Jan. 28, 1918 



Nitrogen- A ssimilaiing Bacteria 



189 



Table III. — Influence of calcium, nitrate on the growth of Azotobacter {strain A) in 



sterilized soil 



One hundred and fifty gm. of soil (dry weight) were weighed into 500- 

 c. c. Erlenmeyer flasks and the nitrates added in solution, as indicated in 

 the following tables. At the same time i per cent of mannit was added 

 in solution and the moisture content was raised to approximately 18 per 

 cent. The flasks were allowed to remain at room temperature for one 

 day, when the contents were thoroughly mixed. The flasks and contents 

 v/ere then sterilized at 15 pounds' pressure for three hours. Upon cooling 

 they were inoculated with 5 c. c. of a suspension of the organisms in sterile 

 distilled water. The cultures were incubated at 28° C. and counts made 

 at the intervals already indicated. Mannit agar was used in pouring the 

 plates. Each number in the following tables represents an average of 

 duplicate plates. Tables I, II, and III show the results of the work vdth 

 strain A and Tables V, VI, and VII the results with strain B. 



It will be seen at a glance that all three nitrates exerted an enormous 

 influence on the growth of the Azotobacter. The smallest concentration 

 did not appear to exert much influence either in increasing or decreasing 

 the number of Azotobacter. There was a slight gain, but it was not so 

 marked as that brought about by higher concentrations of nitrates. 

 When 25, 50, and 100 mgm. of nitrate were present in 100 gm. of soil, 

 very large increases were obtained in practically all instances. In one 

 instance sodium nitrate caused the greatest relative gain, but the most 

 consistent increase was produced by calcium nitrate. Beginning with 

 150 mgm. the number of Azotobacter began to decrease. This decrease 

 was especially noticeable in the cultures containing potassium and 

 sodium nitrates. At the end of the first week, Azotobacter organisms 



