Solubility and Leachability of Fishwaste Compost Phosphorus in Soilless Growing Media

Knowledge of solubility and leachability of fishwaste compost P is needed to effectively use fishwaste compost as a P source in soilless growing media. The fishwaste compost contained 3.6 g P kg −1 compost on a dry weight basis and had high P solubility, with 64.4% of the total P in the compost extractable by six successive extractions with 0.01 M NaCl. In the dilute salt solution, the Ca release from the compost paralleled that of P. The sequential extraction did not substantiate that Ca‐P was the predominant P form in the compost, because substantial amounts of P solubilized in 0.01 M NaCl were from the labile P (NH 4 Cl‐P) and potentially labile P (Fe‐ and Al‐P) pools. High solubility of the compost P resulted in elevation of P concentrations in leachates and cumulative amounts of P leached over 10 wk when the growing media containing 25% or more compost was irrigated with overhead sprinkler irrigation. Young rhododendron ( Rhododendron fortunei ‘Scintillation’) did not significantly ( P > 0.05) reduce P leaching from the 50% or higher compost treatments because the amount of P accumulated in the plant leaves represented 3 ± 0.9 (SD)% of P in the compost and P fertilizer combined. To reduce P leaching and minimize nonpoint P pollution, additions of P fertilizer to the soilless growing media containing the fishwaste compost should be restricted. Otherwise, the proportion of the compost in the media should be limited to 25% or less, unless it is used in conjunction with controlled drip irrigation.