Extractability and adsorption of sulphate in soils

SUMMARY Virtually all of the indigenous sulphate (SO 4 ) 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 SO 4 , the extractability of SO 4 in NaCl and CaCl 2 solutions was dependent on both the ionic strength and cation species. Addition of small amounts (<∼ 10 −2 M) of either NaCl or CaCl 2 actually decreased the amount of SO 4 extracted, but SO 4 extractability increased sharply with concentrations of NaCl or CaCl 2 higher than about 0.1 M. At a similar ionic strength, more SO 4 was extracted by NaCl than CaCl 2 . Sequential extraction with 1 M NaCl removed essentially all of the absorbed SO 4 . The release characteristics of SO 4 were very different to those of phosphate and this difference in behaviour is not easily reconciled with the view that SO 4 is chemisorbed, as is phosphate. Except for a few acid soils with high oxide contents, the capacity of the soils to adsorb added SO 4 was quite small. None of the soils with pH values higher than 6 adsorbed a significant amount SO 4 . The results raise questions regarding the efficiency of SO 4 ‐containing fertilizers in correcting and preventing S deficiency in situations where leaching is important.