Wood ash contains calcium carbonate around 25%, potassium 3% and other useful elements like phosphorous, manganese and zinc. In terms of commercial fertilizer, wood ash contains 0-1-3 (N-P-K).
Phosphorus is useful in the coloration of flowers and in the flavor of the fruits along with potassium, which is useful in the growth and cultivation. There are several other trace elements found in wood ash such as magnesium, iron, copper, boron etc. These contents may vary according to the type of wood you use for combustion.
See more here
You should also see :
build a bench around a tree