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 pulverized and altered rock. The forces that 



