The structure of soils

Before examining more closely the possibility of storing water in soils a brief review of the structure of soils is desirable. As previously explained, soil is essentially a mixture of disintegrated rock and the decomposing remains of plants. The rock particles which constitute the major portion of soils vary greatly in size. The largest ones are often 500 times the sizes of the smallest. It would take 50 of the coarsest sand particles, and 25,000 of the finest silt particles, to form one lineal inch. The clay particles are often smaller and of such a nature that they cannot be accurately measured. The total number of soil particles in even a small quantity of cultivated soil is far beyond the ordinary limits of thought, ranging from 125,000 particles of coarse sand to 15,625,000,000,000 particles of the finest silt in one cubic inch. In other words, if all the particles in one cubic inch of soil consisting of fine silt were placed side by side, they would form a continuous chain over a thousand miles long. The farmer, when he tills the soil, deals with countless numbers of individual soil grains, far surpassing the understanding of the human mind. It is the immense number of constituent soil particles that gives to the soil many of its most valuable properties.

It must be remembered that no natural soil is made up of particles all of which are of the same size; all sizes, from the coarsest sand to the finest clay, are usually present. These particles of all sizes are not arranged in the soil in a regular, orderly way; they are not placed side by side with geometrical regularity; they are rather jumbled together in every possible way. The larger sand grains touch and form comparatively large interstitial spaces into which the finer silt and clay grains filter. Then, again, the clay particles, which have cementing properties, bind, as it were, one particle to another. A sand grain may have attached to it hundreds, or it may be thousands, of the smaller silt grains; or a regiment of smaller soil grains may themselves be clustered into one large grain by cementing power of the clay. Further, in the presence of lime and similar substances, these complex soil grains are grouped into yet larger and more complex groups. The beneficial effect of lime is usually due to this power of grouping untold numbers of soil particles into larger groups. When by correct soil culture the individual soil grains are thus grouped into large clusters, the soil is said to be in good tilth. Anything that tends to destroy these complex soil grains, as, for instance, plowing the soil when it is too wet, weakens the crop-producing power of the soil. This complexity of structure is one of the chief reasons for the difficulty of understanding clearly the physical laws governing soils.

Pore-space of soils

It follows from this description of soil structure that the soil grains do not fill the whole of the soil space. The tendency is rather to form clusters of soil grains which, though touching at many points, leave comparatively large empty spaces. This pore space in soils varies greatly, but with a maximum of about 55 per cent. In soils formed under arid conditions the percentage of pore-space is somewhere in the neighborhood of 50 per cent. There are some arid soils, notably gypsum soils, the particles of which are so uniform size that the pore-space is exceedingly small. Such soils are always difficult to prepare for agricultural purposes.

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