Technical Abstract: Phosphorus transfer from agricultural soils to surface waters is an important environmental issue. Commonly used computer models like EPIC have not always been appropriately updated to reflect improved understanding of soil P transformations and transfer to runoff. Our objectives were to replace EPIC’s constant sorption and desorption rate factor (0.1) with more dynamic rate factors that could more accurately predict changes in soil labile P upon addition to and depletion of P from soils. From published data, methods were developed to easily determine dynamic sorption and desorption rate constants from soil properties. These methods were tested with data from new soil P incubation experiments where changes in soil labile P after P addition to and depletion from nine U.S soils were measured. Replacing constant 0.1 P sorption rate factors with dynamic factors improved prediction soil Labile P with time after P additions, but more so for high clay than low clay soils. EPIC’s constant 0.1 P desorption rate factor greatly under-predicted soil P desorption. Increasing the constant to 0.6 improved predictions, while dynamic P desorption rate factors most accurately predicted P desorption. Soil P simulations showed that replacing constant P sorption and desorption rate factors with dynamic ones may change dissolved P loads (kg ha-1) in runoff for common soil, cropping, and runoff scenarios by only 1-8% in the long-term but by 8-30% in the short-term. These improvements are recommended given the simplicity of making EPIC’s sorption and desorption rate factors dynamic.