Charcoal is Not a Good Soil
Amendment in Colorado
By Carl Wilson, Colorado State University Cooperative Extension Agent,
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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