Terra Preta“, or “dark earth”, is an Amazonian Indian technology which can vastly improve soil fertility and pull carbon dioxide out of the atmosphere, potentially keeping it out for thousands of years. Biomass – plant and animal waste such as manure, waste wood, and crop leftovers – can be turned into charcoal (or “biochar”) and then buried in agricultural soil, making rich black earth that plants grow very, very well in. Charcoal is extremely porous, and provides a perfect environment for beneficial soil microorganisms that help plants grow. It also holds water, and can greatly help crops to survive drought conditions.

Biochar can be used to pull carbon dioxide out of the atmosphere and potentially reverse global warming (if it were used on a wide enough scale). As we know from archaeological carbon-14 dating processes, charcoal lasts a long time; (terra preta was actually “discovered” by archaeologists trying to figure out how the Amazon, with its depleted tropical soil, could have supported the large urban population for which they were finding evidence). Plants pull CO2, carbon dioxide, out of the air to grow – this is where the vast majority of their mass comes from. If that mass is broken down naturally by the composting process, the carbon stays out of the atmosphere for 6-12 years on average. Char it, though, and it’ll be in the ground for hundreds, if not thousands, of years. (Some patches of Terra Preta soil in the Amazon are around 2000 years old.) What’s more, you can weigh exactly how much you’re sequestering – just put it on a scale. Theoretically, this could eventually work into carbon trading schemes.

Photosynthesis, at its most basic, is the process where plants use sunlight to take in carbon dioxide, use the carbon for cell building blocks, and give off oxygen. When it is charred, or burned without access to oxygen, the water vapor and various flammable gases, primarily methane and hydrogen, are driven off. The gases can be collected for cooking gas or electricity generation, or in lower tech production directed below the charcoal container to feed the burn. A company called Epidra is making char, hydrogen, and biodiesel simultaneously – see this article for a good overview on biochar and modern usage.

Charcoal appears to provide a medium which facilitates the transfer of minerals from soils to plants; therefore it is better used in conjunction with mineral fertilizers and compost. Essentially, it provides habitat for symbiotic organisms that help plant roots to grow, and acts as a buffer for soil moisture; it is not a fertilizer but a facilitator. Liquid fertilizers can be applied to the char before it is mixed into soil; an Indian researcher, Dr. Sai Bhaskar Reddy Nakka, has been experimenting with using clay pots full of charcoal as urinals in a boys’ school in India; when the pots start to smell, after 10 days or so, the saturated charcoal is removed and then mixed into potting, bringing a strong dose of easily accessible nitrogen to plants. Photos from the trial showed a much darker, healthier green leaves on the treated plants than on the control sample.

Completely untested personal theory, but I suspect that applying a liquid mycorrhizal rooting enhancer (like Mycogrow, for example) to charcoal would give very nice results as well.

For a home gardener, you could make your own charcoal or buy barbecue charcoal made from tree trimmings – i.e., the natural stuff, not the briquets. The easiest way to make charcoal, though it won’t be burned entirely through, is to place a metal garbage bin full of yard waste upside down on bonfire coals and leave it 24 hours, then pore water over it to make sure no coals survive. Crush the charcoal down to a powder and add it to your garden beds, potting soil, or compost bin.

Anyway, some links:

Watch this first (3 minutes):

Click on the image for a film section in English

Click on image for film section

Full movie (45 min) on youtube – very interesting film about ancient civilizations in the Amazon and tracking them down with archaeology

Treehugger.com’s overview. Lots of links in the comments section, too.

Biochar.org has a bit on simple charcoal making.

Terra Preta on Bioenergylists – big clearing house of information, lots of links on how to make charcoal. There’s also a different section on gasification, which aims at producing burnable gases from biomass primarily but gets char as a side product.

Biochar International – very good summary of what Terra Preta does on this page

Wikipedia’s Terra Preta page

Terra Preta – Science forums – nice active forums talking about terra preta

Note: Terra Preta has become a big interest of mine and something that people really need to know more about (especially my Dad, who’s the main person I’m writing this summary for since he works on environmental projects around Africa). Feel free to link to this post!

Update, Feb 12 2009: There’s now a book on biochar, “Biochar for Environmental Management”, available at http://www.earthscan.co.uk/?tabid=49381 .