Abstract
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 ammoniumacetate extracted most of the exchangcable iron, most of the exchangeable phosphate remained on the surface of the soil particles.
Glasshouse experiments were carried out to observe the respons e
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
was partially determined and related to the fertiliser treatments.