Thursday, 13 July 2006 - 11:45 AM

Agricultural Phosphorus and the Environment: Challenges to Science, Practice and Policy.

Andrew Sharpley and Peter Kleinman. USDA Agricultural Research Service, USDA-ARS-PSWMRU, 3702 Curtin Road, University Park, PA 16802-3702

While phosphorus (P) is essential input for profitable crop and livestock agriculture, its loss in runoff accelerates eutrophication of receiving surface waters. Best Management Practices (BMPs) to mitigate P transfers to surface water must address specific agronomic, environmental and socio-economic conditions. These practices can be categorized as those that related to the management of feed, manure, land, and grazing. “Feed BMPs” are designed to reduce the amount of P imported onto farms, such as by decreasing mineral P supplements, adding enzymes that enhance nutrient utilization and absorption by livestock, and feeding hybrid varieties that contain low levels of relatively indigestible phytate-P. “Manure BMPs” involve decreasing the solubility of P in manure with chemical amendments or physical treatment, moving manure from surplus to deficit areas, and development of alternative uses for manure other than land application. “Land BMPs” are designed to limit runoff, erosion and leaching as important pathways of P loss by use of risk assessment indices to guide the rate, method and timing of P applications, and by targeting critical sources areas of P loss for BMP implementation. These include such practices as conservation tillage, terracing, and stream buffers. “Grazing BMPs” aim to decrease the impact of grazing animals on in-stream export of P and include stream-bank fencing, as well as more intensive pasture and grazing management (e.g., stocking rate and duration).The long-term sustainable management of agricultural P begins with reduced inputs of P at both farm and watershed scales. Specifically, P inputs onto a farm should be matched as closely as possible with P export, such as in animal or crop produce. If a farm's P budget is rich in imports, regardless of any other nutrient management decisions, there will be an ongoing accumulation of P on the farm, which in the long-term will ultimately increase the potential for P loss in surface or subsurface runoff when animal manure is land applied. Nevertheless, the short-term impacts of land applying manure on P loss can be successfully mitigated with implementation and maintenance of BMPs. Even so, it is clear that P management at both farm and watershed scales involves a complex suite of various options, which must be customized to meet site-specific needs.Even though there has been a concerted effort to implement remedial measures through voluntary and regulatory means, the long-term challenges of P surpluses at farm, watershed, and regional scales, has been and remains difficult to overcome. Research that better quantifies the sinks and sources of P as it is transported through a watershed will help develop realistic expectations for BMPs. However, more research is neither the single nor the final solution. Many farmers simply do not have the financial resources to implement and maintain costly remedial measures. Despite there being many cost-share programs to help defray remedial costs, institutional red-tape and conflicting requirements, often limit program enrollment and hinder their widespread adoption. Finally, continuing educational efforts with farmers and the public regarding the importance and impact of BMP's of environmental quality parameters will be essential to reach environmental goals. In some instances, local or regional governmental or agency controls may be necessary to enhance quicker adoption of practices that will have a positive influence on environmental outcomes.

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