Capacity Of Different Soils To Hold Water

The different forms in which water is found in the soil have been mentioned in Chapter II. The water .that is most valuable to the plant is that which is held by the grains as film moisture, although a large part of this may be drawn from the reservoir of free or standing water below. Soils vary widely in their ability to hold film water. In judging the value of a piece of land for cropping, it is fully as important to consider its water-holding capacity as its richness in plant food; a soil may be exceedingly rich in the essential plant foods, yet if it does not hold enough water to dissolve that food and carry it to the plants, it will produce no more than a very poor soil. Fertility consists as much in an abundance of soil water as in an abundance of plant food. The capacity of a soil to hold water depends upon its composition and upon its texture. The lighter a soil is, or the more sand it contains, the less water it will hold. The smaller the grains, the more water the soil holds, since there is more surface for it to cling to and less likelihood that it will leach through. Each soil grain is surrounded by a film of moisture ; if there are over 168,000,000,000 grains in an ounce of soil, as in some alluvial soils, the amount of surface for the water to cling to is much greater than if there are but 56,000,000,000 grains in an ounce, as in some truck soils.

The more humus a soil contains the greater is its water-holding capacity, for humus is vegetable sponge. If small quantities of several kinds of soil are completely dried in an oven, and water is then added to them, it will be found that they will hold about the following amounts : Sharp sand 25%; Clay soil (60% clay) 40%; Heavy clay (80% clay) 61%; Loam 51%; Garden mould 89%; Humus 181%. The same soils do not hold as much water as this in the field, because a large part of it drains off, as it must in order to make the soil congenial for plants. It is far more important to know how much water a soil will hold under its natural conditions in the field, after the excess water that fills the spaces has drained away and only film moisture remains. The amount of film water held by different soils is about as follows: A coarse sand holds but 12 to 15 per cent, by weight of film moisture; a sandy loam from 20 to 30 per cent.; a clay loam from 30 to 40 per cent. ; a heavy clay, or a soil very rich in humus, may hold 40 to 50 per cent, of film moisture.

This means that a mellow loam with a retentive subsoil holds four to five inches of water in the first foot of soil. Although a sandy soil holds less water than a clayey soil this disadvantage is partially offset by the fact that the lighter soils give up to the plants a larger percentage of the water they do contain than the heavier and wetter soils. A light soil may hold 30 per cent, of water and a heavy soil 55 per cent., yet the lighter soil may give nearly three-fourths of its water to the crop while the plants could secure scarcely one-half of the water held by the heavy soil. Influence of Subsoil on the Water-holding Capacity of Soils. — The amount of water held by a soil depends not only upon the character of the upper two or three feet of surface soil, in which the roots of most farm plants chiefly feed, but also upon the character of the subsoil and upon the distance to the water table. Some subsoils are retentive, others are leachy. A layer of gravel or sand three or four feet below the surface may provide perfect natural drainage, thereby increasing the amount of film water that the upper soil can hold. A hardpan of impervious clay, or of rock close to the surface, will greatly reduce the water-holding capacity of the soil, strange as it may seem.

One might think that if the water could pass down only three or four feet before it strikes hardpan, the soil above would be wetter than if the water could pass down through many feet of soil. But the fact is that the shallow soils are dryest; because, in times of abundant rains, the water soon fills the soil, and then flows off as surface drainage; whereas it sinks down into the deep soil for many feet and is stored there for the future use of the crop. The first five feet of a strong loam may contain enough water to make a layer ten to twenty inches deep over the field.

Height of Water Table. — The distance below the surface at which free water is found has an important influence on the amount of film water held by the soil above it. Generally speaking the nearer the water table is to the area in which the roots of cultivated plants forage, the larger will be the amount of film water held by this soil; for a large part of this film water is drawn directly from the free water, and the nearer this is, the more abundant and equable will be the supply. The roots of most cultivated crops rarely go more than five feet deep, hence a soil in which the water table is from four to six feet below the surface is apt to be most abundantly supplied with film water. When wet land is tile drained, the level of the water table is reduced from four to six feet deep, depending upon the depth at which the drains are laid below the surface. The chief reason why wet lands are so valuable after being under-drained is that the water table is lowered only to the point where it can most easily supply the soil above with film moisture; while in lands that need no under-drainage the water table may be thirty feet deep instead of six.