It is the pore-space in soils that permits the storage of soil-moisture; and it is always important for the farmer so to maintain his soil that the pore-space is large enough to give him the best results, not only for the storage of moisture, but for the growth and development of roots, and for the entrance into the soil of air, germ life, and other forces that aid in making the soil fit for the habitation of plants.  This can always be best accomplished, as will be shown hereafter, by deep plowing, when the soil is not too wet, the exposure of the plowed soil to the elements, the frequent cultivation of the soil through the growing season, and the admixture of organic matter.  The natural soil structure at depths not reached by the plow evidently cannot be vitally changed by the farmer.

Hygroscopic soil-water

Under normal conditions, a certain amount of water is always found in all things occurring naturally, soils included.  Clinging to every tree, stone, or animal tissue is a small quantity of moisture varying with the temperature, the amount of water in the air, and with other well-known factors.  It is impossible to rid any natural substance wholly of water without heating it to a high temperature.  This water which, apparently, belongs to all natural objects is commonly called hygroscopic water.  Hilgard states that the soils of the arid regions contain, under a temperature of 15 deg C. and an atmosphere saturated with water, approximately 5-1/2 per cent of hygroscopic water.  In fact, however, the air over the arid region is far from being saturated with water and the temperature is even higher than 15 deg C., and the hygroscopic moisture actually found in the soils of the dry-farm territory is considerably smaller than the average above given.  Under the conditions prevailing in the Great Basin the hygroscopic water of soils varies from .75 per cent to 3-1/2 per cent; the average amount is not far from 12 per cent.

Whether or not the hygroscopic water of soils is of value in plant growth is a disputed question.  Hilgard believes that the hygroscopic moisture can be of considerable help in carrying plants through rainless summers, and further, that its presence prevents the heating of the soil particles to a point dangerous to plant roots.  Other authorities maintain earnestly that the hygroscopic soil-water is practically useless to plants.  Considering the fact that wilting occurs long before the hygroscopic water contained in the soil is reached, it is very unlikely that water so held is of any real benefit to plant growth.

Gravitational water

It often happens that a portion of the water in the soil is under the immediate influence of gravitation.  For instance, a stone which, normally, is covered with hygroscopic water is dipped into water The hydroscopic water is not thereby affected, but as the stone is drawn out of the water a good part of the water runs off.  This is gravitational water That is, the gravitational water of soils is that portion of the soil-water which filling the soil pores, flows downward through the soil under the influence of gravity.  When the soil pores are completely filled, the maximum amount of gravitational water is found there.  In ordinary dry-farm soils this total water capacity is between 35 and 40 per cent of the dry weight of soil.