Environmental Soil Testing for Phosphorus

Abstract

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. Research Question Phosphorus in agricultural runoff and drainage waters can contribute to the eutrophication of surface waters. Many soils in the USA have extremely high soil test P levels as a result of long-term fertilization and manuring. Innovative approaches are needed to assess the potential bioavailability of P in soils and to develop nutrient management programs that reduce nonpoint source pollution by soil P. Soil testing for environmental purposes, rather than to identify situations where crop response to fertilizer P is likely, will require a careful reevaluation of the sampling, analytical, interpretive, and educational roles of soil testing programs. Literature Summary Previous research on nonpoint source pollution of surface waters has clearly identified the importance of soil P management. Many soil testing programs have extensive databases that can assist in prioritizing areas where most intensive soil P management practices are needed. Further, routine soil tests have been shown to be well-correlated with several parameters of importance to eutrophication, such as bioavailable P. Previous research also had identified other analytical procedures for soil P, such as ion exchange resins, iron oxide-impregnated gels, and single-point sorption isotherms, that may have to be adapted as special tests by soil testing labs to enhance existing nutrient management efforts. Study Description This paper reports the results of a survey of labs in four regional soil testing committees, discusses possible approaches to improve interpretation of soil testing results for high P soils, and describes means by which alternative analytical procedures can supplement existing soil testing methods. How extensive is the distribution of soils testing very high or excessive in P? Applied Questions Previous national soil test summaries indicate that, in many states, greater than 50% of commercial soil samples tested are rated as high, very high, or excessive. In the survey conducted for this paper, 86% of the states in the northeastern, north central, mid-Atlantic, and southeastern regional soil testing committees reported that more than 40% of the commercial soil test samples analyzed were high or excessive in P. How can soil testing labs assist in the development of improved, environmentally based nutrient management programs for soil P? More effective use of existing soil test databases can aid in effective allocation of resources to control nonpoint source polluton. Integration of soil testing results with land-use planning information via geographic information systems can contribute to long-term solutions. Development of a consistent approach to rate high P soils in terms of environmental impact is needed and can be accomplished best through the efforts of regional soil testing committees. Soil testing programs should re-evaluate the most appropriate sampling strategies and analytical methods for environmental soil testing for P. New soil testing procedures should be added to lab programs and educational efforts expanded to interpret the results of these tests for farmers, and for individuals in advisory and regulatory agencies.