As per my knowledge and experience regarding gardening, regular wood ash is good for the garden as it contains potassium (K) which is one of the three main plant fertilizers. However, charcoal, even though it is wood, contains chemicals and additives that you probably wouldn't want in your garden. So it is essential to make sure there are no dangerous additives in the charcoal that don't burn off, in order to protect your garden soil by acidity.