6 

 1959; Freriey, 1961; Freney et al. , 1962; Nelson, 1964a; Tabatabai and 



Bremner, 1972a). Tliis fraction may account for 50 to 75% of the total 

 soil S (Alexander, 1977). In a study with 22 soils from eastern 

 Australia, Freney (1961) found an average of 93% of the total S was in 

 organic form. Bonner (19 73) found this value to be 63% for Louisiana 

 soils, but he was unable to account for a large percentage of the total 

 soil S. 



The availability of this organic S to growing plants is dependent 

 upon the rate of decomposition of the soil organic matter and the 

 nature of the organic matter itself. Environmental factors which favor 

 the proliferation of microorganisms such as moisture, temperature, 

 energy sources, aeration, pH, etc. will affect the mineralization cf 

 soil S (Parker and Prisk, 1953; Nelson, 1964a; Burns, 1967). 



Soil organic matter can serve as either a source or a sink fcr 

 plant-available S (Burns, 1967). The decomposition of plant residues 

 low in S can induce S deficiencies of crops growing on a soil. Avail- 

 able S can become assimilated by a growing population of microorgan- 

 isms just as available N can become tied-up when low-N energy sources 

 are added to a soil. Since S is a component of the amino acids, 

 cystine and methionine, it is an essential nutrient of microorganisms 

 as well as of plants and animals. Starkey (1966) stated that the 

 cells of microorganisms may contain 0.1 to 1% S. 

 1.1.2.1 C:N:S relationships 



During the 1960's, much research emphasis was devoted to studying 

 the C:N:S relationships in soils. Williams et al. (1960) found that 

 total soil S was very highly correlated with both C and N in Scottish 

 soils. Thev found a mean C:N:S ratio of 100:7.1:1. These values were 



