Rate-Dependent Effects of Black Soldier Fly Frass on Germination, Shoot Growth, and Nodulation of Forage Legumes

Insect frass fertilisers (IFFs) are increasingly promoted as sustainable soil amendments within circular agricultural systems. However, the compatibility of IFFs with nitrogen-fixing legumes is poorly understood. This study evaluated the effects of a fertiliser produced from Hermetia illucens frass (HexaFrass™; HF) on germination, seedling emergence, shoot growth, and root nodulation in six forage legume species (Trifolium repens L., T. pratense L., T. incarnatum L., T. hybridum L., Melilotus albus Medik., and Medicago lupulina L.). Aqueous HF extracts (1% w/v) had no significant effect on seed germination, whereas higher concentrations (10% w/v) reduced germination in both T. pratense and T. incarnatum. In glasshouse trials, incorporation of HF (3 g per pot) did not affect seedling emergence but significantly increased shoot biomass across all plant species tested, with growth responses comparable to, or exceeding, those obtained with an equivalent mass of organic chicken manure. Across species, the shoot dry weight of the HF-treated plants was over nine times that obtained in the unfertilised control plants. Plant responses to HF application rate were non-linear, with maximum shoot biomass achieved at intermediate doses (~4–5 g per pot). Root nodulation exhibited a similar dose-dependent pattern: low HF application rates slightly enhanced nodulation, whereas higher rates suppressed nodule numbers. These findings indicate that IFFs can promote early growth of forage legumes, but reinforce that for each plant system (plant species, growing conditions, growing medium etc) there is a need to optimise fertiliser application rates to balance nutrient supply while avoiding the inhibitory effects observed at high rates. Further work is needed to establish the compatibility of IFFs with forage legumes in long-term trials performed under field conditions.