Ch. 2] 



CLIMATE 



51 



Whereas the declining content of calcium must be interpreted as a 

 result of leaching, the increasing nitrogen content must be attributed 

 to the greater production of vegetation organic matter, especially 

 roots, as a consequence of increased precipitation. 



The Great Plains area also lends itself to the evaluation of s-T 

 functions. Figure 8 shows the decrease of the total nitrogen content of 

 the surface soils with increasing annual temperature. The samples 



0.4 



0.3 



$0.2 



0.1 







40 



• Bottom-land soils 

 o Terrace soils 



Iowa 



Missouri i Arkansas 



70 



Fig. 8. 



50 60 



Annual temperature in degrees F. 



Soil nitrogen (and organic matter) temperature relationships in the 

 central United States. 



were collected along a transect having fairly uniform annual moisture 

 conditions and extending from Canada to the Gulf of Mexico. 



In the Appalachian mountain chain of the eastern United States, 

 relationships between average clay content of soils (to a depth of 

 40 inches) and latitude (or annual temperature) have been reported 

 (Jenny, 1941). Figure 9 illustrates the increase in soil clay with ris- 

 ing annual temperatures for soils derived from basic igneous rocks, 

 mainly diorite and gabbro. This transect extends from southern New 

 Jersey to Georgia. The annual rainfall varies from about 40 to 50 

 inches. Each dot represents one soil profile. 



Not only does the amount of clay vary with temperature, but so 

 does the chemical composition of the clay. In the northern portion of 

 the above-mentioned belt, the silica-alumina ratio of the soil clay is 



