Partial replacement of NPK with coal-based fertilizer improves yield and profitability of chili pepper on tropical Ultisol soil

Productivity of horticultural crops on tropical acidic soils is frequently constrained by low nutrient availability, poor soil fertility, and limited fertilizer-use efficiency. This study evaluated Futura coal-based fertilizer, a commercial coal-derived carbon fertilizer, as a partial substitute for NPK fertilizer in chili pepper cultivation on Ultisol soil. The field experiment was conducted in Tapin Regency, South Kalimantan, Indonesia, using a randomized complete block design with five practical fertilization regimes involving NPK, coal-based fertilizer, and Celltilizer in selected treatments. The measured variables included plant growth, yield components, fruit quality, Cd, Pb, and As concentrations in fresh fruit, and farm economic performance. Fertilization significantly affected yield and productivity, whereas early growth and most fruit quality variables showed limited treatment responses. The best overall performance was obtained with 50% NPK + 50% coal-based fertilizer without Celltilizer, which produced 10.18 t ha -1 compared with 7.34 t ha -1 under the 100% NPK treatment, representing a 38.69% increase. The yield advantage was mainly associated with a greater fruit number and cumulative fruit weight rather than an increase in individual fruit size. Cd, Pb, and As concentrations remained below their respective limits of quantification in all treatments, indicating no detectable accumulation in harvested fruit during one cropping season. The economic analysis showed that the same treatment generated the highest revenue, net profit, and R/C ratio. These findings support the potential use of coal-based fertilizer as a complementary component of integrated nutrient management, but the underlying mechanisms and long-term environmental safety require further validation.