Long-Term Agronomic Drawdown of Soil Phosphorus in Mid-Atlantic Coastal Plain Soils

Elevated soil P concentrations exist where manure has been repeatedly applied over a long period of time leading to regulatory restrictions on P applications targeted to decrease P loss from fields. Phytoextraction uses growing crops to remove contaminants, including P, from the soil through removal of crop biomass. A long-term study initiated in 1994 compared effectiveness of forage and grain rotations common in the mid-Atlantic region at reducing soil P concentrations in the absence of additional P application. Five soil P concentrations were established in replicated plots at three Maryland locations through application of manure. Grain and forage plots were established as main plots and soil P concentrations were split plots. Mehlich-3 extractable phosphorus (M3P) concentrations were measured biennially or annually and rate of soil P drawdown was calculated as a first-order decay equation. Phosphorus concentration was measured in grain and aboveground biomass from grain and forage plots, respectively, to measure crop P removal. Forage plots had greater P removal than grain plots at all locations studied. Forage plots tended to have greater rate of soil P drawdown at all three locations. An average of 20 yr was estimated for forage plots to return to optimum soil P concentrations, about 100 mg kg(-1) Mehlich-3 P, while an average of 25 yr was estimated for grain plots to return to optimum. Future research should investigate the relative proportions of soil P fractions, as Mehlich-3 P concentrations may not be completely describing soil P drawdown behavior in the absence of P additions.