In-Field and Edge-of-Field Chemical Barriers to Dissolved Phosphorus Losses.

Standard vegetative in-field and edge-of field conservation practices, such as grassed waterways, filter strips and riparian buffers, are effective barriers to particulate phosphorus (P) loss in runoff. However, these practices are not as effective at reducing dissolved P losses. In some soils and landscapes, dissolved P losses can equal or exceed particulate P losses making it impossible to achieve TMDL reductions through implementation of standard conservation practices alone. High levels of dissolved P losses are frequently associated with high soil P concentrations resulting from legacy applications of P in excess of crop requirements. In some cases, soils are so over saturated with legacy P that it will require decades of P removal by crop uptake to remediate the situation. Once these conditions exist, changes in field management practices cannot stem the losses over the short term. Chemical barriers to P losses involve the introduction of P sorbing materials, and several approaches have been proposed. Phosphorus sorbing materials provide a metal cation, such as iron, aluminum or calcium, to react with dissolved phosphorus to create an insoluble compound by sorption processes. Reported reductions in dissolved P range from > 90% in batch studies to < 25% in field studies where variable soil conditions and the dynamics of runoff events complicate simple sorption reactions. Chemical amendments have also been proposed for improving flocculation and thereby reducing colloidal P transport to tile drains. This presentation will discuss in-field and edge-of-field chemical barriers that have been proposed as effective means of reducing P losses in surface and ground water, as well as the challenges to their adoption by farmers and action agencies.