Adding Wood Ash to Gardens

March 31, 2016 – Kenosha News

Question: Hello, I have a question for you. I have a good amount of ash from a wood burner fireplace; would wood ash be good for my garden? I have a small section that is to be tilled up this spring and was thinking of putting some on top before I do so. Thanks. J.T.

Answer: This is one of those questions where the answer varies depending on soil type. In southeastern Wisconsin wood ash is not a good amendment for the garden because of soil pH.

Wood ash is recommended by garden experts as a source of potassium and as a liming agent – material that raises soil pH. Depending on the type of wood, ash can contain 3 to 10 percent potassium (third number on a bag of fertilizer) but southeastern Wisconsin soils are rarely deficient in potassium, especially if you regularly add compost or fertilizer to your garden.

The biggest problem with adding wood ash to our gardens is soil pH. The majority of residential soils in southeast Wisconsin are neutral to slightly alkaline with pH generally ranging from 6.8 to 7.6. Most plants grow best in soils from pH of 6.0 to 6.8. Wood ash combined with water quickly becomes potassium hydroxide which is highly alkaline (pH of 9.0 to 13.5). Adding wood ash to your garden would quickly raise the soil pH even farther than it already is. High soil pH can inhibit plant growth by chemically tying up soil nutrients.

soil test bagIn this situation soil testing is imperative so you know if wood ash would be harmful or helpful in your garden. I strongly suggest sending a sample to a commercial soil testing lab for accurate data. Commercial labs will give you individualized recommendations as part of the results report. University of Wisconsin Soils Lab information and testing instructions are online at, go to the section on lawns and gardens. Mail your sample and $15 fee directly to the lab in Marshfield. You will get test results and recommendations within two weeks.

If the soil pH in your garden is 6.5 or higher based on soil test results, do not add wood ash. If the soil pH is below 6.5, you could mix a small amount into the soil. Never use wood ash around acid-loving plants. Annual soil testing is recommended whenever wood ash is used as a soil amendment.


Kenosha County UW-Extension LogoBarb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at or call 262-857-1942.