Managing Legacy Phosphorus in Artificially Drained Soils.

Long-term over-application of animal manures and fertilizers has led to accumulations of phosphorus (P) in some soils to values that are considerably above agronomic optima and of concern for surface water quality.   Managing this “legacy” P requires long-term strategies and near-term tactics that will sustain agricultural productivity and prevent environmentally significant P losses to water.  Transport of particulate and dissolved P in surface runoff is normally the hydrologic pathway of greatest risk; however, in some settings, leaching and shallow lateral flow of P to nearby surface waters can be a significant contributor to P loss. Installation of artificial drainage systems (ditches, tile drains) to enhance crop productivity in poorly drained soils has been shown to accelerate P transport via subsurface lateral flow, creating unique challenges for our efforts to reduce P export from cropland to water.  Our presentation focuses on two key aspects of this complex issue:  (i) long-term strategies and policies that can address the “legacy” P issue in over-fertilized soils, toward a goal of sustaining soil P values in ranges optimum for crop production; and (ii) practical management practices that can be readily implemented to mitigate P loss from high risk artificially drained soils.