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State-Space Estimation of the Intrinsic Soil Phosphorus Pools from the Mehlich-3 Test

ABSTRACT Widely popular Mehlich-3 phosphorus (P) was not used in the estimation of the intrinsic status of soil P. The objectives of this study were to develop the state-space model to determine the intrinsic soil P pools, a weakly adsorbed P pool, and a tightly adsorbed P pool, from the Mehlich-3 P. A linear relationship between Mehlich-3 P and the intrinsic soil P pools was derived from theoretical analysis of the extraction process of adsorbed P as competitive exchange by aqueous fluoride. Adsorption experiments of soil P were carried out to determine the dynamics of Mehlich-3 P after fertilizer phosphate was applied to soils. The results showed that state-space models of the two soil P pools could describe the dynamics of soil Mehlich-3 P in P adsorption experiments. Mehlich-3 P was a fraction of the weakly adsorbed P, and this fraction showed a linear relationship with soil P-sorption site density.