2i6 SYSTEMS OF PERMANENT AGRICULTURE 



nitrogen in the tops and 2 parts in the roots, thus indicating that 

 73 per cent of the nitrogen contained in the infected plants was 

 secured by the bacteria. The nitrogen in the dry matter of the 

 infected plants varied from 4.09 to 4.33 per cent in the tops, 

 from 1.45 to 1.53 per cent in the roots, and from 5.76 to 6.05 per 

 cent in the tubercles; while the nitrogen in the dry matter of 

 the noninfected plants varied from 2.32 to 2.69 per cent in the 

 tops, and amounted to .88 per cent (in each of three lots) in the 

 roots. From an experiment with soy beans by the Wisconsin 

 Station (Report for 1907), it is computed that only 14 per cent of 

 the nitrogen contained in well-infected plants was secured from the 

 air. The yield of dry matter was practically the same, but the 

 infected plants were richer in nitrogen and protein, and thus of 

 better quality. "The soy beans were grown on low, rich soil in 

 these experiments." 



The Michigan Station (Bulletin 224) reports data showing that 

 33 per cent of the nitrogen in soy beans was' secured by the bacteria, 

 on well-infected plants. 



As an average of 20 untreated plots in one test, and of 16 plots 

 treated with phosphorus and potassium in another test, both over a 

 period of 25 years, in a four-year rotation of corn, oats, wheat, and 

 hay (mixed clover and timothy), the Pennsylvania Experiment 

 Station reports the following yields in pounds per acre per annum : 



POUNDS PER ACRE 



If we compute the nitrogen in the three uncultivated crops (see 

 Table 23), adopt the estimate that the hay was three fourths clover 

 and one fourth timothy, and assume the soil nitrogen for the hay 

 crops to be as indicated by a curve projected from the amounts 

 furnished to the oats and wheat crops, then the clover must have 

 secured from the air 66 per cent of its nitrogen when grown on 



