Jan. 28, i9i8 Nitrogen- Assimilating Bacteria 199 



It will be seen at a glance that a greater relative increase in nitrogen 

 fixation in the presence of nitrates occurred in the soil cultures than on 

 the agar films. But in the latter instance the amount of nitrogen as- 

 similated in the absence of mistakes is far in excess of that assimilated 

 in the soil cultures under similar conditions. The amount of nitrogen 

 fixed in the soil cultures is surprisingly low, but as relative increases or 

 decreases are desired this does not materially influence the results. 



The influence of sodium nitrate on the fixation of nitrogen by pure 

 cultures of Azotobacter in sterilized and unsterilized soil is brought out 

 very clearly in the figures of Tables X and XL In both cases, where 

 no nitrate was added, an equal fixation of nitrogen occurred. Where 10 

 mgm. of nitrate were added to 100 gm. of soil, slightly more nitrogen was 

 assimilated in the unsterilized soil than in sterilized. The reverse seemed 

 to be true when 50 mgm. of nitrate were added. But in the presence of 

 150 mgm. of nitrate, the fixation by the pure culture in sterilized soil 

 did not increase materially in comparison with that which occurred in 

 the 50 mgm. of nitrate concentration. Evidently the maximum fixation 

 under these conditions had been reached. The gain in the unsterilized 

 soil at the highest concentration of nitrate studied almost doubled the 

 amount fixed in the pure culture. It appears evident that the presence 

 of sodium nitrate causes a greater fixation of nitrogen in unsterilized soil 

 than it does under similar conditions in sterilized soil inoculated with 

 Azotobacter. 



In the case of calcium nitrate, somewhat comparable results were ob- 

 tained. The fixation where no nitrate was added was equivalent to 

 that obtained in the controls for the sodium nitrate. Where nitrate 

 was added in amounts equal to 10 mgm. of nitrate in 100 gm. of soil, an 

 increased fixation was obtained in the unsterilized soil, but practically 

 no increase occured in the pure culture in sterilized soil. Fifty mgm. 

 of nitrate in 100 gm. of soil produced an increase in fixation. In the 

 highest concentration of calcium nitrate the difference in nitrogen fixed 

 between the pure culture in sterilized soil and unsterilized soil was not so 

 great as in the case where sodium nitrate was used. 



In the sterilized soil where the two nitrates were present in equal 

 amounts it can be seen that more fixation took place in the presence of 

 sodium nitrate. The diilerence is not marked, but it exists neverthe- 

 less. It will be remembered that calcium nitrate had a detrimental 

 effect on nitrogen fixation by Azotobacter on agar films. However, in 

 soil cultures this same nitrate stimulated Azotobacter to an increased 

 assimilation of nitrogen. This difference is not suprising as it has been 

 shown repeatedly that bacterial activities in soil and in artificial cultures 

 are not always comparable. 



From the results of the experiments performed with reference to the 

 influence of nitrates in soil on the fixation of nitrogen therein, it appears 



