13 

 fraction was explained as probably due to the accumulation of low 

 molecular weight S compounds released by the decomposition of some 

 resistant fraction. The FAA fraction might not lose a large proportion 

 of its S because of the constant addition and faster turnover rate. 

 Bettany et al. (1980) suggested that it might be fractions such as the 

 FAB which show little net change in total S content that are important 

 in supplying available S in the short term. 

 1.1.3 Soil Sulfur Reactions 

 1.1.3.1 Oxidation-reduction 



No other element is known to occur in as many different forms as 

 S. Its valence ranges from -2 to +6 with sulfides and sulfates repre- 

 senting the two extremes (Table 2). As previously mentioned, the re- 

 duced forms of S occur primarily in soil organic matter, amino acids, 

 and the B vitamins including thiamine, biotin, and lipoic acid. 

 Reduced forms of S may also be found in the soil as sulfides of pri- 

 mary minerals such as FeS or as intermediate reaction products such as 

 H ? S, thiosulf ates, thiocyanates , polythionates , or elemental S 

 (Burns, 1967). 



Sulfide is the principal, stable form of S under anaerobic condi- 

 tions, but elemental S and organic compounds of S may persist in some 

 natural anaerobic environments such as in sedimentary rocks associated 

 with S domes and in peat, coal, and oil (Starkey, 1966). 



Reduction occurs through microbial assimilation of sulfate under 

 aerobic conditions and by obligate anaerobes that use sulfate as the 

 H-ion acceptor and organic matter as an energy source under anaerobic 

 conditions. All of the natural organic and inorganic compounds of S 

 are susceotable to microbial attack. Even the S that occurs in 



