146 THE STUDY OF PLANT COMMUNITIES ' Chapter VII 



termine whether the soil solution will be alkaline or acid in reac- 

 tion. 



These and other chemical processes operating more or less con- 

 tinuously, together with physical processes, constitute weathering, 

 which produces soil material that retains few characteristics of 

 parent rock. However, soil is not a product of these processes 

 alone, for biological activity also contributes to its formation. Or- 

 ganic material is an essential part of soil, and its decomposition and 

 incorporation are accomplished largely by microorganisms, whose 



Fig. 75. A soil well that illustrates a soil profile (White Store sandy loam) 

 in which the A horizon is very thin, the sandy gray-white A horizon is 

 sharply distinguished from the plastic red clay of the B horizon, and the 

 rocky C horizon shows in the bottom— Photo by C. F. Korstian. 



