Submitted to: Animal Agriculture and the Environment Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 11, 1996
Publication Date: N/A
Interpretive Summary: Phosphorus (P) is an essential element for plant growth, and its input has long been recognized as necessary to maintain profitable crop production. Phosphorus inputs can also increase the biological productivity of surface waters limiting its use for fisheries, recreation, industry, or drinking. In parts of the northeastern U.S., increasing inputs of P in agricultural runoff have accelerated the growth of algae in surface waters. There are two main reasons for this. Firstly, livestock operations have increased dramatically in the U.S., producing in many cases, more P than local crops require. Secondly, several myths about P still exist and have not been adequately dispelled. The most common myths are: Soils are infinite sinks for P - soils cannot indefinitely fix applied P and increasing amounts will be released to runoff; Erosion control will stop P losses in runoff - erosion control is not the sole answer to P loss concerns; reduction of P loss in runoff can only be achieved by integrating source and transport measures; Most Best Management Practices are permanent solutions - the only permanent solution to reducing P losses in most cases is balancing P inputs and outputs; and Phosphorus management strategies will be universally applied - all fields are not created equal, and remedial measures must be tailored to site vulnerability to P loss and proximity of P-sensitive waters. To dispel these myths, clear documentation is needed on the forms and availability of soil P; its release to and transport in runoff; and the best ways to minimize P loss in runoff through source and transport control.
Technical Abstract: The overall goal of efforts to reduce phosphorus (P) losses from agriculture to surface waters should aim to balance off-farm inputs of P in feed and fertilizer with outputs in produce, along with managing soils in ways that retain nutrients and added P resources. Increasing the use- efficiency of P in agricultural systems may be brought about by source and transport control strategies. Although we know how to and have generally been able to reduce the transport of P from agricultural land in runoff and erosion, less attention has been directed toward P management at the source. Future advisory programs should reinforce the fact that all fields do not contribute equally to P export from watersheds. Often most P export comes from only a small portion of the watershed in relatively few storms. Although soil P content is important in determining the concentration of P in runoff, it is likely that runoff and erosion potential will override soil P in determining P export. If water or soil do not move from a field then P will not move. Clearly, remedial measures will be most effective if targeted to the hydrologically active source areas in a watershed operating during these few storms. Management recommendations will have to account for site vulnerability to runoff and erosion as well as soil P content, however, because not all soils and fields have the same potential to transfer P to runoff. As a result, threshold soil P levels should be indexed against P transport potential, with higher values for source areas than for areas not contributing runoff. Consideration of all these factors will be needed in developing extension and demonstration projects that educate farmers and the general public as to what is involved in ensuring