Biosolids being recycled to agricultural land in the UK have recently contained a higher proportion of Phosphorus (P), iron (Fe), Aluminium (Al) and/or Calcium (Ca), or have been physically altered into pellets (thermal drying) or composts. Whilst P-stripping of wastewater at Sewage Treatment Works reduces P discharge concentrations to water directly, the subsequent addition of sludge with a low N:P ratio to land may also increase diffuse P losses in agricultural runoff through excessive accumulation of soil P. Numerous studies have shown linear or non-linear increases in runoff P as soil extractable P levels increase , however, the solution concentrations and mobility of P in soils is largely governed by sorption and precipitation reactions with Fe, Al and Ca. Since these very reactive elements are also being applied in greater quantities in biosolids, it is probable that they will therefore influence the subsequent availability of P in sludge-amended soils. This paper reports on an incubation study which investigated how changes in extractable P in soil and potential release to runoff might be predicted by soil and sludge properties. Eight biosolids representative of materials applied to land in England were applied to each of five soils with varying physical and chemical characteristics at three rates supplying 200, 1000 and 2000 kg P ha-1 to provide a large range in potential soil P saturation. The biosolids had been advanced-treated to varying degrees by either anaerobic digestion and dewatering with or without Fe, lime stabilization with Fe or without Fe, or thermal drying without Fe. The five amended soils were incubated for 60 days. Sorption isotherms were conducted on the control soils and the soils amended with the varying rates of biosolid. The results of this incubation study have considerable implications for operational guidance on the amounts of biosolid which can be applied to different soil types in England and Wales. In particular, our results show that even when 20 years of sludge P supply was incorporated into the soil, there may be no increase in the release of P into solution depending on the extent to which the degree of P saturation and Phosphorus Binding Capacity (PBC) in the soil has altered. The methodology developed here provides a potential means of quantifying the amount of biosolid that can be applied to land before accelerated release of soil P will occur.