EXTRACTABILITY AND ADSORPTION OF SULFATE 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. Copyright © 1990, Wiley Blackwell. All rights reserved