Normal Soil and Its Requirements 19 



or swelling. Soon, however, it loses its parasitic 

 character and becomes an agent for fixing the free 

 nitrogen of the air, which is then stored up in the 

 root nodule. In this form the nitrogen is consumed 

 by the plant itself. As far as is known, P. radicicola 

 can thrive on the roots of legume plants only. The 

 Rhode Island Experiment Station^ has foimd that 

 an acre of soy beans for instance may fix about 

 1000 pounds of nitrogen from the air during a period 

 of five years, or 200 povinds per jrear. One htmdred 

 and forty pounds of the 200 were removed with the 

 crop, and 60 pounds remained in the field. Since 

 one pound of nitrogen was worth at least i6c., 200 

 pounds would cost $32. We must not, of course, 

 suppose that every acre of soy beans would produce 

 200 pounds of nitrogen every year. This would 

 depend somewhat on the nature of the soil, the degree 

 of moisture, the amoimt of oxygen, and other condi- : 

 tions congenial or unfavorable. What is certain, 

 however, is that every alert gardener and trucker 

 should learn to use legumes more extensively in his 

 system of cropping. 



Soils which have grown leguminous crops for a 

 period of years are well suppHed with P. radicicola. 

 Other soils are deficient in it and must be artificially 

 inoculated. The numerous types of pure cultures 

 of the organism sold in Hquid form have as a rule 

 proven a failure. The organism dies out or loses its 

 effectiveness in the artificial liquid media. The best 

 forms of pure cultures now used are those grown on 



' Rhode Island Agr. Expt. Sta. Bui. 147. 



