The Promise of Terra Preta
by Wade Collins

Bio-char, the product of low oxygen combustion of organic material, may prove invaluable in the fight to stabilize dangerously high global carbon dioxide levels, and in the remediation of agriculturally poor soil. Although relatively new, this technology relies on the two thousand year old agricultural ingenuity of pre-Columbian Amazonian farmers, and their unique culture of land management.

Until very recently, conventional wisdom assumed quite a number of things about the pre-Columbian indigenous societies of Amazonia. Restricted by the poor soil of the tropics (low fertility, low pH, and mineral deficient), human settlements, it was thought, could not achieve the type of urban, sedentary organization and development evidenced in other regions of the New World. The crops that sustained the Inca, the Maya, and the Aztec could not provide the same type of dietary surplus in the Amazon as they could in their native lands. This forced early Amazonian societies to rely more heavily on wild forest products which were hunted and gathered, and small scale, shifting agriculture—so called slash-and-burn agriculture. It turns out, however, that these assumptions, primarily based upon ethnographic data collected well after Western contact had decimated local indigenous populations with various waves of lethal epidemics and genocide, were quite mistaken. Amazonia, like the rest of the New World, was in fact heavily populated prior to European contact—its people living in large, networked communities, sustained by agricultural abundance.

It’s true that the soils of the tropics are not hospitable to most crops. The prominent biological richness and diversity of the above ground rainforest ecosystem stands in stark contrast to the dearth of soil fertility below. Land, once stripped of its forest cover, very rapidly degrades if placed into intensive agricultural production, forcing the extremely long fallow periods common to modern slash-and-burn agriculture. It is also true that the pre-Columbian agriculturists of the Amazon learned how to surmount the limiting nature of their environment, and had been successfully doing so at least 2,000 years before any European would set foot on their shores.

During the late 19th century, accounts began to surface in the reports of western “explorers,” naturalists, and scholars of an anomalous Amazonian soil type which they described as “terra preta,” or black earth. These soils were nothing like that which predominated over most of the landscape. They were rich in fertility and were characterized by “a fine dark loam, a foot, and often two feet thick.”¹ Furthermore, they were littered with pottery fragments “so abundant in some places that they almost covered the ground.” These early observers speculated that these soils were “artificial in nature,” but it wasn’t until the late 1960’s that researchers began to seriously address the issue of attribution.

Led by Wim Sombeck in 1966, a small group of scientists began to put forth the notion that these dark soils of the Amazon were indeed anthrosols—that is, areas of earth that have been modified by human occupation. Furthermore, these scientists believed that the soils were in some, or in all, instances, anthropogenic, or intentionally produced for agricultural purposes and not the by-product of some other settlement activity (such as trash disposal). This scholarship, which has accelerated most appreciably during the last two decades, has revolutionized perceptions of pre-Columbian Amazonian societies. But it has done more than that, for the composition of these soils is such that they are, thousands of years later, still fertile and still biologically stable, something that is quite remarkable in a tropical ecosystem where nutrients are routinely and quickly leached out of the soil. Indeed these cultural artifacts are often mined for potting soil and sold to nurseries and garden stores. The focus, then, has shifted. The question is no longer who created these soils, but rather how they did so.

The most striking feature of any terra preta is its color. It is dark—sometimes nearly black, sometimes gray, but in all cases, quite a contrast to the typical Amazonian soil, which is “yellowish-white to red.”² The color variation in terra preta soils results from the addition of pyrogenic carbon, or more simply, charcoal—which can be found to exceed that in “surrounding soils by a factor of 70.”³ Charcoal is formed when organic material is not burned completely. Typically this occurs when the combustion process proceeds at lower temperatures (350–500° C) and in a reduced oxygen environment. Recent ethnographic studies among the Kayapo Indians of central Brazil have identified a culture of “cool fires” which may in part explain how some of the pyrogenic material originated within historical terra preta.

“To live among the Kayapo is to live in a place where parts of the landscape smolder,” Dr. Susanna Hecht writes. “Fire is used...but not in the manner that typically characterizes recent colonist, or large-scale forest clearing.” These “soft or cool fires are more controllable, and they create incompletely burned residues—charcoal of varying sizes.”⁴

As it turns out, charcoal has some amazing properties that make it a terrific soil amendment. Johannes Lehmann, a soil scientist working at Cornell University, notes that “nutrients from plant and animal remains—like nitrogen, phosphate and potassium—bind to charcoal or biochar, drastically reducing how much is washed away by the...rain. Tiny pores in the charcoal, along with changes in its chemistry, provide more surfaces for nutrients to adhere to, which, in turn encourages microorganisms to colonize the soil...it’s like mopping up nutrients with a magnet that looks like a sponge.”⁵

Charcoal also has the effect of raising soil pH, a significant advantage for farmers working with the highly acidic soils of the tropics. But perhaps one of the most intriguing aspects of charcoal is its possible application in the fight against global climate change.

When organic matter naturally decomposes, the vast majority of the carbon stored in its biomass is released back into the atmosphere as CO2, or carbon dioxide. In fact, within 5–10 years less than 10–20% of that original carbon will have remained intact. If that same organic material is exposed to the low temperature, low oxygen environment necessary to produce charcoal, the results are quite different. Charcoal production preserves about 50% of available carbon, sequestering it within a highly stable form, which, like the pyrogenic carbon in terra preta, remains intact for literally thousands of years. Moreover, “during thermal degradation” (a process often referred to as pyrolysis), “a whole suite of different gases are emitted which can be captured and used as energy carriers. The energy products (created) include thermal energy, hydrogen, bio-oils, and electricity.”⁶ If those products are used to off-set future fossil fuel use, the carbon conserving benefits of charcoal, or bio-char, become even greater.

It is understandable then, that much excitement has been generated in the scientific community regarding the potential of bio-char to make a positive impact not only in the field of agriculture, where it has been demonstrated to improve the fertility and nutrient retention of degraded soils, but as one more tool in the effort to remediate global greenhouse emissions. If in the tropics, for example, a conversion could be made from slash-and-burn agriculture to a “slash-and-char” model, this “could serve as a significant carbon sink that could be an important step towards sustainability in tropical agriculture.”⁷ The benefits, however, are not limited to the tropics. One recent study by Robert Brown at the University of Iowa “revealed that if the U.S. adopted a cap and trade program in CO2 emissions like the one already in place in the EU, farmers in the Midwest could almost double their income by using corn stover—the leaves, stalks and cobs that remain after harvest—to fuel pyrolysis.”⁸

So, we may look forward to the day when modern technology finally catches up to the discoveries of an indigenous culture some 2,000 years in the past. Where agriculture is not solely focused on the production of food for growing populations, but on the fundamentally beneficial transformation of the environment in which it finds itself. And we may yet fashion a truly self-sustaining, self-perpetuating system of sustainability, a system where crops and waste, fuel and fertilizer are completely interchangeable and synonymous with one another. The ancient Amazonians did so. The soil of terra preta is just as alive now as it was 2,000 years ago. That we can say the same for the soil we put our hand to today—that is our challenge.

1. Woods, William.  Development of Anthrosol Research.  In, Lehmann, Johannes, et. al (2003). Amazonian Dark Earths: Origins, Properties, Management (p. 23). The Netherlands: Klumer Academic Publishers.

2. Lehmann, et al, Soil Fertility and Production Potential. In, Lehmann, Johannes, et. al (2003). Amazonian Dark Earths: Origins, Properties, Management (p. 105). The Netherlands: Klumer Academic Publishers.

3. Glaser, et al. (2002). Potential of Pyrolized Organic Matter in Soil Amelioration (p. 422) Beijing: 12th ISCO Conference.

4. Hecht, Susanna. Indigenous Soil Management and the Creation of Amazonian Dark Earths: Implications of Kayapo Practices. In, Lehmann, Johannes, et. al (2003). Amazonian Dark Earths: Origins, Properties, Management (p. 343). The Netherlands: Klumer Academic Publishers.

5. Tenneson, Michael (2007). Black Gold of the Amazon (p. 5). Discover Magazine, April 2007.

6. Lehmann, Johannes (2007). Biochar for mitigating Climate Change: Carbon Sequestration in the Black (p. 15). Forum Geookul, 18 (2), 2007.

7. Christoph Steiner (2006). Slash and Char as Alternative to Slash and Burn: – soil charcoal amendments maintain soil fertility and establish a carbon sink (p. 22, extended summary). Dissertation. Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany. 2006 (English translation from German).
   
   
8. Anne Casselman (2007). Special Report: Inspired by Ancient Amazonians, a Plan to Convert Trash into Environmental Treasure (p.2). Scientific American, web features, May 15, 2007.

Wade Collins
Seeds of Change Seed Cleaning Coordinator