Combined use of charcoal and sago bark ash improves dry matter production, phosphorus uptake, and recovery efficiency of sorghum cultivated on an acid soil

Acidic soils are commonly constrained by toxic concentrations of aluminium (Al), iron (Fe), and manganese (Mn), in addition to low phosphorus (P) availability, thereby limiting crop productivity. Phosphorus plays a critical role in photosynthesis and energy transfer in plants. However, information regarding the combined application of charcoal and sago (Metroxylon sagu) bark ash for sorghum (Sorghum bicolor L.) cultivation on tropical acid soils remains limited. Therefore, a pot experiment was conducted to evaluate the effects of co-applying these amendments with fertilizers on soil chemical properties, P availability, sorghum growth performance, dry matter production, P uptake, and P recovery efficiency. Treatments consisted of charcoal and sago bark ash applied at 25%, 50%, 75%, and 100% of their respective recommended rates, together with the recommended rates of N, P, and K fertilizers. The combined amendments significantly increased soil pH, cation exchange capacity (CEC), total carbon, and total P, while reducing exchangeable acidity and Al3+. Improved soil conditions enhanced sorghum performance, resulting in 3.9-fold greater dry matter production and P uptake, with 4.1-fold higher P recovery efficiency compared with conventional practice. Further field evaluation and economic assessment are recommended to validate these findings.