USE OF PHOSPHATE ROCKS IN THE TROPICS

The use of finely ground phosphate rocks (PRs) as directly applied P fertilizers in tropical farming systems is a cheaper alternative to acidulated, water-soluble P products. However, the effectiveness of PRs in tropical environments depends on the extent to which the required P uptake rate of the crop plant can be maintained by the rate of PR dissolution in that soil. That extent that this outcome is achieved depends on the properties of the PR, the soil, climate, plant factors, and on management practice. Environmental conditions in the surface layers of highly weathered soils in the humid tropics are generally conducive of the attainment of satisfactory rates of PR dissolution, especially as the reactivity of the PR increases. In soils with very high P sorption capacities, however, the agronomic effectiveness of PRs is reduced as the acquisition of dissolved P by plant roots is restricted by competition from P sorption processes in the soil. In determining the required reactivity of PRs for use in tropical regions one must consider the rate of P demand by the crop or pasture and the suitability of the soil environment for PR dissolution. The use of some water-soluble P in combination with the PR might enable PRs of low reactivity to also be used, where alone they would be relatively ineffective. The ability of PRs to provide Ca, in addition to P, needs further study because subsoil Ca deficiency is becoming more widely recognized as a production constraint in highly weathered tropical soils. The future use of PRs in tropical agriculture is expected to expand for plantation crops and pastures and especially for landlocked countries with local deposits of PR. Increased use of PRs will also occur where more reactive PRs can effectively be used to increase the yield of annual food crop.