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Full text of "The principles of soil management"







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LIBRARY 

FACULTY OF FORESTRY 
DIVERSITY OF TORONTO 



Cfje teal Cext=2ook 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 
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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 AUTHORS 1 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. Resume 1 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: 






















|S 


S X 0/ ~ 


CO 




00 

i-i 


O ic o 
00 CI h 




2 a 









a> co 




m *- 












t.g 


c .2 
a^ 


rH 


o 


io 


"* "5 m 
CO <N ro 






i 

is 




M 




m 




_0 




- M <* 3 * 


CC 


w 


h 




jZ 


-^ K- i < ' ~rt CD ^ 


CD CD o 


C3 
W 

Oh 

O 


_o 
"o 


1 o 

SE - 

co i i 

CD,Q 

_o 
"o 
O 


"to 
CD 




-*-* +^ ^^ 

a '5 o 

1 r < rj 








o 




o 










a 


1*$ 


[n. 


O 


t- Tt; cn | * q * -* 


N O! N 




ci 


IN 


im' in" <n" co co" CO co co' 


IN IN CO 




m 












i 












q 


O 


oooNiomoio 


q q o 


05 


N 


co 


id (DO B3 iQ <N t~ CO 


co Tji o 


<! 
S 








- 




1 1 

o 

1-1 

o 


a 

CD 

m 

01 

a 


o 


O 


~ m -r | ^ to m ' 


-Q 

O" mo" 




QQ 


3 


of ^ . r- bio O . g cf 


of^ 


[*< 


a 




o < 


C 


3 






of a 


o 


m 








o M 




o 








94 

o 




g 


i 




C4 


* 




o 
o 

1 

1 J 


a 


o 

*' 2 
o a 

2 


6 

m 


o 

m 


5J 


Si m co co _r 
"^J..^ cp i) Don 

A => cS 03 03 rf) . 

1 hs 3 2 e cq 
<! r/5 "^3 rt S S J5 

-' ^ Jl 0! CO aj""; 


** 1 

O S ft 


n 


oj 
O 




d 


o * w ^ ^ ^ e K 


Bo 


H 








of ^ 






1 




<u . 


S j : : 8 : 


c 


CD '. '. 




CO 

C 






|S ^ o 8 


03 


"5 * 




1 


03 


-jj 


u 2. cd -x. - C 
O -p CD '" '0 


CD 

o o +i 


"3 -2 
M S .13 




"8 


53 oj 


5^ 53 S P, ^ a a a 


. ^03 




a 

c3 


3 




<$ 




fc 


|H 


IN 


CO Tt<lCCOt^CX)050rt 


IN CO 














1 T 1 


il i-i 



(6) 



T3 
O 

a 

'+3 
C 
o 
o 

of 
w 

8 

w 

Cu 

o 

IS 



H 

a 
H 

a 

< 

m 

< 
a 



g 
S 
o 



o 
co 

o 
s 

3 
o 
O 



CD 
< 





In 

arbon- 

ated 

water 














3 7 
3& 


o 














o 


. . o 




. 





.2 




CI 
CO 
CN 


:<n : :S 

. . CO 




j 


: : : : : 






CO 










73 












* 


C 


1* 


d 




.2 o 


Jj CD 3 1) 


0> 
OJ 


o 


CD O 

CD . ^J 


L- 


^ =|3 


c3 ^ CD V 


i 

* 
O 


CO 


d h, . CD d 


O 


a "3 2 


^1 i 

3 "2 5 


CD 60 ^ CD 
CD J, i CD 


cS 


CD T! 4J 
-S d 3 


in 

CO 

03 

PQ 


S| ; i 




3 d ^ 


w cj 8 
pq 


CJ 


^ ^ 


a 


CO 


o 








Specifi 
grav- 
ity 


CO "tf 


CO CN ^i "I" CN 


CO 


C5 


N . "O N 


CN CN 


CN iQ 3 (B N 


CM 


* 


CN CN CN ,1, 


CO 


CO 








T? co 

fc< CO 


o o 


10 lO o io >o 


iO 


o 


o o . o o 


Cj <0 


CN t- 


W cc O iO M 


CM 


CO 


CN i-i ' iH CN 


x - 














O 








w <' 


43 

d 
eu 

CO 

CD 

t 
0. 

CO 

4a 

e 
B 


cc 
K ^ 



CD 

o 


O 
-r O O m _ U 


w 

o" 

CO 

bj 


CO 
cu 


a O CD~ 


a 


60 


03 






s i M a 


H 














03 

o 








M uf 




w 








o 




o 




O 




&S o c Q 



o 

o 


o 


C 


C4 

CN 




O TS + i < 




CN ^5. 

+ c 
G a> 

CO ps< 
c3 60 


<< oj -|- O o3 

o fa O * 

3 CD CO 

^ fe o 


tM 

o 

CO 
CN 

o" 

60 


CO 
CU 


o3 2 cS <5 h 

- 5S ^ -3 CD 

O ^ .2 < -S 


5 


03 


fe 


>3 






o 




CO 














CN 01 


__ 








Tl . 






d 






<1> 


C CD 




. CD 


a 
1 

M 

o 

CD 

s 




a> - -rt - 

i "S 1 a 

^ ca o3 o3 

w X S 4 W 


g 

d 
cu 

& 

l_ 

CD 
CO 


o3 4J 
'55 

03 
CD o 


CD 

4^ 

'C 

O C 

3 c 

O E- 


cd +2 

m .-s a 

CD C O 
4J .3 CD 

N W O 


j iz 


Tfl 10 


CO r- CO O O 


1 I 


CN 


n * io to n 






1 1 H 


i-l i-l i-l r-c CN 


CN 


CN 


CN CM 


W CN CN 



(7) 



8 



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 S 2 + 70 2 + 2H 2 = 2Fe S0 4 + 2H 2 S0 4 

Fe S0 4 + 2H 2 = Fe(OH) 2 + H 2 S0 4 

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 1C. 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: 

K 2 0, A1 2 3 , 6Si0 2 + 6 H 2 = 2KOH + H 2 0, A1 2 3 , 6Si0 2 , 4H 2 

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 


















(A1 2 3 ) 


14.33 


15.23 


16.89 


26.55 


15.30 


18.36 


13.25 


30.75 


3. Ferric iron 


















(Fe 2 3 ) .... 


4.00 


4.39 


9.06 


12.18 


14.25 


20.39 


17.12 


4.31 


4. Ferrous iron 


















(FeO) 


















5. Sulfur trioxid 


















(S0 3 ) 


















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 


















(C0 3 ) 


















9. Magnesia 


















(MgO) 


2.44 


2.64 


1.06 


0.40 


7.38 


3.45 


7.03 


0.61 


10. Soda (Na 2 0). 


2.70 


2.12 


2.82 


0.22 


1.98 


0.14 


2.71 


1.01 


11. Potash (K 2 0). 


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 (Si0 2 ) . . 


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 


















(Fe 2 3 ).... 


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 


















(S0 3 ) 






0.28 


0.07 


0.65 


0.05 






6. Phosphoric 


















acid(P 2 5 ). 






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 


















(C0 2 ) 






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 (H 2 0)] . 


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 

il 


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 


ii 

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 (Fe 2 3 ) . . 

4. Ferrous iron (FeO) 

5. Sulfur trioxid (S0 3 ) . . 

6. Phosphoric acid (P,0 5 ) 

7. Lime (CaO) 

8. Carbon dioxid (C0 2 ). . 

9. Masnesia CMeO.V . 


5 

e 
Bj 

-: 

z 

X 

o 

1 I 


- 

C 
Ph 

r 
iH 


O 

^ 0> 

1 

hj 

'1 

I-H 

CN 

i i 


a 

to 

3 

H 





(34) 



e fl 

CO"-.-* rt C 

c o-a w 
bl to 

OS o 

to O 



jjdoj ajijua 
joj ssoj jiiao jaj 



juaniijsuoo qoraa 
jo ssoj ^uao ja<j 



C5 * 

00ONN0500H101OMO 

n n h M h lo n oi o a o 

-h^hCOOOOCOCOCO^hO 



00 

<M 



OOlIM'I'hiooONO 
;00WOi|iOHHOO00lM 

ioooi><dcdc4iooio<N 

't>COOO5t'-te0Q0OSCOOT 



sssg| 

S ? n 
O " n a> 

^-.SP-gg 
o 



5J0OJ 8JT}U3 

joj ssoj jnao ja<j 



* * # 

C5 CM .MMOONOiOCO 



CM O 
CM 



hQONhMNOh 



L-O 

co 



> juati}i}guoD qoua 
^ ssoj jo juao jaj 



OC5 MOOOiOO^NOO 

l^o ' K"0 io w o rc w ro o 
t~- -t< eorH I -5o6ioi-5ioc35C < i 

CO CO >C CD iO (N O N C-l N 
CM 



S 
o 

S3 . c 
3-5 * 

*55 

0) 



jfooj ajrjna 
joj ssoj }uao jaj 



co 

CM 



co 
, <m 






oo co a 
; o co co 

oo CO 



co 
oo 



X 



juanjijsuoo tjdb9 
jo ssoj juao jaj 






. oo 

00 



. o 
. 

o 

o 



-* LO 

oo crJ t^ 
CM Oi i> 



** 




v- 


~ 


- 


od 


o 


o 


c 




CO 


od 


- 


0J_* 


- 


c 


i- 








< 







jjooj ajijna 
joj ssoj luaa jaj 



to 

,1-0 CO COCMOiOOOCOuOO 

. CO > I I [NTOrHMONOl^ 

o c<i -oi-t-tddddfi 

~T CO 



to 



i-i laanjijsnoo qoraa 
r*i I jo ssoj juao ja,j 



. >o 
.co io 



ONOXcONCCffl 
.NfflOeOCTCOCOtO 



'3 
O 



i I CJ3 
^H 00 



Ol X O c rc C h N 
00 O X i-O C2 rf U5 



> Uh " '.3 






O 
OS 

si 

~T3 



73 95 ^ 

C3'=.H 



C > ^-^ X **; ^-s, w 1 33 

1.S C 5 (, -*-- +j> - c 
<U 3~-= OjjS o O fcli 93 

tNCO^iOCONGOddrHtNoj 



02 

03 
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 (Si0 2 ) 

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 (Fe 2 3 ). . . 

9. Alumnia (A1,0 3 ) . . . 

10. Lime (CaO) 

11. Carbon dioxid (C0 2 ) 

12. Magnesia (MgO) . . . 

13. Soda (Na 2 0) 

14. Potash (K,0) 

15. Phosphoric acid P 2 5 

16. Nitrogen (N) 

17. Sulphuric acid (S0 3 ) . 


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 (Si0 2 ) 

3. Alumina (A1 2 3 ) 

4. Ferric iron (Fe 2 3 ) . . 

5. Ferrous iron (FeO) . . 

6. Sulfur trioxid (S0 3 ) . 

7. Phosphoric acid (P 2 5 ) 

8. Lime (CaO) 

9. Carbon dioxid (C0 2 ) . 

10. Magnesia (MgO). 

11. Soda (Na,0) 

12. Potash (K 2 0) 

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 
0C g 

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 


> 


~? 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 (Si0 2 ) 

Alumina (A1 2 3 ) . . 
Ferric iron (Fe 2 3 ) 
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 



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 (Si0 2 ) 

3. Alumina (A1 2 3 ) 

4. Ferric iron (Fe 2 3 ) 

5. Ferrous iron (FeO) . . . 

6. Sulfur trioxid (S0 3 ) . .. 

7. Phosphoric acid (P 2 5 ) 

8. Lime (CaO) 

9. Carbon dioxid (C0 2 ) . . . 

10. Magnesia (MgO) 

11. Soda (Na 2 0) 

12. Potash (K 2 0) 

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 (Fe 2 3 ) . . 

5. Ferrous iron (FeO) . 

6. Sulfur trioxid (S0 3 ) . 

7. Phosphoric acid (P 2 5 

8. Lime (CaO) 

9. Carbon dioxid(C0 2 ). 

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 


> 




O 


6 
o 


> 


a 

o 


"3 c3 







__ 




4^ 


S 





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 

M s 

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 






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 









o3 03 
O J 
O 


J 


.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 (Si0 2 ) 


8.37 


15.09 


9.00 


16.43 


20.70 


5.33 


3. Alumina (A1 2 3 ) . . 


10.72 


13.61 


9.15 


10.20 


10.54 


8.40 


4. Ferric iron (Fe 2 3 ) 


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 














(P 2 0-) 


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(C0 2 ) 


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 (Na 2 0) 


0.48 


0.40 


0.61 


0.27 


0.33 


0.16 


12. Potash (K 2 0) 


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 






SO 

ej - 

J 

-^ to 
r2 c 
QQ O 



H 



T > 
>> to 

- z 

og 

+3 

DQ 




2 


C m 

a 3 

->S 
o 


o 

IS 
o 

s 
OS 

u 

O 


o a 


a 

_r c 

"3 S 

CO * 

g.ss 




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 (Si0 2 ) 

3. Alumnia (A1 2 3 ) .... 

4. Ferric iron (Fe 2 3 ). . . 

5. Ferrous iron (FeO) 

6. Sulfur trioxide (S0 3 ) . 

7. Phosphoric acid(P 2 5 ) 

8. Lime (CaO) 

9. Carbon dioxid (C0 2 ) . 

10. Magnesia (MgO) 

11. Soda (Na,0) 

12. Potash (K 2 0) 

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 (Si0 2 ). . . 

3. Alumnia (A1 2 3 ) 

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

5. Sulfur trioxid (S0 3 ). . . 

6. Manganese (Mn0 2 ) . . . 

7. Phosphoric acid (P 2 5 ). 

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 (Na0) 

11. Potash (K 2 0) 

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. 










>; 


uj 


5 


1 


Cj 


3 


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^ 


is 


r-s 




l 


N 


=:' 





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fe 


05 


s 


S 


I 






TS 


i 


>, 1 




a 


u 


US 




a 




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q 




4^ 










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q 


10 


43 




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i 1 




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a 

03 
CO 


>q 





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03 


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TEXTURAL (iROUPS 



73 







Table VIII 







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 

^rR3 X 2.65 7rD 3 x2.65 
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 ex r 
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.0375 2 X 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 (Si0 2 ) 

3. Aluminum (A1 2 3 ). 

4. Ferric iron (Fe 2 3 ) . 

5. Phosphoric anhy- 

drid(PA) .... 

6. Sulfur trioxid 

(S0 3 ) 

7. Lime (CaO) 

8. Magnesia (MgO) . . 

9. Soda (Na,Q) 

10. Potash (K 2 0) 

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 


yj 


O 


j 


/. 


o 


Z 


(77 


< 




u 


fa 
O 


as 

fa 


fc 




O 


<l 


H 


fa 


w 


o 


O 




fa 


as 


a 


a 


o 





o 


a 


< 




u 




1 I 


fa 


a 


o 


w 


as 


a 


! 


o 


H 


j 


<j 


<j 




as 


fa 
o 


< 


M 


Hh 


O 


1 


-1 


1 


PS 


> 


fa 


H- 1 


I* 


M 


<n 


fa" 




- 




03 




3 




H 







>> 


co 


CM CN t-- O 




^s? 


00 


q co q q 


o 
^5 


o 


CN 


CN H W uj 


w _ 


t^ 


co co o 5 


^ 


co 


oq co co ^h 


GO 
03 




CN 


-J i-h c4 -* 


o 














Pm 


Ti 


o 


CO t> CN C 




os e~- 


eq 


* co i^- q 




go 


i i 


i i i co 




>> 


co 


T)H i 1 O CO 


,-, 


^6? 


CO 


oq q co co 


o 

s 


o 


1 1 


i I i I _o 








- 


*> _ 


co 


oo -v co i-- 


I I w 
i i '55 


gs 


00 


q ^h oq q 


01 






cn 


a 








fcO 








OS 


T3 


oo 


r- l Ci "^ 05 


^ 


v 


o q io -t 




CO 




T ( 




>. 


-V 


<M lo C3i CO 




56S 


OS 


q q q q 


^_^ 


u 




OS CN CO 


o 








e3 

o 

H ( 




CM 


CO OS O (N 


i* 


GO 


OS i-H CO (N 

O i-H OS 


a 






1 t 


3 


"3 


o 


CO t^ CO OS 




o3t>- 

m 


q 


cn q oj q 






CN i i rf 


_^^ 








d 


>, 
5 


o 


I> * CO iO 


"3 


l>; 


co co co * 












'3 








i* 


CN 


co o co Tt* 


hH 


CN 


CM i-H CN CN 


'C 
O 

JS 














a 


TJ 






en 
O 

j3 
Oh 


to 


t^ 


00 CO >o OS 


q 


cn q ^h ^h 






1 ~P H 








-+J co CS 














S oj m rs t- 




-2 ' os 


ca 








toC in a 




'3 : "| 


'> 








_ -C " 03 , 




co g 


_g 




KM 

o 
a 


to 


1 5 J^ 

t) co .- CD 




O.g J2 

.S '5 






-a 

03 

01 

.O 

a 


"ft 

s 

03 


S^S o S 

>. +=" ^ -2 >. co 
>> 5? O to >> 52 

> >o Ss > g 


a 

H-: 
cc 


:3 O o 'r 
SO rt a 

S a 



^^ 

- 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 = io Q - fp- s Pg r xioo 

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 (Fe 2 3 ) 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 (Fe 2 ;j + 
H 2 0). 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 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 







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 



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 



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 






























































"" 

























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 





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 




































































h26 




































































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 ( (NH 4 ) 2 S0 4 ) . 

Sodium sulfate (Na 2 S0 4 ) 

Sodium nitrate (Na N0 3 ) 

Potassium hydrate (KOH) 

Potassium sulfate (K 2 S0 4 ) 

Wood ashes 


76.8 
75.8 
75.8 
75.1 
75.1 
75.2 


Thomas slag 

Marl 

Lime 

Ammonia (NH 4 OH) 


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 0C. the tension of pure water is 75.6 
dynes per square centimeter, and at 25C. 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; (T 7 ) 
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. 



65 r 

60 

45 

2 40 

I 

o 

35 

z 

u 30 

CO 

s 

u.25 
o 

h 



20 



15 



10 



' 
























































^fcp 


ft* 


^ 


so 
































^ 


































S\ 


&~ 
























































































FINE 


SAf 


ID 










































































3 


FINE 


SAC 


D 
































ARS 


; AN 


D MEDIUM SAt> 


D 




... . 













ooot 


oo ( 


ooc 


000 


300 


JOO 


)OQ( 


ooc 


5 4 FINE GR> 

1 


kVEL 
















/O 









































c 



14 



16 



18 



ao 



8 10 12 

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 TH Q 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 



20 



IS 



16 



* 14 



12 



o 10 

H 
I 
o g 

























. V 




















, * * 


*** 


** 
















* * 


*** 
















,<* 6 


<^ 


* "V. ^ 


















<* 


f*\ 




















X 


>r 






















X 






1 


^SAND 


1 OOO O 


00 00 


3 OO O 


o<* ool 




X 


n ooo 


,oooo 


oOOO 


ooooo 














* 


>" 


























3,<- 


&* 
















f 

o 




---"' 




v^; 


c\U 














40 ,' 


7" 

t 















































8 



a 



10 



11 



12 



12 3 15 6 7 

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. 



50 




* 


SANDY ^ 


LOAM 










. " 


_J ' 


45 


i 


vV ^-* 


s\l.tv 


tOAW_ 


-- ^ 





"* ^^ 






m 40 

Ul 

x or 






^f *"" 
















"35 




>^ 




jj> 














30 

Hi 


I <'' 




















c 25 

IL 


t/ 
' if 




















20 

1- 

15 
111 

1 , 


1/ 

m 
"0 








































10 























n 























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 v are 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 








.. ^n^S 








M> 


ijS^-A 


FTEW^^_ 


SAND 


HNITIAt-PE 


RCENTt-OF 


WATER 






















SAND 














,/~ 






























/ / 
/ / 
















/ 

































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 


93 


Fine sandy loam 


0.67 


Silt loam 


2 71 


Silt loam 


1 60 


Silt loam 


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 


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. 



30 
28 
26 
24 
22 

CO 

20 
o 

S IS 

Z 

-i 16 

< 

< 

* 12 
< 
o 10 

8 

C 



















































































































<& 




















^ 
<>-/ 






















^ 
-e*/ 























* 






















*<' 


^ 










^ 










-^ 














*** 










*> 






*0 * 


*J$ 


*,<* 

#" 
















.^e TT 


Bp2?- 




^ 20 


^60 INCH GAUGE 
INCH GAUGE 1 






s**^ *-~ 





-""^^ 
















' 


^ 


i>" ' 




















>" 





















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 table 1 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 














=^ 


v^ 






co^2 


coNon-:1 s 




^"*^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 



J 1 



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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4 


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^u 





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lb 


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ft 


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4" 


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au 


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X. * ""*> 




t^^^ / 


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1 


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 









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 cl ay- 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 nitrogen 1 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 

x Not 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 bj r 
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 how 7 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 
grow r th, 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 normal 




Grams K2O 
absorbed 


Grams K2O 
absorbed ' 


KCL 


.3124 
.3362 

.5747 


.1990 


K,S0 4 


.2098 


K 2 C0 4 


.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 K 2 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 K 2 had been removed, or at the 
rate of 28,100 parts of water to one part of K 2 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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PH i-H i-H i-H i-H i-H i-H 



(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 KN0 3 , showing (z) cell-wall, (p 1 , p 2 ) 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. 

CaC 2 + 2N = CaCN 2 + 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: 

N 2 + 2 = 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 (Ca 3 
(P0 4 ) 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) 4 P 2 5 . 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: 

Ca 3 (P0 4 ) 2 + 2H 2 S0 4 = CaH 4 (P0 4 ) 2 + 2 CaS0 4 and 
Ca 3 (P0 4 ) 2 + H 2 S0 4 = Ca 2 H 2 (P0 4 ) 2 + CaS0 4 . 

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 (P 2 5 ), and potash 
(K 2 0),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 (K 2 0). 
Per cent sulfate of potash X .54 = per cent potash (K 2 0). 

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 (CaC0 3 ), 
ground limestone (also a carbonate), and calcium sulfate, 
or gypsum (CaS0 4 , 2H 2 0). The application of any of 
these is usually called liming the soil, although gypsum 
does not serve exactly the same purpose as do the other 
forms. Owing to differences in the molecular weights 
of these compounds of calcium, it requires more of some 
forms than of others to furnish the same amount of 
calcium. Approximately equivalent quantities of some 
of the common forms when fairly pure are: 

Quicklime 56 pounds 

Water-slaked lime 74 pounds 

Air-slaked lime, marl and ground limestone .... 100 pounds 

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 


Swine 


5 


Sheep 


3 







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 


-a 


Production 

per 1,000 

pounds live 

weight 


Kinds of 








a 

a 




live stock 






u 






o 













CD 
M 

o 
u 

Z 
0.49 


o 

org 

o * 

Ph 

0.26 


js 
3 
o 

PL, 


a 

M 




E 


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: 

CON 2 H 4 + H 2 = 2 NH 3 + C0 2 . 
2 NH 3 + CO, + H 2 = (NHJ, C0 3 . 

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: 

(NH 4 ) 2 C0 3 + CaS0 4 - (NH 4 ) 2 S0 4 + CaC0 3 . 

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 of 1 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: 

Ca 3 (POJ 2 + 2C0 2 + 2H 2 0=Ca 2 H 2 (POJ 2 + Ca(HC0 3 ) 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: 

K 2 0. A1 2 3 . 6 Si0 2 + Ca(HC0 3 ) 2 = CaO. A1 2 3 . 6 Si0 2 + 2 KHC0 3 

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 NH 3 + 3 2 = 6 H 2 + 2 N 2 . 

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: 

C 72 H 112 N 18 S0 22 + 770 2 = 29 H 2 + 72C0 2 + S0 3 + 18NH 3 . 

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: 

2NH 3 + 30, = 2HN0 2 + 2H 2 0. 
2HN0 2 + 2 = 2HN0 3 . 

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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6 A.M. 



NOON 



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 
3 

a 

03 



25.2 
27.3 
28.6 
31.2 
38.5 



c3 
3 

u 

fa 



24.2 

27.7 
27.8 
30.2 
35.5 



s 



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38.2 
36.6 
35.4 
35.8 



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< 



52.1 
57.5 
53.3 
49.3 
43.8 



03 



61.9 
68.7 
65.1 
60.7 
53.5 



a 

3 



71.0 
78.1 
75.7 
69.9 
61.3 



3 



76.0 
85.1 
81.6 
75.7 
67.4 



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P 
< 






74.5 67.6 
82.9 73.8 
80.172.0 
75.769.2 

69.8 67.6 

I 



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56.7 

57.8 
57.8 
61.3 



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38.7 
41.5 
44.7 
52.2 



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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 





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 





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 

z> 

72 

< 

ui 

I 68 

Ui 

I- 
64 

60 

























































''' 










*> c *-" 


---''" 










c^ .- 

(*?--* 


*" 






















4 

* 
* 


\-^ 


F1NE_SAND_ 


v^-in!_ 








* 

+ -,* 

^ -"^ 












,ai """ 















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 P lanks - 

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- 







*> 



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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 BaC0 3 and 
CaC0 3 , 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 30 

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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