Phosphorus (P) runoff and leaching from agricultural fields has been identified as a major environmental concern for the health of aquatic ecosystems. North Carolina has responded by implementing the Phosphorus Loss Assessment Tool (PLAT). The goal of the PLAT software is to determine the relative P losses from agricultural fields based on several site factors and characteristics including Mehlich 3 P (M3-P) test values. Currently in PLAT, soluble P losses from organic soils are predicted to be significantly greater than from sand, silt, and clay dominated soils with the same M3-P, due to previously derived information that is used in PLAT. Recent research has shown that certain organic soils may contain significant concentrations of Al that may lead to greater P retention than previously expected (Maguire and Sims, 2002). Furthermore, research specific to North Carolina's organic soils has also indicated decreased soluble P release when coupled with high Al concentrations (Buol and Dolman, 1967, Fox and Kamprath, 1971, Johnson, 2005). This study was performed to (i) determine how much Al the organic soils (Typic Medisaprists and Terric Medisaprists) of North Carolina's Lower Coastal Plain (NCLCP) contain, and (ii) how the Al in these soils affects P retention. Four organic soil series in NC were sampled across NCLCP and M3-P, water soluble P, pH, particle size, and total carbon were analyzed. Water soluble P and M3-P were also measured in a 21 day incubation study where P was added at a fertilization rate equivalent to 150kgP/ha. Additionally, a fractionation study of the incubated soils was performed to compare P pools between organic soil series and relate the fractions to the observed P retention. From these results the appropriate algorithms to effectively predict P release from the organic soils within PLAT will be modified. The results of these experiments and their implications will be discussed. References: (1) Buol, S. W., Dolman, J. D. A Study of Organic Soils (Histosols) In the Tidewater of North Carolina. December, 1967. Technical Bulletin Number 181. (2) Fox, R.L., and E.J. Kamprath. 1971. Adsorption and leaching of P in acid organic soils and high organic matter sand. Soil Sci. Soc. Am. J. Proc. 35:154-155. (3) Johnson, A.M. 2004. Phosphorus loss assessment in North Carolina. Ph.D. diss. North Carolina State Univ., Raleigh. http://www.lib.ncsu.edu/theses/available/etd-09012004-082806/unrestricted/etd.pdf (verified 17 June 2005). (4) Maguire, R.O., and J.T. Sims. 2002. Measuring agronomic and environmental soil phosphorus saturation and predicting phosphorus leaching with Mehlich 3. Soil Sci. Soc. Am. J. 66:2033-2039.