24 



interior states than to the coast and valley areas of our own state. 

 In this region there is a modification of low temperatures from the 

 north to the south, for the mountain region is more open to the 

 influence of north and south latitude and is not so fully dominated 

 by local topography and ocean influences, which give to the rest of 

 the state its unique climatic characters. 



CHARACTERISTICS OF CALIFORNIA SOILS* 



In the humid regions of the world, and especially in the humid 

 region of the United States, practically nine-tenths of the soils are 

 either of residual or of glacial origin. The glacial soils have been 

 transported and deposited by ice, and while the glacial deposits may 

 be very deep, the true soil is not deep. The subsoil is usually heavier 

 than the surface, often clayey, and the practical feeding depth of 

 roots is usually less than four feet. The residual soils are much more 



extensive than the glacial soils. 

 They are formed by the destruction 

 of rock masses, the disintegrated 

 and decomposed fragments accumu- 

 lating on the surface of the hard 

 rock to form the soil mass. Resi- 

 dual soils usually have a surface 

 soil six or eight inches deep, resting 

 on heavier material that grades to 

 a clay at two or three feet in depth. 

 At greater depths rock fragments 

 are found in the clay and these 

 grow more numerous until the mass 

 is largely broken or * ' rotten rock, ' ' 

 and finally the solid rock mass is 

 reached. (See Fig. 2.) The total 



LOAMY 

 SOIL 



CLAYEY 



SUBSOIL 



DECOflPOSED 

 ROCK 



ROCK 



Fig. 2. Section of a typical resi- 

 dual soil showing gradation from 

 soil through clay, rotten rock to 

 solid rock. 



depth of the soil mass above the rock varies greatly, but usually is less 

 than four feet. 



In the humid regions, the transplanted soils, other than those 

 formed by glacial action, are of little extent. The flood plains and 

 bench lands along the rivers and creeks are exceedingly productive, but 

 their total area is small compared with the residual and glacial soils. 



In California, as in all sub-humid regions, the residual soils avail- 

 able for agriculture are of limited extent. They are found on hill slopes 

 and on mountain sides and their topographic position makes irrigation 

 exceedingly difficult or impossible, while the shallow soil mass makes 



* By CHARLES F. SHAW, Professor of Soil Technology. 



