E. J. Russell and H. B. Hutchinson 135 



The actual amoimt of nitrogen fixed per gram of mannite supplied 



was: 



Arable soil Garden soil 



Untreated soil 4-7 6-3 mgrams 



Toluenedsoil 3-8 6-2 



Heated soil -5 -2 ,, 



These results confirm our previous conclusion that the increased 

 productiveness of partially sterilised soils is not due to increased 

 nitrogen fixation. 



§ 30. Nitnfication. Both heat and toluene destroyed the nitrifying 

 organisms ; there was no sign of revival even after a month's incubation 

 at 30°. We have already shown (Table 2) that nitrates are not produced 

 in partially sterilised soils except as a result of subsequent infection. 



It has of course been known for many years that volatile antiseptics 

 put an end to nitrification, but it is usually considered that the nitrifying 

 organisms recover after an interval, and even, according to some, work 

 at an increased rate. 



We have made a number of experiments on this point, but in no 



instance have we obtained any evidence of recovery when sufficient 



precautions were taken to guard against re-infection. It was a common 



experience that nitrification would be for a long time suspended in 



toluened soils and would then set in with the production of a large 



amount of nitrate, thus: 



At beginning After 6 weeks After 18 weeks 

 Parts of nitrogen as nitrate \ ■.„ ,„ (64 



per million of dry soil ) (82 



The large amount of nitrate is of course no evidence that nitrification 

 is stimulated, but is simply the result of the increased ammonia 

 production, and accidental inoculation with nitrifying organisms. 



§ 31. When the soil has been heated, however, it becomes unfitted 



for the development of the nitrifying organism. Apparently a toxic 



body is produced, which however only acts on the nitrifying organism 



and not on those producing ammonia. In one experiment soil was 



completely sterilised by heating to 130° for 45 minutes and then infected 



by admixture with a trace of ordinary soil ; the production of ammonia 



and nitrate was as follows : 



At beginning After 21 days After 50 days 



Nitrogen as nitrate 13-6 14-2 15-6 



Nitrogen as ammonia 5*8 26'9 48*6 



Total (parts per million of dry soil) ... 19'4 41'1 64 '2 



Pickering has already demonstrated the formation of a toxic sub- 

 stance by heat, and our results are in complete agreement with his on 

 this point. 



