Biochar is organic matter that is burned slowly, with a restricted flow of oxygen, and then the fire is stopped when the material reaches the charcoal stage. Unlike tiny tidbits of ash, coarse lumps of charcoal are full of crevices and holes which help them serve as life rafts to soil micro-organisms. Biochar alone added to poor soil has little benefit to plants, but when used in combination with compost and organic fertilisers, it can dramatically improve plant growth while helping retain nutrients in the soil.
Amazonian Dark Earths
The idea of biochar comes from the Amazonian rain forests of Brazil, where a civilisation thrived for 2,000 years, from about 500 B.C. until Spanish and Portuguese explorers introduced devastating European diseases in the mid-1500s. Using only their hands, sticks and stone axes, Amazonian tribes grew cassava, corn, and numerous tree fruits in soil made rich with compost, mulch, and smoldered plant matter.
Scientists disagree on whether the soils were created on purpose, in order to grow more food, or if they were an accidental byproduct of the biochar and compost generated in day-to-day village life along the banks of the Earth’s biggest river. However they came to be, there is no doubt that Amazonian dark earths (often called terra preta) hold plant nutrients, including nitrogen, phosphorous, calcium and magnesium, much more efficiently than unimproved soil. Even after 500 years of tropical temperatures and rainfall that averages 80 inches a year, the dark earths remain remarkably fertile.
At Cornell University in Ithaca, N.Y., microbiologists have discovered bacteria in terra preta soils that are similar to strains that are active in hot compost piles. Overall, populations of fungi and bacteria are high in terra preta soils too, but the presence of abundant carbon makes the micro-organisms live and reproduce at a slowed pace. The result is a reduction in the turnover rate of organic matter in the soil, so composts and other soil-enriching forms of organic matter last longer.
How to Make Biochar
To make biochar right in your gardens, 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 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.
Unrestrained open burning releases 95% 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 10 to 100 times slower than rotting organic matter. As long as it is done correctly, this biochar process can result in a zero emission carbon cycling system.
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.