Dec. i6, 1918 Nitrogen-Fixing and Nitrifying Organisms 613 



The quantity necessary to destroy the nitrifying organisms is very 

 much larger than that which is necessary to check their activilty. There 

 are some times sHght increases in the accumulation of nitrate is treated 

 as compared to nontreated soils. This increase, however, can not 

 be attributed to a stimulation of the nitrifying organisms. 



There is never an accumulation of ammonia unless nitrification has 

 been checked, which means that in no instance does the process of 

 ammonia formation exceed that of nitrification, the latter process being 

 limited by the former. Any increase in nitrate formation, therefore, 

 merely means an increased ammonia formation. In those samples in 

 which nitrification has been checked and there is an accumulation of 

 ammonia the total nitrogen present as nitrate and ammonia is not 

 materially different from that present in other samples. 



The effect of carbon disulphid and toluol upon the Azotobacter group 

 of organisms is more definite and pronounced than it is upon the nitri- 

 fying organisms^ This is due to the fact that if the application is suffi- 

 cient to have any appreciable effect, it appears to actually destroy the 

 organisms rather than to check their activity. 



Again the effect of both carbon disulphid and toluol appears to depend 

 more upon the condition of the soil than upon the quantity applied. 

 The data show that under some conditions an application of many 

 times as much is necessary to destroy Azotobacter as under other condi- 

 tions. For these reasons no definite statement can be made as to the 

 effect a given application will have. One-tenth cc. of carbon disul- 

 phid per 100 gm. of soil has been observed to destroy Azotobacter 

 completely, while in other instances 5 cc. were without effect. Simi- 

 larly 0.25 cc. of toluol has been observed to kill, while under other 

 conditions 10 cc. were without effect. 



The elimination of Azotobacter does not eliminate nitrogen fixation. 

 There are other nitrogen-fixing organisms that are able to withstand 

 the heaviest application of carbon disulphid and toluol we have made. 

 The quantity of nitrogen fixed in the absence of Azotobacter is usually 

 from one-fourth to one-half that when they are present, and the quan- 

 tity fixed appears to decrease as the application of carbon disulphid 

 and toluol increases. 



■ As soon as the wide difference in the quantity of carbon disulphid 

 and toluol necessary to destroy Azotobacter under different soil condi- 

 tions was observed, the question of the cause of such differences very 

 naturally arose. As mentioned in the experimental part of this paper, 

 the moisture content offered a working basis. 



In commenting upon the writer's earlier work with carbon disulphid 

 and toluol Kelley ^ suggested that, since these substances are only 

 slightly miscible with water, the noneffectiveness of these agents in 

 decreasing the number of bacteria was possibly due to the high-moisture 



• Kelley, W. p. ammonification and nitrification in Hawaiian soils. Hawaii Agr. Exp Sta 

 Bui. 37, 52 p. 1915. 



