36 RURAL CALIFORNIA 



their total area is small compared with the residual 

 and glacial soils. 



"In California, as in all arid regions, the residual 

 soils available for agriculture are of relatively lim- 

 ited extent, forming about 10 per cent of the arable 

 lands of the state. They are found on hill slopes 

 and on mountain sides and their topographic posi- 

 tion makes irrigation exceedingly difficult or im- 

 possible, while the shallow soil mass makes dry 

 farming precarious. 



"By far the larger portion of the agricultural 

 lands in California are transported soils. For un- 

 counted ages the winter rains have been washing the 

 rock fragments from the mountain sides and carrying 

 the material out to the valleys, spreading the mass 

 out as broad sloping alluvial fans or as relatively 

 flat valley floor. The accumulation of sediments in 

 the valleys is often hundreds or even thousands of 

 feet deep. 



"Soils formed in this way may be quite uniform 

 to great depths or may be made up of successive 

 layers of varying texture, sands, silts, gravels, or 

 clays. As the soils are laid down a little at a time, 

 year after year, they have been acted upon by weath- 

 ering agencies breaking up the particles and making 

 the plant food quite available. Under the climatic 

 conditions that exist, with the hot dry summers and 

 the low rainfall in winter, the weathering action of 

 air and water, the beneficial action of bacteria and 

 the formation of humus in the soil, occur to con- 

 siderable depths and roots ordinarily penetrate to 



