What is Terra Preta?

Biochar is widely associated with Terra Preta, the famously fertile and persistent anthropogenic "black soils" of Amazonia. I had understood Terra Preta soils to be the result of deliberate "slash and char" practices by native Amazonians, as opposed to the much more destructive "slash and burn" agriculture introduced by European settlers, which leaves soils degraded and depleted after only a few growing seasons.

As reported in Charles Mann's book 1491, an extraordinary account of civilizations in the New World prior to contact (everything they taught us in school was wrong!), native peoples lacked the steel tools that would have been required to slash much of anything. He even recounts an experiment wherein rainforest locals were hired to chop down trees using the relatively primitive tools that were known to have existed prior to contact. Slashing your way into burning a forest plot for a brief agricultural fling, without steel tools, just doesn't fit into anyone's energy budget, sustainable or otherwise!

When we look at the agricultural practices of the contemporary descendents of Terra Preta's creators, a different story emerges. An episode of the BBC documentary series Around the World in 80 Gardens tells of Terra Preta soils made by smolder-burning rotting wood and then mixing with ashes, shards of unfired clay pottery, spoiled food, and other organic wastes; as demonstrated in this six-minute video clip:

For a more extensive recounting of the Terra Preta phenomenon, check out the BBC special: The Secret of El Dorado. This more complex view of Terra Preta as a fertilizer that was deliberated produced from char, minerals, and organic debris is consistent with lab analysis of Terra Preta soil particles, which appear to be accretions of organo-mineral nutrients around a char core.

When you think about the way plants acquire nutrients from the soil, and the long timescale of human cultural evolution, it just makes sense that, once the benefits of char as a soil amendment were observed, it would be embellished and improved upon by astute gardeners whose very livelihoods depended on successful experimentation and innovation. The question now is, can we likewise improve on basic biochar by learning a lesson from this ancient agricultural wisdom; only do it on a much grander scale through the judicious application of technology?

Biochar Guru

I had the pleasure of spending some time in the company of Stephen Joseph, co-founder of the International Biochar Initiative, and one of biochar's true pioneers. Stephen was visiting Costa Rica as a consultant to our project, a slight detour from his home in New South Wales, Australia en route to Boulder, CO to attend the North American Biochar Conference

Stephen is a scientific genius, brilliant engineer, and visionary. Drawing on clues from multiple disciplines, ranging from surveys of "fired biomass" agricultural traditions to analyses utilizing state-of-the-art laboratory research tools, he has deduced how Terra Preta soils were created (not simply "biochar", as widely believed) and postulated plausible mechanisms by which they derive their extraordinary fertility and persistence. Next, he reverse-engineered their structure and came up with a recipe for their synthesis. He then went on to perform field trials with this synthetic Terra Preta, and has demonstrated extraordinary plant response, exceeding both biochar and conventional fertilizer treatments. This is an important accomplishment, coming at a time when biochar field research shows generally promising but often inconclusive results, and the mechanisms of the biochar/soil/plant interaction are still subject to speculation.

While Stephen's work is recent and needs to be corroborated by further study, it has important implications for commercialization potential, carbon balance analyses, and future research directions. Following Stephen's lead (a hasty white board sketch and much fast talking and hand waving), we are building a reactor to create synthetic Terra Preta from biochar, soil minerals, and other ingredients. This synthetic Terra Preta will be evaluated in field and greenhouse studies by CATIE (tropical agriculture research institution) along with simple biochar and conventional fertilizer treatments .

Osa Biochar Project Kiln

The kiln built for the Osa Biochar Project was designed by Nikolaus Foidl, through association

with the International Biochar Initiative (IBI). It is a moderate size retort kiln (4M3 capacity) with a removable lid and basket for loading/unloading by means of an overhead hoist. It's a clever design, with a central chimney that passes through the retort from the wood-fired primary combustion chamber below. Once pyrolysis is underway, secondary air inlets promote combustion around the sides of the retort as well, for more even heating.

A unique feature of this kiln design is a water jacket for condensing pyrolignous acids (wood vinegar) and lower temperature pyrolysis gases. Water is circulated through the jacket while temperatures climb through the 150-280C range, and the condensate is routed to an exit spout with a dip tube immersed in water to further condense the smoke. These "smoke chemicals" have been demonstrated to stimulate seed germination and promote plant growth, and are now considered the main actors in fire-response vegetation such as chaparral. (Back in the day when I was studying this stuff, heat of the fire was held to be the trigger that stimulated germination and sprouting.)

Once temperatures in the load have reached 300C, the reaction became self-sustaining (no more need to stoke the firebox with wood), and the chimney outlet holds a strong flame. A few hours later, pyrolysis burns out, and the load needs to cool before exposing to air. The char is then crushed and screened. First production from the new kiln is being used in plant growth field trials conducted by CATIE, the Costa Rican agricultural research institution.

The kiln will eventually be set up at the site of the Sustainable Agricultural Center at La Palma, where diverse feedstocks can be processed into biochar for local farmers. It may be fitted with additional apparatus to utilize waste heat for crop drying and pre-drying on-deck kiln loads. A specific need that has already been identified is powering a drying kiln for timber bamboo grown by the local "Amigos de Bamboo" agricultural cooperative--a key step in promoting commercialization of locally cultured bamboo as an alternative to harvesting rainforest trees for construction.

How Dry, and Why?

Dryness matters, especially when you're working with wood waste, as we are. As discussed in a previous post, excessive moisture content (MC) in the feedstock hurts you on several fronts:

• It takes more time and fuel to reach pyrolysis temperatures;
• More fuel means more ash in the fire chamber that can interfere with thermal flows;
• In a retort kiln pyrolysis of the load will be uneven, compromising the quality and yield of the char;
• In a direct-burn kiln, the result is excessive polluting smoke and difficulty flaring-off waste gases.

A word about Moisture Content (MC) of wood and Relative Humidity (RH) of air. Both are percentages of water, which only serves to mislead, since they have about as much in common as apples and Frisbees. MC refers to the amount of water in the wood as a percentage of dry weight. Depending upon the wood species, freshly cut saturated wood will have an MC in the 30's. Relative Humidity is the percentage of the maximum potential water content of air at a given temperature. Here's the tricky bit: the maximum potential water content of air varies with temperature. A lot.

Under most terrestrial conditions, the equilibrium moisture content of wood (EMC) will be in the teens or drier. Wood waste that's allowed to air dry will approach the EMC as a function of surface/volume and how it's cut (end grain dries faster than other surfaces). Just age it a while, and you're good to go. But in the humid tropics, where temperatures are high and the relative humidity hangs in the 90's, typical EMCs will be in the 20's. Residents of the humid tropics are all too familiar with high EMC's; perfectly clean T-shirts get all moldy-smelling, not because they're dirty, but because the material is damp enough to support fungal growth (yuck!). EMC's in the 20's are high enough to complicate pyrolysis. Simple air drying is not dry enough.

The trick is to elevate the temperature and move air over the surface. Heating air from 30 to 60C takes the RH of the air from 100% to 25%. A given parcel of air that's been heated contains the same amount of water, but now its capacity to do the "work" of drying is much greater. Going from 30 to 60C is a piece of cake with a solar drying kiln. Making a drying kiln that's cheap and simple to use with unwieldy mounds of biomass is the design challenge. Stay tuned...