Modeling the Extraction Reactions of Olsen and Modified Truog Tests to Improve the Measurements of Soil Phosphorus Pools.

The available phosphorus (P) in tropical soils was quantified by two intrinsic pools including the weakly and tightly adsorbed P pools. The objective of this study was to measure the tightly adsorbed P pool by the modified Truog P tests (0.01 mol L^-1 sulfuric acid and 3% ammonium sulfate). The extraction reactions were ligand exchange between sulfate in the modified Truog extractant with the two pools. In the P adsorption kinetics experiments, five levels of fertilizer phosphate (0-800 mg P kg-1 10 soil) were applied to seven tropical soils at the beginning of experiments and soil P was extracted with the Olsen method and the modified Truog method at nine times during 180 days of adsorption. The weakly adsorbed P pool was measured from the dynamics of Olsen test, and the tightly adsorbed P pool at a sampling time was equal to external fertilizer P subtracted by the weakly adsorbed pool at the time. The extraction efficiencies , i.e., percentages of the two pools extracted by the modifed Truog test, were estimated based on the linear relationship between the extractable P by the modified Truog method and the two pools. Results showed that the extraction efficiencies of the tightly adsorbed pool decreased as the increasing of soil P sorption capacity quantified by soil P sorption-site density (PDSS).