Direct and Residual Effects of Integrated Biological Amendments and Mineral Fertilization on Maize Productivity in the Eastern DR Congo

Maize is vital for food systems and rural livelihoods in the Democratic Republic of Congo (DRC). Continuous cultivation depletes soil nutrients, reducing maize production. Inorganic (or mineral) fertilizers provide nutrients rapidly, but their cost and sustainability concerns have prompted interest in alternatives. Biological amendments improve nutrient uptake and soil structure and boost crop resistance, potentially cutting mineral fertilizer use. The present study aims to investigate the direct and residual effects of biological amendments (BAs) on maize productivity in the Eastern DRC, both when applied alone or combined with inorganic fertilizer, with trials conducted in the Kabare, Kalehe, and Ruzizi Plain regions from October 2022 to June 2024. Two trials across four seasons gathered data using a randomized complete block design (RCBD) with nine treatments and three replicates: Control, without fertilizer application; farmer practice; inorganic fertilizer (NPK 17-17-17 and urea); BA_1: Lactobacillus; BA_2: fish serum; BA_3: black soldier fly (BSF) compost; BA_1 + inorganic fertilizer; BA_2 + inorganic fertilizer; and BA_3 + inorganic fertilizer. The results identified three categories: integrated organic and inorganic fertilizers, single applications, and inconsistent uses. The best outcomes emerged from treatments combining Lactobacillus, fish serum, and BSF compost with inorganic fertilizer, positively impacting maize yield parameters. The study confirms that combining biological amendments and mineral fertilizers significantly (p < 0.001) enhances maize productivity in the Eastern DRC. Performance differences across locations emphasize the influence of local soil characteristics and targeted nutrient strategies.