Managing Phosphorus Leaching in Mid-Atlantic Soils: Importance of Legacy Sources

Application of phosphorus (P) inputs to soils saturated with legacy P can significantly increase the risk of P leaching and deteriorate water quality. Our objectives were to quantify the effect of soil type and extent of P saturation on P leaching and determine the suitability of a rapid, inexpensive soil P saturation test for use in P leaching risk assessment protocols. We collected 18 undisturbed lysimeters (30-cm diameter, 50 cm deep), using a tractor-mounted corer, from three typical mid-Atlantic soils in Delaware, where P leaching is a concern. The soils were selected on the basis of Mehlich-3 P saturation ratio (M3-PSR) of optimum (< 0.10) to environmental (> 0.15) thresholds. Lysimeters were irrigated with the equivalent of 50 mm of water each week for a total of 16 wk. Concentrations of dissolved reactive P (DRP) and total P in leachate were not significantly different between optimum and environmental M3-PSR lysimeters before fertilizer application (Weeks 1-8). However, concentrations of DRP and total P significantly increased after fertilizer application at 85 kg P ha(-1) (Weeks 9-16) in the environmental M3-PSR lysimeters. Among three soils in both M3-PSR categories, concentrations and loads of P leached were higher from the Matapeake silt loam and Pocomoke sandy loam due to preferential leaching that limited contact of flowing water and P with the soil P-fixing constituents (Fe, Al, and Ca), while P leaching was lower from Woodstown sandy loam due to matrix flow that resulted in greater interaction of water and P with the soil constituents. These results provide clear evidence of a greater risk of P leaching from P-saturated soils with preferential flow pathways and show that the M3-PSR along with information about preferential flow pathways should be used to predict the risk of P leaching.