Addressing Variable Phosphorus Solubility of Organic Amendments Inthe Chesapeake Bay Program’s Phase 6 Watershed Model

Because land-applied organic amendments (manures and biosolids) have widely differing susceptibilities to phosphorus (P) solubilization by water, total P (TP) content is an unreliable indicator of their potential to impact water quality. While P can be transported by erosion of Penriched soil particles, much research has addressed P loss to surface runoff and subsurface drainage in the form of dissolved reaction P (DRP) because it is this dissolved P that is algae available and which stimulates eutrophication in aquatic systems.Numerous research studies have shown that P loss as DRP is directly correlated with the water extractable P (WEP) content of surface-spreadsoil amendments. To account for this variable P release to runoff and drainage, states (DE, MD, PA, VA) within the Chesapeake Bay Program (CBP) watershed have P indices which allow for differential weighting of applied P sources based on their WEP. Additionally, the APLE (Annual Phosphorus Loss Estimator) model used in the Phase 6 CBP watershed model (CBP WSM) to simulate P losses allows the percent water extractable P (%WEP = WEP/TP × 100) to be supplied as an input variable for conventional manures.