Charcoal is often used to improve soil and drainage when placed underneath a plant in a pot. There are different types of charcoal, however, and grilling charcoal is not the best thing to use on your plants. Horticultural charcoal, on the other hand, can benefit plants and is usually available at local garden centers and nurseries.
Grilling charcoal is designed to ignite quickly and smoulder for a long time. For this reason, charcoal briquettes contain solvents such as paraffin and other hydrocarbons, along with borax, starches and sodium nitrate. However, these are burned off when cooking, according to the Harnett County Cooperative Extension in North Carolina, which concludes that while charcoal ashes won't necessarily hurt your plants, they are not beneficial to plants either. Instead, use a purer form of charcoal.
Horticultural charcoal, also called biochar, comes from hardwood burnt down to charcoal. This is done in an enclosed space, such as a trench covered with an inch of soil or in a metal barrel. Because of the lack of oxygen, the wood burns down to charcoal instead of ash. Like wood ash, horticultural charcoal has a high pH level, which means it adds alkaline to the soil, quickly raising the pH. It can also benefit plants by absorbing toxins and excess water from soil.
Wood ash is what remains if you burn the wood without limiting oxygen. It is primarily composed of calcium carbonate, according to Purdue University's Consumer Horticulture website. It also contains 10 percent potash and less than 1 percent phosphate. Because wood ash is so fine, it shouldn't be used in the same manner as horticultural charcoal. Wood ash should be used to amend soil, whereas horticultural charcoal is primarily used in container planting.
Plants in pots that don't have good drainage benefit from horticultural charcoal. Cover the bottom of the pot with about an inch of the charcoal. Then fill the pot with potting soil or another planting medium. The charcoal helps prevent water from accumulating in the soil and thus helps prevent root rot and other fungal diseases.
In the Garden
Because of the amount of calcium carbonate in it, wood ash can be used to amend garden soil that has a pH level of less than 7.0. Otherwise, the wood ash will raise the pH level too high to benefit plant growth. If your soil's pH level is between 5.5 and 7.0, work about 20 lbs. per 100 square feet into the top 6 inches of soil.
- Texas A&M University -- Aggie Horticulture: House Plants
- Purdue University Consumer Horticulture: Wood Ash in the Garden
- Hardwood Charcoal Basques: Horticulture Charcoal (or Biochar)
- Mother Earth News: Making Biochar to Improve Soil
- Harnett County North Carolina: Ask the Hort Agent -- Will Charcaol Ashes From My BBQ Grill Hurt My Plants?
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