Learning about biochar

Learning about biochar

As a gardener biochar is something I’ve heard about and wanted to experiment with for many years. What is it – why am I interested?

There’s a lot of science and detail around biochar so I’ll keep it simple for now. Biochar is basically charcoal. It’s made by burning material (eg wood, coffee grounds, dry plant material) in a way that reduces the air flow. (Burning these in a regular way gives you ash which is still beneficial for the garden but different to biochar). This carbon product is then activated by wetting it with a beneficial liquid such as vermicast (worm wee). The activated biochar is added to compost to super charge the breakdown process or to the soil to benefit plant growth. How?

One thing to know about biochar is that it isn’t compost, it doesn’t contain beneficial minerals. What it provides to soil is structure to support micro-organisms and help retain minerals and water. This improves the overall health of the soil and therefore benefits plant growth.

make biochar at home

You can buy biochar but of course I want to make it at home without needing special equipment. An easy way I was shown is by using a milo tin in my log burner. This has worked well in my two trials – one using dry coffee grounds and one using small pieces of wood left over from wood chopping. I wet this down with vermicast and plan on putting it in one half of a garden bed so I can test to see if the plants in the half with biochar perform better.

burning biochar

Making biochar

To use a tin in the log burner bang or drill three holes in the end opposite the lid. Fill the tin 2/3 – ¾ with dry material and fit the lid firmly. Place the tin on a bed of coals with the holes on the side so you see them. Add a little more wood to keep the fire burning – it doesn’t need to be burning furiously. I’ve done this as we’ve let the fire burn down for the night but have stayed to watch it burning just in case. It takes about 30-40 minutes.

Liquid and gases will be released when burning. In the milo tin set up you might see this happen dramatically, it depends on what you’re burning. When the vapour stops being released it’s done. Leave it in the fireplace overnight to cool down.

The contents should be black rather than white. White is ash which is still beneficial to the garden but not biochar. If it has a greasy feel or smells it’s also not biochar. Remember to wear a mask when working with the dry product.

Something like coffee grounds burns down to a fairly fine powder. Other material will be chunkier and needs to be broken up by bashing it however you like. The biochar needs to be activated by adding liquid. Use vermicast (worm wee), urine, seaweed or compost teas. If added dry it can draw nutrients from the surrounding soil and it’s also not good to breath it in so wear a mask when handling and keep it moist.


As you can see, experimenting with biochar will be a learning process. Next up for me is making it outside – we’ll stop using the inside log burner soon and I have more material (the feed stock) to use than the little milo tin can handle! I’ve seen various kiln set ups online but I don’t want it to be too complicated to make or use. I’ll keep you updated.

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