Potential of bio-organic amendment of palm oil mill effluent manure and plant growth-promoting bacteria to enhance the yield and quality of maize grains in Cameroon

•Locally produced bio-fertilizer increased the soil nitrogen content compared to the control.•Manure amendment enhanced earthworm abundance compared to the control and NPK fertilizer.•Maize grain weight increased for treatments with manure, microbes and NPK compared to the control.•The protein content in maize grains increased for the treatment that incorporates NPK, manure and microbes. Achieving soil security is essential to improve maize (Zea mays L.) production that is affected by poor soil fertility, which is often mitigated using chemical fertilizers that cause deleterious environmental effects. The potential of bio-organic amendments to boost soil security was investigated in a field study with eight treatments including; Control–no input, chemical NPK (nitrogen:phosphorus:potassium) fertilizer, Manure (palm oil mill effluent–POME), Microbes (plant growth-promoting bacteria), NPK+Manure, NPK+Microbes, Manure+Microbes, and NPK+Manure+Microbes with four replications each. Results revealed significant (P