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Using wood ash in the garden

Each winter in North Georgia, plumes of smoke scatter across the mountains, indicating that families are home enjoying a roaring fire in their fireplaces or wood-burning stove. Whether these fires are used as a source of heat or for enjoyment, once the flames die down, a pile of wood ash remains.

University of Georgia Cooperative Extension agents get calls this time of year from gardeners asking if they can add wood ash to their garden plots. Ultimately, the answer is “yes,” you may add wood ash to your vegetable garden or even sprinkle some over your lawn, but the trick is to not add too much.

When wood burns, nitrogen and sulfur are emitted as gases, but plant nutrients are also left behind in small amounts. The amount of these nutrients largely depends on the type of wood that was burned. In general, hardwoods, such as maple and oak, provide more ash, up to a third more calcium, and slightly more nutrients, than softwoods, such as pine. 

The main nutrient in wood ash is potassium, the third number listed on a fertilizer analysis. It’s left behind in the form of “potash” (potassium bicarbonate). Adding wood ash to a garden plot also adds calcium and magnesium to the soil, similar to applying lime.

If your soil has a neutral pH (pH near 7) or has high levels of potassium, then ashes should not be incorporated into your garden soil. This can sometimes be an issue for individuals who use a standard fertilization program for their garden and lime regularly.

However, if your garden soil is acidic and moderate to very deficient in potassium, then wood ashes can be a viable amendment for the soil. Again, wood ash can be used as a liming material that will work to “sweeten the soil” at about half the rate of ag limestone or dolomite. So, if your soil test report says that you need 20 pounds of lime, then you can try about 40 to 50 pounds of wood ash.

Do keep in mind, however, that only a soil nutrient analysis will give a definitive answer as to your soil needs.

Since adding wood ash can increase the pH level in your soil, when looking for places to use wood ash in your landscape, keep in mind a plant or crop’s preference for soil acidity. Asparagus and eastern red cedars can tolerate a more alkaline soil and therefore, wood ashes, but blueberries and rhododendrons prefer a fairly acidic soil (less than 5.5), so avoid adding wood ashes around these plants or where they’ll be grown.              

Most vegetables and ornamentals perform best in slightly acid soil. A pH level of 6.5-6.8 is just about perfect for a vegetable garden. If you’re not sure about the pH in your garden, then do a soil test first to see if it could benefit from wood ash to avoid adding too much.

Since one batch of wood ash can be different from the next, to safely add wood ash to your home vegetable garden, use this amendment in moderation, as while there are some beneficial components, wood ash tends to contain a relatively high salt (Na) content of up to 20 percent or more.

To check your garden’s existing pH and potassium level, take a soil sample to the Extension office. A best practice is to have the soil tested every couple of years, as the soil test will tell you the pH level and whether you are at a level where you need to hold off on adding more wood ash.

For more information on soil amendments, contact the County Extension office or register for the Fannin-Gilmer County Agriculture and Natural Resources Blog “A Fruitful Discussion” at: https://site.extension.uga.edu/fannin-gilmer/.

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