Biochar enhances growth, yield and nutrient use efficiency of okra under organic and mineral nutrient management

Introduction Increasing nutrient use efficiencies is critical for sustainable crop production. A field experiment conducted over two cropping seasons in southern Taiwan compared the effects of four rates of rice husk biochar (0, 10, 20, 30 t ha −1 ) combined with mineral and organic fertilizers on okra growth, physiology, yield, nutrient use efficiency, and economic returns. Methods Nine treatments were tested: negative control (CK), mineral fertilizer alone (MB0), mineral fertilizer + increasing biochar rates (MB10, MB20, MB30), organic fertilizer alone (OB0), and organic fertilizer + increasing biochar rates (OB10, OB20, OB30). Results All biochar-amended treatments outperformed non-biochar and control treatments in terms of okra performance, with the highest biochar rate (30 t ha −1 ) achieving the best improvements. MB30 increased plant height and SPAD by 37–49.3% and 38.6–62%, while OB30 improved them by 35.3–37.8% and 36–64.2% compared to CK. Net photosynthetic rate rose by 168.2–178.9% (MB30) and 165.0–199.8% (OB30). Fruit yield increased by 45.6–143.0% and 41.8–127.0% under MB30 and OB30, respectively. Biochar contributions to yield increases were 10% higher under organic fertilizers, however, the study reported for the first time that this higher contribution of biochar to yield under organic fertilizers decreases with increasing biochar rates. Biochar also increased nutrient use efficiency, with MB30 showing 25.7% to 43.6% higher PFPN and PFPK than MB0; and OB30 showing 23.4% to 54.0% higher values than OB0, across two seasons, respectively. Although MB0 achieved the highest benefit-cost ratio (BCR) of 2.99 in season one, this trend reversed in season two, where MB30 reached the highest BCR (3.81) through productivity gains. Discussion This study demonstrates that despite higher initial investment, biochar application offers a promising strategy to increase nutrient use efficiency, okra productivity, and economic returns.