220 SYSTEMS OF PERMANENT AGRICULTURE 



to 20 inches in depth. The investigation was made when the sweet 

 clover was full grown and nearly mature. The crop was started 

 the previous season, sweet clover being a biennial plant. 



TABLE 34. ILLINOIS INVESTIGATIONS OF SWEET CLOVER (MELLILOTUS ALBA) 



It will be seen that the yield of sweet clover is very large, amount- 

 ing to 6.4 tons of total dry matter, of which, however, the roots 

 contain only 1.2 tons per acre, or less than one fifth of the total. 

 The tops of sweet clover are nearly as rich in nitrogen as full- 

 grown red clover (40 pounds per ton) , but the roots contain only 

 one seventh, or 14 per cent, of the total nitrogen. Nearly 24 per 

 cent of the total nitrogen was found in the roots, stubble, and sur- 

 face residues (largely of the previous season's growth). 



The sweet clover used in the investigation was well infected; 

 but, in a previous experiment on the same soil (brown silt loam 

 prairie of the early Wisconsin glaciation), it was found that the 

 yield of sweet clover was almost exactly doubled by thorough 

 inoculation, and the percentage of nitrogen in the infected plants 

 was also about one half more than in the noninfected plants, 

 showing that on this soil about two thirds of the nitrogen required 

 for this large crop was secured from the air. 



While sweet clover makes a fair quality of hay, if cut sufficiently 

 early in its growth, and is also used for pasture with some success 



