Excessive soil phosphorus levels cause high concentrations of water-soluble phosphorus in soil, thereby increasing the potential for phosphorus export to streams. Converting water-soluble phosphorus to less soluble forms with lime or calcium-containing coal combustion byproducts can reduce the release of soil phosphorus to surface runoff. A typical agricultural soil at excessive soil phosphorus levels was incubated with four treatments (0 to 20 g kg(-1)) of fluidized-bed combustion fly ash (FBC) and a flue-gas desulfurization (FGD) byproduct. A 10 g kg(-1) application of FBC and FGD to soil reduced the concentration of water-soluble phosphorus by 60% and 50%, respectively. Projection of these results over an agricultural watershed indicates that treating only 4% of the watershed can reduce the loss of water-soluble phosphorus by 30%.