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.