Unified weak and strong soil tests to estimate intrinsic plant available phosphorus pools

Novel approaches for defining and measuring intrinsic P were developed based on the soil P adsorption reactions kinetics in tropical soils. Weakly and tightly adsorbed P pools were proposed as latent variables to describe the internal soil P status. The Olsen extractable P by a weak soil test contained only the weakly adsorbed P pool, and the modified Truog extractable P by a strong soil test contained both pools. The objectives of this study were to develop and test a new method that estimates the tightly adsorbed P pool using mathematical models derived from the state–space modeling technique. Five fertilizer phosphate (0–800 mg P kg^–1 soil) rates were applied to seven tropical soils. Soil P was then extracted with the Olsen and modified Truog methods at nine times during a 180 d incubation. Results showed that the extraction efficiencies by the modified Truog method decreased as soil P sorption site density increased. For the seven soils, 22% to 62% of the weakly adsorbed P pool was extracted by the modified Truog method, and it was 28.8% higher than the Olsen method. For soils with low P sorption capacity, 14% to 36% of the tightly adsorbed P pool was extracted by the modified Truog method whereas only small portions were extracted from soils with high P sorption capacity. This research can be used to develop a mechanistic approach for determining P recommendations designed to optimal yields and minimize the impact of agriculture on the environment.