Can You Put Fireplace Ash in Your Compost?

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Adding fireplace ash to your compost can enrich it with minerals, benefiting your plants and improving soil health.
Use only small amounts of cooled, untreated wood ash—the rule is no more than 5 to 10 percent of your compost pile—to avoid harming microorganisms.
Mix ash thoroughly with other compost materials to prevent lye formation, and always test soil pH before applying.

After a long winter, your fireplace is likely full of ash. However, before you throw it all in the dustbin, consider how you might reuse it. One of the most unexpected scenarios is adding it to your compost pile. In fact, ash has many minerals that can be beneficial to your compost, and, subsequently, your plants.

So how should you add fireplace ash to compost, and what's the best way to use it? Ahead, we spoke to gardening experts about the ins and outs of this useful ingredient.

Petar Ivanov, gardening and plant expert at Fantastic Services
Gurkaren Ark, garden expert and project manager at Ark’s Landscaping

Benefits

Fireplace ash is an excellent source of minerals that improve the quality of compost. “[It's] rich in potassium, which is important for root development and flowering,” says Petar Ivanov, a gardening and plant expert at Fantastic Services.

It can also adjust the pH of your compost. Sometimes compost becomes too acidic during decomposition, Ivanov says—fireplace ash helps neutralize this acidity.

How to Do It

How much fireplace ash should you put in your compost? It depends. If your compost pile is on the larger side, you can use more ash; if it's too small, restrict yourself to less.

“I wouldn’t make more than 5 to 10 percent of my compost fireplace ash,” says Gurkaren Ark, garden expert and project manager at Ark's Landscaping. If you use more than this, your compost pile’s pH may become too alkaline, killing the beneficial microorganisms and worms.

When it comes to the type of ash, stick with natural, untreated wood. “Any plywood, firestarters, scraps, or anything along those lines can introduce contaminants to your compost,” Ark says.

Equally important, use cooled fireplace ash to avoid raising the temperature of your compost pile—too much heat can kill the microorganisms. Finally, ensure that the ash is as fine and burnt down as possible. This will help with absorption.

When to Use It

Use wood ash in your garden whenever you fertilize your plants. “Personally, I prefer using compost in the spring before planting,” Ark says. Just bear in mind that compost made from ash will raise the soil pH.

To apply the compost, sprinkle in a thin layer—don’t add a whole bucket at once. “It’s important that the ash is well mixed with the other materials, not left in a pile,” Ivanov says. This will prevent lye from forming, which can kill plants and negatively impact soil health.

Drawbacks

One drawback is the risk of contaminants mixing into the ash. Unless you only burn natural wood in your fireplace, you might unknowingly blend in other materials—such as ash from burnt newspapers, cardboard, or charcoal.

Another drawback is raising the soil pH too much. “You need to be careful where you’re using the compost. It can hurt your plants if you adjust the pH too much out of their ideal range," says Ark. Always test your soil’s pH level before adding compost that contains wood ash.