Originally Posted by ALDEVAUX

Furthermore, charcoal is quite frequently used in bonsai soil mixtures in France because it contains some potassium (K) and having been dried out it helps keep a more constant low moisture in the soil. It is interresting for trees which don’t like too much moisture (like pines) and thus prevents infection from the water–loving soil-born fungi pathogens. It can also prevent the reproduction of bacteria which are often the cause of root rot in those soil mixtures that retain too much water.
Finally, charcoal releases oxygen during the draining of the soil mixture and helps make the soil a bit lighter.
Of course, charcoal should always be broken up (i.e. never in powder form) and mixed well in the soil.