Wood Ash and Charcoal in the Garden

Posted on December 21, 2023

I am always on the lookout for ways to repurpose “waste” into valuable resource, and one example of this practice is the use of ash and charcoal from the wood stove we use to heat our house during the colder months. Burning around 1.75 cords of firewood every year creates a lot of ash and charcoal, and rather than just dumping it into a pile somewhere outside I have instead applied it towards my soil husbandry practices.

The first step in this process is sifting the ash to remove the larger pieces of charcoal that remain behind. I use a simple compost sifter that I constructed out of untreated lumber and hardware cloth. I just dump the ashes into the sifter, shake it back and forth, and the smaller particles of charcoal and ash pass through while the larger pieces of charcoal are left behind.

Ash and charcoal waiting to be sifted
Charcoal left behind after sifting

The larger charcoal isn’t thrown out, it’s a very valuable substance for increasing soil fertility — not on its own, but because its physical structure consists of multitudes of microscopic cracks and fissures that accumulate water, nutrient, and microbial life. But the larger chunks aren’t yet ready for addition to the garden, so I add them to the deep litter chicken composting system. By incorporating the charcoal into this compost, it can be further broken down by turning and the regular scratching and pecking performed by the chickens, while allowing it to soak up water, nutrients, and microbes present in the compost ingredients to include the chickens’ manure. The charcoal is eventually applied to gardens as part of the finished compost that we rely on to increase fertility. Alternatively, the bio-char can be manually crushed or ground to reduce the size of large particles, and the nutrient can be spiked by adding it to a compost pile and/or soaking it in urine.

Yes, you read that correctly — human urine is one of the most nutrient-dense substances we can find, and I recommend looking for ways to apply it to the garden in order to take advantage of that fact, as well as return excess nutrients from the body to the soil where we live.

Larger chunks of charcoal dumped on the deep litter of the chicken coop
Wood ash and charcoal left behind in wheelbarrow after sifting

What is left behind consists of fine wood ash along with the smaller bits of charcoal that can fit through the hardware cloth screen in the sifter. One potential problem that the addition of biochar can present is that, if uncharged, it can effectively rob nutrients from the surrounding soil until filled with nutrients and microbes. Adding the charcoal with the wood ash during the winter months allows the charcoal to charge slowly over that time and be ready to help boost productivity during the following spring and summer. Furthermore, wood ash is highly alkaline, so it can help to counteract the natural acidity of humic acid produced by compost spread over the surface of garden beds by spreading a light dusting of the surface. Be careful, though — the alkalinity can easily throw off soil pH if applied too heavily.

Garden beds in one garden covered with a light dusting of wood ash
More garden beds with light dusting of wood ash

Many of my garden beds are sown in fall cover crops as a way of enhancing soil life and providing a protective mulch to the soil surface during the winter months. I avoid the wood ash and charcoal in these garden beds, however, instead relying on the natural cycles of life in the soil to maintain a relatively neutral pH and good soil ecology. I rely on a mix of oats, field pea and mustard — and will also add red amaranth to areas sown earlier in the season.

Other vegetable garden contrasting compost-topped bed with wood ash and beds with fall cover crop
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Chad Pilieri

Thanks for sharing, it’s great how you utilize all ‘waste’