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Charcoal is Not a Good Soil Amendment in Colorado

By Carl Wilson, Colorado State University Cooperative Extension Agent, Horticulture

While Colorado gardeners concerned with the environment may be tempted to dispose of barbecue grille ashes in the garden rather than the trash, this is a poor practice because of our soil type.

Charcoal is most commonly made from burned wood. It is highly alkaline (high pH) and also rich in potassium salts. Colorado soils are already highly alkaline and rich in potassium. Adding charcoal ashes to gardens here only adds to soil characteristics already in excess, and doesn't supplement deficient elements such as nitrogen that limit plant growth in our area. Supplementing deficient elements is also known as fertilizing. Wood ash is fertilizer of the wrong type for Colorado.

Increasing alkalinity causes further problems with plants ability to wrest iron and other micronutrients from the soil. The increased salts of a kind not contributing to deficient elements can cause plant salt damage. Further, the fine particle size of ash also tends to plug the pores of clay soils leading to water penetration and drainage problems.

In composting, dry leaves are a far better carbon source to provide energy for composting microorganisms than charcoal ash.  Remember that the compost eventually is placed in the garden where the object is to supplement soil deficiencies, not add to excesses.

In short, ash contributes nothing to aid plant growth in our soil types and can only cause plant growing difficulties. Some gardeners may have applied ashes to eastern U.S. soils that are acid and low in potassium. In Colorado, soil circumstances are different and the practice is not recommended.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010