3 Ways to Use Wood Ashes in the Garden

If your house is equipped with a wood-burning fireplace or you have a fire pit in your backyard, put the left over ashes to work in the garden. As long as you follow a few simple precautions, wood ashes can be used to benefit the garden in three ways: as a fertilizer, as a soil amendment, or as an insect repellent.

Use Wood Ashes as a Fertilizer

As a fertilizer, wood ashes are a good source of potassium, phosphorus, magnesium, and aluminum. They typically contain less than 10% potash, 1 % phosphate, and trace amounts of micro-nutrients such as iron, manganese, boron, copper, and zinc. (Wood ashes do not contain nitrogen.) The exact chemical make-up of ashes varies according to wood type (hardwood ashes contain higher potassium levels than softwood ashes). If compared to a commercial fertilizer, wood ashes would probably read about 0-1-3 (N-P-K).

Use Wood Ashes to Raise pH

Wood ashes contain approximately 25% calcium carbonate and behave like a liming agent when added to your soil (raises the pH). Because ashes are soluble in water, they can quickly correct pH levels when added to acidic soil (pH less than 5.5).

If you're unsure of your soil's pH, don't add wood ashes to your soil until you have it tested. Raising the soil's pH above optimum levels can adversely affect the health of your plants. When pH levels rise above 7.0 (neutral), important nutrients like phosphorus, iron, boron, and potassium start to chemically bond to the soil and become less available to plants.

Use Wood Ashes to Repel Insects

Because of its ability to absorb water, wood ashes can also be sprinkled around plants to prevent attacks from snails and slugs. Once wet, the ashes lose their effectiveness and must be reapplied. To avoid causing chemical burns to your plants, sprinkle small amounts of ashes evenly around the base of the plant, and rinse off any that comes in contact with the foliage.

Tips for Safe Application

• Soils that are slightly acidic (pH 6.0 to 6.5) should be able to safely handle an application of 20 pounds of ashes per 1000 square feet annually (approximately one 5-gallon pail) without changing the pH. This is the equivalent of 6 pounds of ground limestone per 1,000 square feet. If you're trying to raise the soil's pH, you'll need to add more.

• Wood ashes act quickly once added to the soil. If you use them, have your soil tested regularly (every two years) to make sure the proper pH levels are being maintained.

• Wood ashes intended as a fertilizer or used to correct pH should be applied in the winter, or worked into the soil several weeks before planting. Apply the ashes to the soil evenly. Avoid piles and mounds, which can cause salts to build up in the soil.
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• Never use ashes near acid-loving plants like potatoes, rhododendrons and blueberries.

Handling & Storage

• Wood ash is a strong alkaline (like bleach) and should be handled with caution. Wear eye protection, gloves, and a dust mask while handling it, and avoid scattering ash when windy. Make sure ashes are completely "burned out" and cool before collecting them for use. To reduce the risk of fire, store them in a metal container with a secure lid.

Ashes From Charcoal Grills

Ashes from charcoal grills should never be used in the garden due to the chemical residues left by processed charcoal. The same is true of ashes obtained from cardboard, and ashes from wood that has been pressure-treated, painted, or stained. All contain harmful chemicals that could potentially contaminate soil and inhibit plant growth.