16 

 chemoautotrophic, obligate aerobes. The ideal, overall, oxidation 

 reactions of the thiobacilli bacteria are: 



SH~ ■+ S° - S 2 0^- - S.O^- - SO*" . 



sulfide sulfur thiosulfate tetrathionate sulfate 

 Some of the common species of thiobacilli found in soils and the oxida- 

 tion reactions associated with each species are shown in Table 3 

 (Burns, 1967) . 



The acid produced in the oxidation process is of significant 

 agricultural interest. As previously mentioned, the draining of wet- 

 lands may result in acid-sulfate soils or "cat clays." Elemental S, 

 polysulfides, or sulfuric acid may be used to amend alkaline soils or 

 to lower the soil pH for acidophilic plants: 



2S + 30 ? + HO — T. thiooxidans -> H^O^ + (CaC0 3 + H 2 0) ■* 



CaSO, ■ 2H„0 + 2C0„ . 

 4 2 i- 



The H SO produced by microbial oxidation reacts with free lime in the 



2+ + 



soil to form gypsum. The Ca from gypsum replaces adsorbed Na to 



reclaim sodic soils (Reuss, 1975). 

 1.1.3.2 Precipitation reactions 



Reducing conditions can result in the precipitation of FeS and 

 other metal sulfides in soil systems. However, in well-drained soils, 

 metallic sulfides are not a sink for soil S. As previously mentioned, 

 inorganic sulfates are associated with alkaline soils in arid or semi- 

 arid climates. Harward and Reisenauer (1966) noted that sulfate 

 retention in arid soils is merely a consequence of gypsum solubility. 

 Most sulfate salts are quite soluble in water and leach rapidly from 

 soils in humid regions. 



