Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2002
Publication Date: 3/20/2003
Citation: MCDOWELL, R.W., SHARPLEY, A.N. PHOSPHORUS SOLUBILITY AND RELEASE KINETICS AS A FUNCTION OF SOIL TEST P CONCENTRATION. SOIL SCIENCE. 2003. V. 112. P. 143-154. Interpretive Summary: The concentration and risk of phosphorus (P) loss to overland and subsurface flow is dependent upon the ability of the soil to release P into solution. Intuitively, a soil with a much greater total P concentration should have a greater potential for P release and loss in runoff than a soil of lesser P concentration. Previous work has shown that the potential lfor P loss increases greatly once a certain point in soil test P has been exceeded. However, to fully assess this risk of P loss, if overland or subsurface flow should occur, we must consider the solubility of soil P and the rate at which P comes into solution. We show that P can be rapidly released from the outer surfaces of soil particles and diffuse more slowly from inside soil aggregates. Thus, the forms of P in soil and their relative ease and rate of release must be considered if short- and long- term potential for P transport in overland and subsurface flow are to be accurately estimated.
Technical Abstract: The concentration of phosphorus (P) in overland and subsurface flow is related to the concentration, solubility, and release rate of P in soil. To more accurately describe soil P release to runoff water, we investigated P solubility and desorption kinetics of three soils ranging in Olsen P from 9 to 159 mg kg-1. Analysis of solubility diagrams and P fractionations of these soils indicated soil P release to solution was controlled by a combination of Al (variscite), Fe (strengite), and Ca (hydroyapatite) complexes. Soil P desorption kinetics were described (P < 0.01) by a power-function equation (release = a*time^b). The initial rate of release (a) increased (0.3 to 34.9) and release rate with time (b) decreased (0.405 to 0.079) with Olsen P or CaCl2-P (9 to 55 mg/kg and 0.201 to 3.491 mg/L, respectively) in the soils. Relative to the release of CaCl2-P (designed to estimate soil solution and subsurface flow), two desorption processes appeared to occur simultaneously; the rapid release of P from outer surfaces and slower diffusion of P from inner sphere aggregate soil particles. Clearly, these complex and interactive processes should be considered if short- and long-term soil P release to overland and subsurface flow are to be estimated.