Agronomic Performance and Nutrient Dynamics of Pelletized Organo-Mineral Fertilizers Derived from H. illucens and T. molitor Frass in Cabbage (Brassica oleracea L. var. capitata)

The rapid growth of insect farming generates significant volumes of insect frass, a novel organic by-product. Utilizing frass-based organo-mineral fertilizers represents a sustainable alternative to synthetic fertilizers, yet long-term agronomic efficacy and residual effects require further field validation. This study evaluated pelletized organo-mineral formulations for conventional and organic agriculture, derived from Hermetia illucens and Tenebrio molitor frass. Eight field treatments were evaluated: an unfertilized control; an inorganic NPK fertilizer (150 kg N ha−1); a biostabilized municipal solid waste compost at two nitrogen rates (150 and 300 kg N ha−1); and two novel insect-frass-derived organo-mineral fertilizers (VHTI and VHTO) applied at a standard product dose (1000 kg ha−1) and an N-equivalent dose (150 kg N ha−1). Application of organo-mineral and organic treatments at the same nitrogen dose (150 kg N ha−1) doubled cabbage yield compared to the unfertilized control and significantly outperformed the conventional inorganic treatment. These formulations demonstrated enhanced nitrogen and phosphorus use efficiencies, reflecting improved synchronization between nutrient release and crop demand. Furthermore, organo-mineral formulations significantly increased soil available phosphorus without adversely affecting key soil quality parameters. The Integrated Soil–Crop Response Index (ISCRI) confirmed a superior overall agroecosystem response for these treatments. These findings provide important theoretical insights into nutrient synchronization mechanisms in organo-mineral fertilizers, alongside practical applications for sustainable cabbage production. Consequently, frass-based pelletized fertilizers represent a highly sustainable, circular, and efficient alternative to optimize horticultural yield and nutrient use efficiency while enhancing soil health, ultimately supporting the valorization of insect farming by-products as viable substitutes for synthetic fertilizers.