All-natural wood ashes from a fireplace are good in a garden for repelling slugs and snails, and are beneficial for boosting tomato plants, and compost piles. However, soil with a neutral pH level can be harmed by the addition of fireplace ashes.
Fireplace ashes improve soil that is acidic and deficient in potassium. It can be used to lime or neutralize the soil, with about twice as much ash needed as would be needed of lime for the same neutralizing purposes. The only significant soil nutrient contained in fireplace ashes is potassium, though smaller amounts of calcium, phosphorous, and other nutrients are present which can be helpful in particular instances. However, ashes also typically contains approximately 22% sodium content, which can harm the soil.