Phosphorus loss in overfertilized soils: The selective P partitioning and redistribution between particle size separates

Using 12 well-characterised European soils contrasting in their physical and chemical properties, we demonstrate significant differences in the partitioning of phosphorus (P) between various particle size separates. Samples of each soil were subjected to one of three methods of increasingly aggressive dispersion in the order water (WD), mechanical (MD) or chemical (CD). A general, although not exclusive, inverse relationship existed between the concentration of various attributes (these included organic carbon, oxalate and dithionite iron contents, total P, resin and water extractable P) and particle size. The quantity, composition and physico-chemical properties of individual size separates were sensitive to the dispersion method used. This suggested an interaction between the ease of dispersion and particulate properties even within a single size class. Typically the total P concentration was highest in the most easily dispersed (WD) clay fraction. The total P content of the whole soil could be reasonably obtained by summing the appropriate quantities of each particle size material, however, resin and water extractable P were not adequately predicted using this approach. Together these observations have implications for laboratory and field situations where artificial disaggregation and/or selective separation and transport of fine soil material may occur. Finally a comparison of water soluble P between whole soils and their MD dispersed size fractions suggested three broad situations might exist, where either sorption processes, a precipitate or some combined P precipitate and sorption mechanism controlled P release.