Dec. i6. 1918 Nitrogen-Fixing and Nitrifying Organisms 



605 



Cultures for Azotobacter were made from the evaporated samples 

 after the exposure for evaporation and from all cultures after 16 weeks' 

 incubation. Since the principal point to be determined was whether 

 Azotobacter were killed, a total nitrogen determination was not made 

 on the nitrogen-fixing cultures at the 16 Vv^eeks' analysis. Also, previous 

 experiments had shown that the quantity of nitrogen fixed when the 

 characteristic film developed did not vary beyond the experimental 

 error and that the quantity fixed in absence of Azotobacter was usually 

 from one-fourth to one-half than when Azotobacter were present. The 

 character of growth was, therefore, a sufficient index for the object in 

 view. The results are presented in Tables II and III. 



Table II. — Effect of toluol upon nitrogen fixation and nitrification 



"■ Nitrates in milligram per 100 gm. of soil. 



6 Nitrogen fixed in milligram per 50 cc. of culture. 



c Milligrams. 



The results presented in Tables II and III are quite different from those 

 given in Table I. For example, the only samples in which Azotobacter 

 were destroyed and in which nitrification was checked were those treated 

 with 10 CC. of either carbon disulphid or toluol. In case of the evapo- 

 rated samples, even lo cc. of toluol had no effect. Again, in only those 

 samples in which nitrification was checked was there any accumulation 

 of ammonia. 



