EXTRACTABLE SULFATE AND ORGANIC SULFUR IN SOILS AND THEIR AVAILABILITY TO PLANTS

Ten soils collected from the major arable areas in Britain were used t o assess the availability of soil sulphur (S) to spring wheat in a pot experiment. Soils were extracted with various reagents and the extrac table inorganic SO4-S and total soluble S(SO4-S plus a fraction of org anic S) were determined using ion chromatography (IC) or inductively-c oupled plasma atomic emission spectrometry (ICP-AES), respectively. Wa ter, 0.016 M KH2PO4, 0.01 M CaCl2 and 0.01 M Ca(H2PO4)(2) extracted si milar amounts of SO4-S, as measured by IC, which were consistently sma ller than the total extractable S as measured by ICP-AES. The amounts of organic S extracted varied widely between different extractants, wi th 0.5 M NaHCO3 (pH 8.5) giving the largest amounts and 0.01 M CaCl2 t he least. Organic S accounted for approximately 30-60% of total S extr acted with 0.016 M KH2PO4 and the organic C:S ratios in this extract v aried typically between 50 and 70. The concentrations of this S fracti on decreased in all soils without added S after two months growth of s pring wheat, indicating a release of organic S through mineralisation. All methods tested except 0.5 M NaHCO3 - ICP-AES produced satisfactor y results in the regression with plant dry matter response and S uptak e in the pot experiment. In general, 0.016 M KH2PO4 appeared to be the best extractant and this extraction followed by ICP-AES determination was considered to be a good method to standardise on.