Use of soil amendments to reduce soluble phosphorus

The objective of this research was to determine the effectiveness of soil amendments on reducing soluble P in Spodosols under dairy animal land-use. Dairy animal manure is a P source contaminating surface waters of the northern watersheds of Lake Okeechobee in south Florida. Phosphorus contamination has originated from manure-loaded soils found adjacent to milking barns, holding pens, feed lots under intensive animal use, and also developed pastures. In various batch-incubation studies, manure-loaded soils (7.0 to 120.9 g kg- ’ as total organic C) were treated alone or in combination with varying rates of calcium carbonate (to pH 7.5), gypsum (0 to 100 g kg- ’ soil), ferrous sulfate (0 to 1000 mg kg- ’ as Fe), and alum (0 to 1000 mg kg- ’ as Al). The influence of aerobic and anaerobic conditions on soluble P were also studied. Soluble phosphate concentrations were reduced 40 to 63% from gypsum application up to 100 mg kg- ’ soil; nitrate and soluble organic carbon concentrations were similarily reduced by 45 % and 49%, respectively. Increasing water-solu-ble Ca and the soil pH to 7.5 decreased soluble phosphate concentrations in manure-loaded soils. Gypsum amendments were effective under a broad range of manure loading, pH, an