192 



Journal of Agricultural Research 



Vol. XII. No. 4 



Table VII. 



-Influence of calcium nitrate on the growth of Azotobacter {strain B) in 

 sterilized soil 



A glance at the figures of Tables V, VI^ and VII shows that the small- 

 est concentration of nitrate used produced a much more marked relative 

 increase in numbers with strain B than it did with strain A. On the 

 other hand, the greater resistance of this strain to the higher nitrate 

 concentrations is clearly evident. In the potassium- and calcium- 

 nitrate cultures the organisms were present in an active state where 

 the nitrate was added in amounts equivalent to 200 mgm. of nitrate 

 in 100 gm. of soil. However, this same concentration of sodium nitrate 

 prevented the development of the Azotobacter. The first five concen- 

 trations of all three nitrates caused a very large increase in the number 

 of Azotobacter when compared with control cultures where no nitrate 

 was added. In one instance an enormous increase was noted after three 

 weeks' incubation in the presence of 50 mgm. of nitrate as sodium nitrate. 

 This increase far excelled that noted with other concentrations of the 

 same salt. The writer can offer no conjecture as to this occurrence. 



Similar results were obtained by the writer in 19 14 (25) with a strain 

 of Azotobacter isolated -from a silt loam soil at the Pennsylvania Experi- 

 ment Station. It was found that soil and liquid cultures containing 

 small amounts of potassium, sodium, and calcium nitrates caused an 

 increase in the number of Azotobacter in pure culture compared with 

 control cultures containing no nitrate. An increasing concentration of 

 the nitrates continued favorable to the growth of the organism up to a 

 certain limit, but higher concentrations retarded its growth. Finally 

 a nitrate concentration was attained at which Azotobacter growth 

 altogether ceased. 



