Biochar-vermicompost-inorganic N/P₂O₅ integration improves maize yield and soil chemical properties in acidic soils

Improving soil chemical quality while enhancing maize yield is critical for sustainable crop production in acidic soils. We conducted a two-year field experiment on acidic Nitisols in the Burie district, Ethiopia, to evaluate the combined effects of maize cob biochar (BC), vermicompost (VC), and inorganic N/P₂O₅ rates on soil chemical properties and maize productivity. The experiment used a randomized complete block design with a 3 × 3 × 3 factorial arrangement, testing three levels of inorganic N/P₂O₅ (0/0, 120/69, 240/138 kg ha^-1), BC (0, 4, 8 t ha^-1), and VC (0, 5.02, 10.04 t ha^-1). Grain yield was recorded annually, and soil samples were collected before and after the experiment to assess changes in chemical properties. Data were analyzed using factorial ANOVA, and treatment means were separated using Tukey's HSD at 5% significance. Economic feasibility was evaluated via partial budget analysis. Under the conditions of this experiment, integrated application of BC, VC, and reduced N/P₂O₅, alongside lime, significantly improved soil pH (4.87 to 5.82), soil organic carbon, total nitrogen, and available phosphorus, while reducing exchangeable acidity, hydrogen, and aluminum. Maize grain yield increased markedly, with the highest yield (12.13 t ha^-1) observed under 120/69 kg N/P₂O₅ + 8 t BC + 10.04 t VC, a 175.77% increase over the control (4.40 t ha^-1). The combination 120/69 kg N/P₂O₅ ha^-1 + 4 t BC ha^-1 + 5 t VC ha^-1 achieved consistently high yields (12.09 t ha^-1) and the greatest net benefit (1,861 USD ha^-1; 289,124 Ethiopian Birr ha^-1) with a marginal rate of return of 149.1%. Yield was strongly positively correlated with improved soil chemical properties. Among predictive models, Linear Support Vector Machine (LSVM) provided the highest accuracy (R² = 0.923) for estimating maize yield under integrated nutrient management. These results indicate that integrating BC and VC with reduced inorganic N/P₂O₅, in combination with lime, can enhance soil chemical quality and maize productivity in acidic Nitisols. These findings suggest potential for sustainable intensification in the study area and similar agro-ecologies, although multi-location and longer-term validation is needed to confirm broader applicability. Further research is needed across diverse environments and longer timeframes.