We’ve accumulated a lot of wood ash over the winter. Can we add it to our garden soil or compost pile?
Whether using wood ash in the garden is a good idea depends on your garden soil’s pH and fertility levels. If a soil test has shown your garden soil’s pH to be below 6.0 (meaning it’s moderately acidic), adding wood ash could be beneficial, says Garn Wallace, who holds a doctorate in biochemistry and is general manager of Wallace Laboratories in El Segundo, Calif.
In acidic soils, wood ash can increase soil fertility by increasing the availability of phosphorus and potassium as well as some micronutrients — although wood ashes won’t supply any nitrogen. Also rich in calcium, wood ashes are effective for raising soil pH — a potential benefit in places where pH is below the ideal level for most vegetables (6.0 to 7.0).
“Moderation is the key,” Wallace says. “People tend to over-apply nutrients. And after you add something to the soil, you can’t take it away without replacing the soil. If you apply wood ashes without a soil test, it is possible to ruin soil in just one year.”
If, after testing your soil pH, you decide to add wood ash, start with a thin dusting across the soil surface, then work the ashes deeply into the topsoil, because most nutrients won’t move much in the soil. A Purdue Extension publication suggests that gardeners whose soils are below a pH of 6.5 can safely apply 20 pounds of wood ashes per 100 square feet if the ash is worked into the soil about 6 inches.
The following year, test the soil pH and nutrients again. If the pH is still low, work in another thin layer as you did the previous year. When your soil pH has reached 6.5 to 7.0, stop adding wood ash. If you add too much wood ash, you risk raising the pH over the neutral 7.0 to 7.2 range, which can tie up essential nutrients in the soil. Continue to test the soil every two to three years, and adjust soil amendments according to the test results.
As for composting the wood ash, it depends on what stage your compost pile has reached. Adding small amounts of ashes to a new compost pile is probably OK. If the compost is at or near maturity, however, adding wood ash would raise the pH and could increase the availability of heavy metals to harmful levels. “You want these minerals in minute amounts — too much of them is never a good thing,” Wallace says.
One last caution: Never use ashes from treated wood in your garden. Treated wood contains copper, arsenic, chromium and sometimes boron, and ashes that contain these heavy metals could harm soil, plants and animals.
— Vicki Mattern, Contributing Editor
Above: Moderation is key when using wood ash.
Photo Courtesy Spectrum Photofile/James Jeffreys
Vicki Mattern is a contributing editor for MOTHER EARTH NEWS magazine, book editor and freelance magazine writer. She has edited or co-authored seven books on gardening, and lives and works from her home in northwestern Montana. You can find Vicki on Google+.