Extractability and adsorption of sulphate in soils

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Abstract
SUMMARY: Virtually all of the indigenous sulphate (SO4) in a range of UK soils with moderately high pH values (> 6) was found to be present in the soil solution and, as a consequence, was highly susceptible to leaching. For acid soils containing adsorbed SO4, the extractability of SO4in NaCl and CaCl2solutions was dependent on both the ionic strength and cation species. Addition of small amounts (<∼ 10−2M) of either NaCl or CaCl2actually decreased the amount of SO4extracted, but SO4extractability increased sharply with concentrations of NaCl or CaCl2higher than about 0.1 M. At a similar ionic strength, more SO4was extracted by NaCl than CaCl2. Sequential extraction with 1 M NaCl removed essentially all of the absorbed SO4. The release characteristics of SO4were very different to those of phosphate and this difference in behaviour is not easily reconciled with the view that SO4is chemisorbed, as is phosphate. Except for a few acid soils with high oxide contents, the capacity of the soils to adsorb added SO4was quite small. None of the soils with pH values higher than 6 adsorbed a significant amount SO4. The results raise questions regarding the efficiency of SO4‐containing fertilizers in correcting and preventing S deficiency in situations where leaching is important.