRINCIPLES OF SOIL MANAGEMENT

Carbonates of the alkalies, which are present in
many alkali soils, tend to produce a compact soil struc-
ture. The remedy lies in a conversion of the carbonate
into some other form, or in the removal of the alkali.

Fig. 37. A portion of a Meeker harrow, showing its effect on lumpy,
clay soil. (See page 481.)

(See page 314.) Hall has shown that the continued
and extensive use of nitrate of soda on the land may,
even in humid regions, deflocculate the soil.

50. Animal life. — Many forms of animal life affect
the soil structure. Earth-worms, in passing soil through
their bodies, leave it in a granulated condition in the

RAINFALL AND STRUCTURE 119

"casts" which they deposit. Their action is frequently
quite important. (See page 28.) Insects, especially
ants and other burrowing creatures, aid in this and
other ways.

51. Rainfall. — Rain storms compact the surface soil
by washing the fine particles into the interstitial spaces,
and by the actual pressure of the rain-drops. The result
is to form a surface layer having the separate-grain
structure, and which when dry, forms a crust, some-
times capable of preventing germinating plants from
reaching the surface of the ground, and which is con-
ducive to the loss of moisture by evaporation. Some
clay soils are very susceptible to a change of structure
in this way. A heavy thunder-storm may entirely
change the structure of a clay soil to the depth of sev-
eral inches, in a short time. This is due both to the im-
pact of the rain-drops and the saturated condition of
the surface layer.

Surface covering— mulches, sod or any kind of
covering during the summer season — serves to protect
the surface soil from the compacting effect of rain. The
volume weight of a mulched soil will generally be found
to be less, at the end of the growing season, than that
of a well-cultivated one. The benefit to be derived
from sod has already been mentioned.

II. ORGANIC CONSTITUENTS OF THE SOIL

Examination of almost any soil shows it to contain
not only mineral particles but also plant and animal
remains. The forest soil contains in the surface layer

120 THE PRINCIPLES OF SOIL MANAGEMENT

a large amount of partially decayed leaves and stems,
sometimes termed leaf-mold. Sod land is filled with
fine roots, which have served their period of usefulness
to the plant and are being returned to their native
elements. The low swamp areas of soil contain a large
proportion of dark or black material, which, when the
soil is dry, may be burned away, and leaves the residue
a much lighter color. And in all soils there is some of
this same volatile material derived from the growth
and partial decay of plants and animals, and commonly
termed organic — organized matter.

Organic matter may be found in the soil in all stages
of decay, from the fresh tissues to the last oxidation
products of its components. These products of the var-
ious stages in the decay process, comprehended by the
term organic matter, constitute probably the most
important body of material which enters into normal
soil.

52. Sources, derivation and forms. — The organic
matter in the soil is derived from both plants and
animals: plants are the chief source. These materials
undergo decay through the action of bacteria and
fungi, in addition to purely chemical changes. The
character of the intermediate material depends largely
on the relative prominence of the different agencies
concerned in its decomposition. This great variety
in the material, together with the differences in pro-
cesses of decay, gives rise to a number of forms of or-
ganic material which are recognized in the soil. These
materials do not represent any definite composition.
They represent, rather, stages in the general process

ORGANIC MATTER. COMPOSITION 121

of decay. Leaf-mold is the partially decomposed layer
of leaves, twigs, etc., found on the surface of the ground,
usually in well-drained forest areas. Decomposition
is very incomplete. Humus is the black or brown pul-
verent material resulting from a considerably more
advanced stage of decay than is represented by leaf-
mold. When wet, it forms a very fine, gelatinous mass
of a colloidal nature. Peat represents large and usually
deep accumulations of plant remains in the early
stages of decay. Disintegration has usually been stopped
by the saturation of the mass. The products of bacterial
and fungicidal action have accumulated, until the
organisms are killed, and any further growth is pre-
vented until the water is removed and more thorough
aeration is introduced. Plant tissues are plainly evident.
When the peat results from a particular kind of plant,
the name of the latter may be affixed as moss peat.
Peat is generally unproductive as a soil. Muck repre-
sents a much more advanced stage in the decay of peat.
It has a black or brown color, more closely resembling
humus, due to the large proportion of the latter which
it contains. Plant tissues are much less apparent. It
is generally productive, or will very quickly become so
under drainage and cultivation.

53. Chemical composition. — There is no definite
chemical composition of the organic matter in the soil.
It is as variable as the materials from which it is derived
and the conditions under which it is formed. It is
composed of a great variety of carbon compounds,
into which enter nitrogen and all of the mineral ele-
ments which are necessary to plant and animal growth.

122 THE PRINCIPLES OF SOIL MANAGEMENT

These original compounds are broken down in the pro-
cess of decay into other successively simpler com-
pounds. The end of the process is always essentially
the same — the reduction of the elements to their sim-
plest and most stable forms, the carbon to carbon dioxid,
the nitrogen to nitrates, ammonia or even free nitrogen,
and the mineral elements to their simple salts. The
soil constituents which are termed humus, mold, peat,
muck, etc., simply represent stages in the transition
process from the fresh materials to the native elements.
There is no single compound or group of compounds
which imparts definite characteristics. These are the
result of the mixture; and this fact of an infinitely
complex mixture is exceedingly important to keep in
mind, in considering the effects of the organic matter
of the soil. Many of them are acids. Some — as ammonia
and marsh gas — function as bases. They react with
each other in many ways, and, what is more important,
they react with the mineral elements of the soil to form
organic salts. It is by this union that organic matter
has not only a direct effect as a food, but also an indirect
effect in releasing food elements from their less soluble
mineral combinations. Aside from the production of
many complex organic acids, the two most significant
facts of their composition are the per cent of nitrogen
present and the chemical form of part of the carbon.
Nitrogen, which is not a constituent of rocks, is made
available to all higher forms of plants through this or-
ganic decay process, and these various compounds
constitute the soil store-house of the element from which
it gradually changes over into the available forms. The

NITROGEN IN ORGANIC MATTER 123

percentage of nitrogen present varies greatly — viz, from
less than 2 per cent in the humus of some humid soils
to more than 22 per cent in the humus of some arid
soils, as reported by Hilgard. His results show that
under arid and semi-arid conditions the humus is much
more rich in nitrogen than in humid regions, and he
attributes to this fact the large capacity of the former
soils to produce crops with so little organic matter.
His figures on this point are exhibited in the following
table.

Per Cent of Nitrogen in Humus of Soil from
Different Regions

Humid soils, average of sixteen samples 4.58

Sub-irrigated arid soils, average of fifteen samples . . 8.38
Arid upland soils, average of forty-two samples 15.23

The nitrogen is changed under good soil conditions
to forms available to plants.

There is a similar relative increase in the proportion
of carbon in humus over that in the original material.
The coals are metamorphosed muck and peat deposits,
and their value for fuel lies in their carbon content.
Hilgard has shown by a series of analyses that there
is a gradual increase in the carbon content during the
decay process, at least up to the humus stage, which
is shown physically by the darkening of the material.
This darkening, which appears in peat and muck may
be the result of the separation of free carbon which, in
the amorphous form, is black. Its practical significance
in a soil way is its large effect on the color of the soil,
which alters its heat relations. Crops always start first

124 THE PRINCIPLES OF SOIL MANAGEMENT

on black soils, other things equal, and, as has been
stated, this dark color is generally due to humus.

54. Amounts present. — The amount of organic matter
present varies greatly with different soils. Peat and
muck deposits are very largely organic material, the
per cent depending on the state of decomposition.
Some porous, well-drained soils are almost lacking in
this constituent. But nearly all soils have a moderate
per cent. The accumulation is larger in the soil than in
the subsoil, and generally decreases with depth. In
237 types of soil, representing thousands of samples
from all parts of the United States, the soil was found
to contain 2.06 per cent, and the subsoil .83 per cent.
This latter refers to the upper subsoil, and at greater
depths the organic content is very much less. But in
those soils recently formed by stream action the organic
content in the third, fourth and fifth foot may be very
considerable, as is indicated by the color.

In general, arid soils contain less organic matter
than soils of humid regions ; those of cold climates
more than those of warm climates. The soils of the
northern states and Canada are very generally quite
dark colored, while those of the southern states under
similar treatment are much lighter colored, due to dif-
erence in organic content. Wet soils contain more than
dry soils, and clay soils contain more than sandy soils.

These facts are illustrated by the following figures,
showing the amount of organic matter in different soils,
which in the first six lines are the average of ten samples
representing several soil types of approximately the
same natural drainage.

AMOUNT OF ORGANIC MATTER IN SOIL

125

Table XVII

Sandy soils

Loam and clay loam
soils

Soil

Per cent

organic

matter

Subsoil

Per cent

organic

matter

Soil

Per cent

organic

matter

Subsoil

Per cent

organic

matter

Northeastern states

Southeastern states

North Central states

South Central States ....
Semi-arid states

1.66
0.93
1.84
1.16
0.99
0.89

0.60
0.41
0.76
0.55
0.62
0.64

3.73
1.53
3.06
1.80
2.64
1.05

1.35
0.73
1.07
0.65
1.11

Arid states

0.62

Illinois deep peat and muck

Miami black clay loam, average

twelve samples

Portsmouth sandy loam, average

nine samples

Wabash silt loam, average eleven

samples

Soil 0-7 inches

Per cent
organic matter

84.6
5.9
4.1
3.3

Subsoil 7-40 inches

Per cent

organic matter

55.80
2.50
0.92
1.30

The Miami black clay loam is a famous corn soil of
the North Central states, and comprises areas of glacial
clay loam, which were originally very wet and swampy,
but have been reclaimed by drainage. The Portsmouth
sandy loam occurs in the coastal plain of the southern
states, and represents a mild form of swamp soil, but
in which the accumulation of organic matter is not
sufficient to permit its classification as muck. When
well-drained, this soil is a first-class truck soil. The

126

THE PRINCIPLES OF SOIL MANAGEMENT

Wabash silt loam is the much-prized deep, dark silt
loam of the stream bottoms in the North Central states.
It is a close competitor of the Miami black clay in the

production of corn
and grass.

The proportion of
organic matter is the
chief distinction be-
tween soils and sub-
soils. It will be noted
from the table that
in all of the humid
sections this differ-
ence is very marked,
and agrees well with
the color differences
generally observed.
But in the arid re-
gions this distinction
between soil and sub-
soil is not so obvious.
The difference in or-
ganic content is even
less marked than the
figures indicate, for the reason that several of those
for the soil extend to a depth of two feet or more and
those of the subsoil extend often to six feet.

55. Some physical properties. — The physical prop-
erties of the organic constituents of the soil are different
in value from those of the mineral constituents. Though
usually present in small amounts their properties

Fig. 38. A soil of loamy texture in good tilth.

PHYSICAL PROPERTIES OF ORGANIC MATTER 127

are such as to have a large influence on its produc-
tiveness.

56. Solubility. — The organic matter may be divided
into two general classes of materials: (a) If an ordinary
soil or peat or muck be leached with water, particularly
if the water contain a little ammonia, a dark brown
or black color will be imparted to the extract. This
is due to a mixture of organic compounds which have
a colloidal or gelatinous consistency. It is the material
— to which the specific term humus is applied — which
gives the brown color to the drainage water from
swamps and to the leachings from the manure heap.
This color is an indication of the loss of the humus
constituent, and should remind one of the necessity
for precautions against the loss, as far as possible. When
the humus is united with salts like lime to form humates,
this loss is very much reduced. It follows from this
that the loss of humus, by leaching from soils rich in
lime, is very much less than in those soils poor in lime.
Many of the soils in the southern states are very low
in lime, and the streams are generally bordered by
swampy areas. As a result, the drainage water is usually
of a brown coffee-color. On the other hand, in those
northern states where the soils are rich in lime, this
brown color is much less pronounced and is usually
absent. If lime or some other flocculating agent be
added to this brown liquid, the humus separates out
in fluffy masses, which settle to the bottom, leaving
the liquid above almost colorless. This is, in part, what
takes place in the soil when these flocculating materials
are present.

128 THE PRINCIPLES OF SOIL MANAGEMENT

(b) The remainder of the organic material, after
extraction, is composed of the fresh and partially
decomposed fragments of plant and animal remains
more or less stained. It is a light chaffy material, which
by decay may be changed to humus, but in this condition
is not subject to direct loss.

57. Weight. — The organic material is the lightest
constituent in the soil. Warington gives the specific
gravity of humus as 1.2 to 1.5, as compared with 2.68
for the mineral constituents, and Hilgard reports its
volume weight when dry as .33, as compared with
about 1.1 for clay and 1.5 for sand. Therefore, in pro-
portion as a soil contains humus, it is lighter in weight.
On the basis of the above figures, a cubic foot of humus
would weigh about twenty-one pounds. Muck and peat,
however, contain mineral matter washed in with the
organic material and their volume weight is higher.
It ranges from twenty to forty-five pounds per cubic
foot, according to the stage of decay when dry.

58. Absorption properties. — In the form of humus,
organic matter has a very large absorptive power for
gases and salts in solution, similar to that shown by
powdered charcoal. It is much greater than that of
even clay soils, and for this reason its addition to soil
increases this important property.

59. Volume changes. — Like clay soil, when humus
is dried, it shrinks very greatly, and conversely, when
it is moistened it expands. In humus this property
is much more pronounced than in even the heaviest
clay. Warington reports the shrinkage of a very pure
clay, in drying from a saturated state, to be 18 per cent

EFFECTS OF ORGANIC MATTER 129

of the original volume, and that of humus to be 20 per
cent; while others report the shrinkage of muck samples
to be more than twice this amount.

60. Plasticity. — The crude organic matter exhibits no
striking peculiarities, but the humus substance has
many. One of these is its plasticity. Although very
fine, its plasticity is not great as compared with clay.
But it is sufficient to act as a weak cementing material
in soil, which is very important in binding together
light sandy soils, and in lightening up and holding apart
the aggregates or crumbs in clay soil. Thereby it greatly
promotes the granulation of clay soils which are properly
drained.

61. Effects of organic matter. — The effects of organic
matter on the soil, and thereby upon plant growth,
are so numerous and so far-reaching and generally so
beneficial, and further, its maintenance is so important
a part of good soil management that, at the risk of
anticipating some of the subsequent discussions, its
effects are here briefly summarized. They are of two
sorts, (1) Physical, and (2) Chemical.

62. Physical effects. — (a) Physically, it affects both
tilth and granulation. Owing to its weak plasticity
and its great contraction when dried, it is a very potent
factor in hastening the granulation process of clay
soils in the way that has been explained above. And
on light sandy soils which are loose and inclined to be
drifted by the wind or eroded by rains, it has the effect
of binding them together and imparts a much more
loamy character.

(b) By its beneficial effect on the structure of the

130

THE PRINCIPLES OF SOIL MANAGEMENT

soil, it very greatly increases its moisture-holding
capacity, which is further increased by the great ca-
pacity of humus itself to retain water, which amounts*
to 200 or 300 per cent of its dry weight, as compared
with 10 or 15 per cent for sandy loam and 25 to 35 per
cent for clay soils. It therefore improves the drought

Fig. 39. The solid disc harrow. Most efficient on medium heavy soil free

from stone and rubbish.

resistance of soils by increasing their reservoir for
available water.

(c) The dark color which humus imparts to soils
permits them to absorb the heat of the sun's rays very
much more than when the humus is absent, and thereby
their average temperature is decidedly raised. It is for
this reason that the corn first appears in the spring in
the low areas of dark-colored soil, which difference
in time may amount to several days.

MAINTENANCE OF ORGANIC MATTER 131

63. Chemical effects. — The chemical effects are of
two sorts: (a) Vegetable and animal remains contain
all of the essential elements of plant food, and by their
decay these are given back to the soil in a form readily
available as food for other plants. It is therefore a
direct source of food elements.

(b) The products of the decay of organic matter are
many forms of organic acids, the simplest and most
abundant of which is carbon dioxid. In the soil moisture
these act powerfully upon the mineral soil particles
to bring their elements — particularly the bases — into
solution. Because of their presence, the soil water
must be regarded as a weak solution of all of these
products and by their presence its dissolving power is
greatly increased.

64. Maintenance of organic matter. — Two conditions
are necessary to maintain an adequate amount of
organic matter in the soil. These are, first, an adequate
supply, and second, avoidance of a too -rapid loss,
together with the maintenance of those soil conditions
which promote the proper form of decay.

The organic matter derived from the higher plants
is supplemented by that from bacteria and fungi, which
are generally abundant in the soil. Much may be accom-
plished by good soil management to favor the develop-
ment of the lower forms, so that they may be a very
important source of humus. In fact, it has been sug-
gested that they may sometimes be the chief source
of supply.

Any plant may be used as a green manure, to furnish
organic matter to the soil. Plants which have been

132 THE PRINCIPLES OF SOIL MANAGEMENT

much used for this purpose are the clovers, vetch,
field-peas, cowpeas, soy beans, rye, and buckwheat.
When any of these crops are planted in the late summer
to conserve plant food, they are termed "catch crops,"
and when used to cover the ground and protect it from
erosion, they are termed "cover crops." Many forms
of organic manures and waste materials are applied
as a source of humus.

Good tillage and the proper rotation of crops greatly
assist the accumulation of organic matter in the soil,
and to these may sometimes be added amendments
such as lime. Some of the conditions which favor the
accumulation of organic matter in the soil are: (1) The
presence of an excess of water. (2) Low temperature.
(3) Limited aeration. (4) Deficiency of basic elements.
(5) Absence of decay organisms. (6) Application of
organic manures. (7) Accumulation of plant residues
in the soil. (8) Proper rotation of crops. (9) Absence
of tillage.

Some of the conditions which favor the rapid dis-
appearance of humus from the soil are: (1) The presence
of a moderate amount of water. (2) Thorough aeration.
(3) High temperature,— from 75° to 110° Fahr. (4)
Abundance of available basic elements. (5) Abundance
of decay organisms. (6) Failure to maintain the supply
of organic matter. (7) Complete removal of all crops.
(8) Improper crop rotation. (9) Excessive tillage.

Good management seeks to adjust these two sets of
conditions, so that large crops are produced without
imparing the humus supply in the soil.

B. THE SOIL AS A RESERVOIR FOR WATER

I. FUNCTIONS IN PLANT GROWTH

When plants grow, they use water. It circulates
through their vessels, is built into their tissues, and is
evaporated by the leaves. In these capacities it per-
forms three important and vital functions for the plant.
It is (a) a direct food of the plant, and becomes a part
of its tissues either directly as water, or it is broken
up and its elements are used in new compounds, (b)
It is a carrier of food to the plant, and serves as the
medium of transfer for the mineral elements from the
soil and the gaseous elements from the air to their
appropriate points of assimilation and use in the growth
of the plant mechanism, (c) In addition to the last two
functions, water serves as a regulator of the physical
condition of the plant. It equalizes the temperature
of the plant and modifies its stability.

From 60 to more than 95 per cent of the green weight
of the staple crops is due to water.

In the ordinary processes of growth, the amount of
water transpired is many times greater than that used
directly as food. Investigations in different parts of
the world have shown that for the production of each
pound of dry matter ordinary crops transpire from
200 to 500 pounds of water.

Warington has compiled the following figures, show-
ing the amount of water used by different crops in the
production of organic matter.

(133)

134 THE PRINCIPLES OF SOIL MANAGEMENT

Table XVIII. — Water Evaporated by Growing Plants for
One Part of Dry Matter Produced

Lawes and Gilbert
England

Hellriegel
Germany

Wollny
Germany

King
Wisconsin

Beans

Wheat

Peas

Red clover. .
Barley

214
225
235

249
262

Beans

Wheat

Peas

Red clover .

Barley

Oats

Buckwheat .
Lupine ....

Rye

262
359
292
330
310
402
371
373
377

Maize

Millet

Peas

Rape

Barley

Oats

Buckwheat .
Mustard . . .
Sunflower . .

233
416
479
912

774
665
664
843
490

Maize

Potatoes . . .

Peas

Red clover .

Barley

Oats

272
423
447
453
393
557

The variation exhibited by the figures for the crop,
as well as for different crops, illustrates the influence
of climate and soil upon transpiration. Other things
equal, more water will be required in an arid region
than in one of humid climate; more in a warm region
than in a cold region; more on a clay soil than on a
sandy soil; more in a windy section than in a region
of still atmosphere; more with a high soil moisture
content; more on a poor soil; and, finally, more water
is used per pound of dry matter produced in a small
crop than is required in a large crop. All of these figures
agree in indicating the large amount of water used
in the production of crops. Not only is the total seasonal
requirement to be considered, but the maximum de-
mands of the crop at any period of its growth must
be met. King observed that a single corn plant during
the first week of August, when it was coming into tassel
and the ear was forming, used water at the rate of 1,320

TT.-17\E7e REQUIRED BY CROPS

135

grams (one and one-half quarts) per day. Hunt observed
in Illinois that in one week in July the growth of corn
amounted to 1,300 pounds of dry matter per acre. As-
suming the requirement observed in Wisconsin, — 272
pounds per pound of dry matter, — this is equivalent
to 1.55 inches of water.

Assuming the average production of dry matter
to be two tons per acre,, the amount of water required

jz*

Fig. 40. Solid, metal roller. The prevailing type of compacter.

to produce such a yield of the staple crops, under the
best conditions of management, would amount, accord-
ing to the above figures, to from 427 tons to 1,820 tons
of water per acre, which is equivalent to a rainfall of
3.7 and 15 inches, respectively.

II. AMOUNT OF WATER IN THE SOIL

Soils exhibit great differences in moisture content
and in their ability to meet the needs of the plants for
water. In some of the southeastern states, where the

136 THE PRINCIPLES OF SOIL MANAGEMENT

rainfall is from fifty to sixty inches, crops suffer more
from a lack of moisture than they do in some of the
states of the northern Mississippi valley, with only
a third of the rainfall. The light truck soils of the At-
lantic coast suffer much more from a lack of water
than do the interior soils of heavy texture which are
under the same rainfall and general temperature con-
ditions. Plants in a dry greenhouse use more water than
in the more moist outside air. These illustrations serve
to emphasize the three factors which determine the
amount of moisture a soil contains. These are (a) the
available supply of water; (b) the retentive capacity
of the soil for water; (c) the rate and amount of loss of
water from the soil. Each of these factors depends
on many conditions.

65. The supply. — The supply of water is obviously
controlled by conditions external to the soil. These are
the precipitation in the forms of rain and snow, under-
ground seepage, and irrigation.

66. Retentive capacity of the soil. — The retentive
capacity of the soil varies greatly according to its
physical properties, As soils ordinarily occur in the field,
they show the presence of moisture. This moisture
is held quite intimately. Two soils may appear equally
moist, yet have very different capacities to maintain
crops. Plants suffer much more quickly from dry
weather on sand soil than on clay soil, even when the
soils appear equally wet at the outset.

67. Statement of water content. — Five different
methods are commonly used in stating the moisture
content of soils. These are: (1) In terms of per cent

-

"3

o

138 THE PRINCIPLES OF SOIL MANAGEMENT

based on the dry weight of the soil. (2) In terms of
per cent based on the wet weight of the soil. (3) In
terms of the per cent of volume based on the total
volume occupied by the soil. (4) In cubic inches per
cubic foot, or in cubic centimeters per liter or per cubic
meter. (5) In inches in depth of water over the surface
of the soil.

Of these methods the first is most largely used,
because it gives the most definite and constant basis
from which to derive any other quantities. The dry
weight of a soil remains constant, and percentages
referred to that base are always comparable. But it
has several disadvantages which lead to inconsistent
results in practical work. For example, 10 per cent of
water in a cubic foot of clay soil represents a very
different quantity of water from the same percentage
in a sand or a muck soil, because of the very different
volume weights of these materials. In the clay it would
mean about 7 pounds, or 3.5 liters; in the sand soil
10 pounds or 4.5 liters; and in the muck soil 3.5 pounds,
or 1.6 liters, — manifestly very different quantities of
water. Or, to state the matter in a different way, 30
per cent of water in a clay, 12 per cent in sand, and
150 per cent in muck, do not represent as different
volumes of water as is indicated by the figures, because
of the relative weights of the soils. But, because almost
any other figure can be readily derived from the moisture
percentage expressed in terms of dry weight of soil,
it has been very generally used, especially in laboratory
studies. In field practice, a volume method is more
convenient.

STATEMENT OF SOIL MOISTURE CONTENT 139

The second method — -that based on the wet weight
of the soil — is unsatisfactory, because it is not only-
open to the objections made to the first method, but
also because figures on moisture content of the same
sample of soil are not comparable. They do not repre-
sent the same degree of wetness indicated by the per-
centages. For example, 100 grams of wet clay contain-
ing 10 per cent of water would consist of 10 grams of
water in 90 grams of soil, and 100 grams of wet clay
containing 20 per cent would consist of 20 grams of
water in 80 grams of soil. In the first case, the ratio
of water to soil is as 1 to 9; while, in the second, case
the ratio is 1 to 4, instead of 1 to 4.5, as the percentage
comparison would indicate. The difficulty in deriving
other figures from percentages based on wet weight
makes its use undesirable.

The third method, statement of percentage of water
by volume, is the most rational of the first three. It
gives a direct practical basis of comparison for all soils.
It shows the volume of water held by the soil, which
is really the important consideration from the point
of view of the plant. For purposes of comparing the
moisture content of different soils in the field, it is
probably the most satisfactory method. Derivation
of these quantities involves considerable calculation,
and often the determination of some quantities not
readily obtainable.

The fourth method of statement is really a variation
in detail from the third method by which specific quan-
titive statements are made. One hundred seventy-two
and ei.°;ht-tenths cubic inches of water in one cubic

140 THE PRINCIPLES OF SOIL MANAGEMENT

foot of soil, is a cumbersome method of saying the soil
contains 10 per cent of water by volume.

The fifth method is most generally used in field prac-
tice in stating quantities of water. In irrigation practice,
water is often measured in inches in depth per acre of
area. In stating the quantity of water held within root
range by different soils, this method is also direct and
convenient. For example, a sand soil of a certain tex-

Fig. 42. A common type of spike-tooth, iron-framed harrow. It operates
as a shallow cultivator, and may often be very effective in mulching the soil
and conserving moisture.

ture will hold in the four feet surface 9 acre-inches
of water; clay soil, 16; and a muck soil, 40 inches; which
figures are directly comparable for purposes of crop-
production.

The method used in stating the moisture content
of a soil will therefore depend upon the line of investiga-
tion and the application of the results to be made. Both
the percentage of dry weight and the percentage of
volume will be used in this book, according to the
point of view of the discussion.

FORMS OF SOIL MOISTURE

141

68. Forms and availability. — There are three forms
in which water may exist in soils: (1) Gravitational
water, or that which is free to move through the soil
under the influence of gravity. (2) Capillary or film
water, or that which is held against gravity by the
surface tension of the films of water surrounding the
soil particles. (3) Hygroscopic moisture, or that which
condenses from the atmosphere on the surface of the

HYGROSCOPIC

FORMS OF SOIL WATER

CAPILLARY

GRAVITATIONAL

UNAVAILABLE AVAILABLE INJURIOUS

AVAILABILITY OF' SOIL WATER TO PLANTS

Fig. 43. Diagram illustrating the forms, proportions and

availibility of soil water.

soil particles, when the soil is allowed to become air
dry.

There is no sharp change in the moisture condition
of the soil in passing from one form to the other. Still,
it is true that there are certain marked changes in some
of the physical properties of the soil, such as volume,
weight and resistance to penetration, which are in a
general way associated with these transition points.

Not all of the water in the soil is available to use of
plants. It is a matter of general experience that for
most farm crops the saturated condition of the soil
is unfavorable to the best development. There are,
of course, many plants which are adapted to such con-

142 THE PRINCIPLES OF SOIL MANAGEMENT

ditions, as for example the swamp type of vegetation.
About the only cultivated crops of this sort are rice
and cranberries. Practically all of the common culti-
vated crops, from vegetables to fruit trees, are adapted
to growing in soil from which the gravitational moisture
has been removed. The gravitational water is directly
injurious to the growth of these plants, and its practical
removal from the soil constitutes the practice of agri-
cultural drainage, later to be considered as a phase
of soil management, It may therefore be stated that
gravitational water in the root zone is injurious to most
farm crops, and consequently it is in a sense unavailable.
It is the film or capillary moisture which supports plants.
The roots of ordinary crops are adapted to take the
moisture needed by threading their way between the
soil particles, where they may come in intimate contact
with these moisture films and absorb the needed supply
of water, without being excluded from the air supply
which promotes their growth. For, in the capillarily
moist soil, the water is retained chiefly in the very
small spaces, and the large spaces are occupied by air.
While capillary moisture is practically the only form
upon which plants depend, it is not possible for them
to use all of this form of moisture in the soil. They take
their supply most readily when the films are relatively
thick, and when the globules between the particles
are large. But, as the thickness of the films is reduced
by the use of the plant and by evaporation, it becomes
increasingly difficult for the plant roots to take their
needed supply. Before all of the capillary moisture has
been removed, this difficulty becomes so great that it

HYGROSCOPIC MOISTURE IN SOIL 143

practically amounts to the prohibition of further extrac-
tion by the plant. At this stage, if evaporation from
the leaves continues, the plants wilt, because they are
not supplied with moisture by the roots as rapidly as
it is being lost.

Since plants cannot utilize all of the capillary moisture
it is manifestly impossible for them to derive any benefit
from the hygroscopic moisture, which is held much
more intimately by the soil particles than is the capil-
lary moisture. In other words, the hygroscopic moisture
capacity of a soil represents that much water unavail-
able to plants, to which must be added the proportion
of the capillary moisture which is also unavailable.

69. Amounts of each form. — The relative amount
of each form of water varies with the soil, and is deter-
mined by its physical properties. The forms of water
merge one into the other.

70. Hygroscopic water. — The amount of each of
the three forms of soil water depends on the physical
properties of the soil. These are best explained by first
considering the hygroscopic capacity. This depends
on the texture of the particles and the content of organic
matter. Since hygroscopic moisture is a function of
the surface exposed, it results that the larger the surface
area exposed by the soil particles, the greater the hygro-
scopic capacity of the soil. Reference to the table
on page 83 shows fine-textured or clay soils to have the
greatest surface area, and these hold the most hygro-
scopic moisture. Sand soils, with a relatively small
surface area, hold a small amount of this form of water.
This fact is illustrated by the following table.

144 THE PRINCIPLES OF SOIL MANAGEMENT

Per cent of hygroscopic
water at 21° C.

Very fine sand 1.8

Silt 7.3

Clay 16.5

Muck 48.0

The above soils were pure separates derived by
mechanical analysis. These figures serve to show the
direct relation between the (1) surface area exhibited
by soil particles and the hygroscopic moisture retained.
The hygroscopic moisture content of a soil depends
also on the (2) temperature, and the (3) humidity
of the atmosphere. The hygroscopic moisture decreases
with increase in temperature. It varies directly as
the relative humidity of the atmosphere with which
the soil is in contact. Consequently, in the air-dried
condition, while a soil always retains some moisture,
it seldom exhibits its maximum hygroscopic capacity.
Under average conditions of humidity, a light sand
may retain from .5 to 1 per cent, a silt loam from 2
to 4 per cent and a clay from 8 to 12 per cent. This
is, of course, unavailable for the use of plants.

71. Capillary water. — The capillary water capacity
is much larger than the hygroscopic capacity. Its
amount is determined by three things: (1) Texture,
(2) structure; (3) content of organic matter.

72. Texture. — Texture is well known to be the great-
est determining factor in the water-holding capacity
of soils, due to its control of the internal surface,
and this is particularly true with reference to the
capillary form. The following table illustrates this
effect of texture.

CAPILLARY MOISTURE IN SOIL
Table XIX

145

Class

1. Coarse sand

2. Medium sandy loam

3. Fine sandy loam. . .

4. Silt

5. Silt loam

6. Clay loam

7. Clay

Per cent of clay

4.8
7.3
12.6
10.6
17.7
26.6
59.8

Per cent of
moisture retained

against force

2,940 times that

of gravity

4.6
7.0
11.8
12.9
26.9
32.4
46.5

RELATION TEXTURE TO CAPUU'RY

WATER CAPACITY

1

2

3

FJNE GRAVEL

FINE AND

SILT

COARSE AND

VERY FINE SAND

MEDIUM SAND

SOIL SEPARATES

4
CLAY

Fig. 44. Showing the mechanical composition of the soils whose relative
capillary water capacity is given in Table XIX.

No. 1. Coarse Sand.
No. 2. Medium Sandy Loam.
No. 3. Fine Sandy Loam.
No. 4. Silt.

No. 5. Silt Loam.
No. 6. Clay Loam.
No. 7. Clay

146 THE PRINCIPLES OF SOIL MANAGEMENT

It must be remembered that the hygroscopic ca-
pacity of these soils also increases with their fineness,
and that the strictly capillary moisture is represented
by the difference between the total moisture content
given above and the hygroscopic moisture.

The above figures are the most exact available
which show the influence of texture upon moisture
retention. But, while they show the relative effect of
texture, they do not indicate the amount of water
retained by field soils; because these samples have
been subject to a force almost 3,000 times that of
gravity. When under the influence of gravity alone,
these same soils will retain much more water than is
indicated by the figures. However, this influence of
gravity introduces a modification in the moisture content
of the soil which must be constantly kept in mind.
Moisture is retained in the soil as a result of two sets
of forces. These are, first, the attraction of the soil
for water, or adhesion. For example, if a marble is
dipped into water and withdrawn, it carries with it
a film of water over its entire surface. This shows that
for a certain small distance from its surface, the marble
exerts a stronger pull on the water than the water
exerts for itself. If the marble were dipped into mercury
instead of water, it would come out with a dry surface,
because in this case the attraction of the mercury for its
own substance is greater than the attraction of the
marble for the mercury. Quinke estimates the appreci-
able range of this attraction to be approximately .002
millimeter, which, it will be remembered, is equivalent
to the diameter of a medium-sized clay particle. Its

RETENTION OF SOIL MOISTURE

147

tendency is to arrange upon the surface of the soil
particles a film of water molecules equivalent to this
thickness.

But, because of the second set of forces the film is
always thicker than this range of molecular attraction
of the solid. This is due to the attrac-
tion of the water particles for each
other, or cohesion. The water mole-
cules hang together. This cohesion of
the water molecules is exhibited in
surface tension which will permit a
clean steel needle to be suspended
upon the surface of water, or makes
possible the common trick of putting
a handful of nails into a goblet already
level full of water. This surface ten-
sion acts like a stretched elastic mem-
brane, and permits the water to be
piled up. This is what happens in
the soil when capillarity comes into distribution of water

1 J on columns or spnen-

play. As a result of these two sets ca' particles of differ-

r J ent texture. Note the

of attraction, the water hangs on the accumulation of water

in the lower part, also,

particles in thick films; and it drops the approximately

r r equal curvature of water

away only when the weight of the surfaces at each level,
water becomes greater than the surface tension of the
liquid.

It is clear that soil forms a column of considerable
height, and further, that the closer the water film is
drawn around the soil particles, the thinner it will be,
and consequently the less water it will contain. To
illustrate: Suppose a cylinder to have flexible rubber

Fig. 45. Showing the

148

THE PRINCIPLES OF SOIL MANAGEMENT

diaphragms stretched across at frequent intervals from
the top to the bottom. If now a heavy ball is dropped
upon the upper membrane, it will be weighed down
upon the next membrane below, and this in turn will
be depressed, until the ball has brought enough of the
membranes in contact to support its weight. Under these
conditions, the upper membrane will be stretched most
severely, and will therefore be thin, while the lower

ui

40

"J 30

20

<
a

UJ

x

\

I

V

V*.

\

"'^.

%

\

\i A,

\

X

J

-^6

10

30

35

in

15 20 25

PERCENT. OF WATER

Fig. 46. Curves showing the distribution of water in columns of soil
^apillarily saturated, as given in Table XX.

membrane will be very slightly stretched. If, now,
we calculate the actual amount of rubber in each section
of the cylinder it will be found smallest at the top
and largest at the bottom.

In the same way, gravity affects the distribution
of water in the soil. It forms thick, bulging films in
the lower part of the column, and thin, closely drawn
films at the top of the column. Consequently, the sur-
face of a soil of uniform texture is normally less moist
than the subsoil.

DISTRIBUTION OF SOIL MOISTURE

149

As a result of this fact, it is not practicable to say
that any soil contains a definite uniform per cent of
capillary moisture. The content varies with the height
of the column and the plane in the column at which
the determination may be made. This important
principle in the distribution and amount of moisture
in the soil is well illustrated by the following tables
and curves, for soils of different texture, as obtained
by Buckingham:

Table XX

Per cent of water at different distances from
bottom of column in inches

2

10

20

30

40

50

1. Clean dune sand

2. Coarse sand

27.0
23.0

28.5
29.0
35.0

64.0

23
14
25
23
23

55

7
10
16
21

18

47

3.5

7.5

9.5

19.0

15.0

36.0

3

5

7

17

11

20

3. Fine sandy loam

4. Light silt loam

5. Clay

6. Heavy loam, rich in

humus

The above moisture curves illustrate very clearly the
accumulation of the water in the lower part of the soil
column. These columns were permitted to stand in
contact with water for many days, so that, with the
possible exception of the finest textured soils, they had
come to equilibrium. It will be noted that the difference
in moisture content is much greater at the top of the
columns than at the bottom, and decidedly greater
than at a height of about ten inches above the water.

150

THE PRINCIPLES OF SOIL MANAGEMENT

When two soils of different texture are placed in
contact with moisture free to move from one to the
other, they come into moisture equilibrium after a time,
and each holds a certain proportion of the water. The
curvature of the water surfaces between the particles
80r

NO. 2 FINE SAND

VERY FINE SAND

NO. 3 SILT

NO. 4 CLAY

NO.1 FINE GRAVEL
COARSE SAND

MEDIUM SAND

SOIL SEPARATES
Fig. 47. Curves showing the mechanical composition of the soils whose
capillary moisture capacity is shown in Table XX and Fig. 46.

of the two soils is the same. But since in a given volume
of soil the fine texture has so many more of these indi-
vidual drops of water, its total content is greater than
that of the coarse-textured soil. This matter of the
curvature of the water surfaces in the soil will conie up

STRUCTURE AND SOIL MOISTURE 151

prominently in considering the capillary movement
of moisture. The relative adjustment and distribution
of the moisture between small and large particles in
contact is illustrated in Fig. 45. When in capillary
equilibrium, two soils should appear equally moist.

73. Structure. — Structure is the second factor which
determines the moisture capacity of a soil. If the state-
ments in reference to the effect of texture have been
fully understood, the influence of structure will be
readily grasped. The effect of structure is to alter the
effective size of the soil units or granules, and also of
the spaces which they form. In a coarse sand soil,
the general effect of rendering the structure of the soil
more loose is to proportionately reduce its water-holding
capacity, because the spaces are already so large as to
hold a relatively small amount of water, and that to
a very limited height. Change in structure further de-
creases that already deficient capacity. On the other
hand, in a fine clay soil the spaces are all very small,
and all have a capillary efficiency to a great height. This
height is much more than is ordinarily needed to bring
the moisture from the deep subsoil to the root zone.
In such a soil a more loose and open structure has the
effect of increasing the effective moisture capacity,
so long as the spaces are still able to hold water at the
surface of the column. But when this maximum size
of space is exceeded, as in a coarsely clodded soil, the
moisture capacity drops low, as in the case of sand or
gravel, when growth may be seriously interrupted.
Ordinarily, then, it may be said, that loosening the
structure of a coarse sand or gravel soil lowers its

152

THE PRINCIPLES OF SOIL MANAGEMENT

moisture-holding capacity while a reasonable granulation
of a clay soil increases its moisture-retaining capacity.

This effect of structure on the moisture capacity
of two soils is illustrated by the following curves, based
upon the results of Buckingham. The mechanical
analysis of these soils may be found in curves already
given. (See page 150.) The sandy loam is No. 3, and the
clay is No. 5.

50

10

--30

5

>. 20

<
olO

0 10 20 30 40 50

Fig. 48. Curves showing the distribution of water in columns of sand and
clay when loose and compact and capillarily saturated. Figures given in
Table XXI.

Table XXI. — Per Cent of Water in Sand and Clay, Loose

and Compact

i

0

1° -9

%

Vi \*

*n \ o

1

I o

V « o
X \ ^

\ \\

Itn

x^v

\

X

Soil

Structure

Dry
porosity
per cent

Per cent of moisture at different heights
above water level

2 in.

10 in.

20 in.

30 in.

40 in.

Sandy loam. .
Clay

Loose

Compact . .

Loose

Compact . .

50
35
59
52

28.0
27.0
42.5
34.0

25
25
32
23

16.0
17.5
28.0
18.0

9.0
12.5
27.0
15.0

6
10

26
12

ORGANIC MATTER AND SOIL MOISTURE 153

74. Content of organic matter. — Organic matter,
especially in the form of humus, has a larger capacity
for moisture than has the mineral portion of the soil.
Aside from the fact that such material has a large inher-
ent moisture capacity, and that in proportion to its
amount in the soil it increases the water capacity,
no exact figures can be given. The moisture content
of such material varies with the stage of decay, as well
as the general physical properties of the material. The
following figures compiled by Storer illustrate this
capacity.

Per cent of
water retained

1. Humic acid extract from peat 1,200

2. Non-acid humus prepared from peat 645

3. Ordinary vegetable mold 190

4. Peat \ 201-309

5. Garden loam, 54 per cent clay, 7 per cent humus. 96

6. Dark Illinois prairie soil 57

7. Mucky soil (weighing 30 pounds per cubic foot) . 75

Besides its inherent capacity, organic matter affects
the moisture capacity through its influence on soil
structure. In clay it produces a desirable condition
of granulation and therefore increases the absolute
moisture capacity. And its addition to sand has. a similar,
though smaller effect. This is illustrated by the follow-
ing figures, obtained by Detmer, as quoted by Storer,
which resulted from the mixture of sand and muck.
It will be noted that in proportion as muck is substi-
tuted for an equal weight of sand, the water capacity
of the mixture is increased, as is well shown by the
ratio in the last column.

154 THE PRINCIPLES OF SOIL MANAGEMENT

Table XXII

Grams of water
absorbed

Ratio of absorption

Per cent of sand

Per cent of muck

of water in sand
and in mixture

100

12.2

1 :1

80

20

24.0

1 :2

60

40

42.0

1 :3.5

40

60

71.7

1 :6.0

20

80

99.1

1 :8.0

100

114.4

1 :9.3

75. Volume of water held by different soils. — The
columns of soil from which the figures presented on
page 148, with the accompanying curves, were obtained,
were forty-five inches in height, with their lower ends
dipping in water. As they were run for several months,
their moisture content represents the maximum capacity
for each soil. Under these conditions, the mean moisture
content was as follows:

Table XXIII

1. Dune sand

2. Coarse sand. . . .

3. Fine sandy loam

4. Light silt loam .

5. Clay

6. Muck soil

Dry
poros-
ity .
Per
cent

52
51
50
50
59
80*

II

Final mean

water content

Per cent

10.7
10.6
18.0
20.9
30.4
250.0

III

Approximate

per cent of

moisture at

which crops

will wilt

3

3

5

10

17

80

IV

Per cent of
available
moisture

7.7

7.6

13.0

10.9

13.4

170.0

♦Estimated.

WATER CAPACITY OF SOILS

155

Table XXIII, continued

V

Weight of

dry soil per

cubic foot

VI

Volume of available
water per
cubic foot

VII

Inches of avail

able water

to depth of

four feet

1. Dune sand

2. Coarse sand

3. Fine sandy loam .

4. Light silt loam . .

5. Clay

Lbs.
80
81
83
83
68
15

cu. in.
166
170
300
250
252
740

c.c.
2,720
2,790
4,900
4,100
4,140
11,550

4.60
5.20
8.50
6.90
7.03

6. Muck soil

20.50

But all of this moisture is not available to crops.
The third column gives the per cent of water in these
soils which would be unavailable, or the point at which
plants would ordinarily wilt. This per cent, or amount
of water at which plants are just able to survive, is
termed the minimum or critical moisture content, while
the highest per cent at which the plant will survive
is termed the maximum moisture content. The inter-
mediate point at which any crop makes its best growth
is termed the optimum moisture content.

Each of these points, or moisture conditions, is very
definite for each soil and for each crop. The minimum
for different crops on the same soil is not the same as
the results of a number of investigators have shown.
Storer reports that, on a calcareous soil having a hygro-
scopic capacity of 5.2 per cent, the minimum for grasses
was 9.85 per cent, and for legumes 10.95 per cent; while,
on peat (muck) with a hygroscopicity of 42.3 per cent,
the grasses suffered at 50.87 per cent of moisture,

156

THE PRINCIPLES OF SOIL MANAGEMENT

legumes at 52.87 per cent of moisture. Warington
concludes, from the results of Hellriegel and Wollny,
that "when the soil contains 80 per cent of the water
required to saturate it, the proportion was too high;
and that when the water amounted to only 30 per cent
of saturation,, the proportion was too low for the pro-
duction of a maximum crop. The largest crops were
obtained when the proportion of water lay between
40 and 60 per cent of that required for full saturation."

1

2

3

4

5

6

7

8

9

10

ii

12

13

14

15

16

17

18

19

20

21

22

23

24

25

2u

27

28

29

30

14

Z-

13

tn

12

or

11

10

r

"S

CE

9

1

\

/*

h

8

s

,*r

Lir

IE

OF

FX

CE

"*l

Of\

MO

1ST

URE

n

7

<T"/

S

C

vi

<■

u.

o

5

■n'Pat,

F/V

4

/

p-

►

)

3

LINE

OF

DROU

TH

o

2

LU

1

""

0

i

^ t~ CO

^ o i

<= ° RAINFALL IN INCHES ^ d ° °

Fig. 49. Curve showing moisture content of a light sandy loam — early-
truck soil, Union Springs, Alabama, June, 1896.

Cameron and Gallagher have shown that the maxi-
mum and minimum points are marked by distinct
changes in: (1) The cohesion of the soil. (2) Its volume
weight. (3) The freedom with which the soil gives up
moisture. The first of these facts is of especial import-
ance in the tillage of soil. Between the maximum and
the minimum points the soil "works" at its best. It
does not puddle, and it is sufficiently moist to give that
desirable state of granulation which is expressed by
good tilth. The clods of the clay soil are not hard, and

WATER CONTENT OF FIELD SOILS

157

therefore pulverizing operations attain their maximum
efficiency with the minimum of work. (See page 103.)
A soil always tilled in this condition should never get
into bad tilth.

29

30

si

i

-

:;

i

5

G

7

s

9 1 10

11

1- 1 ■ i 1 J L

17 J 18

19

■M

-1

-

26

SS

25

\

ct

24

s

1

23

N

l

22

/

21

»fO

Si

c

rM^**^

20

^

V

/

19

^

y

18

1?

16

15

LINE OF DROUTH

14

IS

12

11

10

9

.33 0.09

.02

.01

.24 1.49

.01 .23
RAINFALL IN INCHES

Fig. 50. Curve showing moisture content of silt loam — blue-grass soil,
Lexington, Kentucky, September, 1896.

76. Available water in some field soils. — The actual
moisture content and fluctuations through a part of the
growing season for different soils is always of prime
concern to the farmer. The following curves, (Figs.

]

2

a

4

5

0

7

B

AVERAGE MOISTURE CONTENT
9 10 11 12 13 14 15 16 17 18

TO

19

DEPTH OF 1 FOOT

20 21 22 23 24 25 26 27

28 29 30 31

1-29

L.5"8

° 3 27

h£26

£iu2S

L~~

«*24

ui *- 23

ii.^'flr

t'.T •_ OUTM LINE

5 22
* 21

| /

■

RAINFALL IN INCHES

.54-1.42

Fig. 51.

Curve showing moisture content of clay soil — black cretaceous
prairie — Macon, Mississippi, July, 1896.

49, 50 and 51) based upon the results of Whitney and
Hosmer, illustrate these fluctuations.

If we assume for the above soils a porosity of 47, 52,
and 65 per cent, respectively, their weights per cubic

158 THE PRINCIPLES OF SOIL MANAGEMENT

foot would be 88, 79 and 58 pounds each. Using the
maximum and minimum moisture contents indicated
by the above curves, the available moisture retained
by each of the soils is as follows:

Table XXIV

Water capacity

Amount of available water

Minimum
Per cent

Maximum
Per cent

Per cent

Cu. in. per
cu. ft.

In. per
acre, 4 ft.

Light sandy

loam

Silt loam

Clay

3
15

23

8
25
40*

5
10
17

122

218
274

3.4
6.0

7.6

It is possible that the maximum assumed for the clay
is too high, in which event the available moisture in
the fifth column for this soil is also too high; but field
experience indicates that it is reasonable.

By reference to page 134, giving the amount of water
required to produce a crop, it will be observed that the
surface four feet of the sand soil will not retain enough
water for the medium crop yield, and that the clay
soil contains less than half enough water for a large
yield of many crops under the best management. This
necessitates the replenishment of the supply by rainfall,
irrigation, or movement up from the subsoil, after the
best tillage practice has been employed to prevent
unnecessary loss by evaporation.

77. Relation of surface tension to capillarity. — In
addition to the three factors mentioned as controlling

♦Assumed.

SURFACE TENSION AND CAPILLARITY 159

the capillary moisture capacity of a soil, one other is to
be considered. The surface tension, or cohesiveness,
of the moisture was described as one of the forces which
acts in conjunction with the texture, structure and or-
ganic content, to retain water. The surface tension
of any liquid is not a constant quantity, and the soil
water is no exception to this rule. Anything which in-
creases surface tension increases moisture retention,
and likewise anything which decreases surface tension
decreases the moisture retention. Soil moisture is sub-
ject to considerable variation in surface tension. Two
things are most active to change this tension, or co-
hesiveness. These are: (1) Materials in solution in
the water. Lime and many other salts increase the
tension, some substances decrease it below the normal
for pure water. (2) Changes in temperature alter the
surface tension.

Whitney and others have determined the surface
tension of a number of salt and soil solutions, some
of which are given in the table on the following page.
The concentrations are not uniform.

The figures show that many salts increase the surface
tension of the soil moisture above that for pure water,
and that certain other substances decrease the surface
tension. Among the latter are some of the most common
constituents of manures which greatly decrease surface
tension. All oily or fatty substances reduce the tension,
and, since both these latter are present in nearly all
soils, the average surface tension of the soil moisture
is less than that of pure water. Consequently, the
tendency is to retain less of such a solution than of pure

160

THE PRINCIPLES OF SOIL MANAGEMENT

Table XXV

Solution

Specific
gravity

Surface tension
dynes per sq. cm.

Water

Common salt (NaCI). . .

1.0000
1.1000
1.1000
1.1000
1.1000
1.1000
1.1000
1.0830
1.0038
1.0012
1.0013
1.0020
0.9600
1.0260
1.0013
1.0000
1.0000
1.0000

73.9
77.6

Muriate potash (KC1)

77.5

Ammonium sulfate ( (NH4)2 S04) .

Sodium sulfate (Na2S04)

Sodium nitrate (Na N03)

Potassium hydrate (KOH)

Potassium sulfate (K2S04)

Wood ashes

76.8
75.8
75.8
75.1
75.1
75.2

Thomas slag

Marl

Lime

Ammonia (NH4OH)

77.4
77.0
75.5
67.5

Urine

Stable manure

Kentucky Blue Grass soil

Wheat soil

64.9
73.2
71.0
69.6

Garden soil

69.4

water. Various salts in solution as fertilizers or otherwise,
tend to overcome this weakness, and therefore to in-
crease the moisture capacity.

Increase in temperature decreases the surface ten-
sion, until near the boiling point it is almost nil. Briggs
reports that at 0°C. the tension of pure water is 75.6
dynes per square centimeter, and at 25°C. it is 72.1.
At the lower temperature more water is held in the
soil, and this is one reason why soils appear more moist
in cool seasons. (See also page 183.)

78. Gravitational water. — By reference to the ori-
ginal illustration (page 141), it will be noted that the
gravitational water was defined as that portion in excess

GRAVITATIONAL WATER

161

of the hygroscopic and capillary capacity of a soil.
It is not retained by the same forces, and is, therefore,
free to move under the influence of gravity, in so far
as the condition and character of the soil will permit.
The amount of gravitational water depends on the total
pore space of the soil on the one hand, and on the total
hygroscopic and capillary capacities on the other hand.
It is the difference between the total capacity of the
soil for water and that held in the other two forms.
It is measured by that amount which will flow from a
soil having all of its pores filled with water.

The maximum water capacity of a soil refers to the
total amount of water which can be put in a given
volume of soil. It is therefore determined directly
by the total pore space of the soil. The pore space may
range from 35 per cent in a clean sand to GO or 70 per
cent in a well-granulated clay, and to 80 or 90 per cent
in a muck soil. If we assume the weight per cubic foot
for these materials given on page 155, the maximum
per cent of water is as follows:

Table XXVI

Dune sand

Coarse sand

Fine sandy loam
Light silt loam. .

Clay

Humus

Weight

per cu. ft.

Pounds

80
81
83
83
68
15

II

Per cent
pore space

52
51
50
50
59
80

III

Pounds of

water
per cu. ft.

32.5
32.0
31.5
31.5
37.0
50.0

IV

Per cejit

of water

in soil at

saturation

40.5
39.5
38.0
38.0
54.5
333.0

162

THE PRINCIPLES OF SOIL MANAGEMENT

One effect of moisture on porosity is to be noted
here. When a dry soil imbibes water it expands, so that,
when the porosity is determined in a wet soil, it is always
found to be larger than in the same soil when dry.
This expansion is greatest in the fine-textured soil,
and in muck it is at the maximum. There are several
factors which enter into this result, one of which is
the tendency of the soil moisture to float the particles,
so that they rest together with less force than when
the soil is dry. Gallagher has shown that a sample of
muck soil having a hygroscopic capacity of above 40
per cent lost 29.2 per cent of its original volume in dry-
ing from a moisture content of 210 per cent to one
of about 80 per cent.

In Table XXIII on page 154 is given the maximum
capacity and approximate wilting point of the soils

Table XXVII

I

II

III

IV

V

VI

'n m

5VS

u

"3

o Z

ional
e in

r cubic

capil-
ure to
ter
ty

■

3 *

si

^ 3

■2.2

o

Gravitat

moistur

pounds pe

foot

Ratio of
lary moist
total wa
capaci

Per cent

Per cent

Per cent

1. Dune sand

80

10.7

40.5

29.8

23.8

1

3.8

2. Coarse sand

81

10.6

39.5

28.9

23.4

1

3.7

3. Fine sandy loam . .

83

18.0

38.0

20.0

16.6

1

2.1

4. Silt loam

83

20.9

38.0

18.9

15.7

1

1.8

5. Clay

68

30.4

54.5

13.9

9.5

1

1.8

6. Muck soil

15

250.0

333.0

83.0

12.5

1

1.3

GRAVITATIONAL WATER AND DRAINAGE 163

recorded in the last table. If this per cent be subtracted
from the per cent given in Column IV of the last table,
the per cent of actual gravitational water in those soils
may be determined. This is shown by the preceding
table.

The amount in Column V represents the pounds
of water per cubic foot which would be lost by drainage
from each of the soils if their pores were all completely
filled with water. Such a soil is said to be saturated.
That plane in the soil to which level all of the pores
are filled with water — saturated — is known as the
water-table. This region of saturation is sometimes
known as the "ground water."

It is possible to have such a structure in a fine clay
soil that all of its spaces are practically filled with water
held capillarily. It will be noted from the table that
the proportion of the total water capacity which is
permanently retained increases with the fineness of
the soil, and consequently with the decrease in the
size of the individual pores, as is shown in Column
VI. The clay in the above tables appears to be very
thoroughly granulated, which is responsible for the
similarity in the ratios for the silt and clay.

Gravitational water is directly injurious to upland
crops, but when it exists at a depth of from four to six
feet below the surface, it may serve as a reservoir from
which moisture is withdrawn by capillarity, to offset
losses by evaporation. Water may be removed by
capillarity from the saturated zone to the point where
the loss is taking place, and under these conditions the
ground water — which then becomes capillary water —

164 THE PRINCIPLES OF SOIL MANAGEMENT

is directly beneficial, and the process constitutes a form
of natural sub-irrigation.

The figures presented above illustrate the effect of
texture on the total water capacity of a soil, and upon
the proportion of gravitational water. Anything which
increases the pore space increases the total water capac-
ity. When there is not a corresponding increase in
the capillary capacity, as happens in a sandy soil, the
total amount of gravitational water is thereby increased.
That is, in such a soil, there is a larger amount of water
which may be lost by percolation. In so far as organic
matter alters the structure of the soil, it modifies the
gravitational water content of a soil in the manner
just outlined.

79. Amount and rate of loss. — Near the outset of
the discussion of soil moisture, it was stated that the
amount of water in a soil depends upon the extent and
rate of loss of water, as well as upon the factors which
have just been explained. For example, fifteen inches
of water is far more efficient in crop production when
applied to a loam soil in a humid region, like the New
England states, than when applied to the sand of the
Imperial Desert, California. In the latter case, the loss
by percolation and evaporation is so great and so rapid
that the amount of moisture available to crops is very
small. The two forms of loss which affect the moisture
in the soil are: (1) Percolation. (2) Evaporation.

Percolation is the gravitational flow of water through
the pores of a soil. Percolation concerns the gravitational
water. The total loss in any given soil will depend upon
the distribution of the rainfall or the irrigation supply.

MOVEMENT OF SOIL WATER 165

Evaporation takes place at the surface, and from
the plants growing in the soil. The rate of such loss
depends on the climatic conditions. In those regions
where the rainfall comes in frequent small showers,
which wet the soil to a depth of only a few inches, a
very large proportion of this water is immediately
returned to the surface by capillarity, and lost by evapo-
ration. On the other hand, if the rainfall occurs at long
intervals and in large amounts, so that it percolates
deeply into the subsoil, it may be held there by appro-
priate surface tillage.

III. MOVEMENT OF SOIL WATER

Soil moisture is subject to movement in three ways.
This movement may be injurious if it facilitates the
loss of moisture, which should be retained for the crop;
it may be beneficial when it serves to replenish the
moisture supply upon which the plant is dependent.
In the discussion of the moisture content and capacity
of soils, it was pointed out that no soil retains within
the surface four feet enough water to meet the needs
of a full-crop yield under average field conditions.
This indicates the necessity for the movement into the
root zone of moisture, to take the place of that removed
by the plant and lost in other ways. The movement
of moisture from adjacent supply in the soil, — as the
deep subsoil — is just as useful as the direct addition
of water to the soil by rainfall. The three types of
movement of soil moisture are (a) gravitational, (6)
capillary and (c) thermal.

166

THE PRINCIPLES OF SOIL MANAGEMENT

80. Gravitational movement. — Gravitational move-
ment is the result of the gravity pull upon the soil
water. The slower the downward movement of water,
the longer the water will be in the root zone of the crop,
and therefore the greater use will the plant be able
to make of that particular supply of moisture. This
gravitational movement concerns primarily the gravi-
tational water, and is not effective to move either the
hygroscopic or the capillary forms of water, although
these are subject to the same gravity pull. The reason
is, so far as these forms of moisture are concerned,
that the gravity pull upon them is overbalanced by
other forces. It will be noticed, in fact, that gravita-
tional water is defined as that part of the soil water
which is free to move under the influence of gravity,
Such movement constitutes percolation.

The rate of percolation depends upon two primary
conditions. These are: (1) The texture of the soil.
(2) The structure of the soil. The rate of movement
depends directly upon the diameter of the individual
soil spaces. The larger the size of spaces, the more freely
will the water descend. King has observed the following
movement of water through sands of different texture
in twenty-four hours:

Table XXVIII

Sands

Clay loam

Black marsh

Mean diameter in m.m.

.50

.35

.27

.25

Inches
301

Inches

160

Inches

73.2

Inches

39.7

Inches
1.6

Inches

0.7

GRAVITATIONAL MOVEMENT 167

The columns were one-tenth of a foot in cross-section
and fourteen inches high, and a head of two inches of
water was maintained above the top of the soil. These
figures show very clearly the reduction in the flow of
water as the texture becomes finer.

Under field conditions, the percolation of water
through the soil is much facilitated by the presence of
numerous cracks, root passages, and worm and insect
burrows, because of their relatively large diameter.

Several other factors affect the percolation of water.
The entrance of rain or irrigation water into the dry soil
where it is applied in a sheet over the surface is hindered
by the presence of the air in the pores in the soil. If
the subsoil is dense, or is filled with water, this inter-
mediate band of air-filled soil serves to hold back the
surface water, except as the air may escape in bubbles
through the upper layer. For this reason, in part, a
heavy shower of rain sinks into the soil to a very small
depth, and is relatively ineffective. Entrance of the
water may be greatly facilitated by a loose condition
of the soil, which affords quite large as well as small
spaces. The large spaces are less likely to be entirely
filled with water, and hence afford means for the escape
of air, while the water passes in through the smaller
pores. There is another hint here in the conservation
of rainfall. If the soil is in a very loose condition to a
depth of eight or ten inches, the water will percolate
into this layer, and its movement will be so much re-
tarded that a larger part will find its way into the deep
subsoil and be permanently retained than if the surface
soil is uniformly fine.

168 THE PRINCIPLES OF SOIL MANAGEMENT

Changes of temperature affect the flow of water
through soils in several ways. It affects the gravita-
tional water directly by changing its viscosity. Warm
water is more limpid and flows more freely than cold
water, just as oil is thinned by heating. Consequently
soils drain more readily in summer than in winter. (See
also page 183.)

Changes in temperature also affect percolation
indirectly through their effect on the free air in the soil,
and the air in the water in the soil. Air, in common
with all gases, expands very greatly with a small in-
crease in temperature, and it thus exerts a pressure
which may force water out of the soil into the larger
drainage channels. Conversely, a lowering of the temper-
ature contracts the air, and causes water to be sucked
into the soil.

In the same way, barometric changes affect the drain-
age of soils. Alternate periods of low and high pressure
sweep over the country at intervals of a few days apart,
and the changes in volume of the outer air are trans-
mitted to the air in the soil, which expands or contracts
and tends to draw water into the soil, or forces it out
as the pressure is decreased or increased. The suctional
effect of winds may have a similar effect. Strong winds
considerably modify the air pressure, and where this
is brought to bear on the soil through a tile drain or
other underground channel it increases the flow of
water.

Water does not necessarily percolate vertically into
the soil. It may flow off nearly horizontally, depending
on the character of the soil and its conditions. A hard

CAPILLARY MOVEMENT 169

subsoil will deflect its movement. Entrapped air will
do the same thing, and this has been found to be a
potent source of contamination of open wells with shal-
low curbing. This is particularly true in heavy soils,
where the escape of entrapped air is especially difficult.
One of the beneficial effects of under drains is that they
facilitate the entrance and movement of rain-water
in the soil by affording a channel for the escape of
entrapped air. (See page 241.)

81. Capillary, or film movement. — Capillary water
has been described (see page 141) as occurring in the
soil in a thin film overspreading the particles, and thick-
ened into a waist-like form at their points of contact.
Toward the bottom of any soil column the film is always
thicker than at the top, owing to the less weight which
the surface tension must bear. This form of distribution
has given rise to the term film water, from which is
derived the idea of film movement, to describe this
type of capillary movement.

Film movement expresses very accurately the actual
condition of affairs, for if there is any translocation of
water at this stage it must be through this film.

82. Principles governing capillary movement. — It
will be remembered (page 147) that, when equilibrium
is established in any mass of wet soil short of saturation,
the water surfaces are comparable to a stretched elastic
membrane. The more closely this film is drawn about
the particles, the more surface there is exposed, and the
greater pull the surface tension exerts. Consequently
the greater the amount of water which will be retained.

In a soil capillarily saturated with water there is

170 THE PRINCIPLES OF SOIL MANAGEMENT

no movement. For the pull at any one point is balanced
by the pulls from every other point, due to the surface
curvature of the film and to the weight of the liquid.
In the bottom of the column, where the weight of the
water acts in conjunction with the curvature of the film,
the curvature is less than at the top of the column,
where the only effective pull is due to the curvature of
the water surfaces. This may be illustrated by the fol-
lowing diagram. (Fig. 52.)

P represents soil particles carrying their maximum
film of water, and therefore in equilibrium at every
point, so that no movement may take place. The force
or pull exerted by the film at the different points is
represented by the arrows at A, B, C, D, E, etc., the
length of the arrow being proportional to the pull exerted
by the film, and in the same direction, or toward the
center of curvature of the surface. The difference
in the pull, and therefore the length of the arrows at
the top and bottom, is compensated by the weight of
the water at the bottom. If water is now taken from the
film into the rootlet at R, the curvature of the film at
that point will be increased. Therefore it will exert a
greater pull than the curvatures in the other spaces,
and water will be moved to R along the lines U, to
replace that taken in by the root. So that the new
adjustment would be represented by the dotted lines
which show the new curvature assumed at each point,
when equilibrium is reestablished, and the water comes
to rest. If water continues to be lost to the root, or by
evaporation from the soil at R, the movement of water
to that point will be continuous as long as movement is

CAPILLARY ADJUSTMENT

171

**«&•&

Fig. 52. Showing the distribution of water around a group of soil particles,
and the distribution of forces and direction of movement in the re-establish-
ment of equilibrium after the removal of water by a rootlet. For further
explanation see text.

possible; the curvatures meanwhile increasing, and the
films become thinner and thinner.

It will be noted that the curvature at every point
in the plane 1 is the same, and that a similar uniformity
prevails for planes 2 and 3. Likewise, in the columns
I, II and III the relative curvatures are the same.

172 THE PRINCIPLES OF SOIL MANAGEMENT

Theoretically, therefore, there is no limit to which
this adjustment might take place in the horizontal plane.
Water might be moved in from a distance of one inch
or one rod. Vertically, however, there would be a limit
to the height to which water could be lifted, because
of the limit to the pull of the surfaces in plane 1.

The larger the number of curves, the greater the total
pull per unit area, and consequently the higher could
water be lifted — just as there is a definite limit to which
water will rise in glass tubes of different sizes. It is
therefore possible to keep trimming off the upper end
of a column of soil, whose lower end dips in water, until
the maximum height through which water may be lifted
and lost by evaporation, or otherwise, is determined.
This is the maximum capillary efficiency of the soil, or
the maximum height to which it could deliver water.

According to the above propositions, the movement
of water would go on freely and uniformly until the mini-
mum thinness of film was reached. This free movement
is modified, however, by another condition. Water,
in moving from any point, as C to R, must pass through
the thin part of the film between the points of contact,
and where it comes in close contact with the soil sub-
stance. In this, friction is developed, and the thinner
the film, and the closer it is drawn about the particle,
the greater does this friction become until it all but stops
movement.

For a period, when the film is thick, the movement
is relatively free; but, after the water comes within the
range of great attraction of the particle, the friction
increases rapidly, and therefore the movement of water

IMPORTANCE OF CAPILLARY MOVEMENT 173

is correspondingly cut down. This factor of friction
greatly limits the effective capillary capacity of a soil
both vertically and horizontally. If the coefficient of
friction is great, it will soon overcome the pull due to
curvature, and water will be quickly moved in from
only a short distance. In proportion as the friction
coefficient is reduced, the range of movement is ex-
tended. It should be noted that friction retards move-
ment rather than stops it. The greater the surface
over which a given volume of water is spread, the slower
therefore will be its movement. (See page 183.)

In the above discussion it was assumed that the
water is uniform in all its properties, and therefore
that corresponding curvatures were the same. If, how-
ever, anything modifies the surface tension of the liquid
at one point — as change of temperature, solution, etc., —
this would be expected to disturb the balance, and result
in film movement. Such is the case, as later examples
will show. (See page 183.) It is probable that, in the
soil, equilibrium is never established, because of these
disturbing variations all through the soil mass. Further,
the last end of the process of adjustment is exceedingly
slow, and probably never actually takes place; because
the force producing the motion is successively reduced
as equilibrium is attained, and because the difference in
curvature of the films is so slight.

83. Extent, rate and importance of capillary move-
ment.— Capillary movement of water is of great conse-
quence to growing plants. Since it concerns the capillary
water, it affects that form of soil water upon which
ordinary crops are directly dependent. The withdrawal

174

THE PRINCIPLES OF SOIL MANAGEMENT

of water at any point by a rootlet is made up by move-
ment of water from the adjacent soil zones. But the
plant is not dependent entirely on the movement of
water to its roots. The roots are themselves constantly
pushing into fresh soil zones, where the moisture, and
perhaps also the food, have not been so thoroughly

Fig. 53. Penetration of root-hairs through the soil, (h, h') root-hairs; (T7)
soil particles; (s, j) air-spaces. Water is indicated by concentric lines.

withdrawn. The roots go to meet the capillary advance
of the soil water. This advance of the fine rootlets is
rapid, and of great consequence in the nourishment
of the plant. It also enables the roots to come into
more intimate contact with the soil; for, as the water
is extracted, it is lost first and most readily from the
large pores. The latter amount of water is found in
the smaller spaces, and consequently the roots are

CAPILLARY MOVEMENT AND TEXTURE

175

led toward these small pores by their attraction for
water.

Three primary soil factors govern the capillary
movement of water. These are: (a) Texture, (6) struc-
ture, (c) dampness of the soil. In addition to these,
the movement is affected by (d) the surface tension
of the soil water, and (e) by the condition of the surfaces
of the soil particles.

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TIME IN HOURS

Fig. 54. Curves showing the height and rate of rise of water in dry soils
of different texture as given in Table XXIX.

84. Texture. — The influence of texture was explained
in the principles outlined above. The finer the soil,
the more surface it will expose, the more points of con-
tact there will be between the particles, and therefore
the greater total curvature the water surfaces will have.
For this reason, a clay containing 20 per cent may draw
water from a sand containing 10 per cent of water.

The capillary capacity of a soil may be measured
in two ways: (1) By the height to which water will

176

THE PRINCIPLES OF SOIL MANAGEMENT

be raised in soils of different texture. (2) By the total
amount of water raised through a given height in a
definite time. The time element enters into both sorts
of measurements, and is an especially important con-
sideration in clay soil where the movement is generally
very slow.

*

Table XXIX
Showing Height of Rise of Water in Dry Soils of Different
Texture, as Shown in the Above Curves

Time

Min.

Hours

Days

15

1

2

1

3

8

13

19

Inches

Inches

Inches

Inches

Inches

Inches

Inches

Inches

1. Silt and very

fine sand . . .

2.7

4.7

7.0

20.0

30

45.0

52.0

56.0

2. Very fine sand

7.6

10.0

12.4

21.0

23

26.0

27.5

28.5

3. Fine sand

9.0

9.5

10.0

11.6

13

14.3

15.2

16.0

4. Coarse and me-

dium sand . .

5.8

6.0

6.3

7.5

9

10.0

11.5

12.5

5. Fine gravel . . .

4.0

5.0

5.3

6.4

8

9.0

10.0

10.8

These materials were sifted to fairly uniform sizes,
according to the scale given above (page 73). The silt
was a natural material, containing a large amount of
very fine sand, together with some clay. It might be
termed a light silt loam. It will be particularly noted
that the smaller classes of particles — silt and clay —
have a relatively large influence on capillary movement.
Above the class of fine sand, there is not much variation
in the height of rise for different textures, the total
height attained being slight.

CAPILLARY MOVEMENT AND TEXTURE

177

Lotighridge has made very careful determinations
of the capillary power of four dry soils of known physical
composition over a period ranging from 6 to 195 days.
These soils range in texture from light sandy loam to
heavy clay adobe of the following mechanical com-
position.

Table XXX

1. Sand soil

2. Light sandy loam

3. Silty loam

4. Clay soil

Per cent of each separate present

Clay
less than

Fine silt

Coarse
silt

Sand

Limits used in diameter of particles

.01

mm.

2.82

3.21

15.02

44.27

.01-025
mm.

3.03

5.53

15.24

25.35

.025- .047

mm.

3.49
15.42
25.84
13.47

.04 7- .5
mm.

89.25
72.05
45.41
13.37

The composition of these soils is also shown in the
following curves:

SANDS
L047. - .5 MM.)

COARSE SILT
(.025 -.047 MM.)

FINE SILT
(.01 - .025 MM.)

Fig. 55. Curves showing the mechanical composition of the soils whose
analysis is given in Table XXX, and whose capillary water capacity is given in
Table XXXI and Figs. 56 and 57.

178 THQ PRINCIPLES OF SOIL MANAGEMENT

The capillary rise of water in these soils was as fol-
lows:

Table XXXI. — Time in Hours

1. Sand

2. Light sandy loam

3. Silty loam

4. Clay

Min.

30

Hours

Height of rise in inches

6.3
0.8

8.0

10.0

12.0

9.0

12.7

19.0

2.7

4.8

8.8

1.4

2.5

5.0

12

13
24
11

Table XXXI, continued. — Time in Days

1. Sand

2. Light sandy

loam

3. Silty loam . .

4. Clay

Days

12 26 48 90 160 195

Height of rise in inches

14.0

15.5

17.0

28.2

30.5

35.0

38

41

44

46.5

13.0

17.0

20.5

25

31.5

35

40.0

45

10.0

14.0

20.0

23

26.5

50

46

As is always the case, the rise is most rapid immedi-
ately after the soil is placed in contact with the water
and the rate of rise decreases progressively as the limit
is reached. The more coarse the texture of the material,
the more quickly is the limit of rise attained.

These figures are shown in the following curves: (1)
For the first twelve hours; (2) for the full period.

CAPILLARY MOVEMENT AND TEXTURE

179

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TIME IN HOURS

Fig. 56. Curves showing the capillary rise of water in twelve hours in dry
soils of different texture as given in Table XXXI.

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20

40

60

140 160 180 200

80 100 120

TIME IN DAYS

Fig. 57. Curves showing the capillary rise of water in 195 days in dry soils of
different texture as given in Table XXXI.

It is evident, from a very simple laboratory experi-
ment, that the rate of capillary movement in dry soil
is inversely proportional to the fineness of texture,
while the total height of rise is directly related to the

180 THE PRINCIPLES OF SOIL MANAGEMENT

texture. Up to a height of three feet, the sandy loam
moves water most quickly. But when it is necessary
to move water to a greater height, a finer soil is required.
The maximum height traversed by capillary water
in the above soils is fifty-six inches in the silt loam,
and in two instances this appeared to be near the
limit of capillary efficiency in dry soil of that texture.
In clay the movement goes on very slowly, and an
excessively long period is required for the limit to be
reached. A crop might perish of drought before water
would move up to meet its needs. That is, in fine-tex-
tured soil, although its capillary capacity is very great,
the surface area of the particles is so large, and therefore
the friction in the movement of water so great, that the
actual capillary movement is very inefficient.

The same fact appears here that appeared in the
discussion of the water capacity of soils, namely, that
it is the soil of intermediate texture — the silt and fine
sandy loam — which most readily meets the needs of
crops for water.

85. Dampness of soil particles. — The capillary move-
ment in dry soil, as given above, does not represent
the true capillary capacity. When a soil is dry, it has
been shown that it resists wetting, and therefore resists
the capillary rise of water. Natural field soils always
contain some oily substances, which are deposited on
the surface of the soil particles when the soil is dried.
This oily matter retards greatly the wetting of the par-
ticles, which takes place only after this material has
been again dissolved, so that a clean surface is exposed
to the solution. Therefore, in a soil which already con-

CAPILLARITY IN MOIST AND DRY SOILS

181

tains a small amount of water, it is to be expected that
the total capillary rise, and the rate of movement of
water, would be most rapid. That is, a slight dampness
of the soil is conducive to the most rapid capillary
movement. Briggs found the limit of capillary move-
ment in dry Sea Island soil — light, fine, sandy loam —
to be about 36 centimeters, or 15 inches, in 14 days;
while, in the same soil in a moist condition, water was
raised through a column 165 centimeters, or 66 inches,
in height — 4.5 as great a height as in the dry soil. Stew-
art found the following limits for three sands of slightly
different texture when dry and wet.

Table XXXII

Dry

Wet

Soil No. 1

Inches

31.8
58.1
86.8

Inches
112.5

Soil No. 2

141.8

Soil No. 3

174.1

These results are in accord with field experience.
The figures for the moist soil most nearly represent
the heights to which soils raise water, and further, under
field conditions, the soil, with the exception of the immedi-
iate surface, seldom becomes air dry in the humid
regions. Consequently, capillary movement concerns
chiefly moist soils.

There are two factors operative to prevent capillary
distribution from moist to dry soil. One of them is
resistence to wetting. The other is the very slow move-
ment of water in thin capillary films, — that is, when

182 THE PRINCIPLES OF SOIL MANAGEMENT

it is reduced to near the minimum capillary content,
or wilting point. At this stage the movement is exceed-
ingly slow, because of the excessive friction. This funda-
mental principle is made use of in soil mulches and de-
termines their usefulness, and should direct their man-
agement. (See page 203.)

86. Structure. — Structure affects capillary movement.
It has been shown how capillary movement is largely
due to the size of the individual spaces in the soil.
The size of the spaces is due, (1) To the size of the par-
ticles. (2) To their arrangement. (See page 203.)
The smaller the particles, and therefore the smaller the
pores, the greater the capillary power and the slower
the movement. In so far as the arrangement of the
particles or structure effects a change in the effective
size of the pores, it affects the capillary movement.
In a puddled structure the movement is much more
slow than in a soil having a granular or crumb structure.
Any tillage operation which alters the structure, in
either one direction or the other, thereby alters the
capillary power and the rate of movement. Compact-
ing a soil is well known as a process which seems to
draw moisture into the compacted zone; while culti-
vation, or loosening the soil structure, has the opposite
effect. Upon this fact vare based many important
tillage operations, such as rolling after seeding small
grains.

87. Surface tension. — Surface tension affects capillary
movement in the same way that it affects the capillary
retention of water. It represents the cohesive properties
of the liquid, and corresponds to an elastic membrane.

CAPILLARY MOVEMENT AND STRUCTURE 183

The stronger such a membrane, the larger the pull it can
exert under a given strain. Consequently, in a soil of
uniform texture, and in moisture equilibrium, any-
thing which changes the surface tension may set up
motion of the soil water. The introduction of fertilizers
may set up such a movement, and this addition to a soil
may enable it to draw and permanently retain more
water than the adjacent soil of same texture. Appli-
cations of magnesium chloride, salt and muriate of
potash, are observed to keep the soil more moist in dry
weather, and a similar effect of some alkali salts has
been noted. These materials all raise the surface tension.
High temperature reduces the surface tension, and there-
fore, in a soil in moisture equilibrium, if one part, as
the surface, is heated, the water will be drawn away
from that region to the cooler zone, where the tension
is higher.

88. Condition of surfaces of particles. — The condition
of the surface of the soil particles affects the tenacity
with which water adheres to them. The application
of oil to a soil tends to destroy its capillary capacity;
and any substance in the soil which will bring about
such a condition reduces the capillary efficiency of the
soil.

The action of capillarity is not limited to any one
direction. It may take place in any direction. It has
usually been measured vertically upward. But it oper-
ates vertically downward, as well, and it moves water
horizontally. The vertical upward movement of capillary
water is modified by the influence of gravity, as is capil-
lary retention. (See page 149.)

184

THE PRINCIPLES OF SOIL MANAGEMENT

The following curves show the capillary transfer of
water in two soils through eight feet horizontally.

18

12

10

* 4

CLAV LOAM INITIAL PERCENT. OF WATER

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SAND

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12345678

DISTANCE FROM EXPOSED END, IN FEET

Fig. 58. Curves showing the initial moisture content of horizontal columns
of sand and clay loam soil and the distribution of moisture after free evapora-
tion from one end of each column, for a period of days. Note the general
movement of water throughout the columns.

It is evident that plants may make use of supplies
of moisture to one side, as well as below their roots even,
in some soils, to a distance of several feet, through the
agency of capillarity. On the other hand, irrigation
farmers have repeatedly noted the very limited lateral
influence upon crops of the application of water in irri-
gation. The limit of the application of water is in some
soils marked almost to the row. In this instance, it
should be remembered that water is added only after
the soil has become relatively dry, at which stage the
moisture films move with great difficulty due to friction,
and probably also to cracks, which of course very effect-
ively break up capillarity. King has concluded from
studies on vertical columns that an adjustment of water
through ten feet of soil may readily take place.

AMOUNT OF WATER MOVED 185

89. Examples of the amount of water moved. — In
crop production, the crucial test of the capillary capacity
of the soil is the amount of water it is able to move.
It must not only be able to move water a long distance,
or to a great height, but it must be able to move a rela-
tively large amount of water, and to move it quickly
if the movement shall be effective. The important
consideration is the amount of water moved a given
distance in a given time. A soil may be able to quickly
move large volumes of water to a height of a foot,
and be utterly ineffective to a height of five feet. On
the other hand, a soil may be so fine as to be able to
lift water to a height of forty feet, and yet the move-
ment be so slow and the amount of water moved be
so small that the result is negligible, — that is the soil
is capillarily ineffective. It therefore appears that,
for any given distance within reason and for any normal
moisture demand of a crop, there is a texture and struc-
ture of soil which will most readily meet those demands.
If the water-table is three feet below the surface, a
very coarse soil may suffice. If the water-table is ten
feet below the surface, a much finer soil will be necessary.
On the other hand, to supply a full-sized pumpkin vine,
having a large evaporation, from a water supply five
feet away, will require a finer soil than is required
to supply a Jersey pine having a small evaporation.
In other words, we need to know the effective capillary
capacity of each soil to different heights and distances,
up to their limits.

Very little data of this sort is available. None is
available for horizontal movement, and the figures

186

THE PRINCIPLES OF SOIL MANAGEMENT

on vertical movement are very incomplete and inade-
quate. King made such a study of sifted quartz sand
having a mean diameter of .47 mm., by means of a
column with an expanded top, and found that the sand
was able to raise water to a height of 6.75 inches at
the rate of 44 inches of water per day, equivalent to
1,340 feet per year. But this same sand failed to lift
any appreciable amount of water to a height of 11.75
inches. King has also found the following movement
to take place to different heights in columns of soil
one square foot in cross section, where the loss was
measured by evaporation from the surface.

Table

XXXIII

Height in feet

1 foot

2 feet

3 feet

4 feet

■*3

<D .

O. cy
m m

c a>
s a

Ph o3

13

C u
0.

s a

n° >>

C ^

a

a .

e s
3 a

Ph o3

T3

hi

-a

Pounds per
day per sq. ft.

U

c ft

1. Fine quartz

sand

2. Clay loam ....

2.37
2.05

166
144

2.07
1.62

146
113

1.23
1.00

86
70

.91
.90

64
63

As remarked by Professor King, these figures prob-
ably do not represent the maximum capacity of these
soils to the heights stated. The shorter the column,
the less accurate are the figures. For in the short col-
umns the evaporation was correspondingly less than
the movement. From the results, it appears that the

AMOUNT OF WATER MOVED

187

clay soil was in a very well-granulated condition, which
brings its rate very near that of the sand. It also appears
from this data, as was shown in the data on height
and time of capillary rise, that, up to three or four feet,
the fine sand is as efficient as the soil of much finer
texture.

In studies on the capillary rise of water in moist Sea
Island cotton soil — a fine sandy loam, — Briggs found

Fig. 59. The weeder, with riding attachment. For very shallow cultivation
in mellow soil free from stone and rubbish.

the movement to be at the rate of 1.3 pounds per square
foot per clay, or 91 inches per year, through a height
of 85 centimeters (34 inches). But when the column
of the sand soil was 165 centimeters long, water was
raised at the rate of .32 of a pound per square foot per
day, or 21.4 inches per year, — a decreased efficiency
from doubling the height of the column of 75.4 per cent,
When the column was 185 centimeters in height, no
appreciable loss took place, — indicating that this sand

188

THE PRINCIPLES OF SOIL MANAGEMENT

was not able to raise water to the height, even when
moist.

Buckingham obtained the following results, which
show a very considerable vertical movement in fine
sandy loam soils to a height of nearly four feet.

Table XXXIV

I

II

III

IV

Height of

column.

Inches

Dry

porosity

Pounds of

water per day

per sq. ft.

Inches of

water per

year

1. Takoma

lawn

46

48

.73

51.6

2. Podunk

fine sandy
loam ....

46

35

.56

39.4

It must be kept in mind, in examining these figures,
that the evaporation conditions in the different experi-
ments were not uniform, and therefore, that the results
are not strictly comparable. They do, however, show
the movement of a very large amount of water in this
way through distances of several feet. The amount
so moved in these sandy soils per year is several times
the total amount required to produce normal crops.
(See page 134.) There is also indication that in the short
period of a day the amount of water moved is sufficient
to meet the needs of a considerable mass of growing
plants. It is regrettable that no figures are available
for silt and clay soils, and to greater heights and hori-
zontal distances, in order that a more complete idea
of the availibility of water supplies at a distance of six,

THERMAL MOVEMENT OF WATER 189

eight and ten feet, or even more, may be had. This
is an important body of information yet to be gained.

90. Thermal movement. — Water moves through the
soil in the form of vapor. If a glass vessel or tube filled
with moist soil be set on a hot surface, the bottom of the
column will be seen to become lighter in color, indicating
a loss of moisture. If the whole column is not heated
and the moisture is determined in successive sections,
beginning at the top, or coldest portion, the moisture
content will be found greatest a short distance above
the heated layer at the bottom.

When the moist soil is heated, steam is formed, which
develops a pressure that forces the vapor rapidly through
the soil. But, at ordinary temperatures, this vapor
movement is the result of simple diffusion, and it obeys
the same laws. Buckingham has shown that the diffu-
sion of air through 'the pores of the soil is exceedingly
slow, and therefore that this phase of soil aeration is
of small effect. (See page 439.) He has also shown
that the diffusion of water vapor through the fine pores
of the soil is very slow. (See table below.)

It is well known that water does not necessarily
evaporate at the surface of the soil. It may evaporate
in the deep pores in the soil if the air at that point
is sufficiently dry. Atmosphere in a moist soil is very
near saturation. In a mulched soil (see page 199) evapo-
ration may take place at the top of the moist layer.
The loss of water will therefore depend very largely
upon the loss of moisture by diffusion through the
mulch. Buckingham obtained the interesting results
given in Table XXXV bearing on this point:

190

THE PRINCIPLES OF SOIL MANAGEMENT

Table XXXV. — Loss of Water by Evaporation from Below
Columns of Different Air-Dried Soils

Soil

Depth of
soil layer

Initial
porosity

Rate of loss
of water
per year

Coarse sand

Inches
2
1
2
4
6
1
2
4
6
2

Per cent
45
48
46
41
46
54
51
49
51
46

Inches
4.30

Fine sandy loam

2 52

Fine sandy loam . . .

1 59

Fine sandy loam

0 93

Fine sandy loam

0.67

Silt loam

2 71

Silt loam

1 60

Silt loam

0 95

Silt loam

0.69

Clay

0.60

It appears from these figures that the thermal move-
ment of water by simple diffusion is determined: (1)
By the size of the individual pores. (2) By the total
amount of pore space in the soil. (3) Upon the thickness
of the soil layer. When equally dry the fine-textured
soil retains moisture as vapor more effectively than does
coarse-textured soil. In so far as the structure of the
soil modifies either the size of the pores or their total
volume, it may modify the loss of water. A coarsely
cloddy mulch would therefore be ineffective. Particu-
larly striking is the small depth of soil which is effective
to prevent the loss of water. Even the one-inch mulch
has a wonderfully high efficiency.

IV. CONTROL OF SOIL WATER

In the control of soil moisture it is desired to accom-
plish one of two things: (a) The average water content

CONTROL OF SOIL MOISTURE

191

of the soil is increased or, (b) the average water content
of the soil is decreased. If the crop is likely to suffer
from a deficiency of water, or from conditions associated
with a deficiency of water — as food, — we aim to in-
crease the moisture supply by conserving the rainfall,
or by direct additions of water. On the other hand,
in soils saturated with water, or which are too cold,
or too poorly aerated because of an excess of water,
it is desired to remove this excess either by drainage
or appropriate tillage
methods.

91. Means of in-
creasing the water
content of the soil. —
The average water
content of the soil
may be increased in
three ways: (1) By
decreasing the losses
from (a) percolation and (6) evaporation. (2) By
increasing the capacity of the soil for water (a) by
modifications of texture and structure, and (b) by in-
creasing the humus content. (3) By the direct addition
of water to the soil, which is irrigation.

92. Decreasing loss. — The water which comes on the
soil is subject to two forms of loss, (a) It may percolate
through the soil and beyond the reach of plant roots.
(b) It may evaporate.

93. Percolation. — The amount of loss in this way
is very great. (See page 192.) Water percolates most
rapidly in large spaces, and whether these large spaces

Fig. 60. One - row toothed cultivator.
Adapted to shallow tillage and the mainte-
nance of a mulch.

192

THE PRINCIPLES OF SOIL MANAGEMENT

are the result of coarse texture or of a loose, cloddy
structure, the final result is the loss of water. The fol-
lowing table shows the average results of the Rothamsted
drain gages for thirty-four years, by months from 1871
to 1904, on a rather heavy loam or clay loam soil, and
twenty, forty and sixty inches in depth. These gages
have an area of one thousandth of an acre each, and are
kept free from vegetation.

Table XXXVI

January

February

March

April

May

June

July

August

September ....

October

November

December

Mean total per
year

Maximum .
Minimum .

Rain-
fall

Drainage through
soil

20

40

60

Depth in inches

Proportion of rainfall
drained through soil

20

40

60

Per cent

2.32
1.97
1.83
1.89
2.11
2.36
2.73
2.67
2.52
3.20
2.86
2.52

28.98

1.82

2.05

1.96

78.5

88.4

1.42

1.57

1.48

72.2

80.0

0.87

1.02

0.95

47.6

55.6

0.50

0.57

0.53

26.5

30.0

0.49

0.55

0.50

23.2

26.1

0.63

0.65

0.62

24.0

27.6

0.69

0.70

0.65

25.3

25.6

0.62

0.62

0.58

23.2

23.2

0.88

0.83

0 76

35.0

32.8

1.85

1.84

1.68

57.8

57.5

2.11

2.18

2.04

76.7

76.3

2.02

2.15

2.04

80.3

85.4

13.90

14.73

13.79

48.2

51.0

84.5
75.2
52.0
28.0
23.6
26.3
23.8
21.7
30.0
52.5
72.4
81.0

48.0

Results for maximum and minimum rainfall

38.70
20.50

23.50
7.32

23.60
7.90

24.30
7.70

60.7
35.7

61.0

38.5

63.0
37.6

LOSS BY PERCOLATION

193

The rainfall and relative loss through gages of differ-
ent depths is shown in the following curves, based upon

the above figures.

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28
26
24
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JAU. rtO. MAK. APHIL MAT JUNL JULY «UU. OCri. UU I . NUV. UtL.

Fig. 61. Curves representing the annual rainfall and percolation through
20, 40 and 60 inches of soil by months. Rothamsted, England. Average of 34
years.

It appears from these figures and curves that about
50 per cent of the rainfall is lost by percolation, under
the climate of England. It also appears that the loss
is slightly less from the sixty-inch than from the twenty-
inch gage. Under a climate less humid, this difference
is greater. This is illustrated in two ways: (1) In the
above table1 it is clear that the proportion of water lost
by drainiage is much less in summer than in winter.
^(See page 195.) The saving is somewhat larger in the
deep than in the shallow gage, as the proportionate

M

194 THE PRINCIPLES OF SOIL MANAGEMENT

capacity of the soil for water is somewhat greater in
this case. (2) It has been estimated that the annual
run-off of the streams in the eastern half of the United
States amounts to about 50 per cent of the rainfall;
but in the basin of the Missouri river the run-off is not
over 20 per cent of the rainfall, and in the Great Basin
it is practically nil.

These figures give some idea of the total amount
of water lost by percolation through the soil, and repre-
sent a supply which it is the aim of good soil manage-
ment to lessen or eliminate, according to the needs of
the crop. Loss from percolation may be reduced in two
ways, which depend upon the fact that the rapidity
of such loss is directly proportional to the size and volume
of the pore spaces in the soil. These are (a) by modifi-
cations of texture, (b) by modifications of the structure
of the soil. The primary method is, or course, that
modification of structure which breaks down the granular
arrangement and permits a greater compactness. When
rain falls on the soil, its fall is not stopped. It continues
to fall through the soil at a reduced rate as gravitational •
water. And, as the movement of this gravitational water
is directly determined by the fineness of the soil spaces,
it is possible to very greatly reduce this type of move-
ment by compacting the soil structures. The greater
compactness of the soil lengthens out the period during
which the soil contains hydrostatic water, and, if the
roots of growing plants are distributed througli the soil,
they are able to make a larger use of this free water
than would be possible if the wave of saturation, as a
result of rainfall or irrigation, quickly passed beyond

LOSS BY EVAPORATION 195

their reach. Therefore, on soils subject to excessive
leaching, water may be conserved by use of the roller
or other compacting implement, and by such manage-
ment as permits the deep subsoil to become more dense.
94. Evaporation. — The second form of soil-moisture
loss is by surface evaporation. It has been shown that,
in the process of growth, a large volume of water is
evaporated directly from the tissues of the plants. In
this process it performs useful functions. But a large
amount of water is also lost by direct evaporation from
the surface of the soil. If the plants which evaporate
the soil water are those of the desired crop, the loss is
proper and not to be avoided. But it frequently happens
that, either before the regular crop is on the land or
mixed with it, are large numbers of worthless plants
through which this same moisture loss occurs. This is
of course a waste of moisture, and is to be avoided by
preventing their growth. It may happen in the spring
that the late plowing of land bearing a heavy growth
of vegetation permits so great a loss in this way that,
unless the subsequent season is one of abundant rain-
fall, the regular crop may suffer from the lack of moisture
which was stored in the soil, and by timely plowing and
preparation could have readily been utilized. In this
connection, it should be kept in mind that green manure
crops may be directly injurious the first season if they
are permitted to grow so late before being turned under
as to unduly deplete the soil moisture. In the manage-
ment of green manure crops, that optimum point when
the excess of water due to heavy spring rain and winter
snow has been removed, but the capillary supply not

196 THE PRINCIPLES OF SOIL MANAGEMENT

impaired, should be selected. In semi-arid regions,
where dry farming — farming without irrigation where it
is usually required — is practiced, it is sometimes advis-
able to grow but one crop in two years, because the
annual rainfall is not sufficient to produce a profitable
crop each season. This practice, of course, implies
those conservation practices which safeguard the rainfall
as it collects, by appropriate tillage methods.

The loss of water by direct evaporation from the soil
may be excessive, and result in direct reduction of the
crop yield. This type of loss is so familiar that examples
hardly need be cited. In the results with the Rotham-
sted rain gages, about 50 per cent of the annual rainfall
was regained in the drainage water. Since the gages
bore no crop, the remaining 50 per cent must have
been lost by evaporation. And it will be noted that in the
summer months under warm temperature this loss was
greatest, amounting to 75 per cent of the rainfall.
Correspondingly, in the semi-arid and arid sections of
the country, where there is little or no drainage, the rain-
fall is all lost by evaporation. Investigations indicate
that about 70 per cent of the precipitation on the land
surface is derived from evaporation from the land sur-
face. Even in the humid sections, where the annual
rainfall is ample for maximum crop production, the crops
are frequently reduced even below the profit point by
prolonged periods of dry weather in the growing season,
during which the loss from the plants, coupled with the
loss from the soil, exhausts the soil supply. If we refer
to page 135, we note that the water absolutely needed
for crop production, and including the necessary losses

CONDITIONS WHICH PERMIT EVAPORATION 197

from the soil, is only a small proportion of the annual
rainfall of most of the cultivated sections. These losses
are therefore preventable; and that this is true is
exemplified by the large difference in average crop
yield on those lands where the best conservation prac-
tices are in vogue over those where they are neglected.
It should be remembered that over the vastly larger
proportion of cultivated land area the crop yields are
controlled more directly by the lack of water than by
the excess of water. It is a common observation that
soils which ordinarily give a low yield in seasons of.
normal or low rainfall give good yields in wet season,
indicating how large a dominating factor is the moisture
supply. For the moisture concerns not only its direct
use as a food and carrier for the plant, but by its influence
on solution, and other essential conditions of plant
growth, its is a chief dominating factor in growth.

Soil evaporation occurs almost entirely at the surface.
Exception may be made where evaporation occurs into
large, deep cracks in heavy clay soil, which is the primary
source of subsoil loss in such cases. If this be prevented,
as it may be, the loss will be very small. Since evapora-
tion is chiefly at the surface, the nearer the available
store of moisture is held to the surface, the larger pro-
portionate loss will occur. This principle has its appli-
cation in the amount and distribution of the rainfall
or irrigation. Frequent small rainfalls are much less
effective than less frequent rains in larger amounts.
For if the rainfall or irrigation produces only shallow
percolation before the water assumes capillary forms,
it may be quickly returned to the surface, and lost.

198 THE PRINCIPLES OF SOIL MANAGEMENT

Also there is a certain inherent loss in the most careful
field practices, which are proportionately greater with
small applications of water than with large ones. It
has been shown (page 182) that as the capillary films
are reduced in thickness the movement becomes in-
creasingly difficult and slow. Therefore in a fine-tex-
tured or dense soil, where evaporation occurs only at
the surface, the top layer may become so dry in warm,
clear weather that capillary movement practically
ceases. Therefore, loss is also stopped. If now there
comes a light rainfall, — sufficient to replenish the super-
ficial moisture films, but not enough to produce deep
percolation, — the result may be the renewal of capillary
movement, which will ultimate in a few days in a greater
total loss than would have occurred had there been no
rainfall. These results have frequently been observed
in practice, and were definitely shown in field moisture
studies made by Stewart. In moisture studies of the
soil in the open, and under a muslin shade used in grow-
ing wrapper tobacco in the Connecticut valley, it was
observed that a small rainfall had a much larger effect
on the soil-moisture content outside than inside the tent.
A rainfall of less than half an inch increased the water
in the surface nine inches of the soil outside the tent to
a larger extent than could be accounted for by the rain-
fall. Careful calculations and observations indicated
that the difference represented movement up from the
subsoil, due to the renewal of film movement. King
has obtained similar results in field studies which he has
checked experimentally. This emphasizes the desira-
bility of storing water as deeply in the soil as is practi-

RETENTION OF WATER. BY MULCHES

199

cable, and of giving a few relatively large applications
rather than many small ones, in the artificial addition
of water.

Surface evaporation may be reduced in two ways:
(1) By the application of some protective covering
to the moist soil. (2) By such surface treatment as will
reduce the tendency to evaporation.

95. Mulches. — The protective covering constitutes
a mulch. That is. a mulch is any material applied to the

Fig. 62. Two types of soil structure . On the right, compact soil, clue
to the use of the roller. On the left, the same soil, loosened at the surface to
form a mulch.

surface primarily for the purpose of preventing evapo-
ration. It may at the same time fulfil other useful
functions, as keeping down weeds and maintaining a
more uniform soil temperature, but its primary use is
to prevent evaporation. Of course, in so far as the
growth of weeds is prevented, moisture loss from that
source is eliminated, and at the same time plant food
is conserved for the regular crop.

Mulches are of two sorts: (1) Foreign material

200 THE PRINCIPLES OF SOIL MANAGEMENT

applied to the surface of the soil. (2) Those composed
of the natural soil modified by appropriate tillage.

The action of both sorts of material depends on the
facts shown on pages 180 and 1S9; namely, that capillary
action may be changed or broken by sufficient change
in the texture or structural properties of the material,
and, second, that the diffusion of water vapor, even after
evaporation has taken place, is exceedingly slow through
small irregular pore spaces, such as exist in all materials
effective as mulch. Any material is effective as a mulch
in proportion as it fulfils these conditions; and their
practical application, therefore, becomes chiefly a matter
of selecting that material which meets these require-
ments, and may be readily applied.

Many kinds of material are used as a mulch. Straw,
chaff, dead weeds, stubble, leaves, sawdust, manure,
boards, canvas, stone, coarse sand — all of these are used,
and many other waste materials which may be available.
They act as a cover to the moist soil, so that water
which is held in the surface of the soil, or is brought up
by capillarity, must evaporate into this stagnant and
therefore soon-saturated atmosphere; under which con-
ditions the loss must be much less than where the vapor
is freely removed, and dry air brought in contact with
the moist soil. All of these materials are very efficient
as a mulch, their efficiency depending upon their thick-
ness and porosity. Straw and leaves, when fresh and
dry, will reduce evaporation below 10 per cent of the
normal, when in a layer three or four inches thick.
As they decay and become water-soaked from succes-
sive rains, their efficiency decreases; but they retain

ARTIFICIAL MULCHES

201

an efficiency of at least 50 per cent for a long period,
or until they are so decayed that they acquire decided
capillary capacity. A practice based upon this effect
is that of growing potatoes under straw. The potatoes
are laid upon the surface of the ground, and covered
deeply with straw, which keeps the surface soil so moist
that the potatoes sprout and will grow a reasonable
crop to maturity, when the straw has simply to be raked
back and the tubers, clean and smooth, are found on or
very near the surface. Leaves, including pine needles,

Fig. 63. A very stony soil. Boulders and gravel serve as a mulch, promote
drainage, and increase the warmth of the soil.

202 THE PRINCIPLES OF SOIL MANAGEMENT

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 cause
injury to its producing power, by its effect on the growing
plant. In some European countries, as well as in a few
places in America, stone has been drawn on the soil,
particularly in orchard and vineyard culture, to serve
as a mulch, and with markedly beneficial effects. Par-
ticularly is this true on those lands too steep to permit
cultivation. And, as a corollary to this practice, it has
been observed in the fruit-growing section of the Ozark
Mountains, and doubtless in other regions, that the
removal of stone from the land not only permits the
soil to become more hard, but also reduces crop yield
by increasing the loss of moisture. It is therefore for
the farmer to decide whether the inconvenience to tillage
or other operations due to the presence of the stone may
not be more than offset by their beneficial effects. A
layer of two or three inches of coarse sand or fine gravel
is a very effective mulch, and is frequently used in green-
house practice.

The above-mentioned mulch materials 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. But
the second type of mulch is almost universal in its
practical availability.

Almost any soil may be converted into an effective
mulch by proper treatment. This treatment will differ
with the character and condition of the soil and the

DUST MULCHES '20'.]

climate. Mulches formed from the natural soil are
commonly termed "dust mulches," or more expressively
"dust blankets." A dust mulch is simply an air-dry
layer of the natural soil covering the moist soil below.
It may be in a compact condition, but ordinarily it is
loose and friable. Its creation is dependent on the prin-
ciples explained on pages 172 and 189 concerning capil-
lary movement and diffusion of water-vapor. Under arid
conditions where the atmosphere is dry and hot, and in
free circulation, the surface soil is quickly dried out
after an application of water. This drying takes place
so rapidly that the capillary 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 so long as it is kept dry it very effec-
tively preserves 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 demonstrated by Bucking-
ham 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, the equilibrium distribution of water in which
is shown in the curve on page 148. At first, the loss under
the arid conditions was very rapid and exceeded the
humid conditions, but the rate of loss soon dropped
considerably below the humid column, and continued
to fall behind during the twenty days of the experiment.
This experiment was conducted under the most difficult
conditions for creating a mulch, since the soil used was
of intermediate fineness and had a large effective capil-

204

THE PRINCIPLES OF SOIL MANAGEMENT

lary capacity, and, further, it had a full supply of water
at the bottom of the column, — conditions seldom found
in practice, and certainly not common under arid-
climate conditions. The curves of water loss, showing
the mulching effect of rapid drying, appear below:

300

< 200

100

0

■=^°

v^

co^2

coNon-:°1s— —

^"*^r

20

25

6 10 15

TIME IN DAYS

Fig. 64. Curves showing the relative evaporation of water from two col-
nm-is of the same soil. One was kept in a dry atmosphere at the immediate
sur.'ace. The other was maintained under normal humid climate conditions of
moisture and temperature.

For the reasons presented, the moisture supply in
arid regions appears to be naturally more effectively
conserved than in humid regions, — certainly a wise
provision. This fact is to be connected with the further
one that capillary movement into the deep subsoil is
very slow.

The mulching effect described above gives further
emphasis to the unwisdom of frequent small applications
of water to the soil.

In humid regions the natural mulching effect is
much less marked than in arid regions. If the farmer
would produce a soil mulch, he must do it by creating
as far as possible the arid conditions. That is, he must
bring about such a rapid drying of the surface soil as
to convert it into a mulch which will retain the moisture

MANAGEMENT OF MULCHES 205

below. Since in humid regions drying is usually slow and
capillary movement strong, the process is hastened by
loosening the top soil by frequently stirring, in order
(1) to hasten the drying of that surface portion to the
point where capillarity is stopped, and (2) to reduce its
capillary conductivity, — both of which hasten the forma-
tion of the mulch. It is for these reasons that a mulch
is generally a loose layer of soil.

The management of the mulch is evident from the
principles involved. It must be kept dry in order to break
up capillarity. In humid regions, where frequent rains
occur, the mulch may be destroyed. After such a rain,
when the soil has reached the proper dryness, it should
be again stirred, to renew the mulch. On heavy clay
soil in fine tilth, a mulch may be destroyed by very
moist foggy weather, or by a number of days of very
humid atmosphere, which, by condensation of moisture
on the clay, hastens the reestablishment of capillarity
with the subsoil, by which moisture may be pumped
up and lost. This is to be overcome by occasional stir-
ring, as conditions may require. Another important
effect of the mulch on clay is to keep the shrinkage
cracks filled up, and thereby prevent the deep drying-
out of such soil.

When perfectly dry, a coarse sand and a pulverized
clay are of almost the same practical efficiency. (See
page 190.) It is only when the structure becomes that
of coarse clods or stone that the efficiency is greatly
reduced. A cloddy surface soil is worse than a smooth
surface with no mulch, for the clods are free to evaporate
water, and offer small protection to the subsoil. On

20G THE PRINCIPLES OF SOIL MANAGEMENT

the other hand, the pulverized clay has so great hygro-
scopic and capillary power that its efficiency as a mulch
may be readily destroyed by natural climate and soil
conditions of common occurrence. It is therefore more

Fig. 65. An example of clean, thorough tillage, and the maintenance of an

effective "dust mulch."

difficult to maintain a dust mulch of clay than of sand.
The strong natural mulching tendency of sand may be
seen on sand-dunes, where, although the surface is dry
and hot, moisture may be exposed by the toe of one's
boot at any season.

A perfectly dry dust mulch need not be very deep, to

DEPTH OF MULCHES 207

be effective. One inch of sand will permit loss by diffu-
sion of less than three inches of water per year, under the
most favorable conditions. In practice, however, it is
found that two or three inches are usually most effective
because of capillary action. And Fortier has concluded
from experiments on irrigated soil in California that a
ten-inch mulch conserves more moisture than one of less
depth. But the efficiency of the ten-inch mulch as com-
pared with the four-inch is very much less in proportion
to depth, and the latter conserves 75 per cent of the
water lost where no mulch was used. Sand mulches may
be thinner than clay mulches. King found in Wisconsin
that, for corn, cultivation with a small toothed culti-
vator to a depth of three inches saved more moisture
in fifteen cases out of twenty than did more shallow
tillage, but that increase in depth resulted in no corre-
sponding increase in efficiency. The sweep or blade type
of cultivator (Fig. 137) may be used more shallow than
an implement producing ridges. The mulch should be no
deeper than is necessary to prevent loss of water, since
this top layer is usually most rich in available plant-food,
particularly nitrates, and the roots are excluded from
it by tillage. Unnecessary depth reduces the root range.
Some results from an experiment conducted at Cornell
University serve to illustrate the relation 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. The figures show the in-
crease or decrease in yield as compared with the nearest
check plots. Moisture determinations were made on

208

THE PRINCIPLES OF SOIL MANAGEMENT

portions of the plots bearing no crop, but otherwise
receiving the same treatment as the remainder of the
plot. The table thus shows the moisture conserved
or lost by treatment, entirely aside from that transpired
by the crop.

Table XXXVII

Increased (+)

or

decreased ( — )

yield

Check plot

Weeds removed, but not
cultivated

Mulched with straw

( 'heck plot

No cultivation; weeds al-
lowed to grow

One cultivation; weeds al-
lowed to grow

Check plot

Pounds

-157

+ 873

-2,888
—109

Yields calcu-
lated to basis
of 100 on
check plots

Soil

moisture

during

August

100

Per cent

21.1

96
121
100

18.2
25.0
18.2

31

9.8

98
100

17.0
17.7

Compari-
son soil
moisture
basis of 100
on check
plots

100

90
130
100

54

95
100

The application of the dust mulch is not confined
to inter-tilled crops like maize, potatoes, vineyards,
fallow, etc. 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 grain field with a sharp-toothed
harrow, the teeth pointing very slightly backward.
This is begun when the plants are small, and may be kept
up until they attain a considerable size or until they
sufficiently shade the ground to greatly reduce surface
evaporation. The surface soil between the plants is
broken up and converted into a mulch. Similar to this

MULCHING PLOW LAND

209

is the use of the harrow in the early stage of growth
of cultivated crops, by which the weeds are kept down
and a mulch created. If the practice is begun when the
plants are very young — even before they appear above
the ground — so that the formation of roots very near
the surface is prevented, it may be kept up to very a
advanced stage of growth without serious injury.

ISO

120

110

100

00

80

70

CO

50

40

SO

k \

//
1/

\\

\

^^s^ -^

ft

\

/

\\

\ \

\ \

/ /
1

\

1

1

\

1
1

1

1
1

.YIELD OF CROP

.MOISTURE IN SOIL

< 5

cr H

=>"•

2

J1

o >

Fig. 66. Curves representing relative yield of dry matter and moisture
content of soil on field plots given different cultural treatments. (See Table
XXXVII.)

But dragging only after the plants are good-sized may
cause serious loss.

96. Mulching plow land. — It frequently happens,
especially on heavy soil, that it is impracticable to
complete plowing before the soil, if left in its natural
condition, becomes too dry for the best results. In such
cases it is frequently practicable to quickly form some-
thing of a mulch by use of the disk or toothed harrow.
Further, this treatment creates numerous lines of weak-
ness which, although drying may progress further

N

210 THE PRINCIPLES OF SOIL MANAGEMENT

than is desired, will cause the soil to break up into a
much better condition than if the surface had not been
treated. The width of the disk or harrow makes it
possible to cover a large area in a short time, and
thereby considerably lengthen the period during which
plowing can be satisfactorily done, as well as conserving
moisture for the succeeding crops.

To summarize briefly the cardinal points in mulch
control: (1) They are more effective and more easily
maintained in an arid than in a humid climate. (2) Their
efficiency depends directly on their dryness and fineness.
(3) Sandy soil is more easily maintained as mulch
than clay soil. (4) From two to three inches is ordi-
narily the most effective depth. (5) After heavy rain,
the soil mulch must be renewed by tillage, and this is
much more urgent on clay than on sand soil. Even
without rain, a clay mulch may become inefficient.
(6) Tillage for mulch purposes must ordinarily be more
frequent in the spring, or humid season, than at other
times of the year. (7) The use of foreign materials
as mulch may be justified under special circumstances.

97. Fall and spring plowing. — Fall and early spring
plowing owe much of their efficiency to the conservation
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 rainfall,
particularly in the 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 the semi-arid section that do not blow
too badly when fall-plowed, and where the winter rain

FALL AND SPRING PLOWING 211

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 amount 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 much more
important. Not only may moisture be conserved, but
the soil is worked at the stage when it yields most
readily to pulverization. Fallow land, and bare stubble
land of fine-textured soil, are most benefited, since they
become 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 station, two adjacent pieces of land
very uniform in character were plowed seven days apart.
At the time 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 earlier plowed
piece had actually gained, doubtless by increased capil-
larity, a slight amount of water over that it contained
when plowed. There was a gain of nearly two inches
of water in the root zone as a result of plowing one week
earlier, enough to produce 1,500 pounds of dry matter
in maize per acre, if properly conserved.

In arid and semi-arid regions, and in other sections
where heavy soil is plowed in the late summer, and
especially where a large crop of green manure or a large
application of coarse strawy manure is plowed under
at any season, it is essential that the lower part of the

212

THE PRINCIPLES OF SOIL MANAGEMENT

furrow slice be brought into close contact with the
subsoil as soon as possible, in order, (1) that the' best
possible capillary contact with the subsoil may be estab-
lished; (2) that there may be sufficient moisture to
promote the rapid decay of the organic matter; (3) to
increase the moisture capacity and cut down loss by
percolation and evaporation. This may be accomplished

Fig. 67. The Campbell subsurface packer

by rolling the plowed land, but in particularly dry
regions the practice of sub-surface packing is advan-
tageous. The aim of sub-surface packing is to pack the
soil and still leave a loose mulch on the surface. The sub-
surface packing may very well be applied to land sub-
soiled in the spring. Land subsoiled in the fall will not,
as a rule, require this treatment, — certainly not in the
humid sections of the country. To accomplish sub-
surface packing, a special group of implements have
been devised, one of which consists of small wheels

SUB-SURFACE PACKING 213

placed five inches apart on an axle. The rim is much
thickened and is triangular in shape, with the thin
edge outward, so that the effect is to give a decided
downward and sidewise pressure, while enough fine
earth is left at the immediate surface to serve as a mulch.
98. Other surface treatments. — Other surface treat-
ments aim to decrease the tendency to evaporation.
When evaporation takes place into a quiet atmosphere,
the layer next to the soil soon becomes so nearly satu-
rated with moisture that the rate of evaporation is
greatly reduced. But if the atmosphere is in free circu-
lation,— that is if there is wind, — the saturated air is
removed, and more dry air is brought over the soil into
which evaporation is continuous. The drying effect of
wind is very generally recognized. Warm winds in
spring and early summer are recognized as particularly
drying, and in the semi-arid section just east of the
Rocky mountains so-called hot winds sometimes do
great damage to growing crops by the rapid evaporation
they produce. Obviously anything which reduces the free
circulation of the air — "breaks the wind" — will reduce
evaporation. In practice, this takes the form of wind-
breaks of various types. Strips of timber are commonly
grown or retained for this purpose. Wooden fences
and walls of one sort or another have a similar effect.
Wind-breaks composed of growing plants have the
disadvantage 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. But artificial shelters
do not have this advantage. Bearing on the efficiency of

214 THE PRINCIPLES OF SOIL MANAGEMENT

wind-breaks, results by King show that when the rate
of evaporation at 20, 40 and 60 feet to the leeward
of a black oak grove 15 to 20 feet high was 11.5 cc, 11.6
cc, and 11.9 cc, respectively from a wet surface of 27
square inches the evaporation was 14.5, 14.2 and 14.7
cc, at 280, 300 and 320 feet distant, — or 24 per cent
greater at the outer stations than at the inner ones. A
scanty hedge-row reduced evaporation 30 per cent at
20 feet, and 7 per cent at 150 feet, below the evapora-
tion at 300 feet from the hedge.

On sandy soil, wind-breaks prevent the blowing of
the dry surface soil, which would expose a fresh surface
of wet soil from which evaporation would be increased.

The glass house reduces evaporation by preventing
winds. Some crops are grown only in the shade of other
crops, where they are not only protected from the sun
but from evaporation by the stagnating effect of the
surrounding vegetation on the atmosphere. Grass pro-
tects the surface of the soil from evaporation, acting
like a mulch. The largest application of this principle
is in the tents used in growing wrapper tobacco in Florida
and the Connecticut valley, and, to a less extent, for
other special crops in various parts of the country. The
most common form of the tent is a frame eight or nine
feet high, over which is spread a loosely woven cloth —
cheese-cloth. Investigations by Stewart 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 per hour,
and 85 per cent when the outside velocity was twenty
miles per hour, there being a small regular decrease in

WINDBREAKS AND TENTS TO SAVE WATER 215

relative efficiency with increased velocity of the wind.
(2) The relative humidity under the tent was higher
than outside, and during a good part of the time
attained a difference of 10 per cent. The effect of this
was to reduce evaporation, by from 53 to 63 per cent
on different days in July, in spite of a higher tempera-
ture inside the tent. (3) The direct effect of these 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, which represent the
per cent of water in the soil to a depth of nine inches
from June 13 to August 1.

20

IB

10

^^]

v^--^^ ,--

Vv,-" \ INSIDE

j^-J

\

A

JUNE 13th

65.U0 .10-1.54-.17

DATE

.22-.03 .17 .44

INCHES RAIN

AUGUST 1ST
.24 .2-.16-.10

Fig. 68. Curves representing the per cent of moisture in a sandy soil to a
d^pth of nine inches inside and outside of a loosely woven cloth tent, July 13
to August 1, Western Connecticut.

Not only was the effect of the tent to prevent evapo-
ration and thereby increase the average moisture con-
tent of the soil, but the soil was able to maintain a more
uniform content, due to the more free movement and
adjustment of the capillary water under the tent — con-
ditions most conducive to rapid crop growth. (See
page 172.)

216 THE PRINCIPLES OF SOIL MANAGEMENT

The velocity of the wind next to the ground may be
checked by ridging the soil. It is doubtful if this prac-
tice conserves moisture, because more 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-toothed culti-
vators. Many experiments have demonstrated the
larger crop yields to be obtained from this practice,
on the average.

The removal of weeds has been mentioned as a means
of conserving moisture. The plants serve to expand
evaporating surface in the same way as ridged culture.
(See page 195.)

99. Increasing the water capacity. — Increasing the
water capacity of the soil may be effective in conserving
soil moisture by holding more of the water which falls.
The first aim should be to get the rainfall or irrigation
water into the soil. It is well known that after a long
dry period when the soil — particularly a fine-textured
soil — has become dry and hard, the first rainfall may
be largely lost by running away over the surface. Sudden
showers are almost entirely lost in this way, because
not only is the water repelled, but the small amount which
is absorbed is held so near the surface that it is quickly
lost. Gentle rains are usually much more effective

INCREASING WATER CAPACITY

217

than sudden showers in soaking up the soil. On the
other hand, if the soil is loose and porous, all the water
which is applied sinks into the soil and may percolate
deeply. It is this condition which should be maintained.
Correlated with the loose surface soil is the rough surface
maintained in level sections of strong winds, where
a considerable part of the precipitation falls as snow.
A rough surface holds the snow against blowing, and
upon melting in the spring it enters the soil.

The moisture taken up by the soil should be retained
and conserved by appropriate cultivation. It will be
apparent from the principles which have been outlined
that all soils may not be managed in the same way,
to increase their moisture capacity. In some the end
is accomplished by loosening the structure, and in others
by compacting the structure. Cultivation, the roller,
the subsoil plow, or fall plowing, are to be adopted
in so far as they accomplish the desired result on the
particular soil in hand. The opposite effect of the same
treatment on different soils is shown by the following
figures.

Table XXXVIII

Soil

Condition

Amount of

water

taken up

Per cent of

water
taken up

Clay loam -J

Silt loam <

Medium sand •]

Compact . .

Loose

Compact . .

Loose

Compact . .

178
85

182
99

158

147

43.6
23.8
44.1
15.3
26.8
23.2

218 THE PRINCIPLES OF SOIL MANAGEMENT

The proportionate increase in the water capacity
of the sand and decrease of the clay loam is here well
shown, and doubtless, if the column had been longer,
the compact sand would have had a greater absolute

capacity than when
loose.

Deep plowing is
greatly to be re-
commended as a
practice to increase
the moisture capa-
city of the soil,

Fio. 69. subsoiier that loosens the subsoil by particularly where
raising and breaking it. organic matter is

well supplied. It creates a deep soil, and should estab-
lish 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 or granulating effect.

The practice of subsoiling aims to locjsen up the struc-
ture of the deep subsoil without turning the material
to the surface. It increases the ease of root penetration,
the rate and depth of percolation, and on clay soil it
increases the water capacity. Subsoiling is unnecessary
and may even be injurious on sandy soil, and on clay
soils must be used with discretion. It is difficult to
secure the proper moisture condition of clay subsoils
for plowing in the spring in time for spring planting.
The soil may be in good working condition, or even dry,
while the subsoil is wet enough to puddle. On the other

SUBSOILING FOR MOISTURE CONTROL

219

hand, if the subsoil gets dry enough to break up, it may
remain so loose and lumpy during the remainder of
the season that capillarity is largely destroyed, and
crops suffer from shallow rooting and lack of moisture.
Decrease in crop yields as a result of subsoiling in spring
are frequently reported. On the other hand, subsoiling
in the fall, although usually more difficult to accomplish,
is more likely to result in benefit. The cloddy condition
which may be developed is largely broken down in re-
gions of heavy winter rain by the saturated condition.
Still the structure does not become nearly so compact
as before the treatment, and good results.

King presents figures which show that, as a result
of the application of 1.34 inches of water, the soil which
had been subsoiled to a depth of twenty-one inches
retained, after a period of four days, 65.6 per cent more
water in the surface four feet than the adjacent land not
subsoiled.

Not only is sub-
soiling effective to
increase the abso-
lute water capa-
city, but it may
strengthen the
capillary or film
movement to such
an extent that an
important amount of water is drawn up from the deeper
subsoil or from adjacent zones not so treated. A
"hardpan" layer below the plow depth may seriously
interfere with the upward movement of water and the

Subsoiler that loosens the subsoil by
breaking through.

220 THE PRINCIPLES OF SOIL MANAGEMENT

penetration of roots. This condition may be largely
corrected by subsoiling.

Coupled with deep plowing and subsoiling, subsur-
face packing is often very beneficial. Particularly is
this true in early fall and late spring plowing, where
the soil is likely to be cloddy and to make poor capillary
contact with the subsoil. Spring crops may be greatly
injured by this condition. The subsurface packer crushes
the clods, presses the furrow slice down more firmly
on the subsoil without compacting the surface soil.

Fig. 71. "Clod crasher" and sub-surface packer.

It leaves a light mulch on the top to hold moisture.
Not only is it useful in improving the soil structure
under the conditions just mentioned, but it promotes
the decay of organic manures and assists plant roots
in penetrating into the subsoil below, where they may
have a larger moisture and food supply.

Increase in the humus content stands next to modifi-
cation in texture and structure as a means of increasing;
the water capacity of the soil in accordance with the prin-
ciples explained on pages 144 and 153. It accomplishes

IRRIGATION 221

this not only through its own large water capacity,
but by its favorable influence on the structure of the
soil. It should be worked deeply into the soil, in order
that its many beneficial effects may be brought to bear
on as large a volume as possible. It is especially favored
as the adjunct of deep plowing and the use of lime for
improving soil condition, particularly clay soil.

The means for increasing the organic content of the
soil have been discussed. (See page 131.) They include
the application of animal manures and other refuse,
and the growth of crops for green manure, together with
that crop rotation which promotes the accumulation
of crop remains, and that type of farming which removes
the smallest proportion of the crop from the farm and
returns the largest proportion to the soil.

100. Irrigation. — Irrigation is the third method by
which the soil moisture may be increased. It is the prac-
tice of directly adding water to the soil, to supplement
the natural rainfall. It is chiefly identified with the
arid and semi-arid sections of the country, where the
annual rainfall is small. It is customary to consider a
region as having a semi-arid climate when the rainfall
is between ten and twenty inches, and arid when it is
less than ten inches. These limits are arbitrary and
necessarily elastic, because the actual aridity of a region
depends on other factors than the total annual rainfall.
It depends on the distribution of the rainfall, the climate,
particularly temperature, and the character of the soil.

While irrigation has been chiefly identified with arid
and semi-arid sections (see map, page 137), it is not
limited to those regions, and is applicable under any

222 THE PRINCIPLES OF SOIL MANAGEMENT

condition where the natural rainfall is deficient at any
period of the growing season. Consequently, irrigation
is practised even under the very humid climate of
Florida, with sixty inches of rainfall, around New
York City and Boston, with forty inches of rainfall, and
at many other places in the United States and Europe,
where a so-called humid climate prevails. In these latter
places it is identified with special crops of high value
which will justify the expense involved. In France, Ger-
many and other European countries, there are extensive
areas of grass land which are artificially watered, often
with sewage, which adds the element of food supply as
well as water. Of course, all greenhouse management in-
volves the practice of irrigation.

Many engineering problems are involved in the prac-
tice of irrigation, and have to do with the collection,
storage and application of water to the land. But the
principles which govern the application — the method,
time and amounts of water — suitable for each crop and
soil are purely agricultural considerations, to be han-
dled in each case as the local conditions may indicate.

The amount of water necessary to be added to pro-
duce a full crop constitutes the "duty," or efficiency,
of water. It is the least amount of water which will
produce a given yield under a given set of conditions.
The "duty of water" depends upon a great many
factors; in fact, is limited by as many things as affect
the moisture supply of soils in humid regions. The dis-
cussion of irrigation which follows presupposes an ade-
quate supply of water, a condition often not fulfilled.
For example, the area of the Western States containing

CONDITIONS REQUIRING IRRIGATION

223

public lands is 973 million acres, of which Newell esti-
mates that about 70,000,000 is of a desert character.
At the present time, irrigation is practiced on less than
1 per cent of this area, and the total water supply is
estimated to be sufficient for less than 10 per cent of the
total area.

Fig. 72. Map of the western portion of the United States, showing in black
the irrigated land, and in dots the area which may be irrigated if all the avail-
able water supply is utilized .

224 THE PRINCIPLES OF SOIL MANAGEMENT

101. Factors affecting the duty of water. — Eleven
factors, as follows, affect the duty of water in irriga-
tion:

(1) The peculiarities of the crop (see page 134).
Some crops require much more water than others for
their growth and maturity. Even certain varieties
may require much more water than others of the same
species.

(2) The physical character of the soil. If the applica-
tion of water is such that leaching may take place,
more water will be lost through sand than through clay.
The character of the soil also determines the effective-
ness of the mulch, which may be maintained.

(3) The character of the subsoil.

(4) The frequency of irrigation.

(5) Amount and distribution of the rainfall. These
last two factors are closely related in their effect on the
duty of irrigation water. Their frequency determines
the proportion of the water which will be lost by surface
evaporation. (See page 198.)

(6) The amount and time of applying water. Water
applied in the evening will be more efficient than when
applied in the morning, because during the cool night
it will have opportunity to diffuse deeply into the soil,
where the hot sun of the following day will have less
effect upon it than if the water were applied in the
morning.

(7) The climate. Other things being equal, more
water will be required in a warm, windy climate than in
one of a cool, quiet atmosphere. This factor, of course,
largely determines the rate of evaporation.

AMOUNT OF WATER USED IN IRRIGATION 225

(8) Method of applying water. The furrow system
is usually more economical of water than the flooding
system, because less opportunity is given for evapora-
tion.

(9) The fertility of the land, as distinguished from its
physical properties, determines the duty of water
through its influence on the size of crop which may be
produced. A large crop is more economical of water
than a small one, but a large crop will require, a larger
total amount of water.

(10) The closeness of planting affects the loss of
water in much the same way as a large or a small crop:
(a) By determining the total amount of water which
must be used directly by the plants; and (b) by shading
the ground and cutting down temperature and wind
movement more or less, it decreases the loss of water
directly from the soil.

(11) The tillage practice affects the efficiency of
water under irrigation as it does the efficiency of rainfall
in humid regions. If lax conservation methods are used,
much more water will be needed than where the best
tillage processes are applied.

For these reasons, it is not possible to specify any
definite amount of water which should be used in the
practice of irrigation. It varies widely for different sec-
tions of the world and, since it is very common to meas-
ure the total amount of water supplied at the head of
the intake canal, it is largely determined by seepage from
the canals and ditches. (See page 134.) The amounts
of water which are applied in different irrigation sec-
tions are given by different authorities as follows:

226 THE PRINCIPLES OF SOIL MANAGEMENT

Table XXXIX

Northern India

Italy

Idaho

Utah

San Joaquin Valley, California
Santa Clara Valley, California

Acres irrigated
per second-foot
of water used

Equivalent to

inches

per ten days

60-150

3.96-1.580

65- 70

3.66-3.-400

60- 80

3.97-2.980

60-120

3.97-1.980

100-150

2.38-1.580

150-300

1.58- .798

In Sefi, on the lower Nile canals in Egypt, one second-
foot is said to be sufficient for 350 acres, as managed.
In the humid regions much less water need be added
by irrigation, and is necessary only to supplement the
rainfall in the drought periods — to fill in the gaps. Ordi-
narily only a few inches per season are needed, usually
toward the latter part. Dr. Yoorhees has compiled the
following figures, which show the percentage of years
in which there was a deficiency of one inch or more per
month in the rainfall, as compared with the average.

Table XL

New York, 1836-1895 . .
Philadelphia, 1868-1895

One month

Per cent
75

88

Two months

Per cent
42
56

Three months

Per cent
21
30

In this region, the deficiency is most likely to occur
in the summer season. The records show that during
one-fourth of the term there is a deficiency of rainfall
covering three months. Considering the monthly rain-

UNITS OF WATER MEASUREMENT

227

fall to be from two to three inches, a deficiency of one
inch amounts to from one-half to one-third of the total,
which must be a serious hindrance to crop growth,
without the most careful soil management.

On light, sandy soils, and with careless tillage in
general, the above figures indicate that there may fre-
quently be occasion for irrigation. The annual rainfall

Fig. 73. Flume for measuring miner's inches.

is ample for full crop production, if it could all be
utilized.

Many units are used in the measurement of water
for irrigation. The two most common methods of stating
the quantity of water used are: (1) In depth of water
over the area, as acre-inches or acre-feet. (2) A given-
sized stream flowing through the growing season. The
two most common units under the latter system are the
second-foot and the miner's inch. It is frequently esti-
mated that a flow of a second-foot of water — one cubic

228 THE PRINCIPLES OF SOIL MANAGEMENT

foot per second — through a growing season of ninety
days, is sufficient to irrigate one hundred acres. This
is sufficient to cover the area 21.3 inches deep, and is
equivalent to a little over seven inches per month.
(See pages 135 and 137.)

The miner's inch varies in value in different sections.
It is most commonly defined as the amount of water
which will flow from an opening one inch square under
a pressure-head of six inches above the top of the orifice,
during a year, and is considered sufficient to irrigate
from 5 to 10 acres. It is equivalent to about 1.5 cubic
feet per minute, or 21.6 inches over 10 acres in a season,
which, it will be observed, is practically the same appli-
cation as one second-foot, as stated above.

102. Methods of applying water. — In his book on
Irrigation and Drainage, King makes the following-
cogent statement with reference to the application of
water in irrigation practice. "When water has been
provided for irrigation, and brought to the field, where
it is to be applied, the steps which still remain to be
taken are far the most important in the whole enter-
prise,— not excepting those of engineering, however
great, — which may have been necessary in providing
a water-supply that shall be constant, ample and moder-
ate in cost; for failure in the application of water to the
crop means utter ruin for all that has gone before."

"To handle water on a given field so that it shall be
applied at the right time, in the right amount, without
injuring the crop, requires an intimate acquaintance
with the conditions, good judgment, close observation,
skillful manipulation, and patience after the field has

METHODS OF APPLYING WATER 229

been put into excellent shape; and just here is where a
thorough understanding of the principles governing
the wetting, puddling and washing of soils, and possible
injury to the crop as a result of irrigation, becomes a
matter of the greatest moment." (See page 103 et seq.)

Mead reports that there are over thirty methods of
distributing water in use in the United States. Each
of these has its special adaptations as to soil, crop,
water supply, climate and land contour. All of these
methods may be grouped under four general heads, the
further differences being in detail of application and
not in essential principles.

These are: (1) Flooding. (2) Furrow distribution.
(3) Overhead sprays. (4) Sub-irrigation.

103. Flooding. — Flooding is practiced in several
ways, and is applied to a much larger area than any other
system. There are two fundamentally different types
of flooding: (1) One covers the surface of the soil with
a thin sheet of flowing water, maintained until the
desired degree of saturation has been reached. (2) The
other covers the surface with a sheet of standing water,
which is allowed to remain until the soil is sufficiently
saturated, when any balance is drawn off, or may be
dissipated by percolation through the soil, as is fre-
quently though unwisely done.

The former system corresponds closely with what is
termed wild flooding, where the water is distributed by
a minute dendric s}^stem of ditches, and the remnant
gathered by a reversed dendric system of ditches, or by
a head ditch at the foot of the slope. The essential
point is to keep a thin sheet of water moving over the

230 THE PRINCIPLES OF SOIL MANAGEMENT

land until the soil is saturated. The second system
agrees with check flooding, in which the water is turned
on a nearly level area to a considerable depth. The
check, or block, may be a small area — a few square rods
on a decided shape, or a large area is possible on very
level land. These may be so arranged that the water
flows successively from one to the other, perhaps at
successively lower levels. The relative advantages of the
two types depend on the character and slope of the soil.
On gently sloping land of moderately porous character,
and not easily washed or puddled, so that the water may
be controlled, wild flooding is the most convenient
method. Grain fields especially lend themselves to the
method. On the other hand, on very level or very steep
land the block type must be used. The water is more
definitely under control, washing is largely prevented
by levees, and puddling is reduced by the almost entire
elimination of current.

The flooding system is best adapted to certain classes
of crops, as follows: (1) Grain fields. (2) Meadows and
hay fields. (3) The soaking of land preliminary to plant-
ing other crops, sometimes termed winter irrigation,
where the water-supply is available only in the winter
season, and is stored in the soil until crop-growing time.
The above crops are adapted to occasional or intermit-
tent flooding; but some crops succeed best under a con-
tinual flood of water, as in: (4) Rice culture and (5)
Cranberry culture. A phase of the flooding system is
the basin system sometimes used in orchard irrigation.

The advantages of the system are: (1) Ease in hand-
ling water. (2) Economy in irrigation works. (3)

IRRIGATION BY FLOODING AND FURROWS 231

Avoids necessity of tearing up the crop to form large
irrigation furrows.

The objections to its use are: (1) The large amount
of water required. (2) The danger of over-irrigation,
with the possible consequent injury from seepage,
and the appearance of alkali salts. (3) The impossi-
bility of conserving water by appropriate cultivation.
(4) On heavy soils possible injury from the crusting
and checking of the surface soil as a result of the lack
of tillage. (5) Direct injury from flooding some crops,
as the potato.

104. Furrows. — Furrow distribution, by which, as
the name implies, the water is not applied to the whole
surface but is distributed in furrows. The length, size
and arrangement of these depends directly on the
soil, chiefly its texture. This includes the subsoil as well
as the soil. In soils which are porous or easity eroded,
the furrows must be shorter than where the opposite
9onditio"ns prevail, in order that the water may reach
the further end of the field before over-wetting the por-
tion near the head ditch. That is, in loose, porous soil,
head or feeder ditches must be nearer together than on
dense, impervious soil.

The furrow system is adapted to all intertilled crops.
Next to the flooding system, it is used on the largest
area, and is adapted to all intensively cultivated crops.
Its advantages are that: (1) It conserves water. (2)
It is especially adapted to inter-tilled crops. (3) It-
permits the conservation of water by appropriate cul-
tural practices. (4) It avoids injury to crops sensitive
to an excess of water. Water should not come in contact

232

THE PRINCIPLES OF SOIL MANAGEMENT

OPENING 6X8"

with the trunk of trees, or, in general, with the stem of
any plant not well shaded. A bright, warm sun in con-
junction with the excess of
water is usually injurious.

(5) It is the more convenient
method to apply to the class
of crops to which it is adapted.

(6) It more readily permits
the avoidance of the injuries
due to seepage by avoiding
the losses to which that is
due. (7) It assists in the con-
trol of alkali soils by permit-
ting tillage.

The supply of soil mois-
ture by capillarity is most
satisfactory to the majority
of cultivated crops, and by promoting this the furrow
system generally gives better results than flooding.

The flooding system has some disadvantages: (1)
It is not so economical of water as is to be desired.
(2) Much attention must be given to forming the furrows,
to the construction of head or supply ditches, to the
collection of the overflow water at the end of the furrows,
and in the general supervision of the flow of the water
over the land to repair broken levees, etc. (3) The water
is not applied uniformly. The head of the furrow invari-
ably becomes more wet than the lower end. (4) Erosion
and puddling occur very readily in cultivated furrows.

105. Overhead sprays. — Overhead spray is used only
on very limited areas, and almost entirely in humid

Fig. 74. Canvas dam with
opening to divide the water in an
irrigating furrow.

IRRIGATION BY SPRAYS 233

sections. It has been applied in the growth of Sumatra
wrapper tobacco in Florida, and of truck crops near
New York, Boston and other large cities. It is therefore
used as a very limited supplement to the regular rainfall.
It is accomplished by the use of a very thorough piping
system with spray nozzles at sufficiently frequent inter-
vals to cover the area. These are connected with a rela-
tively large pressure-head of water — at least five pounds
is necessary.

The advantages of the system: (1) Economy in the
direct application of water to shallow rooted crops.

(2) Convenience in applying water at the desired point.

(3) Absence of injury from erosion or puddling the soil.

(4) No land wasted in irrigation ditches. (5) Natural
climatic conditions developed by such irrigation.

The disadvantages of the system are great: (1) The
large initial cost of the plant. (2) The high operating
expenses ordinarily necessitated to develop the pressure
necessary to distribute water from the nozzles, and to
maintain the system. (3) The limited capacity of the
system. (4) The large evaporation from the spray in
the atmosphere, and from the soil and surface of the
plants.

The spray system is practicable only with special
crops under peculiar conditions.

106. Sub-irrigation. — Sub-irrigation often occurs
naturally. It is the application of water beneath the
surface of the soil. The structure of the land is such
that on many low benches and in river bottoms the
percolation of water through the soil and fissures of
the rock brings it near the surface at these lower levels,

234 THE PRINCIPLES OF SOIL MANAGEMENT

where it maintains a fairly constant supply of water to
those crops which may be growing on the surface.
The ground water is so near the surface in some stream
bottoms, lake shores, etc., that this condition prevails.
Soils ordinarily poor in their moisture relations become
highly satisfactory in such cases. Sandy land is almost
ideal in its crop relations, so far as moisture goes, under
such conditions.

In a limited way it has been attempted to irrigate
the soil from beneath the surface by forming under-
ground channels of porous pipe, properly graded, into
which irrigation water may be turned, which should
diffuse through the soil by percolation and capillarity.

In some situations, as lawns, truck and fruit gardens,
it may be possible to install a drainage system of tile,
which may also serve as a means of irrigation.

The system has a number of advantages, which in
ordinary practice are more than offset by its disad-
vantages. Its advantages may be summarized as fol-
lows: (1) It is very economical of water. (2) In alkali
soil it greatly reduces the surface accumulation of
alkali. (3) It insures deep rooting of the crop. (4) It
avoids waste land. (5) It avoids injury to the physical
condition of the soil. (6) Involves very little super-
vision in the application of water. (7) Possibility of
the use of the system for drainage purposes.

Its disadvantages are: (1) The strong tendency of
roots to enter and clog the pipes. (2) The slow diffusion
of water by capillarity in dry soil. (3) The expense
involved in the installation of a system of pipes ade-
quate to irrigate most soils.

SUB-IRRIGATION 235

Plant roots seek the most moist soil which is short of
saturation, and therefore they are drawn toward and
tend to concentrate around and in the lines of tile,
just as roots are found to do where drain tiles carry living
water through dry soil. This is the greatest disadvantage
of the system. Especially is this true in orchard work.
It is more adapted to shallow-rooted annual crops, and
to soils of strong rapid capillary power, such as fine
sand and coarse silt loam or loam soil.

The amount of water to be added at one time must
be determined chiefly by the texture and structure of the
soil, — or more specifically its water capacity, — and the
supply of water available. Under arid conditions, it is
generally advisable to apply as much water as can be
held within the root zone by capillarity without loss
from percolation. Frequent small applications should
be avoided, because of the large proportionate loss from
surface evaporation. (See page 197.) Also, there is a
stronger tendency to the accumulation of alkali salts at
the surface, because of the larger evaporation. On the
other hand, less frequent large applications of water,
particularly under any but the flooding system, where
a crop occupies the land, permits the creation and main-
tenance of a mulch to conserve moisture; besides which,
the deep distribution of the water insures a deep distri-
bution of the roots, where they are not only in contact
with a larger moisture reservoir, but also with a larger
food-supply than is available to shallow-rooted plants.
It is a fact of common experience that in arid regions
crops generally root deeper than in humid regions.

A common accompaniment of irrigation, certainly

236

THE PRINCIPLES OF SOIL MANAGEMENT

in semi-aricl and arid regions, is the excessive accumu-
lation of soluble salts — "alkali salts" — in the soil.
They may become so concentrated as to injure crops or
prevent their growth. (See page 307.) In the original
condition of such soils they are usually distributed in
relatively small amounts through a deep section of soil.
But by excessive irrigation, which produces seepage and
a general rise in the water-table, aided by those careless

tillage methods which
permit free evaporation
at the surface, these
soluble salts become con-
centrated in the root
zone, and at the surface
as an alkali crust. It
has frequently happened
that land not originally
in a seriously "alka-
line" condition has be-
come so by careless
management. It is obvious that to avoid this injury
there must be (a) conservative irrigation, and (6) the
most thorough tillage methods which shall avoid surface
evaporation. Where an excess of akali salts exists,
they are most successfully removed by means of a deep
thorough drainage system, coupled with heavy irrigation
which shall wash out of the soil the excess of salts.

It is a safe and wise rule to cultivate the soil as soon
after applying water as its moisture condition will per-
mit without injury, and this should be kept up at fre-
quent intervals until an effective dust mulch has been

Fig. 75. Middle breaker plow. Some-
times used in constructing irrigation and
drainage ditches.

PRECAUTIONS IN IRRIGATION 237

3reated. It has been noted (page 204) that in arid
-egions soil mulches are relatively more efficient and
more easily managed than in humid regions.

Soils of intermediate fineness lend themselves most
■eadily to the practice of irrigation. Excessively heavy
;lay is generally to be avoided, because of (a) the slow
liffusion of water, by both capillarity and percolation,
mtl (b) the danger from puddling after an irrigation,
inless cultivation is delayed so long that a large amount
->f water is lost. On the other hand, very light sand should
)e avoided because of its leachy character, and the great
oss of water by percolation or surface evaporation,
,he former, if a large amount of water is added at once;
/he latter, if it is added very frequently.

But in humid regions it is wise to practice irrigation
'or crops easily injured by an excess of water except on
;hose light and porous soils which have thorough
Irainage, because of the possibility of a rainfall following
closely upon the application of water, thereby rendering
he soil over- wet, to the injury of the crop. On the porous
Soil the excess quickly drains away. In the Sumatra
-obacco region of Florida, for example, where there is a
arge rainfall, irrigation has been found successful only
lpon the lighter sandy loam and sand soils. This crop
s particularly sensitive to an unfavorable soil condition,
rhen too, the heavy soil, the clay loam, or clay, has a
arge water capacity, which makes possible the storage
)f a large amount of water against the needs of the crop-
growing season. Consequently it is on these latter that
lry farming of grains is most generally practiced in the
iiVestern states.

238 THE PRINCIPLES OF SOIL MANAGEMENT

As the demand for produce of high value increases,
the maintenance of the moisture supply of the soil by
irrigation may well be extended on large areas of soil
in so-called humid regions, as well as in arid sections.

The highest type of soil-management must seek to
utilize the available water-supply for crops in the three
ways outlined above, that is, by increasing the water
capacity of the soil, by eliminating as far as possible

Fig. 76. An example of poor drainage on level clay soil.

the losses by percolation and evaporation and, lastly,
by supplying any deficiency which may still exist by

wise irrigation.

107. Means of decreasing the water content of the
soil. — The removal of water from the soil may be
accomplished in two general ways. These depend upon
facilitating the two types of loss, by percolation and
evaporation, described on page 191. They are: (1)
Drainage. (2) Surface culture, to hasten evaporation.

108. Drainage by ditches. — Drainage consists essen-

DRAWAGE 239

tially in the direct removal of the gravitational water
from the root zone of the soil by affording free passages
for its percolation and flow. In general, the soil condi-
tions requiring drainage may be divided into two groups,
which are fairly distinct in the problems which they
present. These are: (1) Those lands which are satu-
rated with water throughout the year. (2) Those lands
which are saturated with water for only brief periods.
Into the first group are placed all those lands of an
acknowledged swamp character, which not only retain
a large part of the water which falls upon their own sur-
face, but may receive the water which flows from other
lands. Into the second group is put all those wet lands
which are saturated for a sufficient period to interfere
with the best condition of the soil, or the proper develop-
ment of the crop. It represents a very mild or incipient
stage of the conditions included in the first group.

In the manipulation of soil for the staple upland
crops, the establishment of effective drainage is at the
foundation of all the other practices which must be
employed. If it does not exist, the other farm practices,
such as tillage, fertilization etc., can not be applied
effectively.

An excess of water in the soil has many and far-reach-
ing effects upon the soil as a medium for plant growth,
especially if this condition is intermittent. The manage-
ment of the latter condition is even more crucial than
the former.

109. Effects of drainage. — Twelve of the most
important effects of drainage are as follows: (1) Firms
the soil. (2) Improves the granulation. (3) Increases

240 THE PRINCIPLES OF SOIL MANAGEMENT

the available moisture capacity. (4) Improves the
aeration of the soil. (5) Raises the average temperature.

(6) Promotes the growth of desirable organisms.

(7) Increases the available food supply. (8) Enlarges
the root zone of the soil. (9) Reduces "heaving."

(10) Removes injurious salts from ''alkali soils."

(11) Reduces erosion. (12) Increases crop yields, and
improves sanitary conditions of the region.

110. Firms the soil. — In a saturated soil the particles
are held apart and are partially floated by the water,
with the result that they afford a poor support for plants,
and are largely unable to bear the weight of travel
incident to cultural operations. Heavy objects sink
into the surface, and become mired as a result of the easy
movement of the soil particles from beneath their
weight. This movement is greatly facilitated by the
lubrication afforded by the water between the particles.
It is because of this freedom of movement that a wet
soil may readily be "puddled," that is, the small par-
ticles moved into the spaces between the large ones,
producing a more dense mass, a change not possible
in dry or even moderately moist soils.

111. Improves the structure. — Drainage improves
the granular structure of fine-textured soil. One of the
most important factors in soil granulation is alternate
wetting and drying. (See page 105.) In a wet soil, this
drying and drawing together does not take place. On
the other hand, if a granular soil be kept saturated, the
crumb structure will be broken down and a bad physical
condition results. This is well illustrated by the fact
that nearly all swamp soils are in a puddled, or otherwise

DRAINAGE AND SOIL STRUCTURE

241

bad physical condition, when first drained. Drainage
brings to bear upon the soil all those natural agencies
which promote the granular arrangement. In turn,
the granular structure, particularly in fine-textured

Fig. 77. Section of a 20-year-old tile drain in heavy clay soil. Note the
more open structure above the drain.

soil, affects the movement and capillary retention of
water, the circulation of air, the growth of organisms,
the temperature of the soil, and other conditions depend-
ent on these, in a manner highly beneficial to the crops
generally grown.

112. Increases the available water.

-Drainage in-

242 THE PRINCIPLES OF SOIL MANAGEMENT

creases the available moisture capacity of fine-textured
soil. This is accomplished through the better granulation
and larger porosity which results. The possibilities in
this direction are indicated by the effect of structure
on the moisture capacity of the soil. (See page 151.)
Field experience has many times shown this result to
follow drainage. Instead of plants suffering from lack
of moisture, as a result of drainage, it is found that they
are not only free from the excesses, but that in dry
periods the soil is likely to contain more moisture than
the same kind of soil under poor drainage. This is especi-
ally true of those soils which are wet only a part of the
season. They are subject to great extremes in moisture
content.

113. Improves the aeration. — Drainage improves
the aeration of the soil in two ways. (1) It removes
the gravitational water from the large pores, thereby
permitting the admission of air. (2) Through its effect
on granulation it permits the soil to hold a larger volume
of air and facilitates its circulation. This also is due to
two conditions, especially where the drainage is beneath
the surface. The larger pores resulting from granulation
greatly aid the process. And the underground passages,
formed by tile or other media, afford channels for the
escape of soil air following rain or reduction in baro-
metric pressure, and facilitate its readmission when the
opposite conditions prevail. The net result is a much
larger total change between the outer air and the soil
air. This reacts strongly upon the soil organisms and
upon the general chemical activity of the soil.

114. Raises the average temperature. — Drainage

DRAINAGE AND SOIL TEMPERATURE 243

raises the average temperature of the soil. The specific
heat of water is much higher than that of soil, and there-
fore the larger proportion of water a soil contains the
more heat is required to increase its temperature.
(See page 461.) Further, in a wet soil the surface evapo-
ration is large, and since the evaporation requires several
hundred times as many units of heat as is necessary
to raise the same volume of water from the normal
temperature to the boiling point, it is clear that the
process must consume a large amount of heat. But the
heat supplied to any given area of soil is fairly uniform,
and consequently, if it is used up in evaporating water,
it is not effective to raise the temperature of the soil
mass. If the soil contains water which must be removed
by evaporation, its temperature will be kept correspond-
ingly low; or, what is the same result, the time required
to warm the soil will be correspondingly extended.
For this reason a wet soil is a "late soil," while a well-
drained soil is much "earlier" in attaining the tempera-
ture necessary for the germination and growth of plants.
The practical result of this rapid warming of a well-
drained soil is to lengthen the growing season by per-
mitting its earlier seeding in the spring, and the later
growth of crops in the fall. In some sections of the world,
this margin in the length of the growing season deter-
mines the growth of certain crops, and materially affects
all crops. All of the activities of the soil, both chemical
and biological, are favorably affected by the higher
temperature. In the peat bogs of England, Parkes found
that at a depth of seven inches the drained soil was 15°
warmer than the undrained soil, and at thirtv-one

244 THE PRINCIPLES OF SOIL MANAGEMENT

inches it was 1.7° warmer. King reports the frequent
observation of a difference of 12° between the tempera-
ture at the surface of drained and undrained land.

115. Influences the growth of soil organisms. —
Drainage promotes the development of the desirable
forms of organisms, and hinders the development of the
undesirable forms. As will be shown (page 399), the
soil organisms may be divided into two groups, one of
which requires free oxygen for their growth, the other
does not. These two groups are concerned with different
types of chemical change, — the one producing decay
the other putrefaction. In proportion as the air is ex-
cluded by an excess of water, normal decay is inhibited
and putrefaction promoted. The one is beneficial, the
other is likely to be injurious. Further, the products of
the organisms accumulate in the excess of soil water and
sooner or later may kill most of the forms; as is exempli-
fied in peat bogs, which owe their origin chiefly to this
fact. Not only is the decomposition of organic matter
retarded, but the chemical changes in the mineral
portion of the soil resulting from these processes are
correspondingly reduced by lack of drainage. And most
important of all is the stimulation to the formation of
nitrates which results from good drainage. The supply
of nitrates is often the controlling factor in plant growth,
and consequently, in so far as drainage increases this
supply, it is directly beneficial.

116. Increases the food-supply. — Drainage increases
the available food-supply of the soil in three direct ways:
(1) By holding in the soil a larger proportion of avail-
able moisture which favors a larger chemical activity

DRAINAGE AND THE ROOT ZONE 245

without removing the products from the root zone. (2)
Through direct chemical changes which result from good
aeration. (3) Through the activity of organisms which
not only form nitrates but produce carbonic acid and
other materials which increase the availability of the
mineral portion of the soil. The thoroughness of these
chemical changes is well illustrated by the uniform color
of a well-drained and well-aerated soil, in contrast to the
usually mottled color of poorly aerated and wet soil.
Drainage enables the plant-grower to make better use
of the food stored in his soil.

■■ ■ r ■ jr *' ^ . *= : ~ ^z: — — - — .M-* — v* u — ^ — - — ■ — : — : - r ■■■ = *rv - ■'■■' '

Fig. 78. Cross-section of tile-drained soil, showing the elevation of the water-
table between lines of drains.

117. Enlarges the root zone. — Drainage deepens
and enlarges the root zone of the soil by the removal
of the gravitational water and by the admission of air.
Thereby the plant is brought into intimate relation
with a much larger volume of soil from which it may draw
moisture and food. It is thus enabled to withstand more
protracted periods of dry weather; it enjoys a more uni-
form climate, and has a larger food-supply, all of which
are conducive to a rapid growth and a larger yield.

118. Reduces "heaving." — Drainage reduces "heav-
ing," which results from freezing of a wet soil. When
water freezes, it expands one-eleventh of its volume.
In a saturated soil, this expansion can take place in
only one direction — upward — with the result that the

246

THE PRINCIPLES OF SOIL MANAGEMENT

soil and consequently the crop is lifted. Shallow-rooted
crops are gradually raised out of the ground by repeated
freezing when wet, because the soil settled back into

:;■ «. -'' jl. wtfaa

' _*

Fig. 79. Alfalfa roots raised out of the soil ("heaved") by the repeated

freezing of a wet clay.

place more quickly than the root. Not only is the plant
lifted out of the ground, but many of the smaller roots
are broken off, all of which greatly reduces the vitality
of the plant. It is most serious on clay soil, because this

DRAINAGE AND "HEAVING" 247

texture holds more water and is most likely to contain
an excess of water. Drainage reduces this type of injury
in two ways: (1) By reducing the amount of water
present to freeze. (2) The larger volume of free pore
srace, due to the removal of part of the water and to the
better granulation, permits the expansion due to freezing
to be taken up within the mass of the soil, rather than
produce a lifting of the surface. Serious "heaving"
is always dependent upon an excess of soil water.

119. Removes injurious salts from alkali soils. —
Drainage in conjunction with heavy irrigation is the
most effective means of removing "alkali salts" from arid
soils. These salts are dissolved in the irrigation water
as it passes through the soil, and are then removed
in the drainage system beyond any possibility of further
injury. By this practice it is possible to reclaim the most
pronounced areas of alkali soils to the growth of the most
sensitive crops.

120. Reduces erosion. — Drainage reduces erosion
due to water. This type of injury results from the flow
of water over the surface. Drainage reduces this process:
(1) By increasing the absorption of water. (2) By
affording channels in which it may be removed without
injury, due to a less fall, or in conduits not subject to
erosion, such as tile drains.

121. Increases crop yields and improves sanitary
conditions. — The direct practical result of all of the
above effects is larger and more reliable crop-yields,
together with greater ease in all cultural and harvesting
operations.

Coupled with the direct economic effect of drainage,

248 THE PRINCIPLES OF SOIL MANAGEMENT

is a large improvement in the general sanitary condi-
tions of the region, which was recognized long before
the economic advantages of the practice, and has gener-
ally been sufficient reason for public interest in the prac-
tice. It is only within recent years that the economic
benefits of drainage have been recognized as of sufficient
public concern to warrant regulative legislation.

122. Principles of drainage. — There are two general
types of drains: (1) Open, or "surface drains." (2) Cov-
ered, or "under drains." Each of these types has a partic-
ular range of usefulness and, while they may be substi-
tuted one for the other under some conditions, their
respective spheres of usefulness are fairly distinct.

123. Open, or surface drains. — Open or surface
drains remove water from both the surface and from
the depths of the soil. Their efficiency in removing water
from the subsoil depends upon their depth and fall,
and upon the level of water in the channel. There are
certain conditions to which open surface drains alone
are adapted. These are: (1) Where the volume of water
to be moved is very large. (2) Where the water table
is so near the surface, and the fall so slight, that it is
not possible to place a drain below the surface. (3)
Where the drainage is designed to be for only a short
time.

As open ditches their efficiency depends on the sur-
face flow of water into their channel. They usually
tap the low areas where the water accumulates. Some-
times, as in river bottoms, they may be arranged regu-
larly at intervals, and be of such size as to hold the water
which may fall upon the surface during any ordinary

METHODS OF DRAINAGE

249

rain, until such time, after the subsidence of a general
overflow as it may be removed. They may serve to
remove the water accumu ated as the result of an over-
flow. In every such case their efficiency depends upon
taking advantage of the natural inequalities of the sur-

Fig. 80. Surface ditches for drainage in a grain field. Such drains are usually

of low efficiency.

face of the land. One phase of this practice is to plow the
land in narrow beds, so that the frequent "dead fur-
rows" serve as surface drains and as temporary storage
for the surface water.

As sub-surface drains, their efficiency depends upon
their depth being sufficient to permit percolation lrom

250 THE PRINCIPLES OF SOIL MANAGEMENT

the adjacent subsoil. This, in turn, is determined by
the texture and structure of the soil, and upon all those
other factors which determine the efficiency of closed
drains, later to be discussed.

To be efficient, an open drain should be properly
graded, should have a smooth bottom and sides, should
have sufficiently tenacious walls to resist incidental
erosion, and should have a shape approximately that
of a semicircle, which is the form giving the greatest
carrying capacity per cross-sectional area. Since this
exact shape is difficult to maintain, it is common in
practice to make the depth and bottom width, respec-
tively, one-half the width of the top, with sloping sides.
The farm and grade of the ditch must be governed by
the character of the soil. The steepness of the sides will
be determined by the ability of the soil to form resistant
walls. Clay soil will maintain a much steeper bank than
sand. The fall must not be so great as to produce
serious erosion. A loam or sand soil is much more suscep-
tible to erosion than a clay. The fall should be uniform,
in order that there be no undue accumulation of sediment
at any point. Sedimentation may be reduced by pre-
venting the growth of vegetation in the bottom.

As deep-soil drains, open surface ditches have a
number of disadvantages, some of which are: (1) They
are seldom of sufficient depth. (2) As ordinarily con-
structed, they have a small carrying capacity, due to
their uneven grade and rough bottom and sides. (3)
They are expensive to maintain. (4) They waste much
land. (5) They greatly interfere with cultural opera-
tions. (6) They may be subject to serious erosion.

UNDER-DRAINS

251

124, Covered or under-drains. — Covered or under-
drains are any underground channels constructed for
the removal of water. Many kinds of material have
been used for this purpose. Some of the earlier materials
used were brush, stone, poles, boards, and brick. In
recent years these have been almost entirely supplanted

^- -j?..

•

Fig. 81. Construction of a ditch for tile drains.

by pipes made of clay or cement because of the greater
permanency and efficiency of the latter.

The depth, frequency and size of drains depends on
the character of the soil and subsoil, the amount and
distribution of the rainfall, the topography of the sur-
face, the crop to be grown, the prevalence of under-
ground seepage, and the level of the ground water.
The system should always be arranged with reference
to these conditions.

252

THE PRINCIPLES OF SOIL MANAGEMENT

(a) Depth.— The depth of
the drain must be such that
the water can find entrance
before it shall have caused
serious injury to the crop.
Since water percolates through
sand and gravel so much more
readily than through clay,
drains may be placed much
deeper in the former than in
the latter. In coarse-textured
soil, drains attain their full
efficiency almost at once; but
in clay, owing to its dense
character from long wetness,
there is a gradual increase in
efficiency through several
seasons, as the soil becomes
better granulated and ac-
quires other favorable struc-
tural properties. In sand,
water percolates rapidly into
the drain, but in clay this gen-
eral movement is greatly re-
duced and takes place largely
from the sides and top of the drain. In fact, a dense
clay soil holds its pores almost full of capillary water,
which is not subject to percolation. Under such condi-
tions, a large part of the injury comes from water stand-
ing on the surface. Here the under-drains must be placed
very near the surface, and function chiefly as surface

Fig. 82. Laying tile in the
bottom of ditch by use of the tile
hook. Shows arrangement of tile
preparatory to rilling the ditch.

Construction of under-drains

253

drains. But, as the excess of water is removed, and the
soil structure is improved, they assume more fully the
function of deep drains by removing water from the
joints, or checks, which extend deeply into the soil.
Where deep-rooted crops and trees are to be grown,
deeper drainage is
necessary than where
shallow-rooted crops
are grown. In gen-
eral, it is not desir-
able to lower the
water-table so much
in sandy as in clay
soils, because of the
lesscapillary capacity
of the former. The
water-table should be
lowered to from three
to five feet below
the surface, but it is
not always necessary
to place tile at this
depth, to attain suf-
ficiently thorough
drainage. Where
there is a distinct
change from sand to
clay, or vice versa,
within from two to
four feet of the sur-

» . . 11 i ^ig. 83. Laying double-sole drain-

tace, it is usually best tile by hand.

254

THE PRINCIPLES OF SOIL MANAGEMENT

to place the drain on the boundary between the two.
If the clay is below, the water will percolate along its
surface through the sand and enter the tile. On the
other hand, if the clay is underlain by sand, it is easier
for the water to percolate downward into the coarse-
texture stratum, and through this into the tile, entering
from below.

(b) Frequency. — There are two general systems of
arranging drains: (1) The gridion or regular system.

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in

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&

*

m

*

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ill,

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A

h

4

#.

^j

*

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»

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lb

*

.»

*

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jUi,

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3

Fig. 84. Two systems of arranging tile drains. Compare the amount of
double draining in each system, due to junctions. Note the relative lengths
of tile required for the same area under each system.

(2) The natural or irregular system. In the first, the
drains are arranged at definite regular intervals apart, —
this interval depending chiefly on the texture of the soil.
This is necessary where the surface is very uniform and
the soil very homogenous. It may be applied to a slope
as well as to level land. In clay soil the interval must be
less than in coarse-textured soil. This is because there
is a drainage gradient between the drains. In fine-

CONSTRUCTION OF UN DER-D RAINS

255

8' 3' 3

3 3 3

\

s

3" 3" 3'

textured soil the water level rises rapidly
away from the drain and reaches the sur-
face at no great distance. On sand soil
this gradient is much less. The aim
must be to have the water level reduced
a definite distance below the surface,
after a reasonable interval of time follow-
ing rainfall, and the drains must be
sufficiently frequent to accomplish this.
In heavy clay soil this interval may be
as small as twenty-five feet, while in
coarse-textured soil it may be 200 or 300 o
feet. Usually, it is best to adopt some
minimum interval, and
place the first lines of tile
at two or more times this
interval. If the drainage
does not prove sufficiently thorough,
additional drains may be installed with-
out affecting the general system.

The natural or irregular system is
designed primarily to collect water from
the surface where it has accumulated,
or beneath the surface where it comes
within the range of the plant roots.
Large areas of land are drained by a
single line of tile in the low places.
Where land is kept wet by seepage, the
with ' minimum drains should tap these as near their

number of large . , • 1 1

tile, but having source as is practicable.

many turns and rp,, . r 1 • 1 j ,1

branches. I he size ot drains depends on the

Fig. 85. A more
simple system of
drains, but one re-
quiring more large
tile than in Fig. 86.

s<

256

THE PRINCIPLES OF SOIL MANAGEMENT

0

Fig. 87. The so-called natural or irregular system of arranging drains to remove
water from local wet spots. Shading indicates degrees of wetness.

volume of water to be handled and on the fall. Where
several laterals empty into a main drain, the main must
have a capacity equal to their combined flow; but it is
not possible to calculate the total or relative sizes with
the exactness which is possible in a pressure system of
pipes. This is due to the effect of the soil. It acts as a
sponge to hold the water, and gives it up gradually. The

258

THE PRINCIPLES OF SOIL MANAGEMENT

finer the soil the greater this retentive effect, and con-
sequently the less demand there is for drains capable of
carrying all of the rainfall in a given short time. Drains
run full for only a very small part of the year, and
therefore the normal laws of hydraulics are not entirely
applicable to them. In a general way, doubling the fall
increases the carrying capacity of any given size of tile
by one-third. Where the fall is less than 1 per cent, it is

unwise to use tile smaller
than three inches in di-
ameter, because of their
strong tendency to clog.
Water enters tile al-
most entirely through
the joints between the
sections. Short lengths
are therefore better than
long ones. Through the
walls of even soft brick
tile very little water is
able to percolate. There
is, therefore, no appreci-
able advantage in using
soft tile, while there are
many disadvantages, — such as their weakness and lia-
bility to go to pieces rapidly under alternate wetting
and drying, especially if permitted to freeze when
saturated with water.

Dense, hard-burned tile are most safe to use under
average soil conditions.

"Silting-up" of drains results where the alignment

Fig. 89. Hand tools used in tile-
drain construction. 1, Grade cord; 2, pick;
3, long-handle, round-point shovel; 4
and 7, types of grading shovel for finish-
ing the bottom of the ditch; 5, spade; 6,
tile hook, used in placing tile in ditch from
the bank; 8, grade stakes.

DITCHING MACHINES

259

is bad, the joints too open, or a section is broken. The
joints should be fairly snug, but it is not now considered
necessary to use collars in ordinary soils. The textures
of soil which give most trouble by entering the joints
and stopping flow are very fine sand and silt. These
materials flow readily when saturated with water. Con-
sequently, in laying tile in these materials, precaution
must be taken against this. " Silting-up" is most trouble-

Fig. 90. Traction ditching machine. A modern machine for constructing

tile ditches. (See Fig. 91.)

some immediately after laying the tile, and before the
soil structure has become settled and readjusted. When
this has taken place, the tendency to silting-up is small,
even in fine sand and silt. In clay and coarse sand it is
negligible. This difficulty can be checked or controlled
by using some filtering medium around the joints. Straw,
leaves, chaff, etc., are excellent and undergo slow decay,
coincident with which a resistent structure of soil is

UNDER-DRAINS AND PLANT ROOTS

261

established. Fine gravel or coarse sand is a more per-
manent filtering medium.

Plant roots sometimes enter the joints of tile drains,
and develop so as to stop the flow of water. This occurs
most readily where the tile carries ''living water/' as
where a permanent spring
is drained. During dry
periods and in naturally
well-drained soil, water per-
colates from the joints of
the tile into the adjacent
soil, which conditions at-
tract roots and may lead
them into the tile at the
joints. Depth is not a
decided protection against
this difficulty unless it be
excessive.

There are many points
about the construction of
a tile-drain system about
which special precaution
should be taken. Some of
these are: (1) Uniformity
of grade. (2) Avoid lead-
ing a lateral into a main
with a less fall unless silt
basins are used. (3) Pro-
tection of outlets against

jp • , ,s Fig. 92. Ditch cut by the ma-

caving and freezing. (4) chineshownin Fig.91. goiiaheavy

Protection Of the Outlet clay- Depth 4£ feet.

262

THE PRINCIPLES OF SOIL MANAGEMENT

against the entrance of animals. (5) Free flow of
water from the outlet. (6) Close joints, which may be
more easily attained with round or hexagonal than with

Fig. 93. A poorly constructed outlet for a line of drain tile.

U or soft tile. (7) Junctions should be made at an acute
rather than at a right angle. (8) On hilly land the drain
should run with the slope, as far as possible. (9) In
general, the fall should be as great as the surface fea-

SPECIAL TYPES OF DRAINS 263

tures will permit. (10) Avoid throwing the tile out of
alignment in filling the ditch.

The chief advantages of covered drains, especially
when constructed of tile, are: (1) Permanence. A well-
constructed system will last for many decades. (2)
Greater efficiency where they are suitable. (3) No waste
of land. (4) No interference with cultural operations.
(5) Require very little care for maintenance. (6) Less
cost over a period of years.

125. Other types of drainage. — Drainage may some-
times be accomplished by means of levees. Where land
is subject to overflow at either frequent or infrequent
intervals, such as river bottoms and tidal marshes, their
drainage consists largely in excluding these inundations.
Until this is accomplished, any other form of drainage
may be useless. Frequently direct drainage may advan-
tageously be combined with some form of levee, and
for tidal marshes is useful with the aid of the fresh
water derived from rainfall and upland drainage, in
removing its saltness.

Wells or filter basins may be used to drain certain
sinks or flat areas having no other outlet. This is pos-
sible only where a very porous stratum occurs beneath
the soil within a reasonable depth. Usually this is
practicable where a clay stratum is underlain by sand
or gravel, as occurs in many sections of the country.
Wells are constructed through the clay to the porous
stratum, and this may be filled with stone or brush
as a filtering medium, and covered drains may be
emptied into these.

126. Surface culture. — Surface culture may be em-

264

THE PRINCIPLES OF SOIL MANAGEMENT

ployed to remove a limited excess of water from the soil.
Those practices which may be employed for this purpose
are the opposite of those applied in the conservation of
water. The most applicable ones are: (1) Rolling. (2)
Ridged surface. (3) Growth of plants.

Rolling, or any other practice which compacts the
soil and strengthens capillary movement of water to the
surface, places the moisture in the most favorable position

Fig. 94. Water forced to the surface by the closure of the outlet of a tile drain.

REMOVAL OF WATER BY PLANTS

265

for evaporation. It would be unwise, as a rule, to roll
the soil when it is excessively wet, because of the injury
to the structure of the soil which would result. But the

Fig. 95. A well-constructed outlet for a line of drain tile.

land may be rolled in anticipation of a wet period, which
condition of the soil will facilitate the formation of that
compact surface which most favors evaporation. In the
spring, in regions of cold winters, bare or fallow land has
usually settled into this condition, which, if permitted
to continue, will most rapidly dry the soil.

Ridging increases evaporation by exposing a larger

266

THE PRINCIPLES OF SOIL MANAGEMENT

surface. In some sections of the country where the
wetness is most serious in the spring, the crops are
planted on ridges which are sufficiently raised above
the general surface to be drained; and, by the time the
roots are ready to penetrate deeply, the excess of moist-
ure will have been removed by percolation and evapo-
ration.

Crops of any sort, including weeds, green manures
and cover-crops, may serve to dry the soil by evaporating

Fig. 96. The nine foot evener used in the final filling of the ditch by the
use of the turning plow after laying drain tile. Care should be exercised in
placing the first covering of earth over the tile not to disturb their alignment
or break any of the sections. This is best accomplished by hand, and the earth
should be carefully pressed around the tile.

water from their leaves. It has been seen (page 134) that
the amount of water so used is large because of the
functional activity of the plants and the large surface
which they expose. Growing crops expand the evapo-
rating surface of the soil and are especially useful in
removing a temporary wetness in the spring.

The application of any of the above methods for the
removal of water must be guided by the local conditions
of soil, season, climate, crop and system of farming.

C. PLANT NUTRIENTS IN THE SOIL

I. SOLUBILITY OF THE SOIL THROUGH NATURAL

PROCESSES

Fortunately for mankind, only an exceedingly small
proportion of the soil is at any one time soluble in water
or in the aqueous solutions with which it is in contact.
It is this great insolubility that gives the soil its perma-
nence, for, otherwise, in humid regions, it would be
rapidly carried away in the drainage water. The portion
soluble in the various natural solvents with which it
comes in contact furnishes the mineral-food materials for
plants. The great mass of soil which is relatively in-
soluble is constantly subjected to natural processes
which very slowly bring the constituents into solution.
Those agents concerned in the decomposition of rock
also act upon the soil to bring about its further disin-
tegration, and thereby render it more soluble, while
added to those are the operations of tillage, which con-
tribute to the same end.

The surfaces of the particles alone come into contact
with the decomposing agents, and hence it is these por-
tions of the particles that are rendered most soluble.
The factors that determine how rapidly solution shall
proceed are: (1) The amount of surface exposed, which
we have seen varies with the size of the particles. (2) The
composition of the particles. (3) The strength of the
decomposing and solvent agencies. Were it not for this
process, there would soon be no mineral food available

(267)

268 THE PRINCIPLES OF SOIL MANAGEMENT

to plants, as drainage water and the ash of crops carry
off relatively large amounts of these substances each
year; but in spite of this loss, the soil is able to provide
at least some plant-food material for each crop, when
called upon by the plant.

II. SOLUBILITY OF THE SOIL IN VARIOUS SOLVENTS

For purposes of analysis intended to show the
amounts of mineral plant-food materials in the soil any
one of several different solvents may be used. These
solvents differ in strength, and consequently the per-
centages of the various constituents obtained from
samples of the same soil are different for each solvent.
A chemical analysis of a soil is a determination of the
amounts of the constituents that have been dissolved
in the solvent used. Therefore it will readily be seen
that the interpretation of a chemical analysis must
depend largely upon the nature of the solvent, and,
unless the solvent is equivalent in its action to some pro-
cess or processes in nature, the result must be entirely
arbitrary. The solvents used have generally been in-
tended to show some definite relation of the soil to the
food requirements of crops. Upon the accuracy with
which this is accomplished depends the value of the
chemical analysis.

127. Complete solution of the soil. — By the use of
hydrofluoric and sulfuric acids, the entire soil mass may
be decomposed and all of its inorganic constituents
determined. Such an analysis shows the total quantity
of the plant-food materials except nitrogen, which

SOLUBILITY OF SOIL CONSTITUENTS 269

is never determined in any of the acid solutions, but
by a separate process. A deficiency of any particular
substance may be discovered in this way, but nothing
can be learned as to the ability of the plant to obtain
nutriment from the soil. A rock may show as much
mineral plant-food material as a rich soil. Such an
analysis is used only to ascertain the ultimate limita-
tions of a soil or its possible deficiency in any essential
constituent.

128. Digestion with strong hydrochloric acid. — Analy-
ses made with hydrochloric acid of 1.115 specific gravity
are those usually called "chemical soil analyses."
They are supposed to show the amount of plant food
at the time the analysis is made, which is in a condi-
tion to be ultimately used by the plant, and the plant-
food materials not dissolved by treatment with hydro-
chloric acid are assumed to be in a condition in which
plants can not use them. It may reasonably be ques-
tioned whether these relations hold under field condi-
tions. In fact, it is quite certain that some of them
do not hold. In other words, while treatment with
hydrochloric acid of a given strength marks a definite
point in the solubility of the compounds in the soil, it
does not bear a uniform relation to the natural processes
by which these compounds become available to the
plant.

129. Interpretation of results of analysis of hydro-
chloric acid solution. — This method of analysis was
originally thought to give some indication of both the
permanent fertility and the immediate manurial needs
of a soil; but for both purposes the accuracy of the

270 THE PRINCIPLES OF SOIL MANAGEMENT

deductions are limited by a number of conditions which
make it impossible to predict from an analysis how
productive a soil may be, or what particular manure
may be profitably applied. It is very apparent that the
chemical composition of a soil is only one of the many
factors affecting its productiveness. Unfortunately, not
all of the factors are understood, and consequently
these unknown ones cannot be determined either quali-
tatively or quantitatively. If it ever becomes possible
to determine quantitatively all of the factors entering
into soil productiveness in the field condition, the prob-
lem will be solved.

130. Permanent fertility, and manurial needs. —
Permanent fertility can best be judged by the complete
analysis of the soil, but, with the exception of potash,
the possible deficiency the constituents likely to be
required in manures may be judged from the hydro-
chloric acid solution with a fair degree of accuracy.

Conclusions as to the manurial needs of the soil are
confined to ascertaining whether any constituent is
present in such small amount as to furnish an inadequate
supply for crop production. If, for example, a certain
ingredient is found to be present in very small amount,
it may be concluded that the addition of a manure con-
taining this substance would be profitable; but there is
considerable difference of opinion among analysts
as to what this figure is for each of the ingredients.
This minimum amount may vary with certain conditions
of soil.

131. Relation of texture to solubility.— The relative
amounts of sand and clay in the soil and the distribution

INFLUENCE OF TEXTURE ON SOLUBILITY 271

of the fertilizing materials in these constituents will
affect the minimum amounts required. Hilgard has
shown that the addition of four or five volumes of quartz
sand to one of a heavy but highly productive black clay
soil greatly increased the productiveness, while diluting
the potash content of the mixture to .12 per cent and
the phosphoric acid to .03 per cent. It is evident that
in this soil the plant-food materials were in a condition
to be easily taken up by the plant when the physical
condition of the soil was suitable.

If these small amounts of food elements had been
distributed in the sand particles as well as in the original
clay, the result would doubtless have been different.
Suppose, for example, that 50 per cent of the potash
and phosphoric acid had been in the sand particles and
the remainder in the clay, the former which expose
much the less surface to dissolving liquids would be
proportionately less soluble, and as the minimum quan-
tity is approached, as shown by the more dilute soil
yielding less than the other, the effect would doubtless
have been to decrease the production. (See page 86.)
In some soils, particularly those of the arid region, the
larger particles may carry much of the mineral nutrients,
in which case it is quite evident that a higher percentage
of fertility is required than in soils carrying the plant-
food material largely in the small particles.

132. Nature of subsoil. — The nature and compo-
sition of the subsoil is naturally a factor in determining
soil productiveness, and must be considered as well
as the soil. An impervious subsoil, or a very loose sandy
one, will confine the productive zone largely to the top

272 THE PRINCIPLES OF SOIL MANAGEMENT

soil, and hence require a greater proportionate amount
of fertility.

133. Calcium carbonate. — A determination of the
amount of calcium present as a carbonate is important
as an aid to the interpretation of an analysis of the soil.
Lime not so combined is generally in the form of a
silicate, or possibly phosphate. When there is a large
amount of calcium carbonate in a soil, the potash, phos-
phoric acid and nitrogen are always more readily soluble,
and smaller quantities are sufficient for crop growth than
where the calcium is not found in this form. The effect
of the carbonate of lime upon the nitrogen1 com-
pounds is to furnish a base for the acids produced in the
formation of nitrates and its presence promotes that
process. It probably replaces potassium in certain
compounds where otherwise it would be secured with
more difficulty. It insures the presence of some phos-
phates of lime, in which form phosphorus is more
soluble than when combined with iron. The form of the
manures to be used upon the soil will also depend in
large measure upon the presence or absence of calcium
carbonate. (See page 349.) For instance, where calcium
carbonate is deficient, steamed bone or Thomas slag are
more profitable than superphosphate, and nitrate of
soda than sulphate of ammonium. Finally, the absence
of calcium carbonate indicates the need of liming, and,
if the analyses show a considerable amount of potash
and phosphoric acid, but practice shows them to be
somewhat deficient, it is probable that liming will be
all that is necessary, and that manures carrying these

xNot determined in the hydrochloric acid extract.

EXTRACTION WITH ORGANIC ACID 273

substances may be dispensed with. It must be stated,
however, that there are cases for which these deductions
do not hold, owing to the intervention of other factors.

134. Estimation of deficiency of ingredients. — In a
soil in which the other conditions are normal, one would
suppose it possible to prescribe, with some degree of
accuracy, the content of certain constituents below
which a deficiency exists. The use of a manure contain-
ing this constituent should therefore be expected to
produce beneficial results. However, opinions differ
so widely, depending, apparently, upon the soils with
which the respective analysts have had to deal, that it is
difficult to decide where to set the limit. It is evident
that, as the content of any constituent becomes less,
the probable need for its application becomes greater,
and it thus suggests a practice without assuring its
success.

135. Conclusions. — An analysis of the hydrochloric
acid extract, therefore, cannot be taken as a guide to
the fertilizer needs of the soil, and of itself should not
be relied upon; but in connection with other knowledge,
particularly that derived from fertilizer tests, it may be
useful.

136. Extraction with dilute organic acids. — Other
methods used for dissolving soils for analysis depend
upon extraction with some dilute organic acid, as citric,
acetic, oxalic or tartaric acid. The assumption upon
which these methods are based is that the dilute organic
acids correspond to the solvent agents in the soil, and
thus take from it the amounts of those materials that
the plant could take up if it came in contact with all

R

274 THE PRINCIPLES OF SOIL MANAGEMENT

portions of the soil to the depth represented by the
sample analysed.

137. Advantages in showing manurial needs. — The
action of each of these dilute acids upon the same soil
does not give equal amounts of the various constituents
in solution. Citric acid dissolves especially lime, mag-
nesia and phosphoric acid, and is the most satisfactory
solvent for purposes of analysis. The organic acids
naturally dissolve a much smaller amount of material
from the soil than does hydrochloric acid. The former
acids permit the detection of smaller amounts of easily
soluble phosphoric acid and potash than does the latter,
larger quantities of soil being used. For example, a
chemical analysis of the hydrochloric acid solution is
very likely not to show any increase in the phosphorus
or potassium in a soil that may have been abundantly
manured with these fertilizers, and its productiveness
increased greatly thereby. This is because the amount
of plant-food material added is so small in comparison
with the weight of the area of soil nine inches deep over
which it is spread that the increase in percentage may
well come within the limits of analytical error. An acre
of soil nine inches deep weighs about 2,500,000 pounds.
If to this be added dressings of 2,500 pounds phosphoric
acid fertilizer containing 400 pounds phosphoric acid,
it would increase the percentage of that constituent
in the soil only .016 per cent, which difference could not
be detected by the analysis of the hydrochloric acid
solution.

138. Usefulness of citric acid. — -As shown by Dyer,
the use of a 1 per cent solution of citric acid is well

EXTRACTION WITH SOLUTION OF CARBON DIOXIDE 275

adapted to show the amount of easily soluble phosphoric
acid and potash in certain soils, but for other soils it has
failed to give satisfaction in the hands of a number of
analysts. Shorey, for instance, finds that it fails utterly
for the highly ferruginous soils of Hawaii. It is, doubt-
less, better adapted to soils rich in calcium and low in
iron and aluminum.

The reason urged by Dyer for the superiority of the
citric acid over the hydrochloric acid extraction of the
soil is that the former gave, in his hands, several times
as great a difference in the amounts of soluble phos-
phoric acid in soils needing phosphoric manures as com-
pared with those not needing them.

The application of both the hydrochloric and citric
acid methods to a soil may, when used to supplement
each other, add greatly to a knowledge of the potential
and present productiveness of the soil.

There should be present in a soil for cereals and most
other crops at least .01 per cent phosphoric acid, soluble
in 1 per cent citric acid. A soil containing less than this
amount is deficient in phosphoric acid, unless it exists
largely in the form of ferric or aluminum phosphate,
which is not readily soluble in citric acid, but is fairly
available to the plant. Sod land contains organic com-
pounds of phosphorus that are easily soluble in the citric
acid, but less readily available to the plant; hence such
soil should show by analysis more than .01 per cent
phosphoric acid, to indicate sufficiency.

139. Extraction with an aqueous solution of carbon
dioxide. — As carbon dioxide is a universal constituent
of the water of the soil, and without doubt a potent

276

THE PRINCIPLES OF SOIL MANAGEMENT

factor in the decomposition of the mineral matter, it has
been proposed to use a solution of carbon dioxide as a
solvent in soil analysis. The amounts of soil constitu-
ents taken up by this solvent are much less than by any
of the others heretofore mentioned, but all mineral
substances used by plants are soluble in it to some extent.
The amount of phosphorus is so small as to make its

Fig. 97. The cut-out disc harrow, adapted to hard or stony soil.

detection by the gravimetric method difficult. Like
other methods employing very weak solvents, it is open
to the objection that the extraction fails to remove a
considerable portion of the dissolved matter that is
retained by absorption, and, as this will vary with soils
of different texture, it makes impossible a fair com-
parison of such soils by this method.

140. Extraction with pure water. — When soil is
digested with distilled water, all of the mineral substances

EXTRACTION WITH PURE WATER 277

used by plants are dissolved from it, but in very small
quantities. It has been proposed to use this extract for
soil analysis on the ground that it involves no artificial
solvent, the presence or amount of which in the soil
is doubtful, but shows those substances which are un-
doubtedly in a condition to be used by plants. By
determining the water content of the soil and using
a known quantity of water for the extraction, the per-
centage of the various constituents in the soil water
or in the dry soil may be calculated.

The substances dissolved from the soil by extraction
with distilled water are probably only those contained
in the soil-water solution, including a part of the solutes
held by absorption. The aqueous extract does not con-
tain all of the nutritive salts in solution in the soil water,
and hence is not a measure of the fertility held in that
form. An undetermined amount of nutrients is retained
in the water in the very small spaces and on the surface
of the soil particles. It is, however, a fair comparative
measure of the content of available nutrients.

141. Influence of absorption. — The quantity of ex-
tracted material depends upon the absorptive properties
of the soil, and upon the amount of water used in the
extraction, or upon the number of extractions. Analyses
of the aqueous extract of a clay and of a sandy soil on
the Cornell University Farm serve to illustrate the
greater retentive power of the former for nitrates.
Sodium nitrate was applied to a clay soil, and to a sandy
loam soil at the rate of 640 pounds per acre. Analyses
of aqueous extract; some ninety days later, showed
the following:

278

THE PRINCIPLES OF SOIL MANAGEMENT

Table

XLI

Kind of soil

Fertilizer

Nitrates in soil.
Parts per million

Clay

Sodium nitrate
No fertilizer

Sodium nitrate
No fertilizer

7.8

Clay

Sandy loam

Sandy loam

1.8
150.0

29.7

There was apparently a much greater retention of
nitrates by the clay soil, as shown by a comparison of
the fertilized and unfertilized plats on both soils.

Schulze extracted a rich soil by slowly leaching 1,000

grams with pure water, so that one liter passed through

in twenty-four hours. The extract for each twenty-four

hours was analyzed every day for a period of six days.

The total amounts dissolved during each period were

as follows:

Table XLII

Successive extractions

Total matter
dissolved

Volatile

Inorganic

First

.535
.120
.261
.203
.260
.200

.340
.057
.101
.083
.082
.077

.195

Second

.063

Third

.160

Fourth .

.120

Fifth

.178

Sixth

.123

It will be noticed that the dissolved matter, both
organic and inorganic, fell off markedly after the first
extraction, which was larger on account of the matter
in solution in the soil water. Later extractions were
doubtless supplied largely from the substances held
by absorption and which gradually diffuse into the water

INFLUENCE OF ABSORPTION ON SOLUBILITY 279

extract, as the tendency to maintain equilibrium of
the solution overcomes the absorptive action. With the
removal of the adsorbed substances, the equilibrium
between the soil particles and the surrounding solution
is disturbed, solvent action is increased, and more
material gradually passes from the soil into the solution.
In this way the uniform and continuous body of extrac-
tives is maintained.

142. Other factors. — For purposes of soil analysis,
the quantity of water used for extraction must be placed
at some arbitrary figure, and the method is open to the
objection that it does not represent accurately the soil
water solution. Analyses of soils of different types are
not comparable, and the water extract cannot be con-
sidered to measure the concentration or even the com-
position of the solution existing between the root hair
and the soil particles. However, for studying some of
the changes that go on in the soil, and which are detect-
able in the soil-water solution, the method may be used
to advantage.

III. MINERAL SUBSTANCES ABSORBED BY PLANTS

The plant, in its process of growth, withdraws from
the soil certain mineral matters that are presented to
its roots in a dissolved condition. As the salts in solution
are quite numerous, and as the osmotic process by which
the absorption is accomplished does not admit of the
entire exclusion of any substance capable of diosmosis,
there are to be found in the plant most of the mineral
constituents of the soil. Some of these are concerned in

280 THE PRINCIPLES OF SOIL MANAGEMENT

the vital processes of the plant and are essential to its
growth. Others seem to have no specific function, but
are generally present.

143. Substances found in ash of plants. — The sub-
stances commonly met with in the ash of plants are
potassium, sodium, calcium, magnesium, iron, aluminum,
phosphorus, sulfur, silicon, and chlorine. In addition
to these, nitrogen is absorbed from the soil in the form
of soluble salts.

The substances known to be absolutely essential to
the mature growth of plants are potassium, calcium,
magnesium, iron, phosphorus, sulfur and nitrogen,
while the others are probably beneficial to the plant in
some way not yet discovered.

Of the substances acting as plant nutrients, each must
be present in an amount sufficient to make possible the
maximum growth consistent with other conditions, or
the yield of the crop will be curtailed by its deficiency.
To some extent certain essential substances may be
substituted by others, as, for instance, potassium bjr
sodium; but such substitution is probably possible only
in some physiological role other than that of an ele-
mental constituent of an organic compound. The sub-
stances that are likely to be so deficient in an available
form in any soil as to curtail the yield of crops are potas-
sium, phosphorus and nitrogen, while the addition of
certain forms of calcium is likely to be beneficial on
account of its relation to other constituents and proper-
ties of the soil. It is for the purpose of supplying these
substances, and to some extent to improve the mechani-
cal condition of the soil, that mineral manures are used.

PLANT -FOOD REMOVED BY CROPS

281

144. Amounts of plant-food material removed by
crops. — The utilization of mineral substances by crops
is a source of loss of fertility to agricultural soils. In
a state of nature, the loss in this way is comparatively
small, as the native vegetation falls upon the ground,
and in the process of decomposition the ash is almost
entirely returned to the soil. Under natural conditions,
soil usually increases in fertility; for, while there is some
loss through drainage and other sources, this is more
than counterbalanced by the action of the natural
agencies of disintegration and decomposition, and the
fixation of atmospheric nitrogen affords a constant,
although small supply, of that important soil ingredient.

Fig. 98. A collection of hand-tillage implements. From left to right:
1. Field and garden hoe. 2. Mattock. 3. Weeding -hoe. 4. Stone-hooks.
5. iinger-weeder. 6. Grub-hoe, 7. Scuffle-hoe. 8. Garden rake. 9. Spading-
fork. 10. Garden trowel.

282

THE PRINCIPLES OF SOIL MANAGEMENT

When land is put under cultivation, a very different
condition is presented. Crops are removed from the
land, and only partially returned to it in manure or
straw. This withdraws annually a certain small propor-
tion of the total quantity of mineral substances, but,
what is of more immediate importance, it withdraws
all of this in a readily available form.

The following table, computed by Warington, shows
the amounts of nitrogen, potassium, phosphorus and
lime removed from an acre of soil by some of the common
crops. The entire harvested crop is included.

Table XLIII

Crop

Yield

Total
ash

Nitro-
gen

Potash

Lime

Phos-
phoric
acid

Wheat

Barley

Oats

30 bus.

40 bus.

45 bus.

30 bus.

\\ tons
2 tons
6 tons

17 tons

Pounds
172
157
191
121
203
258
127
364

Pounds
48
48
55
43
49

102
47

192

Pounds

28.8
35.7
46.1
36.3
50.9
83.4
76.5
148.8

Pounds

9.2

9.2

11.6

32.1

90.1

3.4

74.0

Pounds
21.1
20.7
19.4

Maize

Meadow hay. .
Red clover . . .
Potatoes ....
Turnips

18.0
12.3
24.9
21.5
33.1

145. Amounts of plant-food materials contained
in soils. — Comparing the figures given above with those
showing the total amounts of the fertilizing constituents
in certain soils, it is evident that there is a supply in
most arable soils that will afford nutriment for average
crops for a very long period of time. The following
table shows the amount of nutrients contained in the
chief divisions of soil as given on page 30.

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POSSIBILITY OF EXHAUSTING SOILS 285

146. Possible exhaustion of mineral nutrients.—
On the other hand, when we consider that the soil must
be depended upon to furnish food for humanity and
domestic animals as long as they shall continue to
inhabit the earth, at least so far as we now know, the
very apparent possibility of exhausting, even in a period
of several hundred years, the supply of plant nutrients
becomes a matter of grave concern. The visible sources
of supply, to replace or supplement those in the soils now
cultivated are, for the mineral substances, the subsoil
and the natural deposits of phosphates, potash salts,
and limestone, and for nitrogen deposits of nitrates,
the by-product of coal distillation and the nitrogen
of the atmosphere. The last of these is inexhaustible,
and the exhaustion of the nitrogen supply, which a
few years ago was thought to be a matter of less than
half a century, has now ceased to cause any apprehen-
sion. The conservation or extension of the supply of
mineral nutrients is now of supreme importance. The
utilization of city refuse and the discovery of new
mineral deposits are developments well within the range
of possibility, but neither of these promises to afford
more than partial relief. The utilization of the subsoil
through the gradual removal by natural agencies of the
top soil will, without doubt, tend to constantly renew the
supply. The removal of top soil by wind and erosion is,
even on level land, a very considerable factor. The large
amount of sediment carried in streams immediately after a
rain, especially in summer, gives some idea of the extent
of this shifting. This affects chiefly the surface soil and
thereby brings the subsoil into the range of root action.

286 THE PRINCIPLES OF SOIL MANAGEMENT

IV. ACQUISITION OF NUTRITIVE SALTS BY AGRICUL-
TURAL PLANTS

All of the salts taken up by the roots of agricultural
plants are in solution when absorbed. The movement
into the root thus depends upon the presence of moisture,
which is the medium of transfer. The root hairs are the
great absorbing portions of the plant, and through the
cells of their delicate tissues the solutions of the various
salts pass by osmotic action. (See Fig. 53.) The nature
and quantity of material absorbed is determined by the
law of osmosis. From the cells of the root-hairs the dis-
solved salts are transferred to other portions of the plant,
where they undergo the metabolic processes that deter-
mine which constituents shall be retained in the tissues
of the plant. The unused ions which remain in the plant
juices prevent by their presence the further absorption
of those particular substances from the soil water. It thus
happens that the composition of the ash of a plant may
be very different from that of the substances presented
to it in solution. For instance, aluminum, although
always present in the soil, in a very slightly soluble form,
is either absent or present in mere traces in the ash of
most plants. On the other hand, iodine, although present
in sea-water only in the most minute amounts, is present
in large quantities in the ash of certain marine algae.

147. Selective absorption. — A plant will, in general,
take up more of a nutritive substance when presented
in large amount, as compared with the other soluble
substances in the nutrient solution, than if presented
in small amount. Thus, the percentage of nitrogen in

ACQUISITION OF NUTRIENTS BY PLANTS 287

maize, oats and wheat may be increased by increasing
the ratio of nitrogen to other nutritive substances in
the nutrient media. This is also true of potassium and
phosphorus, respectively. This fact is accounted for
by the maintenance of the osmotic equilibrium at a
higher level for a particular ion which is relatively

Fig. 99. Showing the intimate relation of root-hairs and soil particles.

abundant in the nutrient solution, thus preventing the
return of the excess from the plant.

148. Relation between root-hairs and soil-particles. —
In a rich, moist soil the number of root-hairs is very
great, while in a poor or a dry soil there are compara-
tively few. The connection between the root-hairs and
the soil-particles is extremely intimate. When in con-
tact with the particle of soil, the root-hair frequently
almost incloses it, and by means of its mucilaginous

288 THE PRINCIPLES OF SOIL MANAGEMENT

wall forms a contact so close as to practically make the
solution between the particle and the cell-wall distinct
from that between the soil-particles.

There has been considerable difference of opinion
as to how7 the plant can obtain its mineral nutrients
from a substance so difficultly soluble as the soil. It has,
of course, been recognized that the soil-water is aided
in its solvent action by a variety of substances that may
be normally present in solution, beginning with the
gases taken up by the rain in its descent through the
atmosphere, and further aided by the carbon dioxide
and organic and mineral substance obtained from the
soil. It has been held that the plant-roots aid solution
of mineral matter by excretion of acids, which act effec-
tively as solvents. The well-known root-tracings on
limestone and marble have been taken as proof of the
excretion of such acids. Sachs and, later, other investi-
gators grew plants of various kinds in soil and other
media in which was placed a slab of polished marble
or dolomite or calcium phosphate, covered with a layer
of washed sand. After the plants had made sufficient
growrth, the slabs were removed and on the surfaces
were found corroded tracing, corresponding to the lines
of contact between the rootlets and the minerals.

In order to test this theory, Czapek repeated the
experiments, using plates of gypsum mixed with the
ground mineral that he wished to test, and this mixture
he spread over a glass plate. Using these plates in the
same manner as previously described, Czapek found
that, while plates of calcium carbonate and of calcium
phosphate were corroded by the roots, plates of alumi-

ROOT EXCRETIONS AND SOLUBILITY 289

num phosphate were not. He concludes that if the trac-
ings are due to acids excreted by the plant-roots, the
acids so excreted must be those that have no solvent
action on aluminum phosphate. This would limit the
excreted acids to carbonic, acetic, propionic and butyric.
Czapek also replies to the argument that the acids pro-
ducing the tracings must be non-volatile ones, because
of the definite lines made in the mineral, by stating
that the excretion of carbon dioxide alone would be
sufficient to account for the observations, as it dissolves
in water to form carbonic acid, and that carbonic acid
is always present in the cell-walls of the root epidermis,
from which it does not readily exude.

Czapek has also shown that liquids having an acid
reaction exude from root-hairs, and he attributes the
reaction to the presence of acid salts of mineral acids,
having found potassium, phosphorus, magnesium, cal-
cium and chlorine in this exudate. He has not proven,
however, that the exudations were not from dead root-
hairs, or from the dead cells of the root-cap. In either
case they would have some solvent action, but whether
sufficient to make them of importance it is impossible
to say.

Kunze, who followed up this work, discredits the
theory of excretion of acid salts of mineral acids, and
attributes the corrosive action of roots to organic acids.
In his experiments with 200 species, he found that many
plants do not excrete enough acid to be detected by
litmus. He attributes to fungi much the greater activity
in this respect, and considers them more important in
disintegrating the soil than are the higher plants.

290 THE PRINCIPLES OF SOIL MANAGEMENT

The present status of experimental evidence on excre-
tion of acids other than carbonic by the roots of plants
does not admit of any very satisfactory conclusion as to
their relative importance in the acquisition of plant-food
materials. There can be no doubt, however, that carbon
dioxide, resulting from root exudation, and from decom-
position of organic matter in the soil, plays a very promi-
nent part in this operation. The very large quantity
of carbon dioxide in the soil, amounting in some cases
to from 5 to nearly 10 per cent of the soil air, or several
hundred times that of the atmospheric air, must aid
greatly in dissolving the soil-particles.

Whatever may be the concentration of the soil-water,
it seems probable that the liquid to be found where the
root-hair comes in contact with the soil-particle, and
which is separated, in part at least, from the remainder
of the soil-water, must have a density much greater than
that found elsewhere in the soil. The comparatively
rich juices of the plant separated from the soil water
only by the delicate cell-walls of the root-hair insures
a copious transfer of the constituents of these juices
into the intervening water, thus bringing into contact
with the soil mineral salts, of which some are doubtless
acid salts and also mineral salts of organic acids, and,
possibly, some free organic acids. That portion of the
soil-water immediately in contact with the soil grain is
a much stronger solution than the water further from
the soil surfaces on account of the absorptive action of
the particles. These solutions, coming in contact with
the surface of the soil-particles already subjected to the
bacterial and other disintegrating agents of the soil,

ABSORPTIVE POWER OF DIFFERENT CROPS 291

may readily be conceived to start an active transfer
of mineral substances into the plant.

Plants grown in solutions of nutritive salts have
few or no root-hairs, but absorb through the epidermal
tissue of the roots. If the plant depended upon the pre-
pared solution in the soil-water, a similar structure would
doubtless suffice. The special modification by which
the root-hairs come in intimate contact with the soil-
particle, and almost surrounds it, indicates a direct
relation between the soil-particles and the plant, and not
merely between the soil solution and the plant.

New root-hairs are constantly being formed, and the
old ones become inactive and disappear. The contact
of a root-hair with a soil-particle is not long-continued.
Whether the period of contact is determined by the
ability of the root to absorb nutriment from the particle
is not known. Certain it is that only a small portion of
the particle is removed. It may be true that only the
immediate surface which had been previously acted
upon by the disintegrating agents of the soil, and thus
rendered more easily soluble, is affected by the absor-
bent action of the root-hairs.

149. Absorptive power of different crops. — As has
already been pointed out (page 281), crops of different
kinds vary greatly in their ability to draw nourishment
from the soil. The difference between the nitrogen,
phosphorus and potassium taken up by a corn crop
of average size and a wheat crop of average size is very
striking. Corn has the longer growing period, but as be-
tween oats and wheat, where the growing period is nearly
identical, a similar relation exists.

292 THE PRINCIPLES OF SOIL MANAGEMENT

The difference in absorbing power may be due to
either one or both of two causes: (1) A larger absorbing
system. (2) A more active absorbing system. The
former is determined by the extent of the root-hair
surfaces; the latter by the intensity of the osmotic action.

150. Extent of absorbing system. — Plants with
large root systems may, therefore, be expected to absorb
the larger amounts of nutrients from the soil, and such
is usually the case, although the extent of the root-system
is not necessarily proportional to the total area of the
absorbing surfaces of the root-hairs.

151. Osmotic activity. — The osmotic activity of a
plant under any given condition of soil and climate
depends upon: (1) The rapidity and completeness with
which the plant elaborates the substances taken from
the soil into plant substance or otherwise removes them
from solution. (2) The extent to which the exudations
from the root-hairs act upon the soil particles to in-
crease the density of the solution between the root-hair
and the soil-particle.

The first of these is a function of the vital energy
of the plant and its ability to utilize sunshine and carbon
dioxide to produce organic matter. It may be com-
pared to the property which enables one animal to do
more work than another animal of the same weight on
a similar ration.

The removal from the ascending water current in
the plant of substances derived from the soil is accom-
plished in the leaves. By the dissociation of these, ions
are constantly furnished for metabolism into materials
that may be built into the tissues of the plant. The

FACTORS AFFECTING ABSORPTIVE POWER 293

remaining ions are kept in the solution. There is a con-
stant tendency to bring the composition and density
of the solution into equilibrium, by diffusion and dios-
mosis, with the solution between the soil-particle and the
root-hair. The rapidity with which the metabolic pro-
cess removes a substance from the solution in the plant,
therefore, determines the rate at which it is removed
from a solution of given composition and density in the
soil. Plants making a rapid growth remove more nutri-
ents in a given time than those making a slower growth,
when the nutrient solution is of a given composition
and density. A maize plant, for instance, removes more
nutriment from a given solution in one day during its
stage of most rapid growth than does a wheat plant
during a corresponding stage.

Another factor which affects the rate of absorption
of salts from the soil is the solvent influence of exudates
from the root-hairs. This subject has already been
treated (page 287), and it only remains to say that this
action apparently varies with different kinds of plants,
and probably accounts in no small measure for the dif-
ference in the ability of different plants to withdraw
salts from the soil.

These several factors, which, when combined, deter-
mine the so-called "feeding-power" of the plant, are
recognized by the popular terms "weak-feeder" and
"strong-feeder,"— applied, on the one hand, to such
crops as wheat or onions, which require very careful
soil preparation and manuring, and, on the other hand,
to maize, oats or cabbage, which demand relatively
less care. In manuring and rotating crops, this difference

294

THE PRINCIPLES OF SOIL MANAGEMENT

in absorptive power must be considered, not only to
secure the maximum effect upon the crop manured, but
also to get the greatest residual effect of the manure
upon succeeding crops.

Fig. 100. Deep and shallow cultivation for corn. On the right-hand side of
the picture the deep cultivator shovels are destroying the upper roots. On the
left-hand side the shallow cultivation does not reach the roots.

152. Cereal crops. — These plants possess the power of
utilizing the potassium and phosphorus of the soil to
a considerable degree, but generally require fertili-
zation with nitrogen salts. Most of the cereals, like wheat,
rye, oats and barley, take up most of their nitrogen
early in the season, before the nitrification processes
have been sufficiently operative to furnish a large supply

ABSORPTIVE POWER OF CEREALS 295

of nitrogen, and hence nitrogen is the fertilizer consti-
tuent that usually gives best results, and should be
added in a soluble form. Wheat, in particular, needs
a large amount of soluble nitrogen early in its spring
growth. Since it is a delicate feeder, it does best after a
cultivated crop or a fallow, by which the nitrogen has
been converted into a soluble form. Oats can make
better use of the soil fertility and does not require
so much manuring. Maize is a very coarse feeder, and,
while it removes a very large quantity of plant-food
from the soil, it does not require that these be added
in a soluble form. Farm manure and other slowly acting
manures may well be applied for the maize crop. The
long growing period required by the maize plant gives
it opportunity to utilize the nitrogen as it becomes
available during the summer, when ammonification and
nitrification are active. Phosphorus is the substance
usually most needed by maize.

153. Grass crops. — Grasses, when in meadow or in
pasture, are greatly benefited by manures. They are
less vigorous feeders than the cereals, have shorter
roots, and, when left down for more than one year, the
lack of aeration in the soil causes decomposition to
decrease. There is usually a more active fixation of
nitrogen in grass lands than in cultivated lands, but this
becomes available very slowly.

Different soils and different climatic conditions
necessitate different methods of manuring for grass.
Farm manures may well be applied to meadows in all
situations, while the use of nitrogen is generally profi-
table.

296 THE PRINCIPLES OF SOIL MANAGEMENT

154. Leguminous crops. — Most of the leguminous
crops are deep-rooted and are vigorous feeders. Their
ability to acquire nitrogen from the air makes the use
of that fertilizer constituent unnecessary except in a
few instances, such as young alfalfa on poor soil, where
a small application of nitrate of soda is usually bene-
ficial. Lime and potassium are the substances most
beneficial to legumes on the majority of soils.

155. Root-crops. — Many of the members of this
class of crops will utilize very large amounts of plant-
food if it is in a form in which they can use it. Phos-
phates and nitrogen are the substances generally re-
quired, the latter especially by beets and carrots.

156. Vegetables. — In growing vegetables, the object
is to produce a rapid growth of leaves and stalks rather
than seeds, and often this growth is made very early in
the season. As a consequence, a soluble form of nitrogen
is very desirable. Farm manure should also have a
prominent part of the treatment, as it keeps the soil in a
mechanical condition favorable to retention of moisture,
which vegetables require in large amounts, and it also
supplies needed fertility. The very intensive method
of culture employed in the production of vegetables
necessitates the use of much greater quantities of
manures than are used for field crops, and the great
value of the product justifies the practice.

157. Fruits. — In manuring fruits, with the exception
of some of the small, rapid-growing ones, it is the aim
to maintain a continuous supply of nutrients available
to the plant, but not sufficient for stimulation, except
during the early life of the tree, when rapid growth

ABSORPTION BY SOIL PARTICLES 297

of wood is desired. An acre of apple trees in bearing
removes as much plant-food from the soil in one season
as does an acre of wheat.

Farm manure and a complete fertilizer may be used,
of which the constituents should be in a fairly available
form, as a constant supply is necessary.

V. ABSORPTION BY THE SOIL OF SUBSTANCES
IN SOLUTION

If the brown water extract from manure be filtered
through a clay soil not containing soluble alkalies, the
filtrate will be nearly colorless. Many solutions of dye
stuffs are affected in the same way. Solution of alkali or
alkaline earth salts are more or less modified by this
operation, the bases being retained by the soil. Thus
when a solution of the nitrate, sulfate, or chloride of any
one of these bases is filtered through the soil, a part of
the base is absorbed by the soil, while the acid comes
through in the filtrate. If these bases are in the form
of phosphates or silicates, not only the base is absorbed
but the acid as well.

158. Substitution of bases. — Associated with the
absorption of the base from solution, there is liberation
of some other base from the soil, which combines with
the acid in the solution and appears in the filtrate as a
salt of that acid.

When absorption takes place from solution, the base
is never entirely removed, no matter how dilute the
solution may be. A dilute solution of potassium chloride
filtered through a soil will produce a filtrate containing

298

THE PRINCIPLES OF SOIL MANAGEMENT

some calcium chloride or sodium chloride, or both, and
some potassium chloride. The more dilute the solution,
the larger the proportion retained. Peters treated 100
grams of soil with 250 c.c. of a solution of potassium
salts, and found that the potassium of different salts was
retained in different proportions, and that the stronger
solutions lost relatively less than the weaker.

Table XLV

Strength of solution

fo normal

2 0 normal

Grams K2O
absorbed

Grams K2O
absorbed '

KCL

.3124
.3362

.5747

.1990

K,S04

.2098

K2C04

.3134

The same bases are not always absorbed to the same
extent by different soils; one soil may have a greater
absorptive power for potassium, while another may retain
more ammonia. They seem to be interchangeable, as any
absorbed base may be released by another in solution.

159. Time required for absorption. — The amount of
absorption depends upon the time of contact between the
soil and the solution. While a large part of the dis-
solved base is taken up in a short time after being in
contact with the soil, the maximum absorption is only
effected after considerable time. Ammonia, according
to Way, reaches its maximum absorption in half an
hour, while Henneberg & Stohmann found that phos-
phorus required twenty-four hours to reach the same
degree of absorption.

PROPERTIES OF ABSORBED SUBSTANCES 299

160. Insolubility of certain absorbed substances. —
Although bases once absorbed may be easily displaced
by other bases, it is a difficult matter to dissolve them
from the soil with pure water. Peters treated 100 grams
of soil with 250 c.c. of water containing potassium
chloride, of which .2114 grams of K20 were absorbed.
The soil was then leached with distilled water, using
125 c.c. of water daily for ten days. At the end of that
time .0875 grams of K20 had been removed, or at the
rate of 28,100 parts of water to one part of K20 dis-
solved from the soil. Henneberg and Stohmann found
that it required 10,000 parts of water to dissolve one
part of absorbed ammonia from the soil.

161. Influence of size of particles. — The surface
area of the soil-particles determines to some extent
the amount of substance absorbed. For this, and other
reasons, a fine-grained soil absorbs a greater quantity
of material than a coarse-grained soil. In fact, it was
early shown by Way that the absorption phenomenon
is largely a function of the silt, clay and humus of the
soil.

162. Causes of absorption. — A number of causes have
been assigned for the absorption of substances by soils,
and there can be no doubt that the phenomenon is not
due to any one process. Several distinct causes are now
quite generally recognized and, while others that have
been suggested may have a part in the result, they
cannot all be taken up at this time. The bexter-known
and more important absorption processes are the fol-
lowing:

163. Zeolites. — As stated on a preceding page,

300 THE PRINCIPLES OF SOIL MANAGEMENT

Way demonstrated that sand had little absorbing power
as compared with clay, and further, that when the
zoelitic silicates were removed from clay by digestion
with hydrochloric acid, the clay largely lost its power
of absorption. Way produced an artificial hydrated
silicate of alumina and soda, and Eichorn found natural
hydrated silicates or zeolites that removed bases with
the substitution of other bases, in the manner of natural
soil. A further characteristic of these zeolites is that
the replaced base is present in the filtrate in amounts
chemically equivalent to the base removed.

It has further been shown that the absorptive power
of soils is more or less proportional to the amount of
acid soluble silicates it contains. The zeolites being
rather easily soluble in strong mineral acids, it is held
that the bases so combined are more readily available
to plants than in most combinations found in the soil,
and yet are not readily leached out of it.

Soluble bases added to the soil in manures are taken
up and held by zeolites, instead of being removed in the
drainage water. However, nitric acid, important as it
is to agriculture, is not absorbed, and, together with the
sulfuric and hydrochloric acid, is quickly but not com-
pletely removed from the soil by drainage water.

164. Other absorbents. — Humus, ferric and alumi-
num hydrates, and calcium carbonate, exercise absor-
bent properties, but to what extent and of what import-
-ance it is difficult to say. Soils rich in humus, without
doubt, owe much of their fertility to the retention by
that constituent of a large supply of readily available
plant-food material. Many prairie soils that have been

ADSORPTION 301

reduced in productiveness under cultivation respond
to the application of organic matter in a remarkable
manner. Humus in these soils seems to be the chief
conserver of readily available plant-food materials.
Ferric and aluminum hydrate aid in the retention
of acids, notably phosphoric, by forming highly in-
soluble compounds.

165. Adsorption. — There is a physical absorption,
termed adsorption, due to the concentration of the soil
solution in contact with the surface of the particles.
The phenomenon is familiarly exemplified in the clari-
fying effect of the charcoal filter. This process results
in the retention of considerable soluble material in fine-
grained soils, that would otherwise be washed out.
In the case of nitrates, which are not retained by the
zeolites, adsorption is an important factor. (See page
325.) If a solution of a known quantity of nitrate of
soda be added to a clay soil, and it is then attempted to
extract the nitrate from the soil with distilled water,
it will be found impossible to recover a very appreciable
per cent of the amount added. While adsorption prob-
ably does not account for all of the nitrates retained,
there can be no doubt that it plays an important part.
Nutritive salts held in this way are readily available
to the plant whose root-hairs come in contact with the
soil particles.

166. Occlusion. — According to Wiley, clay in a col-
loidal state has the property of dissociating to a certain
extent potash salts, and entangling the basic ion in the
meshes of the colloid structure. How extensive or
important this action is has not been demonstrated.

302

THE PRINCIPLES OF SOIL MANAGEMENT

167. Absorption as related to drainage. — The drainage
water from cultivated fields in the humid region, and
to a less extent in the semi-arid and arid region, except
where irrigation is practiced, carries off very consider-
able amounts of plant-food material. The loss of this
material is due to the operation of the various natural
disintegrating agents upon the soil mass, and to the

Fig. 101. Wasting manure by leaching.

application of fertilizing materials in a soluble form.
The various absorptive properties stand between the
natural solubility of the soil and the tendency to loss in
drainage, and hold these materials that would otherwise
be lost, in a condition in which they may readily be used
by the plant.

168. Substances usually carried in drainage water. —
However, some material is always lost in drainage
water, of which, among the bases of the soil those most

SUBSTANCES REMOVED IN DRAINAGE WATER 303

likely to be found are soda, magnesia and lime, and of
the acids nitric, carbonic, hydrochloric and sulfuric.
Nitric acid and lime undergo the most serious losses.
The former may be curtailed to a great extent by keeping
crops growing on the soil, during all of the time that
nitrification is going on, and if the crop does not mature
or if, for any other reason, it is not desired to harvest
the crop, it should be plowed under, to return the nitro-
gen in the form of organic matter. A crop used for this
purpose is called a "catch crop." Rye is used quite
commonly as a catch crop, as it continues growth until
late in the fall, and resumes growth early in the spring,
conserving nitrates whenever nitrification may occur,
and it may then be plowed under to prepare the land
for another crop. Rye also has the advantage of small
cost for seed.

The loss of calcium cannot well be prevented, and
the use of commercial fertilizers always greatly in-
creases such loss. The only remedy is the application
of some form of calcium to the soil.

169. Drainage records at Rothamsted. — Drainage
water from a series of plats at the Rothamsted Experi-
ment Station, which have been manured in various
ways and planted to wheat each year since 1852, have
been analysed at certain times, and the results of these
analyses, as compiled by Hall, give some idea of the loss
of salts from cultivated soils. The drainage water was
obtained from the tile drains, one of which extended
under each plat from one end to the other, and opened
into a ditch, so that the water could be collected when
desired. The analyses shown in the accompanying table

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(304)

SUBSTANCES REMOVED IN DRAINAGE WATER 305

were made by Dr. A. Voelcker, and represent the mean
of not more than five collections made in December,
May and January and April during a period of two years.
They can not be regarded as showing accurately the
annual removal of salts from the soil but are still sig-
nificant.

From this table it will be seen that lime is the in-
gredient lost in largest amount from this soil and that
the character of the manure applied influences this loss
to some extent. The sulfates of sodium, potassium,
and magnesium have notably increased the loss of lime,
as have also the ammonium salts. The loss of lime from
all of the manured plats was notably greater than from
the unmanured.

Potash was not removed in large amount by the
drainage water from any of the plats. Ammonium
salts with superphosphate and with magnesia occasioned
only a slight loss of potash, as did also the absence of
manure. The plats receiving mineral manure alone and
farm manure lost the greatest quantities of potash.

The quantity of sulfuric acid leached from the soil
is quite large and highly variable. It is frequently,
but by no means uniformly, large on those plats from
which lime is removed in large amounts. The plat
receiving farm manure lost the largest quantity of sul-
furic acid.

Phosphoric acid was removed in small amounts and,
except in the case of the unmanured plat, those plats
losing the least phosphoric acid gave the largest yields.
The loss of phosphoric acid seems to be a matter of
failure on the part of the crop to utilize it, rather than
its liberation by any manurial substance.

T

306 THE PRINCIPLES OF SOIL MANAGEMENT

Ammoniacal nitrogen in the drainage water is very
small in amount, but nitrate nitrogen is present in
amounts sufficient to make the loss of some concern.
The use of sodium nitrate occasioned the greatest loss
of nitrogen while ammonium salts and farm manure
contributed nearly as much. Forty to fifty pounds of
nitrogen per acre may be lost annually in this way,
which amount would have a commercial value of from
eight to nine dollars.

The most serious losses are those of nitrogen and
lime, and both are to an extent unavoidable. Potassium
and phosphorus, which must also be purchased in ma-
nures, are lost only at the rate of a few pounds per acre
but had lime been applied to any of these plats, the loss
of potassium would probably have been larger. Nitro-
gen and phosphorus are best conserved by keeping
crops growing on land as much of the time as possible,
and the former may also be protected by applying the
soluble nitrogen salts only at a time when they can be
utilized by crops. The loss of calcium frequently amounts
to several hundred pounds per acre annually, and, as
the presence of calcium carbonate is essential to a
healthy condition, of the soil this loss, particularly
from the soil receiving salts like sulfates and chlorides,
the bases of which are absorbed by plants in larger
amounts than the acids, is likely to result in a very bad
condition of the soil. The only method of obviating this
is to lime the soil from time to time.

170 Relation of absorptive capacity to productive-
ness.— The absorptive capacity of a soil is not so much
a measure of its immediate as of its permanent produc-

ALKALI SOILS 307

tiveness. It is well known that a very sandy soil responds
quickly to the application of soluble manures, but that
the effect is confined mainly to one season; while a clay
soil, although not so quickly responsive to fertilization,
shows the effect of the application much more markedly
the second or third year than does the sandy soil. Me-
chanical absorption holds the nutritive material in a very
readily available condition, while absorption by zeo-
litic bodies renders these substances somewhat less
readily available. There are also other reasons why the
sandy soil is more responsive.

It cannot be said that there is a relation between the
absorptive capacity of a soil and its productiveness when
manured or when nearly virgin, but soil long-cultivated
and unmanured frequently show such a relation. King,
in working with eight types of soil in different portions
of the United States, found that those soils removing
the most potassium from solution gave the largest yield
of crop. It would not be permissible, however, to adopt
this test as a method for determining productiveness
in soils.

VI. ALKALI SOILS

As already explained (page 14), soils are acted upon
by a great variety of agencies, which gradually render
soluble a portion of the particles. The soluble matter
is taken up by the soil water, and in humid regions
where a large amount of water percolates through the
soil and passes off in the drainage, the soluble matter
is found only in small quantity at any time. In arid
regions the loss by drainage is slight or entirely wanting,

308

THE PRINCIPLES OF SOIL MANAGEMENT

and under such conditions the soluble materials accumu-
late in the soil, being transposed downward with the
percolating water and upward again with the capillary
rise of water during the dry period. The lower soil may
at one time contain considerably more soluble salt
than the upper soil, while at another time the upper

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Fig. 102. Bare spot, marking the first appearance of injurious quantities of
alkali salts in the surface layer of soil. Utah.

soil may contain more of these salts, in which case the
solution in contact with plant-roots may, and often
does, contain so much soluble matter that vegetation
is injured or destroyed. This excess of soluble salts may
or may not have a marked alkaline reaction, but in
any case produce what are termed alkali soils.

KINDS OF ALKALI 309

171. Composition of alkali salts. — The materials
dissolved in the soil water consist of all of the sub-
stances found in the soil, but, as the rates of solubility
of these substances vary greatly, there accumulates
a much larger quantity of some substances than of
others. Carbonates, sulfates and chlorides of sodium,
potassium, calcium and magnesium occur in the largest
amounts. Sodium may be present as carbonate, sulfate,
chloride, phosphate and nitrate. Potassium may be
similarly combined. Magnesium is likely to appear as a
sulfate or chloride, and calcium as a sulfate, chloride
or carbonate. In some soils one salt will predominate,
and in other soils other salts will prevail. A base may be
present in combination with several different acids.
The nature of the prevailing salt influences greatly the
effect upon vegetation. Table XLVII gives the composi-
tion of the soluble salts from a number of alkali soils.

172. White and black alkali. — Sulfates and chlorides
of the alkalies when concentrated on the surface of the
soil produce a white incrustation, which is very common
in alkali regions during a dry period, as a result of evapo-
ration of moisture. Alkali in which these acids predomi-
nate is called white alkali.

Carbonates of the alkalies dissolve organic matter
in the soil, thus giving a dark color to the solution
and to the incrustation, and for this reason alkali con-
taining large quantities of these salts is called black
alkali.

Black alkali is much more destructive to vegetation
than is white. A quantity of white alkali that would
not seriously interfere with the growth of most crops

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312

THE PRINCIPLES OF SOIL MANAGEMENT

might completely prevent the growth of useful crops
if the alkali were black.

173. Effect of alkali on crops. — The presence of rela-
tively large amounts of salts dissolved in water and
brought in contact with a plant cell has been shown by
DeVries to cause a shrinking of the protoplasmic lining
of the cell, the shrinking increasing with the concentration

a

b

Fig. 103. Showing plasmolysis of plant cells produced by strong solutions
of salts, (a) Normal cells; (b) cell subjected to action of 5 per cent solution
of KN03, showing (z) cell-wall, (p1, p2) plasmatic membranes, (s) vacuole;
(c) cell subjected to action of 2.3 per cent solution of KNO3, causing a slight
contraction of the plasmatic membranes.

of the solution. This causes the plant to wilt, cease
growth and finally die. The nature of the salt, and the
species and even the individuality of the plant, deter-
mine the point of concentration at which the plant
succumbs.

174. Direct effect. — The directly injurious effect of
the chlorides, sulfates, nitrates, etc., of the alkalies
and alkali earths is due to this action on the cell con-
tents. The carbonates of the alkalies have, in addition,
a corroding effect upon the plant tissues, dissolving the
portions of the plant with which they come in contact.

EFFECTS OF ALKALI ON CROPS 313

175. Indirect effect. — Indirectly alkali salts may in-
jure plants by their influence upon the soil tilth, soil
organisms, and fungous and bacterial diseases.

176. Effect upon different crops. — The factors that
determine the tolerance of plants to alkali are: (1) The
physiological constitution of the plant. (2) The rooting
habit.

The first is not well understood, but resistance varies
with species, and even with individuals of the same
species. So far as the rooting habit influences tolerance
of alkali, the advantage is with the deep-rooted plants
like alfalfa and sugar-beets, probably because at least
a part of the root is in a less strongly impregnated portion
of the soil.

Of the cereals, barley and oats are the most tolerant,
being able in some cases to produce a fair crop on soil
containing one-tenth per cent of white alkali. Of the
forage crops, a number of valuable grasses are able to
grow with somewhat more than one-tenth per cent of
alkali. Timothy, smooth brome and alfalfa are the cul-
tivated forage plants most tolerant of alkali, — although
they do not equal the native grasses in this respect.
Cotton will also tolerate a considerable amount of alkali.

177. Other conditions influencing the action of alkali.
— The larger the water content of the soil, the less the
injury to plants from alkali; but, should the same soil
become dry, the previous large quantity of water would,
by bringing into solution a larger amount of alkali,
render the solution stronger than it would otherwise have
been, and thus cause more injury.

The distribution of the alkali at different depths may

314 THE PRINCIPLES OF SOIL MANAGEMENT

have an important bearing on its effect upon plants.
Young plants and shallow-rooted plants may be entirely
destroyed by the concentration of alkali at the surface,
when the same quantity evenly distributed through the
soil, or carried by moisture to a lower depth, would have
caused no difficulty.

A loam soil, by reason of its greater water-holding
capacity, will carry more alkali without injury to plants
than will a sandy one.

Certain of the alkali salts exert a deflocculating
action upon clay soils, and effect an indirect injury in
that way.

178. Reclamation of alkali land. — The alkali salts,
being readily soluble, are carried by the soil-water where
there is any lateral movement, as frequently occurs
where land slopes to some one point. Low-lying lands
adjacent to such slopes are thus likely to contain con-
siderable alkali, and the "alkali spots" of semi-arid
regions and the large accumulations of alkali in many
of the valley lands of arid regions are traceable to this
cause.

179. Irrigation and alkali. — In irrigated regions,
the injurious effect of alkali is frequently discovered
only after irrigation has been practised for a few years.
This is due to what is known as a "rise of alkali," and
comes about through the accumulation, near the surface
of the soil, of salts that were formerly distributed
throughout a depth of perhaps many feet. Before the
land was irrigated, the rainfall penetrated only a slight
depth into the soil, and when evaporation took place
salts were drawn to the surface from only a small volume

RECLAMATION OF ALKALI LAND 315

of soil. When, however, irrigation water was turned
upon the land, the soil became wet for perhaps fifteen
or twenty feet in depth. During the portion of the year
in which the soil is allowed to dry, large quantities of
salts are carried to the upper soil by the upward-moving
capillary water. These salts are in part carried down again
by the next irrigation, but the upward movement con-
stantly exceeds the downward one. This is because the
descending water passes largely through the non-capillary
interstitial spaces, while the ascending water passes en-
tirely through the capillary ones. The smaller spaces,
therefore, contain quite a quantity of soluble salt after
the downward movement ceases and the upward move-
ment commences. In other words, the volume of water
carrying downward the salts in the capillary spaces is less
than that carrying them upward through these spaces.
Surface tension causes the salts to accumulate largely in
the capillary spaces, and it is therefore the direction of
the principal movement through these that determines
the point of accumulation of the alkali.

There are large areas of land in Egypt, India and even
in France and Italy, as well as in this country, that have
suffered in this way, and not infrequently they have
reverted to a desert state.

There are a number of methods that have been used
with more or less success to reclaim alkali land.

180. Underdrainage. — Of the various methods for
removing an excess of soluble salts the use of tile drains
is the most thorough and satisfactory. When this is
used in an irrigated region, heavy and repeated appli-
cations of water must be made, to leach out the alkali

316 THE PRINCIPLES OF SOIL MANAGEMENT

from the soil and drain it off through the tile. When
used for the amelioration of alkali spots in a semi-arid
region, the natural rainfall will in time effect the removal.

In laying tiles, it is necessary to have them at such
a depth that soluble salts in the soil beneath them will
not readily rise to the surface. This will depend upon
those properties of the soil governing the capillary move-
ment of water. Three or four feet frequently suffices,
but the capillary movement should first be determined.

After drains have been placed, the land is flooded with
water to a depth of three or four inches. This is allowed
to soak into the soil and pass off through the drains,
leaching out part of the alkali in the process. Before the
soil has time to become very dry the flooding is repeated
and the operation kept up until the land is brought into
a satisfactory condition.

Crops that will stand flooding may be grown during
this treatment, and they will serve to keep the soil from
puddling, as it is likely to do if allowed to dry on the
surface. If crops are not grown, the soil should be har-
rowed between floodings.

The operation should not be carried to a point where
the soluble salts are reduced below the needs of the
crop, or to lose entirely their effect upon the retention of
moisture.

181. Correction of black alkali. — The use of gypsum
on black-alkali land has sometimes been practiced for
the purpose of converting the alkali carbonates into sul-
fates, thus ameliorating the injurious properties of the
alkali without decreasing the amount. The quantity
of gypsum required may be calculated from the amount

NEUTRALIZATION OF BLACK ALKALI

317

and composition of the alkali. The soil must be kept
moist, in order to bring about the reaction, and the
gypsum should be harrowed into the surface, and not
plowed under.

Fig. 104. Bromus inermis growing on reclaimed alkali land.

When soil containing black alkali is to be tile-drained,
it is recommended that the land first be treated with
gypsum, as the substitution of alkali sulfates for carbo-
nates causes the soil to assume a much less compact
condition and thus facilitates drainage, as well as pre-
venting the loss of organic matter dissolved by the alkali

318 THE PRINCIPLES OF SOIL MANAGEMENT

carbonates, and soluble phosphates, both of which are
precipitated by the change.

182. Retarding evaporation. — As evaporation of
moisture from the surface of the soil is the cause of rise
of alkali, it is important to reduce evaporation to a
minimum, either in drained or in undrained land.
Especially where irrigation is practiced without drainage,
it becomes desirable to use as little water as is necessary
to produce good crops, and to conserve this to the utmost
by checking evaporation from the surface of the soil.

The methods used for checking evaporation are the
maintenance of a soil or other mulch, and of a good
tilth. (See page 195.) In handling alkali spots in the
semi-arid region, it is very important to reduce evapo-
ration to the smallest amount practicable.

183. Cropping with tolerant plants. — Certain alkali
soils that are strongly impregnated with alkali may be
gradually improved by cropping with sugar-beets and
other crops that are tolerant of alkali, and which re-
move large amounts of salts. This is more likely to be
efficacious where irrigation is not practiced.

184. Other methods. — Numerous other methods of
disposing of alkali or ameliorating its effects have been
used or proposed. Among these are the following:
(1) "Leaching," which consists of flooding the surface
of the soil for the purpose of carrying the soluble salts
down to a depth of three or four feet, where they will
not effect the roots of ordinary crops. If natural drain-
age exists, this plan is effective and without danger;
otherwise evaporation must be reduced to the smallest
possible amount. (2) Removal of alkali by scraping the

MANURES 319

surface when the salts have accumulated there in time
of drought. While this may aid in the work of ameliora-
tion, it is not a final solution of the difficulty. (3) Wash-
ing the alkali from the land by turning on a rapidly
moving body of water, when the alkali is encrusted on
the surface of the soil, has been tried, but with poor suc-
cess, as the alkali is largely carried into the soil, instead
of being removed by the water passing over the surface
of the land.

185. Alkali spots. — In semi-arid regions, small areas
of alkali are frequently found, varying from a few square
yards to several acres in size. The quantities of alkali
in these are usually not sufficient to prevent the growth
of crops in years of good rainfall, but in periods of
drought the concentration of the salts and the compact
condition they tend to produce combine to injure the
crop. The methods already mentioned for treating alkali
land are of service on these small areas, and, in addition,
the plowing under of fresh farm manure has been found
to improve their productiveness. This, with surface
drainage, deep tillage and good cultivation, to prevent
the soil from drying out, will usually remedy the diffi-
culty. Frequently these spots become highly productive
under proper treatment.

VII. MANURES

A manure is any solid substance added to the soil to
make it more productive. This it may do: (1) By im-
proving the physical condition of the soil, as usually
results from the application of lime and the incorporation

320 THE PRINCIPLES OF SOIL MANAGEMENT

of organic matter. (2) By favoring the action of useful
bacteria, which is one of the most beneficial results of
farm manure, and also of lime. (3) By counteracting
the effects of toxic substances, as, for instance, the con-
version of sodium carbonate into sulfate by gypsum,
or the neutralization of acidity, or possibly the removal
of toxic organic substances by certain salts. (4) By
adding to the soil the nutrient materials absorbed by
plants, which results in the case of almost all substances
used as manures.

186. Early ideas of the function of manures. —
Manures were at one time supposed to pulverize the soil,
and the French word manceuvrer, from which the word
manure comes, means to work with the hand. This idea
probably originated through the observation that farm
manure, which was the only manure in use at that time,
made the soil less cloddy.

It has been argued, notably by Jethro Tull, that as
tillage pulverizes the soil it may be used as a substitute
for manures. There are, however, conditions aside from
tilth that are influenced by manures, and good tilth
alone will not suffice to maintain a permanently intensive
agriculture. It is true in the United States, as it is in
Europe, that a large consumption of manures goes hand-
in-hand with a highly developed and intensive system of
farming.

187. Development of the idea of nutrient function of
manures. — While the use of animal excrement on cul-
tivated soils was practiced as far back as systematic
agriculture can be definitely traced, the earliest record
of the use of mineral salts for increasing the yield of

HISTORY OF COMMERCIAL FERTILIZERS 321

crops was published, in 1669, by Sir Kenelm Digby. He
says, "By the help of plain salt petre, diluted in water,
and mingled with some other fit earthly substance,
that may familiarize it a little with the corn into which
I endeavored to introduce it, I have made the barrenest
ground far outgo the richest in giving a prodigiously
plentiful harvest." His dissertation does not, however,
show any true conception of the reason for the increase
in the crop through the use of this fertilizer. In fact,
the want of any real knowledge at that time of the com-
position of the plant would have made this impossible.

In 1804, Theodore de Sausure published his chemical
researches upon plants, in which he, for the first time,
called attention to the significance of the ash ingredients
of plants, and pointed out that without them plant-life
is impossible, and further, that only the ash of the plant
tissue is derived from the soil.

Justus von Liebig, in his writings published about
1840, emphasized still more strongly the importance
of mineral matter in the plant, and its extraction from
the soil. He refuted the theory, at that time popular,
that plants absorb their carbon from humus, but made
the mistake of attaching little importance to the pres-
ence of humus in the soil. He showed the importance
of potassium and phosphorus in manures, but, in his
later expressions, failed to appreciate the value of
nitrogenous manures, holding that a sufficient amount
is washed from the atmosphere in the form of ammonia.

A true conception of the necessity for a supply of
combined nitrogen in the soil was even at that time enter-
tained by Boussingault and by Sir John Lawes, although

u

322 THE PRINCIPLES OF SOIL MANAGEMENT

the elaborate experiments conducted by Lawes, Gilbert
and Pugh, in 1857, were required to fully demonstrate
the fact. Their care in conducting the experiments
resulted in their sterilizing the soil with which they
experimented, and hence their failure to discover the
utilization of free atmospheric nitrogen by legumes.

Between 1840 and 1850, Sir John Lawes began the
manufacture of bone superphosphate, and, about the
same time, Peruvian guano and nitrate of soda were
introduced into Europe. The commerical fertilizer
industry thus dates from this time.

188. Classes of manures. — While manures are very
numerous as to kind, and a certain manure may have
a number of distinct functions, they may yet be roughly
divided into classes. They will accordingly be treated
under the following heads: (1) Commercial fertilizers.
(2) Farm manures. (3) Green manures. (4) Soil amend-
ments.

189. Commercial fertilizers. — Although the commer-
cial fertilizer industry is little more than half a century
old, the sale of fertilizers in this country amounts to
about $50,000,000 annually. Animal refuse and phos-
phate fertilizers are exported, while nitrate of soda and
potassium salts are imported.

Of the fertilizers sold in 1899, about 70 per cent was
consumed in the North Atlantic and South Atlantic
states, in an area lying within 300 miles of the seaboard.
Nearly one-half of the remainder was purchased in four
states, Ohio, Indiana, Alabama and Louisiana.

190. Function of commercial fertilizers. — Primarily
the function of commercial fertilizers is to add plant

324 THE PRINCIPLES OF SOIL MANAGEMENT

nutrients to the soil, usually in a form more readily
soluble than those already present in large quantity.
While other beneficial effects may be produced by
certain fertilizers, they are usually of secondary import-
ance, as compared with the addition of the plant nutri-
ents.

191. Fertilizer constituents. — Prepared fertilizers, as
found on the market, are usually composed of a number
of ingredients. As these are the carriers of the fertilizing
material, and as it is upon their composition and solu-
bility that the value of the fertilizer depends, a knowl-
edge of the properties of these constituents is of interest
to every user of fertilizers, and is a valuable aid in their
purchase.

192. Fertilizers used for their nitrogen. — Nitrogen
is the most expensive constituent of manures, and is of
great importance, as it is very likely to be deficient in
soils. A commercial fertilizer may have its nitrogen
in the form of soluble inorganic salt, or combined as
organic material. Upon the form of combination de-
pends to a certain extent the value of the nitrogen,
as the soluble inorganic salts are very readily available
to the plant, while the organic forms must pass through
the various processes leading to nitrification before the
plant can use the nitrogen so contained. The inorganic
nitrogen fertilizers are sodium nitrate, ammonium sul-
fate, calcium nitrate and calcium cyanamid.

193. Sodium nitrate. — This fertilizer now constitutes
the principal source of inorganic nitrogen in commercial
fertilizers. The salt occurs in the crude condition in
Northern Chili, and is believed to be due to the action

NITROGEN BEARING FERTILIZERS 325

of soil organisms acting through a very long period, and
leaving the product finally in the form of sodium nitrate
that has crystalized out of solution in which it has some-
time been held. The crude salt is purified by crystalli-
zation, and, as put upon the market, contains about
96 per cent sodium nitrate, or about 16 per cent of nitro-
gen, 2 per cent of water, and small amounts of chlorides,
sulfates and insoluble matter. The cost of nitrogen in
this form is from fifteen to eighteen cents per pound.

On account of its easy availability, sodium nitrate
acts quickly in inducing growth. For this reason it is
used much by market gardeners, and for other purposes
when a rapid growth is desired. It is the most active
form of nitrogen. A light dressing on meadow land in the
early spring assists greatly in hastening growth by fur-
nishing available nitrogen before the conditions are
favorable for the process of nitrification. On small
grain it serves a similarly useful purpose where the soil
is not rich.

Owing to the fact that it is not absorbed by the soil
in large quantities, it is easily lost in the drainage water;
for which reason it should only be applied when crops
are growing upon the soil, and then only in moderate
quantity.

The continued and abundant use of sodium nitrate
upon the soil may result, through its deflocculating
action, in breaking down aggregates of soil-particles,
thus compacting and injuring the structure. This effect
is attributed to the accumulation of sodium salts, par-
ticularly the carbonate, as the sodium is not utilized
by the plant to the same extent as is the nitrogen.

326 THE PRINCIPLES OF SOIL MANAGEMENT

194. Ammonium sulfate. — When coal is distilled,
a portion of the nitrogen is liberated as ammonia, and is
collected by passing the products of distillation through
water in which the ammonia is soluble, forming the
ammoniacal liquor. The ammonia thus held is distilled
into sulfuric acid with the formation of ammonium
sulfate and the removal of impure gases.

Commercial ammonium sulfate contains about 20 per
cent of nitrogen. It is the most concentrated form in
which nitrogen can be purchased as a fertilizer, having
from sixty to eighty pounds more of nitrogen per ton
than sodium nitrate. It is, therefore, economical to
handle. Its effect upon crops is not so rapid as that of
sodium nitrate, but it is not so quickly carried from the
soil by drainage water, as the ammonium salts are
readily absorbed by the soil. A pound of nitrogen in the
form of sulfate has about the same value as the same
amount in the form of nitrate.

The long and extensive use of ammonium sulfate on
a soil has a tendency to produce an acid condition,
through the accumulation of sulfates which are not
largely taken up by plants.

Ammonium sulfate, like sodium nitrate, should
not be applied in the autumn, as the ammonia is con-
verted into nitrates and leached from the soil in sufficient
quantities to entail a very decided loss of nitrogen.
There is not likely to be so large a loss of nitrogen from
ammonium salts as from nitrates, and, as would naturally
be expected, there is greater loss of nitrogen when these
salts are used alone than when they are combined with
other fertilizing ingredients.

LOSS OF SOIL NITROGEN

327

Hall has estimated the loss of nitrogen from certain
drained plats, of the Rothamsted Experiment Station.
This estimate is based upon the concentration of the
drainage from the different plats, of which there was
no record of total flow, but for which the measurements
of flow from the lysimeter draining 60 inches of soil
were taken, and the total loss of nitrates calculated
on this basis. Estimated in this way, the effects of sev-
eral different methods of manuring are shown in the
accompanying table.

Table XLVIII
Pounds per Acre Nitric Nitrogen in Drainage Water

Treatment

Unmanured

Mineral fertilizers only

Minerals + 400 pounds amnion, salts . .
Minerals + 550 pounds nitrate of soda .
Minerals + 400 pounds ammon. salts

applied in autumn

400 pounds ammon. salts alone

400 pounds ammon. salts + sulfate of

potash

Estimated drainage in inches

1879-80

Spring
sowing

to
harvest

1.7

1.6
18.3
45.0

9.6

42.9

19.0
11.1

Harvest

to
spring
sowing

10.8
13.3
12.6
15.6

59.9
14.3

16.4

4.7

1880-81

Spring
sowing

to
harvest

0.6

0.7

4.3

15.0

3.4
7.4

3.7

Harvest

to
spring
sowing

17.1
17.7
21.4
41.0

74.9
35.2

25.3

18.8

This table, in addition to confirming the statements
already made in regard to the loss of nitrogen in drain-
age waters, also shows how closely the supply of avail-
able nitrogen was used by the crops on those plats,

328 THE PRINCIPLES OF SOIL MANAGEMENT

evidently in need of nitrogen fertilization, as these plats
lost very little nitrogen during the growing season,
while during the remainder of the year they lost nearly
as much as did some of the nitrogen manured plats.
It also indicates that the loss when nitrate is used is
greater than when ammonium salts are applied, as the
amount of nitrogen in the 550 pounds of nitrate is really
eight pounds per acre more than in the 400 pounds
ammonium sulfate, which is not sufficient to account
for the difference in the loss. However, half of the
nitrate treated plat received no other manure, and
produced only a small crop, which would naturally
result in a a greater loss by drainage.

195. Calcium cyanamid. — The vast store of atmos-
pheric nitrogen chemically uncombined, but very inert,
will furnish an inexhaustible supply of this highly valu-
able fertilizing element, when it can be, with reasonable
economy, combined in some manner that will result
in a product commercially transportable, and that will,
when placed in the soil, be or become soluble without
liberating substances toxic to plants. The importance
of the nitrogen supply for agriculture may be appreciated
when we consider that nitrates are being carried off
in the drainage water of all cultivated soils at the rate
of from twenty-five to fifty pounds and even more per
acre, annually, and that nearly as much more is removed
in crops.

The exhaustion of the supply of nitrogen in most
soils may be accomplished within one or two generations
of men, unless a renewal of the supply be brought about
in some way. Natural processes provide for an annual

LIME NITROGEN 329

accretion through the washing down of ammonia and
nitrates by rain-water from the atmosphere, and through
the fixation of free atmospheric nitrogen by bacteria;
but, without the frequent use of leguminous crops,
the supply could not be maintained. Farm practice
of the present day requires the application of nitrogen
in some form of manure, and, as the end of the commer-
cial supply of combined nitrogen is easily in sight, there
is urgent need of discovering a new source. This has
lately been done by combining calcium with atmospheric
nitrogen in the forms of calcium cyanamid and cal-
cium nitrate.

The most successful process for the production of
cyanamid consists in passing nitrogen into closed retorts
containing powdered calcium carbide heated to a tem-
perature of 1,100° C, the product being calcium cyana-
mid, and free carbon.

CaC2 + 2N = CaCN2 + C.

The free carbon remains distributed in the cyanamid
and gives it a black color. A modification of the process
provides for the use of lime and coke instead of calcium
carbide, but this has not yet been used on a commercial
scale. The nitrogen required for the process is obtained
either by passing air over heated copper, or by the frac-
tional distillation of liquid air.

The fertilizer, as placed on the market, is a heavy,
black powder with a somewhat disagreeable odor. At
present it is not manufactured in America and is not
obtainable except in small amounts. Plants for its
production are being promoted, which will doubtless/

330 THE PRINCIPLES OF SOIL MANAGEMENT

result in its being placed on the market in the near
future.

There are, at present, two calcium cyanamid ferti-
lizers being manufactured. One is called lime-nitrogen,
and is made in Italy; the other is called nitrogen-lime,
and is made in the province of Saxony,. Germany. The
former contains 15 to 23 per cent nitrogen, 40 to 42
per cent calcium, and 17 to 18 per cent carbon dust.
The latter is said to contain somewhat less nitrogen,
and to have in it some calcium chloride, which is some-
times injurious to plants.

The value of calcium cyanamid as a fertilizer has not
yet been definitely and conclusively ascertained. The
cyanamid must be decomposed before becoming avail-
able to the plant. Under favorable conditions, the nitro-
gen of the cyanamid is converted into ammonia; but,
if the conditions for decomposition are not favorable,
the dicyanamid may be formed, which has a poisonous
effect upon plants. Another objection which sometimes
obtains is that acetylene is produced from the carbide,
which remains unchanged in the manufacture of the
cyanamid. Acetylene is also injurious to plants.

By incorporating the calcium cyanamid in the soil
eight to fourteen days before the seed is planted, this
difficulty may be overcome. It is also important that
the cyanamid be plowed under, and not left on or near
the surface of the soil, as, under these circumstances,
decomposition does not go on properly, and the poisonous
action above referred to takes place.

Upon heavy soil the value of cyanamid as a fertilizer
is not greatly below that of sodium nitrate, but upon

CALCIUM NITRATE 331

.sandy soil it ranks much lower. Indeed, it appears to be
but poorly suited to use on sandy soils.

196. Calcium nitrate. — The other process for com-
bining atmospheric nitrogen is of even more recent
invention than that for the manufacture of calcium
cyanamid and, like it, is not conducted on a commercial
scale in this country; but, with the vast opportunities
for developing electric power which are offered in certain
localities, factories for the manufacture of calcium nitrate
will soon be established.

The process employs an electric arc to produce nitric
oxide by the combustion of atmospheric nitrogen, ac-
cording to the simple equation:

N2 + 02 = 2NO.

A very high power is required for this synthesis,
involving a temperature of 2,500° to 3,000° C, and the
expense of the operation is determined almost entirely
by the cost of the electricity.

The nitric oxide gas is passed through milk of lime,
giving calcium nitrate.

The calcium nitrate produced by this process has a
yellowish white color, and is easily soluble in water,
but deliquesces very rapidly in the air. This last prop-
erty can be overcome by adding an excess of lime in the
manufacture, thus producing a basic calcium nitrate,
which contains only 8.9 per cent nitrogen. Another
way of avoiding the difficulties involved by the deliques-
cent property of the nitrate is practiced by the factory
at Nottoden, Norway. This consists in first melting
the product, then grinding it fine, and packing it in

332 THE PRINCIPLES OF SOIL MANAGEMENT

air-tight casks. The fertilizer thus prepared contains

11 to 13 per cent nitrogen.

Calcium nitrate contains its nitrogen in a form
directly available to plants. It resembles sodium nitrate
in its solubility, availability, and lack of absorption by
the soil. It may be spread upon the surface of the ground,
as it exerts no poisonous action, and does not tend to
form a crust, as does sodium nitrate.

The relative values of the different soluble nitrogen
fertilizers vary with a great many conditions and can
be accurately judged only by a large number of tests.
At present, both the calcium nitrate and the cyanamid
are being produced at less cost per pound of nitrogen
than is sodium nitrate, when laid down in the neighbor-
hood of the factories in Europe. It seems quite certain
that, when the processes have been further improved,
the result will be to greatly reduce the cost of the avail-
able nitrogen.

197. Organic nitrogen in fertilizers. — The commercial
fertilizers containing organic nitrogen include cotton-
seed-meal, which contains 7 per cent nitrogen, when
free from hulls; linseed-meal, with 5.5 per cent nitrogen;
castor pomace, having 6 per cent nitrogen; and a number
of refuse products from packing-houses, among which
there are red-dried blood and black-dried blood, the
former having about 13 per cent nitrogen, and the latter
6 to 12 per cent; dried meat and hoof meal, carrying

12 to 13 per cent nitrogen; ground fish containing 8 per
cent nitrogen; and tankage, of which the concentrated
product has a nitrogen content of 10 to 12 per cent, and
the crushed tankage, 4 to 9 per cent; also leather-meal

OTHER FORMS OF ORGANIC NITROGEN 333

and wool-and-hair waste, which last two, on account
of their mechanical condition, are of practically no value.

The meals made from seeds are primarily stock-foods,
but are sometimes used as manures. They decompose
rather slowly in the soil, owing to their high oil content,
and are much more profitably fed to live stock than
applied as farm manure. They contain some phosphorus
as well as nitrogen.

Guano consists of the excrement and carcasses of
sea-fowl. The composition of guano depends upon the
climate of the region in which it is found. Guano from
an arid region contains nitrogen, phosphorus and potas-
sium, while that from a region where rains occur con-
tains only phosphorus — the nitrogen a-nd potassium
having been leached out. In a dry guano the nitrogen
occurs as uric acid, urates, and, in small quantities,
as ammonium salts. A damp guano contains more
ammonia. The phosphorus is present as calcium phos-
phate, ammonium phosphate, and as the phosphates of
other alkalies. A portion of the phosphate is readily
soluble in water. All of the plant-food is thus either
directly soluble, or becomes so soon after admixture
with the soil. The composition is extremely variable.
The best Peruvian guano contains from 10 to 12 per cent
of nitrogen, 12 to 15 per cent phosphoric acid, and 3 to 4
per cent of potash.

Guano was formerly a very important fertilizing
material, but the supply has become so nearly exhausted
that it is relatively unimportant at the present time.

Of the abattoir products, dried blood is the most
readily decomposed, and therefore has its nitrogen

334 THE PRINCIPLES OF SOIL MANAGEMENT

in the most available form. In fact, it produces results
more quickly than any other form of organic nitrogen.
It requires a condition of soil favorable to decomposi-
tion and nitrification, which prevents its exerting a
strong action in the early spring. It should be applied
to the soil before the crop is planted. The black dried
blood contains from 2 to 4 per cent of phosphoric acid.

Dried meat contains a high percentage of nitrogen,
but does not decompose so easily, and is not so desirable
a form of nitrogen. It can be fed to hogs or poultry to
advantage, and the resulting manure is very high in
nitrogen.

Hoof-meal, while high in nitrogen, decomposes slowly,
being less active than dried blood. It is of use in in-
creasing the store of nitrogen in a depleted soil.

Ground fish is an excellent form of nitrogen, and is as
readily available as blood, but has a lower nitrogen
content.

Tankage is highly variable in composition, and the
concentrated tankage, being more finely ground, under-
goes more readily the decomposition necessary for the
utilization of the nitrogen. Crushed tankage contains
from 3 to 12 per cent of phosphoric acid, in addition
to its nitrogen.

Leather-meal and wool-and-hair waste are in such
a tough and undecomposable condition that they may
remain in the soil for years without losing their struc-
ture. They are not to be recommended as manures.

198. Fertilizers used for their phosphorus. — Phos-
phorus is generally present in combination with lime,
iron or alumina. Some of the phosphates also contain

PHOSPHATE FERTILIZERS 335

organic matter, in which case they generally carry some
nitrogen. Phosphates associated with organic matter
decompose more quickly in the soil than untreated
mineral phosphates.

199. Bone phosphate. — Formerly, bones were used
entirely in the raw condition, ground or unground.
When ground, they are a more quickly acting fertilizer
than when unground. Raw bones contain about 22
per cent phosphoric acid and 4 per cent nitrogen. The
phosphorus is in the form of tricalcic phosphate (Ca3
(P04)2).

Most of the bone now on the market is first boiled or
steamed, which frees it from fat and nitrogenous matter,
both of which are used in other ways. Steamed bone
is a more valuable fertilizer than raw bone, as the fat
in the latter retards decomposition, and also because
steamed bone is in a better mechanical condition. The
form of the phosphoric acid is the same as in raw bone,
and constitutes 28 to 30 per cent of the product, while
the nitrogen is reduced to H per cent.

Bone tankage, which has already been spoken of as a
nitrogenous fertilizer, contains from 7 to 9 per cent
phosphoric acid, largely in the form of tricalcium phos-
phate. All of these bone phosphates are slow-acting
manures, and should be used in a finely ground form,
and for the permanent benefit of the soil rather than as
an immediate source of nitrogen or phosphorus.

200. Mineral phosphates. — There are many natural
deposits wof mineral phosphates in different portions of
the world, some of the most important of which are in
North America. The phosphorus in all of these is in the

336 THE PRINCIPLES OF SOIL MANAGEMENT

form of tricalcium phosphate, but the materials asso-
ciated with it vary greatly.

Apatite is found in large quantities in the provinces
of Ontario and Quebec, Canada. It occurs chiefly in
crystalline form.

The tricalcium phosphate of which it is composed
is in one form associated with calcium fluoride, and
in the other with calcium chloride. The Canadian
apatite contains about 40 per cent phosphoric acid,
being richer than that found elsewhere. Phosphorite
is another name for apatite, but is chiefly applied to
the impure amorphous form.

Caprolites are concretionary nodules found in the
chalk or other deposits in the south of England, and in
France. They contain 25 to 30 per cent of phosphoric
acid, the other constituents being calcium carbonate and
silica.

South Carolina phosphate contains from 26 to 28 per
cent of phosphoric acid, and but a very small amount
of iron and alumina. As these substances interfere with
the manufacture of superphosphate from rock, their
presence is very undesirable, — rock containing more than
from 3 to 6 per cent being unsuitable for that purpose.

Florida phosphates occur in the form of soft phos-
phate, pebble phosphate, and boulder phosphate.
Soft phosphate contains from 18 to 30 per cent of phos-
phoric acid, and, on account of its being more easily
ground than most of these rocks, is often applied to
the land without being first converted into a superphos-
phate. The other two, pebble phosphate and boulder
phosphate, are highly variable in composition, ranging

SUPERPHOSPHATE 337

from 20 to 40 per cent phosphoric acid. Tennessee phos-
phate contains from 30 to 35 per cent of phosphoric
acid.

Basic slag, or, as it is also called, phosphate slag or
Thomas phosphate, is a by-product in the manufacture
of steel from pig-iron rich in phosphorus. The phos-
phorus present is in the form of tetracalcium phosphate,
(CaO)4P205. It also contains calcium, magnesium,
aluminum, iron, manganese silica and sulfur. On ac-
count of the presence of iron and aluminum, and because
its phosphorus is more readily soluble than the trical-
cium phosphate, the ground slag is applied directly
to the soil without treatment with acid.

201. Superphosphate fertilizers. — In order to render
more readily available to plants the phosphorus con-
tained in bone and mineral phosphates, the raw material,
purified by being washed and finely ground, is treated
with sulfuric acid. This results in a replacement of phos-
phoric acid by sulfuric acid, with the formation of
monocalcium phosphate and calcium sulfate, and a
smaller amount of dicalcium phosphate, according to the
reactions:

Ca3 (P04)2 + 2H2S04 = CaH4 (P04)2 + 2 CaS04 and
Ca3 (P04)2 + H2S04 = Ca2H2(P04)2 + CaS04.

The tricalcium phosphate being in excess of the sul-
furic acid used, a part of it remains unchanged.

In the treatment of phosphate rock, part of the
sulfuric acid is consumed in acting upon the impurities
present, which usually consist of calcium and magnesium
carbonates, iron and aluminum phosphates, and cal-

338 THE PRINCIPLES OF SOIL MANAGEMENT

cium chloride or fluoride, converting the bases into sul-
fates and freeing carbon dioxide, water, hydrochloric
acid and hydrofluoric acid. The resulting superphos-
phate is therefore a mixture of monocalcium phosphate,
dicalcium phosphate, tricalcium phosphate, calcium
sulfate, and iron and aluminum sulfates.

In the superphosphates made from bone, the iron
and aluminum sulfates do not exist in any considerable
amounts. However, as long as the phosphorus remains
in the form of monocalcium phosphate, the value of a
pound of available phosphorus in the two kinds of fer-
tilizer is the same; but the remaining tricalcium phos-
phate has a greater value in the bone than in the rock
superphosphate.

The superphosphates made from animal bone con-
tain about 12 per cent available phosphoric acid, and
3 or 4 per cent of insoluble phosphoric acid. They also
contain some nitrogen. Bone-ash and bone-black super-
phosphates contain practically all of their phosphorus
in an available form, but they contain little or no nitro-
gen. South Carolina rock superphosphate contains from
12 to 14 per cent available phosphoric acid, including
from 1 to 3 per cent reverted phosphoric acid. The best
Florida rock superphosphates contain from 17 per cent
downward of available phosphoric acid, part of which
is reverted. The Tennessee superphosphates vary from
14 to 18 per cent available phosphoric acid.

202. Reverted phosphoric acid. — On standing, a
change sometimes occurs in superphosphates by which
a part of the phosphoric acid becomes less easily soluble,
and to that extent the value of the fertilizer is decreased.

FORMS OF PHOSPHATE 339

This change, known as "reversion," is much more likely
to occur in superphosphates made from rock than in
those derived from bone. It will also vary in different
samples, — a well-made article usually undergoing little
change, even after long standing. It is supposed to be
caused by the presence of undecomposed tricalcium
phosphate, and of iron and aluminum sulfates.

203. Double superphosphates. — In making super-
phosphates, a material rich in phosphorus must be used,
— not less than 60 per cent tricalcium phosphate being
necessary for their profitable production. The poorer
materials are sometimes used in making what is known
as double superphosphates. For this purpose they are
treated with an excess of dilute sulfuric acid; the dis-
solved phosphorus and the excess of sulfuric acid are
separated from the mass by filtering and are then
used for treating phosphates rich in tricalcium phosphate
and forming superphosphates. The superphosphates
so formed contain more than twice as much phosphorus
as those made in the ordinary way.

204. Relative availability of phosphate fertilizers. —
Superphosphates and double superphosphates contain
their phosphorus in a form in which it can be taken
up by the plant at once. They are therefore best applied
at the time when the crop is planted, or shortly before,
or they may be applied when the crop is growing. Crude
phosphates, on the other hand, become available only
through the natural processes in the soil. The presence
of decomposing organic matter is a great aid to the
.decomposition of crude phosphates.

Reverted phosphorus, although not soluble in water,

340 THE PRINCIPLES OF SOIL MANAGEMENT

is readily soluble in dilute acids. It is now quite gener-
ally believed that it furnishes an available supply of
phosphorus to the plant. In a statement of fertilizer
analyses it is termed "citrate soluble," and this and the
"water soluble" are termed "available."

The degree of fineness to which the material is ground
makes a great difference in the availability of the less-
soluble phosphate fertilizers, especially in the ground-
rock phosphates, and in ground bone. This material
should be ground fine enough to pass through a sieve
having meshes one-fiftieth of an inch in diameter.

205. Fertilizers used for their potassium. — The pro-
duction of potassium fertilizers is largely confined to
Germany, where there are extensive beds varying from
50 to 150 feet in thickness, lying under a region of
country extending from the Harz mountains to the
Elbe river, and known as the Stassfurt deposits. De-
posits have lately been discovered in other parts of
Germany.

206. Stassfurt salts. — The Stassfurt salts contain
their potassium either as a chloride or a sulfate. The
chloride has the advantage of being more diffusible in
the soil, but in most respects the sulfate is preferable.
Potassium chloride has an injurious action on certain
crops, among which are tobacco, sugar-beets and pota-
toes. On cereals, legumes and grasses, the muriate
appears to have no injurious effect.

The mineral produced in largest quantities by the
Stassfurt mines is kainit. Chemically it consists of mag-
nesium and potassium sulfate, and magnesium chloride,
or magnesium sulfate and potassium chloride. Kainit

POTASH-BEARING FERTILIZERS 341

has the same action on plants as has potassium chloride.
It contains from 12 to 20 per cent of potash, and 25 to
45 per cent of sodium chloride, with some chloride and
sulfate of magnesium.

Kainit should be applied to the soil a considerable
time before the crop for which it is intended is planted.
It should not be drilled in with the seed, as the action
of the chlorides in direct contact with the seed may
injure its viability. In addition to the potassium added
to the soil by kainit, there are also in this fertilizer
magnesium and sodium. The magnesium may be objec-
tionable if there is much already present in the soil.
(See page 350.) Sodium may to some extent replace
potassium in the soil economy, and in that way may be
beneficial.

Silvinit contains its potassium both as chloride and
as sulfate. It also contains sodium and magnesium
chlorides. Potash constitutes about 16 per cent of the
material. Owing to the presence of chlorides, it has the
same effect on plants as has kainit.

The commerical form of potassium chloride generally
contains about 80 per cent potassium chloride, or 50
per cent potash. The impurities are largely sodium
chloride and insoluble mineral matter. The possible
injury to certain crops from the use of the chloride has
already been mentioned. For crops not so affected,
potassium chloride is a quick-acting and effective carrier
of potassium, and one of the cheapest forms.

High-grade sulfate of potassium contains from 49
to 51 per cent of potash. Unlike the muriate, it is not
injurious to crops but is more expensive.

342 THE PRINCIPLES OF SOIL MANAGEMENT

There are a number of other Stassfurt salts, consisting
of mixtures of potassium, sodium and magnesium in
the form of chlorides and sulfates. They are not so
widely used for fertilizers as are those mentioned above.

207. Wood ashes. — For some time after the use of
fertilizers became an important farm practice, wood
ashes constituted a large portion of the supply of potas-
sium. They also contain a considerable quantity of lime
and a small amount of phosphorus. The product known
as unleached wood ashes contains 5 to 6 per cent of pot-
ash, 2 per cent of phosphoric acid, and 30 per cent of
lime. Leached wood ashes contain about one per cent of
potash, 1^ per cent of phosphoric acid, and 28 to 29 per
cent of lime. The}^ contain the potassium in the form of a
carbonate, which is alkaline in its reaction, and may be
injurious to seeds when in large amount. They are
beneficial to acid soils through the action of both the
potassium and calcium salts. The lime is valuable for the
other effects it has on the properties of the soil. (See
page 348.)

208. Insoluble potassium fertilizers. — Insoluble
forms of potassium, occurring in many rocks, usually
in the form of a silicate, are not regarded as having
any manurial value. Experiments with finely ground
feldspar have been conducted by a number of experi-
menters, but have, in the main, given little encourage-
ment for the successful use of this material. An insoluble
form of potassium is not given any value in the rating
of a fertilizer, based upon the results of its analysis.

209. Fertilizer practice. — The purchase and use of
commercial fertilizers is an art that requires some

BRANDS OF FERTILIZERS 343

technical knowledge for its efficient conduct. There are
many fertilizing materials put up under numerous
brands that must be selected from and applied to a great
variety of crops grown on innumerable types of soil.
The result is that an economical fertilizer practice is
difficult to establish, and the use of fertilizers is usually
conducted in an entirely empirical manner.

210. Brands of fertilizers. — Each manufacturer or
compounder of commercial fertilizers places on the
market a number of brands of fertilizers that have some
trade name, frequently implying the usefulness of the
fertilizer for some particular crop, but without reference
to the character of the soil on which it is to be used.
Each brand of fertilizer is usually composed of several of
the constituents that have been described. If those sub-
stances are used that are difficultly soluble, the ferti-
lizer is not so valuable as if composed of easily soluble
substances. The solubility, as well as the percentage
of each ingredient of the fertilizer, should be known by
the purchaser.

A fertilizer is known in the market as a high-grade or
a low-grade product, depending upon the percentage of
fertilizing constituents that it contains. Low-grade
fertilizers are cheaper than high-grade merely because
they contain less plant-food, although the price per
pound of plant-food may be no less, — and, in fact, is
usually more. The low-grade product is encumbered
with a large amount of inert material, that adds to the
cost of transportation and handling, without adding
to the value of the fertilizer. For these reasons, the high-
grade material is almost always the cheaper fertilizer.

344 THE PRINCIPLES OF SOIL MANAGEMENT

A ton of low-grade fertilizer may contain 500 or 600
pounds more inert material than a high-grade fertilizer,
upon which freight must be paid, and which must be
hauled from the station and spread upon the field.

211. Fertilizer inspection. — Some thirty states have
enacted legislation providing for the inspection and con-
trol of the sale of commercial fertilizers. Each package
of fertilizer must bear a certificate stating the percentage
of nitrogen, phosphoric acid and potash, and more or less
information in regard to the forms in which these are
held and their rates of solubility. This must be guaran-
teed to be correct by the manufacturer.

The guarantee does not always state the percentage
of nitrogen (N), phosphoric acid (P205), and potash
(K20),but often uses other terms that imply the presence
of these substances, but so combined that the percentage
of the carrier is larger, — as, for instance, ammonia, bone
phosphate and sulfate of potash. To convert one term
into another, factors have been devised which greatly
simplify the process.

Per cent ammonia X .8235 = per cent nitrogen (N.)
Per cent nitrate of soda X .1647 = per cent nitrogen (N).
Per cent bone phosphate X .458 = per cent phosphoric acid

(PA)-

Per cent muriate of potash X .632 = per cent potash (K20).
Per cent sulfate of potash X .54 = per cent potash (K20).

212. Trade values of fertilizers. — It has been custom-
ary for the authorities charged with fertilizer inspection
in the states concerned to adopt each year a schedule of
trade values for nitrogen, phosphoric acid and potash,
in each of the various forms in which they appear in

TRADE VALUE OF FERTILIZERS 345

fertilizers. These values are based on the cost of the
unmixed constituents, if purchased in wholesale lots
from the manufacturer, and are secured by averaging
the wholesale prices per ton of all the various fertilizer
supplies for the six months preceding March 1, to which
is added about 20 per cent of the price, to cover cost of
handling. The trade values for 1907 were as follows:

Value per pound
Cents
Nitrogen, in nitrates 18.5

Nitrogen, in ammonium salts 17.5

Organic nitrogen, in dried and finely ground fish meat

and blood, and in mixed fertilizers 20.5

Organic nitrogen, in finely ground bone and tankage. .20.5
Organic nitrogen, in coarsely ground bone and tankage. 15.0

Phosphoric acid, soluble in water 5.0

Phosphoric acid, soluble in ammonium citrate 4.5

Phosphoric acid, insoluble, in fine bone and tankage. . 4.0
Phosphoric acid, insoluble, in coarse bone and tankage . 3.0

Phosphoric acid, insoluble, in mixed fertilizers 2.0

Phosphoric acid, insoluble, in finely ground fish, cotton-
seed meal, castor pomace and wood-ashes 4.0

Potash, as muriate 4.5

Potash, as sulfate, and in forms free from muriates. . 5.0

213. Computation of the commercial value of a ferti-
lizer.— The percentage of each fertilizing constituent of a
fertilizer, and its form or rate of solubility being known, it
is possible to calculate its commercial value. Suppose a
fertilizer costing $48 per ton contains the following:

Per cent
Nitrogen in sodium nitrate 4

Nitrogen in fine bone 3

Phosphoric acid, available, in rock superphosphate

(corresponds to soluble in ammonium citrate) .... 6

Phosphoric acid, insoluble, in fine bone 22

Potash, water soluble, in muriate of potash 10

346 THE PRINCIPLES OF SOIL MANAGEMENT

The number of pounds of each constituent per ton
of fertilizer is then found thus:

Nitrogen as nitrate 4X20= 80 pounds per ton

Nitrogen in fine bone 3X20= 60 pounds per ton

Phosphoric acid, available. . . . 6X20 = 120 pounds per ton
Phosphoric acid, insoluble . . .22X20 = 440 pounds per ton
Potash, muriate 10X20 = 200 pounds per ton

The trade values, as published by the fertilizer in-
spection authorities, are then applied to the several
constituents.

Nitrogen, as nitrates 80 X. 185 = $14 80

Nitrogen in fine bone 60 X .205 = 12 30

Phosphoric acid, available 120 X .045 = 5 40

Phosphoric acid, insoluble 40 X .040= 1 60

Potash, muriate 200 X .045 = 9 00

$43 10

The computed value may then be compared with the
market price. It must be remembered that this is the
commercial value, and not necessarily the agricultural
value, which is determined by the profits from its use,
and will depend upon many other factors. For instance,
a soil markedly deficient in nitrogen will not respond to a
phosphate fertilizer alone to an extent which would
justify its use.

214. Mixing fertilizers on the farm. — It has been
shown by several of the Experiment Stations that the
raw materials may be purchased from the manufacturers
and mixed on the farm at a considerably lower cost than
they can be bought in fertilizer mixtures, and that the
results obtained from them are fully as satisfactory.

METHODS OF APPLYING FERTILIZERS 347

Other advantages from home-mixing are, that it per-
mits the farmer to use exactly the proportions of the
several constituents that he desires, and that it makes
unnecessary the handling of a large amount of inert
material frequently contained in mixed fertilizers.
It is thus possible for him to ascertain by fields test
the best proportions of the various fertilizer constituents
to use upon his own land for each of the crops he is grow-
ing, which knowledge makes it possible to decrease
greatly the expenditure for fertilizers.

215. Methods of applying fertilizers. — The distribu-
tion of the fertilizer by means of machinery is much more
satisfactory than is broadcasting by hand, as the former
method gives a much more uniform distribution. Cereals
and other crops planted with a drill or planter are now
usually provided with an attachment for dropping the
fertilizer at the same time that the seed is sown, the ferti-
lizer being by this method placed under the surface of the
soil. Broadcasting machines are also used, which leave
the fertilizer uniformly distributed on the surface of the
ground, thus permitting it to be applied and harrowed-in
sufficiently, before the seed is planted, to prevent injury
to the seed by the chemical activity of the fertilizing
material.

Corn planters with fertilizer attachments deposit the
fertilizer beneath the seed, so as not to bring the two in
contact. Grain drills do not do this, and, where the
amount of fertilizer used exceeds 300 or 400 pounds per
acre, it is better to apply it before seeding. Grass seed
and other small seeds should be planted only after the
fertilizer has been mixed with the soil for several days.

348 THE PRINCIPLES OF SOIL MANAGEMENT

For crops to which large quantities of fertilizers are to
be added, it is desirable to drop only a portion of the
fertilizer with the seed, the remainder to be broadcasted
by machinery and harrowed in earlier, and, as is fre-
quently better for crops requiring very liberal fertiliza-
tion, a later application may be made.

216. Soil amendments. — Certain substances are some-
times added to soils for the purpose of increasing pro-
ductiveness through their influence on the physical
structure of the soil, and thereby upon the chemical
and bacteriological properties. These substances are
called soil amendments. It is true that they may add
essential plant ingredients to the soil, but that function
is of minor importance.

217. Salts of calcium. — Calcium, although essential
to plant growth, need seldom be added to the soil to
supply the plant directly; but, on account of its effect
upon the soil properties, its use is beneficial to a great
number of soils.

218. Effect on tilth and bacterial action. — On clay
soils, the effect of lime is to bring the fine particles into
aggregates which are loosely cemented by the calcium
carbonate. The effect of this structure upon tilth has
already been explained. (Seepage 117.) On sandy soils,
the carbonate of calcium serves to bind some of the par-
ticles together, making the structure somewhat firmer,
and increases the water-holding power. It should be
used only in small amounts on sandy soils.

There is a tendency for most cultivated soils to be-
come acid, owing to the formation of organic acids in
decomposition and to the greater removal of mineral

SOIL AMENDMENTS 349

bases than acids by plants, but particularly because
of the loss of lime and the alkali salts in the drainage
water. Acidity may reach a point where it becomes
directly injurious to certain plants, but it becomes
indirectly injurious before that point is reached. One
way in which this occurs is by curtailing the action of
certain bacteria in their processes of rendering plant-food
available. A slightly alkaline reaction and an easily
available base to combine with the organic acids affords
the most favorable condition for the decomposition
processes due to bacterial action, and hence the best
results cannot be obtained where carbonate of lime is not
present. Its action in improving tilth also facilitates
desirable forms of bacteriological activity by increas-
ing the permeability of the soil for air.

219. Liberation of plant-food materials. — It has been
stated (page 297) that the alkalies and alkaline earths are
more or less interchangeable in certain compounds in
the soil. The addition of lime may in this way liberate
potassium, when otherwise it would be difficult for
crops to obtain a sufficient supply from a particular
soil. Magnesium, although rarely deficient, may also
be made available in this way. The use of calcium salts
may also render phosphorus more useful, probably by
supplying a base more soluble than iron or alumina
with which in soils deficient in calcium the phosphorus
might otherwise be combined.

Boussingault, as quoted by Storer, found that the
addition of lime to a clover crop increased greatly the
calcium, potassium and phosphorus contained in the
crop.

350 THE PRINCIPLES OF SOIL MANAGEMENT

Table IL

Kilos per hectare

Lime

Potash

Phos-
phoric
acid

Crop not limed (first year)

Crop limed (first year)

Crop not limed (second year)

Crop limed (second vear)

32.2

79.4

32.2

102.8

26.7
95.6
28.6
97.2

11.0

24.2

7.0

22.9

Calcium salts may also increase greatly the rate at
which nitrogen becomes available by its effect upon
bacterial action, as before explained.

220. Effect on toxic substances and plant diseases. —
Free acids are toxic to most agricultural plants. Some
plants are much more sensitive than others. Clover and
alfalfa, for instance, should have a slightly alkaline
medium for their best growth, and any acid is very
injurious. Calcium salts in neutralizing acidity remove
this toxic condition.

Certain toxic substances of an organic nature are
also said to be rendered innocuous by the presence of
calcium carbonate. Magnesium salts, when present in
excess, may exert a toxic action upon plants. The
relative proportion of calcium and magnesium, accord-
ing to Loew, determines whether or not magnesium is
toxic. The exact limits of the ratio of magnesium to
calcium beyond which the former is toxic depends upon
the combinations and solubilities of the two, and also
upon the crop grown. An actually greater amount of
magnesia, as shown by a strong hydrochloric acid diges-

LIME AS A SOIL AMENDMENT 351

tion analysis, is not present in very fertile soils of any
region, according to Loew. If injury from magnesium
is suspected, the obvious means of correction is to
increase the proportion of calcium by its addition in
some form.

The use of limestone, ground or burned, that contains
a large percentage of magnesium may be injurious to
some soils, as may also those Stassfurt salts containing
magnesium.

The presence of soluble calcium, with its effects
upon the soil, retards the development of certain plant
diseases, like the "finger and toe" disease of the cruci-
ferae. On the other hand, it may promote some diseases,
as, for instance, the potato "scab."

221. Forms of calcium. — Calcium is used on the
soil in the form of calcium oxide, or quicklime (CaO),
water-slaked lime (Ca(OH)2), air-slaked lime (CaC03),
ground limestone (also a carbonate), and calcium sulfate,
or gypsum (CaS04, 2H20). The application of any of
these is usually called liming the soil, although gypsum
does not serve exactly the same purpose as do the other
forms. Owing to differences in the molecular weights
of these compounds of calcium, it requires more of some
forms than of others to furnish the same amount of
calcium. Approximately equivalent quantities of some
of the common forms when fairly pure are:

Quicklime 56 pounds

Water-slaked lime 74 pounds

Air-slaked lime, marl and ground limestone .... 100 pounds

Caustic lime, or the hydrate, when added to the soil,
eventually assume some of the more insoluble forms of

352 THE PRINCIPLES OF SOIL MANAGEMENT

combination or remain as the carbonate, never being
present as the oxide. It is always desirable to have
present in the soil at least a small amount of calcium
carbonate.

222. Caustic lime. — Quicklime and water-slaked lime
have a markedly alkaline reaction, and hence neutralize
quickly any acidity that may exist in the soil. They act
also quickly in liberating plant-food, particularly nitro-
gen. Some soils respond more rapidly to quick- or water-
slaked lime than to carbonate of lime, especially when
the carbonate is in the form of marl or ground limestone,
in which cases it is never in such a finely pulverized con-
dition. The use of the caustic forms of lime has been
said to result in the loss of nitrogen by the decomposition
of organic compounds.

Upon clays, the granulating effect of caustic lime is
more marked than that of the carbonate, and for this
reason the former has a distinct advantage for use on
heavy clay. An occasional moderate dressing is, for the
same reason, better than a heavy dressing given less
frequently.

223. Carbonate of lime. — Air-slaked lime has the
advantage of being in a finely divided condition, and
does not produce the injurious action upon organic
matter attributed to caustic lime. Its effect upon the
granulation of clay soils is probably less pronounced
than that of caustic lime.

Marl differs from air-slaked lime principally in its
property of being in a less finely pulverized condition.
It acts less quickly than does caustic lime. Owing to
the fact that marl deposits differ greatly in the compo-

!

FORMS OF LIME AND CROP VALUE 353

sition of their products, it is well to know the quality
of the material before purchasing it. The carbonate of
lime in marl may vary from 5 or 10 to 90 or 95 per cent
in different samples.

Ground limestone has been used as a substitute for
marl. It is very important that it be finely ground, as
upon the comminution of the material much of its effi-
ciency depends. As there was some question as to the
value of ground limestone, experiments in which it was
compared with caustic lime have been conducted at
some of the experiment stations. These have, in the
main, given results very favorable to finely ground lime-
stone.

Frear reports tests in which plats treated with slaked
lime, at the rate of two tons per acre once in four years,
were compared with plats treated with ground limestone,
at the rate of two tons per acre every two years. The
records, at the end of twenty years, show that in every
case the total yields were greater on the plats receiving
ground limestone. After the treatment on these plats
had been continued for sixteen years, a determination of
nitrogen showed the upper nine inches of soil on the
limestone-treated plats to contain 2,979 pounds of nitro-
gen per acre, and the siaked-lime plats to contain 2,604
pounds. It may be inferred from these figures that the
slaked lime caused a greater destruction of organic
matter than did the limestone.

Patterson also conducted experiments for eleven
years with caustic lime produced by burning both stone
and shells, and the carbonate of lime in ground shells
and shell marl. The average crops of maize, wheat

w

354 THE PRINCIPLES OF SOIL MANAGEMENT

and hay were all larger on the carbonate -of- lime
treated plats.

224. Sulfate of lime. — Gypsum, in which form calcium
sulfate is usually applied to soils, is effective in liberating
potash, and possibly other substances, from the more
difficultly soluble combinations. Its action in improving
tilth is less marked than that of caustic lime, or of the
carbonate. Whether it eventually contributes to the
presence of carbonate of lime is a matter regarding
which there is still a difference of opinion. It has the
disadvantage of introducing into the soil an acid radical,
which is removed by plants only in small amounts, and
which tends to produce an acid condition of the soil.
On the whole, gypsum is not an adequate substitute for,
nor so desirable a form of, calcium as the oxide, hydroxide
or carbonate.

225. Common salt. — Sodium chloride has a marked
effect upon some soils, but wherein its effectiveness lies
is not well understood. The addition of sodium and of
chlorine as plant constituents is clearly not the reason,
as these substances are always present in soils in avail-
able form far in excess of their requirements.

The effect of sodium chloride upon clay-bearing soils
is to liberate certain plant nutrients, among which are
calcium, magnesium, potassium, calcium and phos-
phorus. This action, although limited in amount, is,
in some cases at least, partly responsible for the bene-
ficial action of common salt.

The structure of the soil is improved by the applica-
tion of sodium chloride, just as it is by lime, — although
usually not to the same extent.

OTHER SOIL AMENDMENTS 355

Another effect of salt is to conserve and distribute
soil moisture. Its conserving action is probably due to
an increase in the density of the soil-water solution re-
tarding transpiration. The film movement of water is
likewise increased by the presence of salt in the solution,
and in this way the upward movement of bottom water
is facilitated, and the supply within reach of the roots
maintained in time of drought.

It is not all soils, however, that are benefited by salt,
its usefulness not being of such wide application as that
of lime. Certain crops, as previously mentioned (page
340), are injured by the presence of chlorine.

226. Muck. — The effect of muck is to change the
structure of soils; making heavy clay soils lighter and
more porous, and binding together the particles of a
sandy one. Both classes of soils, but particularly the
sandy type, have a greater water-holding capacity after
treatment with muck, owing to its great absorptive
power, amounting to 70 per cent or more of its own
weight. (See page 153.) It is to its content of organic
matter that the physical effects of muck are due.

Muck contains 1.0 to 2.0 per cent of organic nitrogen,
calculated to dry matter which does not readily undergo
ammonification. The addition of farm manure which
ferments readily, and of lime, serves to hasten ammoni-
fication. Its use as an absorbent in the stable fits it well
for use on the land.

Very large applications of muck are necessary when
it is used to improve the structure of the soil. From
ten to forty or fifty tons per acre are frequently
applied.

356 THE PRINCIPLES OF SOIL MANAGEMENT

227. Factors affecting the efficiency of fertilizers. —

The potentially available nutrients in a soil, whether
natural or added in manures or fertilizers, are only in
part utilized by plants, and the extent of their utilization
depends upon the operation of certain limiting factors.
This is a very important consideration in the manuring of
land, for under conditions as they frequently exist the
use of fertilizers is wasteful and extravagant.

The factors within the control of man that effect the
availability of fertilizing material are the following:
(1) Soil moisture content. (2) Soil acidity. (3) Organic
matter in the soil. (4) Structure or tilth of the soil.

An undesirable condition of any one or more of these
factors is a very common and apparent occurrence, and
yet fertilizers are expected to produce profitable returns,
in spite of these adverse conditions. It must be remem-
bered that fertilizers are primarily only nutrient materials,
and that the supply of nutrients is only one of the con-
ditions that influence plant growth. Furthermore, an
economical use of fertilizers requires that they merely
supplement the natural supply in the soil, and that the
latter should furnish the larger part of the soil material
used by the crop. Finally, most fertilizers are ren-
dered more or less difficultly soluble, or in some cases
practically insoluble in pure water, by the absorptive
properties of the soil, and the release of these sub-
stances for plant use depends to a great extent upon the
factors mentioned above.

For instance, when a potassium fertilizer, as potas-
sium sulfate or chloride, is placed in the soil, a consider-
able portion of the potassium is (page 297) fixed by ab-

EFFICIENCY OF FERTILIZERS 357

sorption as one of the bases in a poly-silicate, and thus
held in a condition very sparingly soluble in pure water.
Other reactions take place, and a portion of the potas-
sium in some form is doubtless mechanically held by the
soil particles. While this added potassium is more
readily obtained by plants than that contained naturally
in many soils, it must become available largely by the
processes by which the natural supply is rendered soluble.
Ammonium sulfate undergoes a somewhat similar pro-
cess, while the nitrate of soda remains in a soluble form.

It is evident, therefore, that the conditions which
contribute to the natural fertility of the soil also apply
to that added as fertilizers, with the possible exception
of the nitrate.

Phosphate fertilizers may be rendered practically
insoluble in pure water, when added to the soil, and in
the presence of a large amount of iron and aluminum it
forms more or less ferric and aluminum phosphate,
which becomes soluble very slowly, even under the
action of soil-water and plant-roots. When converted
into tricalcium phosphate, the phosphorus becomes
soluble more readily; but, in any case, its rate of solu-
bility depends upon those conditions which are most
favorable to the solubility of the natural soil phosphates.

It is generally recognized that a sandy soil responds
more promptly to the application of fertilizers than does
a clay soil. There may be two reasons for this: (1)
Absorption may not be so complete both on account of
the particles being larger, and because in many sandy
soils the particles are largely composed of quartz, which
does not have the property of forming combinations

358 THE PRINCIPLES OF SOIL MANAGEMENT

with bases as does clay. (2) Drainage and aeration are
likely to be better, as are all those conditions that con-
duce to solubility of plant-food. For these reasons, a
sandy soil generally gives larger returns the first year
from the application of manures, but shows less effect
in subsequent years unless the treatment is repeated.
Clay soils are, for these reasons, more likely to involve
a wasteful use of fertilizers than are sandy soils, except
in respect to loss of nitrogen in drainage, in which the
sandy soil is more likely to be at fault, especially if there
is no crop on the land.

228. Soil-moisture content. — Soils in a humid region
-commonly suffer from an excess of water in the spring,
and a deficiency in the summer. Cereals and many other
crops require the largest quantity of water at the time
of heading and blossoming, and the largest production
of crop can be secured only where the supply is adequate
at that time. It is safe to say that in the great majority
of cases crops raised, even in the humid region, suffer
at some time from a deficient water-supply. On the other
hand, it is well known that crops, almost without excep-
tion, suffer either by lateness of planting, or by delayed
early growth from an excess of moisture in the spring.

A control of the soil-moisture supply should, there-
fore, remove the excess of moisture in a time of large
rainfall, and conserve it in time of drought.

There are three means that may be employed to
bring this about: (1) Drains, especially by means of tile.

(2) Use of green manures or other organic matter.

(3) Good tillage. (See page 190.)

Viewed purely from the standpoint of soil fertility,

EFFICIENCY OF FERTILIZERS 359

tile drainage does much to increase crop production,
and to effect economy in the use of fertilizers. The rela-
tion of soil drainage to soil fertility may be summarized
as follows. (See, also, page 239.)

(1) Aeration provided by the removal of water
greatly facilitates nitrification. This relieves the con-
stant necessity for the use of soluble nitrogen fertilizers,
and makes it possible to rely largely upon the use of
leguminous crops for nitrogen fertilization. Aeration
also renders the other fertilizing constituents of the soil
more easily soluble.

(2) By quickly removing the excess moisture in the
early spring, and thus increasing the length of the grow-
ing period, plants secure more nutriment, there is a cor-
responding increase in the length of time in which nitri-
fication can take place, also in other action brought
about by aeration. Available nitrogen thus produced
at an early period in the crop growth is more effective
than a later supply would be.

(3) By removing an excess of water from the soil, a
larger proportion of the available fertility, both natural
and that added in manures, is absorbed by the crop.
This is because the solution is less dilute, and conse-
quently a larger amount of mineral nutrients pass
through the plant by transpiration.

229. Soil acidity. — An acid condition of the soil
renders ineffective a large proportion of the fertilizing
material that might otherwise be available. A good
illustration of this is the comparison of the crops grown
on acid soil when treated with lime with a similar soil
not so treated. The size of the crop on contiguous plats

360 THE PRINCIPLES OF SOIL MANAGEMENT

has been increased several hundred per cent by the use
of lime at a number of the Experiment Stations. The
amount of the acidity determines the injury it occasions.
There is always a great waste of fertilizers when they are
added to an acid soil. The acidity should be corrected
by the application of lime, in order that manuring shall
be most effective.

There are several ways in which an acid condition
of the soil operates to render ineffective the natural
and applied fertility.

(1) Bacteria which are concerned in the processes of
rendering plant-food available do not usually thrive
in an acid media, preferring a neutral or slightly alka-
line condition. Acidity for this reason checks nitrifi-
cation, as well as the bacteriological processes by which
phosphorus is rendered soluble.

(2) Bacteria concerned in the acquisition of atmos-
pheric nitrogen in symbiosis with legumes are greatly
injured by an acid condition of the soil. Nitrogen con-
servation, one of the most important features of the use
of legumes for green manuring, cannot be effectively
carried out on an acid soil.

(3) The liberation of potassium from zeolitic combi-
nations is best effected only where there is a basicity
that will permit the replacement of one base by another.
The presence of at least a small amount of calcium car-
bonate in the soil is essential for this, as it is for many
other desirable processes, and an acid condition of the
soil means that no basicity exists.

(4) Lime, when present in large amount, reacts
with the very insoluble phosphates of iron and alumina,

EFFICIENCY OF FERTILIZERS 361

and by producing phosphate of lime, renders the phos-
phoric acid more available for the plant.

230. Organic matter. — The ways in which organic
matter contributes to economy in the use of fertilizers
are: (1) By improving the soil structure. (2) By con-
serving moisture. (3) By producing through decompo-
sition carbon dioxide which, dissolved in water, is a
weak but continuously acting solvent of the mineral
fertilizers; also by forming organic acids that act in a
similar way. (4) It furnishes a source of food and energy
for bacteria, which aid in rendering soluble the absorbed
fertilizing constituents.

It is particularly in rendering available to plants the
more difficultly soluble phosphate fertilizers that organic
matter directly aids in making fertilizers more effective.

Farm manure is undoubtedly the best all-round ferti-
lizer to be had. In addition to adding organic matter and
certain mineral plant-food materials, it introduces into
the soil, and furnishes a favorable medium for the growth
of large numbers of bacteria that are of great value in
rendering available the plant nutrients contained in soils.

The use of raw or untreated phosphates to replace
superphosphates in soil manuring has received much
attention in Germany and to some extent in this country
in recent years. Raw phosphates, being much more
difficultly soluble than the superphosphates, do not,
under most conditions of the soil, give as marked
returns. On the other hand, the raw phosphate has the
advantage of being very much cheaper, and of not con-
taining sulfuric acid. The extent to which raw phosphates
will become available in the soil depends largely on the

362

THE PRINCIPLES OF SOIL MANAGEMENT

extent of decomposition of organic matter. A soil poor
in humus, and which has not been treated with farm
manure or green manure, is not likely to respond very
strongly to an application of raw phosphate. The fact
that superphosphate is available under these conditions
is likely to lead to its use without any attempt to im-
prove the humus-content of the soil, and thus increase
those difficulties that arise from a deficiency of organic
matter. It is this condition that makes it necessary
to constantly increase the dressings of fertilizer in order
to maintain productiveness.

Experiments by Thorne have shown that the use of
farm manure in conjunction with raw phosphates serves
to increase greatly the availability of the latter. In
these experiments stall manure was used at the rate of
eight tons per acre, in one case alone, and in another
in connection with 320 pounds of rock phosphate.
The manures were applied to clover sod, and plowed
under for maize in a rotation of corn, wheat and clover.
In the following table, average yields from the manured
plats and from the unmanured ones are given.

Table L
Effect of Stall Manure on Availability of Rock Phosphate

Average

Average

Average

yield eleven

yield

yield

crops

ten crops

seven crops

maize

wheat

hay

Bushels

Bushels

Tons

Stall manure, 8 tons per acre

57.7

20.3

1.6

Stall manure, 8 tons per acre and

rock phosphate, 320 pounds per

acre

64.0

25.6

2.2

No manure

34.6

10.4

1.0

EFFICIENCY OF FERTILIZERS 363

It will be seen from this table that the combination
of stall manure and rock phosphate produced larger
crops than did the same quantity of stall manure alone;
from which it may be fairly concluded that, under these
conditions, the raw phosphate becomes available to an
extent sufficient to make its use practical. Whether
raw phosphate can be used without supplementing
them with superphosphate will depend upon the natural
fertility of the soil and the amount of decomposing
organic matter it contains.

231. Structure or tilth of the soil. — Tillage aids the
plant in several ways to obtain nutrients from ferti-
lizers added to the soil: (1) By promoting aeration.
(2) By permitting the plant-roots to come in contact
with a large area of soil. (3) By conserving moisture
in time of drought.

232. Cumulative need for fertilizers. — It is often
remarked that on fertilized soils there is a gradually
increasing need for greater quantities of fertilizers.
This is doubtless the case in many instances, and arises
from neglect of other factors affecting soil productive-
ness. As we have seen, certain fertilizers induce a loss
of lime from the soil, which, if allowed to continue/'equires
an increased amount of fertilizer to maintain the yield
of crops. Organic matter is allowed to decrease and this,
as well as loss of lime, causes the soil to become compact
and poorly aerated, and so, one bad condition leading
to another, crops become poorer in spite of increased
applications of fertilizer.

233. Farm manures. — The original components of
farm manure are the solid excreta from the animal, the

364

THE PRINCIPLES OF SOIL MANAGEMENT

urine, usually from the same animal or animals, and the
litter used as bedding and also for the purpose of absorb-
ing the liquid manure and to render the whole easier to
handle. As these constituents differ greatly in their
physical and chemical properties, the proportions in
which they exist affect appreciably the properties of the

manure.

Fig. 106. A striking example of waste of manure. Leaching and fermentation
will remove over half of its value in six months.

234. Solid excreta. — The solid excreta furnishes most
of the body of the manure, and as it is already in a stage
of partial decomposition, and in a condition both physi-
cally and chemically to favor the further processes of
decomposition, it is largely to this constituent that the
fermentative action of manure is due. It is particularly
valuable for the effect it has upon the physical condition
of the soil and the encouragement it gives to decompo-
sition processes.

Chemically, it is not so valuable as the liquid excreta.

FARM MANURES

365

It represents, in part, the food materials that have
passed undigested through the alimentary canal, and
also the secretions this has received on the way, and
these substances are not all held in a soluble form, as
are those in the urine.

Stoeckhardt states the composition of the solid
excreta of different farm animals to be as follows:

Table LI

Horses (winter food)

Cows (winter food)

Swine (winter food)

Sheep (two pounds hay per day)

Water

Per cent
76
84
80

58

Composition of dry matter

Nitro-
gen

Per cent
2.08

1.87
3.00

1.78

Phos-
phoric
acid

Per cent
1.45
1.56
2.25
1.42

Alkalies

Per cent
1.25
0.62
2.50
0.71

Calculated to 1,000 pounds of solid excrement, these
figures show the following number of pounds of each
constituent.

Table LII

Water

Nitro-
gen

Phos-
phoric
acid

Alkalies

Horse

Pounds
760
840
800
580

Pounds
5.0
3.0
6.0
7.5

Pounds
3.5
2.5
4.5
6.0

Pounds
30

Cow

1 0

Swine

5 0

Sheep

3 0

The smaller percentage of water in the sheep excre-
ment makes it, pound for pound, the richest of any. Next

366

THE PRINCIPLES OF SOIL MANAGEMENT

to it stands hog excrement, and cow excrement is the
poorest in fertilizing materials.

235. Urine. — The urine represents a portion of the
food which has been digested by the animal and excreted
as a waste product through the kidneys. The proportion
of the nitrogen and mineral matter retained by the tis-
sues depends upon the age of the animal and upon the
nature of the food. An animal receiving a large amount
of easily digestible nitrogenous food excretes more nitro-
gen in the urine than a poorly fed animal.

The composition of urine, as given by Stoeckhardt is
as follows:

Table LIII

Horse (hay and oats)

Cow (hay and potatoes)

Swine (winter food)

Sheep (two pounds hay per day)

Water

Per cent
89.0
92.0
97.5
86.5

Composition of dry matter

Nitro-
gen

Per cent
10.9
10.0
12.0
10.4

Phos-
phoric
acid

Per cent

trace

trace

5.0

3.7

Alkalies

Per cent

13.6

17.5

8.0

14.9

These figures show the following number of pounds
of each constituent in 1,000 pounds of urine.

Table LIV

Water

Nitro-
gen

Phos-
phoric
acid

Alkalies

Horse

Pounds
S'.H)
920
975
865

Pounds
12

8

3
14

Pounds

1^25
0.50

Pounds
15

Cow

14

Swine

2

Sheep

20

COMPOSITION OF ANIMAL MANURES 367

The liquid excreta of the sheep contains in a given
quantity more fertilizing material than that of any of the
other animals.

Comparing the solid and liquid excreta of these
animals as a whole, it will be seen that, in general, the
urine is richest in nitrogen and alkalies, while the solid
excrement is richest in phosphoric acid.

The amount and composition of the urine is more
constant than that of the solid excrement. Both are
influenced by the character and amount of feed, but the
urine much less so than the solid excrement. Experi-
ments conducted at the Rothamsted Experiment Sta-
tion have shown that from 57 to 79 per cent of the total
nitrogen of the food is excreted in the urine, and from
16 to 22 per cent in the solid excrement.

236. Litter. — The use of a bulky absorbent, like
straw, sawdust or leaves, is almost universal where live
stock are kept in a stable. This is useful in providing
a soft bed for the animal, in absorbing the liquid excre-
ment, in lightening the manure, making it easier to
handle, less likely to undergo undesirable fermentation,
and more effective in improving the physical condition
of heavy soils.

Straw is the absorbent usually used, and is, all
things considered, the most satisfactory. It decomposes
readily in most soils and, in decomposing, adds to the
soil considerable fertilizing material. Of the different
kinds of straw, oat straw has the greatest fertilizing
value. A ton of oat straw contains about 16 pounds
nitrogen, 4 pounds phosphoric acid, 26 pounds of potash,
and 9 pounds of lime. As this is more nitrogen and

368 THE PRINCIPLES OF SOIL MANAGEMENT

potash than is contained in a ton of average manure,
the use of this absorbent increases the fertilizing value
of the manure. It is, however, undesirable on some soils
to have a very large proportion of straw, on account
of its effect in retarding decomposition.

Sawdust and shavings are sometimes used, but,
while they are good absorbents, they decompose very
slowly in the soil, making them objectionable on light
soils, and they have practically no plant-food materials.
Dry leaves absorb well, and decompose satisfactorily in
the soil. They do not add much fertility.

237. Manures produced by different animals. — There
is a great difference in the amount and value of manure
produced by different kinds of live stock. This is due to
a number of causes, among which are the size of the
animal, the nature of its food, and the mechanical con-
dition in which the digestive processes leave the solid
excrement. The differences affect not only the amount
of fertilizing constituents in the manures, but, what is of
more importance, they determine the nature and rapidity
of the decomposition processes, and hence affect the loss
of manurial substances and the value of the manure as a
fermentive agent in the soil.

238. Horse manure. — A well-fed, moderately worked
horse will produce from 45 to 55 pounds of excrement per
day, of which from 12 to 15 pounds consists of urine.
The straw used for bedding will amount to from 4 to 6
pounds. Roberts has computed the value of the excre-
ment to be nearly one-half the cost of the food, while
from Wolff's tables, based on a large number of determi-
nations in Europe, the combined solid and liquid excreta

COMPOSITION OF ANIMAL MANURES 369

contains the following average percentages of the organic
matter, nitrogen and mineral substances originally
present in the food consumed:

Per cent

Organic matter 33.90

Nitrogen 39.50

Mineral substances 56.25

In Robert's calculations, the value of the manure is
based entirely upon its content of nitrogen, phosphoric
acid and potash, valued at 15 cents, 7 cents and 4.5 cents,
respectively. It is difficult to get a true idea of the value
of animal manure, as its content of fertilizing substances
is only a part of its manurial value, of which its physical
and bacteriological effects upon the soil are extremely
important.

Horse manure has the fibrous matter of the food less
well broken down than has cow manure, and this, with
its lower water content, produces a light, easily ferment-
able substance that readily loses its nitrogen, which
passes off as ammonium carbonate. The dry fermen-
tation, indicated by a whitish appearance of the interior
of the manure heap and a slight smoke, is the cause of
this loss. The values calculated for the excrement are
never realized in practice because of the losses that
occur between the stable and the « field. To preserve
horse manure to the best advantage, it should be mixed
with cow manure, — the wet, compact character of
which lessens the amount of fermentation by changing
the physical condition of the manure.

239. Cow manure. — A mature cow, given good feed,
will produce from 60 to 90 pounds of excrement daily.

x

370 THE PRINCIPLES OF SOIL MANAGEMENT

depending upon the weight of the animal. Of this
20 to 35 pounds is likely to be urine. Even the solid
excreta contains a large percentage of water, and, accord-
ing to Boussingault, only about one-eighth of the total
excreta is dry matter.

The very watery nature of cow excreta causes it to
require a large amount of litter. In spite of the lighten-
ing effect of the litter, it decomposes slowly as compared
with other manures. When applied alone to the soil,
action is slow, but it is prolonged over a considerable
number of years.

The loss of ammonia in the decomposition processes
is much less than with horse manure. The admixture
of other manures adds much to the rapidity of fermen-
tation and to the ease of handling.

The percentage of organic matter, nitrogen and min-
eral substances contained in the food of cattle that
appear ultimately in the excrements are as follows:

Per cent

Organic matter 27

Nitrogen 42

Mineral matter 50

This corresponds fairly well with the percentage for
horse manure, and would justify the belief that the value
of the manure would hold about the same ratio to that
of the food as in the case of the horse.

240. Swine manure. — The quantity of excrement
voided by swine varies greatly even for mature animals,
the amounts per 1,000 pounds live weight varying from
less than 50 to more than 100 pounds per day. A more
concentrated ration produces less excreta, but causes

COMPOSITION OF ANIMAL MANURES 371

it to be much richer in fertilizing ingredients. Roberts
calculated the value of the manure produced in one
year by a 150-pound pig fed on a highly nitrogenous
ration to be $3.24, and that of a pig of similar weight
fed on a carbonaceous ration to be $1.84 for the same
period.

The manure of swine is wet, but not quite so much so
as cow manure. According to Boussingault, about one-
sixth of the solid excrement is dry matter. It decomposes
slowly. As the urine contains by far the larger part of
the nitrogen, it should be saved.

241. Sheep manure. — The total amount of excre-
ments voided by mature sheep is from 30 to 40 pounds
per 1,000 pounds of live weight, of which about one-
fourth is dry matter. Although drier than horse manure
and generally richer in nitrogen it is less likely to lose
that constituent by fermentation, as the compact nature
of the solid excreta is not so favorable to rapid decom-
position as is the physical structure of horse manure.
It is however, when placed in the soil, a readily acting
manure and is frequently used by gardeners for that
reason. To obtain the best results, it should be mixed
with horse and cow manure.

242. Relative values of animal manures. — Extensive
experiments conducted by Roberts, Wing and Cava-
naugh at Cornell University Experiment Station, with sev-
eral different kinds of animals fed on the common Ameri-
can feeds, but perhaps in somewhat heavier rations than
the average, and kept under normal conditions, may well
be taken to show the relative values of animal manures,
although the absolute values may be somewhat above

372

THE PRINCIPLES OF SOIL MANAGEMENT

the average. In calculating the values of the manures
produced by these animals, nitrogen is reckoned at
fifteen cents per pound, phosphoric acid at six cents,
and potash at four and one-half cents. The composition,
amount and value of the manures without litter are
given in the following table.

Table LV

Composition, Amount and Value of Manures (Without

Litter) from Different Animals

Percentage compo-
sition

Pounds ingredi-
ents per ton

manure

c

0
-»a

Production

per 1,000

pounds live

weight

Kinds of

a

a

live stock

u

o

0
CD
M

o
u

Z
0.49

o

org

o *

Ph

0.26

js
3
o

PL,

a

M

0

E
0

o d
Ph

J3

CD

3

O

Ph

1

Ph O.

Mi

a

Horses . . .

48.70

0.48

9.00

5.20

9.60

$2.21 '48.8

$27.74

Cows

75.25

0.43

0.29

0.44

8.60

5.80

8.80

2.02 74.1

29.27

Calves ....

77.73

0.50

0.17

0.53

10.00

3.40

10.60

2.18 67.8

24.45

Swine

74.13

0.84

0.39

0.32

16.80

7.80

6.40

3.29, 83.6

60.88

Sheep

59.52

0.77

0.59

15.40

7.60

11.80

3.30

1

34.1

26.09

243. Poultry manure. — The droppings of poultry
are nearly twice as valuable, pound for pound, as cow
manure, when calculated on the value of the nitrogen,
phosphoric acid and potash they contain. It is in the
former constituent particularly that poultry manure is
rich. A thousand pounds live weight of fowls produce
from thirty to forty pounds of droppings daily. These
contain when fresh between 50 and 60 per cent of water
and over 1 per cent of nitrogen. The nitrogen is largely
present as ammonium compounds. It quickly undergoes
fermentation, with loss of nitrogen. Lime or alkalies

COMPOSITION OF ANIMAL MANURES 373

decompose the ammonium compounds with liberation
and loss of free ammonia. An absorbent, such as land
plaster, superphosphate, kainit or dry earth will greatly
lessen the loss of nitrogen. Mixing it with other manures
is also advisable.

When applied to the soil, poultry manure decom-
poses rapidly, and is used by market gardeners on account
of its rapid action.

244. Factors affecting the values of farm manures. —
The value of animal excrements for manurial purposes
depends upon a number of factors, among which are:
(1) The relative proportions of solid excrement and
urine. (2) The species of animal producing the manure.
(3) The age of the animal. (4) The character of the food
the animal receives. (5) The use to which the animal
is being put. In addition to the factors affecting
the excrement, the manure may always be modified
by the litter or other absorbent added, and by the
method of handling. The effects of solid and liquid
excreta, and of the species of animal, have already been
discussed.

245. Age of animal. — A young and growing animal
requires more nitrogen and phosphoric acid to build
bone and muscle than does an animal that has completed
its growth. This is taken from the food, and not excreted
in the urine or other excretory products, and hence does
not appear in the manure.

246. Food of the animal. — Since the large part of the
nitrogen phosphorus and potassium contained in the
food is contained in either the solid or liquid excrement,
it follows that the richer the food in these constituents

374

THE PRINCIPLES OF SOIL MANAGEMENT

the more of them the manure will contain. A highly
carbonaceous ration produces a poor manure largely
because it is low in nitrogen. The manurial value of a
food-stuff is generally increased by passing through the
animal, provided it can largely be recovered, because
the digestion process leaves it in a condition more favor-
able to decomposition and to thorough mixing with the
soil.

247. Use of the animal. — The amounts of the ferti-
lizing constituents recovered in the excrement vary to
some extent with the use that is being made of the ani-
mal. Animals that are being fattened, or that are pro-
ducing milk, divert a portion of the fertilizing constit-
uents to their products. Experiments by Laws and Gil-
bert with different classes of animals used for different
purposes show the following disposition of some of the
constituents of the food. As the excrements include
the perspiration, the small amount of matter passing
off in that form is, of course, not recovered in the manure.

Table LVI

Nitrogen

Mineral matter

Contained

in

product

Contained

in
excrement

Contained

in

product

Contained

in
excrement

Horse at rest

Per cent

None

None

24.5

3.9

14.7

4.3

Per cent
100.0
100.0
75.0
96.1
85.3
95.7

Per cent

None

None

10.3

2.3

4.0

3.8

Per cent
100.0

Horse at work

100.0

Milking cows

89.7

Fattening oxen

97.7

Fattening pigs

96.0

Fattening sheep

96.2

DETERIORATION OF MANURE 375

It will be seen from these experiments that milch
cows divert more of the fertilizing constituents from the
manure than do any other class of animal, that fattening
pigs divert much more of the nitrogen than do cattle
or sheep similarly employed, and that the work of the
horse does not affect the composition of the manure.

248. Deterioration of farm manure. — There is always
a loss in the value of farm manure on standing. The two
processes most operative in bringing this change about
are: (1) Fermentation. (2) Leaching. The first of
these is a natural process, common to all farm manure,
and not occasioned by any outside agencies; the second
is due to the running off of the liquid portion of the
manure, and to the exposure of the manure to rain.

249. Fermentations. — The fermentations occurring
in heaps of farm manure are produced both by aerobic
and anaerobic bacteria, that is, by bacteria requiring
oxygen for their activity, and by those that do not. The
fermentations of the outside of the heap are constantly
different from those on the interior, where air does not
readily penetrate; but, as fresh manure is thrown upon
the pile from day to day, most of the manure first under-
goes aerobic fermentation before the anaerobic bacteria
begin their work.

It is through the action of bacteria on the nitrogen-
ous compounds of the manure that loss of value through
fermentations occurs. The action of the aerobic bacteria
is to convert the nitrogen of the organic matter into
ammonia, which, owing to the large formation of carbon
dioxid, is partly converted into ammonium carbonate.
Both of these substances being volatile, there is danger

376 THE PRINCIPLES OF SOIL MANAGEMENT

of their passing off from the heap into the air. The drier
the heap, the more apt these substances are to escape.

The production of ammonia is very rapid from some
of the compounds in farm manure. Urea, in which form
the nitrogen of urine is largely found, undergoes con-
version into ammonia very rapidly, and some loss in
this way is inevitable even under the best management.
Chemically the process is a simple one, which may be
represented by the following equation:

CON2H4 + H20 = 2 NH3 + C02.
2 NH3 + CO, + H20 = (NHJ, C03.

The use of certain preservatives makes it possible
to decrease the loss of ammonia from manure. The
preservatives are intended to convert the ammonia into
a less volatile compound. For this purpose gypsum,
kainit, superphosphates and ground phosphate rock are
used. The action of gypsum, for instance, in the manure,
is to convert ammonia or ammonium carbonate into
the form of ammonium sulfate, which is not volatile.
The reaction is as follows:

(NH4)2 C03 + CaS04 - (NH4)2S04 + CaC03.

It is customary to sprinkle the preservative in the
stall of the animal, where it comes in contact with the
excreta as soon as they are voided. Salts of calcium
other than the sulfate, cannot be used, on account of
their action in decomposing ammonium salts.

The decomposition of proteins forming, among other
products, hydrogen sulphide, which becomes oxidized
to sulfuric acid, causes a part of the ammonia to natu-

WASTE OF MANURE BY LEACHING 377

rally take the form of a sulfate, which protects this por-
tion from volatilization.

The other fermentation resulting in the loss of nitro-
gen is due to the action of certain anaerobic bacteria
that convert ammonium salts into free nitrogen. Certain
of these organisms are able to reduce nitrates to nitrites,
and the latter to ammonia, but the greatest loss is doubt-
less due to the ammonium salts formed directly from
proteins. This process occurs only in the poorly aerated
portions of the heap. There does not appear to be as
great loss of nitrogen through the action of the anaerobic
ferments as through the loss of ammonia, which makes
it advisable, in practice, to keep the manure heap as
compact as possible, and to prevent the heap from be-
coming very dry by the application of water in amounts
sufficient to keep the heap moderately moist without
leaching it. In the arid and semi-arid parts of the coun-
try, this is an important precaution to be taken in the
preservation of farm manure.

250. Leaching. — When water is allowed to soak
through a manure heap and to drain away from it, there
is carried off in solution and in suspension a certain
quantity of organic and inorganic compounds contain-
ing nitrogen as urea, other organic nitrogen in small
amounts, ammonium salts and nitrates, some phos-
phorus and considerable potassium, with other mineral
substances of less importance. The amount of loss to the
manure in this way may be very great; and, without
doubt, in the humid portions of the country leaching
is the greatest source of loss. Protection of manure from
the rain is therefore very important.

378

THE PRINCIPLES OF SOIL MANAGEMENT

Experiments conducted by Roberts serve to show
the rate and extent of deterioration of manure in a region
having a rainfall of about twenty-eight inches in the six
months from spring until autumn, during which period
the tests were made. The loss arising from fermentation
and leaching combined was determined in these experi-
ments.

Horse manure was lightly packed in a wooden box,
not water-tight, surrounded with manure, and left
exposed to the weather from March 30 to September 30.
Analyses made at the beginning of and at the end of the
experiment showed the following:

Table LVII

Gross weight . .

Nitrogen

Phosphoric acid
Potash

April 25

Pounds

4,000.00

19.60

14.80

36.00

September 30

Pounds

1,730.00

7.79

7.79

8.65

Loss

Per cent
57
60
47
76

At the same time, cow manure was similarly treated,

except that 300 pounds of gypsum were mixed with it.

This, doubtless, protected some of the nitrogen, and the

greater body of material would also decrease loss of all

constituents.

Table LVII I

Gross weight . .

Nitrogen

Phosphoric acid
Potash

April 25

Pounds

10,000

47

32

48

September 30

Pounds

5,125

28

26

44

Loss

Per cent

49

41

19

8

HANDLING MANURE 379

The greater loss suffered by the horse manure was
doubtless due in part to the more rapid fermentation
accompanied by volatilization of ammonia, and to its
less compact nature making it more permeable to the
rain water.

Roberts also reports an experiment in which a block
of undisturbed manure one foot deep, consisting of both
horse and cow excrement mixed with straw and solidly
packed by trampling of animals in a covered shed,
was exposed from March 31 to September 30 in a gal-
vanized iron pan with perforated bottom. The losses
were as follows: Loss

Per cent

Nitrogen 3.2

Phosphoric acid 4.7

Potash 35.0

This shows a great saving to both kinds of manure
when they are mixed and tramped. The enormous
difference in the nitrogen lost, without a corresponding
difference in the loss of potash, indicates that the volatili-
zation of ammonia, which is greatly reduced by com-
pacting, is responsible for a very large share in the
deterioration of manure, even in a humid climate.

251. Methods of handling. — The least opportunity
lor deterioration of farm manure occurs when it is hauled
directly to the field from the stall and spread at once.
This is not always possible, and manure must be stored
on every farm for longer or shorter periods. In holding
manure, the two important conditions are, a sufficient,
but not excessive supply of moisture, and a well-com-
pacted mass. Water draining away from a manure heap,

380 THE PRINCIPLES OF SOIL MANAGEMENT

and a fermentation producing a white appearance of the
manure under the surface of the pile ("fire fanging"),
are both sure indications of unnecessary loss in its ferti-
lizing value.

Composting farm manure increases the availability
of its fertilizing constituents; but, even when carefully
conducted, is accompanied by some loss of nitrogen.
The total amount of organic matter is decreased by
reason of the decomposition, in which process carbon
dioxid and water are formed, part of which escapes, and
part remains in the manure. The mineral constituents
increase percentagely, due to the loss of organic matter;
and the water increases for the same reason, and because
it is sometimes added to the compost. The mineral con-
stituents are not materially changed in their solubilit}*,
but the organic matter becomes more soluble. The
nitrogen, after conversion into ammonium salts, is
oxidized finally into nitrates, but only in small amounts,
and after considerable time. The beneficial effects of
composing are only in small part due to the chemical
changes in the manure, but chiefly to the good physical
condition of the composted material, and to the fact
that the operations preliminary to the formation of
nitrates have largely been effected in the compost, and
when applied to the soil nitrification is rapid. Composting
manure with soil, sod, muck or other absorbent material
increases the manurial value of the latter by increasing
its decay, and therefore its availability, and by reducing
loss by leaching.

The following analyses, by Voelcker, show the com-
position of fresh and rotted farm manure:

HANDLING MANURE
Table LIX

381

Water

Soluble organic matter. . . .
Soluble organic nitrogen . .
Soluble inorganic matter . .
Insoluble organic matter . .
Insoluble inorganic matter

Fresh

Rotted

66.17

75.42

2.48

3.71

0.15

0.30

1.54

1.47

25.76

12.82

4.05

6.58

In applying farm manure to the field, it is customary
either to throw it from the wagon into small heaps, from
which it is distributed later, or to scatter it as evenly
as possible immediately on hauling it to the field. The
use of the automatic manure spreader accomplished
the latter procedure in an admirable manner. As be-
tween these two methods, the advantage, so far as the
conservation of the manurial value is concerned, is
with the practice of spreading immediately. When piled
in small heaps, fermentation goes on under conditions
that cannot be controlled, and that may be very unfavor-
able. The heaps may dry out, and thus lose much of
their nitrogen; or they are likely to leave the field un-
evenly fertilized by leaching into the soil directly under
and adjacent to the heap. On the other hand, when
spread immediately, little fermentation takes place,
as the temperature is generally low and the soluble
compounds are leached quite uniformly into the soil.
Plowing should follow as closely as possible the spread-
ing of the manure, and, except in winter, at which time
deterioration is not likely to be great, this can well be
done.

382 THE PRINCIPLES OF SOIL MANAGEMENT

The amounts and frequency with which farm manure
should be applied must depend, to some extent, upon the
nature of the farming and upon the character of the soil.
Farm manure tends to render all soils more porous and

Fig. 107. The wrong way to distribute manure. There is large loss by decay
and an uneven growth of crop.

light. A naturally light soil may be rendered less pro-
ductive by the application of heavy dressings of manure;
particularly in a dry climate is this the case. In regions
where so-called "dry farming" is practiced, the return
of organic matter to the soil is a great problem, on
account of the difficulty in accomplishing its decay

PLACE FOR MANURE 383

when plowed under. Composting, or plowing under after
it has been applied to sod for several months, or incorpo-
rating with a green manure, are methods that must be
used with "dry farming."

Even on heavy soils in a humid region, there is an
advantage in applying small dressings of farm manure
frequently, rather than large amounts at long intervals.
Organic matter decomposes more rapidly when present
in the soil in relatively small amounts, and its influence
on the solubility of plant nutrients is therefore greater
in proportion to the amount of manure used. There can
be no doubt that the bacterial flora introduced into the
soil by the incorporation of farm manure is an important
factor in its usefulness, and when this occurs at frequent
intervals it has a marked effect on productiveness.
Applications of ten tons to the acre are better than
twenty tons at twice the interval.

252. Place in crop rotation. — When a crop rotation
includes grass or clover as one of the courses, the appli-
cation of farm manure may well be made at that time
as a top-dressing. The spreading can be done at times
when cultivated land would not be accessible, and the
crop of hay will profit greatly. The sod, when plowed,
is frequently planted to corn — a crop that is rarely
injured by farm manure. On light, dry soils this practice
is of advantage, as already explained.

Most cultivated crops, with the exception of tobacco,
and occasionally sugar-beets, are much benefited by
farm manure. Small grains are usually benefited when
grown on poor, heavy soils with plenty of rainfall; but
in a dry region farm manure should not be applied

384 THE PRINCIPLES OF SOIL MANAGEMENT

for these crops, and on rich soils manure is likely to cause
small grain to lodge.

Farm manure, in judicious amounts, may be plowed
under in orchards to great advantage.

253. Functions. — The useful function which farm
manures perform in the soil are as follows: (1) To
improve the physical condition of the soil by the intro-
duction of organic matter, with its favorable influence
on the structure and moisture content. (See page 129.)
(2) To add a certain quantity of plant-food in a compara-
tively readily available condition. (3) To introduce
a new bacterial flora capable of increasing the rapidity
of decomposition of organic matter, and of thereby in-
creasing the amount of available fertility.

254. Green Manures. — Crops that are grown only for
the purpose of being plowed under to improve the soil
are called green manures. They may benefit the soil
in one or all of four ways: (1) By utilizing soluble plant-
food that would otherwise escape from the soil. (2) By
incorporating vegetable matter with the soil. (3) Le-
guminous crops, when used, add to the nitrogen content
of the soil through the fixation of atmospheric nitrogen.
(4) Plant-food from the lower soil may be brought to the
surface soil.

A large number of crops may be used for this purpose,
but certain ones are more useful than others, while the
climate determines to some extent which crops should
be used. Leguminous crops have the great advantage
of acquiring nitrogen from the air. Crops that can be
planted in the fall and grow during the cool weather
can be utilized when otherwise the land would frequently

GREEN MANURES 385

lie bare. Deep-rooted crops usually accumulate a large
amount of nutriment from the soil, and considerable
from the lower depths. They are therefore useful in
bringing plant-food to the upper layer of soil. Succulent
crops decompose easily, and dry out the soil less, when
plowed under, than do woody crops. Crops with exten-
sive root-systems prevent loss of soluble matter more
thoroughly than do plants with small roots.

255. Leguminous crops. — A soil that has become less
productive under cultivation, and that must be improved
before profitable crops can be grown, receives more
benefit from the use of leguminous crops than any
other. The legume to use is naturally the one best
adapted to the region in which the soil is located. Red
clover, mammoth clover and field peas on the soils to
which they are adapted in the northern states; alsike
clover in the wet soils of that region; cowpeas and crim-
son clover in the South, and alfalfa, clovers, soy beans
and cow peas in the West, are the principal leguminous
green-manuring crops. More recently a positive effort
has been made in certain northern states to grow sweet
clover (Melilotus alba), which is a vigorous wild legume,
as a green manure crop. Marked success has followed
its use, but, like alfalfa and the clovers, it requires a soil
well stocked with lime.

The legumes have the important property of securing
nitrogen from the air, which is added to the soil from
the decomposition of the tops and roots when the crop
is plowed under. The nitrogen contained in a ton of the
green crop, when in a condition to plow under, is as
follows:

386

THE PRINCIPLES OF SOIL MANAGEMENT

Table LX

Red or mammoth clover

Crimson clover

Alsike clover

Alfalfa

Cowpeas

Soy beans

Field peas

Nitrogen
per ton

Probable

yield per

acre

Pounds

Tons

10

6

9

6

10

5

14

8

8

6

10

6

11

5

Nitrogen
per acre

Pounds
GO
54
50
112
48
60
55

Not all of the nitrogen contained in these crops is
taken from the air. On soils rich in nitrogen, a consider-
able proportion may be obtained from the soil. On poor
soils, the proportion derived from the atmosphere is
considerably larger. The soils needing the nitrogen
most are those that benefit most largely.

As the legumes need other fertilizing material in an
available form to produce a good yield, mineral ferti-
lizers or farm manure should be added to the soil.
Especially on run-down land this treatment is profitable.

The crops should be plowed under while green and
succulent, as they decompose most readily at that stage.
On sandy soils and in dry regions, the soil may be
rendered so porous by plowing under a crop of dry
vegetation that the capillary rise of water is greatly
decreased, and the movement of air through the soil
causes it to become very dry.

The perennial clovers (red, mammoth and alsike)
and alfalfa do not make a rapid growth after seeding,
which is a disadvantage when quick results are desired,

COVER CROPS AND GREEN MANURE

387

as on a badly run-down soil. Crimson clover is an annual,
and in the central and southern states may be sown in
the fall and plowed under in the late spring, thus making
use of a period of the year when the soil is most likely
to be unoccupied by a crop. Cowpeas, soy-beans and
field peas must be grown during the summer months.
Vetch promises to be a useful green manure for winter
growth in the northern states.

256. Cereal crops. — Where it is desired to keep a
crop on the soil during the autumn, winter and spring,
for the purpose of utilizing the soluble plant-food, the
cereals, especially rye, are useful. Rye has the advan-
tage of being an inexpensive crop to seed, besides being
very hardy, and capable of growing on poor soil. It
furnishes fall pasture, but should not be pastured in the
spring if intended for green manure. It is important
that it be plowed under while green.

Buckwheat, on account of its ability to grow on poor
soil, is adapted to use as a green manure, but it must
be grown in the summer.

D. ORGANISMS IN THE SOIL

A vast number of organisms, animal and vegetable,
live in the soil. By far the greater part of these belong
to plant life, and these comprise the forms of greatest
effect in producing those changes in structure and
composition which contribute to soil productiveness.
Most of the organisms are so minute as to be seen only
by the aid of the microscope, while a much smaller
proportion range from these to the size of the larger
rodents. They may thus be classed as macro-organ-
isms and micro-organisms.

I. MACRO-ORGANISMS OF THE SOIL

Of the macro-organisms in the soil the animal
forms belong chiefly to (1) rodents, (2) worms, (3)
insects; and the plant forms to (1) the large fungi and
(2) plant roots.

257. Rodents. — The burrowing habits of rodents,
of which the ground-squirrel, mole, gopher and prairie-
dog are familiar examples, result in the pulverization
and transfer of very considerable quantities of soil.
While their activities are often not favorable to agri-
culture, the effect upon the character of the soil is
quite beneficial, and analogous to that of good tillage.
Their burrows also serve to aerate and drain the soil,
and in permanent pastures and meadows are of much
value in this way.

(388)

SOIL MICRO-ORGANISMS 389

258. Worms. — The common earthworm is the most
conspicuous example of the benefit that may accrue
from this form of life. Darwin, as the result of care-
ful measurements, states that the amount of soil
passed through these creatures may, in a favorable
soil in a humid climate, amount to ten tons of dry
earth per acre annually. The earthworm obtains its
nourishment from the organic matter of the soil, but
takes into its alimentary canal the inorganic matter
as well, expelling the latter in the form of casts
after it has passed entirely through the body. The
ejected material is to some extent disintegrated,
and is in a flocculated condition. The holes left in
the soil serve to increase aeration and drainage, and
the movements of the worms bring about a notable
transportation of lower soil to the surface, which aids
still more in effecting aeration. Darwin's studies
led him to state that from one-tenth to two-tenths
of an inch of soil is brought to the surface of land in
which earthworms exist in normal numbers.

Instances are on record of land flooded for a con-
siderable period so that the worms were destroyed,
and the productiveness of the soil was seriously
impaired until it was restocked with earth-worms.

Wollny conducted experiments with soil, in one
case containing earthworms, and in another destitute
of them. Although there was much variation in his
results, they were in every case in favor of the soil
containing the worms, and, in a number of the tests,
the yield on rich soil was several times as great as where
no worms were present.

390 THE PRINCIPLES OF SOIL MANAGEMENT

Earthworms naturally seek a heavy, compact soil,
and it is in soil of this character that they are most
needed, on account of the stirring and aeration they
effect. Sandy soil and the soils of the arid regions,
in which are found few or no earthworms, are not
usually in need of their activities.

259. Insects. — There is a less definite, and probably
less effective, action of a similar kind produced by
insects. Ants, beetles, and the myriads of other bur-
rowing insects and their larva? effect a considerable
movement of soil particles, with a consequent aeration
of the soil. At the same time they incorporate in the
soil a considerable amount of organic matter.

260. Large fungi. — The larger fungi are chiefly con-
cerned in bringing about the first stages in the decom-
position of woody matter, which is disintegrated
through the growth in its tissues of the root-mycelia
of the fungi. These break down the structure, and
thus greatly facilitate the work of the decay bacteria.
Action of this kind is largely confined to the forest
and is not of much importance in cultivated soil.

Another function of the large fungi is exercised
in the intimate and possibly symbiotic relation of the
fungal hyphae to the roots of many forest trees, in
soil where nitrification proceeds very slowly, if at all,
for nitrates are apparently never present in forest
soils. This enveloping system of hyphse, which may
consist of masses in a definite zone of the cortex,
with occasional filaments passing outward into the
soil, or which may surround the root with a dense
mass of interwoven hyphse, is called mycorhiza.

SOIL MICRO-ORGANISMS 391

The cereal, cruciferous, leguminous and solanaceous
plants are not associated with mycorhiza. Mycotropic
plants are usually those that live in a humus soil
filled with the mycelia of fungi. It is thought that
the mycorhiza aid the higher plants to obtain nutri-
ment that they must strive for in competition with
the fungi.

Mycotropic plants are also able to grow with a very
small transpiration of moisture, as is well known to
be the case with many conifers; and this restricted
transpiration would doubtless result in lack of nutri-
ment were it not for the assistance of the mycorhiza.

261. Plant roots. — The roots of plants assist in pro-
moting productiveness of the soil both by contributing
organic matter and by leaving, upon their decay,
openings which render the soil more permeable to
water and which also facilitate drainage and aeration.
The dense mass of rootlets, with their minute hairs
that are left in the soil after every harvest, furnish
a well-distributed supply of organic manure, which is
not confined to the furrow slice, as is artificially incor-
porated manure. The drainage and aeration of the
lower soil, due to the openings left by the decomposed
roots, are of the greatest importance in heavy soil,
and the beneficial effects of clover and other deep-
rooted plants are due in no small measure to this
function.

II. MICRO-ORGANISMS OF THE SOIL

Of the micro-organisms commonly existing in
soils, the great majority belong to plant rather than

392 THE PRINCIPLES OF SOIL MANAGEMENT

to animal life. Of the latter, the only organisms of
economical importance are the nematodes, whose
injurious effect upon plant growth is accomplished
through the formation of galls on the
roots, in which the young are hatched
and live to sexual maturity.

262. Plant micro-organisms. — The
microscopic plants of the soil may be
classed as slime-molds, bacteria, fungi
and algae.

263. Plant micro-organisms injurious
Nematodes enter- to higher plants. — Injurious plant

mgaroot. micro- organisms are confined mostly
to fungi and bacteria. They may be entirely para-
sitic in their habits, or only partially so. They
injure plants by attacking the roots. Those attacking
other portions of plants may live in the soil during
their spore stage, but these are not strictly micro-
organisms of the soil. Some of the more common dis-
eases produced by soil organisms are: Wilt of cotton,
cowpeas, watermelon, flax, tobacco, tomatoes, etc.,
damping-off of a large number of plants, root-rot,
galls, etc. .

These fungi or bacteria may live for long periods,
probably indefinitely, in the soil, if the conditions
necessary for their growth are maintained. Some of
them will die within a few years if their host plants
are not grown upon the soil, but others are able to
maintain existence on almost any organic substance.
Once a soil is infected, it is likely to remain so for a
long time, or indeed indefinitely. Infection is easily

PLANT MICRO-ORGANISMS

393

carried. Soil from infected fields may be carried on
implements, plants, rubbish of any kind, in soil used
for inoculation of leguminous crops, or even in stable
manure containing infected plants, or in the feces
resulting from the feeding of infected plants. Flooding
of land by which soil is washed from one field to another
may be a means of infection.

Prevention is the best defense from diseases pro-
duced by these soil organisms. Once disease has pro-
cured a foothold, it is practically impossible to eradi-
cate all its organisms. Rotation of crops is effective
for some diseases, but entire absence of the host crop
is more often necessary. The use of lime is beneficial
in the case of certain diseases. Chemicals of various
kinds have been tried with little success. Steam-
sterilization is a practical method of treating green-
house soils for a number of diseases. The breeding of
plants immune to the disease affecting its particular
species has been successfully carried out in the case of
the cowpea and cotton plants and can doubtless be
accomplished with others.

264. Plant micro-organisms not injurious to higher
plants. — The vegetable micro-organisms of the soil
all take an active part in removing dead plants and
animals from the surface of the soil, and in bringing
about the other operations that are necessary for the
production of plants. The first step in the preparation
for plant growth is to remove the remains of plants and
animals that would otherwise accumulate, to the ex-
clusion of other plants. These are decomposed through
the action of organisms of various kinds, the inter-

394 THE PRINCIPLES OF SOIL MANAGEMENT

mediate and final products of decomposition assisting
plant production by contributing nitrogen and certain
mineral compounds that are a directly available source
of plant nutriment, and also by the effect of certain
of the decomposition products upon the mineral
substances of the soil, by which they are rendered
soluble and hence available to the plant.

Through these operations the supply of carbon and
nitrogen required for the production of organic matter
is kept in circulation. The complex organic compounds
in the bodies of dead plants or animals, in which con-
dition plants cannot use them, are, under the action
of micro-organisms, converted by a number of stages
into the very simple compounds used by plants. In
the course of this process, a part of the nitrogen is
sometimes lost into the air by conversion into free
nitrogen, but fortunately this may be recovered and
even more nitrogen taken from the air by certain other
organisms of the soil.

The slime molds, bacteria, fungi and algse all play
a part in these processes, but none of them so actively
during every stage of the process as do the bacteria.
Molds and fungi are particularly active in the early
stages of decomposition of both nitrogenous and non-
nitrogenous organic matter. Molds are also capable
of ammonifying proteins, and even reforming the
complex protein bodies from the nitrogen of ammonium
salts. Certain of the molds and algse are apparently
able to fix atmospheric nitrogen, and contribute a
supply of carbohydrates required for the use of the
nitrogen-fixing bacteria.

BACTERIA OF THE SOIL

395

265. Bacteria. — Of the several forms of micro-
organisms found in the soil, bacteria are the most
important. In fact, the abundant and continued growth
of plants upon the soil is absolutely dependent upon
the presence of bacteria, as through their action chemi-
cal changes are brought about which result in making
soluble both organic and inorganic material necessary

for the life of higher plants, and
which, in part at least, would
not otherwise occur.

Bacteria are thus transform-
ers, and not producers, of fer-
tility in the soil, although, as
we shall see later, certain kinds
of bacteria take nitrogen from
the air and leave it in the soil.
With this exception, however,
they add no plant food to the
soil. It is their action in render-
ing available to the plant ma-
terial already present in the soil
that constitutes their greatest
present value in crop-produc-
tion. It is to their activity in conveying nitrogen from
the air to the soil that we are indebted for most of
our supply of nitrogen in virgin soils.

It is not usually the entire absence of bacteria
from the soil that is to be avoided in practice, for all
arable soils contain bacteria, although sometimes not
all of the desirable forms; but, as great bacterial
activity is required for the large production of crops,

"^

Fig. 109. Some types of
soil-bacteria, highly magnified.
a, Nitrate formers; b, nitrite-
formers; c. Bacteria graveolens;
d, B. fusiformis; e, B. nebtilis;
f, Closteridium pasteurianum.

396 THE PRINCIPLES OF SOIL MANAGEMENT

the practical problem is to maintain a condition of
soil most favorable to such activity.

266. Distribution. — Bacteria are found almost uni-
versally in soils, although they are much more numer-
ous in some soils than in others. A number of investi-
gators have stated that in soils from different locali-
ties and of different types that they have examined,
the numbers of bacteria were proportional to the
productiveness of the soils. The number of bacteria
present has, in some cases, been shown to be propor-
tional to the amount of humus contained in the soil.
It is natural to expect that within certain limits
both of these findings will hold. The conditions ob-
taining in a productive soil are those favorable to
the development of certain forms of bacteria, and these
kinds constitute a very large proportion of those gen-
erally found in soils. However, there is evidence
that comparatively unproductive soils may contain
a large number of bacteria which are presumably
not favorable to plant-growth.

Samples of soil taken from certain productive
and relatively unproductive portions of a field on
Cornell University farm contained a larger number
of bacteria in the poor soil, although the two soils
were equally well drained, and the good soil had slightly
more organic matter. They had also received practi-
cally the same treatment during the preceding few
years.

Character of Number of bacteria

soil per gram of dry soil

Good 1,200,000

Poor 1,600,000

ABUNDANCE OF SOIL BACTERIA 397

After wheat had been growing for two months
on these soils in the greenhouse, and maintained at
the same moisture content, they were again sampled.

Character of Number of bacteria

soil per gram of dry soil

Good 760,000

Poor 1,120,000

Another reason why this relation between the
number of bacteria and soil productiveness does not
hold is that those bacteria having the same functions
in relation to plant-food do not always have the same
physiological efficiency. In other words, they do not
have the same virulence, a small number in some
cases being able to bring about the same changes that
in other cases require a much larger number.

Bacteria are found chiefly in the upper layers of
soil, although not at the immediate surface of the
ground. The layer between the first and sixth or
seventh inches contains, in most soils, the great bulk
of the bacteria present. Below that depth they de-
crease in numbers, and below a depth of six to eight
feet there are usually none.

267. Numbers. — The number of bacteria in any
soil will naturally vary with the conditions that favor
or discourage their growth. In sandy soils, forest
soils, desert soils, acid soils, waterlogged soils and
soils low in humus, the bacteria are either absent or
very few in numbers. In soils very rich in organic
matter, especially where animal manure has been
applied, or where a carcass has been buried, the num-
ber becomes very large, as many as 100,000,000 per

398

THE PRINCIPLES OF SOIL MANAGEMENT

gram having been found; while in soil of ordinary
fertility and tilth the numbers range from 1,000,000
to 5,000,000 per gram. The extreme rapidity with
which reproduction occurs makes it possible for the
number to increase enormously when conditions are
favorable for their growth. While, therefore, very
few bacteria are present in soils of the northern states
during the winter, the number increases with great
rapidity in the spring. Marshall Ward has shown
that in the mild winters in England some soil bac-
teria at least continue their activity throughout the
winter. In the southern states of America the same
is doubtless true.

The following table shows the number of bacteria
per gram of soil found in different parts of the United
States during some portion of the growing season:

Table LXI

State

Soil

Crop

Investi-
gator

Number

Delaware . .

Grass, 12 yrs.

Chester

425,000

Delaware . .

Grass, 4 yrs.

Chester

425,000

Delaware . .

Clover, follow-
ing fallow

Chester

1,880,000

Delaware . .

Woodland

Chester

70,000

Delaware . .

Rich garden

Vegetables

Chester

1,860,000

Kansas ....

Loam

(humus 2.19%)

Mayo &
Kinsley

33,931,747

Kansas ....

Loam
(humus 3.07%)

Mayo &
Kinsley

53,596,060

Kansas ....

Thin soil,
gumbo subsoil

Mayo &
Kinsley

78,534

Kansas ....

Loam, low in
humus

Mayo &
Kinsley

8,543,006

Kansas ....

Loam, low in
humus

Mayo &
Kinsley

3,192,131

SOIL BACTERIA, CONDITIONS FOR GROWTH 399

268. Conditions affecting growth. — Many conditions
of the soil affect the growth of bacteria. Among the
most important of these are the supply of oxygen
and moisture, the temperature, the presence of organic
matter, and the acidity or basicity of the soil.

269. Oxygen. — All soil bacteria require for their
growth a certain quantity of oxygen. Some bacteria,
however, can continue their activities with much less
oxygen than can others. Those requiring an abundant
supply of oxygen have been called aerobic bacteria,
while those preferring little or no air are designated
anaerobic bacteria. This is an important distinction,
because those bacteria which are of the greatest benefit
to the soil are, in the main, aerobes, and those bac-
teria that are injurious in their action are chiefly
anaerobes. However, it seems likely that an aerobic
bacterium may gradually accommodate itself within
certain limits to an environment containing less
oxygen, and an anaerobic bacterium may accommodate
itself to the presence of a larger amount of oxygen.
Thus a bacterium may be most active in the presence
of an abundant supply of oxygen; but, when subjected
to conditions in which the supply is small, growth
continues, but with lessened vigor. The term facultative
bacteria has been used to designate those bacteria
that are able to adapt themselves to considerable
variation in oxygen supply. The structure, tilth and
drainage of1 the soil consequently determine largely
whether aerobic or anaerobic bacteria shall be most
active.

270. Moisture. — Bacteria require some moisture

400 THE PRINCIPLES OF SOIL MANAGEMENT

for their growth. A notable decrease in the moisture
content of the soil may temporarily decrease the
number of bacteria by limiting their development to
the films of moisture surrounding the particles. With
a decrease in the moisture content of any soil, there
occurs an increase in the oxygen in the interstitial
spaces. Those bacteria thriving in the presence of
oxygen are thereby favored, and the character of the
bacterial flora is correspondingly changed. When the
soil remains saturated, or nearly so, for any consider-
able period, the anaerobic forms assert themselves,
and the usually beneficial activities of the aerobic
bacteria are temporarily suspended. The most favor-
able moisture conditions for the activity of the most
desirable bacteria is that found in a well-drained soil.
271. Temperature. — Soil bacteria, like other plants,
continue life and growth under a considerable range
of temperature. Freezing, while rendering bacteria
dormant, does not kill them, and growth begins slightly
above that point. Warrington has. shown that nitri-
fication goes on at temperatures as low as 37° to 39°
Fahr. It is not, however, until the temperature is
considerably higher that the functions of any of the
soil bacteria are pronounced. From 70° to 110° Fahr.
their activity is greatest, and it diminishes perceptibly
below or above those points. The thermal death points
of most forms of bacteria is found at some point
between 110° and 160° Fahr., but the spore forms
even resist boiling. Only in some desert soils does the
natural temperature reach a point sufficiently high
to actually destroy bacteria, and there only in the

SOIL BACTERIA, CONDITIONS FOR GROWTH 401

upper surface. In fact, it is seldom that soil tempera-
tures become sufficiently high to curtail bacterial
activity.

272. Organic matter. — The presence of a certain
quantity of organic matter is essential to the growth
of most, but not all, forms of soil bacteria. The or-
ganic matter of the soil, consisting as it does of the
remains of a large variety of substances, furnishes a
suitable food-supply for a very great number of forms
of organisms. The action of one set of bacteria upon
the cellular matter of plants embodied in the soil
produces compounds suited to other forms, and so
from one stage of decomposition to another this con-
stantly changing material affords sustenance to a
bacterial flora the extent and variety of which it is
difficult to conceive. Bacteria not only affect the or-
ganic matter of the soil, but, in the case of certain
forms, their activities produce changes in the inorganic
matter that cause it to become more soluble and more
easily available to the plant.

A soil low in organic matter usually has a lower
bacterial content than one containing a larger amount,
and, under favorable conditions, the beneficial action,
to a certain point at least, increases with the content
of organic substance; but, as the products of bacterial
life are generally injurious to the organisms producing
them, such factors as the rate of aeration and the
basicity of the soil must determine the effectiveness
of the organic matter.

273. Soil acidity. — A soil having an acid reaction
makes a poor medium for the growth of bacteria. A

402

THE PRINCIPLES OF SOIL MANAGEMENT

neutral or slightly alkaline soil furnishes the most
favorable conditions for bacterial growth. The activi-
ties of many soil bacteria result in the formation of
acids which are injurious to the bacteria themselves,
and, unless there is present some basic substance
with which these can combine, bacterial development
is inhibited by their own products. This is one of the

Fig. 110. Spring-toothed walking cultivator. For thorough, shallow tillage.

reasons why lime is so often of great benefit when ap-
plied to soils, and especially to those on which legumi-
nous crops are growing. For the same reason, the
presence of lime hastens decay of organic matter in
certain soils, and the conversion of nitrogenous ma-
terial with a minimum loss into compounds available
to the plant. As showing the value of lime in the
process of nitrification, it has been pointed out that
in the presence of an adequate supply of lime the
availability of ammonium salts is almost as high as

FUNCTIONS OF SOIL BACTERIA

403

that of nitrate salts, but where the supply is insufficient
the value of ammonium salts is relatively quite low.

274. Functions of soil bacteria. — Bacteria have
a part in many of the processes of the soil which
greatly affects its productiveness. It has become
customary to refer to the changes produced by certain
forms of bacteria as their function in contributing
to soil-productiveness.

275. Decomposition of mineral matter. — Certain
bacteria decompose some of the mineral matter of
the soil and render it more easily available to the
plant. While the nature of the processes and their
extent are not known, there is sufficient evidence to
justify the above statement. It is well known that
several forms of bacteria are instrumental in decom-
posing rock, and that sulfur and iron compounds are
acted upon by other forms. Again, the much greater
efficiency of difficultly soluble phosphate fertilizers,
when used in conjunction with a quantity of organic
matter, is evidence of the relation of bacterial action
to the decomposition of mineral substances. Stocklasa
has shown that, when B. megatherium and B. fluor-
escens are added to soil fertilized with insoluble
phosphates, plants grown thereon take up a larger
amount of phosphorus than those on uninoculated
soils.

Organic acids and carbon dioxid are constantly
produced by soil bacteria. These in soil water are
weak but ever-acting solvents, the effect of which
must in the end be considerable. It seems likely,
however, that there is a more direct effect of certain

404 THE PRINCIPLES OF SOIL MANAGEMENT

bacteria upon mineral matter than merely the solvent
action of these acids. That rock may be disintegrated
through the action of bacteria has been already com-
mented upon. Although it has not yet been demon-
strated, bacteria such as are capable of decompos-
ing rock may, in all probability, exist in the soil
where their activities result in the "weathering" that
always goes on in soils even when no organic mat-
ter is present.

It has been suggested that carbon dioxid dissolved
in water may act on the very difficultly soluble tri-
calcium phosphate, producing di-calcium phosphate,
a more soluble form, and calcium bicarbonate, thus:

Ca3(POJ2 + 2C02 + 2H20=Ca2H2(POJ2 + Ca(HC03)2

The calcium bicarbonate thus produced, as well
as that derived from other sources, may then act on
the double silicates of aluminum and one of the alkalies,
thus:

K20. A1203. 6 Si02 + Ca(HC03)2 = CaO. A1203. 6 Si02 + 2 KHC03

There is then another nutrient rendered available to
the plant.

It has been shown by Van Delden and by Nadson
that several forms (M. desulfuricans, M. cestuarii,
Proteus vulgaris and B. mycoides) are able to reduce
sulfates, while transformations of iron, silicon and
calcium are effected by Proteus vulgaris.

276. Decomposition of non-nitrogenous organic
matter. — The organic matter commonly decomposed
in soils contains a large proportion of compounds

FUNCTIONS OF SOIL BACTERIA 405

containing no nitrogen. The non-nitrogenous sub-
stances decompose quite rapidly, and the organic
nitrogen disappears less rapidly than the carbon,
hydrogen and oxygen of organic bodies.

Humus always contains a higher percentage of
nitrogen than do the plants from which it is formed
(page 123).

The non-nitrogenous substances consist of cellulose
and allied compounds forming the cell-walls of plants,
and the carbohydrates, organic acids, fats, etc., con-
tained in them. The dissolution of cellulose is brought
about by the action of the enzyme cytase secreted
by a number of fungi, and is also probably accomplished
by the Bacillus amylobacter, but whether through the
secretion of an enzyme is not known. ' Other bacteria
have been reported to secrete a cytase that acts on
certain constituents of the cell-wall. It is probable
that numerous organisms capable of fermenting cellu-
lose and allied substances exist in the soil, which
decomposition they accomplished through the pro-
duction of cytase.

The effect of cytase upon cellulose and other fiber
is to hydrolyse it with the formation of sugar, as glu-
cose, mannose, zylose, aribinose, etc.

Starch is converted into glucose by a ferment
(diastase) either present in the plant itself or possibly
secreted by fungi or bacteria. All the sugars are finally
converted into organic acids which may combine with
mineral bases. Distinct organisms have been isolated
that can utilize for their development formates, acetates
propionates, butyrates, etc., the final product being

406 THE PRINCIPLES OF SOIL MANAGEMENT

carbon dioxid and water. Thus, step by step, the non-
nitrogenous matter incorporated in the soil is carried
by one and another form of organisms from the most
complex to the simplest combinations.

The final product of the decomposition of carbon-
aceous matter being carbon dioxid, there is a return
to the air of the compound from which the carbon of
the decomposing substance was originally derived.
In the plant, unless it is saprophytic, the carbon of
the tissues comes directly from the carbon dioxid of
the air, from which more complex carbon-bearing
compounds are produced and utilized in its functions
or in its tissues. A portion of the carbon is returned
to the air by the plant in the form of carbon dioxid,
the remainder is retained by the plant, and may be
returned by the process of decay, or may be consumed
by an animal, and, as the result of its physiological
processes, either exhaled as carbon dioxid or deposited
in the tissues to be later decomposed and converted
into carbon dioxid. The soil is thus the scene of at
least a part of the varied transformations through
which carbon is continually passing, as it is utilized
by higher plants, animals, bacteria and fungi.

The non-nitrogenous organic substances in their
various stages furnish food for a large number of
bacteria, among which are those concerned in the
decomposition of mineral matter and in the processes
of nitrification and nitrogen-fixation. There are, there-
fore, two ways in which these substances are of great
importance in soil fertility: (1) As a source of organic
acids. (2) As a food-supply for useful soil bacteria.

DECAY BACTERIA

407

277. Decomposition of nitrogenous organic matter.
— The decomposition of nitrogenous organic matter is
accomplished by a series of changes from one compound
to another, as we have seen was the case with the non-
nitrogenous materials. The final products are carbon
dioxid, water, and usually some hydrocarbon gases
resulting from the carbon and hydrogen of the organic

Fig. 111. The large-shovel riding cultivator.

matter, and also some hydrogen sulfide or other gas
containing sulfur or a final oxidation of the sulfur of
the proteids into sulfates, while the nitrogen is ulti-
mately converted into nitrates, or into free nitrogen,
although a portion of the original nitrogen some-
times escapes into the air in the intermediate stage,
ammonia.

The processes will be discussed under the following
heads, which represent certain more or less definite
stages in the decomposition: (1) Decay and putre-

408 THE PRINCIPLES OF SOIL MANAGEMENT

faction. (2) Ammonification. (3) Nitrification. (4)
Denitrification.

278. Decay and putrefaction. — Decomposition of
the nitrogenous organic matter of the soil, consisting
largely of the proteins, begins with either one of two
processes — decay or putrefaction. Decay is produced
by aerobic bacteria, and naturally occurs when the
conditions are most favorable for their development.
When the conditions are otherwise, the growth of
these bacteria is checked, and then further decom-
position would be extremely slow were it not for the
other process — putrefaction. Putrefaction is produced
by anaerobic bacteria. In the same body, and conse-
quently in the same soil, decay and putrefaction may
be in progress simultaneously, decay taking place on
the outside and on the surfaces of other parts exposed
to the air, while putrefaction occurs on the interior,
where the supply of oxygen is limited. By means of
the two processes, decomposition is greatly facilitated.

Decay produces a very rapid and complete decom-
position of the substance in which it operates, most of
the carbon and hydrogen being quickly converted into
carbon dioxid and water, and the nitrogen into am-
monia and probably some free nitrogen. The latter
is possibly due to the oxidation of ammonia, thus

4 NH3 + 3 02 = 6 H20 + 2 N2.

The sulfur of the proteins finally appears in the form
of sulfates.

What the intermediate products are has not been
determined, but in the decay of meat, where there was

PUTREFACTION BACTERIA 409

an abundant supply of oxygen, succinic, palmytic,
oleic and phenyl-propionic acids have been found.

Putrefaction results in a large number of complex
intermediate compounds and proceeds much more
slowly. Many of the substances thus produced are highly
poisonous and most of them have a very offensive
odor. They may be further broken down by decay when
the conditions are suitable, or by a continuation of the
process of putrefaction. In either case, the poisonous
properties and the odor are removed.

In the process of decomposition of organic matter
two classes of substances are produced: (1) Those
which have been excreted or secreted by the bacterium,
and therefore have passed through the metabolic
processes of the organism. (2) Those that have been
formed because of the removal of certain atoms by
bacteria or enzymes from compounds, thus necessi-
tating a readjustment of the remaining atoms and the
consequent formation of a new compound.

Putrefaction is carried on by a large number of
forms of bacteria, the resulting product depending upon
the substance in process of decomposition, and upon
the bacteria involved. Some of the characteristic,
although not constant products, formed in the putre-
faction of albumin and proteins are albumenoses, pep-
tones, and amino-acids, followed by the formation
of cadaverin, putrescin, skatol and indol. Where an
abundant supply of oxygen is present, or where a
sufficient supply of carbohydrates exist, these sub-
stances are not formed. There are many other products
of putrefaction, including a number of gases, as carbon

410 THE PRINCIPLES OF SOIL MANAGEMENT

dioxid, hydrogen sulfide, marsh gas, phosphine, hy-
drogen, nitrogen, etc.

It will . be noticed that these changes, like those
occurring in the non-nitrogenous organic matter,
involve a breaking down of the more complex com-
pounds and the formation of simpler ones; that a very
large number of bacteria are concerned in the various
steps, while even the same substances may be decom-
posed and the same resulting compounds formed
by a number of different species of bacteria.

Present-day knowledge of the subject does not
make it possible to present a list of the bacteria con-
cerned in each step, or to name all of the intermediate
products formed; but for the student of the soil the
principal consideration is a knowledge of the circum-
stances under which the nitrogen is made available to
plants, and the conditions which are likely to result
in its loss from the soil.

279. Ammonification. — Decay and putrefaction
may be considered as a continuation of ammonification,
or the latter process as the beginning of the former.
Ammonification, as its name implies, is that stage of
the process during which ammonia is formed from the
intermediate products.

Like the other processes of decomposition, there
are many species of bacteria capable of forming am-
monia from nitrogenous organic substances. Differ-
ent forms display different abilities in converting nitro-
gen of the same organic material into ammonia, some
acting more rapidly or more thoroughly than others.
In tests by certain investigators where the same bac-

AMMONIFICA TION 411

teria are used upon different substances, the order of
their efficiency is changed with the change of sub-
stance. It seems likely, therefore, that certain forms
are most efficient when acting on certain organic com-
pounds. That, in other words, each species is best
adapted to the decomposition of certain substances,
while capable of attacking others, although less effec-
tively.

Among the bacteria producing ammonification
are B. mycoides, B. subtilis, B. mesentericus vulgatus,
B. janthinus and Proteus vulgaris. Of these, B. mycoides
has been very carefully studied, and the findings of
Marchal may be taken as representative of the process
of ammonification. He found that when this bacterium
was seeded on a neutral solution of albumin, ammonia
and carbon dioxid were produced, together with small
amounts of peptones, leucin, tyrosin, and formic,
butyric and proprionic acids. He concludes that in
the process, atmospheric oxygen is used, and that
the carbon of the albumin is converted into carbon
dioxid, the sulfur into sulfuric acid, the hydrogen
partly into water, and partly into ammonia by com-
bining with the nitrogen of the organic substance.
He suggests that a complete decomposition of the al-
bumin occurs according to the following reaction:

C72H112N18S022 + 7702 = 29 H20 + 72C02 + S03 + 18NH3.

The greatest activity occurred at a temperature of
86° Fahr., and as low as 68° Fahr. action was quite
strong. Access of an increased amount of air, produced
by increasing the surface of the liquid, increased the

412 THE PRINCIPLES OF SOIL MANAGEMENT

rate of ammonification. A slightly acid reaction in the
liquid produced the maximum activity, but in a neu-
tral or even slightly acid medium the process was
continued, although much less actively.

He found that B. mycoides was also capable of
ammonifying casein, fibrin, legumin, glutin, myosin,
serin, peptones, creatin, leucin, tyrosin and asparagin,
but not urea and ammonium salts.

280. Nitrification. — Some agricultural plants can
utilize ammonium salts as a source of nitrogen. This
has been determined for maize, oats, barley and po-
tatoes. Other plants, such as beets, show a decided
preference for nitrogen in the form of nitrates. Whether
any of the common crops can thrive as well on ammo-
nium salts as upon nitrates has not been finally demon-
strated. In all arable soils the transformation of
nitrogen does not stop with its conversion into am-
monia, but proceeds by an oxidation process to the
formation of first nitrous and then nitric acids. This
may be considered to proceed according to the fol-
lowing equations:

2NH3 + 30, = 2HN02 + 2H20.
2HN02 + 02 = 2HN03.

The acid in either case combines with one of the bases
of the soil, usually calcium, so that we have calcium
nitrate resulting.

Each of these steps is brought about by a distinct
bacterium, but they are closely related. Collectively
they are called nitro-bacteria. Nitrosomonas and
Nitrosococcus are the bacteria concerned in the

NITRIFICATION 413

conversion of ammonia into nitrous acid or nitrites.
The former are supposed to be characteristic of Euro-
pean, and the latter of American soils. They are
sometimes referred to as nitrous ferments.

Nitrobacter are those bacteria that convert nitrites
into nitrates. They are also designated nitric ferments.
There seem to be some differences in bacteria from
different soils, but the differences are slight, and the
conditions favoring their actions are similar. It is
also true that the conditions favoring the action of
Nitrosomonas and Nitrobacter are similar, and they
are generally found in the same soils, although some
experiments show that, in the same soil, nitrites
may sometimes accumulate, indicating conditions
more favorable to the development of the Nitrosomonas
bacteria. The formation of nitrates usually follows
closely on the production of nitrites, so that there is
rarely more than a trace of the latter to be found in
soils. A soil favorable to the process of nitrification
is usually well adapted to all of the processes of nitro-
gen transformation.

Marked differences have been found in the nitri-
fying power of bacteria from different soils. Highly
productive soils have generally been found to contain
bacteria having greater nitrifying efficiency than those
from less productive soils, but this may not always
be the case, as other factors may limit the productive-
ness.

281. Effect of organic matter on nitrification. —
A peculiarity in the artificial culture of nitrifying
bacteria is that they cannot be grown in artificial

414

THE PRINCIPLES OF SOIL MANAGEMENT

medium containing organic matter. This property
for a long time prevented the isolation and identifi-
cation of these organisms, as it was hardly conceivable
that organisms living in the dark, where energy can-
not be obtained from sunlight, could exist without
using the energy stored by organic matter. It has
been suggested, in explanation of this, that the energy

OCTOBER

Fig. 112. Curves showing the relation of the moisture and temperature of
the soil to the formation of nitrates which are given in parts per million of dry
soil. Depth of sampling, eight inches. These curves bring out clearly the fact
that the warmer soil temperature, combined with a moderately high soil mois-
ture content favors the formation of nitrates.

produced by the oxidation involved in the process
of nitrification makes possible the growth of the or-
ganisms under these, apparently impossible, condi-
tions. Some experimenters report having grown
nitrobacteria in organic media, but it is generally
believed, at present, that this is not possible, and that
there has been some error in their work.

The presence of peptone in the proportion of 500
parts per million completely prevents the develop-

NITRIFICA TION 415

ment of nitrobacteria and one half that quantity checks
it, while 150 parts of ammonia per million has a similar
effect. In a normal soil, the quantity of soluble am-
monium salts is well below this amount, as must also
be that of soluble organic matter. In confirmation of
the inhibiting effect of organic matter on the nitrobac-
teria, cases have been reported of soils very rich in
organic matter in which no bacteria of this type occur.

It has also been stated that very heavy manuring
with organic manures results in decreased nitrification
in the soil. While this may be true where farm manure
is used in the quantities sometimes applied in garden-
ing operations, it is not likely to occur in soils on which
ordinary field crops are grown. The principle is well
illustrated by the dry-earth closet. Manure mixed with
earth in relatively small proportions and kept aerated
by occasional mixing undergoes a very thorough decom-
position of the manure but without any corresponding
increase in nitrates. On the other hand, under field con-
ditions, manure used in relatively small amounts does
not undergo this serious loss.

The application of twenty tons of farm manure
per acre to sod on a clay loam soil for three consecu-
tive years, at Cornell University, resulted in a larger
production of nitrates on the manured soil than upon
a contiguous plat of similar soil left unmanured.
This was true during the third year of the applications,
when the land was in sod, and also the fourth year
when no manure was applied to either plat, and when
both were planted to corn, as may be seen from the
following table:

416

THE PRINCIPLES OF SOIL MANAGEMENT

Table LXII. — Nitrates Produced on Heavily Manured and

on Unmanured Soil

Land in timothy —

April 23

May 3

May 14

May 30

June 1

June 13

June 20

July 24

August 14

Land in maize —

May 19

June 22

July 6

July 28

August 10

NO3 in parts per million, dry soil

Twenty tons ma-

Unmanured soil

nure per acre for

three years

8.2

21.0

4.1

4.6

3.3

4.5

2.0

40

2.4

2.0

0.8

1.1

1.3

3.0

2.2

2.8

1.8

3.0

17.5

20.1

42.8

79.3

50.0

105.0

195.0

304.0

151.0

184.0

282. Effect of soil aeration on nitrification. —
Probably the most potent factor governing nitrifi-
cation in the soil is the supply of air. In clay and even
in loam soils, the tendency to compactness is such
as to exclude air sufficient to enable nitrification to
proceed as rapidly as desirable unless the soil be well
tilled. Columns of soil eight inches in diameter and
of the same depth were removed from a field of clay
loam on Cornell University farm, and carried to the
greenhouse without disturbing the structure of the
soil as it existed in the field. At the same time, simi-
lar-sized vessels were filled with soil dug up from a

SOIL CONDITION AND NITRIFICATION

417

spot nearby. These may be termed unaerated and
aerated soils. Both were kept at the same temperature
and moisture content in the greenhouse, but no plants
were grown upon them. The production of nitrates
was as follows:

Table LXIII

Nitrates in dry soil, parts per million

Date of analysis

Unaerated soil

Aerated soil

When taken from field

3.2
4.2
9.0

3.2

After standing one month

After standing two months

17.6
45.6

283. Effect of sod on nitrification. — Nitrification
proceeds slowly on sod land, especially if the soil is
heavy. On the same type of soil as that used in the
experiment last described, the average quantities of
nitrates for each month of the growing season in the
surface eight inches of sod land as compared with
maize land under the same manuring were as follows:

Table LXIV

Month

April .
May ..
June. .
July . .
August

Nitrates in dry soil, parts per million

AA

418 THE PRINCIPLES OF SOIL MANAGEMENT

The amount of nitrogen removed by the maize
crop was greater than that removed by the timothy,
consequently the greater amount in the former soil
can not be due to the effect of the crop.

So far as the> conservation of nitrogen is concerned,
sod is an ideal crop, for nitrates are formed very little
faster than they are used, and are not carried off in
large amounts by the drainage water.

In the corn land as much as 500 pounds of nitrates
were present in the first twelve inches of one acre, or
fully five times as much as was used by the crop.

284. Depth at which nitrification takes place. —
Warington concluded from his experiments that
nitrification takes place only in the surface six feet of
soil. Hall has pointed to the fact that no more nitrates
were leached from the 60-inch lysimeter at Rotham-
sted than from the one 20 inches deep; which is very
good evidence that in that particular soil nitrification
does not take place below 20 inches from the surface.
In more porous soils, however, nitrification probably
extends deeper, especially in the rich and porous
subsoils of the arid and semi-arid regions.

In all probability, nitrification is largely confined
to the furrow slice, where the opening up of the soil
by tillage has provided the necessary air, and where the
temperature rises to a point more favorable to the action
of nitrifying bacteria. The results from the aerated
and unaerated soils cited above represent the differ-
ences that doubtless exist between the furrow slice and
the subsoil so far as nitrification is concerned.

285. Loss of nitrates from the soil. — Nitrogen hav-

SOIL CONDITION AND NITRIFICATION

419

ing been converted into the form of nitric acid, it im-
mediately combines with available bases in the soil*
forming salts, all of which are very easily soluble, and
which are carried in solution by the soil water. In a
region of large rainfall, the removal of nitrates in the
drainage water is very rapid. Hall states that nitrates
formed during the summer or autumn of one year are
practically all removed from the soil of the Rotham-

Fig. 113. The modern, small, eight-shovel riding cultivator.

sted fields before the crops of the following year have
advanced sufficiently to utilize them. It was formerly
customary to fertilize with ammonium salts in the
autumn, but the drainage water showed on analysis
such a large quantity of nitrates during the months
intervening between the time of fertilizing and the
opening of the growing season that the practice was
discontinued.

In regions of less rainfall or of greater surface
evaporation, the loss in this way is less, reaching a

420 THE PRINCIPLES OF SOIL MANAGEMENT

minimum in an arid region when irrigation is not prac-
ticed. Under such conditions, there is a return of ni-
trates to the upper soil, as capillary water moves
upward to replace evaporated water. In fact, wherever
evaporation takes place to any considerable extent,
there is some movement of this kind. The need for
catch crops to take up and preserve nitrogen is there-
fore greater in a humid region than in an arid or semi-
arid one. An arrangement of crops that allows the
land to stand idle for some time, or a crop that requires
intertillage, as does maize, fails to utilize all of the
nitrates produced, and promotes the loss of nitrogen
in drainage water.

286. Denitrification. — The nitrogen transforming
bacteria thus far studied have been those that cause
the oxidation of nitrogen as the result of their activi-
ties. We may now consider a number of forms of bac-
teria that accomplish a reverse action. The several
processes involved are commonly designated by the
term denitrification, and comprise the following:

(1) Reduction of nitrates to nitrites and ammonia.

(2) Reduction of nitrates to nitrites, and these to
elementary nitrogen.

The number of organisms that possess the ability
to accomplish one or more of these processes is very
large, — in fact greater than the number involved in
the oxidation processes, — but, in spite of their numbers,
permanent loss of nitrogen in ordinary arable soils is
unimportant in amount, although in heaps of barnyard
manure it may be a very serious cause of loss.

Some of the specific bacteria reported to bring about

DENITRIFICATION 421

denitrification are: B. ramosus and B. pestifer, which
reduce nitrates to nitrites; B. mycoides, B. subtilis, B.
mesenteric us vulgatus and many other ammonification
bacteria which are capable of converting nitrates into
ammonia.

Bacterium denitrificans alpha and Bacterium deni-
trificans beta reduce nitrates with the evolution of
gaseous nitrogen.

In addition to these nitrate-destroying bacteria,
there are other bacteria which also utilize nitrates;
but, like higher plants, they convert the nitrogen into
organic nitrogenous substances. However, as they
operate in the dark and cannot obtain energy from
sunlight, they must have organic acids or carbohy-
drates as a source of energy. While these bacteria
cannot be considered to be denitrifiers, they help to
deplete the supply of nitrates when conditions are
favorable for their development. What these condi-
tions are is not well understood, nor can any estimate
be made as to the extent of their operations.

Most of the nitrifying bacteria perform their func-
tions only under a limited access of oxygen, while
others can operate in the presence of a more liberal
supply; but, in general, thorough aeration of the soil
practically prevents denitrification. Straw and dung
apparently carry an abundant supply of denitrifying
organisms, and also furnish a supply of carbohydrates
which favors their action, so that stable manure is
very likely to undergo denitrification, and straw or
coarse stable manure are conducive to the growth of
denitrifying bacteria in the soil.

422 THE PRINCIPLES OF SOIL MANAGEMENT

Under ordinary farm conditions, denitrification is
of no significance in the soil where proper drainage
and good tillage are practiced. Warington showed that,
if an arable soil be kept saturated with water to the
exclusion of air, nitrates added to the soil are decom-
posed, with the evolution of nitrogen gas. As lack of
drainage is usually most pronounced in the early
spring, when the soil is likely to be depleted of nitrates,
it is not likely that much loss arises in this way unless
a nitrate fertilizer has been added. Of the many diffi-
culties arising from poor drainage, denitrification of
an expensive fertilizer may be very considerable item.

The addition of a nitrate fertilizer to a soil receiving
stable manure is not likely to result in a loss of ni-
trates unless the dressings of manure have been ex-
tremely heavy. Hall states that at Rothamsted, where
large quantities of nitrate of soda are used every
year in connection with annual dressings of farm
manure, the nitrate produces nearly as large an in-
crease when added to the manured as when added
to the unmanured plat. There appears, in other words,
to be no loss of nitrate by denitrification.

It is possible to reach a point in manuring where
denitrification may take place. Market gardeners
sometimes reach this point where fifty tons or more
of farm manure, in addition to a nitrate fertilizer,
are added to the soil. Plowing under heavy crops
of green manure may produce the same result. In
either case, the best way to overcome the difficulty is
to allow the organic matter to partly decompose
before adding the fertilizer. The removal of the easily

NITROGEN FIXATION BY BACTERIA 423

decomposable carbohydrates needed by the denitri-
fying organisms decreases or precludes their activity.

287. Nitrogen fixation through symbiosis with
higher plants. — It has long been recognized by farmers
that certain crops like clover, alfalfa, peas, beans, etc.,
improve the soil, making it possible to grow larger
crops of cereals after these crops have been upon the
land. The benefit was, within the past century, traced
to an increase in the nitrogen content of the soil, and
the specific plants so affecting the soil were found to be,
with perhaps a few exceptions, those belonging to the
family of legumes. It has furthermore been demon-
strated that these plants utilize, under certain con-
ditions, the uncombined nitrogen of the atmosphere,
and that they contain, both in the aerial portions
and in the roots, a very high percentage of nitrogen.
In consequence, the decomposition of even the roots
of the plants in the soil leaves a large amount of
nitrogenous matter.

288. Relation of bacteria to nodules on roots. — ■
It has also been shown that the utilization of atmos-
pheric nitrogen is accomplished through the aid of
certain bacteria that live in nodules (tubercles) on
the roots of the plants. These bacteria acquire the
free nitrogen from the air in the soil, and the host plant
secures it in some form from the bacteria or their
products. The presence of a certain species of bacteria
is necessary for the formation of tubercles. Legumi-
nous plants grown in cultures or in soil not containing
the necessary bacteria do not form nodules, and do
not utilize atmospheric nitrogen, the result being that

424

THE PRINCIPLES OF SOIL MANAGEMENT

the crop produced is less in amount and the percentage
of nitrogen in the crop is less.

It has for some years been the belief that the or-
ganism which produces the nodules and utilizes the
uncombined nitrogen is the Pseudomonas radicicola,

but this has very lately
been called in ques-
tion.

The nodules are not
normally a part of
leguminous plants but
are evidently caused
by some irritation of
the root surface, much
as a gall is caused to
develop on a leaf or
branch of a tree by an
insect. In a culture
containing the proper
bacteria, the prick of
a needle on the root
surface will cause a
nodule to form in the
course of a few days.
The entrance of the
bacteria is effected
through a root -hair
which it penetrates,
and may be seen as

Fig. 114. Nodules on the roots of an cl ■,•

alfalfa plant Bacteria live in these nodules, a Ulament extending

or tubercles, and have the power of utilizing . i , ■ i ,1 <• , 1

the free nitrogen of the air in their growth. the entire length 01 the

TUBERCLE BACTERIA 425

hair, and into the cells of the cortex of the root,
where the growth of the tubercle starts.

Even where the causative bacteria occur in cultures
or in the soil, leguminous plants may not secure any
atmospheric nitrogen, or perhaps only a small quantity,
if there is an abundant supply of readily available
combined nitrogen upon which the plant may draw.
The bacteria have the ability to utilize combined
nitrogen as well as uncombined nitrogen, and prefer to
have it in the former condition. On soils rich in nitro-
gen legumes may, therefore, add little or no nitrogen
to the soil, while in properly inoculated soils deficient
in nitrogen an important gain of nitrogen results.

While P. radicicola has been considered the organism
common to all leguminous plants, it is now known
that the organisms from one species of legume are not
equally well adapted to the production of tubercles
on each of the other species of legumes. They show
greater activity on some species than on others, but
do not develop so successfully on any species as on
the one from which the organisms were taken. It was
quite generally believed at one time that the longer any
species of legume is in contact with the organisms
from another species the more active they become,
and the greater the utilization of atmospheric nitrogen.
Considerable doubt has been cast upon this view in
recent years, and it is now generally conceded that
the bacteria of certain legumes are not capable of
inoculating certain other species of legumes.

289. Transfer of nitrogen to the plant. — It has
been shown by several investigators that bacteria

426 THE PRINCIPLES OF SOIL MANAGEMENT

from the nodules of legumes are able to fix atmos-
pheric nitrogen even when not associated with legumi-
nous plants. There would seem to be no doubt, there-
fore, that the fixation of nitrogen in the tubercles of
legumes is accomplished directly by this organism,
and not by the plant itself, or through any combina-
tion of the plant and organism, — both of which hy-
potheses have been advanced. The part which the
plant plays is doubtless to furnish the carbohydrates
required in large quantities by all nitrogen-fixing
organisms and which the legumes are able to supply in
large amounts. The utilization of large quantities
of carbohydrates by the nitrogen-fixing bacteria in
the tubercles may also account for the small proportion
of non-nitrogenous organic matter in the plants.

Kow the plant absorbs this nitrogen after it has
been secured by the bacteria is less well understood.
Early in the growth of the tubercle, a mucilaginous
substance is produced which permeates the tissues
of the plant in the form of long, slender threads, and
which contain the bacteria. These threads develop
by branching or budding, and form what have been
called Y and T forms known as bacteroids, which are
peculiar to these bacteria, and not produced by them
when grown in the media of the laboratory. The threads
finally disappear, and the bacteria diffuse themselves
more or less, through the tissues of the root. What
part the bacteroids play in the transfer of nitrogen is
not known. It has been suggested that in this form
the nitrogen is absorbed by the tissues of the plant.
It seems quite likely that the nitrogen compounds

SOIL-INOCULATION 427

produced within the bacteria cells are diffused through
the cell-wall and absorbed by the plant.

In a recent report, De Rossi states that Pseudo-
munas radicicola is not the causative agent in the
fixation of nitrogen in the nodules of leguminous
plants, and that he has isolated other bacteria that
do possess this property. These bacteria produce the
Y and T forms in artificial media, which is in itself an
indication of their identity with the bacteria concerned
in nitrogen-fixation. De Rossi's work may also explain
why what was formerly considered to be one form
of bacterium, Pseudomonas radicicola, common to
all leguminous plants, is not capable of inoculating
one species of legume when transferred from another.
It may be that there are a number of different forms,
each adapted to certain species of legumes.

290. Soil-inoculation for legumes. — The possibility
of securing a better growth of leguminous crops on
soils not having previously grown such a crop success-
full}', was conceived immediately following the dis-
covery of the nitrogen-fixing bacteria. Extensive
experiments showed the practicability of inoculating
land for a certain leguminous crop by spreading upon
its surface soil from a field on which the same crop is
successfully growing. It is manifestly much better
to apply the organisms or a certain species of legume
from a field having grown the same species than to
attempt to use organisms from another species of le-
gume. The fact that soil-inoculation by means of soil
from other fields may possibly transmit weed seeds
and fungous diseases, and also necessitates the trans-

428 THE PRINCIPLES OF SOIL MANAGEMENT

portation of a great bulk and weight of material,
has led to numerous efforts to inoculate soil by means
of pure cultures. The pure culture may also make it
possible to bring to the soil bacteria of greater physio-
logical efficiency than those already there.

The first attempt at inoculation by pure cultures
was made in Germany, the cultures being sold under the
name of "Nitragin." Careful experiments made with
this material previous to the year 1900 did not show
it to be very efficient; but, of recent years, improve-
ments in the method of manipulating the cultures
have resulted in much greater success. In " Nitragin,"
the medium used for growing the organisms is gelatin,
and, before use, this was formerly dissolved in water;
but now a solution of greater density is used in order
to prevent a change of osmotic pressure, which may
cause plasmolysis and result in the destruction of the
bacteria.

Within recent years, a number of cultures for soil-
inoculation have been offered to the public. The first
of these utilized absorbent cotton to transmit the
bacteria in a dry state from the pure cultures in the
laboratory to the user of the culture, who was to
prepare therefrom another culture to be used for
inoculating the soil. Careful investigation of this
method showed that its weakness lay in drying the
cultures on the absorbent cotton which frequently
resulted in the death of the organisms. More recently,
liquid cultures have been placed on the market in this
country, but they have not yet been sufficiently well
tested to prove their efficiency. It is undoubtedly

NON-SYMBIOTIC NITROGEN FIXATION 429

only a question of time until a successful method of
inoculating soil from artificial cultures will be found.
In the meantime, inoculation by means of infested
soil is the most practical method.

291. Nitrogen-fixation without symbiosis with
higher plants. — If a soil be allowed to stand idle, either
without vegetation or in grass, it will, under favorable
moisture conditions, in the northern states, accumu-
late in one or two years an appreciable amount of
nitrogen not present at the beginning of the period.
At the Rothamsted Experiment Station, one of the
fields in volunteer plants, consisting mainly of grass
without legumes, gained in the course of twenty years
about twenty-five pounds of nitrogen per acre, annually.
According to Hall, the nitrogen brought down by rain
would account for about five pounds per acre per
annum, and dust, bird-droppings, etc., for a little
more. As pointed out by Lipman, there must also
have been a greater total accretion of nitrogen during
the twenty years than appears in the final result, as
considerable must have been lost through removal
of nitrates in drainage and escape of nitrogen in the
ordinary processes of its transformation.

292. Nitrogen-fixing organisms. — Direct experi-
ment has shown that certain bacteria have the ability
to utilize atmospheric nitrogen and to leave it in the
soil in a combined form. A bacillus — Clostridium
pasteurianum — was first found to produce this result.
Later, a commercial culture called " Alinit" was placed
on the market in Germany, which culture it was claimed
contained Bacterium ellenbachensis , with which the

430 THE PRINCIPLES OF SOIL MANAGEMENT

soil was to be inoculated, and that a large fixation
of atmospheric nitrogen would result. A number of
tests of this material failed to show that it caused any
marked fixation of atmospheric nitrogen.

A number of other nitrogen-fixing organisms
have since been discovered. There are: (1) Several
members of the group designated Azotobacter, which
are aerobic bacteria, and which some investigators
hold to be capable of fixing atmospheric nitrogen
when grown in pure cultures, and others believe to
be able to do so, at least in large amounts, only in
the presence of certain other organisms. (2) Mem-
bers of the Granulobacter group, which are large
spore-bearing bacilli of anaerobic habits. (3) B.
radiobacter, which appear to be closely related to or
identical with the B. radicicola of legume tubercles.
The latter has been shown to be able to fix atmospheric
nitrogen even when not growing in symbiosis with
legumes.

There are doubtless many other nitrogen-fixing
organisms still to be discovered.

A peculiarity of these nitrogen-fixing organisms
is their use of carbohydrates, which they decompose
in the process of nitrogen-fixation. They secure more
atmospheric nitrogen when in a nitrogen-free medium.
The presence of soluble lime or magnesium salts, es-
pecially carbonates, is necessary for the best per-
formance of the nitrogen-fixing function, as is also
the presence of a somewhat easily soluble form of
phosphorus. They are exceedingly sensitive to an acid
condition of the soil.

NITROGEN-FIXATION IN PURE CULTURES 431

293. Mixed cultures of nitrogen-fixing organisms. — -
Mixed cultures of the various organisms mentioned
fix larger amounts of nitrogen than do the pure cultures
of any one of them, while some forms are incapable
of fixing nitrogen in pure cultures. Certain algse, par-
ticularly the blue-green alga?, aid greatly in promoting
growth and nitrogen-fixation by these organisms.
This they probably do by producing carbohydrates,
which are used by the bacteria as a source of energy
for nitrogen-fixation, the bacteria furnishing the alga?
with nitrogenous compounds. To what extent the
relation is symbiotic is not known at present, but it
seems probable that a relation may exist similar to
that between leguminous plants and the nitrogen-
gathering bacteria in their nodules.

294. Nitrogen-fixation and denitrification antagonis-
tic.— Nitrogen-fixation and denitrification are reverse
processes. The former is, for most bacteria, favored
by an abundant air-supply and a moderately high
temperature. Thus, at 75° Fahr., fixation was rapid;
at 59° Fahr., it was decreased, and at 44° Fahr,
there was none. Denitrification is favored by a some-
what limited supply of oxygen.

There is no reason to believe that the practical
importance of nitrogen-fixation without legumes is
equal, under the most favorable conditions, to that
with legumes. A further knowledge of the organisms
effecting fixation and of their habits will doubtless
make possible a greater utilization of their powers,
to supplement the use of legumes, as a source of com-
bined nitrogen in the soil.

E. THE SOIL AIR

I. FACTORS DETERMINING VOLUME

The amount of air that soils contain varies with
different soils, and in any one soil it varies with cer-
tain changes to which it is subject from time to time.
The factors affecting the volume of air in soils are:
(1) The texture. (2) The structure. (3) The organic
matter. (4) The moisture content.

295. Texture. — The size of the soil particles affect
the air capacity of the soil in exactly the same way
as it does the pore-space (see page 92), since the
two are identical. A fine-textured soil in a dry condi-
tion would, therefore, contain as large a volume of
air as a coarse-textured one, provided the particles
were spherical and all of the same size.

Under the conditions actually existing in the field,
those soils composed of small particles generally
possess the larger air-space.

296. Structure. — The volume of air in a water-free
soil being identical with the pore space, the formation
of aggregates of particles is favorable to a large air
volume. The volume of air in any soil, therefore,
changes from time to time; and particularly is this true
of a fine-grained soil, in which the changes in structure
are greater than in a soil with large particles. A change
in soil structure may greatly alter the volume of air con-
tained, by altering the pore space, thereby influencing
the productiveness. Clay is most affected in this way.

(432)

THE SOIL ATMOSPHERE 433

297. Organic matter. — Organic matter being more
porous than any size or arrangement of mineral particles,
the effect of that constituent is always to increase the
volume of air. While this is generally beneficial in a
humid region, it is often very injurious in an arid one.
Unless sufficient water falls upon the soil to wash the
soil particles around the organic matter and to maintain
a supply sufficient to promote decomposition, the pres-
ence of vegetable matter leaves the soil so open that the

Fig. 115. Blade cultivator, with hammock seat. For surface work.

capillary rise of moisture is interfered with, and the large
movement of air keeps the soil dry, with the result that
the portion of the soil layer mixed with and lying above
the organic matter, is too dry to germinate seeds or
support plant growth.

298. Moisture content. — It is quite evident that the
larger the proportion of the interstitial space filled with
water the smaller will be the quantity of air contained.
This does not necessarily mean that the higher the per-

BB

434 THE PRINCIPLES OF SOIL MANAGEMENT

centage of water in the soil the smaller the volume of
air, as the amount of pore space determines both the
water and the air capacity. A soil with 30 per cent
moisture may contain more air than one with a water
content of 20 per cent because of the tendency of mois-
ture to move the soil particles further apart.

In soils in the field, the average diameter of the cross-
section of the pore space is the most potent factor in
determining the volume of air. Small spaces are likely
to hold water, while the larger ones, not retaining water
against gravity, are filled with air.

In a clay soil, the volume of air is increased, other
things being equal, by the formation of granules, and
decreased by deflocculation or compaction.

II. COMPOSITION OF SOIL AIR

The air of the soil differs from that of the outside
atmosphere in containing more water vapor, a much
larger proportion of carbon dioxid, a correspondingly
smaller amount of oxygen, and slightly larger quantities
of other gases, including ammonia, methane, hydrogen
sulphid, etc., formed by the decomposition of organic
matter.

299. Analyses of soil air. — The composition of the
air of several soils, as determined by Boussingault and
Lewy, is quoted by Johnson in the table on the follow-
ing page.

There are several factors influencing the composition
of the soil air, those of greatest importance being the
production and the escape of carbon dioxid, while of

COMPOSITION OF SOIL AIR
Table LXV

435

Character of soil

Sandy subsoil of forest . .
Loamy subsoil of forest.
Surface soil of forest

Clay soil

Soil of asparagus bed not

manured for one year.
Soil of asparagus bed

freshly manured

Sandy soil, six days

after manuring

Sandy soil, ten days after

manuring (three days

of rain)

Vegetable mold compost

Volume in one

acre of soil to

depth of 14 inches

Air

Cu. ft.
4,416
3,530
5,891

10,310

11,182
11,182
11,783

11,783
21,049

Carbon
dioxid

Cu. ft.
14
28
57
71

80

172

257

1,144

772

Composition of 100 parts
soil-air by volume

Carbon
dioxid

0.24
0.79
0.87
0.66

0.74

1.54

2.21

9.74
3.64

Oxygen

Nitro-
gen

19.66
19.61
19.99

19.02

18.80

10.35
16.45

79.55
79.52
79.35

80.24

79.66

79.91
79.91

less influence is the excretion of carbon dioxid and utili-
zation of oxygen by plant roots.

300. Production of carbon dioxid as affecting com-
position.— -Although the formation of carbon dioxid
in the soil depends upon the decomposition of organic
matter, it is not always proportional to the quantity
of organic matter present. The rate of decomposition
varies greatly, and where this is depressed, as is some-
times seen in muck or forest soils, the content of carbon
dioxid is low. A high percentage of organic matter is in
itself likely to prevent a proportional formation of carbon
dioxid by the accumulation of the gas inhibiting further
activity of the decomposing organisms.

436

THE PRINCIPLES OF SOIL MANAGEMENT

Ramann states that the percentage of carbon dioxid
in the soil air has the following relations:

The carbon dioxid increases with the depth.

In general the percentage of carbon dioxid rises and
falls with the temperature, being higher in the warm
months and lower in the cold months.

r-^

Fig. 116. Disc cultivator fitted with fenders.

Changes in temperature and air pressure change the
percentage of carbon dioxid.

In the same soil the content of carbon dioxid varies
greatly from year to year.

An increase of moisture in the soil increases the per-
centage of carbon dioxid.

The amount of carbon dioxid varies in different parts
of the soil.

301. Escape of carbon dioxid as affecting composition.

FUNCTION OF THE SOIL AIR 437

— The movement of carbon dioxid from the soil depends
chiefly upon diffusion into the outside atmosphere.
The conditions governing diffusion, which will be dis-
cussed later (page 439), therefore largely determine
the rate of loss of carbon dioxid from the soil.

302. Effect of roots upon composition. — The absorp-
tion of oxygen and excretion of carbon dioxid by roots
has a real, but as yet unmeasured influence upon the
composition of the soil air. It is worthy of note, however,
that the carbon dioxid thus excreted is in a position
where its aqueous solution can be of the greatest benefit
to the plant in its solvent action upon the soil, as it is
in direct contact with the absorbing portion of the
roots.

III. FUNCTIONS OF THE SOIL AIR

Both carbon dioxid and oxygen as they exist in the
air of the soil have important relations to the processes
by which the soil is maintained in a habitable condition
for the roots of plants. Deprived of these gases, the soil
would soon reach a sterile condition.

303. Oxygen. — An all-important process in the soil
is that of oxidation, because by it the organic matter
that would soon accumulate to the exclusion of higher
plant life is disposed of, and the plant-food materials
are brought into a condition in which they may be
absorbed by plant-roots. The presence of oxygen is
essential to the life of the decomposing organisms and
to the complete decay of organic matter. Through this
process, roots of past crops, as well as other organic matter
that has been plowed under, are removed from the. soil.

438

THE PRINCIPLES OF SOIL MANAGEMENT

The process of decay gives rise to products, chiefly
carbon dioxid, that are solvents of mineral matter, and
leaves the nitrogen and ash constituents more or less
available for plant use.

Oxygen is also necessary for the germination of seeds
and the growth of plant-roots. These phenomena,
although not involving the removal of large quantities

of oxygen, are
yet entirely de-
pendent upon
its presence in
c o n s i de r a b 1 e
amounts.

304. Carbon
dioxid. — T h e
solvent action
of carbon dioxid
is its most im-
portant func-
tion in the soil. By its solvent action it prepares for
absorption by plant-roots most of the mineral substances
found in the soil. Although a weak acid when dissolved
in water its universal presence and continuous formation
during the growing season results in a large total effect.
Carbonic acid dissolves from the soil more or less
of all the nutrients required by plants. The amounts so
dissolved are appreciably greater than those dissolved
in pure water. The constant formation of carbon dioxid
by decomposition of organic matter keeps this solvent
continually in contact with the soil.

Carbon dioxid serves a useful purpose in combining

Fig. 117. Hand cultivator, or wheel hoe, with
attachments.

MOVEMENTS OF THE SOIL AIR 439

with certain bases to form compounds beneficial to the
soil. Particularly is this the case with calcium carbo-
nate, which is of the greatest benefit to the soil in main-
taining a slight alkalinity very favorable to the develop-
ment of beneficial bacteria and to the maintenance
(if good tilth.

When combined as sodium or potassium carbonate
in considerable quantity, as in certain alkali soils, a very
injurious action upon plant-roots, and upon soil-struc-
ture results. Upon plants it acts as a direct poison.
(See page 312.) The effect upon soil structure is to de-
flocculate the particles producing the separate grain or
compact arrangement. (See page 116.)

IV. MOVEMENT OF SOIL AIR

There is a constant movement of the air in the inter-
stitial spaces of the soil, and an exchange of gases between
the soil atmosphere and the outside atmosphere, as well
as a more general but probably less effective, movement
of the air out of. or into the soil, as the controlling-
conditions may determine.

The movement may be produced by any one or more
of the following phenomena: (1) Gaseous diffusion.
(2) Movement of water. (3) Change of atmospheric
pressure. (-1) Change of temperature in soil or atmos-
phere. (5) Suction produced by wind.

305. Diffusion of gases. — The wide difference in the
composition of soil ami atmospheric air gives rise to a
movement of gases due to a tendency for the external
and internal gases to come into equilibrium. According

440 THE PRINCIPLES OF SOIL MANAGEMENT

to Buckingham, the interchange of atmospheric and soil
air is due in large measure to diffusion.

The rate of movement of the soil air due to diffusion
is dependent upon the aggregate volume of the interstitial
spaces, and not upon their average size. Thus it is the
porosity of the soil that influences most largely the dif-
fusion of the air from it, and consequently the size of the
particles is not a factor, but good tilth permits diffusion
to take place more rapidly than does a compact condi-
tion of soil, as the volume of the pore space is thereby
increased. Compacting the soil in any way, as by rolling
or trampling, has the opposite effect.

306. Movement of water. — As water, when present
in a soil, fills certain of the interstitial spaces, it thus
decreases the air space when it enters the soil and
increases it when it leaves. The downward movement
of rain-water produces a movement of soil air by forc-
ing it out through the drainage channel below, while at
the same time a fresh supply of air is drawn in behind
the wave of saturation, as the water passes down from
the surface. The movement thus occasioned extends to
a depth where the soil becomes permanently saturated
with water. Twenty-five per cent of the air in a soil
may be driven out by a normal change in the moisture
content of the soil.

307. Changes in atmospheric pressure. — Waves of
high or low atmospheric pressure, frequently involving
a change of .5 inches on the mercury gage, cross the
continent alternately every few days. The presence of a
low pressure allows the soil air to expand and issue from
the soil, while a high pressure following, causes the out-

FACTORS AFFECTING SOIL AERATION 441

side air to enter in order to equalize the pressure. An
appreciable, but not important movement of soil air is
produced in this way.

The size of the interstitial spaces is more potent than
their volume in effecting soil ventilation by this and the
following methods.

308. Changes in temperature. — A movement of soil
air may be induced by a change of temperature in the
atmosphere or in that of the soil itself. Changes in atmos-

Fig. 118. The hillside plow. The hinged share and moldboard permit con-
tinuous plowing on one side of the land.

pheric temperature act in the same way as do changes
in atmospheric pressure; in fact, it is the effect of tem-
perature upon air pressure that causes the movement.
Like the movement due to atmospheric pressure, it is
not great; but where the soil immediately at the sur-
face of the ground attains a temperature of 120° Fahr.
at mid-day, as occurs in the corn-belt, the movement
must be appreciable.

The diurnal change in soil temperature decreases
rapidly from the surface downward, due to the absorp-
tion and slow conduction of heat. (See page 455.) At
the Nebraska Experiment Station, the average diurnal
range for the month of August, 1891, was as follows:

442 THE PRINCIPLES OF SOIL MANAGEMENT

Diurnal Range of Air and Soil Temperatures

Degrees Fahr.

Air 5 feet above ground 14.4

Soil 1 inch below surface 17.9

Soil 3 inches below surface 14.8

Soil 6 inches below surface 9.2

Soil 9 inches below surface 6.6

Soil 12 inches below surface 4.3

Soil 24 inches below surface 0.5

Soil 36 inches below surface 0.0

This soil contains about 50 per cent of pore space, in
the upper foot of which 40 per cent is normally filled
with water during the summer months. This leaves 518
cubic inches of air in the upper cubic foot of soil. With
an increase in temperature, the air expands 79T m volume
for each degree Fahr. The average increase of tempera-
ture is, in this case, about 11° Fahr. for the first foot.
The air exhaled or inhaled by each cubic foot of soil
would then be

' — r— = 11.6 cubic inches.

491

As this is slightly over 2 per cent of the air contained
in the upper foot of soil, and as the movement below
that depth is negligible, the change in composition at any
one time is not great; but this pumping effect is kept up
day after day, although less energetically in the cooler
portion of the year. In proportion as poor drainage
equalizes the temperature it would prevent this type of
circulation. The total effect assisted by diffusion is to
aid materially in ventilating the soil. Owing to diffusion
of air in the interstitial spaces, the air expelled is dif-
ferent in composition from that inhaled.

MODIFICATION OF SOIL AERATION 443

309. Suction produced by wind. — The movement of
wind, being almost always in gusts, alternately increases
and decreases the atmospheric pressure at the surface
of the soil. There is a tendency, therefore, for the soil
air to escape and for atmospheric air to penetrate the
soil with each change in pressure. The effect presumably
influences only the superficial air spaces, but it must be
very frequent in its action. No measurements have
been made and no definite estimate of its effect can be
arrived at.

V. METHODS FOR MODIFYING THE VOLUME AND
MOVEMENT OF SOIL AIR

The conditions that affect the ventilation of soils are:
(1) The volume and size of the interstitial spaces. (2)
The moisture content. (3) The daily and annual range
in temperature.

Although the size of the interstitial spaces does not
appear to influence greatly the diffusion of gases from
a soil, it has a marked effect upon certain of the other
processes by which air enters and leaves the soil. A
sandy soil, a soil in good tilth, and, particularly, a soil
composed of clods, permit of more rapid movement of
air than does a compact soil.

While a certain movement of air through the soil is
desirable, and indeed necessary, for the reasons already
stated, a very large movement is injurious unless there
is an abundant rainfall. The effect of air movement
through the soil is to remove soil moisture. In a region
of small rainfall and low atmospheric humidity, this

444

THE PRINCIPLES OF SOIL MANAGEMENT

may be disastrous if the soil is not kept compact by
careful tillage. On the other hand, in a humid region and
in clay soil, there is likely to be too small a supply of
oxygen for the use of crops and lower plant life unless
the soil is well stirred.

310. Tillage. — The ordinary operations of tillage
influence greatly the ventilation of the soil. When a soil
is plowed, the soil at the bottom of the furrow is exposed
directly to the air at the surface, and, by the separation

Fig. 119. The Acme harrow. An efficient pulverizer on clean soil,

free from stones.

of adhering particles and aggregates of particles, air
is brought in contact with particles that may previously
have been completely shut off from the air. It is largely
because of its effect upon soil ventilation that plowing
is beneficial, and the necessity for its practice is greater
in a humid region and upon a heavy soil than in a region
of small rainfall and on a light soil. The practice of list-
ing corn, by which the soil is sometimes left unplowed
for a number of years, although in the semi-arid region,
productive of crops of sufficient yield to make them

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446 THE PRINCIPLES OF SOIL MANAGEMENT

profitable; would fail utterly on the heavy soils of a
humid region.

Subsoiling by loosening the subsoil increases the
ventilation to a greater depth. Rolling and sub-surface
packing both diminish the volume and movement of air.
Their essential difference is in their effect upon moisture
rather than upon air. (See page 111.) Harrowing and
cultivation have the opposite effect, and both increase
the production of nitrates in the soil by promoting
aeration. The tillage which is most beneficial is that
which increases the porosity of the soil, and not the size
of the interstitial spaces.

311. Manures. — Farm manures, lime and those
amendments that improve the structure of the soil, have
to the same degree a beneficial action upon soil aeration.
By their effect upon the physical condition of the soil,
they increase its permeability, and by their action in con-
tributing to the production of carbon dioxid they stimu-
late diffusion.

It is chiefly through its effect in increasing the volume
of air space in soils that farm manure is injurious in light
soils of the semi-arid region. It may thus be injurious
as well as beneficial, if used under certain conditions.

312. Under drainage. — By lowering the water table,
unclerdrainage by means of tiles removes from the soil
the water from all but the small capillary spaces, and
leaves free to the air the remainder of the interstitial
spaces. There is also a very considerable movement
of air through the drains, and a movement of air upward
from the drains to the surface of the soil, which, serves
to aerate, to some extent, this intervening layer. The

PLANT ROOTS AND SOIL AERATION 447

aeration of the soil brought about by underclrainage
is one of its beneficial features.

313. Irrigation. — The influence of irrigation upon
the soil is much like that of rainfall. The alternate
filing and emptying of the interstitial spaces with water
and air causes a very considerable change of air.

314. Cropping. — The roots of plants left in the soil
after a crop has heen harvested decay and leave channels
in the soil through which the air penetrates. Below
the furrow slice, where the soil is not stirred and where
it is usually more dense than at the surface, this affords
an important means of aeration. The growth of legumi-
nous plants and other deep-rooted crops is in this way,
among others, beneficial to the soil. The absorption of
moisture from the soil by roots also causes the air to
penetrate, in order to replace the water withdrawn.

F. HEAT OF THE SOIL

I. FUNCTION OF THE HEAT OF THE SOIL IN ITS
RELATION TO PLANT GROWTH

The heat of the soil has three general functions with
reference to plant growth. These are: (1) Biological.
(2) Chemical. (3) Physical.

315. Biological. — Heat is the motive power in plant
growth. A certain degree of heat is necessary for the
normal action of all of the functions of the plant. When
the soil, as well as the atmospheric temperature, passes
beyond a certain maximum or minimum degree, growth
is inhibited. These points differ for different species
and groups of plants, and they may be different for
different individuals of the same species. Somewhere
between the maximum and the minimum temperature
which any plant can withstand and still live, is the
optimum or best temperature for growth. These rela-
tions may be divided into the following three groups. The
best soil temperature for: (1) Germination. (2) Growth
and vegetation. (3) Proper activity of the soil organ-
isms.

316. Germination. — This takes place at widely dif-
ferent temperatures for different plants. Ordinarily,
the optimum temperature for germination is several
degrees below the optimum temperature for growth
during the average period of vegetation.

The range for a few common plants is shown in the
following table:

(448)

SOIL TEMPERATURE FOR PLANT GROWTH 449

Table LXVI

Melons

Tobacco

Maize

Red clover and alfalfa

Barley and vetch

Turnips

Oats

Flax

Rye

Mustard

Temperatures for germination in degrees
Fahrenheit

Minimum Optimum

55-65
50-60
45-50
40-45
38-45
36-45
32-45
32-40
32-40
32-38

88-100
75- 90
75- 85
75- 95
60- 75
85- 90
70- 85
75- 80
60- 75
60- 85

Maximum

110-120

90-110

90-100

100-110

100-105

100 110

90-100

85- 95

90-100

90-100

These figures show that germination may take place
as low as 32° Fahr. for some seeds, but that the best
temperature is from 60° to 90° Fahr., with the average
near 85°. Few seeds germinate at temperatures much
above 100° Fahr. At temperatures below the optimum,
the time required is correspondingly increased, — as
shown, for example, by Nobbe, who found that musk-
melon seeds required 290 hours to germinate at 60.5°
Fahr., but at 88° Fahr. they germinated in forty-eight
hours. The long period may give opportunity for
certain fungous diseases to destroy the seed.

317. Growth and vegetation. — Growth seldom takes
place below a temperature of from 40° to 50° Fahr., and
a much higher temperature is necessary for vigorous
growth. Hall presents the following table, showing the
relation of temperatures to the growth of some common
crops.

cc

450 THE PRINCIPLES OF SOIL MANAGEMENT

Table LXVII

Temperature for growth in degrees Fahrenheit

Minimum

Optimum

Maximum

Mustard

Barley

Wheat

Maize

Kidney bean

Melon

32
41
41
49
49
65

81.0
83.6
83.6
92.6
92.6
91.4

99.0
99.8
108.5
115.0
115.0
111.0

The figures in the above two tables indicate that the
temperature of the soil has a large influence on germi-
nation and growth of different plants. Those indivi-
duals which require a high temperature should not be
planted until the soil attains the desired degree of heat.
If planted before this point is reached, the seed will be
slow to germinate and may be destroyed by disease.
If it succeeds in germinating, the growth will be slow
and unsatisfactory; and, even if the proper soil tempera-
ture is attained, the vigor of the plant will have been so
reduced that the maximum yield can not be produced.
The soil temperature also makes it impossible to grow
certain crops where others thrive. This is a large factor
in the distribution of crops and wild species of plants.

318. Activity of the soil organisms. — The activity of
all soil organisms is reduced by low temperatures.
Consequently those biological changes which increase
soil fertility are less pronounced during periods of low
than during periods of high temperature. One of the
most important of these relations is the formation of

SOURCES OF HEAT OF THE SOIL 451

nitrates, which takes place most actively at a tempera-
ture of 80° to 100° Fahr., and ceases at about 40° Fahr.

319. Chemical changes. — In the soil chemical changes
are greatly accelerated by a high temperature, and are
correspondingly retarded by low temperature. But,
unlike biological activity, they never wholly cease as a
result of temperature changes, though the type of
change in the different compounds may be altered.
Warm temperatures increase particularly the solubility
of the soil constituents, by which they are made available
to plants.

320. Physical changes. — As a result of temperature,
physical changes are less marked than the chemical and
biological, except when the freezing point is reached,
when the soil moisture is solidified and renders nutrition
of higher plants impossible. The movement of moisture
and gases through the soil is greatly facilitated by the
higher temperatures within the range of plant growth.

II. SOURCES OF THE HEAT OF THE SOIL

There are three direct sources of heat which reach
the soil. These are: (1) Solar radiation. (2) Conduction
from the interior of the earth. (3) Organic decompo-
sition.

Under field conditions, the first of these sources is
far the most important.

321. Solar radiation. — Solar radiation of heat reaches
the soil in three ways.

(1) By direct radiation from the sun in the form
of sunshine.

452

THE PRINCIPLES OF SOIL MANAGEMENT

(2) Indirectly through the radiation which is im-
parted to the atmosphere, from which it is radiated to
the soil or is given up by direct contact of the atmosphere
with the soil. Clouds in the atmosphere reflect back to
the soil some heat which has been received by the

soil and is again
given off. They may
serve as a cover or
blanket.

(3) In the spring,
rain-water carries a
large amount of
heat into the soil.
The percolation of
warm spring rain is
a means of rapidly
warming up the soil,
and its strong in-
fluence is shown by the large quickening of growth
which follows such rainfall.

322. Conduction. — Conduction of heat from the
interior of the earth is negligible as an appreciable
source of soil heat.

323. Organic decay. — Organic decay liberates heat,
and may be so rapid as to greatly change the tempera-
ture of the soil. This is exemplified by the heating of
manure heaps and in the use of the hotbed. The same
amount of heat is set free bjr decomposition as would
result from ignition of the material, but its liberation
is distributed over a much longer period of time accord-
ing to the conditions for decay.

Fig. 121. Mean annual sunshine of Canada
and the United States. The figures indicate the
number of hours of bright sunshine in a year.
(From Bartholomew's Atlas of Meteorology.)

FACTORS AFFECTING SOIL TEMPERATURE 453

III. TEMPERATURE OF THE SOIL

The temperature which the soil in any given position
will attain depends upon a number of factors. The more
important of these are as follows: (1) Heat supply.
(2) Specific gravity of the soil. (3) Specific heat of the
soil. (4) Color of the soil. (5) Attitude of the surface.
(6) Conductivity of the soil. (7) Circulation of air above
the soil. (8) Water-content of the soil.

324. Heat supply. — The heat supply is obviously
the most direct factor contributing to the soil tempera-
ture. This is reflected in the seasonal, daily and hourly
variations in the temperature. The hourly variations
in temperature at a depth of one foot below the surface
are shown by the following table and curves:

Table LXVIII

June 1 — Readings in two-hour periods

6

8

61.0

70.0
71.0

10

61.0
61.0
70.0
69.5

N

61.3
60.3

69.8
69.8

2

62.0
60.3
70.0
69.5

4

62.5
60.6
69.8
69.2

6

63.0
61.2
69.7
69.0

8

63.5
61.7
69.5
69.0

10

63.6
62.0
69.3

68.8

M

63.6
62.1
69.0
68.0

2

63.3
62.1
68.5
67.5

4

1. Clay loam, Penn.

2. Loam, Penn. . . .

3. Silt loam, N.C.. .

4. Sandy loam, N.C.

60.5

72.0
72.5

63.0
62.1
68.0
67.2

June 2 — Readings in two-hour periods

6

8

62.5
61.8
67.3
66.5

10

62.5
61.5
67.2
66.0

N

62.7
61.4
67.2
66.5

2

63.0
61.3
67.8
67.5

4

64.0
61.8
68.3
68.2

6

64.5
62.2

68.7
68.8

8

65.0
62.8
68.8
69.0

10

65.0
63.3

68.8
68.7

M

65.0
63.3
68.5
68.3

2

64.8
63.3
68.0
68.0

4

1. Clay loam, Penn.

2. Loam, Penn

3. Silt loam, N.C. . .

4. Sandy loam, N.C.

62.8
62.0
68.0
67.0

64.5
63.2
67.8
67.5

454

THE PRINCIPLES OF SOIL MANAGEMENT

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Fig. 122. Curves showing the daily range of soil temperature near the
surface on soils of different texture in Pennsylvania and North Carolina.
Table LXVIII.

The following table and curves show the average
mean monthly range in temperature of the air and soil
at different depths at Lincoln, Nebraska for a period of
twelve years.

Table LXIX. Temperature in Degrees Fahrenheit

Position

1. Air

2. Soil, 1 in..

3. Soil, 6 in..

4. Soil, 12 in

5. Soil, 36 in

03
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25.2
27.3
28.6
31.2
38.5

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24.2

27.7
27.8
30.2
35.5

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35.8
38.2
36.6
35.4
35.8

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52.1
57.5
53.3
49.3
43.8

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61.9
68.7
65.1
60.7
53.5

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71.0
78.1
75.7
69.9
61.3

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76.0
85.1
81.6
75.7
67.4

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74.5 67.6
82.9 73.8
80.172.0
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69.8 67.6

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55.5
56.7

57.8
57.8
61.3

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38.7
38.7
41.5
44.7
52.2

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o

Q

28.3
31.6
32.0
35.2
43.3

There is a large daily as well as annual range in the
temperature of the soil. At the surface, the range is

SUNSHINE AND SOIL TEMPERATURE

455

considerably greater than in the air above, and this
excess extends to a depth of nearly one foot. At greater
depths in the soil, the range in temperature is less than
in the air and much less than at the surface, and the
waves of temperature change fall successively behind

SEPT. OCT-

TIME IN MONTHS

Fig. 123. Curves showing the mean monthly range in temperature of (he
air, and of the soil at different depths, as given in Table LXIX. Note the in-
fluence of the rate of heat conduction, as shown by the curves.

those of the atmosphere. These variations are asso-
ciated directly with the amount and intensity of the
sunshine.

325. The specific gravity and specific heat. — The
first of these directly affects the temperature to only a
small degree. The larger the mass, the more heat required
to change its temperature. Hence, the more dense the
soil, the more heat absorbed in each layer.

The specific heat 'of the soil has a considerable in-
fluence on its temperature and, because of its marked

456

THE PRINCIPLES OF SOIL MANAGEMENT

difference from that of water, has an important prac-
tical bearing. Drainage owes one of its largest beneficial
effects to this fact.

Warington quotes from Lang the following table
of specific heat of soil constituents.

Table LXX

Water

Ferric oxide

Calcium carbonate

Magnesium carbonate
Quartz, orthoclase, granite

Humus (peat)

Clay •...

Relative specific heat of

Equal weights Equal volumes

1.000
0.163
0.206
0.260
0.189
0.477
0.233

1.000
0.831
0.561
0.754
0.499
0.587
0.568

In the above table, the specific heat of equal vol-
umes is more nearly representative of field conditions
than is that of equal weights. On this basis, dry soil
has about one-half the specific heat of water; that is,
a given amount of heat would raise a mass of soil to
nearly twice the temperature that it would the same
volume of water.

326. Color of the soil. — A dark-colored soil absorbs
heat much more rapidly than does a light-colored one,
and therefore warms up more rapidly. The effect of a
thin layer of carbon-black and chalk on the tempera-
ture of dry, fine sand, one inch below the surface,
when exposed to the sun in thick wooden boxes, is
shown in the following table:

COLOR OF SOIL AND TEMPERATURE
Table LXXI

457

Time in minutes from start

Fine Sand Soil

0

10

20

30

40

50

60

70

1. Carbon black .

2. Chalk (white)..

Deg. F,
61
61

Deg. F.

65.2
63.5

Deg. F.
71.6

65.8

Deg. F.
75.3

68.6

Deg. F.

78.6
69.5

Deg. F.

81.5
70.8

10.7

Deg. F.
84.3
72.2

Deg. F.
87.0

73.5

Difference

0

1.7

5.8

6.7

9.1

12.1

13.5

These figures agree with those of Schubler, who
found that, at one-eighth of an inch below the surface,
blackened soil attained a temperature from 12° to 15°
Fahr. warmer than the same soil whose, surface was
made white by magnesia.

Humus, because of the black or dark color it im-
parts to the soil, has a large effect on the soil tempera-

100

96

t

UJ

I 92

z

UJ

E

1 88
£

2 84
u

cc
o
g 80

z

Z 76
cc

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£ 72

<£

ui

I 68

Ui

I-
64

60

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v^-in!_—

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10

20

30 40

MINUTES FROM START

60

60

70

Fig. 124. Curves showing the temperature of a dry sandy loam soil,
covered by a very thin layer of powdered chalk and carbon black respectively,
after exposure in bright sunshine.

458

THE PRINCIPLES OF SOIL MANAGEMENT

ture. Its effect due to color is reduced by the higher
water-content which such a soil normally retains.
(See page 101.) Red soils absorb more heat than yellow
or gray ones, and yellow soils absorb more heat than
gray ones.

327. Slope of the soil. — A smooth surface absorbs
more heat than a rough or rigid surface. The effect
of the direction and angle of slope on the amount of
heat received from the sun is shown by the following
diagram.

Fig. 125. Diagram illustrating the influence
ot the slope of the land surface upon the
amount of sunshine received.

On the 21st of June, on the 42d parallel, the sun-
beam which falls on a given level area would be dis-
tributed over almost 6 per cent less area when the slope is
toward the sun at an angle of 20°, while on a slope of 20°
away from the sun the same amount of sunshine would
fall upon 16 per cent greater area. The area which

HEAT CONDUCTIVITY OF THE SOIL

459

sloped away from the sun would also receive the sun's
rays for a shorter period of each day. Wollny found
in Germany that the temperature of a sandy soil at
six inches depth on a south slope of 30° averaged 3.1°
Fahr. warmer than the corresponding slope to the
north. King found the following differences in tempera-
ture between the level and an 18° south slope, in Wis-
consin, in July.

Table LXXII

First foot

Second foot

Third foot,

South slope, 18 degrees . . .
Level

Degrees Fahr.
70.3
67.2

Degrees Fahr.
68.1
65.4

Degrees Fahr.
66.4
63.6

Difference

3.1

2.7

2.8

The north slope ordinarily has the most uniform
temperature.

328. Conductivity. — The conductivity of the soil
for heat depends upon four factors. These are: (1) Com-
position. (2) Texture. (3) Structure. (4) Moisture
content. The relative influence of these factors, as
reported by Warington from the results of Pott, are
shown in Table LXXIII on page 460.

Quartz has the largest power to conduct heat of
any of the soil constituents studied. The effect of lime-
stone and quartz stone is probably a textural one, as
is shown by the fact that the coarser the texture the
greater the conductivity. A compact soil conducts
heat more readily than a loose one. But, while a com-
pact soil will receive heat most rapidly, it also gives

460

THE PRINCIPLES OF SOIL MANAGEMENT

Table

LXXIII

Compo-
sition

Texture

Structure
Loose quartz
powder =100

Moisture
content

Quartz

powder

= 100

Fine
quartz

sand
= 100

Loose

Com-
pact

Dry
quartz
powder

= 100

Quartz, sand fine

Quartz sand, medium. . .
Quartz sand, coarse ....

Quartz powder

Chalk

100.0
85.2
90.7
90.7
94.1
112.1
115.6

100.0

103.6
105.3

ldo. b

85.2
90.7
90.7

106.7

92.6
98.1
96.4

201.7
153.2

Peat

94.3

Kaolin

155.6

Clay

Clay with limestones . . .
Clay with quartz stones..

....

it up most readily. The effect of the mulch is therefore
to maintain a more uniform soil temperature. The
presence of stone in the soil increases its temperature.
The movement of heat through the soil is also increased
decidedly by the presence of moisture. Pott found that
when a dry sand conducted 100 units of heat, the same
sand in a moist state conducted 174 units, and when
wet, 189 units, or nearly twice that for the dry sand. The
operation of rolling by compacting the soil increases
its conductivity for heat, and consequently its average
temperature. King found, as an average of several
trials on different soils, that at a depth of 1.5 inches,
rolled soil was 3.1° Fahr. warmer than the unrolled
soil, and at a depth of three inches the difference in
favor of rolling was 2.9° Fahr. In extreme cases, he
has found differences nearly three times as great as

WATER CONTENT AND TEMPERATURE 461

the above figures between the temperature of rolled
and unrolled land. Rolling generally favors deep
warming. The movement of heat in the soil is illustrated
by the curves of soil temperature on page 455. The
change in temperature in the subsoil lags considerably
behind that at the surface, and is also more uniform.

329. Circulation of air. — This is due, first, to direct
conduction between the air and the soil; and, second,
to the influence of wind on evaporation. Tillage of the
soil, particularly in the spring, increases the rate of
warming, because at that season the air is usually warmer
than the soil, and, by bringing all parts of the soil to
the surface successively, it is warmed by contact with
the air and by the direct receipt of the sun's heat. Wind
hastens the change in temperature of the soil in either
direction by increasing the volume of air with which
the soil comes in contact.

330. Water-content. — The water-content of the soil
is the largest factor, after the heat supply, in determining
the temperature of the soil. This is due to two things:

(1) The high specific heat of water as compared to soil.

(2) The heat absorbed in the evaporation of water.
The specific heat of water, as compared with an

equal volume of soil, is shown by the table on page 456
to be nearly twice as great. Consequently, the more
water a soil contains, the more slowly will its tempera-
ture change with a given heat supply. The tempering
influence of large bodies of water upon adjacent land
areas is an example of this fact.

In the evaporation of water, a large amount of heat is
absorbed. The vaporization of one pound of water at

462 THE PRINCIPLES OF SOIL MANAGEMENT

the boiling point requires 5.3 times as much heat as is
necessary to raise its temperature from the freezing point
to the boiling point. It is this large absorption of heat
which renders evaporation such a large cooling operation.
The more evaporation which takes place from the soil
moisture, the more will the temperature be kept clown.
Any treatment which reduces evaporation, such as the
mulch, will favor a higher soil temperature.

This influence of the moisture content has given
rise to popular descriptive terms, such as "warm,"
and "cold" soils; "early" and "late" soils. A "warm
soil" is one which retains naturally a relatively small
amount of water, that is, soils of coarse texture. "Cold
soils," on the other hand, are those which retain a rela-
tively large amount of water, that is, those of fine tex-
ture. The difference in the amount of heat required
to warm the water contained in the soil, as well as that
lost in evaporation, which is of course greatest in the
soil containing most water, is the source of their normal
differences in temperature.

An "early soil" is one which retains a relatively
small amount of water. It therefore warms up most
rapidly under a given heat supply, and is in condition
to permit seeding earlier in the season. A late soil
retains much water, and, consequently, is slow in
warming up. Its planting must therefore be deferred
until later in the season. Coarse-textured soils are
"early," and fine-textured ones are "late." Wollny
concluded, from extensive experiments, that in summer
sandy soils are warmest, followed by humus, lime and
loam soils. In winter this order is reversed.

MEANS OF MODIFYING THE SOIL TEMPERATURE 463

The large effect of drainage on the soil temperature
is due to these heat relations of the soil moisture. If
the excess of water is removed by evaporation, it keeps
the soil unduly cold. King observed differences in tem-
perature of from 2.5° to 12.5° Fahr. between drained and
undrained soil on different days in April. These results
are abundantly borne out by practical experience. The
removal of the excess water by drainage conserves heat.

IV. MEANS OF MODIFYING THE SOIL TEMPERATURE

The means of modifying the soil temperature are
obvious from the above principles. The practices which
may be used for this purpose are:

(1) Modification of the texture and structure of the
soil by appropriate tillage.

(2) Modification of the color
of the soil, chiefly through the
addition of organic matter.

Fig. 126. The spading disc. Adapted to much more stony,
hard soil than the solid disc.

464 THE PRINCIPLES OF SOIL MANAGEMENT

(3) Modification in the moisture content by the
use of mulches, irrigation, and especially by drainage,
where there is an excess of water.

(4) The attitude of the surface may be somewhat
changed by tillage, especially in the matter of rough or
smooth surface. Of course, the general slope cannot
be altered.

(5) Promotion of organic decay through the addi-
tion of organic matter to the soil, in such a state and
under such conditions as will promote favorable decay
by which its heat may be liberated. The high tempera-
ture attained in hotbeds in the winter and early spring
exemplifies this practice. The application of manure
under field conditions may appreciably alter the soil
temperature, due perhaps to several effects. Wagner
observed an increase of 5° Fahr. as a result of the
application of twenty tons of manure per acre, and
during a period of several weeks there was an average
excess of 1° of temperature on the manured land.
Georgeson observed, through a period of twenty days
following the application of different amounts of manure
in the fall, temperature differences amounting to .9°
for ten tons, 1.7° for twenty tons, 2.3° for forty tons,
and 3.4° for an application of eighty tons per acre.

(6) Construction of shelters may modify the soil
temperature. Coldframes and greenhouses make use
of this principle by preventing the circulation of air and
by entrapping the sun's rays. Partial shade influences
the soil temperature, usually producing a lower average
and a greater uniformity.

G. EXTERNAL FACTORS IN SOIL MANAGEMENT

In the foregoing chapters, some of the principles
underlying the management of the soil have been
pointed out. In addition to these are several practices
associated with soil management resting upon the prin-
ciples that have been explained, which are so funda-
mentally important as to warrant their separate discus-
sion in this connection.

I. MEANS OF MODIFYING THE SOIL

In the art of soil management, one has a number
of practices which may be used to modify the soil.

331. Summary of practices. — The most prominent
of these practices are: (1) The manipulation of the soil
by means of implements. (2) Drainage. (3) Irrigation.
(4) Application of amendments, including all forms
of organic materials. (5) Application of chemical ma-
nures. (6) Inoculation. (7) Rotation. (8) Crop-
adaptation.

Each of these practices has a primary function.
That of drainage is to remove excess water from the
soil; of chemical manures to add food elements; of inocu-
lation, to introduce organisms; of tillage, to modify
the structure of the soil. But, in the exercise of their
primary function, each practice also has many second-
ary or indirect effects on the soil, which may sometimes
be more important to the productive qualities of the
dd (465)

466

THE PRINCIPLES OF SOIL MANAGEMENT

soil than its direct effect. This complex effect is well
illustrated by drainage, which not only removes excess
water and admits air, but it thereby affects the soil
temperature, growth of organisms and the elaboration
of plant-food. Similarly, tillage, first of all, is designed
to alter the structure of the soil, and through this alter-
ation in structure, the retention of moisture, aeration
and root-penetration, not to mention many other

Fig. 127. Bottom view of a modern plow, showing the parts. 1, share; 2,
moldboard; 3, landside; 4, frog; 5, brace; 6, beam; 7, clevis; 8, handle.

relations, are changed. In fact, every practice which
may be applied to the soil influences in some degree
every phase of the soil mechanism. The relative promi-
nence of these different effects depends on the character
and condition of the soil.

The application of these various practices has been
indicated in the foregoing pages, in connection with
the principles discussed.

II. TILLAGE

Tillage, or the manipulation of the soil by means of
implements, is so general in its application and so

OBJECTS OF TILLAGE 467

pronounced in its effects, as well as complex in its
modes of operation, that it is given a separate treatment.

332. Objects of tillage. — Tillage rests upon three
primary objects. These are: (1) Modification of the
texture and structure of the soil. (2) Disposal of rubbish
or other coarse material on the surface, and the incor-
poration of manures and fertilizers in the soil. (3) To de-
posit seeds and plants in the soil in position for growth.

The most prominent of these objects is the modifi-
cation of the soil structure.
No perceptible change in the
soil texture can be effected but
through changes in structure,
by which it is made either
more open *or more compact.
Thereby the retention and fig. 128. Heel plate for regukt-
movement of moisture is ing the width at the heel.

affected, aeration is altered, the absorption and reten-
tion of heat is influenced, the growth of organisms
is either promoted or retarded; through all of these the
composition of the soil solution is affected and, lastly,
the penetration of plant roots is influenced. The crea-
tion of a soil mulch is simply a change in the structure
of the soil at such time and in such manner as will
prevent evaporation of moisture. For this reason, it
is essential to appreciate the relation of soil structure to
movement of moisture in managing the mulch. In fine-
textured soils, where the granular or crumbly structure
is most desired, tillage may have an important influ-
ence on the promotion or destruction of these granules.
As has been pointed out (page 105), any treatment

4G8

THE PRINCIPLES OF SOIL MANAGEMENT

which increases the number of lines of weakness in the
soil structure facilitates the action of the moisture
films in solidifying the soil granules. Tillage shatters
the soil and breaks it into many small aggregates of
particles, which may be further drawn together and
loosely cemented by the further evaporation of moisture.
The more numerous the lines of weakness produced,
the more pronounced the granulation; and, conversely,
the fewer the lines of weakness which result, the more
coarse and cloddy the structure.

Fig. 129. The modern sulky riding plow.

333. Implements of tillage. — The number of imple-
ments adapted to the manipulation of the soil is very
large, and they embrace many types and patterns.
Many operations are comprehended by the term tillage.
It includes the use of all those implements which are
used to move the soil in any way in the art of crop-pro-
duction. It includes the smallest hand implements, as
well as the largest traction implements.

334. Effect on the soil. — All these operations may be
divided into two groups, according to their effect on

OPERATION OF TILLAGE IMPLEMENTS 469

the soil: (1) Those which loosen the soil structure.
(2) Those which compact the soil structure. In the
subsequent paragraphs of this chapter the effect of the
more common types of tillage implements on the soil
are pointed out as a guide to their selection for the ac-
complishment of a particular desired modification.
For, good soil management consists, first, in analyzing
the soil conditions, to determine the change which
should be effected; second, in the selection of the im-
plement or other treatment which will most readily
and economically accomplish the object.

335. Mode of action. — According to their mode of
action, tillage implements may be divided into three
groups: (a) Plows. (6) Cultivators, (c) Crushers and
packers.

336. Plows. — The primary function of a plow is to
take up a ribbon of soil, twist it upon itself, and lay
it down again bottom side up, or partially so. In the
process two things result. (1) If the soil is in proper
condition for plowing, it will be shattered and broken up.
(2) The soil is inverted, and any rubbish is put beneath
the surface.

337. Pulverizations. — In twisting, the soil tends to
shear into thin layers, as pointed out by King. These
layers are moved unequally upon each other, as, when
the leaves of a book are bent, they slip past each other.
The result should be a very complete breaking up of
the soil. How thorough the breaking-up will be will
depend upon (a) the condition of the soil, and (b) the
type of plow. As to the condition of the soil, there is
a certain optimum moisture content at which the best

470 THE PRINCIPLES OF SOIL MANAGEMENT

results will be obtained. Any departure from this
moisture content will result in less efficient work. It
has been said that, in proportion to the amount of energy

Fig. 130. Sulky Lister

required, the plow is the most efficient pulverizing
implement available to the farmer. The optimum
moisture content for plowing is indicated by that nicely
moist condition in which a mass of the soil when pressed
in the hand will adhere without puddling, but may be
readily broken up without injury to the intimate
soil structure. This is a much more critical stage for
fine-textured soils than for coarse-textured ones. Sandy
soils are not greatly altered by plowing when out of
optimum moisture condition. On the other hand, if a
clay soil is plowed when it is saturated with water, it

TYPES OF TILLAGE IMPLEMENTS

471

will be thoroughly puddled, and will dry out into a hard
lumpy condition. Such a structure requires a considera-
ble time to overcome.

As to the second factor, there are two general types
of turning plows: (1) The common moldboard plow.
(2) The disc plow. The mode of action of the two is
quite different, although, so far as the soil is concerned,
the result is much the same. The moldboard plow
seems to have a wider application than the disc plow,
although both have a particular sphere of usefulness.

For any given texture of soil and any given soil
condition, there is a type of plow, a shape of mold-

Fig. 131. Types of coulters. Lower right hand, knife coulter; lower left
hand, rolling coulter; upper right hand, fin coulter; upper left hand, jointer.
The last-named attachment assists in turning trash under the surface as well
as to cut the soil.

472 THE PRINCIPLES OF SOIL MANAGEMENT

board, and a depth of furrow slice, which are calculated
to give the best results. This fact is to be kept constantly
in mind in plowing soil. Sod land requires a different
shape of plow from fallow land, sandy land from clay
land. Rubbish on the surface may be handled by one
plow and not by another. Wet clay should have the
use of a different shape of plow from dry soil.

There are several different shapes of plow. Among
these the most prominent types are the moldboard,
the disc, the hillside and the subsoil.

Of the moldboard type there are two general shapes :

(1) The long, sloping moldboards, with little or no over-
hang, found on what is called the sod plow. This neatly
cuts off the roots at the bottom of the slice, and slowly
and gradually twists the soil over without breaking the
sod, and lays it smoothly up to the previous furrow-
slice. It is seldom desirable to completely invert the
soil. According to depth of plowing, the furrow-slice
should be laid at an angle with the horizontal of from
25° to 50°, so that the projecting edge of the slice may
be worked down for a seed-bed, while the roots and
rubbish on the surface is somewhat uniformly distri-
buted through a considerable depth of soil, instead of
occupying a single layer in the bottom of the furrow.

(2) The short, steep moldboard with a marked overhang.
This is not adapted to sod land, because it breaks up
the sod and shoots it over in a rough, jagged manner
with uneven turning. But on fallow land, to which it
is adapted, it very completely breaks up the soil and
throws it over in a nearly level mellow mass. The pul-
verizing effect is obviously much greater than with the

THE PLOW AS A TILLAGE IMPLEMENT

473

sod plow. Since the steep moldboard or fallow-ground
plow exerts the most force on the soil in a given time
at a given speed of movement, it follows that if a
particular soil is over-wet it should be plowed with
the sod-plow, while, if it must be plowed when too dry,
the fallow-ground plow will be more effective, — disre-
garding the draft which will probably be large in the
latter case.

Fig. 132. Six-gang plow. Usually operated by steam engine. Adapted to
large, level areas of uniform soil, relatively free from stone.

There is a general relation between the width of the
furrow-slice and its depth. In general, it may be said
that this ratio is about two in width to one in depth.
The greater the depth, the less in proportion may he
the width of the furrow-slice.

On clay soil in particular, there is also a relation
between depth and condition. A wet soil should be
plowed more shallow, other things equal, than a dry soil,
because the puddling action is less. On a dry soil, the
depth should be increased, to increase the pulverization.

474 THE PRINCIPLES OF SOIL MANAGEMENT

Combining these principles, then, it may be said that
if a clay soil must be plowed when too wet, it should be
plowed with a sod plow, and to as shallow a depth as
is permissible. But, on an over-dry soil, the opposite
conditions should be fulfilled, — that is, steep mold-
board and increased depth. Likewise, on sandy soil,
where the aim is generally to compact the structure,

Fig. 133. The modern garden seeder. It modifies the soil structure

this may be furthered by deep plowing with steep
moldboard when the land is over-wet.

In connection with this phase of the subject, it is
important to consider what Professor Roberts called
the plow sole. That is, the soil at the bottom of the
furrow which bears the weight of the plow and trampling
of the team, and which, under uniform depth of plowing,
does not become loosened. In clay soil, especially, it
gradually becomes more compact, in time developing
something of a "hard-pan" character, which is detri-

TYPES OF PLOWS

475

mental to the circulation of air and moisture and inter-
feres with the penetration of plant roots. Consequently,
occasional deep plowing or even subsoiling is recom-
mended to break up this unfavorable soil structure,
commonly called the " plow sole." There is less tendency
for the disc than the moldboard plow to form the "sole."
The hillside plow is a modified form of the mold-
board plow, which has a double curvature to the mold-
, board, so that it is essentially two plows in one. This
swings on a swivel
in such a way that
it may be locked
on either the right
or the left side. It
removes the neces-
sity of plowing in
beds, and, by per-
mitting all of the work to be done from one side, enables
the plowman to lay the furrow slices in one direction.
On the hillside this direction is down the slope, because
of the greater ease in turning the soil in that direction.
It also removes the difficulty of pulling up and down the
hill. There is another type of compound moldboard
plow designed to eliminate "dead furrows" and "back
furrows." The former is developed by turning the last
furrow slices of two lands in opposite directions, thereby
leaving a gulley between which, by reason of its fre-
quent unproductive character, is termed the "dead
furrow."' The back furrow consists of two furrow slices
thrown together, usually forming a ridge more productive
than the average of the land.

Fig. 134. Berry hoe or ridger. For close tillage
of berries, vines and low-headed trees.

476

THE PRINCIPLES OF SOIL MANAGEMENT

The disc plow is essentially a large revolving disc
set at such an angle that it cuts off and inverts the soil,
at the same time pulverizing it quite effectively after
much the same manner as the moldboard plow. One

Fig. 135. Disc plow.

advantage claimed for it is its lighter draft for the same
amount of work done, because it has rolling friction in
the soil instead of sliding friction. In practice, it appears
to be especially effective on very dry, hard soil and in
turning and covering rubbish.'

338. Covering rubbish. — The secondary function of
the plow is to cover weeds, manure and rubbish which
may be upon the surface. This also the turning plow
does very effectively. The cutting and turning of the
sod, rubbish and weeds is facilitated by several attach-
ments. These are: (1) Coulters. (2) Jointers. (3) Drag-
chains. There are several types of coulters. Blade
coulters are attached to the beam or to the share in such
a manner as to cut the furrow slice free from the land
side. They should be adjusted so as to cut the soil after

THE PLOW FOR COVERING RUBBISH

477

it has been raised and put in a stretched condition, when
the roots are most easily severed. This position is a
little back of the point of the share. A knife edge
attached to the share is commonly called a fin coulter.
A jointer is a miniature moldboard attached to the
beam for cutting and turning under the upper edge of
the furrow slice, so that a neat, clean turn is effected
without the exposure of a ragged edge of grass which
may continue growth. This is used chiefly on sod land.
A drag-chain is an ordinary heavy log-chain, one end
of which is attached usually to the central part of the
beam, and the other to the end of the double tree on
the furrow side, and with enough slack so that it drags
down the vegetation on the furrow slice just ahead of
its turning point. It is used, primarily, in turning under
heavy growths of weeds or green-manure crops.

There is a third type of plow, the so-called subsoil

Fig. 136. The orchard disc. Adjustable and suited to working close up to

low-headed trees.

478 THE PRINCIPLES OF SOIL MANAGEMENT

plow. The purpose of this implement is to break up
and loosen the subsoil without mixing the material
with the soil. It consists essentially of a small mole-
like point on a long shin. This implement is drawn
throng! i the bottom of the furrow, and fractures and
loosens the subsoil to a depth of eighteen inches or two
feet. It is often useful on soils having a dense, hard
subsoil, but its use requires the exercise of judgment,
as the process may prove very injurious if done out of
season. As a general rule, it is best to use the subsoiler
in the fall when the subsoil is fairly dry, and in order that
the subsoil may in a measure be recompacted by the
winter rain. Spring subsoiling is seldom advisable in
humid regions, owing to the danger of puddling the sub-
soil or the possibility of its remaining too loose for best
root development, if performed when the subsoil is too
dry to puddle.

339. Cultivators. — There are more types of cultiva-
tors than of any other form of soil-working implements.
These may be grouped into: (1) Cultivators proper.
(2) Leveler and harrow type of cultivators. (3) Seeder
cultivators. These implements agree in their mode
of action on the soil, in that they lift up and move it
sidewise with a stirring action which loosens the struc-
ture and cuts off weeds, and to a slight degree covers
rubbish. However, the action is primarily a stirring
one, and, in general, it is much more shallow than that
of the plow. One important fact should be kept in mind
in cultural operations, especially just following the plow.
That is, to do the work when the soil is in the right
moisture condition. Particularly is this true in the

TYPES OF CULTIVATORS

479

pulverization following the plow. Plowing, if it be prop-
erly done, leaves the soil in the best possible condition
to be pulverized. It is properly moistened, and if the
clods are not shattered they are reasonably frail and may
be much more readily broken down than when they are
permitted to dry out. In drying, they are somewhat
cemented together and thereby hardened. Not only is
it desirable in almost all cases to take advantage of this

Fig. 137. "Sweeps" used extensively in the southern states, particularly
for shallow cultivation of cotton and corn. (Hartley.)

condition of the soil, but the leveling and pulverizing
of the soil reduces drying and improves the character
of the seed bed.

340. Cultivators proper. — There is a great va-
riety in types and patterns of cultivators. .They may be
divided into: (a) Large shovel forms, (b) Small shovel
forms. The former have a few comparatively large
shovels set rather far apart, which vigorously tear
up the earth to a considerable depth and leave it in
large ridges. There is a lack of uniform action, and

480

THE PRINCIPLES OF SOIL MANAGEMENT

the bottom of the cultivated portion is left in hard
ridges. Such implements are now much less used than
formerly, and may be considered to supplant in a meas-
ure the use of the plow, where deep working without
turning is desired. Some of the wheel-hoes used in or-
chard tillage belong to this type. The old single and
double shovel -plows are earlier types of the same
implement.

The small shovel-cultivators have very generally
supplanted the large shovel type in most cultural work.

Fig. 138. Broadcast seeder, which also cultivates the soil.

The decrease in size of shovels is made up by the great
increase in number. Ordinarily they operate shallow,
but very thoroughly and uniformly. They are now much
preferred in all inter-tillage work for eradication of
small weeds and the formation of a loose surface mulch.
A modification from these in shape of shovel is the
sweep, much used in the southern states, especially
in cotton-growing. It consists of broad blunt knife-
like blades, which pass along a few inches beneath the
surface of the soil and raise it an inch or two, then
permit it to drop back in place in a much broken con-
dition. It works best on soil relatively free from stone.

CULTIVATORS PROPER

481

In addition to being a good implement to form a shallow
mulch and keep the surface level, it is very effective as
a weed-killer.

Fig. 139. Small, one-horse grain drill for seeding in standing corn. Its use

is equivalent to cultivation.

Another classification, which has less relation to
utility than to the convenience and comfort of the
operation, is based on the presence or absence of wheels.
There is a strong movement toward the use of wheel-
cultivators, carrying a seat for the operator. These
have a wider range of operation as to depth and facility
of movement than have the cultivators without wheels.

Still further, there is the distinction of shovels from
discs. Discs are used on the larger cultivators; seldom
on the small ones.

Fig. 140. Meeker disc pulverizer. See also Fig. 37.

EE

482

THE PRINCIPLES OF SOIL MANAGEMENT

Cultivators are also adapted to till one or more rows
at a time.

341. Leveler and harrow type of cultivator. — In
this group come the spike-toothed harrow, smoothing
harrow,, the spring-toothed harrow, disc harrow, spading
harrow, weeders and the Acme harrow.

The spike-toothed harrow is essentially a leveling
implement, adapted to very shallow cultivation of loose
soils. It is also something of a cleaner, in that it picks

Fig. 141. Plow for loosening beets and other root crops.
Cultivates the soil deeply.

up surface rubbish. The spring-toothed harrow works
more deeply than the spike-toothed harrow, and can there-
fore be used in many situations to which the latter is
not adapted. In working down cloddy soil it brings the
lumps to the surface, where they may be crushed.
The disc harrow depends for its primary advantage
upon the conversion of sliding friction into rolling
friction. Its draft is, therefore, less for the same amount
of work done. It has a vigorous pulverizing action simi-
lar to the plow, and more so than shovel-cultivators.

LEVELERS AND HARROWS 483

Disc implements are not adapted to stony soil, whereas
toothed forms are as effective here as on soil free from
stone, so long as the stones are not large enough to collect
in the implement. On the other hand, on land full of
coarse manure, sod, etc., the disc implement is the
more efficient. The spading harrow (cutaway disc) is
very little different from the disc harrow, except that
it takes hold of the soil more readily. A recent attempt
to accomplish a large amount of pulverization, and with

Fig. 142. Riding cotton- and corn-planter. It also cultivates the soil.

greater uniformity, is represented by the double-disc
implements. In these implements there are two sets
of discs, one set in front of and zig-zagged with the
other, and also adjusted to throw the soil in opposite
directions.

Weeders are a modified form of the spring-toothed
harrow, adapted to shallow tillage of friable, easily
worked soil, where the aim is to kill weeds and create
a thin surface mulch. They are wide and are fitted with
handles, and therefore stand intermediate between

484

THE PRINCIPLES OF SOIL MANAGEMENT

cultivators proper and harrows. They are much used
for the intertillage of young crops.

The Acme harrow consists of a series of twisted
blades which cut the soil and work it over. They are
most useful in the latter stages of pulverization on soil
relatively free from stone. The Meeker harrow is a

Fig. 143. Stubble digger used to fit light, mellow soils for seeding.

modified form of disc, used primarily for pulverization.
It consists of a series of lines of small discs arranged
on straight axles, and is especially adapted to breaking
up hard, lumpy soil. In this particular, it may be con-
sidered to belong to the third set of implements, the
clod crushers. But, as compared with the roller on hard
soil, it is more efficient.

342. Seeder cultivators. — Many implements used

SEEDER CULTIVATORS

485

primarily for seeding purposes are also cultivators,
and their use is equivalent to a cultivation. The grain
drill is a good example of this group. It is essentially a
cultivator — either shoe or disc — adapted to depositing
the grain in the soil at the proper depth. All types of
planters which deposit the grain in the soil have a similar
action on the structure of the soil. The ordinary two-
row maize planter, the potato planter, etc., while of
low efficiency, as cultivators, still have an effect which

Fig. 144. Grain drill with either hoes or discs, and having
fertilizer-spreading attachment.

is measureable. This action is well seen in the lister,
used for planting maize, by which the grain is deposited
beneath the furrow, which is filled by cultivation after
the grain is up. The lister is generally used without
previously plowing the ground, and its use is limited
to regions of low rainfall where the soil is aerated by
natural processes. Lately, plowed ground listers have
been introduced, which combine the advantages of deep
planting with proper preparation of the soil.

There is also a very considerable tillage action in

486 THE PRINCIPLES OF SOIL MANAGEMENT

many harvesting implements. The potato-digger, for
example, very thoroughly breaks up and cultivates the

Fig. 145. Corn planter; also compacts the soil over the seed and establishes
capillarity with the lower soil, thus bringing more moisture in contact with
the seed.

soil, which process is one important reason for the
general high yield of crops following the potato crop.
Bean-harvesters and beet-looseners also have a similar
action on the soil.

343. Packers and crushers.— These may be divided
into two groups: (a) Those implements which aim to

Fig. 146. Scotch chain harrow. A good pulverizer and very effective on
pastures in breaking up and spreading "droppings."

PULVERIZERS AND PACKERS 487

compact the soil, (b) Those whose primary purpose
is to pulverize the soil by crushing the lumps. Both
sets of implements have something of the same action
on the soil. That is to say, any implement which com-
pacts the soil does a certain amount of crushing; and,
conversely, any implement which crushes the soil does
some compacting.

344. Rollers. — The type of the first group is the solid
or barrel roller, which by its weight aims to force the
particles of soil nearer together and to level the surface.

Fig. 147. The bar roller and pulverizer.

The smaller the diameter in proportion to its weight,
the greater the effectiveness of the roller. Its draft is
correspondingly greater. As a crusher, the roller is
relatively inefficient on hard, lumpy soil, because of its
large bearing surface. Lumps are pushed into the soft
earth rather than crushed.

It should be mentioned that there is one condition
where the roller is effective in loosening up the soil
structure. This is on fine soil on which a crust has
developed as a result of light rainfall. Here the roller
may break up the crust and restore a fairly effective
soil mulch.

488

THE PRINCIPLES OF SOIL MANAGEMENT

Another form of roller is the sub-surface packer. One
type of this implement consists of a series of wheels with
narrow V-shaped rims, which press into the soil and com-
pact it, while leaving the surface loose. (Fig. 67.) They
are designed primarily to level the land after plowing,
and to bring the furrow slices close together and in good
contact with the subsoil, in order to conserve moisture

Fig. 148. Potato digger which is also very effective in stirring the soil.

and promote decay of organic material, which may be
plowed under. This implement has been developed
chiefly in semi-arid and arid sections of country where
the conservation of moisture is especially important,
but they might well have a much larger use for the same
purpose in those sections of the country which are sub-
ject to late summer and fall droughts. While compacting
the soil, these implements leave a mulch behind.

WEEDS AND THEIR CONTROL 489

345. Clod-crushers. — The aim of these implements
is to break up lumps. As to mode of action, there are
several forms. The bar roller and the "clod-crusher"
(see Fig. 71) concentrate their weight at a few points,
and are open enough so that the fine earth is forced up
between the bearing surfaces. They are very effective
in reducing lumpy soil to comparatively fine tilth. They
have very little leveling effect further than the breaking
down of lumps.

The planker, drag or float, variously so-called, con-
sists essentially of a broad, heavy weight without teeth,
which is dragged over the
soil. The lumps are rolled
under its edge and ground
together in a manner which
very effectively reduces their fig. 149. Float or smoother made
size. At the same time, the of Planks-

soil is leveled, smoothed, and, to a degree, compacted.
It may well be used in the place of the roller as a pul-
verizer, on many occasions. It is constructed in many
forms.

III. OTHER PHASES OF TILLAGE OPERATION

In addition to the modification of food, moisture,
air and heat of the soil, through changes in its structure
as a result of tillage and other cultural practices, other
important soil conditions may be changed. Two of the
most important of these are: (1) The destruction of
weeds. (2) The control of erosion.

346. Weeds in their relation to crop-production. —
A weed has been defined as a plant out of place. By

490 THE PRINCIPLES OF SOIL MANAGEMENT

this definition any plant which grows where it is not
desired is a weed.

347. Objectionable qualities of weeds. — Weeds are
objectionable for several reasons. Some of the objec-
tionable effects of weeds are: (1) They may remove
moisture needed by the crop. (2) They may use food
needed by the crop. (3) They usurp the light and heat
supply. (4) They interfere with tillage and harvesting,
operations, and perhaps also with the planting of the
following crop. (5) They leave the soil in a condition
unfavorable to the growth of the following crop. (6)
They decrease the value of the crop by introducing im-
purities which are both injurious and expensive to
eliminate.

348. The control of weeds. — The control of weeds
depends on their character and habits of growth. Each
situation develops its own peculiar crop of weeds. They
arise as a result of the character and condition of the
soil, and the character and habits of the regular crop.
It is a type of the natural association of plants. In the
wheat fields of the Northwest, mustard is troublesome;
in maize, it may be quack-grass, sonchus, daisy or morn-
ing-glory. In meadows, it may be the thistle, yarrow or
daisy. These weeds gain a foothold because their cycle
of growth so closely corresponds with that of the
crop. According to the character and occurrence of the
weed, one of two methods of control or eradication must
be employed: (a) If its propagation is dependent on
seed-production, then seed-production should be pre-
vented. (6) If propagated vegetatively, then the develop-
ment of the aerial portion must be prevented for a

WEEDS AND THEIR CONTROL

491

sufficient time to kill the root or other propagative parts.
In this direction, much may be accomplished through
change in the rotation and in cutting the weeds at the
proper time. But much may be accomplished by tillage.
This may be largely accomplished in tillage for other
purposes. Some of the practices which aid the process
are:

(1) Early and frequent tillage. Weeds are most
easily killed when young. Soon after the seed has ger-
minated, they are most delicate. Stirring the soil at this
period may so change their relation to it as to cause

Fig. 150. Erosion on a gravelly hillside.

492 THE PRINCIPLES OF SOIL MANAGEMENT

their death. Tillage in hot, dry weather is especially
effective in killing most weeds. They soon dry out from
lack of moisture.

(2) Small-toothed implements which very thoroughly
stir the soil are more effective in killing small weeds
than are large shovels which may slide past the weed.
Thorough stirring of the soil is the essential point to
be aimed at.

(3) Where weeds are beyond the reach of the culti-
vator, as in the row in maize that has reached a con-

■«*>

. : l^0^$M ^SSMSS.^

Fig. 151. Terracing to prevent erosion of hillside.

siderable size, they may often be killed by covering
with soil by use of large shovels.

Shading by a rapid-growing leafy crop and spraying
with chemicals for some species are also effective aids
in weed control.

349. Erosion. — Erosion is often a serious menace to
the productiveness of the soil. It may result from two
causes: (1) The action of running water. (2) The action
of wind. The soil is removed and causes injury to the
productiveness of the land, first, by carrying away the

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494 THE PRINCIPLES OF SOIL MANAGEMENT

most fertile portion; second, by such changes in the
physical condition of the land as greatly interferes with
all cultural operations. This is especially true where
large gullies are formed, as happens on some soil types,
or where ridges and mounds are formed by wind action.
In some sections and on certain classes of soil, wind
erosion is most serious; notably in dry regions of high
winds. Under other conditions, erosion by water is
most serious.

350. Erosion by water. — This type of erosion is a
function of flowing water. It therefore occurs almost
entirely on sloping land. The exception is where the
soil is underlain by a stratum of fine sand which flows
with the water when saturated. The removal of sand
below permits the soil to cave down. As has been noted
in another connection, erosion is greatly increased by
material carried by the water and which becomes its
tool. Some of the most effective practices for the con-
trol of this type of erosion are: (1) Deep plowing on
heavy soil, by which a larger part of the rainfall is
absorbed and retained. (2) Increased granulation of
the soil, which may be produced by the means explained
on page 104. The absorptive power and water capacity
of the soil is thereby increased so that there is a less
amount to flow away. (3) Addition of organic matter,
which not only aids granulation, but binds the soil
together. It also increases the water capacity of the
soil. (4) Underdrainage reduces erosion where the soil
is saturated with water. Instead of its flowing away
violently in rills, it is gradually removed in the drainage
channels, which are not subject to erosion. (5) Various

EROSION

495

protective coverings and binding materials may be kept
on the soil. The most effective of these are fine-rooted
crops, which not only hold the soil togther, but protect
It against the force of the water. In those sections where

Fig. 153. Characteristic erosion of loess. The binding power of roots is illus-
trated by the tree roots at the surface.

496 THE PRINCIPLES OF SOIL MANAGEMENT

it thrives, blue grass is permitted to occupy those areas
of the hillside most subject to erosion. Trees afford a
similar protection and are valuable in reclaiming eroded
land. It is a general custom to retain in some cover-
crop those steep areas of land most subject to erosion.

(6) Contour farming, that is, the performance of all
tillage operations around the hill at a uniform level,
instead of up and down the slope, creates a succession
of small ridges which hold the water, to a certain extent.

(7) Side-hill ditches are employed where contour farm-
ing does not create sufficiently large ridges to hold the
water. The two are usually combined. These side-hill
ditches are usually given a small grade along the face
of the slope, to gradually carry away the water. (8)
Terracing is preferred in some sections as a method to
prevent erosion as well as to facilitate tillage. The water
is, of course, held on each level strip throughout the suc-
cession of terraces. Where gullies have already formed,
there extension may usually be prevented by filling with
some porous material, such as straw, brush or stone, which
checks the flow of water and accumulates the sediment.
Cross-embankments are also useful. When these are
combined with the growth of grass, trees or other plants,
to bind the soil together and further protect it, such
land can frequently be reclaimed.

351. Erosion by wind. — Erosion by wind, including
the drifting of sand, may be checked by means of:
(1) Windbreaks, and in some cases, by keeping the sur-
face rough. (2) A surface covering such as stone or
vegetation, the latter to bind the soil together and
break the force of the wind. (3) The addition of organic

CROP ADAPTATION 497

matter, which will hold the soil together and increase
its moisture content, which latter also greatly aids the
process. (4) In fine-textured soil, such as silt and very
fine sand, by the promotion of granulation and by the
avoidance of a loose fallow surface at that season of
the year when wind erosion is likely to be serious.

The aggregate of soil moved from tilled fields by
erosion of these two types is large, and it usually con-
cerns the most productive portion. The encroachment
of sand-dunes upon valuable land is often a serious
menace. Reforestation and the planting of sand-binding
grasses are the chief protective measures available.

IV. ADAPTATION OF CROPS TO SOIL

It is a matter of common observation that all crops
do not grow equally well upon the same soil.

352. Philosophy of crop-adaptation. — Each plant is
adapted to make its best growth on a particular soil
and under a particular climate. Any departure from
these ideal conditions results in changing the character
of the plant and reduction in its value. This peculiar
adaptation of crop to soil is the result of centuries of
natural selection. The basis of all the tillage operations
which have for their object the modification of the soil
conditions is to bring the soil more nearly to the ideal
condition required to nourish plants. This wide differ-
ence in the preferences of crops is well known. On the
other hand, there are hundreds of different kinds of
soil, — that is, soils which normally maintain different
conditions for growth. Some are fine, others are coarse;

FF

498 THE PRINCIPLES OF SOIL MANAGEMENT

some are deep, others are shallow; some have one chemi-
cal composition, others have a different composition;
some are dark, others are light-colored; some are wet,
others are dry; some occur under one climate, others

Fig. 154. Lettuce and celery growing on muck soil. Such soil usually requires

special fertilization.

under another climate. It is the combination of all
these factors which affect plant growth that gives rise
to the great variety of soil conditions. The great variety
of plants is a reflection of this great variety in the con-
ditions of growth. In any given situation, those crops
which are adapted to that situation persist and thrive.

FACTORS IN CROP-ADAPTATION 499

The range of conditions upon which a particular crop
will grow is limited. It is wider for some crops than
for others. Likewise, the range of crops which can be
grown on any particular soil is also limited. The more
extreme the soil condition, the more limited is this
range of crop-adaptation. It is the soil of intermediate
properties — texture, organic content, drainage and
food supply — which is adapted to the greatest variety
of crops. In the largest utilization of any particular
soil, this adaptation of crop to soil must be made use of,
as well as modification of the soil.

353. Factors in crop-adaptation. — The determining
factors in crop-adaptation are of two sorts: (1) The
physiological requirements of the plant. (2) The ca-
pacity of a given soil and climatic condition to fulfil
those physiological requirements.

354. Physiological requirements of the plant. — The
physiological requirements of the plant are of both a
physical and a chemical character.

(1) The physical requirements relate to the habits
of growth of the plant, particularly the type of its root
system and the intensity of sunshine, temperature and
wind it is able to withstand. Especially important is
the root system. Deep- or tap-rooted plants have a very
different feeding ground from shallow-, fibrous-rooted
plants.

(2) The chemical requirements relate to food ele-
ments necessary to growth, and especially to the presence
or absence of accessory substances which the plant is
able to withstand. For example, some plants will not
grow in a soil rich in lime; others require this condition.

FACTORS IN CROP-ADAPTATION 501

In arid soils some plants are able to withstand alkali
conditions where others quickly succumb. There
may be toxic substances in the soil injurious to one
plant and not to another. These may arise from the
growth of other crops, and so determine the plants which
may be associated with that crop. This bears on crop-
rotation.

355. Requirements for growth supplied by the soil. —
The internal conditions of different soils may be very
different. On a puddled clay soil saturated with water,
only a few plants may thrive. On a dry sandy soil,
only certain other plants can secure the essentials for
growth. On a very shallow soil, shallow-rooted, early-
maturing crops may be grown where trees would utterly
fail. On soils subject to midsummer drought, early-
maturing crops may be grown where late-maturing
crops would fail. Thus, the soil conditions are the arbiter
in the selection of crops to be produced. The distribu-
tion of different crops and types of agriculture is a re-
flection of this adaptation. Many failures result from
failure to recognize these relations.

Full knowledge for the accurate adaptation of crops
to soil, or soil to crops, is yet to be gained. Such infor-
mation is often not to be derived by definite experi-
mentation. It comes of long experience. But many
striking examples of adaptation are known. They are
governed by soil conditions broadly considered, rather
than by any single factor. One of the most general
of these relations is the adaptation of early truck crops
to light, sandy soil; of grass, to heavy soil. Certain
varieties of apple grow to their highest perfection on

CROP-ROTATION 503

certain types of soil. Cherries and peaches succeed best
on a lighter soil than apples may be best grown on.
Muck soils are eminently adapted to the growth of celery,
onions, etc. With the extension of careful soil and crop
surveys, these relations are becoming better known and
are extending from groups of plants to species and
varieties of plants. By the same methods our informa-
tion concerning these relations must be extended until
the production of crops rests upon definite knowledge
of the plant requirements on the one hand, and the soil
capacity and the means available to alter the soil
environment on the other hand. Really intelligent hus-
bandry can rest only upon the basis of exact knowledge
concerning these two groups of facts and principles.

V. RELATION OF SOIL PRODUCTIVENESS TO CROP-
ROTATIONS

At an early time in the development of agriculture,
it was understood that a succession of different crops
upon any piece of land gave better returns than one
crop raised continuously. The plan of changing the
crops grown each year thus became customary, and
the universality with which it was practiced by Euro-
pean peoples shows that its value must have been dis-
covered independently in many communities, as ideas,
particularly agricultural ones, traveled very slowly in
the middle ages.

In Great Britain and some of the countries of Europe,
crop rotations have been most systematically and effec-
tively developed. This has been the natural result of

504 THE PRINCIPLES OF SOIL MANAGEMENT

the incentive arising from diminishing productiveness
of the soil consequent upon long-continued cultivation,
coupled with an increasing population. Countries
having undepleted and uninfested soil, or an unpro-
gressive people, have done little with crop-rotations.

Another condition that discourages the use of crop-
rotation is the suitability of a region to the production
of some one crop of outstanding value, combined, per-
haps, with a relatively cheap supply of fertilizing ma-
terial. The abundant use of fertilizers may postpone
for a long time the recourse to crop rotations.

356. Principles underlying crop-rotation. — There are
many benefits to be derived from a proper rotation of
crops that are not directly concerned with soil-produc-
tiveness. The practice of crop-rotation must depend
upon certain principles in soil management, some
of the most prominent of which are mentioned below,
and are modified by climatic, topographic, geographic
and economic features, and many other factors, that
cannot be treated here.

357. Nutrients removed from the soil by different
crops. — Some crops require large amounts of one
fertilizing constituent, while others take up more of
another. As before pointed out (see page 294), cereal
crops are able to utilize the potassium and phosphorus
of the soil to a considerable degree but have less ability
to secure nitrogen. They are, therefore, usually much
benefited by the application of a nitrogenous manure and
leave a considerable residue in the soil. A number of
other crops, as, for instance, beets and carrots, can
utilize this residual nitrogen. Grasses remove compara-

PRINCIPLES OF CROP-ROTATION 505

tively little phosphoric acid. Potatoes remove very-
large amounts of potassium. A rotation of crops is,
therefore, less likely to cause a deficiency of some one
constituent than is a continuous growth of one crop,
and it utilizes more completely the available nutrients.

358. Root systems of different crops.— Some crops
have roots that penetrate deeply into the subsoil, while
others are only moderately deeply rooted, and others
quite shallow-rooted. Among the deeply rooted plants
are alfalfa, clover, certain of the root crops, and some
of the native prairie grasses. Representing those having
moderately long roots, are oats, maize, wheat, meadow
fescue, grass, etc., and among those having shallow roots
are barley, turnips and many of the cultivated grasses.
As plants draw their nourishment from those portions
of the soil into which their roots penetrate, the deeper
soil is not called upon to provide food for the shallow-
rooted crops, and the deep-rooted crops remove rela-
tively less of the nutrients from the surface soil. It
therefore happens that a rotation involving the growth
of deep- and shallow-rooted crops effects, by utilizing
a larger area of the soil, a more economical utilization
of plant nutrients than would a continuous growth of
either kind.

359. Some crops or crop treatments prepare food
for other crops. — It is quite evident that the growth of
leguminous crops, even when not plowed under, leave
in the soil an accumulation of organic nitrogen trans-
formed by bacteria from atmospheric nitrogen. This,
in the natural course of decomposition and nitrification,
becomes available to cereal or other crops that may follow

506 THE PRINCIPLES OF SOIL MANAGEMENT

in the rotation. The presence of a grass crop upon the
land for several years favors the action of non-symbiotic
nitrogen-fixing bacteria, as already explained (see page
429). The grass crops also leave a very considerable
amount of organic matter in the soil, which by its gradual
decomposition contributes both directly and indirectly
to the supply of available nutrients. As the organic
matter left by the legumes and grasses decomposes
slowly, these crops should be followed by a coarse
feeding crop, like corn or potatoes, and one which
is at the same time a cultivated crop, as are these.
Stirring the soil at intervals during the summer greatly
facilitates decomposition, and leaves a supply of easily
available food for more delicate feeders, like wheat or
barley, that may follow the cultivated crop. The intro-
duction of cultivated crops in the rotation thus serves
to prepare food for the non-cultivated ones. Although
practical difficulties sometimes make it impossible to
follow the cultivated crops with winter wheat, the prac-
tice, where proper preparation of the seed-bed is pos-
sible, is a good one.

360. Crops differ in their effect upon soil structure. —
Plants must be included among the factors affecting
the arrangement of soil particles. The result of practi-
cally all root growth is to improve the physical condi-
tions of the soil, to a greater or less degree. In general,
crops with rather shallow and very fibrous roots are
most beneficial, at least to the surface soil. Millet, buck-
wheat, barley, and to a less extent wheat, leave the soil
in a friable condition. It is upon heavy soils that this
property is most beneficially exercised.

508 THE PRINCIPLES OF SOIL MANAGEMENT

Tap-rooted plants, and others with few surface roots,
do not exhibit this action. Alfalfa and root crops are
likely to leave the soil quite compact as compared
with the crops mentioned above.

The effect of sod is generally beneficial, and this is
one of the reasons for using a grass crop in a rotation.

361. Certain crops check certain weeds. — By rotating
crops, the weeds that flourish during the presence of one
crop upon the land may be greatly checked by succeed-
ing crops. Some weeds are best destroyed by smothering,
for which purpose small grain and notably corn or sor-
ghum sown for fodder are effective. Others are most
injured by cultivation, to accomplish which the hoed
crops are needed; while others can best be checked by
the presence of a thick sod on the ground for a number
of years. In the warfare against weeds that must be
carried on wherever crops are raised, the use of different
crops involving different methods of soil treatment is
of great service.

362. Plant diseases and insects checked by removal
of hosts. — Many plant diseases and many insects spend
their resting stages and larval existence in the soil.
A continuous growth of any one crop upon the soil
favors the increase of these species by providing each
year the particular plant upon which they thrive. A
change of crops, by removing the host plants, causes
the destruction of many diseases and insects through
their inability to reach their host plants. A long rota-
tion, such as is frequently used in Great Britain, is
particularly effective in eradicating those diseases that
persist in the soil for a number of years. In the case of

REASONS FOR CROP-ROTATION 509

diseases that affect more than one species of plant, as
does the beet and potato scab, there is need for special
care in arranging the rotation. Such considerations
may frequently make it desirable to change the plan of
a rotation.

Another feature of the relation of crop rotation to
plant diseases is that the more thrifty growth obtainable
under rotation assists the crop to withstand many dis-
eases.

363. Loss of plant-food from unused soil. — A system
of crop-rotation permits a more constant use of the
land than is possible with most annual crops. As a
soil bearing no crop upon it always loses more plant-
food than one bearing a crop, it is thus possible, by a
well-chosen rotation, to save plant-food that would
otherwise be lost.

364. Accumulation of toxic substances. — That the
soil frequently contains organic substances that exert
an injurious effect upon the growth of certain plants
is indicated by recent experiments and was surmised
by some early writers upon the subject. De Candolle
was probably the first to advance the idea in 1832.
He suggested that at least some plants excrete from
their roots substances that are injurious to themselves,
although harmless or even beneficial to other plants.
This he considered one of the reasons for the failure of
many crops to succeed when grown continuously upon
the land, while that same soil may be productive under
a rotation of crops. Liebig, in his first report to the
British Association in 1840, made a similar statement.

Recently, Pouget and Chonchak, working with alfalfa

510 THE PRINCIPLES OF SOIL MANAGEMENT

soils, have reached the conclusion that alfalfa plants
excrete a toxic substance which, gradually accumu-
lating in the soil, injuriously affects the growth of
alfalfa plants. Whitney, Livingston, Schreiner and
their associates conclude that certain soils contain toxic
substances of organic nature which may be produced
by plant roots, or possibly by certain processes of de-
composition of organic matter. They have isolated
from soils organic compounds that are poisonous to
plants.

It is found, for instance, that cumarin, which is a
normal constituent of sweet clover (Medicago alba, L
and M., officinalis, P), may be obtained from certain
soils, and that it is toxic to wheat seedlings, — from
which it may be supposed that it is more or less toxic
to other plants. Dihydroxystearic acid was isolated
from certain soils by Schreiner and Shorey, who found
that it is acid to litmus and decomposes BaC03 and
CaC03, forming the corresponding salts. The extracts
of the soil containing this substance were toxic to wheat
seedlings. The relation of soil acidity and soil toxicity
is thus suggested.

Working with different media in which wheat and
other seedlings were grown, it was shown that, where
the nutrient solutions were very dilute, so as not to
enable the plant to overcome the effects of small quan-
tities of toxic matter, the wheat plants grew much
better when following other plants; and that, in spite
of a renewal of the supply of nutrients, the wheat plants
grew less well when one crop succeeded another. The
cause of the lessened growth was attributed to the

CROP ROTATION AND TOXIC MATERIALS 511

excretion from the plant roots of substances which,
while more or less toxic to other plants, are especially
so to plants of the same species.

Although there are yet many phases and details of
this subject to be worked out, there seems to be some
relation between the presence in the soil of organic
substances poisonous to plants and the continuous
growth of one crop; and this may be considered to be
one reason for the benefit derived on some soils, at least,
from the practice of crop-rotation.

INDEX

PAGE

Absolute specific gravity jf soil ... 94

Absorption by the soil ..... 297

Causes of ... 299

Effect of adsorption .301

Effect of aluminum hydrate . . .300

Effect of calcium carbonate 300

Effect of ferric hydrate 300

Effect of humus 300

Effect of size of particle 299

Effect of zeolites 299

Influence in soil analysis 277

Insolubility of absorbed sub-
stances 299

Occlusion 301

Relation to drainage 302

Relation to productiveness 306

Time required 297

Absorption of nutrient salts 286

Absorptive, physical 102

Absorption properties of humus. . 128
Abundance of common minerals . . 8
Acidity of soil, effect of ammonium

sulfate 326

Effect of lime 349

Effect on availability of fertiliz-
ers 360

Effect on availability of phos-
phorous 361

Effect on bacteria 360

Effect on liberation of potas-
sium 360

In relation to bacteria 401

Acme harrow, efficiency of 484

Adaptation of crops to soil, exam-
ples of 503

Part in soil management, 465

To soil, factors in 499

To soil, philosophy 497

To soil, lack of knowledge 501

Adjustment of soil moisture 170

Adobe soil, relation to wind forma-
tion 62

Adsorption by the soil 301

Effect on nitrates 301

Relation to plant nutrition. . . .301

PAGE

yEolian rocks 11, 14

yEolian soils, deposition of 60

Composition of 63

Relation to loess 60

Aeration, effect on availability of

fertilizers 359

Effect of drainage on 242

Effect of nitrification 416

Agencies of rock decay 14

Plants and animals 28

Agricultural classes of soil 74, 77

Air (oxygen) of soil, as factor in

plant growth 1

Circulation of, affects soil tem-
perature 461

Air of soil, effect of on percolation . 167

Air of the soil 432

Analyses 434

Composition 434

Carbon dioxide in 438

Effect of carbon dioxide produc-
tion on 435

Effect of cropping on 447

Effect of irrigation on 447

Effect of manures on 444

Effect of organic matter on. . . .433
Effect of roots on composition. .437

Effect of soil moisture on 433

Effect of structure 432

Effect of texture 432

Effect of tillage on 444

Effect of underdrainage on. . . .445

Escape of carbon dioxide 436

Functions 437

Modifying volume and move-
ment 443

Movements 439

Movement due to atmospheric

pressure 440

Movement due to gaseous dif-
fusion 439

Movement due to temperature. 441

Movement due to water 440

Movement due to wind 443

Oxidation 437

GG

(513)

514

INDEX

PAGE

Algae 394

Alinit 429

Alkalies, carbonate of, affect struc-
ture 118

Alkali, relation to irrigation 230

Alkali salts, relation of drainage to

removal 247

Alkali soils 307

Black alkali 309

Composition 309

Correction of black alkali 316

Direct effect on plants 312

Effect on different crops 313

Effect on plants 312

Indirect effect on plants 313

Reclamation 314

Reclamation by growing toler-
ant plants 318

Reclamation by leaching 318

Reclamation by retarding evap-
oration 318

Reclamation by underdrainage.315

Relation to irrigation 314

Relation to water content 313

Rise of alkali 314

White alkali 309

Alkali spots 314

Reclamation 319

Alluvial soils 47

Composition of 52, 53

Alternate, cropping in semi-arid

regions 196

Aluminum hydrate, effect on ab-
sorption 300

Amount of water in soil 136

Amount of water used per irriga-
tion 235

Amendments 348

Salts of calcium 348

Use of, part in soil management . 465

Ammonia, conservation in manure 376

Formation in farm manures. . . .376

Ammonification 410

Ammonium sulfate 326

Composition of 326

Effect on acidity of soil 326

Loss from soil 326

Manufacture of 326

Amount of water moved by soil . . . 185
Amount of water used in irrigation. 226

Analysis of marine clay soils 50

Arid and humid soils 65

Coastal plain soils 49

PAGE

Analysis of common minerals 6

Cumulose (muck) soils 44

Dust and loess soils 63

Earth's crust 4

Glacial lake soils 51

Glacial soils 57

Humus for nitrogen 123

Mechanical 70

Residual soils 32

Soil air 434

Soil separates 85, 86

Animal, age of, effect upon ma-
nures 373

Use of, effect on value of ma-
nure 374

Animals and plants, agencies in

rock decay 28

Manures produced by different. 368
Animal life, effect on structure. . . 118
Apparent specific gravity table . . 96
Applying water in irrigation, meth-
ods 228

Aqueous rocks 11, 12

Areas of residual soil, in America . . 31
Arid and humid climates, forma-
tion of mulches in 198

Arid and humid soils 64

Properties 79

Arid soils, composition of 65

Organic matter in 125

Arrangement and porosity 88

Arrangement of soil particles 88

Ash of plants, substances found in . 280
Atmosphere, as agency of rock

decay 16

Composition of 16

Atmosphere in saturated soil 189

Atmospheric pressure, effect on

movement of soil air — 440

See, also, Air.

Attachments for plow 476

Available water in field soils 157

Availability of fertilizers 356

Availability of phosphate fertiliz-
ers 339

Soil water 141

Back furrow, meaning of 475

Bacteria 395

Ammonification produced by. .410
Conditions affecting growth. . . .399
Decay and putrefaction pro-
duced by 408

INDEX

515

PACE

Bacteria, decomposition of cellu-
lose 405

Decomposition of mineral matter403
Decomposition of nitrogenous

organic matter 407

Decomposition of non-nitrogen-
ous organic matter 404

Decomposition of starch 405

Distribution 396

Effect of drainage on 244

Relation to organic matter. 361, 401
Relation to oxygen in the soil. .399
Relation to temperature. 400, 450

Relation to soil acidity 401

Relation to soil moisture 399

Soil, functions 403

Bacteria, nitrification produced by 412

Nitrobaeter 413

Nitro-bacteria 412

Nitrogen fixing 429

Nitrosococcus 412

Nitrosomonas 412

Nitrous ferments 413

Numbers in the soil 397

Relation to nodules on roots. . .423

Symbiotic relation 423

Y and T forms 426

Bartlett, determination of expan-
sion of rock 20

Bases, substitution in the soil. . . .297

Basic slag 337

Basins, filter, for drainage 263

Biological processes, relation to

soil temperature 448

Biological enemies in soil, as fac-
tors in plant growth 1

Black alkali 309

Bobb, sediment carried by chief

rivers 26

Bone phosphate 335

Tankage 335

Brands of fertilizers 343

Briggs, amount of water moved by

soils 1S7

Figures on capillary movement. 1S1

Figures on surface tension 160

Buckingham, amount of water

moved by soil 188

Diffusion of soil moisture 1S9

Calcium, as plant-food element . . 3
Calcium carbonate, effect on ab-
sorption 300

PACE

Calcium carbonate, reasons for de-
termination 272

Calcium cyanamid 328

Application to soil 330

Composition of 330

Manufacture of 329

Calcium nitrate 331

Composition of 332

Manufacture of 331

Calcium salts as amendments . . . .348

Carbonate, effect on soil 352

Effect on plant diseases 3£0

Effect on soil bacteria 348

Effect on tilth 348

Effect on toxic substances 3C0

Forms used on soil 351

Liberation of plant food 349

Oxide, effect on soil 352

Water-slaked 352

Calculation, of apparent specific

gravity 96

Number of particles 81

Porosity 92

1/ Surface area 83

Cameron, soil moisture and physi-
cal properties 156

Campbell, sub-surface packer 212

Capacity of soil for water 136

Capillarity affected by dampness

of particles 175

Organic matter 153

Temperature and solution 1 73

Capillary efficiency of soil 185

Capillary efficiency determined by

texture 180

Maximum 172

Capillary movement, affected by

friction 1 72

Affected by oily substances. . . . 183

And surface tension 182

Effect of structure 182

Extent and rate of 1 73

Horizontally 184

In damp and dry soil 180

In dry soil 1 76

Influenced by fertilizers 182

Measurement of 1 75

Modified by gravity 183

Of water 169

Under cloth tent 215

Capillary water, character and

amount of 144

Distribution affected by gravity. 148

516

INDEX

PAGE

Capillary water, relation to texture 144

Relation to structure 151

Supplies plants 142

Carbonates, of alkalies, affect soil

structure 118

Carbon, as plant-food element.. . . 3

Carbon, in humus 123

Carbon dioxide in relation to or-
ganic matter 361

Carbon dioxide in soil air 438

Carbonate of calcium, effect on soil .352

Case-hardening 101

Caustic lime, effect on soil 352

Cellulose, decomposition by bac-
teria 405

Cementation of granules 106

Cementing materials 99

Cereal crops, absorption of nutri-
ents 294

As green manures 387

Chain harrow, use of 4S6

Chamberlin and Salisbury, char-
acter of minerals 8

Characteristics of minerals 8

Characteristics of the soil 2

Checking and formation of gran-
ules 106

And moisture content 106

Checking of soil 98

Relation to evaporation 99

Relation to drying 106

Relation to weakness 106

Chemical, agencies of rock decay. . 14
Chemical analysis of soil, complete

solution 268

Extraction by distilled water. .276

Interpretation 270

Manurial needs 270

Permanent ferlility 269

Strong hydrochloric acid 269

Use of carbon dioxide for 275

Use of organic acids 273

Chemical composition of soils. ... 30
Chemical decay of rocks, decom-
position 14

Chemical effects of organic matter. 131
Chemical precipitates, rocks. . .11, 12
Chemical processes in soil, depend-
ent on temperature 451

Chemical properties of arid and

humid soil 64

Chert, influence on soil formation. 40
Chief groups of soils 67

PAGE

Citric acid, use in soil analysis. . . . 274

Clark, abundance of minerals. ... 9

Proportion of element in earth's

crust 4

Classes of manures 322

Soil textural 74, 76

Number of particles in 82

Classification of rocks 10, 11

Classification of soils 30

Classification of soil textural 70

Clay soil 74

Clay soil, evaporation in checks. .197

Organic matter in 125

Climate, influence on percolation . 193
Humid conditions requiring

irrigation 226

Relation to irrigation practice. .224

Clod-crushers, types of 489

Clod, relation granule 91

Cloth tent, effect on soil moisture. 214

Cohesiveness of soil 98

Cold and heat, agency of rock

decay 18

Cold soils, meaning of 462

Colloidal clay and plasticity 97

Colluvial soils, characters of 45

Colors of soil 101

Color of soil, affected by humus. . . 130

Effect on temperature 456

Commercial fertilizers 322

Constituents 324

Function 322

Compacting soil, importance of

drainage in 240

Composition of alluvial soils . . . 52, 53

Arid and humid soils 65

Atmosphere 16

Effect of roots on composition .437

Escape of carbon dioxide 436

Glacial soils 57

Marine soils 49, 50

Residual soils 32

Rocks and residual soils 32

Soil air 434

Soil air, effect of carbon dioxide

production on 435

Soil-forming minerals 6,7

Soil separates 85

Soil, relation to rock 37

Soil, relation to texture ...... 87

Soils, chemical 30

"Wind-formed soil 63

Composting manure 380

INDEX

517

PAGE

Computation of value of fertiliz-
ers 345

Conditions affecting growth of

bacteria 399

Conditions affecting structure . . . .103
Conditions requiring irrigation . . . .221

Conductivity of heat by soil 459

Conduction of heat, effect on tem-
perature 452

Constituents of soil 69

Constituents of soil, organic 119

Control of erosion 494

Control of soil moisture, means

of 155, 191

Coulters, for plow, use of 476

Covered drains. See Underdrains.

Cow manure 369

Crop adaptation, examples of . . . .503

And texture 80

Factors in 499

Lack of knowledge 501

Part in soil management 465

Philosophy of 497

Crop, peculiarities of, in water re-
quirement 224

Crop rotations 503

Crop rotation, effect on soil struc-
ture 506

Nutrients renewed by 504

Place of manure in 383

Principles of 504

Relation to diseases and insects. 508
Relation to loss of plant food. . .509
Relation to toxic substances .... 509

Relation to weed growth 508

Root systems of different crops .505
Some crops prepare food for

others 505

Crop value reduced by shrinkage. 98
Crop yields, effect of drainage on . . 247
Crop yields often controlled by soil

moisture 197

Cropping in alternate years 196

Cropping, effect on soil air 447

Crops, absorptive power, for nutri-
ents 291

Relation of to use of irrigation

water 235

Relation to texture 79

Crumb structure 91

Cultivation, best time for 478

Cultivators, types of 478

Effect on soil 478

PAGE

Cultivators, harvester type 486

Seeder types 484

Cultures for soil inoculation 428

Cumulose soils, characteristics of. 41

Composition of 44

Occurrence of 41

Czapek, experiments on solvent

action of roots 288

Dead furrow, meaning of 475

Decay, agencies of rock 14

Types of 14

Decay of organic matter 120

Promoted by 132

Retarded by 132

Rock, type of and composition

of soil 36

Decay, place of bacteria in 408

Decomposition, type of rock decay 14
Decreasing moisture content of

soil 238

Damp and dry soil, capillary

movement 180

Deep plowing, advantages of .218, 221
Deficiency of mineral nutrients. . .280

Deficiency of plant nutrients 273

Deflocculation, effect of sodium

nitrate 325

Denitrification 420

DeRossi, experiments with Pseu-

domonas radicicola 427

Depth best for mulches 207

Depth to which nitrification ex-
tends 418

Deterioration of farm manure 375

Detmer, effect of humus on sand,

water capacity 153

Diameter of individual pores. ... 94

Dicyanamid 330

Diffusion of moisture vapor in soil . 189

Diffusion of soil air 439

Disc harrows and cultivators 482

Disc plows, efficiency of 476

Diseases of plants, relation to crop

rotation 508

Disintegration, by atmospheric

agency 18

Type of rock decay 14

Distribution of bacteria 396

D'Orbigny, abundance of minerals 8

Double superphosphates 339

Drainage, by surface culture 263

Availability of fertilizers 359

518

INDEX

PAGE

Drainage, conditions requiring .. .238

Depth of underdrains 254

Effects of on soil 239

Frequency of underdrains 254

Late soils 243

Methods of 248

Part of soil management 465

Principles of 248

Relation to absorption 302

Soil temperature 463

Special types of 263

See Underdrains.

Drainage water, composition 303

Records at Rothamsted 303

Dried blood 333

Dried meat 334

Dry and damp soil, capillary move-
ment in 180

Dry farming, place of mulch in . . 208

Use of manure in 383

Drv matter, water used in produc-
tion 134

Dry soil, capillary movement in. .176
Drying of soil, influence on struc-
ture 107

Relation to checking 106

Dust, blankets 203

Mulch, management of 205

Storms 17

Soil, composition of 63

Volcanic 62

Duty of water in irrigation 222

Duty of water in irrigation, factors

affecting 224

Early soils, causes of 462

Earthworms, effect on structure. . 118
Effects of organic matter on soil. . 129
Efficiency, maximum capillary. . .172

Elements, abundance of 4

Of plant food 3

Proportion lost in residual soil

formation 34, 36

Engineering in irrigation 222

Erosion, agencies causing 492

By water, conditions permitting 494

Control of 494

Relation of drainage to 247

Evaporation 164

Evaporation, affected by checking 99

Affected by winds 213

At Rothamsted 196

Effect of on soil temperature. . .461

PAGE

Evaporation, from weeds and green

manure 195

From soil, wastes water 195

Occurs at surface 196

Prevented by 199

Prevented by special treatment . 213
Proportion of rainfall lost in

United States 196

Relation to irrigation 197

Relation to mulch formation . . . 198

Ridge culture to prevent 216

Excreta, solid of manure 364

Exhaustion of mineral nutrients. .285

Exhaustion of nitrogen 328

Exhaustion of plant food 285

Expansion, of minerals and rock

by heat 19, 20

Soil due to water 162

•j Factors in plant growth 1

Factors which determine soil tem-
perature 453

Failyer table, composition of soil

separates 86

Fall plowing 210

Farm manures 363

Fermentations of manure 375

Ferric hydrate, effect on absorp-
tion 300

Fertility of land, relation to irri-
gation 225

Fertilizer, brands 343

Ammonium sulfate 326

Availability of phosphates 339

Basic slag 337

Bone phosphate 335

Bone tankage 335

Calcium cyanamid 328

Calcium nit rate 33 1

Commercial 322

Computation of value 345

Constituents 324

Containing phosphorus 334

Cumulative need for 363

Double superphosphates 339

Dried blood 333

Dried meat 334

Effect of soil acidity on availa-
bility ...359

Effect of soil moisture on availa-
bility 358

Factors affecting efficiency 355

Function 322

INDEX

519

PAGE

Fertilizer, Guano 333

High grade 343

Hoof meal 334

In relation to organic matter . . .361

Insoluble potassium 342

Inspection 344

Kainit 341

Lime-nitrogen 330

Low grade 343

Methods of applying 347

Mineral phosphates 335

Mixing on the farm 346

Muriate of potash 341

Nitrogen lime 330

Organic nitrogen in 332

Part in soil management 465

Practice 342

Reverted phosphoric acid 338

Silvinit 341

Sodium nitrate 324

Stassfurt salts 340

Steamed bone 335

Sulfate of potash 341

Superphosphates 337

Tankage 334

Trade value 344

Used for their nitrogen 324

Used for their potassium 340

Wood ashes 342

Fertilizing, for cereal crops 295

Fruit crops 296

Grass crops 295

Leguminous crops 296

Root crops 296

Vegetables 296

Field soil, surface area 83

Available water in 157

Film water 146

Film movement of water 169

Horizontally 184

Film water and structure 105

Film moisture, renewal of wastes

water 198

Flocculation, affected by soluble

salts 116

Produced by carbonate of lime. 352

Produced by caustic lime 352

Relation to structure 116

Flooding in irrigation practice. . . .229
Crops and conditions permitting 230
Disadvantages of 231

Food, in soil, as factor in plant

growth 1,2

PAGE

Food supply in soil, affected by

drainage 44

Food of animal, effect on value of

manure 375

Force of frost in formation 20

Formation of soils, elements lost

in 34,36

Forms of soil 141

Friction, effect on movement of

water 172

Friction determined by texture. . . 172
Determines capillary efficiency . 180
In movement of soil moisture,

time element 172

In moisture movement, relation

to mulch 182

Retards movement of moisture .180
Frost, force of in disintegration. . . 20
Fruits, absorption of nutrients. . .296

Functions of manures 384

Functions of soil 1

Soil air 437

Soil bacteria 403.

Water in soil and plant 133

Fungi, large, effect on the soil. . . .390

Microscopic, in the soil 392

Furrow back, meaning of 475

Dead, meaning of 475

Irrigation, conditions and crops

permitting 231

Irrigation, disadvantage of 231

Irrigation, principles of 231

Proper width and depth of 472

Gallagher, expansion of soil by

water 162

On soil shrinkage 98

Soil moisture and physical prop-
erties 156

Geological classification of soils. . . 30

Geology, not soils 2

Relation to soil study 2

Georgeson, on effect of manure on

soil temperature 464

Germination, effect of soil tempera-
ture on 448

Gilbert, and Laws, water used by

plants 134

Glacial ice, agency in rock decay. . 27

Influence on topography 28

Glacial soils, character of 54

Chemical character of 56

Composition of 57

520

INDEX

-PAGE

Glacial soils, modified by water . . 59

Occurrence of 56

Physical character of 57

Relation to underlying rocks . 55, 60

Grain, separate, structure 89

Granular structure 91

Granulation, relation to texture. . . 92

Due to ice crystals 108

Relation to soil moisture 156

Granules, cementation of 106

Relation to checking 106

Grass crops, absorption of nutri-
ents 295

Protection to surface 113

Gravitational water 160

Capacity 162

Injurious to crops 163

Movement of water 165

Relation to porosity 161

Relation to texture 161

Water, affected by percolation. 164
Gravity, affects capillary distribu-
tion 148

As agency of soil transportation . 45

Green manures 384

Green manures, and humus supply. 132

Cereal crops 387

Leguminous crops 385

May be injurious 195

Ground limestone, effect on soil. . .353
Growth of plants, influence of tem-
perature on 449

Growth, requirements of plants for . 1

Guano, composition 333

Gypsum, effect on soil 354

Hall, effect of sodium nitrate on

structure 118

Hardpans, broken up by subsoil-

ing 219

Hardpan soil, objections to 475

Hard surface soil, repels rain

water 216

Harrow, as cultivator 478

Scotch chain 486

Types of 482

Use in mulching land 209

Heat and cold, agency of rock de-
cay 18

Frost action on rock 20

Mechanical action of 19

Heat, increases solvent action .... 19
Heat of the soil, functions of 448

PAGE

Heat of soil as factor in plant
growth 1

See, also, Temperature.
Heat, specific, of soil and water. . .455
Heaving, relation of drainage to . . . 245
Hellriegel, best soil moisture con-
tent 156

Water used by plants 134

Henneberg and Stohmann, vexperi-

ments with soil absorption. . .297

High grade fertilizers 343

Hilgard, nitrogen in humus 123

Volume weight humus 128

Hillside plow, advantages of 475

Hoof meal 334

Horizontal movement of water. . .172
Horizontal movement under field

conditions 184

Horse manure 368

Hosmer and Whitney, soil moisturel57

Humates 127

Humid and arid soils 64

Conditions, efficiency of mulch . 198

Properties 79

Humid climate, requirements of

irrigation in 226

Organic matter in 125

Regions, difficulty of keeping

mulch 204

Soils, composition of 65

Humus, defined 121

Humus, carbon in 123

Affects water capacity 153

Effect on absorption 300

Effect on color and temperature

of soil 457

Effect on structure 113

Nitrogen in .123

Plasticity 129

Some properties of 115

Volume change 128

Hunt, growth of corn crop 135

Hydration, defined 2]

Hydration, example of 2]

Hydrogen, as plant food element . 3
Hygroscopic water, character and

amount 143

Relation to texture 144

Use of 194

Ice as agency of rock decay 27

Ice crystals form lines of weakness .108
Relation to granulation 108

INDEX

521

PAGE

Ice-formed soils, character of . . . . 54

Igneous rocks 11, 12

Implements for creating mulch . .207

Implements of tillage, group 469

Implements used to increase water

capacity 217

Inoculation, part in soil manage-
ment 465

Of soil by means of cultures. . . .428

Of soil for legumes 425, 427

Inorganic constituents of soil .... 69

Insects, effect on the soil 390

Relation to crop rotation 508

Insolubility of the soil 267

Inspection of fertilizers 344

Iron as coloring material 101

As plant food element 3

Oxides as cementing material . . 100
Irrigation, amount of water to

use 226, 235

Conditions requiring 221

Duty of water in 222

Effect of on alkali 236

Effect on soil air 447

Engineering and agricultural

phases 222

Evaporation affects practice. . .197

Kind of soil most suited to 237

Land in United States requir-
ing 223

Of tobacco in Florida 237

Part of soil management 465

Relation of climate to practice

of 224

Relation of tillage to practice. .225

Relation to alkali 314

Skill required in 228

Time to apply water in 224

Units for measuring water in . . . 227
When to apply water 197

Jointer, for plow, use of 476

Kainit 3

Kaolinite, character of 7

King, amount of water moved by

soils 186

Effect of drainage on soil tem-
perature 244

Effect of plow on soil structure. Ill
Effect of trees on evaporation. .214
Experience with dust mulch. . .207
Experiment in spring plowing. .211

PAGE

King, extent of soil moisture ad-
justment 184

Figures on effect of drainage on

temperature 463

Figures on effect of subsoiling. .219

Figures on percolation 166

Observations on action of plow. 469

On temperature of soil 459

Skill required in irrigation 228

Small rainfall increases loss of

soil water 198

Suggestions on structure 103

Water used by plants 134

Kunze, experiments on solvent
action of roots 289

Lake or lacustrine soils 47

Land in United States requiring

irrigation 223

Lang, figures on specific heat of

soil 456

Late soil, causes of 462

Relation of drainage to 243

Lawes and Gilbert, water used by

plants 134

Leaching of manure 377

Organic matter 127

Leaf mold 121

Legumes, bacteria on roots 423

Leguminous crops for green ma-
nure 385

Absorption of nutrients 296

Legumes, inoculation of soil for. . .427
Legumes, inoculation of soil for. . .425

Levees used in drainage 263

Level culture, generally best 216

Lime, carbonate effect on soil. . . .352

As cementing material 100

Lime, caustic effect on soil 352

Effect on humus 127

Effect on plant diseases 350

Effect on soil bacteria 348

Effect on structure 116

Effect on tilth 348

Effect on toxic substances 350

Forms of, in relation to structure 117
Ground limestone, effect on soil .353

Liberation of plant food 349

Lime-nitrogen 330

Lime, relation to magnesium 350

As a soil amendment 348

Limestone soils 37

Lister, conditions where useful. . . .485

522

INDEX

PAGE

Loess, composition of 63

Physical characters of 61

Relation to wind formation. ... 60
Soil, extent of 61

Loose top soil absorbs rainfall. . . .217

Loss of nitrates from soil 418

Water from soil 164

Loughridge, table, composition

soil separates 85

Low-grade fertilizers 343

Macro-organisms of the soil 38S

Maintenance of organic matter. . .131
Magnesium, as plant food element . 3

Magnesium, relation to lime 350

Management of dust mulch 205

. External factors in 463

\ Mulches, summarized 210

Soil and texture 80

Soil moisture, highest type of. .238

Manures 319

Manure, early ideas of function . . . 320
As affected by food of animal . . .373
As affected by use of animal. . . .374
As affected by age of animal . . . .373

Composting 380

Conservation of ammonia 376

Deterioration of 375

Different classes 322

Effect on volume and movement

of soil air 444

Factors affecting value 373

Farm, formation of ammonia. . .376

Farm, loss of free nitrogen 377

Fermentations of 375

For cereal crops 295, 387

For grass crops 295

For leguminous crops 290

For root crops 296

For vegetables 296

From animals, relative values. .371

From poultry 372

From sheep 3 1 1

From swine 370

From the cow 369

From the farm 363

From the horse 368

Function of 3S4

Green 3S4

Green, leguminous crops 3S5

In maintaining humus 132

Leaching 377

Litter 367

PAGE

Manure, methods of handling . . . .379

Nutrient function 320

Place in crop rotation 383

Produced by different animals . . 368
Relation to soil moisture supply 221

Sawdust 368

Solid excreta 364

Straw as litter 367

Urine 366

Use of in dry farming 383

Mass of soil 2

Marine soils 46

Marine soils, composition of 49

Marl, associated with cumulose

soil 43

Maximum capillary efficiency .... 172

Moisture content 155

Water capacity 161

Materials, coloring in soil 101

Mead, number of methods of apply-
ing water 229

Means of decreasing soil moisture . 238

Methods of applying fertilizers. . . .347

Applying water in irrigation .... 229

Handling manure 379

Mechanism of the soil 2

Mechanical action of water in rock

decay 24

Analysis defined 70

Decay, of rocks, disintegration . . 14
Support by soil as factor in plant

growth 1

Meeker harrow, efficiency of 484

Merrill, composition of residual

soils 33

Example of atmospheric disin-
tegration 18

Examples of expansion of rock. . 20
Method of calculating loss of

rocks in decay 31

Table, composition soil separates 84

Metamorphic rocks 11, 14

Micro-organisms, plant 392

Micro-organisms in the soil 391

Miners' inch, defined 228

Minerals, characteristics of 8

Absorbed by crops 291

Amounts removed by crops. . . .2S1
Composition of important soil

forming 5

Constituents of soil, absorption

by roots 279

Deficiency 273, 280

INDEX

523

PAGE

Minerals, exhaustion of 285

Groups of 5

Important soil forming 4

Matter, decomposition by bac-
teria 403

Nutrients, amounts contained

in soils 282

Phosphates, composition . . .335, 336

Relative abundance of 8

Table of chemical and physical

properties 6, 7

Minimum capillary content and

wilting 182

Minimum moisture content 155

Mixing fertilizers on the farm. . . .346
Moisture in soil, as factor in plant

growth 1

Affected by manures 221

As related to soil air 433

Capacity of soil effect of texture

and structure 216

Capillary form 144

Content and structure 105

Control of 190

Critical content 155

Decreasing loss of 191

Diffusion of vapor through soil . 190
Effect of early spring plowing. .211

Effect of salt 355

Effect of structure on move-
ment 182

Effect on porosity 162

Effect of tent on 214

Effect on heat conductivity. . . .460

Effect on soil temperature 461

Forms and availability 141

Gravitational form 160

Hygroscopic form 143

Increased loss through rainfall . 198

Maximum content 155

Means of decreasing 238

Minimum content 155

Of soil, adjustment of 170

Prevention of percolation 191

Relation to bacteria 399

/ Should be stored deep 198

Special treatment to prevent

evaporation 213

Used by different crops 185

Use of hydrostatic form 194

See Water.

Modification of structure 104

Soil temperature means 463

PAGE

Modification of structure, texture. 87

Molds, slime 394

Moldboard, relation of shape to

character of soil 472

, Movement of plant roots in soil . . . 174

Soil air 439

Soil air, met hods for modifying . 443
Soil moisture, affected by damp-
ness 175

Soil moisture, amount of 185

Soil moisture, due to texture. . .172
Soil moisture supplemented by

roots 1 74

Soil moisture, thermal 1S9

Water, affected by friction . . . .127

Water affected by solution 173

Water, affected by structure. . .182
Water by soil, lack of data on. . 185

Water horizontally 172

Water in soil 165

Muck 355

Muck, defined 121

Ammonia extract, effect on struc-
ture 115

Composition 355

Crude, effect on structure. . 115, 355

Organic matter in 125

Effect on soil moisture 355

Formation of 41

Quantity added to soil 355

Mulches, principles of 199

Depend on rapid evaporation . . 198
Depend upon preventing diffu-
sion 189

Depth of 207

Effectiveness determined by

friction 182

Effect on structure 119

Example of effect, Cornell Uni-
versity 207

Equalize soil moisture 198

Implements for creating 207

Importance of texture 205

Kinds of 199

Management of dust 205

Management of, summarized . . .210

May be compact soil 205

Most effective in arid regions. . .204
Natural formation in arid and

humid climates 198

Relation to irrigation practice. .236

To prevent wind erosion 496

When may be formed by roller . 487

524

INDEX

PAGE

Mulching, grain fields 208

Plow land 209

Muriate of potash 341

Nature, method of tillage 18

Nematodes in the soil 391

Nitrogen, as plant food element. . . 3

Fixation by symbiosis 423

Fixation without symbiosis. . . .429

Fixing organisms 429

Per cent in humus 123

Transfer from nodules to plant. 425

Nit rogen lime 330

Free, loss from manures 377

Nitrification 412

Nitrification, depth to which it

extends 41S

Effect of aeration on 416

Effect of organic matter on. . . .413

Effect of sod on 417

Nitrates in soil, affected by drain-
age 244

Loss from soil 418

Of soda, effect on structure. ... 118

Reduction to ammonia 420

Reduction to nitrites 420

Nitragin 428

Nitrites, reduction to free nitrogen420

Nitro-bacteria 412

Nitrobacter 413

Nitrosococcus 412

Nitrosomonas 412

Nitrous ferments 413

Nobbe, on germination and tem-
perature 449

Nodules 423

Nodules, transfer of nitrogen to

plant 425

Number of bacteria in soil 397

Particles, calculation 81

Particles in soil 81

Nutrient salts, absorption of 286

Selective absorption 286

Occlusion, relation to soil absorpt'n301
Oily substances on soil particles

affects capillarity 183

Open drains, advantages and dis-
advantages of 250

Construction of 250

Conditions permitting 248

Organic acids, use for soil analysis . 273
Organic constituent of soil 119

PAGE

Organic matter, amount in soil. . .124

Absorptive properties 128

Affects water capacity 153, 361

And raw phosphates 361

As coloring material 101

Composition 121

Derivation of 120

Effect of decay on temperature

452,464

, Effects of on soil 129

Effect on nitrification 413

Effect on structure 113,361

In relation to bacteria 361, 401

In relation to carbon dioxide . . .361

In relation to fertilizers 361

In relation to soil air 433

Loss by leaching 127

Maintenance of in soil 131

Nitrogenous, decomposition by

bacteria 407

Non-nitrogenous, decomposition

by bacteria 404

Properties of 126

Sources of 120

Used to prevent erosion 497

Volume change 128

Organic nitrogen in fertilizers. . . .332

Organic soils 43

Organisms in the soil 3S8

In soil, effect of drainage on ... . 244

Macro-, in the soil 388

Micro-, in the soil 391

Of soil, importance of tempera-
ture for 450

Osmotic equilibrium in plants. . . .287
Outlet of underdrains, importance

of 261

Oxygen, as plant food element. ... 3

Relation to soil bacteria 399

Oxidation, a function of soil air. . .437

Packers and crushers 4S6

Packing of subsoil, benefits of . . . .212
Parkes, effect of drainage on soil

temperature 243

Particles of soil, constitution of . . . 69

Number of in soil 80

Number of calculation 81

Of soil, mineral character 70

Of soil, shapes 70

Surface area of 82

Peat, defined 121

Formation of 41

INDEX

525

PAGE

Percolation 164

Percolation of water, factors affect-
ing 167

Direction of 168

King's figures on 166

Prevention of 191

Relation to climate 193

Rothamsted figures on 192

Phosphate fertilizers 334

Physical agencies of rock decay . . 14

Absorption 102

Character of soil, relation to irri-
gation 224

Chief groups of soil 66

Effects of organic matter 129

Of arid and humid soil 64, 79

Of soil 68

Processes in soil, relative to tem-
perature 431

Properties of soil-forming miner-
als 6,7

Properties of soil most suited to

irrigation 237

Piedmont soils 40

Planker, drag or float, efficiency of .4S9

Plants amount of water used by . . 133

And animals, agencies in rock

decay 28

Plant food, elements of 3, 291

As factor in growth 1

Derived from rocks 3

Derived from air and water. ... 3

Diseases, effect of lime 350

Elements, abundance of 4

Elements in soil, abundance. 6S, 282

E xhaustion of 285

Groups of 3

In the soil, amounts removed by

crops 281

Relation to crop rotations 509

Plant growth, factors in 1

Influence of soil temperature on 449
Plants, injured by gravitational

water 163

Micro-organisms in the soil 392

Physiological requirements in

adaptation 499

Requirements for growth 1

Result of the inherent capacity

of seed 1

Result of environment 1

Roots, effect on the soil 391

Roots, effect on soil structure . . .113

PAGE

Plants, roots, importance of ad-
vance through soil 174

Roots, strike deep in drained

soil 245

[/Plasticity, cause of 97

Of humus 129

Properties of 97

Plows, types of 471

Attachments for uses 476

Efficiency of depends on 470

Effect on soil structure Ill

Hillside 475

Mode of action 469

Moldboard, adaptation to soil. .471
Relation of use to soil moisture. 470

Subsoil 477

Plowing deep, advantages of 218

Relation to humus supply . . . .218

Plow-land, mulching 209

Plow-sole, character of 474

Pore space in soil 93

Diameter of individual 94

Porosity of rocks 21

Affected by moisture 162

Calculation of 92

Of soil 92

Potassium fertilizers 340

Potato growing, straw mulches in . 202

Harvester, as cultivator 486

Potts, figures on heat conductivity

of soil 459

Poult ry manure 372

Practices used in soil management .465
' Practices in soil management, pri-
mary and secondary functions

of 465

Pressure of air, effect on percola-
tion 168

Primary minerals 5

Process of atmospheric rock decay . 16

Productiveness of soil depends on . 36

In relation to crop rotations. . . .503

Properties of soil 68

Properties of soil separates 80

Puddled structure 90

Puddled soil, changed by lime. ... 117

Danger from subsoilins 478

Movement of water in 182

Pulverization, action of plow in. . .469
Putrefaction 408

Quinke estimates molecular attrac-
tion 146

526

INDEX

PAGE

Rainfall, United States, map of . . 137

Effect on structure 119

Relation to irrigation practice . . 222
When small, may cause loss of

water 198

Rains, gentle, best 216

Raw phosphates and organic mat-
ter.. 361

Reclamation of alkali soil by drain-
age 247

Reduction of nitrates to nitrites. .420

Nitrates to ammonia 420

Nitrites to free nitrogen 420

Relation of lime to magnesium .... 350

Residual soils 31

Characteristics of section 40

Proportionate loss of elements

in formation, table 34

Texture of 41

Reverted phosphoric acid 338

Ridge culture.effect on evaporation 216

Ries, on plasticity 97

Rise of alkali 314

Roberts, plow sole 474

Rodents effect on the soil 388

Rocks, as source of soil 2

Aggregates of minerals 10

And its products 2

Aqueous 11, 12

Classification of 10

Decay, agencies of 14

Decay, atmospheric 16

Decay, by solution 22

Decay, ice as agency 27

Decay, type of and composition

of soil 36

Decay, type of and texture of

soil 36

Expansion due to heat 20

Igneous 11, 12

Important soil-forming 9

Porosity of 21

Roller, conditions where effective .487

And packers, types of 486

Effect on soil temperature 459

Relation to percolation 194

When used to mulch soil 487

Roots, absorbing system 292

Absorption of mineral matter. . . 277
As related to crop rotations. . . .505
Crops, absorption of nutrients. .296
Conditions where enter drains
253,261

PAGE

Roots, effect on composition of soil

air 437

Excretion of acids 290

Hairs, relation to soil particles. .287
•j Of plants, strike deep in drained

soil 245

Osmotic activity 292

Osmotic equilibrium in 287

Plants, effect on the soil 391, 113

Rotation of crops 503

Rotation of crops, principles of. . .504

Effect on soil structure 506

Nutrients removed by 504

Part in soil management 465

Place of manure in 383

Relation to diseases and in-
sects 508

Relation to loss of plant food . . . 509
Relation to toxic substances. . .509

Relation to weed growth 508-

Root systems of different crops . 505
Some crops prepare food for

others 505

Rothamsted, figures on evapora-
tion 196

Tables of percolations 192

Rubbish, covering by plow 476

Salisbury, and Chamberlin, char-
acter of minerals 8

Sachs, experiments on solvent ac-
tion of roots 288

Salt, effect on soil 354

Sand-dunes, damage by 497

Naturally mulched 206

Sandstone soils 39

Sandy soil 74

Sandy soil, organic matter in 12

Sanitary conditions, effect of drain-
age on 247

Saturation, total water for 161

And gravitational water

Wave of in soil, useful 194

Sawdust as litter in manure 368

Schubler, figures on effect of color

on temperature 457

On soil shrinkage 98

Season, length of increased by

drainage 243

Secondary minerals 5

Sedimentary rocks 11,12

Sedentary soils 30

Sedentary soils, divisions of 31

INDEX

527

PAGE

Sediment, in water of chief rivers . . 26

Seeder types of cultivators 485

Selective absorption of nutrient

salts 286

Semi-arid soils, composition of ... . 65

Separate grain structure 89

Separates, properties of soil 80

Series, soil, defined 78

Sewage irrigation 2L>2

Shale soils 39

Shapes of soil particles 70

Sheep manure 371

Silica as cementing material 101

Silting up of tile drains 258

Silvinit 341

Size of particle, influence on soil

absorption 299

Slime molds 394

Slope of soil, influence on tempera-
ture 458

Shrinkage of soil 9S

Sod, effect on nitrification 417

Sodium nitrate 324

Sodium nitrate, composition of. . .325

Deflocculating action 325

Effect on structure 118

Effect on plant growth 325

Preparation 325

"Soil" defined 68

Adaptation to crops 497

Air 432

Amendments 348

Analysis, influence of absorp-
tion 277

Broader than geology 2

Characteristics of 2

Clay 74

Cohesiveness 98

Environment of plant 1

Factors in plant growth 1

Forming rocks, important 9

Functions of 1

Management and texture 80

Mechanism 2

Moisture, adjustment of 170

Number of particles in classes. . 82

Organic constituent 119

Organic matter in 125

Organisms 3S8

Particles, constitution of 69

Particles, influence surface cool-
ing on movement of moisture . 183
Particles, relation to root-hairs . 287

PAGE

Soil, plasticity 98

Productiveness in relation to

crop rotation 503

Productiveness of arid and humid 69
Requirements of growth fur-
nished by 501

Sandy 74

Separates, composition of 84

Series, defined 78

Structure, defined 88

Study, viewpoint 2

Surface area of 83

Types 78

Units 80

Solar radiation, relation to soil

temperature 451

Soluble material in water of chief

rivers 24

Solubility of the soil 267

Solubility of the soil, surface ex-
posed 267

Composition of particles 267

Organic matter 127

Relation to texture 270

Strength of solvent agents 267

Soluble salts, affect flocculation. . . 116
Soluble salts.as cementing materiallOO

Effect on structure 116

Solution, affect movement of waterl73

By water in rock decay 22

Surface tensions 173

Sorting power of water 27

Source of soil material 2

Specific gravity of soil, absolute. . 94

Of humus 128

Table of apparent 96

Specific heat of soil and water. . . .455
Spray irrigation, advantages and

disadvantages of 233

Spray irrigation, conditions where

used 232

Spring plowing 210

Starch, decomposition by bacteria .405

Stassfurt salts 340

Steamed bone 335

Stewart, figures on capillary move-
ment 18J

Effect of cloth shade on soil

moisture 214

Rainfall causes loss of soil water 198
Storer, quotes Detmers, water

capacity 153

Reports soil moisture for crops . . 155

528

INDEX

PAGE

Storing water in soil, importance

of depth 198

Straw as litter in manure 367

Straw mulches, in potato growing. 201

Stream-formed soils 47

Structure of soil, defined 88

Structure, conditions affecting. . . . 103

Affected by animal life 118

Affected by carbonate of lime. .352

Affected by caustic lime 352

Affected by crop rotations 506

Affects capillary water 151

Affected by organic matter. . . .114

Affected by rainfall 119

Affected by soluble salts 116

Affected by surface covering ... 1 19

And moisture content 105

Crumb 91

Effect on capillary movement. .182

Heat conductivity 459

Percolation 167

Tillage on 111,469

Lime 116

Nitrate of soda 118

Structure, effect of plow on 469

Affected by roots 13

Granular 91

Ideal aspects 88

Influence of drainage on 240

Modification of 104

Puddled 90

Relation to texture 88, 92

Relation to soil air 432

Relation to moisture 156

Relation to porosity 88

Structure, separate grain 89

Sub-irrigation, natural 233

Advantages and disadvantages . 234

Method of artificial 234

"Subsoil", defined 68

Organic matter in 125

Plows, operation of 218

Plow, purpose of 477

Plow, when to use 478

Productiveness of arid and

humid 69

Subsoiling, aims of 218

Conditions requiring 218

Increases water capacity 219

May be injurious 218

Relation to sub-surface packing . 212

Time to perform 212

Substitution of bases in the soil . . . 297

PAGE

Substitution of one mineral nutri-
ent for another 280

Packer 488

Packing necessary in subsoiling . 220

Sub-surface packing 212

Sulfate of lime, effect on soil 354

Sulfate of potash 341

Sulfur, as plant-food element 3

Sunshine distribution of in United

States 452

Relation to soil temperature 452, 453
Superphosphates, composition of. 338

Fertilizers 337

Manufacture of 337

Supply of soil water 136

Surface area, in field soils 83

Calculation of 83

Relation to absorption 103

Relation to texture 82

Surface covering, influence on struc-
ture 119

Surface tension.and capillarityl58, 182

Affected by 159, 173

And plasticity 97

Movement of water by 182

Relation of fertilizers to 182

Relation to granulation in tillagell2
Surface drains. See Open drains.
Swamp soil, organic matter in. . . . 125
Sweep, cultivators, efficiency of . .480

Swine manure 370

Symbiosis, fixation of nitrogen by .423
Systems of textural classification 71, 73

Table of surface tensions 159

Tankage 334

Temperature affects, movement of

moisture 1 73

Of color on 456

Percolation 168

Surface tension 173

Temperature of soil. <See, also, Heat.

Affected by wind 461

And air, monthly range 454

At different depths 454

Daily range of 453

Effect of drainage on 463

Effect of manure on 464

Effect of moisture on 461

Effect on chemical processes . . . .451

Effect on conductivity 459

Effect upon movement of soil air 441
Factors which determine 453

INDEX

529

PAGE

Temperature of soil, heat from in-
terior of earth 452

Influence of drainage on 242

Influence of organic decay 452

Influence of shelter on 464

Influence of slope on 458

Means of modifying 463

Relation to bacteria 400

Relation to physical processes. .451

Relat ion to specific heat 455

Relation to sunshine 453

Sources of heat for 451

Tensile strength and plasticity ... 97

Tent of cloth, effect on soil moist-
ure 214

Texture, defined 70

And moisture capacity 80

And soil management 80

Classification 70

Classification, agricultural 74

Determines structure 92

Effect on capillary movement. .176

Effect on heat conductivity 459

Effect on percolation 167

Groups, limits in size 73

Importance in mulch 205

In relation to soil air 432

Modification of 87

Relation to absorption 103

Relation to capillary water 144

Relation to composition 87

Relation to crops 79

Relation to gravitational water. 161
Relation to hygroscopic water. . 144
Relation to movement of moist-
ure 172

Relation to pore space 93

Relation to solubility 270

Relation to shrinkage 98

Relation to surface area 83

Soil, relation to type of plow. . .470

Thermal movement of soil moisture 189

Tile drains. See Underdraws.

Tillage, objects of 467

Action of implements in 469

And granulation Ill

As means of drainage 263

By cultivators 478

By plows 469

Early and late to kill weeds. . . .491

Effects on the soil 468

Effect on volume and movement
of soil air 444

PAGE

Tillage, implements of 468

Influence on structure Ill

In grain fields 209

In spring, effect on soil tempera-
ture 461

Nature's method 18

Part of soil management 465

Practice, best for holding water. 217

Principles of 466

Relation to alkali control 236

Relation to irrigation practice. .225
Relations to structure and water

movement 182

Rollers, packers and crushers for 486
Time elements in capillary move-
ment 180

Element in movement of soil

moisture 173

To apply irrigation water 224

Tobacco, irrigation of in Florida. .237
Toxic substances, effect of lime on . 350

Relation to crop rotation 509

Trade value of fertilizers 344

Transported soils 30

Transported soils, agencies of . . . . 44
Transporting power of flowing

water 24

Tubercles 423

Types, soil 78

Underdrains, arrangement on hill

land 262

Construction of 251

Depth of 252

Effect on soil air 445

Freezing of 261

Frequency of 254

Grade of 258, 263

Importance of outlet 261

Materials used in construction. .251
Plants roots enter, when . . . 253, 261

Silting up of 258

Size of 255

Systems of arrangement 254

Types of tile used in 258

Units of soil, defined 80

Units, used in measuring irrigation

water 227

Use of lime on soils 348

Urine in manures 366

Vegetables, absorption of nutrients 296
View point, of soil study 2

HH

530

INDEX

PAGE

Volume change of humus 128

Of soil air, methods for modify-
ing 443

Of water held by soils 154

Weight, table of 96

Wagner, on effect of manure on soil

temperature 464

Warington, effect of lime on clay

soil 117

Figures on water used by plants . 133
Quotes figures on heat conduc-
tivity 459

.Quotes figures on specific heat. .456
Quotes on adequate soil moist-
ure 156

Quotes, on shrinkage 98

Shrinkage of clay and humus. . . 128

Specific gravity of humus 128

Suggestions on structure 103

Warm soils, meaning of 462

Water, amount of used by plants . 133

Amount in soil 135

As agency of rock decay 21

As agency of soil transportation . 45

Available in field soils 157

Capillary movement of 169

Capacity, maximum 161

Capacity of humus 130

Capacity of soil 136

Capacity of soil, effect of texture

and structure on 216

Capacity of soil increased by

subsoiling 219

Capacity of soil, influence of

drainage on ' 241

Capacity of soil, means of increas-
ing 216

Carrying power 24

Causes expansion of soil 162

Character of soils deposited by. 46
Chemical action of in rock decay 21
Content of soil, effect on temper-
ature 461

Content, statement of 136

Content of soil, relation to crops .155
Composition of soils deposited

by 49,50,51,52,53

Critical content 155

Division of soils deposited by . . . 46

Erosion by 494

Extract of soil 276

Film 146

PAGE

Water, films and checking of soil . 98

Films and structure 105

Forms of 141

Function of in plants 133

Gravitational form 160

Increased loss through rainfall . 198

In soil, availability 141

In soil, density of 290

In soil, means of decreasing 238

In soil, effect on movement of

air 440

Lack of, often controls crop

yields 197

Loss of from soil 164

Material transported in chief

rivers 26

Maximum content 155

Minimum content 155

Mechanical action of in rock

decay 24

Movement of in soil 165

Soil as reservoir 133

Soil supply dependent on 136

Solvent action 21

Sorting power 27

Supply of 136

Used by different crops 185

Volume of, held 154

Way, experiments with soil absorp-
tion 298

Weed, defined 489

Control of, principles 490

Deplete soil moisture 195

Early and late tillage to kill. . . .491

How, injurious 490

Implements useful to kill 492

Relation to crop rotation 508

Removal of saves water 216

Special methods of control 492

Weeders, type of cultivator 483

Weight of soil 94

Weight of soil, per cubic foot 96

Per acre-foot 96

Weight of organic matter in soil . . 128

Weight of peat and muck 128

Wet soil, organic matter in 125

White alkali , 309

Whitney and Hosmer, soil moist-
ure 157

Suggestions on structure 103

Surface tension of solutions. . . . 159
Wind-blown material, character-
istic of 18

INDEX

531

PAGE

Windbreaks, disadvantages of . . .213

Effectiveness of 213

To prevent erosion 496

Wind, control of erosion by 496

Deposits, composition of 63

Effect of cloth tent on 215

Effect on evaporation 213

Effect on percolation 168

Effect upon movement of soil

air 443

Formed soils, deposition of 60

Formed soils, relation to loess . . 60

PAGE

Wollny, best soil moisture content .156
Conclusions concerning soil tem-
perature 462

On temperature of soil 459

Water used by plants 134

Wood ashes 342

Worms, effect on the soil 389

Zeolites, effect on absorption by
the soil 299

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