TRANSFORMATION OF SULFUR IN SOIL BY MICROBES 171 



but they use the sulfide or sulfur as their specific foods, oxidizing 

 them to sulfate. Even in the absence of these organisms, sulfates 

 are formed, presumably in some way associated with the develop- 

 ment of various heterotrophic organisms including both bacteria 

 and fungi. The process of sulfur oxidation can be utilized for 

 increasing the acidity of the soil, and in view of the fact that certain 

 organisms producing plant diseases cannot thrive at certain acid 

 concentrations, it may be desirable to create such acid conditions 

 as are inhibitive to their development. This is brought about by 

 adding sulfur to soils. In the soil, it is oxidized to sulfuric acid 

 and brings about the desired effects as considered in more detail 

 elsewhere. This process can also be utihzed for reducing the alka- 

 linity of black alkali soils. The sulfuric acid formed by the oxida- 

 tion of sulfur interacts with the sodium carbonate, giving sodium 

 sulfate. The latter salt of sodium is much less injurious to plants 

 than the former; it has less undesirable effects on the physical 

 condition of the soil and it can also be more readily removed by 

 irrigation and drainage waters. The oxidation process is further 

 utilized in transforming the insoluble phosphate of the rock phos- 

 phate to more soluble forms. When sulfur is mixed and composted 

 with rock phosphate and soil, in the proper proportions, the sul- 

 fur becomes oxidized to sulfuric acid and changes the phosphate 

 to the di-calcium and mono-calcium forms. The application of 

 such material to soils gives much the same effects as the applica- 

 tion of " superphosphate." Similar solution of potassium occurs 

 when certain insoluble minerals containing potassium are substi- 

 tuted for the rock phosphate in these composts. 



Large amounts of sulfide may be produced in nature by the 

 reduction of sulfates as well as elementary sulfur itself. These 

 processes consume appreciable amounts of energy which must be 

 supplied from other sources, such as the oxidation of organic 

 compounds. Further, the reaction proceeds only under anaerobic 

 conditions. In such an environment the reactions take place 

 somewhat as follows : 



2S 



+ 2H2O + C6H12O6 = 2CH3-COOH 4- 2H-C00H + 2H2S 



Glucose .\cetic acid Formic 



acid 



3CaS04 + 2(C3H503)Na 



Sodium lactate 



= SCaCOa -f Na^^COa + 2H2O -f- 2CO2 + 3H2S 



