72 



SOILS: PROPERTIES AND MANAGEMENT 



however, the chemical forces are the determining factor 

 as to the character of the soil. Arid soils are therefore 

 usually coarser soils and their color is very likely to be 

 light. Such soils are deep and uniform, there being 

 but little difference between the surface and the subsoil. 

 The soils of the humid regions are usually of fine texture, 

 particularly in residual regions, since the chemical agencies 

 have been so active. Various colors may develop because 

 of oxidation, 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 con- 

 4ain_jriQre ol the i mportant -mineral plant-food elements. 

 The following analyses bring out the differences in a 

 striking manner : — 



Average 

 Composition of 

 l.ithosphere 1 



Insoluble residue and soluble 



Si0 2 



A1 2 3 



Fe 2 3 



P2O5 



CaO 



MgO 



Na 2 



K 2 



Water and ignition . . . 

 Humus 



59.36 (Si0 2 ) 

 14.81 

 6.34 

 .29 

 4.78 

 3.74 

 3.35 

 2.98 



1 Hilgard, E. W. The Relation of Soil to Climate. U. S. 

 Weather Bur., Bui. 3. 1892. 



2 Clarke, F. W. Data of Geochemistry. U. S. Geol. 

 Survey, Bui. 491, p. 33. 1911. 



