PHOSPHORUS ACQUISITION AND CYCLING IN CROP AND PASTURE SYSTEMS IN LOWFERTILITY TROPICAL SOILS

Soil-plant processes which enhance P acquisition and cycling in low-P Oxisols were investigated in a crop rotations and ley pasture systems experiment on the Colombian eastern plains. Comparison of rooting patt erns indicated that, despite low available P at depth, there are impor tant differences in root size and distribution among native savanna, i ntroduced forage and crop species which affect their ability to acquir e P from these soils. Differences in crop/forage residue decomposition and P release rates suggest that managing the interaction of residue with soil may help slow P fixation reactions. Despite these difference s, soil P fractionation measurements indicate that applied P moves pre ferentially into labile inorganic P pools, and then only slowly via bi omass production and microbes into organic P pools under both pastures and crop rotations.