ICir. 120— A.] 



TESTING CULTURES OF NODULE-FORMING BACTERIA/ 



By Kakl F. Kellkrman, ' Phys;i()I(jffiiit in Charge of ISuil-Bacterioloyy and Plant- 



Nutrition Inrrnliyations. 



INTRODUCTION. 



The importance of le<iiiininoii.s crops in maintaining soil fertility 

 or in rejnvenating overcropped and worn-out fields has long been 

 recognized by practical farmers. Critical investigations carried 

 on both in the laboratory and in the field have shown that this 

 method of improving soil conditions is dependent chiefly upon the 

 simultaneous development of a leguminous crop and the variety of 

 bacteria which can produce root nodules and fix atmospheric nitrogen 

 in suitable form for plant food. 



Although these nodule-forming bacteria are widely distributed in 

 ]iature, there are regions where thev are lacking, and thev should be 

 artificially introduced into the soil in these regions. This may be ac- 

 complished either by transferring from 200 to 500 pounds of soil per 

 acre from a field Avhere the leguminous crop shows abundant root 

 nodules to the new field Avhere the same legume is to be planted, or by 

 the ap]3lication of artificially prepared cultures of bacteria to the seed 

 before jDlanting or directly to the field itself. Though inoculation 

 b}" the pure-culture method has proved less universally successful 

 than the soil-transfer method, the artificial cultures possess certain 

 important advantages, especially the greater ease of their trans- 

 portation and application, as well as their freedom from danger of 

 introducing weeds or plant diseases.^ 



VARIATION IN INOCULATING POWER. 



A plausible explanation for the occasional failure of cultures of 

 Bacillus radicicola to properly inoculate a crop is that the bacteria, 

 though able to grow vigorously in the culture medium, have actually 

 deteriorated in the essential quality of being able to infect the 

 leguminous roots and to produce nodules. Testing the cultures at 

 frequent intervals on potted plants in greenhouses or on small plats 



1 Issued Apr, .5, 1913. 



Messrs. F. L. GoU and L. T. Leonard, scientific assistants, liavc assisted in working out 

 the details of construction of tlie testing apparatus and its utilization in the laboratory. 



2 These facts are discussed in more detail in previous publications of the Bureau of 

 Plant Industry. 



3 

 [Cir. 120] 



