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 io nic strengths of the equilibrating solutions did not mimic real field conditions. We investigated the time-dependent P sorption and transfor mation of five benchmark soils from the semiarid part of northeast Bra zil with the aid of rival kinetic models and sequential extraction. Ou r objectives were to ascertain whether commonly employed kinetic model s can describe slow P sorption at field moisture capacity and soil ion ic strength, and to determine the changes in labile and nonlabile P wi th time. The time-dependent P sorption was described by the rival kine tic models in the order: Elovich equation = fractional power function > apparent first order > parabolic diffusion, all of which are indicat ive of diffusion kinetics with a slow chemical reaction, involving pos sible ligand exchange between H2PO4- or HPO42- and OH-. Sequential P e xtraction 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 (A L(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 v acant sites in the micropores of AL oxides or Al-substituted Fe oxides . Such internal sites may not be directly accessible to soil solution P.