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Chelsea Green Blog

How to Make Biochar

Doing some spring cleaning around your property? By making biochar from brush and other hard-to-compost organic material, you can improve soil—it enhances nutrient availability and also enables soil to retain nutrients longer. This excerpt from The New Farmer’s Almanac, Volume 3, explains how to get started.


To make biochar right in your garden, start by digging a trench in a bed. (Use a fork to loosen the soil in the bottom of the trench and you’ll get the added benefits of this “double-digging” technique.) Then pile brush into the trench and light it. You want to have a fire that starts out hot but is quickly slowed down by reducing the oxygen supply. The best way to tell what’s going on in a biochar fire is to watch the smoke. The white smoke, produced early on, is mostly water vapor. As the smoke turns yellow, resins and sugars in the material are being burned. When the smoke thins and turns grayish blue, dampen down the fire by covering it with about an inch of soil to reduce the air supply, and leave it to smolder. Then, after the organic matter has smoldered into charcoal chunks, use water to put out the fire. Another option would be to make charcoal from wood scraps in metal barrels.

Unrestrained open burning releases 95 percent or more of the carbon in the wood, weeds, or whatever else that goes up in smoke. However, low-temperature controlled burning to create biochar, called pyrolysis, retains much more carbon—about 50 percent—in the initial burning phase. Carbon release is cut even more when the biochar becomes part of the soil, where it may reduce the production of greenhouse gases, including methane and nitrous oxide. This charcoal releases its carbon ten to one hundred times more slowly than rotting organic matter. As long as it is done correctly, controlled charring of weeds, pruned limbs, and other hard-to-compost forms of organic matter, and then using the biochar as a soil or compost amendment, can result in a zero-emission carbon cycling system.

If each of one million farmers around the globe incorporated biochar into 160 acres of land, the amount of carbon locked away in the earth’s soil would increase fivefold.

Burning responsibly requires simple common sense. Check with your local fire department to make sure you have any necessary permits, wait as long as you must to get damp windless weather, and monitor the fire until it’s dead.


Image courtesy of the Oregon Department of Forestry on Flickr.


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