Winter means many homeowners have plenty of wood ashes left over from wood-burning stoves and fireplaces. Wood ash is a compostable material that, with some caution and attention to the chemistry of your compost pile, can be a good addition to the compost mix. So, instead of discarding all that wood ash, consider recycling it as part of your compost mix or as a standalone soil amendment.
The first word of caution for adding ash to a compost pile is to only add ashes generated by burning firewood. Ash from charcoal, coal and other sources can contain chemicals toxic to soil and plants. As always, never add to the compost pile any materials that may contain chemicals that could be harmful to your plants or soil.
The second word of caution involves understanding the chemistry of wood ash. Ash is very alkaline. This means that, like lime, wood ash has a high pH. Raising the pH of your compost pile, by adding too much wood ash or other alkaline materials, can reduce the amount of nitrogen available in the pile. Wood ash can also raise soil pH, and ash that lands directly on plants can even “burn” the plants with its alkalinity. Compounding the challenge for composting is that wood ash more rapidly raises pH when it gets wet – and your compost pile should be a moist environment.
This all means that, while wood ash may be composted, it should be added sparingly to the compost bin or pile. Most composting guidelines we’ve seen advise adding about one-half cup of ash for every five gallons of compost. Since that is a small amount, consider storing wood ashes from this winter in a covered container, like a trash barrel, to be slowly added to your compost pile during warmer months. Keeping the ashes covered and dry will help maintain their suitability for sprinkling into your compost pile later.
The best way to recycle wood ash as compost could be to avoid adding it to the compost pile, by cautiously using it as a soil amendment. When used properly, wood ash is a suitable soil amendment for raising soil pH and providing potassium to plants. We found university guidelines recommending no more than 20 pounds of ash be used annually per 1,000 square feet of garden space; the ash should be evenly spread and immediately incorporated into the soil. Always avoid using wood ash, or other pH-raising materials, near acid-loving plants like azaleas and blueberries.