Wood ashes benefit gardens when applied in moderation

Posted Dec. 19, 2008, at 3:59 p.m.

A lot of Maine gardeners rely on wood-burning stoves and fireplaces to stay warm in winter, resulting in an abundance of wood ashes. An average cord of wood, depending on wood type and combustion efficiency, will yield a 5-gallon bucket of ashes. Should these ashes be used in the garden?

There was a time when wood ashes were valued as a primary source of potassium for both field and garden, as well as a means of raising the pH of acidic soils. While those days may be gone, home gardeners can still take advantage of a supply of wood ashes for use as fertilizer and for amending the soil.

Wood ashes, used in moderation, can benefit most garden soils. They contain between 5 percent and 7 percent potassium and up to 2 percent phosphorus, both major plant nutrients. Hardwood ashes have higher potassium levels than softwood ashes.

Wood ashes also contain 25 percent to 50 percent calcium, another essential plant nutrient, along with a number of minor elements. They do not contain a significant amount of nitrogen, so you will need to supply this essential element from compost, green manures and other sources.

There are two potential problems associated with use of wood ashes: excess soluble salts and alkalinity. Since wood ashes are up to 90 percent water-soluble mineral salts, excessive applications can result in a buildup of these salts in the soil, resulting in root injury and plant death.

Because ashes are alkaline, you should avoid using them in soils above pH 6.5. Also, avoid using them around rhododendrons, blueberries and other acid-loving plants. Wood ashes act much faster than lime in raising soil pH, so have your garden soil tested at least every other year to make sure the use of wood ashes is not raising soil pH above the optimum level.

A safe application rate for most garden soils would be 20 pounds per 1,000 square feet, the equivalent of a 5-gallon pail of ashes. This is the equivalent of 6 pounds of ground limestone per 1,000 square feet, an amount considered appropriate for yearly applications without changing proper soil pH.

Wood ashes can be raked into freshly tilled soil in spring, three to four weeks before planting, or they can be side-dressed around growing plants. Apply the ashes evenly, avoiding lumps or piles.

To avoid burning sensitive plant tissues, do not apply ashes over germinating seeds or young seedlings. Also, rinse applied ashes off foliage.

Store your wood ashes in a metal container to keep them dry. Potassium and other water-soluble nutrients are leached out of wood ashes that are left standing in the rain. This leaching not only reduces the nutrient value of the ashes but also leaves them more alkaline.

Never put coal or charcoal ashes on the garden. Coal ashes tend to be high in toxic heavy metals while charcoal ashes contain sodium borate, a chemical toxic to plants.

A final caution: Protect yourself when applying wood ash. Use the same precautions you would use when handling household bleach, another strongly alkaline material. Wear eye protection and gloves and, depending on the fineness of the ash and the direction of the wind, you may want to wear a dust mask.

In moderation, application of wood ashes can be an effective part of garden soil management. And gardeners know, from experience, that the garden demands moderation in all things.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

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