Phosphorus in Soil and Subsurface Tile Drainage as Affected by Poultry Manure Application for Corn-Soybean Rotations.

This study evaluated effects of poultry manure application for corn-soybean rotations on soil P and P loss through subsurface tile drainage. Manure from egg layers was applied from 1998 until 2006 at an Iowa site with Nicollet soil (Aquic Hapludoll). Corn and soybean were planted each year to 0.4-ha plots. The manure was applied before corn at 0, 168, and 336 kg N ha^-1, which over time averaged zero, 60, and 120 kg P ha^-1. Tiles were installed at a 1.2-m depth, and collected the combined drainage from the two crop sides of each plot. We sampled soil and dissolved reactive P (DRP) in tile drainage during the last seven years. Soil samples (0-15, 15-30, 30-60, 60-90 and 90-120 cm) were collected each year and were analyzed for Bray-1 P. The shallowest samples also were analyzed for Mehlich-3 P, Olsen P, and a P saturation index [P/(Al+Fe) molar ratio]. Manure application resulted in a large STP increase in the 15-cm top layer and a small increase in the 15-30 cm layer. Bray-1 P in the surface layer was 20-30, 65-75, and 90-130 mg P kg^-1 over time for manure rates zero, low, and high, respectively. Mean annual DRP concentrations or loads did not increase consistently over time. The seven-year averages were 12, 10, and 30 ug L^-1 and 88, 58, and 143 g P ha^-1 for manure rates zero, low, and high, respectively. Soil-test P and soil P saturation were highly correlated. The DRP concentrations ad loads did not increase with increasing STP until values of 80-100 mg P kg^-1 (Bray-1 or Mehlich-3). Phosphorus leaching from poultry manure application does not pose a water quality concern for conditions similar to those in this study until soil  is four times higher than optimum levels for corn and soybean production.