If you burn wood in a fireplace or woodstove, you have lots of ash by spring. Although I know wood ash can be applied to a garden, I'm never sure how much to spread and what the effects on my plants may be. Dan Sullivan, a soil scientist at Oregon State University, has the lowdown on wood ash.
The type of wood burned influences the quality and quantity of ash. Hardwoods produce three times as much ash as softwoods and contain five times the nutrients. According to Sullivan, ash from burned trash, cardboard, and painted or stained wood is toxic and shouldn't be used in the garden.
Wood ash contains 10 to 25 percent calcium, 1 to 4 percent magnesium, 5 to 15 percent potassium, and 1 to 3 percent phosphorus. If it were a chemical fertilizer, the grade would be 0-2-10.
Ash will increase soil pH, but not as dramatically as lime: two pounds of wood ash equals 1 pound of ground limestone. Don't apply ashes to your garden if your soil pH is already 7.0 or greater, as in most of the western United States.
Finally, wood ash promotes nitrogen loss from ammonia-based fertilizers such as ammonium sulfate or ammonium nitrate. Apply the wood ash at least 1 month before adding high-nitrogen fertilizer to lawns or gardens.
How much wood ash should home gardeners use? If you live in the eastern or northwestern United States where soils are acidic, use 1/2 to 1 pound of wood ash per year for shrubs and roses, 10 to 15 pounds per 1,000 square feet for lawns, and 10 to 30 pounds per 1,000 square feet for vegetable and flower gardens.