Because firewood is a readily available resource in our cold area, wood burning fireplaces and furnaces are not uncommon in the North Country. Depending on the efficiency of a wood burning furnace and the type of wood being burned, an average cord of wood creates about 20 pounds of ash. Over the course of one winter the ash can really add up! Many people spread their wood ashes in the garden, which can be a benefit if done properly.
Wood ashes contain about 1 percent phosphorous, 5 percent potassium, and small amounts of other elements our plants require. At first this sounds great, but due to the way the potassium reacts in the soil, wood ashes can be degrade your soil's health.
When the potassium combines with water in the soil an alkaline substance chemically known as potassium hydroxide and commonly called lye is formed. This lye then raises the pH of the soil and soil pH effect nutrients availability and the microorganisms that can live in the soil.
Before putting any wood ashes in your garden, get the pH tested. If your soil pH is 7.0 or higher, you won't want to use any wood ashes in your garden. If your pH is below 7.0 you can consider using wood ashes.
At the same time you get your pH tested, request a recommendation for adding limestone (or lime). The general rule of thumb is to use two pounds of wood ash for every pound of ground limestone. Apply no more than 5 pounds of wood ash per 100 square feet of soil per year. If you add more than this you can run into problems of excess salinity, alkalinity, and plant nutrient availability.
The best time to apply wood ashes is during the winter. Applying the wood ashes during the growing season can cause poor germination or chemical burn.
Never use coal ashes or charcoal briquettes. Coal contains toxic heavy metals. Charcoal briquettes are held together with a binder made from sodium borate, which is toxic to plants.
Anne Lenox Barlow is the horticulture educator with Cornell Cooperative Extension in Clinton County. CCE offices may be reached in Clinton County at 561-7450; Essex County, 962-4810; and Franklin County, 483-7403. E-mail your questions to askMG@cornell.edu.