PHYSIOGRAPHIC FACTORS 155 



less of the origin or nature of the materials; likewise, that the 

 ultimate soil group for similar climatic regions will be the same. 



Since climatic conditions determine the activities and kinds of 

 organisms of a region and these organisms in turn contribute to 

 soil development, it is not surprising that vegetation types and soil 

 types are closely related. The development of a soil is paralleled 

 by vegetational changes, the vegetation contributing to soil ma- 

 turation and the soil controlling the rate of progressive succession 

 of plant communities, until a mature soil for a given climate sup- 

 ports a climax community of organisms. Mapping soils on the basis 

 of mature profile and mapping vegetation on the basis of climax 

 vegetation should produce closely similar results. 



The recognition of climatic soil types originated in Russia. The 

 approach is well illustrated by Glinka's (1927) grouping of the 

 great soil groups of the world primarily on a climatic basis. Ac- 

 ceptance of the idea has become rather general although sometimes 

 in modified form. The use of specific climatic factors, such as the 

 relationship between precipitation and evaporation, for delimiting 

 effective climate produces regions that correspond closely to the 

 major soil groups. 131 In the United States, 169, 17 ° soils are most 

 often grouped on the basis of mature profiles. Since only the ma- 

 ture profile is considered, it is a recognition of the same basic ap- 

 proach used by those determining regional limits through climate, 

 although it requires that the profile must exist in reality, not as a 

 potentiality. 



Profile Development —Three major processes of soil develop- 

 ment are concerned in the production of the profiles characteristic 

 of different climatic conditions. 



Podsolization occurs typically in humid, cold temperate regions 

 where rainfall exceeds evaporation and where vegetation produces 

 acid humus. The acid decomposition products from the litter in- 

 crease the solvent power of the plentiful percolating water so that 

 soluble materials and colloids are almost completely removed from 

 the surface soil, which is, therefore, of single grain structure at 

 maturity. Although podsolization occurs under hardwood and 

 pine forests, its strongest development takes place where spruce, 

 fir, or hemlock are dominant. The process is partially a product 

 of the vegetation, for the content of bases in the needles of these 



