Cultivar-specific responses of hydroponically grown lettuce to volatile fatty acids: implications for using anaerobic digester effluents as organic fertilizers

• Anaerobically derived organic fertilizers need volatile fatty acid (VFA) control • Negative crop responses started at 1.2 and 0.6 mM acetate and isovalerate levels • Effluent dilution can be safe if ≤8-30 mM (C 2 ) or ≤6-22 mM (C 5 ) at crop N levels • Higher crop carbon content at increasing VFA levels indicates accumulation of VFAs • Digesters promoting aceto-methanogenesis can produce easier applicable fertilizer