II 



Iowan soils. Values for C:N:S relationships in soils and soil-S frac- 

 tions reported in the literature are compiled in Table I. 



Bettany et al. (1973, 1979) found a gradual increase in both the 

 C:S and N:S ratios of soils along an environmental gradient in Sas- 

 katchewan, Canada. Brown chernozemic soils (Aridic Haploborolls) had 

 a C:N:S ratio of 61:6.5:1 with 52% of the total S as Hi-reducible while 

 grey luvisolic soils (Typic Cryoboralfs) had ratios of 112:9.7:1 with 

 32% Hi-reducible S. These values for the C:N:S ratios agree with 

 earlier values for similar soils in the northern United States and 

 Canada (Evans and Rost, 1945; Lowe, 1965). Intermediate soils were the 

 dark brown and black chernozemic soils (Typic Haploborolls and Udic 

 Haploborolls) with a mean C:N:S ratio of 80:7.8:1 and 50% Hi-reducible 

 S. However, differences in C:N:S ratios of humic acid (HAA) fractions 

 of these soils were greater than those of the total soil organic mat- 

 ter. Differences in the comparable ratios of conventional fulvic acid 

 (FAA) , clay-associated humic acid (HAB) , and less than 2-um humin were 

 smaller than for HAA along the environmental gradient. In 1973, 

 Bettany proposed that C-bonded S is more likely to be incorporated 

 into the strongly aromatic humus whereas the Hi-reducible S fraction 

 would be associated with active side-chain components. 



A more recent, refined proposal suggests FAA, HAB, and less than 

 2-um humin as the sources of potentially labile organic S (Bettany et 

 al., 1979). However, a study of cultivated versus uncultivated soils 

 showed that the HAA, HAB, and humin fraction accounted for 80% of the 

 total S lost upon cultivation. The FAA fraction had narrower C:N:S 

 ratios and contained more Hi-reducible S but contributed only 14% of 

 the total S loss (Bettany et al., 1980). The small loss by the FAA 



