82. How to modify structure. ВЂ” The structure of a

coarse-grained soil cannot be greatly affected, since it is

always fairly good; but with a clay, constant care is

necessary to prevent its becoming puddled. Many a

farmer has learned through sad experience that he can,

by cultivating a clay soil when too wet, so injure the tilth

that several years are required to get the soil back into

good condition.

The structure of a soil is affected by almost everything

that causes a movement of soil particles. Among the

most common factors are the following: (1) tillage, (2)

Physical Properties of the SoU

— Hardpaa !□ arid Hoit three f c

(Wl^

86 The Principles of Agronomy

the growth of roots, (3) freezing and thawing, (4) alter-

nate wetting and drying, (5). organic matter, (6) soluble

salts, (7) animal life, and (8) storms. The tilth is the

result of the combined action of a number of these factors,

all of which improve it except certain kinds of storms like

hail, and certain soluble salts like sodium carbonate.

83. Specific gravity of soils. — The weight of a soil

may be expressed as the real or the apparent specific

gravity. The real specific gravity, referring to the weight

of the individual particles in comparison with water, is

not affected by the pore-space. The apparent specific

gravity, on the other hand, refers to the relative weight

of a given volume of soil and the weight of the same

volimie of water. This is greatly affected by pore-

spaces.

Clay is often spoken of as a " heavy " soil ; sand is said

to be " light." This does not refer to weight, but means

that clay is difficult, and sand easy, to till. An average

sand weighs about 110 pounds to the cubic foot, but clay

weighs only about 80 pounds.

84. Air in the soil. — Since air is necessary to the

growth of all plants, it is impossible to have a fertile soil

without spaces through which air can circulate. Seeds

in germinating, and plant roots in growing, require oxygen

which is absorbed while carbon dioxide is given off.

The decay of organic matter requires oxygen and forms

carbon dioxide, which accumulates in the soil-air with

the depletion of oxygen. If the condition of the soil

does not favor the free movement of air, the oxygen supply

soon becomes reduced to the point where plant growth is

retarded. The aeration of the soil is dependent on its

texture, structure, drainage, and a number of other

factors. In a coarse sand air moves readily, but in a clay,

especially if compact, the movement is slow. Puddling

Physical Propert^s of the Soil 87

a soil greatly reduces its aeration^ while flocculating its

particles into groups promotes the ready movement of

air. The size of particles cannot be changed, but their

arrangement is affected by plowing and harrowing, which

thereby indirectly influence aeration.

A water-logged soil usually has its producing power

reduced by lack of oxygen; and the free circulation 6f

air, resulting from placing tile drains under such a soil,

is in part responsible for the increased yields following

drainage. The beneficial nitrifying and nitrogen-fixing

bacteria of the soil require an abundant supply of oxygen

for their best growth, and their action is practically

discontinued when the air supply is reduced below a

certain point. In some soils the aeration may be so

great as to result in the loss of excessive quantities of

water. This condition, however, is rarely met, and may

be remedied in most cases by packing.

85. Heat of the soil. — The temperature of the soil

is important because of its influence on the germination

of seeds and on the growth of plants ; also because of its

effect on chemical changes and bacterial action in the

soil. When the soil is cold, its life is dormant and all

chemical action is reduced. The earlier a soil is warmed

in the spring and the later it is kept warm in the fall,

the longer is its growing-season. This may have con-

siderable practical importance in regions where early crops

bring the best prices and where the season is so short

that crops do not fully mature.

Soil heat comes largely from the sim, the rays of which

are most effective when striking perpendicularly. A

south slope, therefore, is considerably warmer than one

facing the north, and a sandy soil is much warmer than

clay. The high specific heat of water makes it slow to

warm, and as a consequence, a wet soil is usually late in

88 The Principles of Agronomy

starting the growth of plants in the spring. The exces-

sive evaporation from a wet soil also reduces its tempera-

ture. Such factors as color, specific heat, tillage, and a

number of others play a very important r61e in regulating

the temperature of a soil.

86. The organic matter of the soil is without doubt one

of its most important parts since it influences so greatly

the physical, chemical, and biological changes that take

place. The tilth of a soil, its water-holding capacity, its

temperature, and a niunber of other physical properties

are improved by the presence of organic matter, which,

on decaying, increases the availability of mineral matter

in the soil and hastens desirable chemical changes.

Bacteria, which are so important to the soil, could not do

their work without organic matter, since they secure their

energy by its decomposition. The fertility of a soil,

therefore, depends as much on the presence of organic

matter as on any other factor; and the maintenance of

fertility must include the keeping up of this important

constituent.

87. Maintaining the organic matter. — The organic

matter of soils is derived largely from the decay of roots,

leaves, and stems, although a part of it comes from the

remains of animals. For ages, accumulation has been

going on until some soils have a large percentage of organic

material. In arid soils, however, where the growth

of vegetation has been light, the organic content is

low; hence, one of the chief problems in the manage-

ment of arid soils is to increase the proportion of organic

matter.

Organic matter in the soil is maintained by the addition

of farm manure and other organic refuse, and by the

raising of crops to be plowed under. The wise farmer

will, if possible, apply large quantities of manure in order

Physical Properties of the SoU 89

to maintain organic matter as well as to add plant-food.

The continuous raising of grain crops on the same land

year after year and the burning of straw and stubble is a

procediu'e most ruinous to land. The regular use of

green-manure crops, preferably the legumes, and the

returning of all plant residues to the land will serve to

maintain the proper proportion of organic matter in the

soil.

SUPPLEMENTARY READING

Soils, Lyon, Pippin, and Buckman, pp. 83-197.

Physical Properties of the Soil, R. Warington.

Soils, E. W. Hilgard, pp. 83-187.

Fertilizers and Crops, L. L. Van Slyke, pp. 117-144.

The Soil, A. D. Hall, pp. 60-88.

Physics of Agriculture, F. H. King, pp. 49-68.

Cyclopedia of American Agriculture, Vol. I, pp. 349-357.

CHAPTER IX

THE WATER OF THE SOIL

All plants and aninlals require water for life and growth.

Plants may live for considerable time without receiving

any outside supply of mineral food, but if water is with-

held, they very soon wilt and cease to function. The

yield of crops during any particular year is usually a

reflection of the moisture conditions during the growing-

season. Even in humid regions, the lack of available

moisture often reduces crop-yields. Over more than

half of the earth's tillable surface, the shortage of moisture

is the chief limiting factor concerned in crop growth;

while in parts of the humid regions, the excess of water in

the soil prevents the cultivation of vast areas of otherwise

fertile soil. On the whole, therefore, no factor connected

with agriculture needs to be more carefully studied and

more thoroughly understood than the water of the soil.

88. Origin of soil water. — The water of the soil has

at some time been precipitated from the atmosphere.

That contained in the soil of any given field may have

come in by percolation, or by flooding the surface ; but

it has at some time been vapor. The quantity of mois-

ture in the soil of any large area, therefore, will be de-

pendent on the precipitation of that region. The rain-

fall cannot be influenced by man, but he can do much to

save water after it falls. In dry regions he may increase

the amount of soil water by irrigation, or he may reduce

90

The Water of the Sail 91

it by drainage when there is too much for the best growth

of the crops. He may also increase the efficiency of the

moisture in the soil by certain tillage operations.

89. Variations m soil moisture. — The quantity of

moisture in the soil is not so stable as the phosphorus,

lime, or silica; but it varies from season to season and

from day to day. It seldom remains the same even for

a short period. More is being added from time to time,

and losses occur through a number of channels. Even

if for a short period no water is added or lost, there is a

constant movement from place to place with a tendency

to establish an equilibrium which is seldom, or never,

reached. Many forces are at work making it difficult

to determine all the laws by which soil moisture is in-

fluenced.

90. The condition of the soil moisture depends largely

on* the quantity present and the nature of the soil. -If

the soil is saturated, the action is not the same as if only a

small quantity of water is present. The soil is able to hold

only about a certain amount of moisture and when more

is added it percolates rapidly. As the quantity decreases,

the tenacity with which it is held increases. A sandy

soil reaches the point of saturation with much less water

than does a clay soil. The condition of the moisture,

therefore, will not always be the same with a given per-

centage, but will vary according to the nature of the soil.

The water of the soil is usually divided into three

classes, determined by the percentage present. These

are : (1) free, or gravitational, (2) capillary, or film, and

(3) hygroscopic water.

91. Free water. — When the soil becomes saturated

with water, a part of it drains away, due to the action of

gravity. This drainage water is known as free, or grav-

itational, water. The attraction of the soil for it is not

92 The Principles of Agronomy

so great as that of gravity. This water is found between

the grains of soil, taking the place of air. Gravitational

water, held in surface soil for any great length of time,

excludes the air needed by roots. Normally, after a

heavy rain, the top soil has its air spaces filled with

water, but this rapidly sinks to moisten the drier soil

below, in which case it ceases to be free water. Drainage

is practiced to remove the free water that cannot drain

away unaided.

92. Capillary water. — After all the free water has

drained out of the soil, there is still remaining a great

deal of moisture that is held in a thin film around each

soil particle. Most of the water in ordinary cultivated

soils under field conditions is of this nature, and it is

this form that supplies water to plants. The quantity

of capillary water that can be held by a soil depends on

the surface area of its particles. Since many fine particles

have more surface than a large one occupying the same

volume, a fine-grained soil, such as clay, will hold much

more film water than a coarse-grained soil like sand.

Capillary water moves by going from the wetter to the

drier particles in the soil. The films on different soil

grains tend to become of the same thickness, thus exert-

ing a pull on the thicker films.

93. Hygroscopic water. — A part of the moisture is

retained by the soil even when it seems to be dry. Road-

dust, on being heated, will give off water vapor which

may be condensed on a cold body. This last moisture

which a soil retains is called hygroscopic water. If a

soil is dried completely with heat and then allowed to

stand in the open, it will absorb moisture from the air.

This water is held in a thin film around the particle in a

way similar to that in which capillary water is held, only

much more firmly. It does not move from particle to

The Water of the Soil 93

particle as does the capillary water. Hygroscopic water

is of no direct use to plants, since the soil has a greater

attraction for it than have the plants.

94. Other critical points. — It has been shown in late

years that there are a number of critical points in the

percentage of soil moisture besides those already men-

tioned. For example, there is a point at which plants

wilt. This occurs when there is a small quantity of

capillary water in addition to the hygroscopic water.

There is another point in the capillary water below which

the movements are very slow. Above this point the

capillary movement is much more rapid.

95. Quantity of water in field soils. — The quantity

of moisture found in field soils depends on a number of

conditions. Of these, the amount and frequency of

rainfall is perhaps most important. In arid regions,

the soil is seldom near the point of saturation, while

in regions of great rainfall, it is kept constantly wet.

Every gradation between these extremes is found. The

amount of moisture that a soil will hold depends almost

entirely on the size of particles and the amount of organic

matter which it contains. The finer the soil the greater

its water capacity. A clay that seems fairly dry really

contains more water than a moist sand. In one experi-

ment the maximum water-holding capacity of different

classes of soils was as follows :

Sand 8 per cent

Silt loam 25 per cent

Clay 40 per cent

The clay was able to retain five times as much as the

sand. Organic matter, or humus, greatly increases the

power of soils to retain water. This is one reason why

much humus is desirable.

'

&4 The Principles of Agronomy

96. Methods of expressing the quantity of water. —

Soil moisture is usually expressed in percentage of the

soil by weight. This may be based either on the total

weight of the soil and water, or on the dry soil alone. For

example, if on heating 100 pounds of soil there was a

loss of ten pounds, there would be 10 per cei^t of water

on the wet basis, there being ninety pounds of soil and

ten pounds of water. On the other hand, since ten pounds

is 11.1 per cent of ninety pounds, there would be 11.1 per

cent of water in the soil on the dry basis. The quantity

of water may also be expressed in percentage of the soil

by volume. The depth of water over the surface of a

given area of land is a common method of expressing

quantity in an irrigated district.

97. Loss of soil moisture. — The water that falls on

the soil can be lost in three ways : (1) run-off from the

surface, (2) percolation through the soil, and (3) evapora-

tion from the surface.

In arid regions, it is desirable to reduce run-off to

a minimum, but it may be necessary to increase it in

sections of excessive rainfall. Too much run-off under

any condition is undesirable, as it is likely to cause de-

structive erosion. This loss is diminished by keeping

the soil open and receptive in order that it may absorb

the rain as fast as it falls. Percolation can be reduced

only by increasing the water-holding capacity of the soil.

This is done by keeping the soil loose and increasing its

organic matter.

The loss by evaporation is, in part, under the control

of man. When moisture is once in the soil, it should be

held there until needed by plants. This is accomplished

by some protecting cover such as a mulch.