BACTERIA OF THE SOIL 257 



form of some soluble compound and that the plant can 

 benefit by the presence of the bacteria without destroying 

 them. 



It already has been noted that there probably should be 

 recognized more than one species of the genus Bhizobium. 

 This is evidenced particularly when attempts are made to 

 inoculate legumes of one kind growing in sterile sand with 

 the organisms originating from the root of some other 

 species. In general leguminous species of plants which are 

 closely related botanically may be cross-inoculated readily. 

 For example, the various species of the genus Medicago, to 

 which the alfalfa belongs, readily cross-inoculate. Further- 

 more, bacteria from the roots of sweet clover belonging to 

 the closely related genus Melilotus also cross-inoculate 

 readily with alfalfa. Forms, howevei-, so distantly related 

 as soy beans, lupine, and the clovers seem quite impossible 

 of cross-inoculation. It is entirely probable that the organ- 

 isms producing the nodules on these three plants belong to 

 different species. 



It is evident from the preceding discussion that although 

 bacteria capable of causing root nodules to form on the roots 

 of leguminous plants are widely distributed and present in 

 most soils, it is nevertheless true that in many soils those 

 particular forms, possibly species, capable of inoculating 

 the roots of certain legumes may be absent. Care must be 

 used, however, to differentiate between the lack of organ- 

 isms capable of inoculating roots and physical or chemical 

 soil conditions unfavorable to the growth of plants. Many 

 of the clovers, for example, prefer to grow in a soil con- 

 taining a considerable quantity of lime, one which is, there- 

 fore, neutral or somewhat alkaline in reaction. With such 

 plants, lack of a good stand may be quite as apt to be due 

 to wrong soil conditions as to the lack of the proper inocu- 

 lating organism. Various methods of artificial introduction 

 of organisms capable of inoculating the roots of certain 



