The water that falls upon dry-farms is very seldom sufficient in quantity to reach the standing water-table, and it is necessary, therefore, to determine the largest percentage of water that a soil can hold under the influence of gravity down to a depth of 8 or 10 feet—the depth to which the roots penetrate and in which root action is distinctly felt. This is somewhat difficult to determine because the many conflicting factors acting upon the soil-water are seldom in equilibrium. Moreover, a considerable time must usually elapse before the rain-water is thoroughly distributed throughout the soil. For instance, in sandy soils, the downward descent of water is very rapid; in clay soils, where the preponderance of fine particles makes minute soil pores, there is considerable hindrance to the descent of water, and it may take weeks or months for equilibrium to be established. It is believed that in a dry-farm district, where the major part of the precipitation comes during winter, the early springtime, before the spring rains come, is the best time for determining the maximum water capacity of a soil. At that season the water-dissipating influences, such as sunshine and high temperature, are at a minimum, and a sufficient time has elapsed to permit the rains of fall and winter to distribute themselves uniformly throughout the soil. In districts of high summer precipitation, the late fall after a fallow season will probably be the best time for the determination of the field-water capacity.
Experiments on this subject have been conducted at the Utah Station. As a result of several thousand trials it was found that, in the spring, a uniform, sandy loam soil of true arid properties contained, from year to year, an average of nearly 16-1/2 per cent of water to a depth of 8 feet. This appeared to be practically the maximum water capacity of that soil under field conditions, and it may be called the field capacity of that soil for capillary water. Other experiments on dry-farms showed the field capacity of a clay soil to a depth of 8 feet to be 19 per cent; of a clay loam, to be 18 per cent; of a loam, 17 per cent; of another loam somewhat more sandy, 16 per cent; of a sandy loam, 14-1/2 per cent; and of a very sandy loam, 14 per cent. Leather found that in the calcareous arid soil of India the upper 5 feet contained 18 per cent of water at the close of the wet season.
It may be concluded, therefore, that the field-water capacities of ordinary dry-farm soils are not very high, ranging from 15 to 20 per cent, with an average for ordinary dry-farm soils in the neighborhood of 16 or 17 per cent. Expressed in another way this means that a layer of water from 2 to 3 inches deep can be stored in the soil to a depth of 12 inches. Sandy soils will hold less water than clayey ones. It must not be forgotten that in the dry-farm region are numerous types of soils, among them some consisting chiefly of very fine soil grains and which would; consequently, possess field-water capacities above the average here stated. The first endeavor of the dry-farmer should be to have the soil filled to its full field-water capacity before a crop is planted.
Downward movement of soil-moisture
One of the chief considerations in a discussion of the storing of water in soils is the depth to which water may move under ordinary dry-farm conditions. In humid regions, where the water table is near the surface and where the rainfall is very abundant, no question has been raised concerning the possibility of the descent of water through the soil to the standing water. Considerable objection, however, has been offered to the doctrine that the rainfall of arid districts penetrates the soil to any great extent. Numerous writers on the subject intimate that the rainfall under dry-farm conditions reaches at the best the upper 3 or 4 feet of soil. This cannot be true, for the deep rich soils of the arid region, which never have been disturbed by the husbandman, are moist to very great depths. In the deserts of the Great Basin, where vegetation is very scanty, soil borings made almost anywhere will reveal the fact that moisture exists in considerable quantities to the full depth of the ordinary soil auger, usually 10 feet. The same is true for practically every district of the arid region.
Such water has not come from below, for in the majority of cases the standing water is 50 to 500 feet below the surface. Whitney made this observation many years ago and reported it as a striking feature of agriculture in arid regions, worthy of serious consideration. Investigations made at the Utah Station have shown that undisturbed soils within the Great Basin frequently contain, to a depth of 10 feet, an amount of water equivalent to 2 or 3 years of the rainfall which normally occurs in that locality. These quantities of water could not be found in such soils, unless, under arid conditions, water has the power to move downward to considerably greater depths than is usually believed by dry-farmers.
In a series of irrigation experiments conducted at the Utah Station it was demonstrated that on a loam soil, within a few hours after an irrigation, some of the water applied had reached the eighth foot, or at least had increased the percentage of water in the eighth foot. In soil that was already well filled with water, the addition of water was felt distinctly to the full depth of 8 feet. Moreover, it was observed in these experiments that even very small rains caused moisture changes to considerable depths a few hours after the rain was over. For instance, 0.14 of an inch of rainfall was felt to a depth of 2 feet within 3 hours; 0.93 of an inch was felt to a depth of 3 feet within the same period.
To determine whether or not the natural winter precipitation, upon which the crops of a large portion of the dry-farm territory depend, penetrates the soil to any great depth a series of tests were undertaken. At the close of the harvest in August or September the soil was carefully sampled to a depth of 8 feet, and in the following spring similar samples were taken on the same soils to the same depth. In every case, it was found that the winter precipitation had caused moisture changes to the full depth reached by the soil auger. Moreover, these changes were so great as to lead the investigators to believe that moisture changes had occurred to greater depths.
In districts where the major part of the precipitation occurs during the summer the same law is undoubtedly in operation; but, since evaporation is most active in the summer, it is probable that a smaller proportion reaches the greater soil depths. In the Great Plains district, therefore, greater care will have to be exercised during the summer in securing proper water storage than in the Great Basin, for instance. The principle is, nevertheless, the same. Burr, working under Great Plains conditions in Nebraska, has shown that the spring and summer rains penetrate the soil to the depth of 6 feet, the average depth of the borings, and that it undoubtedly affects the soil-moisture to the depth of 10 feet. In general, the dry-farmer may safely accept the doctrine that the water that falls upon his land penetrates the soil far beyond the immediate reach of the sun, though not so far away that plant roots cannot make use of it.
Importance of a moist subsoil