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Can Fireplace Ash Be Composted (& How Much to Use)?

We don’t run our wood fireplace all the time, but when we do we have a large amount of ash left over. Rather than tossing it, we wanted to find another use for the ash and thought about using it in our compost bin for the garden. So, I did some research to learn more. Here’s what I found.

Good Ash for CompostingBad Ash for Composting
PaperTreated Wood
WoodTrees Near Industrial Sites
Most Other Wood-Based AshTrash or Plastic

Fireplace ash from paper, wood, and charcoal is compostable. Avoid using ash from coal, treated wood, and trash as they have contaminants and heavy metals. Too much ash can make a compost pile alkaline, so use a 1/2 inch layer of ash for every 1 vertical foot of compost. Ash is best for soil with a pH less than 7.0.

So, while certain types of fireplace ash are good for compost, how much ash should you use, and can you apply fireplace ash directly to the garden? Let’s take a closer look.

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How Much Ash Can You Use in Compost?

using ash in a compost bin

If you like a quick guideline of how much ash to use, apply a 1/2 inch layer of ash for every vertical foot of compost.

However, many gardeners recommended not applying wood ash until you know the pH of your compost pile.

This is because while ash is mostly carbon, it has high alkalinity and can mess with the pH of compost.

ph scale fireplace tips

Wood ash has a pH of 10-12, while finished compost should have a pH of about 5.5 to 8.0.

So, adding too much ash quickly turns compost piles alkaline.

Alkalinity slows the beneficial microbes from decomposing the compost and limits the nutrients absorbed by the plant’s roots.

The reason why plants prefer a slightly acidic pH (commonly between 6.0 to 7.0) is it helps dissolve the nutrient solids in the soil to be absorbed by the plant’s finer roots.

Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.

Donald Bickelhaupt, Instructional Support Specialist, Department of Forest and Natural Resources Management

However, that doesn’t mean you should skip using fireplace ash. It’s full of nutrients that are useful for compost and gardens (more on this later). This just means it’s smart to keep an eye on your compost’s pH.

Two good ways to check your compost or soil pH are to use a pH strip or a pH meter. I prefer using a pH meter since they’re affordable and easy to use.

Before using ash, check your soil or compost’s current pH. If it’s under 5.5, apply a 1/2 to 1-inch layer of ash at a time. Avoid using ash if your soil or compost is above a 7.0 pH.

If you find that your compost pile is too alkaline, add acidic amendments such as coffee grounds, peat moss, or pine needles.

Of course, if your compost pile is too acidic, add wood ash! You can also use charcoal and biochar.

Perhaps more important than the pH of a compost pile is balancing its carbon and nitrogen ratio.

Is Fireplace Ash Brown or Green Compost?

using wood ash in a compost pile
Brown (Carbon) MaterialsGreen (Nitrogen) Materials
Dried leavesGreen leaves
HayCoffee grounds
Wood chipsTea bags
SawdustFood scraps
Pine needlesSeaweed

Fireplace ash (and other ash) is brown compost as it’s mostly carbon.

Aim for your compost pile to have a carbon-nitrogen ratio of 30:1.

It’s difficult to apply too much carbon (brown material) as the worst case will slow the rate of decomposition.

However, too much nitrogen (green material) in compost leads to a stinky and too potent pile.

For proper decomposition, aim for your compost pile to have a pH between 5.5 to 8.0 and a carbon-nitrogen ratio of 30:1.

Pro-tip: If your compost is getting smelly or has flies, add a 1-inch layer of ash (or any other fine brown/carbon material). The carbon smothers the smell and prevents flies and other surface bugs from accessing the compost.

Can You Apply Fireplace Ash Directly to the Garden?

ash placed on a garden bed

You can apply fireplace ash directly to the garden if it’s from paper, wood, or charcoal.

Wood ash contains about 25% calcium (in the form of calcium carbonate), which makes it a natural and common liming material to increase soil alkalinity. It also has other secondary ingredients such as potassium (4%).

Here’s what Rosie Lerner, a horticulture writer at Purdue University, has to say about using wood ash in the garden.

Acidic soils (pH less than 5.5) will likely be improved by wood ash addition. Soils that are slightly acidic (pH 6.0 to 6.5) should not be harmed by the application of 20 pounds per 100 square feet annually, if the ash is worked into the soil about 6 inches or so. However, if your soil is neutral or alkaline (pH 7.0 or greater), find another way to dispose of wood ash.

Rosie Lerner, Purdue University

Because wood ash is a fine material, it reacts quickly in the soil. While there are valuable nutrients in wood ash, the main ingredient is calcium (a liming agent), so avoid applying it if soils are already neutral or alkaline in pH (7.0 or above).

On the other hand, if you’re working with acidic soil, then wood ash is one of the best amendments you can get.

Before you apply wood ash, make sure the plants you’re growing aren’t acid-loving plants.

To give you a head start, here are some plants that prefer acidic soils and some that prefer alkaline soils.

Acid-Loving PlantsAlkaline-Loving Plants
CranberriesSweet Potato
Fruit TreesBeets

Pro-Tip: Apply wood ash while the soil is wet so the ash sticks and doesn’t blow away.

Overall, start by applying small amounts of fireplace ash to your compost or garden soil.

If you’re applying larger amounts (more than 1/2 inch), first measure your compost’s or soil’s pH before applying more.

Wood ash that has been exposed to weather (mostly wind and rain) has lost most of its liming and alkalizing effect, so you should be safe in applying more.

Increasing your garden soil’s organic matter also means increasing its water and nutrient retention, so mix the ash with compost for the best results!

Remember to keep your compost pile between a pH of 5.5 to 8.0, and at a carbon-nitrogen ratio of 30:1, and it’ll be much more effective.

Other Uses for Fireplace Ash

a snowy driveway

Aside from using it in your compost or garden, there are many other uses of fireplace ash, including:

  • Cleaning
  • Soap Making
  • Oil Spills
  • Melting Ice and Snow
  • Chicken Dustbaths

For example, fireplace ash is a cheap and eco-friendly way to melt ice and snow and make your drive or walk outside much easier. This is in part because ash sticks to snow like glue, providing a good amount of traction. It also isn’t corrosive to cars like salt, and it’s better for the environment.

If you’d like to see even more uses (and instructions for how to use them), check out my other post: 10 Creative Ways to Use Wood Ash (Don’t Toss It!)

Need More Help?

You can always ask us here at Fireplace Tips, but you should know the other resources available to you! Here are the resources we recommend.

  • Chimney Safety Institute of North America (CSIA): The CSIA is your BEST resource for fireplace and chimney safety at home. They’re a non-profit governed by a volunteer board of directors dedicated to the education, training, and certification of chimney and industry related professionals.
  • Self-Sufficiency and Off-Grid: If you’re like me and passionate about off-grid and self-sufficiency, see my number 1 resource—Abundance Plus. Check out their Frugal Homesteading Course on growing 90% of your own food. Get 7-days free and 10% off with the code: TYLER10