Dissolution and agronomic efficiency of phosphate rock and partially acidulated phosphate rock fertilizers in some acid lowland rice soils in tropical Asia

A kinetic experiment was conducted to study the dissolution behavior of some sparingly soluble phosphorus(P) sources such as phosphate rock(PR) and partially acidulated phosphate rock(PAPR) in acid lowland rice soils of the Philippines, Vietnam, Thailand, Indonesia and India. The relative effectiveness of these sources in maintaning soluble P was assessed and compared with that of triple superphosphate(TSP) at different periods of reaction in soils. Further, an incubation experiment was conducted to determine the percent dissolution of the given sources at several periods of incubation in the acidic soils studied. Findings of the study show that the PAPR sources were better providers of soluble P than the PR, throughout the reaction or incubation periods, and that the reactivity among the PAPR products increased with degree of acidulation. These trends were attributed to the higher inherent reactivity of the parent PR for the two PAPR sources, as well as the partial acidulation of the former, leading to the formation of some water-soluble monocalcium phosphate component. The performance of these P sources relative to TSP improved with the period of reaction. This was especially true in soils of relatively low exchangeable calcium content, which appeared to encourage sustained dissolution of the PR and PAPR sources in the given acidic soils. The mean percent dissolution of the P sources over several incubation periods was also found to be strongly dependent on exchangeable calcium content of the soils, and less so on their P-fixing capacity, or on pH. A maximum in soluble P concentrations was apparent from the kinetic study, for instance, at a 3-day reaction period at intermediate P-fixing capacity of the soils. This was attributed to the possibility that although a high P-fixing capacity of soils was necessary for the initial dissolution of a PR in an acidic soil, a too high P-fixing capacity may, however, leave a low soluble P concentration in soil on continued reaction. The relative agronomic effectiveness of the P sources obtained from some field trials in tropical Asia was further related to the dissolution studies in the laboratory. Results suggest that the maximum benefits of using the given sparingly soluble P sources may be expected in acid soils with low exchangeable calcium, and an intermediate P-fixing capacity. In these soils, PAPR sources may perform better than PR materials