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WASTE MANAGEMENT

The Longer-Term Phytoavailability of Biosolids-Phosphorus

Soil and Water Sci. Dep., Univ. Florida, Gainesville, FL 32611-0510

^* Corresponding author (millrtym{at}ufl.edu).

Phosphorus based nutrient management could limit biosolids application to rates based on crop fertilizer P recommendations (P-based rates). Determinations of biosolids P phytoavailability relative to inorganic P fertilizers are necessary when P-based rates are mandated. Previous shorter-term (4 mo) studies successfully distinguished P phytoavailability differences among biosolids, but the longer-term (>4 mo) phytoavailability of biosolids-P is incompletely characterized. Furthermore, no a priori tool exists to distinguish biosolids relative P phytoavailability differences. A 16-mo greenhouse study was conducted to characterize the longer-term phytoavailability of biosolids-P and to identify a useful measure of biosolids-P phytoavailability. Seven biosolids and triple super phosphate (TSP) were used as P sources and applied to an Immokalee soil (sandy siliceous, hyperthermic Arenic Alaquod) at three P application rates: 56 (P-based rate), 112, and 224 kg ha^–1 (N-based rate). Bahiagrass (Paspalum notatum Flugge) was grown continuously in soil columns and harvested every 4 to 8 wk to characterize P uptake. The longer-term relative P phytoavailability of less soluble-P biosolids was 50 to 80% that of TSP, but more soluble-P biosolids were as phytoavailable as TSP. Estimates of biosolids relative P phytoavailability were well correlated with biosolids phosphorus saturation index (PSI; the molar ratio of oxalate-extractable P to oxalate-extractable iron and aluminum) values, suggesting that biosolids PSI values could be used to distinguish P phytoavailability differences among biosolids. Biosolids application rates should increase to account for the reduced relative P phytoavailability of less soluble-P biosolids, but no application rate adjustment is warranted more soluble-P biosolids.

Abbreviations: BPR, biological phosphorus removal • GRU, Gainesville Regional Utilities • Lakeland NS, Lakeland north side • MCP • monocalcium phosphate • OCUDS, Orange County utility district south • PAN, plant available nitrogen • PSI, phosphorus saturation index • PWEP, percent water-extractable phosphorus, RPA, relative P adsorption • RPP, relative phosphorus phytoavailability • SRP, soluble reactive phosphorus • TSP, triple super phosphate • WEP, water-extractable phosphorus

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Received for publication November 18, 2008.