Wood ash is such a good fertilizer that when the United States became a British colony one of the things America sent over to England was shiploads of wood ash for use as fertilizer because England had badly depleted soil. Even in contemporary times poor farmers around the world use it as a cheap and readily available fertilizer. I have a fireplace and after a fire I collect a plastic bag full of the finest ash I can find with very few pieces of charcoal. I add this to my plants not during the veg cycle but about a week into the flowering cycle. This gives the plant a little extra NPK to help it grow big juicy fat buds. If you do not believe me, try it. I grew two clones cut from the exact same plant and both were planted in 3 gallon smart pots. One plant was given just wood ash and the other plant I fed the Fox Farm nutrient line up. The plant I gave straight wood ash had bigger thicker healthier buds and the overall look of the plant was more healthy than the one I fed nutrients.