46 



jenny. ORIGIN OF SOILS 



[Ch. 2 



Parent Material (Lithofunctions and Lithosequences) 



In the time functions in Fig. 2, the left end of each curve shows the 

 value of the soil property CaC0 3 at the beginning of soil formation. 

 The soil at zero time is designated as parent material. It may repre- 

 sent consolidated or unconsolidated rock in the broadest sense of the 

 word. 



Contrasting soil formation on granitic rocks with soil formation on 

 basaltic rocks has little pedologic significance unless the magnitudes 

 of the remaining soil-forming factors are indicated. Theoretically, the 

 role of different parent materials in soil formation can be assessed only 

 if cl, o, r, t are either constant or ineffective. 



An illustration of a parent material function or lithofunction is given 

 in Table 1. It refers to soils derived from Winona glacial till in north- 



TABLE 1 



Lithofunctions op Glacial Soils in Illinois 



(Kellogg et al, 1949) 



eastern Illinois. It is proper to assume that cl, o, r, t are the same for 

 all profiles. The higher the clay content of the till, the higher is the 

 amount of clay in the B horizon of the derived soil. The Winona glacial 

 tills also vary in their content of calcium carbonate (CaC0 3 ). Dur- 

 ing soil formation, calcium carbonate is leached from the surface into 

 lower strata. The depth of leaching is controlled by the clay content 

 of the till. Its influence overshadows the inverse relationship of the 

 carbonate content of the till. 



