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

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