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– Composting Wood Ashes –
Free Fertilizer or Toxic Waste?

As we enter into the depths of winter, I am often asked about the merits of composting wood ashes. I can only assume that this is due to all of the wood stoves and fireplaces that have kicked into high gear in recent months.

composting wood ashes

The questions people ask are varied but they all have one underlying concern – will composting wood ashes harm their compost piles? Actually, I’d bet we need to take this concern one step further and ask, “If I add wood ashes to my compost pile and then use that compost in my gardens, will it harm my plants?” The answer to both questions is the same…”No!” Actually, quite the opposite is true. Wood ashes can be a great addition to compost piles and/or to your garden.

Ash versus Charcoal

Just to clarify, when I use the term “wood ash” in this article, I am referring to the entire remnants of a wood burning fire. In reality, what remains after a typical fire in a wood burning stove or fireplace is both ash and charcoal. Ash is the fine grey substance, whereas charcoal describes the black chunks of material that accompany the ash. Together, I am calling these two substances “wood ashes”.

Both ash and charcoal can offer tremendous benefits to your compost and garden soil.

Benefits of Composting Wood Ashes

Phosphorus, Potassium, and Micro-Nutrients

Wood ashes are a source of phosphorus and a great source of potassium. If wood ashes were sold as a fertilizer, the nutrient breakdown would read “0-1-10”. That is, 10% potassium and 1% phosphorus. That’s not bad for an organic fertilizer, assuming you’re in need of potassium in your garden.

During the composting process, these nutrients will be shuttled through, and tied up in, the bodies of your compost microbes. Once your compost is applied to your garden, these nutrients will gradually make their way into your plants via plant-microbe exchanges.

By composting wood ashes, you will also be adding the following micro-nutrients back into your pile – iron, manganese, boron, copper and zinc.

Nutrient-Holding Capacity

The charcoal component of wood ash is a great addition to your compost pile for several reasons, both of which have to do with its porosity. Similar to clay, charcoal has a huge surface area.

“Just 1 gram of activated charcoal has a surface area in excess of 500 m2 (about one tenth the size of a football field)1.

This increased surface area introduces a large amount of oxygen into your pile, which your microbes love. The second is that it helps to adsorb (not absorb) nutrients in your compost pile and prevent them from leeching out.

This adsorbing characteristic of charcoal is one of the main reasons it is used in the medical community to treat alcohol poisoning. The alcohol attaches to the charcoal molecules, preventing further absorption into the bloodstream and gets shuttled out of the body.

Increasing pH

compost ashes

Whether you’re composting with wood ashes or using them directly in your garden, they tend to increase pH levels. Wood ashes are 25% calcium carbonate, which makes them quite alkaline (pH greater than 7.0) by nature. Calcium is a VERY critical nutrient to your composting and gardening success.

If your soils are quite acidic, the addition of wood ash is very beneficial. If you’re composting a lot of vegetable wastes (quite acidic), you’ll benefit from the alkalizing effects of ash. However, if your soils are already alkaline, which mine are in Ontario, using an excessive amount of wood ash in my compost or garden can be detrimental.

This is one reason to avoid using wood ashes in excess. If you swing your pH to far in one direction, it will lock up specific nutrients in your soil, which is not what we want. This is one reason I always recommend regular soil testing.

Note – Ideal soil pH is slightly acidic.

Avoid Doing this with Wood Ashes

1. Avoid using wood ashes in excess – they can contain trace amounts of heavy metals, however, the harmful effects of such metals can be mitigated by ensuring a healthy microbial population in your soils. Just avoid using ashes in excess.

2. Do not use wood ashes to fertilize acid-loving plants (e.g. blueberries).

3. Avoid spreading wood ashes around newly planted seedlings or seeds.

4. If you’re not composting wood ashes before applying them to your garden, try mixing the ashes with some soil. Do this prior to applying them to your garden. Wood ashes can be quite caustic when combined with water, so pre-mixing them with soil before applying can reduce many mishaps.

All in all, as I sit in my basement with my back to my big ol’ wood burning stove, I have to give thanks, not only for the heat, comfort, and sounds that it generates, but also for the wonderful free compost and garden additive it produces.

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Want to learn other uses for charcoal in your garden? Check out what our other Tribe members are recommending.


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