Extractability and adsorption of sulphate in soils

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 SO₄ in NaCl and CaCl₂ solutions was dependent on both the ionic strength and cation species. Addition of small amounts (<～ 10^?2M) of either NaCl or CaCl₂ actually decreased the amount of SO₄ extracted, but SO₄ extractability increased sharply with concentrations of NaCl or CaCl₂ higher than about 0.1 M. At a similar ionic strength, more SO₄ was extracted by NaCl than CaCl₂. Sequential extraction with 1 M NaCl removed essentially all of the absorbed SO₄. The release characteristics of SO₄ were very different to those of phosphate and this difference in behaviour is not easily reconciled with the view that SO₄ is chemisorbed, as is phosphate. Except for a few acid soils with high oxide contents, the capacity of the soils to adsorb added SO₄ was 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.