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

Elevated soil P concentrations may exist where manure has been repeatedly applied over a long period of time leading to regulatory restrictions on phosphorus applications targeted to decrease P loss in field drainage.  Management practices designed to decrease P losses typically do not alleviate the problem of elevated soil P concentrations. 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 the 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 at 0, 400, 800, 1200, 1600 kg-P ha^-1. Grain and forage plots were established as main plots and soil P concentrations were split plots. Mehlich-3 extractable P concentrations were measured biennially 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, with an effect of initial soil P concentration on soil P drawdown rate identified at one location. An average of 20 years (8 to 30 yr over the range of initial soil P concentrations) was estimated for forage plots to return to optimum soil P concentrations, about 100 ppm Mehlich-3 P, while an average of 25 years (10 to 40 yr range) was estimated for grain plots to return to optimum. Future research should investigate the relative proportion of soil P fractions, as Mehlich-3 P concentrations may not be completely describing soil P behavior over time in the absence of P applications.