Organic matter can reduce P adsorption/precipitation (PA/P) by: release of organic acids which compete for phosphate adsorption sites; trough the formation of P- compounds in the soil solution; and by building complexes with Fe and Al and thus decreasing PA/P. The aim of this study was to evaluate the PA/P in Oxisols, by addition of organic acids: citric (CA), oxalic (OA), salicylic (SA), and humic acids (HA). An amount of 2.5 cm³ samples of a clayey Red Latosol (Rodic Hapluxtox) and a medium-texture Red-Yellow Latosol (Hapluxtox) were placed in erlenmeyer. K2HPO4 and/or organic or humic acids were added in different application forms (phosphate added before, together, or after the acid application) with organic acid/P molar ratios ranging from 0 to 2:1. The humic acid doses ranged from 0 to 89.28 mg cm-3, equivalent to organic fertilization with additions of 0 to 40 t ha -1 of organic matter. The effect of organic or humic acids on the reduction in PA/P followed the order: CA > OA > HA > SA. The application form also influenced the PA/P in both soils. In the Red Latosol, the greatest reduction in PA/P was caused by the combined application of P and organic and humic acids, indicating a retention of phosphate by organic acid/humic acids. In the Red-Yellow Latosol, the greatest reduction in P adsorption was observed when P was applied after organic or humic acids, suggesting the blockage of P sorption sites by these acids.
phosphorus; organic matter; competitive adsorption
