Wood Ashes: A valuable source of potash. Wood ashes generally contain from 1 to 10 percent potash and 1-1/2 percent phosphorus. They can be mixed with other fertilizing materials or side-dressed around growing plants. Apply about 5 to 10 pounds per 100 square feet. Avoid using wood ashes around blueberries or other acid-loving plants and don’t use heavily in alkaline soils. Only wood ashes, and not coal ashes, should be used in the soil or compost. Wood ashes are also an effective tool in controlling slugs.