170 TRANSFORMATION OF MINERAL SUBSTANCES 



Direct evidence is still lacking to indicate that microorganisms 

 convert sulfur, associated in organic compounds, directly to sulfate. 

 Some sulfur may be liberated in this way, but a greater portion is 

 first changed to sulfide and appears as sulfate only after secondary 

 attack by organisms able to oxidize inorganic sulfur compounds. 

 Wherever protein materials undergo decomposition by the agency 

 of bacteria, sulfide is formed. Under anaerobic conditions it 

 is not transformed further, but under aerobic conditions it soon 

 disappears and is oxidized to sulfate, as may be represented by the 

 following equation: 



+ 30 

 H2S + 0-> H2O + S — > H2SO4 



Various specific bacteria (largely of the Thiobacillus group) are 

 capable of bringing about these processes in the soil (see Fig. 15), 

 The hydrogen sulfide is first oxidized to elementary sulfur. In the 

 case of some bacteria, this sulfur actually accumulates within or 

 without the cells. Other bacteria, however, immediately oxidize 

 the sulfur to sulfuric acid. This acid interacts with the soil bases: 



H2SO4 + CaCOs = CaSOi + H2O + CO2. 



Sulfide may be considered as an intermediate decomposition 

 product. It is unavailable to plants as such, is injurious rather 

 than beneficial to the activity of most soil microorganisms, and its 

 presence in soils in any appreciable amounts is indicative of par- 

 tial anaerobic conditions unsuited for plant growth. Sulfide, like 

 any other decomposition product such as ammonia, is a waste 

 product left by the microorganisms as unnecessary in their further 

 development. Its formation as a product of decomposition of 

 organic matter indicates that the compounds from which it was 

 produced contained more sulfur than was required by the organisms 

 in their growth while using this organic matter as food. As 

 hydrogen sulfide it represents a source of considerable potential 

 energy which will become liberated during its transformation to 

 sulfate. 



After elementary sulfur is added to soil, it is oxidized directly 

 to sulfuric acid if the proper organisms are present. The oxidation 

 proceeds particularly rapidly where some of the specific sulfur 

 bacteria are concerned. These organisms are similar in their 

 nutrition to the nitrifying bacteria in that they are autotrophic, 



