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A sound prediction of soil organic phosphorus (Po) mineralization would be helpful to improve fertilizer recommendations. However in most mineral soils expensive and time consuming isotopic dilution experiments are required to assess Po mineralization. A proposed method to estimate Po mineralization is the quantification of P in the coarse fraction (CF >53 µm) of the soil. The aim of this study was to evaluate if P in the CF can effectively predict Po mineralization. Soil samples from three North American long term arable field experiments (>10 years) with different textures (sand content 15 to 82) and different fertilization treatments (non P fertilized mineral P fertilizer poultry litter) were analyzed. Soil samples were physically fractioned into CF and fine fraction (FF

Surface application of broiler litter to grasslands can increase concentrations of ammonium (NH₄+-N) and dissolved reactive phosphorus (DRP) in surface runoff. It is not known, however, for how long after broiler litter applications that NH₄+-N and DRP concentrations remain elevated. Five 0.75-ha, fescue-bermudagrass paddlocks received four broiler litter applications in 1995 and 1996, and only inorganic fertilizer N in 1997 and 1998. Runoff from each paddock was measured, sampled, and analyzed for NH₄+-N and DRP. Flow-weighted NH₄+-N and DRP concentrations increased from background values of 0.5 and 0.4 mg L-1, respectively, to values as high as 50.7 mg NH₄+-N L⁻¹ and 18.8 mg DRP L-1 in a runoff event that occurred immediately after the third litter application. Concentrations remained high while broiler litter was being applied but decreased steadily after the last application, reaching values near 1 mg L⁻¹ (for NH₄+-N and DRP) by 19 months after the final application. Among the factors that affected the average concentration of NH₄+-N and DRP in cumulative runoff after a litter application were cumulative runoff, rates of total N and NH₄+-N applied, and cumulative total litter N, total litter P, and water-soluble litter P applied during the four years of the study. Soil test P also affected DRP concentrations, but its effect depended on when the paddocks last received broiler litter. There is a need for tools to identify situations in which the application of broiler litter may enrich surface runoff with P. One such tool is the simulation model Erosion Productivity Impact Calculator (EPIC). EPIC's ability to simulate runoff volume and losses of dissolved reactive P (DRP) was evaluated. Data from the five 0.75-ha, tall fescue-bermudagrass plots that were fertilized with broiler litter during two years, and received only inorganic fertilizer N for the two subsequent years, were compared with EPIC estimates. EPIC simulations of runoff volume in individual events did not show bias in three of the plots but underestimated runoff in one plot and overestimated runoff in another. On an annual basis, the runoff volumes simulated by EPIC were similar to the observed values. A modified version of EPIC yielded better estimates of event DRP losses than the original EPIC and generated estimates of annual DRP loss that were similar to observed values. These results suggest that the modified EPIC may be useful for identifying situations where there is a high risk of large annual P losses from grasslands fertilized with broiler litter. Ph. D.