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