198 Nitrogen Fixation— Nonsymbiotic 



Table 42. Nitrogen Fixation by Strains of Azofobacter with and without a 

 Contaminant (from Lind and Wilson) 



Nitrogen Fixed f 



Strain Pure Culture Plus 



Hours No.* of Azotobadcr Contaminant 



Azotobactcr and BacUhis from Mixed Culture 

 21 1 6.9 11.1 



2 9.5 16.2 



3 8.2 16.2 



12 1 7.0 19.8 



2 (5.7 27.2 



3 8.3 26.4 



12 1 5.0 20.1 



2 8.9 22.3 



3 7.3 21.7 



41 1 16.5 31.4 



2 20.0 32.5 



3 18.9 32.5 



12 2 10.8 18.8 



3 10.4 26.7 



Azotobactcr and Clo.stridlum pa.sfrurianuin 

 12 2 (5.7 19.5 



12 3 8.9 21.0 



* Strain 1 was Azotobactcr stock culture; strains 2 and 3 were pure Isolates of .(;.')/()- 

 bacter from mixed culture. 



t Values in milligrams N per 100 ml; this includes nitrogen in inoculum, which was 

 1 ml in 15 ml. 



antagonistic or competing agents are among the factors that control 

 the presence and abundance of this organism in the soil. 



Out of 105 soil samples examined, Burri found Azotohacter miss- 

 ing in 34, chiefly heavy clay soils. Jones and Murdoch isolated Azo- 

 tohacter from 9 out of 17 soil types examined, and from 22 out of 29 

 soil samples representing 9 types. The maximum number of Azoto- 

 hacter cells found per gram of soil was 18. Rossi reported an average 

 of 1,815 cells per gram of Italian soil, the number ranging from to 

 21,400. Swaby found Azotohacter in 26 per cent of all soil samples 

 examined, usually 30 cells per gram of soil. Clostridium hutijlicum 

 was found, however, in most of the soils, usually 100 spores per 

 gram. 



