84 The Substratum 



face. The circulation of soil air provides for the transport of oxygen 

 and carbon dioxide necessary for or resulting from the metabolism 

 of the plant roots, soil animals, and microorganisms. 



The diagram also indicates the photosynthesis of the vegetation and 

 the transfer of organic substances formed by the foliage to the roots 

 deep in the soil. Organic compounds in great variety are elaborated 

 by the growth of associated plants and animals. As described earlier, 

 the deciduous parts of plants, the excrement of animals, and finally 

 the bodies of all these organisms, when they die, are added to the 

 soil. These organic remains accumulate on the surface, or are carried 

 to deeper levels by the soil fauna, where they begin the slow process 

 of decomposition. 



This interplay of biological and climatic agents working on the 

 parent material results in the production of the soil profile as indi- 

 cated in the diagram. The A horizon is the scene of the major 

 biological activity, and here organic matter chiefly accumulates. 

 Rain entering this layer and percolating through tends to leach out 

 the soluble salts. The A horizon is usually darker in color and lighter 

 in texture than the B horizon below it, from which it is often sharply 

 distinguished. The B horizon is characterized by less intense biologi- 

 cal activity. Fine particles tend to accumulate here and mineral 

 salts are often concentrated in this layer. The material of the B 

 horizon is usually bright in color and densely compact. The B hori- 

 zon tends to grade rather indistinguishably into the C horizon below 

 it where there is little or no biological activity. As the soil matures, 

 the A and B horizons tend to deepen until equilibrium conditions are 

 reached. 



The exact nature of the mature soil profile depends upon the par- 

 ticular balance reached between the climatic factors, the biological 

 agents, the contour of the land, and the type of the parent rock in 

 the given area. The many possible variations and complications 

 should be borne in mind in a consideration of the simplified ex- 

 ample outlined above. It should be amply clear, however, that the 

 condition of the soil is a result of the elaborate interaction between 

 organisms and their environment. The dynamic viewpoint of the 

 modern ecologist is, therefore, especially appropriate in the further 

 investigation of the problems of the soil that have been merely touched 

 upon here. As one further illustration let us consider briefly the in- 

 teraction between the vegetation and the substratum that underlies 

 the differentiation of two great categories of soils. 



Soil-Group Divisions. Soils are classified into groups, and in the 

 United States soil groups fall into two great divisions: the pedocals 



