226 The National Geographic Magazine 



A Field of Vetch, Showing the Effect of Inoculation 



The plat on the left was not inoculated, while that on the right was inoculated. 



Even more startling than this picture is a comparison of the figures of yield of uninoculated 

 and inoculated plots. Two patches of hairy vetch grown side by side under precisely the same 

 conditions yielded crops as follows : Uninoculated patch, 581 pounds; inoculated patch, 4,501 

 pounds. Crimson clover under similar conditions yielded, uninoculated, 372 pounds ; inocu- 

 lated, 6,292. pounds. The difference in the amount of nitrogen obtained from the two crops was : 

 hairy vetch, uninoculated, 7 pounds; hairy vetch, inoculated, 105.5 pounds; crimson clover, 

 uninoculated, 4.3 pounds; crimson clover, inoculated, 143.7 pounds. 



portrayed the possibilities of such a ca- 

 tastrophe. This view of the situation 

 is greatly exaggerated, but the fact re- 

 mains, nevertheless, that the main reason 

 of once fertile lands becoming unpro- 

 ductive is loss of nitrogen in the soil. 



The difficulty has been to get the nitro- 

 gen back into the ground. Fertilizers 

 are expensive and not satisfactory ; but 

 there is an inexhaustible supply of free 

 nitrogen in the air if it could be captured. 



The problem of how to utilize this free 

 nitrogen has now been solved. 



It was discovered some time ago that 

 leguminous plants— clover, alfalfa, peas, 

 etc. — were able to put back nitrogen into 

 the soil and thus fertilize it. This is the 

 reason why a wheatfield after a crop of 

 alfalfa will yield a much heavier har- 

 vest. The plants absorb the free nitro- 

 gen by means of bacteria- tubercles grow- 

 ing on their roots, the tubercles varying 



