Black Soldier Fly-Composted Organic Fertilizer Enhances Growth, Yield, and Nutrient Quality of Three Key Vegetable Crops in Sub-Saharan Africa

Worldwide, French beans (Phaseolus vulgarisL.), tomato (Solanum lycopersicumL.), andkales (Brassica oleraceaL.var. acephala) are considered economically important foodcrops. There is a rapid decline in their yield due to severe soil degradation. Thus, highcommercial fertilizer inputs are crucial, though they remain expensive and inaccessibleto resource poor farmers. We investigated the comparative performance of compostedblack soldier fly frass fertilizer (BSFFF), conventionallycomposted brewer’s spent grain(BSG), commercial organic fertilizer (Evergrow), and mineral [nitrogen, phosphorus, andpotassium (NPK)] fertilizer on growth, yield, N use efficiency, and nutritional quality (crudeprotein, 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 ofN ha−1. For each crop, theplots 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 controltreatment consisted of unfertilized soil. Results show that vegetable yields achievedusing a combination of BSFFF and NPK were 4.5, 2.4, and 5.4-folds higher than theyield from the control treatment for tomatoes, kales, and French beans, respectively. Thecombined 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, underboth greenhouse and open-field conditions. The highest agronomic N use efficiency wasachieved in sole BSFFF-treated plots compared to sole BSG and Evergrow. The N takenup by the vegetables was significantly higher when BSFFF and NPK were integrated.Vegetables grown using a combination of BSFFF and NPK had thehighest crude protein Anyega et al.Frass Fertilizer Boosts Vegetable Productivity and ash concentrations. Our findings demonstrate that the integration of BSFFF andNPK in vegetable cropping systems at the recommended rate of1.24 t ha−1BSFFFand 322 kg ha−1NPK would improve soil health, boost yield, and nutritionalquality ofvegetable crops.