CHAPTER V

DRY-FARM SOILS

Important as is the rainfall in making dry-farming successful, it is not more so than the soils of the dry-farms. On a shallow soil, or on one penetrated with gravel streaks, crop failures are probable even under a large rainfall; but a deep soil of uniform texture, unbroken by gravel or hardpan, in which much water may be stored, and which furnishes also an abundance of feeding space for the roots, will yield large crops even under a very small rainfall. Likewise, an infertile soil, though it be deep, and under a large precipitation, cannot be depended on for good crops; but a fertile soil, though not quite so deep, nor under so large a rainfall, will almost invariably bring large crops to maturity.

A correct understanding of the soil, from the surface to a depth of ten feet, is almost indispensable before a safe Judgment can be pronounced upon the full dry-farm possibilities of a district. Especially is it necessary to know (a) the depth, (b) the uniformity of structure, and (c) the relative fertility of the soil, in order to plan an intelligent system of farming that will be rationally adapted to the rainfall and other climatic factors.

It is a matter of regret that so much of our information concerning the soils of the dry-farm territory of the United States and other countries has been obtained according to the methods and for the needs of humid countries, and that, therefore, the special knowledge of our arid and semiarid soils needed for the development of dry-farming is small and fragmentary. What is known to-day concerning the nature of arid soils and their relation to cultural processes under a scanty rainfall is due very largely to the extensive researches and voluminous writings of Dr. E. W. Hilgard, who for a generation was in charge of the agricultural work of the state of California. Future students of arid soils must of necessity rest their investigations upon the pioneer work done by Dr. Hilgard. The contents of this chapter are in a large part gathered from Hilgard's writings.

The formation of soils

"Soil is the more or less loose and friable material in which, by means of their roots, plants may or do find a foothold and nourishment, as well as other conditions of growth." Soil is formed by a complex process, broadly known as _weathering, _from the rocks which constitute the earth's crust. Soil is in fact only pulverized and altered rock. The forces that produce soil from rocks are of two distinct classes, _physical and chemical. _The physical agencies of soil production merely cause a pulverization of the rock; the chemical agencies, on the other hand, so thoroughly change the essential nature of the soil particles that they are no longer like the rock from which they were formed.

Of the physical agencies, _temperature changes _are first in order of time, and perhaps of first importance. As the heat of the day increases, the rock expands, and as the cold night approaches, contracts. This alternate expansion and contraction, in time, cracks the surfaces of the rocks. Into the tiny crevices thus formed water enters from the falling snow or rain. When winter comes, the water in these cracks freezes to ice, and in so doing expands and widens each of the cracks. As these processes are repeated from day to day, from year to year, and from generation to generation, the surfaces of the rocks crumble. The smaller rocks so formed are acted upon by the same agencies, in the same manner, and thus the process of pulverization goes on.

It is clear, then, that the second great agency of soil formation, which always acts in conjunction with temperature changes, is _freezing water. _The rock particles formed in this manner are often washed down into the mountain valleys, there caught by great rivers, ground into finer dust, and at length deposited in the lower valleys. _Moving water _thus becomes another physical agency of soil production. Most of the soils covering the great dry-farm territory of the United States and other countries have been formed in this way.

In places, glaciers moving slowly down the canons crush and grind into powder the rock over which they pass and deposit it lower down as soils. In other places, where strong winds blow with frequent regularity, sharp soil grains are picked up by the air and hurled against the rocks, which, under this action, are carved into fantastic forms. In still other places, the strong winds carry soil over long distances to be mixed with other soils. Finally, on the seashore the great waves dashing against the rocks of the coast line, and rolling the mass of pebbles back and forth, break and pulverize the rock until soil is formed._ Glaciers, winds, _and _waves _are also, therefore, physical agencies of soil formation.

It may be noted that the result of the action of all these agencies is to form a rock powder, each particle of which preserves the composition that it had while it was a constituent part of the rock. It may further be noted that the chief of these soil-forming agencies act more vigorously in arid than in humid sections. Under the cloudless sky and dry atmosphere of regions of limited rainfall, the daily and seasonal temperature changes are much greater than in sections of greater rainfall. Consequently the pulverization of rocks goes on most rapidly in dry-farm districts. Constant heavy winds, which as soil formers are second only to temperature changes and freezing water, are also usually more common in arid than in humid countries. This is strikingly shown, for instance, on the Colorado desert and the Great Plains.

The rock powder formed by the processes above described is continually being acted upon by agencies, the effect of which is to change its chemical composition. Chief of these agencies is _water, _which exerts a solvent action on all known substances. Pure water exerts a strong solvent action, but when it has been rendered impure by a variety of substances, naturally occurring, its solvent action is greatly increased.

The most effective water impurity, considering soil formation, is the gas, _carbon dioxid. _This gas is formed whenever plant or animal substances decay, and is therefore found, normally, in the atmosphere and in soils. Rains or flowing water gather the carbon dioxid from the atmosphere and the soil; few natural waters are free from it. The hardest rock particles are disintegrated by carbonated water, while limestones, or rocks containing lime, are readily dissolved.

The result of the action of carbonated water upon soil particles is to render soluble, and therefore more available to plants, many of the important plant-foods. In this way the action of water, holding in solution carbon dioxid and other substances, tends to make the soil more fertile.

The second great chemical agency of soil formation is the oxygen of the air. Oxidation is a process of more or less rapid burning, which tends to accelerate the disintegration of rocks.

Finally, the _plants _growing in soils are powerful agents of soil formation. First, the roots forcing their way into the soil exert a strong pressure which helps to pulverize the soil grains; secondly, the acids of the plant roots actually dissolve the soil, and third, in the mass of decaying plants, substances are formed, among them carbon dioxid, that have the power of making soils more soluble.

It may be noted that moisture, carbon dioxid, and vegetation, the three chief agents inducing chemical changes in soils, are most active in humid districts. While, therefore, the physical agencies of soil formation are most active in arid climates, the same cannot be said of the chemical agencies. However, whether in arid or humid climates, the processes of soil formation, above outlined, are essentially those of the "fallow" or resting-period given to dry-farm lands. The fallow lasts for a few months or a year, while the process of soil formation is always going on and has gone on for ages; the result, in quality though not in quantity, is the same--the rock particles are pulverized and the plant-foods are liberated. It must be remembered in this connection that climatic differences may and usually do influence materially the character of soils formed from one and the same kind of rock.

Characteristics of arid soils

The net result of the processes above described Is a rock powder containing a great variety of sizes of soil grains intermingled with clay. The larger soil grains are called sand; the smaller, silt, and those that are so small that they do not settle from quiet water after 24 hours are known as clay.

Clay differs materially from sand and silt, not only in size of particles, but also in properties and formation. It is said that clay particles reach a degree of fineness equal to 1/2500 of an inch. Clay itself, when wet and kneaded, becomes plastic and adhesive and is thus easily distinguished from sand. Because of these properties, clay is of great value in holding together the larger soil grains in relatively large aggregates which give soils the desired degree of filth. Moreover, clay is very retentive of water, gases, and soluble plant-foods, which are important factors in successful agriculture. Soils, in fact, are classified according to the amount of clay that they contain. Hilgard suggests the following classification:--

Very sandy soils 0.5 to 3 per cent clay Ordinary sandy soils 3.0 to 10 per cent clay Sandy loams 10.0 to 15 per cent clay Clay loams 15.0 to 25 per cent clay Clay soils 25.0 to 35 per cent clay Heavy clay soils 35.0 per cent and over

Clay may be formed from any rock containing some form of _combined silica _(quartz). Thus, granites and crystalline rocks generally, volcanic rocks, and shales will produce clay if subjected to the proper climatic conditions. In the formation of clay, the extremely fine soil particles are attacked by the soil water and subjected to deep-going chemical changes. In fact, clay represents the most finely pulverized and most highly decomposed and hence in a measure the most valuable portion of the soil. In the formation of clay, water is the most active agent, and under humid conditions its formation is most rapid.