Sorption of phosphorus by soil, part 1: Principles, equations and models

The very extensive literature on phosphorus (P) sorption studies is reviewed with the intention of selecting equations and parameter values for use in a soil P dynamics model. Processes considered are fast reversible sorption of P onto surface sites, and various slower processes including reactions which deposit P at depth below surfaces of iron or aluminium oxide minerals in soil or precipitate calcium phosphate. Sorption is considered to take place both onto static soil components and onto mobile sediments or colloids. Phosphorus transport in sediments moving in surface runoff flows has been extensively studied, but problems of colloid facilitated through soil P pollution flows have only recently received attention. There is almost no published information about sorption onto such colloids. Equations are considered for processes, column experiments and P dynamics systems studies. Isotherm equations representing fast reversible sorption have been identified. Instantaneous equilibrium can be assumed for most applications of the fast sorption equations, with the exception of surface erosion studies. Details of some very complex mechanistic models of the slow reaction and deposition processes are presented and discussed. Some simpler equations for these processes from existing field-scale P dynamics models are also presented. It is concluded that, at least in the short term, the mechanistic approach is too complex for incorporation into a systems model of the whole range of P processes, and that further development should represent time-dependent processes by adaptation of the simple equations. (C) 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved.