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

Core Ideas Rate of soil P drawdown is a slow process having economic impacts to producers. Cropping system affected drawdown of soil P concentrations at all locations. Forage cropping systems exhibited faster soil P drawdown. Lower initial soil P status resulted in faster soil P drawdown at one location. Soils could take 18 to 44 yr to return to agronomic optimum P concentrations. Phytoremediation remains the only management option to lower soil P concentrations. 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.