SW—Soil and Water: Sorption of Phosphorus by Soil, Part 2: Measurement Methods, Results and Model Parameter Values Review Article

The very extensive literature on phosphorus (P) sorption studies is reviewed with the intention of selecting rate constants and other parameter values for the constituent equations in a soil P dynamics model. Processes considered are fast reversible sorption of P onto surface sites, and slow 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 immobile soil components and onto mobile colloids, but there is almost no published information about the latter. Experimental methods are discussed (including various ‘soil P tests’) for determining both quantities of P in various categories, and parameters of equations (previously presented in Part 1 of this paper) representing sorption processes. Some data from literature sources are presented. These include parameter values (for a few soils) for isotherm equations representing fast reversible sorption, suitable for most applications where instantaneous equilibrium can be assumed. Some rate constants have been identified for a simple representation of the various slow reaction and deposition processes, and also for fast sorption where time dependence needs to be considered (only for surface erosion studies). Plant availability of P depends on desorption as the reverse of the fast sorption process, and the extent of this is limited to the P which has not undergone the slow deposition process. However, the level of biologically available phosphorus (BAP) in a soil or soil-derived sediment, which leads to eutrophication of water bodies, includes much of the P deposited by slow reactions as well as plant available components.