How Water Is Held In The Soil

All fertile soils contain many tons of water, which is present in the soil in several forms. First, and most conspicuous, is what is variously called free water, ground water, standing water or bottom water. This fills all the spaces between the particles up to a certain height, which varies with different soils, and even different parts of the same field. Free water is supplied by rainfall; it frequently comes to the surface as springs and is often the source of supply of wells. If a hole is dug in any soil water will stand in it up to a certain point, which may be several inches or many feet below the surface. This point is called the "water table." The height of the water table may be judged in a general way by the depth of surface wells, but this evidence is not always reliable. It may vary at different times during the year, according to the dryness of the season. We must consider, then, that beneath all farm soils, at some depth, is standing water; that we plow and harrow above subterranean lakes, which are no less lakes because the water is not entirely free but merely fills the spaces between the particles of soil. The importance of this fact lies in its influences upon the production of a crop. If it is only two or three feet from the top of the soil to the surface of the lake, there is not enough dry soil on top for roots to grow in and the plants drown.

Such soils are said to be shallow; they are of little value for ordinary farm crops until ditched or under-drained and the level of the underground lake lowered thereby. The draining of land is considered in detail in Chapter IX. Film Water. — Water is also present in all farm soils as film moisture. Above the water table is the soil in which the roots of farm crops forage. This soil must be moist, else plants would not grow in it ; but water does not fill all the spaces between the soil grains, as it does below the water table. If we look at a handful of this soil we cannot see water standing in it, but it feels moist. The water is sticking to the soil grains, covering them with a very thin film, as when small stones are dipped in water. It is held close to the grains by surface tension, or adhesion. If this soil were put in an oven and heated, the film water would be driven off as water vapour, and the soil would be left perfectly dry. There is always a large amount of film water clinging to the grains of every soil, even in the dryest season. The dryest road dust has some film water clinging to it. The amount of water that can adhere to a single grain of soil is, of course, infinitely small, but the amount of water that can cling to all the soil grains of a field is enormous, especially when we consider the vast surface area of the grains, in the aggregate.

A good farm soil often holds more than one-half its weight of film water. Film water is far more important in farming operations than free or bottom water, for it is the direct supply of plants. No common farm crops can thrive in free water, but all must have a large area of soil that is moist with film water. Much of this supply of film water, however, is drawn from the natural reservoir of free water below. Water absorbed from the air. — Under certain conditions the soil absorbs a small amount of water from the air. The air that fills the spaces between the particles of soil usually contains much water vapour; if the soil becomes very dry it may absorb some of this. The surface soil may also absorb water vapour from the air, especially when there are heavy fogs. This "hydroscopic" water, however, is not of much importance as a means of supplying plants with water, except in a time of great drought.