ABSTRACT:
The contribution of phosphorus (P) to non-point source (NPS) pollution of surface and groundwaters is a serious environmental problem in Delaware. In 1999, the Delaware Nutrient Management Act was passed limiting application of P on “high” P soils to a “three year crop removal” rate or to the amount recommended by a University of Delaware P site index. The Delaware P site index was developed and evaluated on seven farms in Delaware, through a joint effort between the universities of Delaware and Maryland. Results showed that 78% of fields evaluated were in the “low” risk category, with the remaining 22% falling into the “medium” (6%), “high” (7%), and “very high” (9%) risk categories. The components of the index found to have the greatest influence on P site index ratings were soil erosion, subsurface drainage, leaching potential, distance from field to surface water, soil test P and organic P application rates and methods. P site index ratings were found to vary by year, depending on manure applications, suggesting a need for yearly P site index evaluations or averages over a cropping rotation. The P site index worked well for identifying fields with differing relative potential risks of P loss; however, validation of these P loss assessments is needed to ensure that the risk categories assigned are sufficiently protective of water quality. Continual monitoring, analysis, and improvement of the P site index are needed to ensure that it remains a useful tool for P based nutrient management planning in the future.
Footnotes
April B. Leytem is a soil scientist with the U.S. Department of Agriculture-Agricultural Research Service, Northwest Irrigation and Soils Research Laboratory, Kimberly, Idaho. J. Thomas Sims, is a professor with the Department of Plant and Soil Science at the University of Delaware in Newark, Delaware. Frank J. Coale is a professor with the Department of Natural Resource Sciences and Landscape Architecture at the University of Maryland in College Park, Maryland.
- Copyright 2003 by the Soil and Water Conservation Society
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