414 MICROBIOLOGY OF SOIL 



uninoculated plots. After such favorable results, it was but a natural 

 step to try the effect of similar applications of soil rich in the nodule- 

 forming bacteria to ordinary cultivated soils of varying character. 

 While results in some cases were eminently satisfactory, in others there 

 was no increase in the vigor or amount of the crop as a result of the 

 inoculation. 



METHODS or SOIL INOCULATION. From these early experimental 

 results there evolved two general methods of inoculation, namely, the 

 application of soil from an already inoculated field, and the application 

 of pure cultures of the nodule-forming bacteria to the seed before 

 sowing. 



Inoculation with Legume-earth. The use of soil as inoculating 

 material -was tried by various experiment stations of the United States, 

 with results not varying widely from those secured in the pioneer 

 experimental work at Bremen. It was found in general that the 

 commonly grown crops, such as the common clovers, peas and beans, 

 made little or no increase as a result of inoculation with old legume-soil. 

 With new crops, however, such as alfalfa and soy beans when they were 

 first introduced, it was found impossible in many places to secure a 

 successful stand until the fields on which these crops were to be grown 

 had received a top-dressing of soil from land that had already grown 

 the crop in question; and it became a common practice to inoculate 

 soil in this manner before seeding with these new crops. It was early 

 observed, however, that this method of soil transfer for inoculation 

 purposes was not an unmixed benefit. Aside from the expense and 

 difficulty of handling and transportation of soil, fungus and bacterial 

 diseases, not only of legumes but of other crops, as well as the seeds 

 of noxious weeds, were transmitted from one field to another and even 

 from one section of country to another. It was to avoid this difficulty 

 that the preparation of pure cultures was introduced. 



Inoculation with Pure Cultures. Nitragin. The first pure culture 

 method was launched in 1896 by Nobbe and Hiltner, German investi- 

 gators, who prepared cultures of the legume bacteria on nutrient gelatin 

 and arranged with a firm of manufacturing chemists to place them on 

 the market under the trade name of 'Nitragin. 



Dried Cultures. In the United States the matter of pure cultures 

 was first taken up by the Department of Agriculture about 1902. 

 Cultures of the nodule-forming bacteria were cultivated in nitrogen- 



