Ammonium fluoride extraction for determining inorganic sulphur in acid forest soils

Current methods for determining inorganic sulphur (S) in aerated mineral so il horizons often result in underestimates. To overcome this defect we deve loped a new method combining a batch extraction with 0.5 m NH4F solution at a soil:solution ratio of 1:5 with a subsequent analysis of the mobilized S O42- by ion chromatography. The ammonium fluoride extraction enables us to characterize inorganic sulphate in non-calcareous forest soils. It is more efficient than conventional procedures in which inorganic S is extracted wi th phosphate or bicarbonate solution. In contrast to the extraction with st rongly alkaline reagents (NaOH, KOH, LiOH), the NH4+-NH3 buffer system in N H4F prevents the pH of the suspension from exceeding 9.0 and thus the undes ired conversion of organic S into SO42- by auto-oxidation and hydrolysis of ester sulphate. In a comparison we demonstrated that the inorganic S in si x German forest soils is underestimated by up to 50% or 200 kg S ha(-1) in the uppermost 60 cm, if it is assessed by extraction with 0.016 m KH2PO4 or 0.5 m NaHCO3 instead of 0.5 m NH4F. Conversely, the pool of ester sulphate is overestimated almost threefold.