CYRIL GEORGE HOPKINS 



cannot afford to purchase commercial nitrogen for the pro- 

 duction of his common farm crops. Doctor Hopkins therefore 

 taught the necessity of depending upon legume nitrogen, which 

 is obtained by clovers, alfalfa, soybeans, etc., by the aid of 

 symbiotic bacteria from the inexhaustible supply in the air, 

 provided the soil conditions are favorable to their growth and 

 development. In the Illinois system of permanent soil fer- 

 tility he provided not only that a legume occur in each rota- 

 tion but that the legume hay or chaff produced be carefully 

 conserved and returned to the soil, either as farm manure or 

 green manure crops. Further, he urged that the utmost use 

 be made of legume cover crops grown in connection with the 

 production of wheat and other cereals. 



In the development of this use of legume cover crops the 

 research work of Doctor Hopkins is particularly outstanding. 

 Sweet clover was a favorite crop with him for this purpose and 

 he was among the first to call attention to its great possibilities. 



Unfortunately, legumes, so essential for soil improvement, 

 cannot be successfully grown on many soils in Illinois, as they 

 now exist, because of their acid condition, the acidity fre- 

 quently being so great as to prevent absolutely the growth 

 of legumes. Limestone is of fundamental importance in soil 

 fertility. That a limestone soil is a rich soil is an age-old 

 truth. Thruout the world soils that have become famous for 

 their persistent fertility are limestone soils. This is true of 

 the soils of the far western United States, of the blue-grass 

 regions of Kentucky, of the valley of the Nile, and of the 

 black soils of India and Russia. Unfortunately, limestone is 

 of all soil constituents probably the most readily lost in drain- 

 age water, and hence humid soils are as a rule deficient in 

 this essential constituent. The first principle of soil fertility 

 is that limestone must be added to those soils in which it is 

 not already present. While the limestone is added primarily 

 for the purpose of creating conditions favorable to the growth 

 of the necessary legume crops, it also has markedly favorable 



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