Dec. i6, 1918 Nitrogen-Fixing and Nitrifying Organisms 603 



Fifty-gm. samples containing 6.5 gm. of water each were treated 

 with varying amounts of toluol and carbon disulphid as indicated in 

 Table I. The soil was then immediately placed in 500-cc. bottles, 

 tightly stoppered, and left for three days. At the end of this time the 

 water content of the nonevaporated samples was made up to 12 cc. or 

 one-half saturation, and the stopper replaced with cotton. The samples 

 were incubated under conditions which would retard loss of moisture and 

 were not opened except when analyses were made. The evaporated 

 samples were treated exactly alike, except that at the end of the first 

 three days the soil was emptied into petri dishes, left thus for 48 hours, 

 replaced in the bottle, and the moisture content made up to one-half 

 saturation. Tests for nitrogen fixation were made at the end of four 

 weeks and again after six months' incubation, and the nitrate content 

 was determined at the end of six months. 



Before the six-months' Analyses were made the moisture content of all 

 samples had fallen very low, some samples being practically air-dry. 

 This apparently had no eflfect upon nitrogen-fixing organisms, but did 

 obscure the recover)^ of active nitrification. In other words, before 

 sufficient time had elapsed for the recovery of active nitrification, the 

 moisture content of many samples had fallen so low that nitrification 

 was impossible. The results are reported in Table I. 



There is little difference between the evaporated and nonevaporated 

 samples in the amount of nitrogen fixed; 0.25 cc. of toluol per 100 gm. 

 of soil destroyed the Azotobacter, and there is no evidence of recovery. 

 The same quantity checked nitrification, but the quantity necessary to 

 destroy the nitrifying organisms is very much higher, the samples re- 

 ceiving even 5 cc. having partly recovered after six months. As men- 

 tioned above, the failure to recover was probably due to low water con- 

 tent rather than to the toluol. Ammonia accumulated only when 

 nitrification was checked. 



The quantity of carbon disulphid necessary to destroy Azotobacter 

 was only o.i cc. per 100 gm. of soil, and there is no evidence of recovery 

 even after six months. In the nonevaporated samples, o.i cc. checked 

 nitrification, while 0.25 cc. were required to check the process in evapo- 

 rated samples. There is evidence of recovery of nitrification even with 

 5.0 cc. of carbon disulphid. Ammonia accumulated only when nitrifica- 

 tion was checked. 



Even when Azotobacter were destroyed, the ability to fix nitrogen 

 was not destroyed with the largest quantity of carbon disulphid or toluol 

 applied in these experiments. However, fixation by organisms other 

 than Azotobacter appeared to decrease as the application of chemicals 

 increased. 



A second set of experiments was conducted, in which the soil had been 

 in the laboratory much longer and was almost air-dry, containing only 



