Phosphorus solubility and release kinetics as a function of soil test P concentration

The concentration of phosphorus (P) in overland and subsurface flow is related to the concentration and release rate of P in soil. To more accurately describe soil P loss to runoff water, we investigated P species controlling P release, and the kinetics of P release in three soils ranging in Olsen P from 9 to 159 mg kg(-1). Using solubility diagrams and P fractionation data, soil P release to solution was likely controlled by a combination of Al (variscite), Fe (strengite), and Ca (hydroxyapatite) complexes. Soil P release kinetics were described (P<0.01) by a power-function equation (release=alphat(beta), where t=time). The initial rate of release (alpha) increased (0.3 to 34.9) and release rate with time (beta) decreased (0.405 to 0.079) with Olsen P or CaCl2-P (9-55 mg kg(-1) and 0.201-3.491 mg l(-1), respectively) in the soils. Relative to the release of CaCl2-P (designed to estimate P in soil solution and subsurface flow), two processes appeared to occur simultaneously, a rapid release of P from soil immediately in contact with the solution and a slower diffusion of P from inside soil particles. These processes should be considered when interpreting the release of P to overland and subsurface flow in soils of different P concentration. (C) 2002 Elsevier Science BY All rights reserved.