Sulphate retention by New Zealand soils in relation to the competitive effect of phosphate

Abstract

Retention of sulphate and phosphate ions added in solution, alone and in combination, was determined in percolation experiments using individual horizons from soils differing widely in parent material, degree of weathering, and level of native adsorbed sulphate. Weakly weathered soils, formed from moderately siliceous sedimentary parent materials and containing mainly 2:1 type (micaceous) clays contained little native adsorbed sulphate, and had little capacity to retain added sulphate. Moderately to strongly weathered soils, with mainly 1:1 type (kaolinitic) clays contained significant amounts of native adsorbed sulphate and showed an appreciable capacity to retain added sulphate. Weakly weathered soils from volcanic parent materials contained low levels of native adsorbed sulphate but had a greater capacity to adsorb added sulphate than did the “sedimentary” soils. The more weathered “volcanic” soils showed very high levels of native adsorbed sulphate and very high sulphate-retention capacities. In soils from both sedimentary and volcanic parent materials phosphate retention followed similar trends to sulphate retention, although at a higher level. Phosphate retention was little affected by the presence of sulphate at 1:1 and 3:1 S:P ratios (by weight), whereas sulphate retention was strongly impeded by the presence of phosphate. Weakly sulphate-retentive soils retained almost no sulphate at the 1:1 S:P ratio, but exhibited some retention at the 3:1 ratio. Strongly sulphate-retentive soils, on the other hand, were able to retain appreciable amounts of added sulphate even in the presence of phosphate. It is suggested that on many weakly weathered soils with a low level of native adsorbed sulphate the use of superphosphate alone is likely to result in rapid depletion of available sulphur because of the combination of a low sulphate-retention capacity and competition from the more strongly adsorbed phosphate ion.