Abstract
Chemical changes are described, together with some possible explanations, which have occurred in a tropical red-yellow podzolic clay soil after fertilization with ammonium sulphate, mono-calcium phosphate, potassium chloride and the subsequent growth of dry, hill rice. The reactions of mono-calcium phosphate were mainly controlled by iron and aluminium. The latter dominated the exchange complex and soil acidity reactions while both iron and aluminium played major roles in phosphate transformations. Aluminium, iron, calcium and organic-bound phosphates were available to plants, yet withstood leaching. Maize growth after eight months and natural regrowth during the next three months were significantly better on those plots high in non-occluded phosphates. The order of residual effects on natural regrowth was mono-calcium phosphate > potassium chloride > ammonium sulphate (t values at 18 d.f. for the differences between levels 0, 1 were 16.70, 1.79 and 1.50). Potassium chloride reduced the concentration of exchangeable aluminium and interacted with mono-calcium phosphate to increase the amount of iron-and aluminium-bound phosphates. The ‘available’ P determination gives a useful prediction of crop response on this soil.
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