PHOSPHORUS SORPTION AT FIELD-CAPACITY AND SOIL IONIC-STRENGTH - KINETICS AND TRANSFORMATION

An array of kinetic equations has been used to describe time-dependent sorption of P by soils. In these studies, soil/solution ratios and ionic strengths of the equilibrating solutions did not mimic real field conditions. We investigated the time-dependent P sorption and transformation of five benchmark soils from the semiarid part of northeast Brazil with the aid of rival kinetic models and sequential extraction. Our objectives were to ascertain whether commonly employed kinetic models can describe slow P sorption at field moisture capacity and soil ionic strength, and to determine the changes in labile and nonlabile P with time. The time-dependent P sorption was described by the rival kinetic models in the order: Elovich equation = fractional power function > apparent first order > parabolic diffusion, all of which are indicative of diffusion kinetics with a slow chemical reaction, involving possible ligand exchange between H2PO4- or HPO42- and OH-. Sequential P extraction by the modified Hedley procedure indicated that 60 to 90% of sorbed P was in the labile form. Anion-exchange resin extractable P (AER-P) correlated negatively with dithionite-citrate-extractable AL (AL(d)). An improved correlation between AER-P and Aid with the time (r = - 0.53 [P = 0.05] at 10 d, r = - 0.55 [P = 0.01] at 100 d, and r = -0.60 [P = 0.01] at 200 d) is consistent with diffusion kinetics. Most likely, the slow P sorption involved surface-adsorbed P diffusing to vacant sites in the micropores of AL oxides or Al-substituted Fe oxides. Such internal sites may not be directly accessible to soil solution P.