ENVIRONMENTAL SOIL TESTING FOR PHOSPHORUS

Many soils in the USA have extremely high soil test P levels from long-term fertilization and manuring. Sediment-bound and soluble P in runoff from these soils may contribute to eutrophication of surface waters. A field rating system, the ''P index,'' has been developed to assess the potential for soil P to contribute to nonpoint source pollution. A critical component in this index is soil test P. The primary objective of this paper is to discuss the roles soil testing programs can play in the development of nutrient management strategies, such as the P index, that are needed to minimize nonpoint source pollution by soil P. A survey of soil testing labs participating in four regional soil testing committees (North Central, Northeast, Mid-Atlantic, Southeast) was conducted in 1991-1992 to determine current approaches to soil P testing, the percentage of soils testing in the high or excessive range, and major concerns with high P soils. Results indicated a need for more consistency in defining and identifying soils that are excessive in P, from an environmental standpoint, and that P management in animal waste-amended soils was the major environmental issue for most states. Soil P testing for environmental purposes will require a careful re-evaluation of the sampling, analytical, interpretive, and educational roles of soil testing programs. Alternatives considered in this paper include integration of soil testing databases with land-use planning information via geographic information systems, the use of special soil tests for biologically available P, or to estimate P sorption/desorption, and expanded educational efforts focused not only on farmers, but on advisory and regulatory agencies and the general public.