Aerobic Bacteria 199 



Tahi.k 4;J. Efkixt ok Soiuc k ok Iuon on Xituockn Fixaiion uy Pikk and MrxKi) 

 CuLTUHES OF Azolohaclrr (from Lliid and Wilson) 



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

 1 ml in 15 ml. 



Azotobacter usually cannot develop in a soil having a pH of less 

 than 6.0. As soon as the reaction of the soil is adjusted by means of 

 lime, so that the pH becomes higher than this minimum, a txpical 

 Azotobacter flora will develop. The carbonate-phosphate ratio in 

 soil was found to influence markedly the development of this organ- 

 ism in soils of different reaction. In Malayan soils, however, certain 

 strains of Azotobacter have been found capable of growing at a 

 wide pH range, 3.6 being the limit on the acid side. Starkey and 

 De isolated an organism {Az. indictim) capable of growing at reac- 

 tions lower than pH 6.0. 



Abundance of Azotobacter in the soil was found to be influenced 

 by cropping and by fertilizer treatment, the numbers being higher 

 in unfertilized than in fertilized soils. 



Azotobacter may live symbiotically with algae, especially with 

 Nostoc and Anabaena, as well as with other bacteria. The quanti- 

 ties of nitrogen fixed by this association are frequently considerable, 

 as pointed out in Tables 42 and 43. The symbiotic action between 

 CI. pasteurianum and Azotobacter, whereby the latter uses up the 

 oxygen, making conditions favorable for the former, has also been 

 demonstrated. The various acids produced by Clostridium are 

 neutralized by the soil bases and can be utilized by Azotobacter as 



