Submitted to: Soil Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/10/2000
Publication Date: 2/20/2001
Citation: Mcdowell, R.W., Sharpley, A.N., Brookes, P., Poulton, P. 2001. Relationship between soil test phosphorus and phosphorus release to solution. Soil Science. 166(2):137-149. Interpretive Summary: Continued long-term fertilization of land can lead to available phosphorus concentrations in topsoil that exceed those required for optimum plant growth, increasing the potential for phosphorus loss to surface waters and eutrophication. In comparison to surface runoff, little attention has been paid to phosphorus losses in subsurface drainage waters. However, phosphorus loss by subsurface runoff is now recognized as a significant problem in phosphorus-saturated soils, soils that are poorly drained and high in organic matter, and where soils have received large amounts of manure. Thus, we studied the release of phosphorus from a wide range of agricultural soils form the United Kingdom, New Zealand, and the U.S. We found that when the amount of phosphorus in a soil rises above a critical change or threshold level, the potential for phosphorus release to surface runoff or subsurface drainage is also greatly increased. From an environmental standpoint, fertilizer and manure applications to soil shoul be carefully managed to minimize the risk of this threshold soil phosphorus value being exceeded.
Technical Abstract: Continued fertilizer applications in excess of those required for optimum plant growth can increase soil P concentration and the potential for P movement to surface waters, which can contribute to freshwater eutrophication. Although soil test methods were developed for soil fertility assessment and fertilizer recommendations, they are frequently used for environmental risk assessment. This is due to a lack of consensu on what constitutes a technically defensible environmental soil P test. Several studies have found soil test P (STP) is related to the concentration or release of P into soil solution, surface runoff, or subsurface drainage by two linear relationships of significantly different slopes (p < 0.05), either side of a change point for a limited number of soils. Thus, we investigated the existence and behavior of a change point in soil P release for a wide range of variously managed soils from the U.K., New Zealand, and U.S., varying in pH, organic C and P. Soil P release was determined by CaCl2 extraction (5:1 solution to soil ratio for 30 min). For all soils, CaCl2-P increased with soil test P as either Olsen or Mehlich-3 P (representing a quantity/intensity relationship typical of sorption-desorption isotherms). Statistically significant (p < 0.05) change points for Olsen P occurred in most soils (20 - 112 mg P/kg) and for Mehlich-3 P for the U.S. soils (120 - 190 mg P/kg).