Function
Wood ash is about 40 to 45 percent calcium carbonate, which makes it a good substitute for lime in rendering the soil more alkaline for good fruit tree growth.
Amounts
Use about one gallon of ash to a square yard of ground under your trees annually, and about half a gallon per yard if the soil is sandy. Under small trees, use two pints of wood ash around the base.
Benefits
Wood ash is high in calcium content and roughly half as effective as lime in neutralizing acidic soil. Check your soil pH levels every year or two. Ash is an effective way to increase soil pH if it's too acidic.
Considerations
Do not use wood ash around acid-loving trees such as pine, spruce and junipers, or around azaleas, rhododendrons or blueberries. Keep the wood ash dry when storing it.
Warning
Do not use ashes from treated wood, wax logs or other treated wood products. These contain chemicals that may be harmful to your soil and trees.