| Summary: | Incubation experiments were carried out to study the chemical changes which occur under waterlogged, and alternate flooded and moist conditions using four soils with and without added urea. Samples were analysed periodically to determine the concentrations of water and ammonium-acetate extractable phosphate, iron, manganese, calcium and potassium. The redox potential, pH and specific conductance of the samples were also measured. In addition, mineralisation of soil organic phosphorus and isotopically exchangeable pool of phosphate and iron were studied. The results showed that with one exception, the redox potential became highly reducing after four weeks of waterlogging and soil pH values tended towards a neutrality. The amounts of extractable iron and manganese increased substantially within four weeks of waterlogging. The concentrations of calcium also increased whilst the concentration of potassium remained almost stable. The concentration of extractable phosphate increased in all soils but to different extents. The results for the alternate flooded and moist conditions were very variable due to non-uniform aeration. The application of urea had no substantial effect on any of the values measured. Water-soluble amounts of iron, manganese, calcium and potassium generally reached peak values after four weeks. The specific conductance values increased substantially within four weeks of waterlogging in all four soils. The studies on mineralisation of organic soil phosphorus showed that, using existing techniques for phosphorus determination, it was difficult to draw any firm conclusions. Isotopic exchange studies indicated a substantial increase in both exchangeable phosphate and iron compared with fresh, aerobic soils. However, whilst ammonium-acetate extracted most of the exchangeable iron, most of the exchangeable phosphate remained on the surface of the soil particles. Glasshouse experiments were carried out to observe the response of rice to added fertiliser phosphorus. These studies showed that higher dry matter yield can be achieved by application of fertiliser phosphorus in a phosphate deficient soil although physical factors are also probably involved. The chemical composition of the rice plants was partially determined and related to the fertiliser treatments.
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