DRY-FARM SOILS 51 
that will be rationally adapted to the rainfall and 
other climatic factors. 
It is a matter of regret that so much of our infor- 
mation 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 Cali- 
fornia. Future students of arid soils must of neces- 
sity 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 pulverizéd and altered rock. The forces that 
