50 



jenny. ORIGIN OF SOILS 



[Ch. 2 



ceeds from the semi-arid to the semi-humid zone, soil calcium declines 

 in exponential fashion. 



Figure 7 shows the trend of total soil nitrogen along the 11° C. an- 



3.0 



I I I II I 



18 20 



22 24 26 28 30 

 Rainfall in inches 



Fig. 6. Variation of calcium content of soil with rainfall in Nebraska. The total 

 CaO content is the sum of the acid-soluble and acid-insoluble CaO content. 



nual isotherm which lies slightly south of Alway's regions. Each dot 

 represents the total nitrogen content of a soil sample taken to a depth 

 of 10 inches. As mean annual rainfall increases, soil nitrogen also in- 

 creases. Since soil nitrogen and organic carbon are closely related, 



0.25 



0.20 



2 0.15 



c 

 a> 



1 0.1.0 

 0.05 



J L 



J L 



10 20 30 



Rainfall in inches 



40 



25 50 75 



Rainfall in cm. 



100 



Fig. 7. Increase in soil nitrogen (and organic matter) with increasing mean 

 annual rainfall in the Great Plains area. 



the curve also indicates the trend of soil humus. Multiplying the ni 

 trogen percentage by 20 gives the approximate humus percentage of 

 the soils in this region. 



