Method to determine elemental sulphur in soils applied to measure sulphur oxidation

A method was established for the determination of elemental S (So) in soils. The So added to four arable soils was extracted with chloroform, digested with fuming HNO3KNO3 and determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The method gave recoveries of between 96.6 and 100.4% in air-dried soils and in the same soils at 50% of the water holding capacity (WHC), with CVs varying between 0.4 and 5.6%. Complete recoveries were also obtained at soil moisture contents of 2 and 4 times of WHC, although precision was poorer. In contrast, the digestion method using HClO4HNO3 was not suitable, because it resulted in only about 80% recovery of So. An incubation study showed that the net amount of sulphate-S produced was substantially less than the decrease of So in four soils. After 38 days of incubation at 25°C, the percentages of total oxidised S appearing as sulphate-S were greater in the two sandy soils (64–78%) than in the two clayey soils (34–40%). The difference between the oxidation of So and the production of sulphate-S was probably due to microbial immobilisation. It is suggested that both measurements are needed to study the oxidation of So in soils and the availability of oxidised S to crops.