METHOD TO DETERMINE ELEMENTAL SULFUR IN SOILS APPLIED TO MEASURE SULFUR OXIDATION

A method was established for the determination of elemental S (S degre es) in soils. The S degrees added to four arable soils was extracted w ith chloroform, digested with fuming HNO3-KNO3 and determined using in ductively coupled plasma atomic emission spectroscopy (ICP-AES). The m ethod gave recoveries of between 96.6 and 100.4% in air-dried soils an d in the same soils at 50% of the water holding capacity (WHC), with C Vs varying between 0.4 and 5.6%. Complete recoveries were also obtaine d at soil moisture contents of 2 and 4 times of WHC, although precisio n was poorer. In contrast, the digestion method using HClO4-HNO3 was n ot suitable, because it resulted in only about 80% recovery of S degre es. An incubation study showed that the net amount of sulphate-S produ ced was substantially less than the decrease of S degrees in four soil s. After 38 days of incubation at 25 degrees C, the percentages of tot al oxidised S appearing as sulphate-g were greater in the two sandy so ils (64-78%) than in the two clayey soils (34-40%). The difference bet ween the oxidation of S degrees and the production of sulphate-S was p robably due to microbial immobilisation. It is suggested that both mea surements are needed to study the oxidation of S degrees in soils and the availability of oxidised S to crops. Copyright (C) 1996 Elsevier S cience Ltd