30 



CIRCULAR NO. 131, BUREAU OF PLANT INDUSTRY. 



Table I. — Effect of avimonium sulphate upon the nitrogen-fixing power of Azotobacter 

 chroococcum and A. beijerinckii in nutrient solutions. 



Inocui.ated with Azotobacter Chroococcum. 



Inoculated with Azotobacter Beijerinckii. 



At the conclusion of the incubation period the sokitions wore tested 

 for mannitc and found to contain a considerable portion of the 1.5 

 per cent added, but as no accurate determination of the quantity of 

 mannite remaining in the solution was made the gain in nitrogen 

 can not be calculated for each gram of mannite consumed. The solu- 

 tions to which ammonium sulphate was added were also tested for the 

 presence of ammonia. The method employed was to remove a few 

 drops of the solution by means of a pipette, dilute with 10 cubic centi- 

 meters of distilled water, and treat with Nessler's reagent. Strongly 

 positive tests were secured in every instance, thus showing that a 

 considerable portion of the ammonium sulphate added still remained 

 in the solution at the end of the experiment. 



The quantities of nitrogen fLxed for both Azotobacter chroococcum 

 and A. heijerincTcii are rather low for these organisms; however, the 

 results seem to indicate that Azotobacter species not oidy fix atmos- 

 pheric nitrogen in the presence of an abundant supply of combined 

 nitrogen in the form of ammonium sulphate, but the presence of the 

 combined nitrogen in the nutrient solution stimulates these organ- 

 isms to greater activity, so that a larger quantity of atmospheric 

 nitrogen is fixed than under conditions requiring the entire nitrogen 

 su{)ply to l)e drawn from the free nitrogen of the air. 



[Gil-. 131] 



