OXIDATION OF SULFUR IN THE SOIL 



615 



inoculated with crude cultures of Th. thiooxidans has become so 

 acid due to long incubation (one jear) that only the latter or- 

 ganism survaves. If Thiohacillus B. was originally in the com- 

 post, it has died out, due to the intense acidity. The alkali 

 compost consisting of black alkali, to which 0.5 per cent of sulfur 

 has been added and incubated for 6 months, definitely indicates 

 the presence of the second organism. 



The sulfide (table 3) was oxidized to some extent by both 

 organisms, and particularly by a mixture of the two organisms. 

 This is explained by the fact that the sulfur produced from the 

 sulfide by the action of Th. B. is rapidly oxidized to sulfuric acid 



TABLE 3 



The oxidation of K2S by microorganisms 



Medium 8 used as a basis, 0.5 per cent KjS in place of the thiosulfate 



• In some cases, particularly at lower pll values, the oxidation of the sulfide 

 by the Th. thiooxidans was much more extensive. 



by Th. thiooxidans. This is well demonstrated by the column of 

 sulfates formed. A detailed study of the morphology of these 

 two organisms will form the subject of a succeeding paper. 



SUMMARY 



1. At least two organisms can be shown to take part in the 

 oxidation of sulfur in the soil; while, in acid soils, Th. thiooxidans 

 will rapidly oxidize the elementary sulfur, in alkali soils, particu- 

 larly black alkali soils, still another organism, Thiohacillus B., 

 similar to Th. thioparus of Beijerinck, takes an active part in 

 this process. 



2. Th. thiooxidans is usually not present in common culti- 

 vated soils, but is found abundantly in soils previously treated 

 with suKur. Thiohacillus B. is commonly present in cultivated 

 soils. 



