Managing P loss from poultry litter amended pastures has regional importance to the health of sensitive ecosystems. Typical sampling methodology considers that which passes through a 0.45 m filter as dissolved P but many P sorbing particles that behave differently than truly dissolved aquo-ions are <0.45 m and this misclassification may cause errors in predictive models. We assessed the contribution of colloid and nanoparticle-bound P (nPP; 1 nm 415 nm) to P released from these soils after oxic, anoxic, and pH adjusted laboratory incubations and to runoff P in fescue watersheds that typically received bi-annual poultry litter applications since 1995. Soil incubations indicated anoxic conditions in seasonally saturated fields were potentially a major source of generating nPP in solution. An average of 10% of <0.415 m runoff P was present as nPP. Understanding conditions responsible for P loss in agricultural systems and the forms of this P may lead to preventing such loss.