Response of Robusta Coffee (Coffea canephora Pierre ex A. Froehner) Seedlings to Solid Organic Fertilizer Derived from Coffee Husk Waste and Corn Cob Biochar

Indonesia is the fourth-largest coffee-producing country in the world, following Brazil, Vietnam, and Colombia. Approximately 67% of the country's total coffee production is exported, while the remaining 33% is consumed domestically. This study aimed to evaluate the effectiveness of coffee husk compost and corn cob biochar as alternative organic amendments in growing media for robusta coffee (Coffea canephora) seedlings. The primary issue addressed in this research was the limited availability of topsoil for nursery production, necessitating the use of alternative materials that are inexpensive, readily available, and environmentally sustainable. The experiment was arranged in a factorial randomized complete block design (RCBD) consisting of two factors: coffee husk compost at rates of 0, 75, and 150 g polybag⁻¹, and corn cob biochar at rates of 0, 100, and 200 g polybag⁻¹. These factors were combined into nine treatment combinations with three replications. Observed variables included leaf number, leaf area, plant height, stem diameter, and root length, which were measured periodically over a 12-week observation period. Data were subjected to analysis of variance (ANOVA), and significant differences among treatments were further evaluated using Duncan’s Multiple Range Test (DMRT) at the 5% significance level. The results indicated that coffee husk compost significantly affected plant height and root length, whereas corn cob biochar significantly influenced root length only. No significant interaction was observed between the two factors for any of the measured growth parameters. Nevertheless, the highest treatment combination (K2B2: 150 g compost + 200 g biochar per polybag) produced the greatest mean values for most growth variables, particularly root length (16.29 cm) and plant height (13.38 cm), suggesting a potential synergistic effect despite the absence of statistical significance.