10 

 materials and was more stable than the Hi-reducible fractions. How- 

 ever, Lowe and DeLong were unable to account for all of the organic 5 

 in their soils, and made no conclusions as to the significance of this 

 organic-S fraction on plant S nutrition under field conditions. 



Freney, Melville, and Williams (19 70) found that Fe and Mn could 

 interfere with the determination of C-bonded S by the method of Lowe 

 and DeLong. An average of 23% of the organic S in 15 Australian sur- 

 face soils could not be accounted for by this method even when it was 

 modified to reduce interferences. They calculated C-bonded S by the 

 difference of total S and Hi-reducible S. Each form accounted for 

 about 50% of the total soil S in Australian soils. Similar values 

 were obtained for organic S fractions in 37 Iowa soils (Tabatabai and 

 Bremner, 1972a). Bonner (1973) reported that in 23 Louisiana soils, 

 44% of the total S was Hi-reducible and 25% was C-bonded. This was 

 slightly more than the 63% organic S present in the soil. He also 

 found 14% was calcium phosphate-extractable sulfate S but did not 

 account for all of the total S. No significant relationship was found 

 among total S, organic S, and the various organic-S fractions with 

 yield, S uptake, and S concentration in the plant tissue of a sorghum- 

 sudangrass hybrid. 



Analyses of S fractions in some surface soils from Brazil and sur- 

 face soils from Iowa in the United States indicated that the Brazilian 

 soils contain much more adsorbed inorganic S than lowan soils (Neptune 

 et al., 1975). The average percentage of total S as ester sulfate S 

 and as C-bonded S was 50 and 7%, respectively, for the Brazilian soil 

 and 50 and 11% for the lowan soils. However, they were unable to 

 identifv 42% of the organic S in the Brazilian soils and 34% in the 



