72 SOILS: PMOPEBTIES AND MANAGEMENT 



however, the chemical forces are the determining factor 

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

 usually coarser ^oils 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 hmnid 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- 

 tain more of the important mineral plant-food elements. 

 The following analyses bring out the differences in a 

 striking manner : — 



Insoluble residue and soluble 



SiOa 



AlaOs 



FegOa 



P2O5 



CaO 



MgO 



NaaO 



xx2t) 



Water and ignition . . . 

 Humus 



Arid Soils 

 aveba-ge op 

 573 Samples^ 



Humid Soils 

 AvKEAQB or 

 690 Samples^ 



75.87 



88.21 



7.21 



3.66 



5.48 



3.88 



.16 



.12 



1.43 



.13 



1.27 



.29 



.35 



.14 



.67 



.21 



5.15 



4.40 



1.13 



1.22 



AVEEAGK 



Composition ob 

 Lithospheke2 



59.36 (SiOs) 

 14.81 

 6.34 

 .29 

 4.78 

 3.74 

 3.35 

 2.98 



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

 Weather Bur., BuL 3. 1892. 



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

 Survey, BuL 491, p. 33. 1911. 



