44 



HOW ROCKS AND CLIMATE AFFECT SOILS 



these crusts are white the name ^^ white alkah'^ is used, and ^' black 

 alkaU^' when the crusts are brown or black. ^' Black alkali'' dis- 

 solves organic matter; hence the black crusts. 



Alkali Spots. — In sub-humid and humid climates small areas 

 of alkali soils may be found varying in extent from a few square 

 rods to several acres. These areas are usually depressions or areas 

 kept wet by seepage water. In either case there has been an 

 accumulation of salts in these areas, the quantity is usually not 

 sufficient to prevent entirely the growth of crops; though in some 

 cases much injury does result. 



Fig. 18, — An alkali area in Western United States. Alkali soils like these do not 

 form in humid regions, because the salts which are formed as a result of rock weathering 

 are dissolved and carried away. (U. S. Dept. of Agriculture.) 



■ > 



Other Effects of Climate.— Soils in arid ^ and semi-arid climates 

 are usually coarser textured and generally lighter in color than 

 those of humid regions. 



Arid soils are deep and uniform with but little difference in 

 texture between soil and subsoil; while those of humid or sub- 

 humid regions are generally of fine texture and have subsoils which 

 contain more clav than their surface soils. 



^ Sections receiving less than 10 inches of rainfall annually are designated 

 "arid," and ''semi-arid'' when the annual rainfall is between 10 and 20 inches. 



