Fireplace ashes for compost can be used to help maintain the neutral condition of the compost. It can also add nutrients to the soil. Decomposing materials in the compost pile can become somewhat acidic and wood ash can help offset this, as it's more alkaline in nature. But Adding too much ash to the pile can be detrimental to the makeup of the compost because of its alkalinity. Ashes tend to raise the soil pH, so you shouldn't use it directly on plants, especially acid-loving ones like rhododendrons, azaleas, and blueberries. Also, in high amounts, wood ash can inhibit plant growth by restricting nutrients, like iron.
