52 



jenny. ORIGIN OF SOILS 



[Ch. 2 



greater than 2.0; in the southern portion it is considerably below 2.0. 

 A few decades ago the reaction against geologic concepts in soil 

 formation was extreme, and many attempts were made to classify soils 

 solely according to climate. Although such systems have many at- 

 tractive features, the oversimplification 

 leads to gross misrepresentations. The 

 interplay of all soil-forming factors can- 

 not be ignored. 



Biotic Factor (Biofunctions and Bio- 

 sequences) 

 In soil investigations, the biotic factor 

 is usually restricted to aspects of vegeta- 

 tion. The vegetational factor refers to 

 kinds of species of plants (flora) and not 

 to the abundance and yields of plants. 

 The latter aspect is a dependent variable, 

 being itself conditioned by soil and en- 

 vironment. 



In nature it is difficult to evaluate the 

 role of vegetation on soil formation. It 

 is necessary to locate soils which carry 

 different kinds of plants, but which, at 

 the same time, have identical conditions 

 of climate, parent material, topography, 

 and time. An extensive region which sat- 

 isfies these conditions is found in the Middle West, more specifically, 

 the prairie-timber transition zone. Accurate comparison of prairie 

 and forest soils shows that forests tend to accelerate soil formation. 

 The soils are more acid, lime is leached to greater depth, and trans- 

 location of clay is enhanced. 



10 12 14 16 



Annual temperature 



in degrees C. 



18 



Fig. 9. The average clay 

 content of soils derived from 

 basic igneous rock increases 

 from north to south (eastern 

 United States). 



PROCESSES OF SOIL FORMATION 



The functional relationships between soil properties and soil-form- 

 ing factors hitherto discussed are formalistic in nature. They record 

 observed dependencies among variables. They are not concerned with 

 mechanisms of soil formation, and they are not based on physical, 

 chemical, or biological theories. 



In contrast, the elucidation of processes of soil formation requires 

 the application of knowledge and concepts developed by the basic 

 sciences. 



