Blackwell Publishing Journal Backfiles 1879-2005
Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
Various extractants used in current analytical procedures for the fractionation of sulphur (S) in soils were compared with respect to their ability to dissolve Al hydroxy sulphate minerals of defined composition (basaluminite, K alunite, mixtures of basaluminite and Na alunite). The minerals were synthesized and aged in the laboratory at 20°C and 50°C. The dissolution kinetics at 20°C of these Al hydroxy sulphates in deionized water, 0.02M HCl and 0.02M NaOH were also investigated.The dissolution stability of the Al hydroxy sulphate minerals increased in the order basaluminite 〈 K alunite 〈 Na alunite. The dissolving power of the reagents used increased in the order H2O ≤ 0.016M KH2PO4≪ 0.02M HCl ≈ acidic NH4 oxalate ≈ 0.5M NaHCO3 〈 Na2 CO3/NaHCO3 〈 0.1M NH4F 〈 0.5M NH4F 〈 0.05M NaOH ≤ 0.05M LiOH. In Al hydroxy sulphate-containing soils, inorganic S is probably understimated and ester sulphate S overestimated, if the inorganic S pool is assessed by extraction with KH2PO4, Ca(H2PO4)2 or NaHCO3 solutions. The dissolution of all studied Al hydroxy sulphates, particularly that of K and Na alunite, in H2O and 0.02 M HCl is strongly delayed by kinetic restraints. Thus it seems unlikely that Al3+ or SO2−4 activites in soil solutions are strictly regulated by precipitation/dissolution equilibria of these minerals except for horizons with extraordinarily slow seepage water movement.
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