Phosphorus Uptake from Green Manures and Phosphate Fertilizers Applied in an Acid Tropical Soil.

Quantifying the relative contribution of different phosphorus (P) sources to P uptake can lead to greater understanding of the mechanisms that increase available P in integrated P management systems. The 32 P– 33 P double isotope labeling technique was used to determine the relative contribution of green manures (GMs) and P fertilizers to P uptake by Setaria grass ( Setaria sphacelata ) grown in an amended tropical acid soil (Bungor series) in a glasshouse study. The amendments were factorial combinations of GMs ( Calopogonium caeruleum , Gliricidia sepium and Imperata cylindrica ) and P fertilizers [phosphate rocks (PRs) from North Carolina (NCPR), China (CPR) and Algeria (APR), and triple superphosphate (TSP)]. Dry matter yield, P uptake, and P utilization from the amendments were monitored at 4, 8, and 15 weeks after establishment (WAE). The GMs alone or in combination with P fertilizers contributed less than 5% to total P uptake in this soil, but total P uptake into Setaria plants in the GM treatments was three to four times that of the P fertilizers because the GMs mobilized more soil P. Also, the GMs markedly increased fertilizer P utilization in the combined treatments, from 3% to 39% with CPR, from 6–9% to 19–48% with reactive PRs, and from 6% to 37% with TSP in this soil. Both P GM and the other decomposition products were probably involved in reducing soil P‐retention capacity. Mobilization of soil P was most likely the result of the action of the other decomposition products. These results demonstrate the high potential of integrating GMs and PRs for managing P in tropical soils and the importance of the soil P mobilization capacity of the organic components. Even the low‐quality Imperata GM enhanced the effectiveness of the reactive APR more than fourfold. [ABSTRACT FROM AUTHOR]