Soil Phosphorus Dynamics, Acquisition and Cycling in Crop–Pasture–Fallow Systems in Low Fertility Tropical Soils: A Review from Latin America

Knowledge of the phosphorus (P) dynamics in the soil–plant system, and especially of the short- and long-term fate of P fertiliser in relation to different management practices, is essential for the sustainable management of tropical agroecosystems. A series of field trials was conducted in the tropical savannas and Andean hillsides in Colombia to follow the dynamics of P under different management systems. In tropical savannas in the Llanos of Colombia, in cereal–legume rotations (maize–soybean or rice–cowpea) and ley pasture systems, measurements of soil P fractions indicated that applied P moves preferentially into labile inorganic P pools, and then only slowly via biomass production and microbes into organic P pools under both introduced pastures and crop rotations. Field studies conducted to quantify the residual effectiveness of P fertiliser inputs in crop rotations in terms of both crop growth response and labile P pool sizes, indicated that soluble P applications to oxisols of Colombia remain available for periods that are much longer than expected for ‘high P-fixing’ soils, such as the oxisols of Brazilian Cerrados. In Andean hillsides of Colombia, the impact of short-term planted fallows to restore soil fertility in N and P-deficient soils by enhancing nutrient recycling through the provision of organic matter, was investigated. Results indicated that the fractionation of soil organic matter and soil P could be more effective for detecting the impact of planted fallows on improving soil fertility than the conventional soil analysis methods. Litterbag field studies contributed to characterisation of the rate of decomposition and nutrient release from green manures and organic materials that could serve as biofertilisers. The data sets from these field and greenhouse studies are valuable for further testing and validation of APSIM.