Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

by Eugene W. Hilgard

How Nature Tills.—When we examine the undisturbed soil of woods or prairie in the humid region, we will as a rule find the natural surface soil in a very good condition of tilth; the obvious cause being the presence in it of an abundant network of surface roots and rootlets of grasses and herbs, which in connection with the fallen foliage prevent the beating and compacting of the soil surface; which can be seen to happen before the observer’s eyes whenever a heavy rain falls on a bare land surface, however well tilled.

Crusting of Soils.—In some soils, especially of the Gulf States, the beating of rain followed by warm sunshine so effectually compacts the surface that in the case of taprooted plants like cotton, it becomes necessary to cultivate after each rain, so as to break the crust that would otherwise not only prevent the proper circulation of air, but would also serve to waste the moisture of the land. The same land in the wild condition suffered no such change, being protected by the native vegetation, and by fallen leaves. ([See chapt. 8]).

Soils of the arid region.—In the regions of deficient rainfall the conditions are modified in several respects. Grass sward rarely exists, nearly all grasses assuming the habit of growing in tufts or bunches some distance (a foot or two) apart; hence the name of “bunch grass” commonly used, which however means not any one definite kind of grass, but serves to distinguish the grasses of the uplands from those of the moist lowlands, where true sward may be found. Between these bunches of grass the soil is fully exposed, and being free from roots and leaf-covering is compacted, unless its nature is such that the usually gentle rains do not produce a serious crusting of the surface.

That such is actually the predominant nature of the soils formed under arid influences has already been stated; and thus the hard-baked soil-surface so often seen in the Eastern United States in unplowed bare land, or during the prevalence of a drought, is rarely seen in the arid region. The clay lands that do exist are usually sufficiently calcareous to possess the property of “slaking” into crumbs whenever wetted after drying. But where this is not the case, the stony hardness brought about by the long dry and warm season is long in being removed by the winter rains.

Changes of soil-volume on wetting and drying.—The behavior of colloidal clay in the above respects has already been described above ([see chapt. 4, page 59]). It is obvious that whenever soils contain a large proportion of such clay, their behavior on wetting and drying will approximate to those of the pure clay. This is exemplified in the heavy clay, or so-called “prairie soils” of the United States, which when thoroughly wetted in spring will, during a dry summer, form wide, gaping cracks. These in the long summers of the arid region may extend to the depth of several feet, with a width of as much as three and more inches at the surface of the ground. This, of course, contributes greatly to the drying-out of the soil to the same depth, and results as well in the mechanical tearing of the root-system of growing plants; sometimes causing the total destruction of vegetation. In some clay soils it happens that after a rain or irrigation, the shrinkage occurring upon the advent of warm sunshine will cause the surface crust to so contract around the stem, e. g., of grain, as to constrict and injure the bark, causing serious injury to the crop. In soils of this character very thorough tillage in preparing for a crop, and the maintenance of a loose surface during its growth, are of course extremely essential.

In the arid region it will frequently happen that such soils when not tilled to a sufficient depth, will during the later part of the summer so shrink and crack beneath the shallow-tilled surface layer that the latter will bodily fall into the cracks, exposing the roots to all the deleterious influences of mechanical lesion and drying-out. It is thus obvious that the cultivation of such soils should not be undertaken at all by those not naturally able and willing to bestow upon them, to the fullest extent, the deep and thorough tillage which is absolutely essential in the utilization of their usually high productive power.

Extent of Shrinkage.—The extent of this shrinkage in drying, and subsequent expansion in wetting, have been measured by the writer by the use of the sieve cylinder described below ([chapt. 11, p. 209]), as serving for the determination of the water capacity of soils. When a soil of the kind above referred to is placed in the sieve cylinder in the tilled (flocculated) condition, then allowed to absorb its maximum of water and then dried at 100 degrees C., the contraction in drying can be very strikingly seen, and its amount measured by filling up the empty space with mercury; then measuring the latter after expelling the surplus by means of a ground glass plate laid on top. The contraction of several heavy clay soils, thus measured, has been found by the writer to range from 28 to as much as 40 per cent of the original bulk.[32] The soil thus contracted, when again wetted, does not return altogether to its original bulk, but remains in a more or less compacted condition, like that of a soil which has been rained upon.

The expansion and contraction of a heavy clay soil on wetting and drying are well illustrated in the [figure below], in which the soils are shown in the shallow cylinder which serves for the determination of water-holding power ([see chapt. 11, p. 209]). The middle figure shows in profile the expansion of a dry, pulverized “black adobe,” struck level, when allowed to absorb its maximum of water; it rises above the rim of the sieve-box to nearly the half height of the latter. The outside figure to the right shows the same soil after drying; that to the left, a red clay soil similarly treated. It is easily seen that these variations in volume may bring about very marked results in the fields; the surface of which, apart from the cracks usually formed, may be several inches lower in the dry season than during wet weather.

Red Clay Soil. Black “Adobe” Clay Soil.