jaM. 28. i9i8 Nitrogen-Assimilating Bacteria 211 



From the results as a whole it appears that it is the nitrate radical 

 and not the combined salt which causes the increase in the number of 

 B. radicicola when small amounts of nitrates are present. A stimulation 

 occurred, resulting in an increase in number which is quite comparable 

 to that obtained with potassium, sodium, and calcium nitrates. The 

 highest concentration of ammonium nitrate used did not appear to have 

 such an inhibiting effect as did the corresponding concentrations of the 

 three other salts. 



Throughout the work with B. radicicola in sterilized soil compara- 

 tively low numbers of these organisms were found. Whether or not 

 this depression was due to toxic substances formed as a result of steriliza- 

 tion can not be stated. If this decrease in numbers as a result of the 

 presence of toxic substances is true, it is very evident that the detrimental 

 effect had not become materially lessened at the end of the incubation 

 period. However, in any event the validity of the outcome is not im- 

 paired, since comparative and not absolute data are of importance and 

 since in all probability the same conditions obtained throughout the 

 cultures. 



It seems certain from the results of these studies on the effect of 

 potassium, sodium, calcium, and ammonium nitrates on the growth of B. 

 radicicola in sterilized soil that small amounts of nitrate stimulate the 

 growth of the organisms. It is also shown that B. radicicola is much 

 more resistant than Azotobacter to higher concentrations of potassium, 

 sodium, calcium, and ammonium nitrates. 



INFLUENCE OP BACILLUS RADICICOLA ON NITRATES IN SOLUTION 



The series of soil culture experiments just discussed served to give an 

 idea concerning the effect of nitrates on the legume organism. It was 

 found that in small amounts nitrates stimulated the bacteria to increased 

 reproduction. But no study was made as to the effect of Bacillus radicicola 

 on the nitrate. Does the organism break up the nitrate, reducing it to 

 nitrite or ammonia ? Does it cause any loss in nitrate when grown in a 

 solution containing that salt? Beijerinck {2, p. J62) as a result of 

 physiological experiments with B. radicicola, states that the organism does 

 not reduce nitrate. Prucha {41) also states that B. radicicola does not 

 reduce nitrates. However, Zipfel (49) found that B. radicicola will reduce 

 nitrates to nitrites but not to ammonia. 



The following experiments, somewhat similar to those already cited in 

 relation to Azotobacter, were carried out in an endeavor to answer these 

 questions. 



To twenty 500-c. c. Erienmeyer flasks containing 200 c. c. of mannit 

 solution, potassium, sodium, calcium, and ammonium nitrates were 

 added as indicated in Tables XXIV, XXV, and XXVI. Quadruplicate 

 flasks were prepared for each concentration of nitrate and for the control 

 cultures without nitrate. The flasks and contents were sterilized at 



