234 Transformation of Mineral Substances 



and can be carried out by a great many bacteria. This type of reac- 

 tion has recently attracted much attention as an outgrowth of an 

 effort to increase the supply of available sulfur by utilizing the waste 

 products. In well-aerated soils, however, sulfur does not accumu- 

 late as a result of oxidation of hydrogen sulfide, but is rapidly oxi- 

 dized further to sulfate, as shown by the second reaction. 



When elementary sulfur is the starting point, only very few bac- 

 teria are able to bring about its oxidation to sulfuric acid. The acid 

 interacts with the bases and other buffering substances present in 

 the soil to give rise to various sulfates. When it is desirable to reduce 

 the alkalinity of certain soils, such as black alkali soils, or when it is 

 advisable to make the soil more acid for control of certain disease- 

 producing organisms, such as potato scab, addition of sulfur, and its 

 resultant oxidation to sulfuric acid, may become of great economic 

 importance. This reaction can be summarized as follows: 



Ca3(P04)2 + ^2H2S04 = 3CaS04 + Ca(H2P04)2 



Among the other transformations of sulfur compounds in the soil 

 and in other natural substrates, the reduction processes are of great 

 significance. The reduction of sulfate to hydrogen sulfide is brought 

 about by certain specific bacteria, usually designated as sulfate- 

 reducing organisms. One of these organisms has been designated by 

 Beijerinck as Microspira dcsulfiiiicans, and more recently by Starkey 

 as Sporovibrio desttlftiricans. The reduction process takes place in 

 the presence of a suitable source of energy as follows: 



CaS04 + C Hg C OOH = H.S + CaC Og + ( O2 + HgO 



Anaerobic conditions, or absence of free atmospheric oxygen, is 

 essential for this reaction, and a form of organic matter must be 

 available. The sulfate is used by the bacteria as a source of oxygen 

 for the oxidation of the organic substances; the energy liberated in 

 the oxidation process is partly consumed in the reduction of the 

 sulfate. The hydrogen sulfide produced in this reaction is character- 

 istic of certain water basins, of peat bogs, and of other water- 

 saturated environments, where there is a lack of free oxygen. In the 

 presence of iron, black iron sulfide is produced, which is character- 

 istic of the organic muds laid down under anaerobic conditions. 

 This reaction may lead to corrosion of iron in steel pipes, which may 

 become of considerable economic importance. 



