12 



NATURE AND PROPERTIES OF SOILS 



commonest sulfide of this element. Although sulfur is no 

 more abundant in the average soil than phosphorus, it is 

 generally not considered as an extremely important fertilizing 

 constituent. 



It is interesting to note at this point the amounts of the 

 above elements in ordinary mineral soils. Generally the nitro- 



% 



5% 10% i$% 



Si0 2 



80.ll 



r« z 5 +Al 2 5 — 9.0l 



Na 2 2.0H 



K 2 1.5  



CaO .61 



M9O 5* 



P 2 5 .11 



SO3 .11 



M Z\ 



organic- 



fig. 2. — Chemical composition of a representative productive soil. 



gen (N) may range from .1 to .2 per cent., the phosphoric 

 acid (expressed as P 2 5 ) from .05 to .30 per cent, and the 

 potash (expressed as K 2 0) from 0.5 to 2.0 per cent. Of the 

 plant nutrients in the soil nitrogen, although usually present 

 in small quantities, is relatively more available than is 

 phosphoric acid or even potash. Phosphoric acid may be in 

 the minimum because of its unavailability as well as because 

 of the small quantity. Potash is commonly present in rela- 



