132 THE BOOK OF WHEAT 



Bacteria providing nitrogenous food for plants seem to be 

 of three classes. One of these works on the nitrogen con- 

 tained by the soil humus, and comprises three genera, each of 

 which has an essential function in reducing nitrogen to a 

 form available as plant food. Another class develops symbi- 

 otically with the growing plants, swarming in colonies upon the 

 rootlets. Their vital activity oxidizes atmospheric nitrogen. 

 The third class apparently secures the same result without 

 symbiosis. 



Efforts were made to inoculate soils with artificial pure cul- 

 tures of the third class and thus increase the nitrogen content 

 without the aid of manure or mineral fertilizer. While some 

 very successful experiments were made, the percentage of fail- 

 ures was too great for practical purposes. The root tubercle 

 bacteria seem to give the greatest promise of success. All of 

 those which have yet had any practical importance were found 

 exclusively on the roots of legumes. Some cultures of these 

 organisms, known as nitragin, were placed upon the market a 

 few years ago by German experimenters. They were adapted 

 to specific crops only, for it was claimed that each kind of 

 leguminous plant had a special germ which was more success- 

 ful upon it than any other form. There were so many fail- 

 ures that the manufacture of nitragin was abandoned. 



Previous to 1902 the United States department of agricul- 

 ture inaugurated extensive practical experiments in an effort 

 to find improved methods of soil inoculation. The reasons for 

 the failure of the German pure culture method were worked 

 out. Improved ways of handling and preserving pure cultures 

 were discovered, as well as means of rapidly and enormously 

 increasing them after they were received by the farmer. 

 Great progress was made toward developing an organism effect- 

 ive for all legumes, and the virility of the bacteria was so in- 

 creased that they fixed over five times as much nitrogen as 

 formerly. When the department could send in perfect condi- 

 tion to any part of the United States "a, dry culture, similar 

 to a yeast cake and no larger in size," the nitrogen-fixing 

 bacteria of which could be "multiplied sufficiently to inoculate 

 at least an acre of land," the prospects of an early and com- 

 plete solution of the nitrogen problem seemed to have a rosy 

 hue indeed. 



