Charcoal Ash and Volatile Matter Effects on Soil Properties and Plant Growth in an Acid Ultisol

There is a growing interest in converting organic wastes to charcoal for use as a sustainable soil amendment with a potential to improve soil productivity and sequester C. Three consecutive greenhouse experiments were conducted to investigate the effects of charcoal with different ash and volatile matter (VM) contents on soil properties and maize (Zea mays) growth and to evaluate the effect of time on charcoal performance. Five charcoal amendments (high-VM corncob, low-VM corncob, Kiawe, Binchotan, and a gasification charcoal of Leucaena leucocephala) applied at a 2.5% (wt/wt) rate were compared with a zero-charcoal control with and without fertilization. Only the gasification charcoal significantly increased maize growth without fertilization. The low-VM corncob charcoal with fertilization significantly increased maize growth by 164% compared with the fertilized control in the first planting cycle. Maize growth in the high-VM corncob charcoal supplemented with fertilizer treatment was significantly lower than that of the fertilizer-alone treatment in the first planting cycle. The negative effect of the high-VM charcoal on the fertilizer was caused by bioavailable carbon in the charcoal, which increased soil microbial activity and could have caused N immobilization. Both the beneficial and detrimental effects of charcoal did not persist beyond the first planting cycle, suggesting that charcoal impacts are temporary. Whereas charcoal ash and VM content seem to be important parameters for predicting charcoal behavior in the short-term, more research is needed to examine a broader spectrum of feedstocks exposed to varying thermal treatments.