Oxidation and transformation of elemental sulphur in soils

The oxidation of 35 S‐S 0 ( 35 S‐S 0 , sulphur flowers) added to four cultivated soils was well described using first‐order kinetics. The half‐life ( t l/2 ) of 35 S‐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 35 S‐S 0 oxidised remained primarily as SO   4 2− ‐S in most of the test soils. The amount of KH 2 PO 4 ‐ extractable 35 S‐SO   4 2− ‐S as a percentage of total oxidised 35 S‐S was at a maximum when approximately half of the added 35 S‐S 0 was oxidised. By the end of the 56‐day incubation period, with the exception of the Ultisol, 54–70% of the oxidised 35 S‐S in the test soils was present as 35 S‐SO   4 2− ‐S. The remaining oxidised S is presumed to have been converted into organic matter fractions. This 35 S‐organic‐S could be fractionated into the 0.1 M KH 2 PO 4 ‐extractable organic‐ S (5–13% of total oxidised 35 S‐S) and the 0.01 M NaOH‐extractable organic‐S (14–23% of total oxidised 35 S‐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%).