MINERAL MATTEB FOR CORN LAND 



137 



98. Hopkins ' believes it fair to " assume for a rough 

 estimation that the equivalent of 2 per cent of the nitrogen, 

 1 per cent of the phosphorus, and J of 1 per cent of the total 

 potassium contained in the surface soil can be made avail- 

 able during one season by practical methods of farming." 



The above statement is borne out by results in many of 

 the prairie soils of Illinois. The amount of nitrogen, 

 phosphorus, and potassium in the surface, and the amount 

 available annually on the above basis, is shown by Hopkins 

 to be as follows : — 



TABLE XXVII 



Fertility in Illinois Soils and Amount Annually Avail- 

 able IN Pounds per Acre (roughly estimated) 



From the above tj^jical examples, it appears that many 

 of these soils do not meet the requirements of a 50-bushel 

 corn crop in all the three elements considered. 



1 Hopkins, C. 



2 Ibid., p. 82. 



G., I.e., p. 107. 



'Ibid., p. 110. 



