Effects of Using Vermicomposted Black Soldier Fly Larval Frass as a Germination Substrate on Emergence, Growth, and Antioxidant Content in Kale, Bell Pepper, and Tomato Seedlings

This study evaluated the effects of thermocomposting followed by vermicomposting on the physicochemical properties of insect frass and its suitability as a germination and growth substrate for kale, tomato, and bell pepper. Vermicomposting improved frass stability by reducing pH, electrical conductivity, carbon content, and the C/N ratio, while increasing total nitrogen, cation exchange capacity, and calcium and magnesium availability, indicating enhanced maturity and nutrient retention. Peat–frass mixtures (20–100%), increased pH from acidic conditions in the control to near neutral in 100% frass and raised electrical conductivity from 0.67 dS m−1 to the highest values in the pure frass treatment. Tomato seedlings exhibited strong tolerance and enhanced growth at all frass proportions, with seedling heights exceeding 33 cm compared with the control. Kale showed optimal growth at 20–60% frass, while 80–100% reduced early development. In bell pepper, emergence declined at high frass proportions, although seedlings grown with ≥40% frass reached heights of approximately 8.3–8.6 cm. Vermicomposted frass also influenced plant metabolism, increasing flavonoid accumulation and modifying antioxidant activity. These findings demonstrate that stabilized frass can serve as a sustainable substrate component, contributing to organic waste valorization and improved seedling production when applied at crop-specific proportions.