Is Wood Ash a Good Soil Amendment?

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Is wood ash good for the garden? People have been tossing their fireplace ashes in their gardens for centuries but what affect does wood ash have on our soil and can it help or hurt our plants? The answer depends on several factors.

If you’re interested in alternative soil amendments, also see Biochar For the Garden: The Pros and Cons.

Garden path with archway and image of wood burning fire.

Should I Put Wood Ash in My Garden?

If you have a wood-burning fireplace you use on a regular basis, you will accumulate a lot of leftover wood ash. Most of it just gets thrown out, ending up in landfill, but many of us would rather use it in our gardens if it is indeed beneficial.

So, the burning question is:

Is wood ash actually good for plants in the garden?

Or, is it just filler? Or, can it be harmful?

And, the quick answer is, it depends.

Along with some other factors, how wood ash affects soil and plants depends on the composition of the wood ash itself, the composition of your soil (including the pH level, available nutrients and deficiencies indicated by lab soil tests), and the volume of wood ash applied.

And because it depends, like many other soil amendments it is best to only add wood ash if there is a specific, measurable need and you know how to apply it.

Trivial amounts are not a concern but it’s larger volumes—enough to make measurable changes including escalating the soil pH level—that we’ll discuss here.

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What is Wood Ash?

Wood ash is the powdery residue remaining after the combustion of wood with oxygen present.

Biochar and charcoal are created when organic matter like wood, grass, agricultural waste, or manure are pyrolyzed (heated to high temperatures) without oxygen present.

Like biochar, wood ash is also the remnants of burned organic matter, but there is one major difference.

While biochar contains carbon, burning wood literally sends the carbon up in smoke, into the atmosphere.

So, from an environmental perspective, it’s not a great energy choice since it does not sequester carbon, but, if the deed is done, what about making use of those ashes?

We know wood ash is used on agricultural and forest soils in many parts of the world but it’s a big leap to assume it would also benefit our home gardens without more information.

In those wider applications, they have studied the soil and its needs, and apply wood ash (with specific traits) tailored to those needs.

Contrast that with our home gardens that vary considerably.

Plus, not all wood ash is the same. The composition depends on the types of trees burned, the kind of soil they grew in, and differences between trunks and branches. The wood ash from your fireplace will very likely different than mine.

So, the more you know about your soil and wood ash, the better decisions you can make.

Let’s have a look at some of the ways wood ash can affect our plants and soil.

How Wood Ash Affects Soil pH

Soil pH level is a measure of the concentration of hydrogen ions in the soil and offers clues to how well our plants can or cannot take up available (and needed) nutrients. The nutrients may be in the soil, but when the pH level is whacky, the plants cannot get what they need.

Wood ash is highly alkaline, usually reading somewhere between 9 and 13.5. Looking at the chart (above), you can see how extreme that is. Here in Canada and the United States (zones 4 to 8), most of our gardens clock in with a pH level between 5 and 7.5 and most of our plants do well in this range.

Right away you can see how introducing a large volume of wood ash could adversely disrupt your soil conditions. And you certainly wouldn’t want to add it near plants like blueberries or azaleas that do best with slightly acidic soil.

Again, it won’t matter for small quantities sprinkled here and there, but larger amounts could push your pH level out of the desired range (for a period of time), interfering with water and nutrient uptake.

How much is enough or too much? The answer will depend on your soil test results. It’s impossible to know the dose if you don’t know the need.

If I wanted to pursue this ongoing, I would get tests done on my soil and the actual wood ash to understand if they are a good match and in what proportions.

Wood Ash for Raising pH

If you do have acidic soil (low 5s or lower) and are looking to raise your soil pH, then wood ash can be used as a liming agent—acting like traditional lime or calcium carbonate.

Wood ash is not as potent as lime, but it usually works more quickly—for a while. Any changes to soil pH are temporary and it’s a never-ending project if you want to adjust it ongoing. That’s why you hear of gardeners with acidic soil liming their gardens annually.

If we compare lime with wood ash, wood ash wins for nutrients.

Traditional lime has calcium but no other nutrients.

Wood ash from a home wood stove is often around 30% calcium, which sounds great except that garden soil is rarely calcium-deficient.

Wood ash also contains phosphorus and magnesium but little or no nitrogen. It is also typically pretty high in potassium (a soil macronutrient), although potassium is highly water soluble and tends to wash out pretty quickly. Easy come, easy go.

There’s no benefit to adding excess nutrients to your soil: plants can’t use them and they can do harm to your garden and the environment.

If your soil is acidic and deficient in potassium, then wood ash starts looking pretty good as a soil amendment.

And that is the case for a lot of the world’s agricultural soils—especially in tropical and subtropical areas—and forest soils, even in North America, where they’ve been damaged by acid rain and suffer from calcium depletion.

Know What You’re Burning

The same way we need to research the type of wood used in wood chip mulch, it is important to make an informed decision about the type of wood used for intentional fires and, consequently, what remains in the wood ash.

You don’t want wood that has been chemically-treated or exposed to toxins.

If you’re just using locally cut trees for firewood, it’s probably not a problem. If you’re buying wood pellets be sure to check the ingredients and avoid any unsavory additives. If you’re buying cords of wood, find out all your can from the seller.

Also be careful handling ashes—you don’t want to inhale them, get them in your eyes, or on your skin. If you recall the warnings from high school chemistry class: anything with high (or low) pH can burn and wreak havoc.

Adding Wood Ash to the Compost Pile

If, like us, you’ve ruled out the use of wood ash as a direct soil amendment, that doesn’t mean you’re stuck with throwing the ashes out.

You can add wood ash to your compost pile with these considerations.

The bacteria that are primarily responsible for turning your scraps into compost can be pretty sensitive to changes in pH—much more so than fungi. But there are bacteria that thrive in alkaline conditions—so raising the pH in your compost pile may favor some bacteria over others, but everything should work out okay.

Just don’t overwhelm your compost with wood ash—bacteria are good up to about a pH of 8 and you probably have acidic wastes in your compost pile that will help offset the wood ash.

And, while there is very little carbon or nitrogen in wood ash, meaning it won’t do much for your C-to-N ratio (see Composting 101), it will add nutrients that will eventually be slow-released in the garden over time.

There are people who would love to see wood ash collected for use in forests and also for restoring lakes that have also become acidified, but, unfortunately, that’s proven to be difficult to organize.

If you’re in that same situation with acidic soil and a potassium deficiency, then wood ash may make sense as a soil amendment. Otherwise, you can put the ashes in the compost pile—in moderation— and at least get some of the nutrients back.