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.     

Leaching of nitrate,calcium and magnesium under maize cultivation on an oxisol in Togo

In tropical regions, crop yields generally decrease with time, partly due to a decline in the levels of exchangeable bases linked to acidification of the upper layers of the soil. Nitrogen, calcium and magnesium balances were studied on an oxisol in southern Togo under continuous maize cropping with mineral fertilization and crop residue return, by measuring inputs and outputs.The balance was near equilibrium for nitrogen (urea dressing, 120 kg N ha^–1 y^–1) and negative for calcium and magnesium. Leaching represented between 29% and 85% of the total output for nitrogen and accounted for practically all the calcium and magnesium outputs.In the upper horizons, nitrate, calcium and magnesium contents in the soil solution were correlated. This was interpreted as an exchange between the calcium and magnesium ions adsorbed on the soil and the protons released by the transformation of ammonium into nitrate. The nutrient content of the soil solution increased in the case of no potassium fertilization, probably because of a lower nitrogen immobilization by microorganisms, the crop residue yield being reduced by the induced deficiency. An insufficient potassium fertilization increases leaching losses of calcium, magnesium and nitrogen.In the deeper horizons, the nitrate content of the soil solution was not correlated with the calcium and magnesium contents. Nitrogen was probably taken up by roots and some calcium and magnesium ions exchanged during their transport through the soil.     

Response of rice cultivars to phosphorus supply on an oxisol

Genotypic differences in absorption or utilization of P might be exploited to improve efficiency of fertilizer use or to obtain higher productivity on P-deficient soils. The objective of this study was to evaluate responses by 75 genotypes of upland rice (Oryza sativa L.) to two soil P levels in two field experiments. In the first experiment, soil P levels (Mehlich 1) were 1.5 mg kg^–1 and 5 mg kg^–1, and in the second experiment, 3 mg kg^–1 and 4.7 mg kg^–1 of soil, respectively. Rice cultivars differed significantly in shoot dry matter production at flowering, grain yield, and plant P status. Based on a grain yield efficiency index, cultivars were classified as P-efficient or P-inefficient. Shoot dry matter was more sensitive to P-deficiency but was not related to grain yield. Phosphorus use efficiency was higher under the low P treatment. Phosphorus uptake was significantly correlated with dry matter, P concentration and P-efficiency ratio. Results of this study indicate that genetic differences in P-use efficiency exist among upland rice cultivars and may be exploited in breeding programs.Contribution from National Rice and Bean Research Center of EMBRAPA, Goiania, Goias, Brazil and Appalachian Soil and Water Conservation Research Laboratoy, Beckley, WV, USA.