Determination of available sulphate in some soils of Ghana using five extraction methods

Summary A study was undertaken to measure the sulphate-S in some representative soils cropped to cocoa (Theobroma cacao) and coffee (Coffea canephora var. robusta) in Ghana using five extraction methods at two soil-extractant ratios. The least extracting power was shown by 0.1 N HCL. A soil-extractant ratio of 1:10 extracted higher amounts of SO ₄ ^2– –S than a 1:5 ratio. There were highly significant differences among the extractants and among the soils. From the 15 soil samples, the highest amount of SO ₄ ^2– –S was extracted at the 1:5 ratio and at the 1:10 ratio by H₂O and the KH₂PO_4– extracting solution, respectively. There was no correlation between arylsulphatase activity and the various amounts of extractable SO_4/2– –S, and therefore soil arylsulphatase activity seems less appropriate as an indicator of plant-available inorganic SO ₄ ^2– –S. In the three soil profiles analysed, the amounts of H₂O-extractable SO ₄ ^2– –S decreased with depth while those extracted by sodium acetate buffer increased with depth. The possibility of large amounts of extractable SO ₄ ^2– –S in the subsoils should be considered along with the SO ₄ ^2– –S in the surface samples if an estimation is made of the available –S status of cocoa and coffee rooting zones.