Laboratory study of phosphate desorption kinetics in soils of Galicia (N.W. Spain)

The kinetics of phosphate (P) desorption from surface horizons of 31 soils of Galicia (N.W. Spain) varied with the dose of added P (D = doses studied were equivalent to 200, 500, 1000, and 2000 μg P per g soil) and the soil‐P contact time (T = 7,180, and 360 days). Kinetic data for P desorption in aqueous suspensions of soil were fitted by zero order (Z), first order (F), second order (S), parabolic diffusion (PD), Elovich‐type (ET), and two‐constant equation (TC). TC equations gave the best fits (mean r2 = 0.947±0.082). In linearized plots of Z, ET, and PD equations, the slope (K), which gives a measure of the rate of P desorption, increases with D and decreases with increasing T. In corresponding plots of F, S, and TC equations, K describes the constancy of the rate as P desorption progresses, and decreases with increasing D and increases with T. The y‐intercept of the above plots corresponded to a function of Co (the concentration of initially desorbable phosphate) in all cases. For all equations, Co increases dis‐ proportionately with D and exponentially with T. Since the kinetic parameters Co and K vary with sorption conditions, they cannot be considered characteristic of the tendency of a soil to release phosphate. A modified Freundlich equation with independent variables T, D, and t (soil‐water contact time) was also fitted to the kinetic data (mean R2 = 0.959+0.031); the exponents (ß1, ß2, and ß3, respectively) of these independent variables indicate how each one influences P desorption in a given soil.