612 SEIiMAN A. WAKSMAN 



but rapidly deteriorates on artificial culture media. Its absolute 

 purity, particularly as far as contamination with Th. thio- 

 oxidans or a modified strain of this organism was not established 

 as yet. 



When sulfur is added to a neutral or acid soil, it is rapidly 

 oxidized by Th. thiooxidans when present or introduced. When 

 sulfur is added to an alkali soil, particularly a black alkali soil, 

 the oxidation is very slow, even in the presence of Th. thiooxidans. 

 However, after a little time, the suKur is oxidized. When some 

 of this black alkali soil, to which sulfur has been added, is intro- 

 duced into flasks with media 6 and 8, an excellent growth will 

 take place in the latter; while, in the former, growth will take 

 place only if the oxidation is at an advanced stage. A series of 

 studies has led the author to conclude that, while, in acid soils, 

 the sulfur is oxidized primarily by Th. thiooxidans, in alkali soils 

 Thiobacillus B. also takes an active part in the process. This 

 part consists either in a direct oxidation of the suKur, whereby 

 the reaction of the soil is made sufficiently acid so that Th. 

 thiooxidans (if present) may oxidize the sulfur further, or by a 

 certain modification of the sulfur, so that it is more readily acted 

 upon by Th. thiooxidans under alkaline conditions. The chemis- 

 try of the process of sulfur oxidation both in acid and alkali soils 

 by the two organisms has been reported in detail elsewhere 

 (Waksman and Joffe, 1921, Waksman, 1922 c). 



The following three tables show clearly the relation between 

 initial reaction, source of sulfur and sulfur oxidation. 



The following tables definitely estabUsh the fact that Th. thio- 

 oxidans primarily oxidizes elementary sulfur under acid condi- 

 tions. The possibility that it may oxidize elementary sulfur in 

 the soU, under alkahne conditions is thereby not excluded, par- 

 ticularly in the presence of other organisms. It also oxidizes 

 thiosulfate with an intense acid formation. Thiobacillus B. 

 oxidizes very httle elementary sulfur in the soil or in solution, 

 but the fact that it can act upon sulfur under highly alkahne condi- 

 tions suggests the possibility of the associative action of the two 

 organisms in changing the reaction of black alkali soil to which 

 sulfur has been added. Thiobacillus B. oxidizes thiosulfate 



