Phosphorus Saturation in Spodosols Impacted by Manure

ABSTRACT Significant amounts of phosphorus (P) accumulate in soils receiving animal manures that could eventually result in unacceptable concentrations of dissolved P loss through surface runoff or subsurface leaching. The degree of phosphorus saturation (DPS) relates a soil's extractable P to its P sorbing capacity, and is reportedly a predictor of the P likely to be mobilized from a system. A DPS value (DPS‐1) was derived that expressed the percentage of Mehlich 1–extractable P to the sorbing capacity of a Spodosol (expressed as the sum of oxalate‐extractable Fe and Al). Values of DPS‐1 were determined in various horizons of soil in current and abandoned dairy systems in South Florida's Lake Okeechobee watershed to assess P release potential. Land use within the dairies was classified as highly impacted by cattle (intensive and holding), and minimally impacted by cattle (pasture, forage, or native) areas. The A and E horizon of soils in heavily manure‐impacted intensive and holding areas for both active and abandoned dairies generally had higher DPS‐1 values than the pasture, forage, and native area soils, which were minimally impacted by manure. Degree of P saturation was also calculated as a percentage of Mehlich 1–extractable P to the sum of Mehlich 1–extractable Fe and Al (DPS‐2). Both DPS‐1 and DPS‐2 were shown to be significantly (P = 0.0001) related to water‐extractable P for all soil horizons, suggesting that either index can be used as an indicator for P loss potential from a soil.