Ammonium fluoride extraction for determining inorganic sulphur in acid forest soils

Current methods for determining inorganic sulphur (S) in aerated mineral soil horizons often result in underestimates. To overcome this defect we developed 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 SO₄^2- 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 with phosphate or bicarbonate solution. In contrast to the extraction with strongly alkaline reagents (NaOH, KOH, LiOH), the NH₄⁺-NH₃ buffer system in NH₄F prevents the pH of the suspension from exceeding 9.0 and thus the undesired conversion of organic S into SO₄^2- by auto-oxidation and hydrolysis of ester sulphate. In a comparison we demonstrated that the inorganic S in six 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.