OXIDATION AND TRANSFORMATION OF ELEMENTAL SULFUR IN SOILS

The oxidation of S-35-S-0 (S-35-S-0, Sulphur flowers) added to four cultivated soils was well described using first-order kinetics. The half-life (t(1/2)) of S-35-S-0 ranged from 12 days in a paddy soil to 176 days in an Ultisol. During the early phase of oxidation, a large proportion of the oxidised S was incorporated into organic fractions. After the first week any additional S-35-S-0 oxidised remained primarily as SO42--S in most of the test soils. The amount of KH2PO4-extractable S-35-SO42--S as a percentage of total oxidised S-35-S was at a maximum when approximately half of the added S-35-S-0 was oxidised. By the end of the 56-day incubation period, with the exception of the Ultisol, 54-70% of the oxidised S-35-S in the test soils was present as S-35-SO42--S. The remaining oxidised S is presumed to have been converted into organic matter fractions. This S-35-organic-S could be fractionated into the 0.1 M KH2PO4-extractable organic-S (5-13% of total oxidised S-35-S) and the 0.01 M NaOH-extractable organic-S (14-23% of total oxidised S-35-S). With the exception of the Ultisol where the undetermined organic S was about 36%, the amounts of unaccounted for S (not directly measured), in all soils were very small (typically less than 4%).