What is the commonly used woodc ash in garden fertilizer?

Hardwoods make the best fertilizer- oak, ash, hickory, etc- but any wood ash can be used. Avoid any ash from treated lumber- much of the "green" lumber is treated with some very toxic metals you do NOT want in your tomatoes. We mix wood ash from our wood burning furnace in our compost pile (be CERTAIN that ashes are cold). The alkali from the ash helps offset the acidity of the compost.