190 



Journal of Agricultural Research 



Vol. XII, No. 4 



were still found in the potassium-nitrate cultures where 200 mgm. were 

 present. However, at the end of the second week the organisms were 

 dead. The same concentration of sodium and calcium nitrates proved 

 even more toxic. No evidences were secured, indicating that these 

 organisms can resist concentrations in excess of 300 mgm. of nitrate per 

 100 gm. of soil. 



The question may be raised in regard to the influence of sterilization 

 on the nitrate present in the soil. Does the prolonged heating in the 

 presence of soil organic matter reduce the nitrate? In order to study 

 this point, a few cultures were prepared similar to those already described. 

 They were subjected to sterilization under pressure of 15 pounds for two, 

 three, and five hours. Nitrate determinations at the end of these periods 

 failed to show any reduction. In the presence of i per cent of mannit 

 the nitrate content remained unchanged during sterilization. 



From these results it is evident that small amounts of nitrate up to 

 150 mgm. of nitrate in 100 gm. of soil greatly increased the reproduction 

 of Azotobacter. In regard to the toxicity of higher concentrations, 

 sodium nitrate appeared to exert the greatest influence in this direction, 

 followed by calcium and potassium nitrates in the order named. The 

 results of the experiment are recorded in Table IV. 



Table IV. — Inflttence of ammonium nitrate on the growth of Azotobacter {strain A) in 



sterilized soil 



That the nitrate radical and not the combined metal was the causal 

 agent in the increase in the number of Azotobacter was indicated from 

 the results of the next test. Here ammonium nitrate was used. 



It will be seen from the data of this experiment that ammonium nitrate 

 caused an increase in the number of Azotobacter when present in small 

 amounts. However, the increase in the presence of ammonium nitrate 

 was less marked than when equal quantities of the other nitrates were 

 used. Since the experiments with ammonium nitrate were not made at 

 the same time as the preceding experiments (discussed on pp. 189-190), it 

 is possible that conditions varied sufficiently to account for the less pro- 

 nounced results. When 200 mgm. of nitrate were present in 100 gm. of 



