“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 .