How to Compost Wood Ashes

Up here in Vermont, the winters are long.

For me, this means months spent curled up by the wood stove with a book and a cat on my lap.

It also means the creation of a lot of ash – which always leads me back to the question of whether or not I can compost ashes.

As it turns out, the answer is yes, with a couple of crucial caveats.

A vertical picture of a wood fire burning with ash collecting. To the center and bottom of the frame is green and white text.

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This article will explore how to use wood ashes in the compost and in your garden, and when it is appropriate to do so.

Benefits of Wood Ash

Wood ash from your fireplace contains a number of nutrients that can be very beneficial to a garden – in the right circumstances.

But never use the ash from charcoal, trash fires, or treated wood, which can contain toxic chemical residue from additives.

Wood ash contains potassium and calcium in considerable quantities, as well as lesser amounts of magnesium and phosphorus, and micronutrients such as copper and zinc.

A close up of a metal bucket full of wood ashes from the fireplace, with a small shovel and cultivator, set on a lawn.

Due to its high level of calcium, it can increase the pH of soil, making it an ideal natural substitute for lime, an amendment often used to balance soil that is too acidic.

It can be a very useful amendment where acidity is too high for growing most veggies, in a pH range of 6.0 and below.

But you’ll need to be cautious. If the soil is already neutral or alkaline, adding ashes will cause excess alkalinity and add soluble salts, ultimately doing more harm than good.

So how do you know when it makes sense to add ashes to your compost or garden?

Let’s explore.

Test Your Soil

Before adding ashes (or any other amendment, for that matter) to your garden, be sure to get your soil tested!

You can easily request a test through your local agricultural extension office.

You can buy home pH and soil test kits or pH meters at your local hardware store or online, though I would recommend getting a test from your local extension office at least once.

The results of these tests are more comprehensive and they will tell you a whole lot about your soil, including information about any other nutrient deficiencies.

A close up of a soil pH and moisture meter placed in the soil with a flower in soft focus in the background.

If you don’t have access to professional testing or a kit, it is possible to DIY a basic test of the pH with just two cups full of soil, some vinegar, and some baking soda.

Pour vinegar into the first cup. If the soil begins to fizz, it is alkaline.

Add some water to moisten the soil in the second cup, then add baking soda. Fizz this time means it is acidic.

This method is not especially accurate, and you won’t be able to determine the exact pH level of your soil this way. It is still a good idea to get a more accurate test when you are able.

In the meantime, however, this simple method should at least give you a general sense of whether the soil is acidic or alkaline.

When to Compost: Timing and Moderation

The key is to add small amounts of cooled ashes to a new or uncooked pile. Because it has such a high pH value, it is important that you don’t add too much to your compost.

According to Olivia Saunders, Extension Field Specialist in Food and Agriculture at the University of New Hampshire Extension, “it should not make up more than 5% of your compost.”

Additionally, once the compost nears maturity, the addition of ash could raise the pH too much, increasing the bioavailability of heavy metals to harmful levels.

How to Compost

Before you start, be sure to suit up with gloves, eye protection, and a mask to avoid any potential irritation to the skin, eyes, or lungs. Also, ensure that the ashes have completely cooled before handling them.

A close up of a wooden home composting bin filled with garden and kitchen waste in the garden.

Sprinkle the ash onto your compost pile along with the appropriate ratio of brown and green material.

What does this mean? Add about a quarter inch for each 18-inch section of browns and for every six inches of greens. Be sure to turn the pile each time you add new material.

As a reminder, browns include carbon rich materials such as straw, hay, and dried leaves, while greens are more nitrogen heavy items such as kitchen scraps and fresh grass clippings.

To learn more on the basics of composting, check out this article.

If you have a hot compost pile, add a small amount of ash along with other new materials every month or so while it is active.

If the pile is cold or rarely added to, only add ash in the fall or late summer, allowing time for everything to break down before being used in the garden during the growing season.

You can collect ashes and store them in a covered container through the winter.

Adding It Directly to the Garden

If you have determined via a test that your pH is low – below 6.5 – you can also choose to add ashes directly to the garden to reduce acidity.

A close up of a gloved hand from the top of the frame spreading ash from the fire around strawberry plants growing in the garden, pictured in bright sunshine.

Incorporating ashes can also increase the bioavailability of potassium, phosphorus, and various micronutrients, thereby increasing fertility.

A Note of Caution:

Never mix ashes with nitrogen fertilizer, it can cause a reaction that releases ammonia gas. Always wear eye protection, a face mask, and gloves when handling wood ashes.

Spread on calm days to prevent it from blowing around and scattering to unwanted areas – including all over your clothes.

Apply in moderation, lightly dusting a small amount on the garden surface and working it into the soil several inches deep with a fork.

According to Rosie Lerner, Horticulture Specialist at Purdue University, “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 six inches or so.”

Be sure to test the soil again the following year. You can reapply if the pH is still too low, but if it has reached 6.5, don’t add any more. If you raise the pH too much, this can deplete the bioavailability of essential nutrients.

Do not apply to acid-loving plants such as blueberries, rhododendrons, or azaleas.

Continue to test your soil every few years and amend as necessary.

Ashes to Dust

While it is never wise to dump a whole bucket on your compost or in the garden beds, used in moderation with careful planning and an understanding of your soil, wood ashes can be repurposed as a useful amendment.

A close up of a trowel scooping wood ash out of a fireplace for placing on a compost pile.

Though my wood stove churns out far more than I can safely use each winter, I am still able to recycle much of it back into my compost.

Have you used wood ashes in your garden? Please share your experience in the comments below!

If you enjoyed this article, you can learn more composting tricks in these guides:

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