Worldwide, French beans (Phaseolus vulgaris L.), tomato (Solanum lycopersicum L.), and kales (Brassica oleracea L. var. acephala) are considered economically important food crops. There is a rapid decline in their yield due to severe soil degradation. Thus, high commercial fertilizer inputs are crucial, though they remain expensive and inaccessible to resource poor farmers. We investigated the comparative performance of composted black soldier fly frass fertilizer (BSFFF), conventionally composted brewer's spent grain (BSG), commercial organic fertilizer (Evergrow), and mineral [nitrogen, phosphorus, and potassium (NPK)] fertilizer on growth, yield, N use efficiency, and nutritional quality (crude protein, crude fiber, crude fats, ash, and carbohydrate concentrations) of tomatoes, kales, and French beans under greenhouse and open-field conditions for two seasons. The fertilizers were applied at rates equivalent to 371 kg of N ha−1. For each crop, the plots were treated with sole rates of BSFFF, BSG, Evergrow, and NPK to supply 100% of the N required. Additional treatments included a combination of BSFFF and NPK, and BSG and NPK so that each fertilizer supplies 50% of the N required. The control treatment consisted of unfertilized soil. Results show that vegetable yields achieved using a combination of BSFFF and NPK were 4.5, 2.4, and 5.4-folds higher than the yield from the control treatment for tomatoes, kales, and French beans, respectively. The combined application of BSFFF and NPK produced 22–135%, 20–27%, and 38–50% higher yields than sole NPK for tomatoes, kales, and French beans, respectively, under both greenhouse and open-field conditions. The highest agronomic N use efficiency was achieved in sole BSFFF-treated plots compared to sole BSG and Evergrow. The N taken up by the vegetables was significantly higher when BSFFF and NPK were integrated. Vegetables grown using a combination of BSFFF and NPK had the highest crude protein and ash concentrations. Our findings demonstrate that the integration of BSFFF and NPK in vegetable cropping systems at the recommended rate of 1.24 t ha−1 BSFFF and 322 kg ha−1 NPK would improve soil health, boost yield, and nutritional quality of vegetable crops.

Although the black soldier fly frass fertiliser (BSFFF) is globally recognised as a promising and potential high-quality organic fertiliser, there is inadequate information on its impact on the growth performance and nutritional status of maize. Furthermore, no information exist on their combination with mineral fertiliser (NPK) as well as economic impact on maize production. This study evaluated the comparative impact of BSFFF, NPK and commercial organic fertiliser (Evergrow®) on growth, nitrogen use efficiency, yield, nutritional quality, and profitability of maize under greenhouse conditions. The treatments included: (1) sole application of BSFFF, conventional compost of brewers’ spent grain (BSG), Evergrow, and NPK at rates equivalent to 100 kg nitrogen (N) per hectare; (2) BSFFF and BSG combined with NPK so that each fertiliser supplies 50% of the N required; and (3) the control (unamended soil). Our findings revealed that BSFFF increased maize grain yield at higher rates: 2-25, 25-113 and 153-212% than NPK, BSG and Evergrow, respectively. Similarly, the BSF frass application led to higher maize growth and yield than the control treatment. The agronomic N use efficiency of maize grown using BSFFF was 2 and 3 times higher compared to that of BSG and Evergrow, respectively. Maize grown using BSFFF and NPK had higher crude protein and crude fibre content compared to the other treatments. The net income generated from the commercialisation of maize grown using a combination of BSFFF and NPK was 2, 163 and 173% higher than those achieved using sole NPK, mixture of BSG and NPK, and sole BSFFF, respectively. Our results imply that developing and promoting BSFFF alone or in integration with NPK can enhance the food security and livelihoods of smallholders, while safeguarding planetary health.