Relating Soil Phosphorus Indices to Potential Phosphorus Release to Water P. S. Hooda,* A. R. Rendell, A. C. Edwards, P. J. A. Withers, M. N. Aitken, and V. W. Truesdale ABSTRACT Relationships between soil test phosphorus (STP) and release of P in surface and subsurface runoff are needed to help identify source areas for implementing management strategies to limit P loss to water. To determine whether soil P release could be predicted either by STP values, sorption-desorption indices, or the degree of soil satura- tion with phosphorus (DSSP), 11 sites with contrasting chemical prop- erties and management histories were sampled from long-term field trials in the UK. Each site offered up to three treatments, resulting in a total of 29 soil samples. The results showed that the amount of P desorbed using a successive dilution procedure had no relationship with either total soil P content or P sorption capacity. The most significant property was the extent of P saturation. There was little desorption for DSSP values below 10%; above this point, the amount of P desorbed increased linearly with the DSSP. Five STP methods (Olsen, Mehlich-3, acidified ammonium oxalate-oxalic acid, Fc 2 O,- coated paper strip, and distilled water) were compared to predict their effectiveness in predicting potential P release to water. While STP values obtained using acidified ammonium oxalate proved to be least effective, those extracted with water correlated best with the amount of P desorbed, accounting for 96% of the variability in differential P release from the soils. P HOSPHORUS transport in agricultural runoff can ac- celerate eutrophication in surface waters (Sharpley P.S. Hooda, A.R. Rendell, and V.W. Truesdale, School of Biological and Molecular Sciences, Oxford Brookes Univ., Oxford OX3 OBP, UK; A.C. Edwards, Macaulay Land Use Research Institute, Craigie- buckler, Aberdeen AB15 8QH; P.J.A. Withers, ADAS Bridgets, Win- chester SO21 1AP; and M.N. Aitken, Scottish Agricultural College, Auchincruive, Ayr KA6 5HW, UK. Received 11 Aug. 1999. '''Corre- sponding author (pshooda@brookes.ac.uk). Published in J. Environ. Qual. 29:1166-1171 (2000). and Menzel, 1987). The loss of P from agricultural soils in surface runoff or subsurface drainage therefore can have potentially serious implications for water quality. The magnitude of P loss from soils is influenced by various factors, including land use (Nearing et al., 1993), soil type (Miller, 1979; Hooda et al., 1997), soil P (Heck- rath et al., 1995; Pote et al., 1996; Hooda et al., 1999), amount and intensity of rainfall (Edwards and Daniel, 1993), and time and rate of fertilizer and manure appli- cation (Edwards and Daniel, 1993; Hooda et al., 1999). However, soil P is the underlying source of P in agricul- tural runoff, and increases in soil P status due to surplus P inputs therefore represent a long-term eutrophica- tion hazard. The potential loss of P from soils has been assessed using STP methods (e.g., Olsen, Mehlich-3, and acetic acid) that were originally designed to estimate plant available P (Miller et al., 1993). A certain degree of relationship has been demonstrated to exist between STP and the loss of P in runoff. These relationships generally have been tested for situations having similar soil type (Heckrath et al., 1995; Austin et al., 1996; Pote et al., 1996; Hooda et al., 1999). However, recent research suggested that an approach based upon STP might lead to ambiguous conclusions, particularly when different soils are compared, due to differences in the nature of P-adsorbing surfaces. For example, Sharpley (1995), assessing P loss from a group of 10 soils amended with poultry litter, showed that two soils of 200 mg kg" 1 Abbreviations: STP, soil test phosphorus; DSSP, degree of soil satura- tion with phosphorus; DRP, dissolved reactive phosphorus; PSI, phos- phorus sorption index; PSM, phosphorus sorption maxima.