Many folks who burn wood in fireplaces and wood stoves are faced with an abundance of wood ash. An average cord of wood can yield up to 20 – 30 pounds of ashes.
Wood ashes are fine for raising the pH of the soil (making it more alkaline or “sweetening” it) in either your garden or even on the lawn. They contain potassium, and smaller amounts of phosphorus and magnesium, depending on the variety of the wood.
Be careful though, especially if your garden is small and your supply of ashes is large, as you can overdo it. Don’t use more than about 20 pounds per 1,000 square feet each year (or about ¼ – ½ inch of ashes per year).
If your soil is in the proper pH range, this rate would be safe for yearly treatments, especially around deciduous trees and shrubs, fruit trees, vegetables (except around potatoes because increased alkalinity can cause potato scab), perennials, roses, bulbs and annuals.
You may want to test to see if your pH range is below 7.5 before adding ashes in ratios higher than this though. Otherwise, over-application can increase the likelihood of soil related problems. Your local Extension Service should be able to assist you with having your soil tested.
Avoid using wood ashes around acid-loving plants such as azaleas, junipers, conifers, blueberries, rhododendrons and camellias.