SOIL-FOEMING PROCESSES 



31 



been so active. Various colors may develop because o£ oxida- 

 tion, hydration, and the presence of organic matter. Such 

 soils usually are not excessively deep, and are likely to be 

 underlaid by subsoils heavier than the surface. The general 

 physical condition and tilth of arid soil is uniformly better 

 than that of regions of plentiful rainfall./ 



Chemically, because of less leaching, the arid soils contain 

 more of the important mineral elements. The following 

 analyses bring out the differences in a striking manner : 



Table II 



COMPABATIVE ANALYSES OE ARID AND HUMID SOILS^ 





Arid Soils 



Humid Soils 



Constituents 



Average op 



AVIBAGE OF 





313 Samples 



466 Samples 



Insoluble 



77.82 . 



88.24 =». 



AI2O3 



7.89 ' 



4.30 



Fe^Oj 



<l) * I D i> 



3.13 



CaO 



1.36 



.11 



K,0 



.73' 



.22 



P.O. 



.12, 



.11 



MgO 



1.41 



.23 



Volatile 



4.94 ^ 



3.64 



It is immediately apparent that the arid soil is poorer in 

 silica than the humid soil, but richer in iron and alumina, in- 

 dicating a less weathered condition of the feldspars. Due to 

 a greater amount of leaching, the humid soil is much lower 

 in phosphoric acid, lime, magnesia, and potash. The hu.mus 

 in arid soils is somewhat lower than in the soils under better 



^Hilgard, E. W., Die Boden ander %nd humider Lander; Internat 

 Mitt. Bodenkunde, Bd. I, pp. 415-529. 1912. 



