Is Burnt Wood Ash Good For A Garden

A fire burns in a wood stove. The ash left behind can be used in the garden.

Question: I have a wood stove and generate a lot of ashes this time of year. Is it OK to put wood ash in the garden, compost or lawn?

Answer: Wood ash contains most of the 13 essential nutrients that the soil supplies for plant growth. When wood burns, it gives off nitrogen and sulfur as gasses. But calcium, potassium, magnesium and other trace elements don't volatilize; they remain with the ash. Some of what is left in the ash include carbonates and oxides. These are valuable liming agents that can raise pH and help neutralize acid soils.

The nutrients in and volume of wood ash depends on the type of wood you are burning. Hardwoods produce about three times the ash and five times the nutrients per cord as softwoods. A cord of oak provides enough potassium for a garden 60 by 70 feet. A cord of Douglas fir ash supplies enough potassium for a garden 30 by 30 feet.

Both hardwood and softwood ash will reduce soil acidity slightly. Since soils west of the Cascades tend to be slightly acid and low in potassium, wood ash can be beneficial to most garden plants if used judiciously.

Wood ash is alkaline (caustic), which means it has a high pH level. Do not use it if your soil is alkaline.

Quite a number of plant groups, those that evolved in alkali or calcium-rich environments, will benefit from ash including apples, figs and pears; bulbs; broccoli, Brussels sprouts, cauliflower, kale and collards; rosemary; Oriental poppies; deciduous vines, such as Chinese mountain yam; and alkaline-loving shrubs, including lilacs and lavender.

I don't recommend using wood ash in your compost. I have found that if I add much wood ash at all to my compost piles, that my red-wiggler worm population tends to drop off precipitously.

Carol Savonen is a naturalist and writer. She is an associate professor emeritus at OSU and tends a large garden in the Coast Range Hills west of Philomath with her husband and dogs. She can be reached at Carol.Savonen@oregonstate.edu or c/o: EESC, 422 Kerr Admin. Bldg., OSU, Corvallis, OR 97331.

MORE: Find more local garden tips from Carol Savonen

Using wood ash safely

Use the same precautions when using wood ash as when handling other strongly alkaline materials, such as household bleach. Here are some safety tips from the OSU Extension Service:

Wear eye protection, gloves and a dust mask.
Do not scatter ashes in the wind. Apply recommended amounts to moist soil, and rake lightly to mix.
Do not use ash from burning trash, cardboard, coal or pressure-treated, painted or stained wood. These materials can contain potentially harmful substances. For example, the glue in cardboard boxes and paper bags contains boron, an element that can inhibit plant growth if found in excessive levels.
Do not use wood ash where potatoes will be planted, as it encourages scab disease, or on acid-loving plants such as conifers, mountain laurels, camellias, oaks, strawberries, caneberries, blueberries, azaleas or rhododendrons.
Never leave wood ash in lumps or piles. If it is concentrated in one place, excessive salt from the ash can leach into the soil and create a harmful environment for plants.
Do not apply ash at time of seeding. Ash contains too many salts for seedlings.
Do not add ash with nitrogen fertilizers such as ammonium sulfate (21-0-0-24S), urea (46-0-0) or ammonium nitrate (34-0-0). These fertilizers lose their nitrogen as ammonia gas when mixed with high pH materials such as wood ash. For a lawn, wait at least a month after wood ash application before applying nitrogen fertilizer. This will allow time for the soil to reduce the alkalinity of the wood ash.