CLIMATE AND CHEMICAL COMPOSITION OF SOIL 7 



the types and amount of vegetation, and the kinds of residues 

 resulting and accumulating in the soil. In general, the cooler the 

 temperature and the lower the precipitation, the slower the trans- 

 formation of the soil materials. Table 5 presents certain differ- 

 ences obtained by the determination of the average composition 

 of 466 soils in humid regions of the southern portion of the United 

 States as compared with the composition of 313 soils from arid 

 areas within the states of California, Washington and Montana. 



TABLE 5 



Comparative Chemical Composition of Soils of Humid and 

 Arid Regions (from Clarke) 



Humid Soil 



Arid Soil 



Insoluble in HCl 



Soluble SiOa 



AI2O3 



FejOg 



Mn304 



MgO 



CaO 



NazO 



K2O 



P2O5 



SO3 



Water and organic matter 



Totals 



Per Cent 

 84.03 

 4.21 

 4.30 

 3.13 

 0.13 

 0.23 

 0.11 

 0.09 

 0.22 

 0.11 

 0.05 

 3.64 



100.25 



Per Cent 

 70.57 

 7.27 

 7.89 

 5.75 

 0.06 

 1.41 

 1.36 

 0.26 

 0.73 

 0.12 

 0.04 

 4.95 



100.41 



It is quite apparent that arid conditions are conducive to the 

 accumulation of much more soluble substances than the humid 

 environment. These arid soils are frequently very fertile and owe 

 their scant plant growth entirely to the small amounts of water 

 which reach them. It requires only the introduction of moisture 

 to transform them from dry, brown deserts to flowering gardens. 



The mineral compounds in the rocks undergo modifications 

 which result in the solution and fragmentation of the large aggre- 

 gates. Many of the more soluble substances initially present in 

 the parent materials, such as chlorides and sulfates of the alkalies, 

 are removed from the soil quite rapidly in humid regions and 



