How dense is Ian Macfarlane?, page-75

You should read more on the subject (hint: Google "terra preta"). The charcoal itself isn't metabolised, but it greatly aids both microbes and plants alike. The surface of carbon is fantastic at binding and holding mineral nutrients so they don't wash away in water - but they're mostly still available to organisms (barring some irreversible binding sites). If you want healthy soil, adding lots of finely-ground charcoal (preferably pre-adsorbed with minerals - otherwise you'll see a transient initial depletion from the surrounding soil) is a great starting point.