Use of soil amendments to reduce soluble phosphorus in dairy soils

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(-1) 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(-1) soil), ferrous sulfate (0 to 1000 mg kg(-1) as Fe), and alum (0 to 1000 mg kg(-1) 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(-1) soil; nitrate and soluble organic carbon concentrations were similarily reduced by 45% and 49%, respectively. Increasing water-soluble 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, and redox conditions. Bacterial activities were also affected by gypsum-amended soils. Calcium carbonate effectiveness was limited to lower pH soils (< pH 7.0). Although Fe and Al soil amendments increased P retention to over 400% from unamended soils, source costs and questionable biological toxicities may limit their usefulness. Evidence indicated that both precipitation and microbial mechanisms are involved in P retention/desorption in soils loaded with animal manures.