Water exists in soils in three different states, which may be termed hydrostatic, capillary and hygroscopic. Hydrostatic water is that which fills the soil spaces and would drain away if given opportunity. Its upper surface stands at a certain level, which is known as the water table. Capillary (or film) water is that which is capable of rising in the soil or of moving from a more moist to a less moist part of the soil under the influence of surface tension with out regard to the force of gravity. Hygroscopic water is that which closely surrounds the soil particles, but is not affected by gravity and does not move through the soil under the in fluence of surface tension. Capillary water is often drawn from the hydrostatic supply and is the most important from the standpoint of plant growth. Hygroscopic water is not of great value to vegetation. Heinrich found that plants begin to wilt before the water content of soils is reduced to the hygroscopic limit.
Soil water when present in sufficient amount is constantly in motion. In case of rainfall, irrigation or the melting of snows, the moisture sinks into the soil, carrying along with it oxy gen, carbonic acid, nitric acid, ammonia, etc., and rendering plant food available, a part of which may be lost in the drainage if the water supply is excessive. This downward movement or percolation of water due to gravity is most rapid in soils of small retentive power, that is, coarse-grained open soils. Agencies like lime, which flocculate the soil particles and loosen the soil, promote percolation, and those like alkali salts, which tend to compact the soil and puddle its particles, retard it. When the supply of water ceases evaporation commences, and the soil water begins to rise by capillarity, carrying along with it dissolved plant food which ac cumulates in the surface soil within easy reach of the roots of plants. A harmful accumulation of soluble salts (alkali), however, is thus some times brought about in regions of deficient or irregular rainfall.
The capillary properties of soils are of great importance since they determine largely the available water supply of soils. Different soils vary widely as regards the amount of capillary water they retain. Schiibler has shown that the water content of a soil is a function of its structure. Coarse sand sometimes retains as little as 15 per cent, while heavy clay loam and soils containing a large amount of humus often retain as much as 50 to 60 per cent. Ordinary barns retain about 40 per cent. As King has pointed out, however, the amount of capillary water actually found in soils in the field are as a rule smaller than the theoretical amounts that the soils are capable of holding, and are con trolled to a large extent by the depth of the water table, which is constantly fluctuating.
The capillary movement of water has been ob served to extend to a depth of seven feet, but declines as the depth of the water table in creases. As a rule the amount of capillary water in the soil decreases from the water table upward toward the surface. In the finer soils the moisture as a rule rises more slowly, but to a greater height than in the coarser. The force, surface tension. which causes water to rise in a soil also produces a lateral movement, but this is much slower than the other. All movements of water become more rapid as the soil approaches saturation.
Under ordinary conditions moisture is con stantly escaping from the soil by evaporation. Evaporation may be reduced by tillage and mulching which interfere with the capillary rise of moisture in the soil, and by the use of wind breaks which lessen the drying effect of winds. The belief is common that the moisture condi tions of soils may be materially modified by the use of fertilizers, more especially by the appli cation of common salt, but careful observations and experiments on this point have failed to show any decided effect in increasing the water content of soils by ordinary applications of salt and similar substances.
Colloidal substances, in humus, clay and the like, appear to play an important part in soils, influencing especially their absorptive properties. Unfortunately exact knowledge on this point is comparatively limited and not yet sufficient for generalization of much practical significance.
Biological Action in the Soil, A large and very important part of the changes which go on in soils is due to biological processes, that is, to the actions of living organisms, in cluding the roots of plants, earthworms and other living agencies, but especially the micro organisms, generally grouped under the term bacteria. These play an important part in fix ing the free nitrogen of the air in the soil for the use of plants, in converting the organic nitrogen of soils into readily available forms by the process of nitrification, or in causing a loss of the soil nitrogen by denitrification. They are also active agents in the disintegra tion of rocks and the formation of soils; and in the formation of acids from organic matter in the soil, thus rendering the application of lime or other neutralizing agents necessary. It is the object of good soil management to so control the conditions of moisture, aeration, temperatures, etc., that the beneficial biological processes are promoted and the harmful re strained.