Organic fertilizers and Azotobacter: effects on onion growth, yield, metabolites, and soil fertility

Abstract This study assesses the effects of organic fertilizers (vermicompost (V) and rabbit manure (R), combined with the bio-fertilizer Azotobacter chroococcum (AZ), on onion growth, yield, soil properties, and chemical metabolites over two growing seasons (2021/2022 and 2022/2023). The experiment, conducted at the Experimental Farm of Al-Azhar University, Assiut, Egypt, used a split-plot design with organic fertilizers as the main plot and bio-fertilizer treatments as subplots. The combined application of rabbit manure and Azotobacter (R + AZ) produced the tallest plants (59.64 cm and 63.23 cm), highest leaf number (10.71 and 10.82), and greatest bulb diameter (10.29 cm and 10.39 cm) in the first and second seasons, respectively. This treatment also resulted in the highest bulb length (9.49 cm and 9.66 cm), bulb weight (2.54 kg and 2.61 kg per 10 plants), and total yield (63.86 and 65.64 ton ha−1) in the first and second seasons, respectively. Additionally, organic fertilization improved several soil properties, including increased organic matter content, pH reduction, and changes in electrical conductivity. Azotobacter inoculation also increased the concentration of bioactive metabolites, such as phenolic compounds, flavonoids, and lipids, with the R + AZ treatment showing the most significant increases. This study demonstrates that the combination of rabbit manure and Azotobacter can enhance onion yield and nutritional quality, providing a sustainable approach to onion production by improving soil health and reducing the need for synthetic fertilizers. Graphical abstract A graphic abstract explains the importance of adding organic fertilizersand Azotobacter to improve the growth and yield of onion plants, as wellas the fertility and health of the soil.