Biochar has been around for over 2500 years and only now have our soil scientists begun looking into it. The Terra Preta de Indio (or Indian Black Earth) is a Pre-Columbian dark earth mass re-discovered in the Brazilian Amazon region and several other countries in South America.[1] Soil scientists noticed the dark soil was fertile where identical soil adjacent to it was poor at best. Analysis showed the only difference was the dark soil contained charcoal. Yet these dark soils have remained fertile without additional amendments for hundreds of years. (Some other Terra Preta-like soils are currently being studied in Holland, Japan, South Africa and Indonesia, according to Allan Balliett in the Acres U.S.A. article Terra Preta, Magic Soil of the Lost Amazon.)
What biochar does in the soil is several things that increase soil health. Initial testing supports increased water retention, plant growth, soil stability and reductions of organic fertilizers. The porous texture of the biochar provides an environment for microbes to flourish. “Biomass from woody charcoal has an interior layer of bio-oil condensates that microbes consume, and is equal to glucose in its effect on microbial growth.” ~ Christoph Steiner, EACU 2004. Biochar also absorbs nutrients in the soil. What is special about biochar is its effectiveness at retaining most nutrients and making them available to plants much better than other organic materials such as compost or manures. This is also true for phosphorus (P) which is not at all retained by ‘normal’ soil organic matter.[2] "Soils with biochar additions are typically more fertile, produce more and better crops for a longer period of time." ~ Johannes Lehmann[2]
Making charcoal has been around for hundreds, maybe thousands of years, and the process has changed very little. Traditionally, wood was piled in a round stack having a central ‘flue’ and covered with a wet clay mud. The wood was set to a slow burn (Pyrolysis) in the absence of almost all oxygen other than small holes around the perimeter to control the burn. It could take up to 3 weeks and constant attention for a complete burn. Once cooled, the charcoal was bundled and sold by the colliers (charcoal makers) as a heating and cooking fuel.

Wood Mound to make charcoal, circa 1900
Modern wood mounds in Crete

Biochar is made active by thoroughly wetting the “char” in a liquid fertilizer like fish emulsion, soluble mineral fertilizer, compost tea, etc. and then drying before crushing. Leaving a small moisture content keeps the dust down when crushing (which can be done with a 2x4 in a heavy metal bucket, or a concave rock). It is suggested NOT to use manufactured charcoal briquettes (used for your barbeque grill) as they contain char plus other ingredients like starch, borax, paraffin and a hydrocarbon solvent in the ‘instant light’ briquettes . A quick biochar can be made from a commercially available hardwood lump charcoal by the name “Cowboy Charcoal” and available at places like Ace Hardware and Lowe’s. You can also easily and cheaply make your own charcoal.

Cowboy Hardwood Lump Charcoal
Charcoal Close-up

After crushing, screen the biochar with ¼” hardware cloth. Gravel sized pieces of biochar will work fine in the soil. Retain any larger chunks for further crushing. Biochar dust is better for the plants and soil but is also a concern for air quality. The finished biochar may then be broadcast and tilled in, or applied as a side dressing. The amount to use varies with your total soil condition: tilth, fertility, pH, etc. A rough estimate I saw mentioned was 1 pound per square foot although significantly less also produced substantial results. Biochar produces faster and more noticeable results on poor soils. Biochar is also very effective added to your compost pile. Add up 15-25% by volume and mix it in. Add it to your bokashi. The microbes will love it! After you add your biochar to the garden, it is important to test the pH. Biochar is slightly alkaline, and soil scientists don’t yet know how it affects pH in the soil when added with organic fertilizers and minerals. Some think biochar plus an organic fertilizer may produce a neutral pH. More research is needed.

The very significant other advantage of biochar is its ability to sequester carbon. We know oil and coal deposits long ago sequestered carbon in the earth… until we dug them up. Plant matter absorbs CO2 from the atmosphere while growing. The process of making biochar does release some of the plant’s CO2, using about +/- 30% of the biomass to carbonize the other +/- 60%. We can sequester that larger amount in our soils. If plant material is fully burned to ash or left alone to rot, that same biomass would all become carbon dioxide in the atmosphere, contributing to the greenhouse effect, or global warming. Think of all the scrap around of used pallets, lumber removed from houses due to termite infestation, corn cobs and stalks, peanut shells… even straw, dead leaves and dried weeds. They all can be used to make biochar, and sequester the carbon that otherwise would go into our atmosphere.

Biochar research is still in its infancy and Cornell University faculty member Johannes Lehmann is one of the foremost biochar researchers. Soil scientists around the world are actively researching biochar to combat global warming and to enrich soils to feed the world.

The Secret of El Dorado clip

Black Gold Agriculture

Biochar - agrichar - Terra Preta


2. Lehmann J 2007 Bio-energy in the black. Frontiers in Ecology and the Environment 5, pp. 381-387

Photo Credits:

B&W wood mound, Flominator, GNU Free Documentation License
Abandoned Charcoal Kiln near Walker, AZ, 6/21/07 by Benjamin Cody, GNU Free Documentation License
Terra_Preta photo, GNU Free Documentation License
Carbon footprint, #5536407, © Daniel Cooper, Used by Permission
Charcoal Pyres, #696422, © Paul Cowan, Used by Permission

Other photos are by the author.