Forms of Phosphorus in Soil Receiving Cattle Feedlot Waste

Abstract Cattle feedlot waste (FLW) was applied (176 to 1614 Mg ha−1) to irrigated continuous‐grain sorghum [Sorghum bicolor (L.) Moench] grown on Pullman clay loam (Torrertic Paleustolls) over an 8‐y period. The FLW applications increased the total, inorganic, organic, and available P content and decreased the P sorption index of surface soil (0–30 cm). Amounts of P in the surface soil were highly correlated with the total amount of FLW‐P applied and time since the last application. The proportion of total P as inorganic P increased (34 to 71%) with larger FLW applications. Increases in the amounts of surface soil inorganic and organic P with FLW application were due mainly to increases in labile fractions of these P forms. When FLW applications were stopped, however, soil organic P contents decreased to pretreatment levels more rapidly than inorganic P contents as a result of labile organic P mineralization. Increased P contents of surface soil following FLW applications will increase the potential for soluble and sediment‐bound P to be transported in runoff.