CHAPTER IX 

 SOIL TYPES 



Ever since the mutual dependence of soil formation and plant com- 

 munities was first recognized and it was learned that soils as well as 

 plant communities are to be regarded as biological units, efforts have 

 been made to relate the units of vegetation with pedologically deter- 

 minable units, the "soil types." 



While the farmer considers only the superficial layers under the 

 term "soil," in soil science all of the living part of the earth's crust is 

 understood thereby as far as biological processes are demonstrable at 

 all. Below this is the mineral layer, the solid crust of the earth, an 

 inorganic dead mass. 



The soil profile tells the character of the soil. Only on the basis of 

 the profile can a scientifically accurate, natural classification of soil 

 types be made. Each soil type has certain morphological and physico- 

 chemical properties which render it suitable for definite plant com- 

 munities. The soil types distinguished to date, it must be admitted, 

 are described only in roughest outline, and their derivatives have been 

 studied only in part. Like ecological research, pedology has become an 

 independent science only in most recent times. 



Mineral Weathering. — In mineral weathering water initiates the 

 process by hydrolysis. Simultaneously, or perhaps slightly later, 

 oxidation joins in the process of weathering. The importance of 

 temperature in this process is shown by the work of Ramann. He 

 shows that the intensity of hydrolysis at 50° is about eight times that 

 occurring at 0°C. For this reason solid rock will show considerable 

 differences in the first stages of weathering in different climatic regions 

 (Gschwind, 1931). Chemical changes proceed very quickly and 

 thoroughly in the weathering of rocks rich in carbonate, while as Niggli 

 (1926, p. 337) has shown, the weathering products of the sihcates retain 

 the chemical nature of the parent rock for a longer time. Under these 

 conditions only acid humus seems to bring about a noticeable change 

 of the chemical nature of the soil. Of course, acid humus appears 

 invariably in humid, temperate, and cold climates, wherever there is 

 adequate plant growth. 



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