Applications of food waste-derived black soldier fly larval frass as incorporated compost, side-dress fertilizer and frass-tea drench for soilless cultivation of leafy vegetables in biochar-based growing media

•Food waste-derived black soldier fly larval frass and waste-wood biochar could substitute for peat compost and soil.•Sidedress application of frass yielded results comparable to those of the liquid inorganic fertilizer control.•The direct application of frass was more effective in promoting plant growth than frass tea application.•Frass and frass tea applications were free of heavy metals and foodborne pathogens. Black soldier fly (BSF) larval bioconversion can recycle nutrients in organic wastes into larval biomass and frass. While the frass has been commonly marketed as a soil amendment, its usefulness in soilless cultivation remains largely unexplored. Growth experiments were conducted to investigate the effectiveness of surplus food-derived and okara-derived BSF larval frass as an incorporated compost, side-dress fertilizer and frass-tea drench for the cultivation of pak choi and lettuce in waste-wood derived biochar growing media. Pak choi yields from treatments with surplus food-derived frass and biochar at a 10:90 (v/v) ratio and inorganic fertilizer were comparable to those of the control which consisted of soil, peat-based compost and inorganic fertilizer. However, yields decreased with increasing frass incorporation rates owing to high salinity and potentially low oxygen conditions in the growing media. When used as a fertilizer on biochar-coir growing media, the direct application of frass as a side-dress fertilizer was 1.6–6.8 times more effective in promoting lettuce growth than the application as a frass-tea drench. Frass fertilizers derived from surplus food outperformed those derived from okara by 1.3–5.3 times. Lettuce yields were not significantly different between the treatment with surplus food-derived frass applied as a side-dress fertilizer and the control of liquid inorganic fertilizer. Variations in fertilizing potential were attributed to nutrient availability and the presence of plant growth promoting microbes in the growing media. BSF larval frass derived from food waste shows promise in partially replacing unsustainable agricultural inputs for leafy vegetable cultivation, including soil and inorganic fertilizers.