Charcoal is a lightweight, black, porous material similar to coal. It consists of carbon (about 50 to 95 %) and volatile chemicals and ash. Charcoal is usually produced by slow pyrolysis, the heating of wood, sugar, bone char, or other substances in the absence of oxygen. It can also be produced by some enzymatic conversion of organic mater. The backbones of charcoal are polycondensed aromatic moieties which are chemically and biologically persistent. Charcoal is a valuable soil amendment (see also terra preta), retaining nutrients at their surface and improving the soil structure leading to enhanced water and air retention.