The longer-term phytoavailability of biosolids-phosphorus
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.