USE OF SOIL AMENDMENTS TO REDUCE SOLUBLE PHOSPHORUS IN DAIRY SOILS

The objective of this research was to determine the effectiveness of s oil amendments on reducing soluble P in Spodosols under dairy animal l and-use. Dairy animal manure is a P source contaminating surface water s of the northern watersheds of Lake Okeechobee in south Florida. Phos phorus contamination has originated from manure-loaded soils found adj acent 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 carbonat e (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 influ ence of aerobic and anaerobic conditions on soluble P were also studie d. Soluble phosphate concentrations were reduced 40 to 63% from gypsum application up to 100 mg kg(-1) soil; nitrate and soluble organic car bon concentrations were similarily reduced by 45% and 49%, respectivel y. Increasing water-soluble Ca and the soil pH to 7.5 decreased solubl e phosphate concentrations in manure-loaded soils. Gypsum amendments w ere effective under a broad range of manure loading, pH, and redox con ditions. Bacterial activities were also affected by gypsum-amended soi ls. Calcium carbonate effectiveness was limited to lower pH soils (< p H 7.0). Although Fe and Al soil amendments increased P retention to ov er 400% from unamended soils, source costs and questionable biological toxicities may limit their usefulness. Evidence indicated that both p recipitation and microbial mechanisms are involved in P retention/deso rption in soils loaded with animal manures.