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TERRA PRETA DO INDIO

via Daily Kos

by brasilaaron
Thu Jun 17, 2010 at 11:20:12 AM PDT

Terra Preta do Indio means "Indian Black Earth" in Portuguese (in Brazil Indian is a term frequently used where in the US we would say Native American) and is a remnant of indigenous agricultural practices of the Amazon region. They are often also referred to as Amazonian Dark Earth (ADE). It is generally a highly fertile and dark soil that is prized by Amazonian farmers whereas most native upland soils of the Amazon are highly weathered, nutrient poor red or yellow clays and easily exhausted by even brief bouts of agriculture.

ADE are a somewhat magical combination of charred vegetable remains, bones and pottery shards. I say magical beause nobody has been able to figure EXACTLY what made them this way or how to reproduce them effectively.

I became fascinated by ADE while writing a term paper in grad school and pretty much everything that I am going to write about has been researched, presented and written about extensively in the excellent book called "Amazonian Dark Earths: Explorations in Space and Time" eds. Bruno Glaser and William Woods which covers pretty much every aspect of their discovery, description and research as does the excellent book "Amazonian dark earths: origin properties management" by Johannes Lehmann,C. Dirse Kern and Bruno Glaser. Most of my diary will draw from these books. There is also an excellent Terra Preta Nova (TPN means New Black Earths) research group at Cornell University headed by Dr. Johannes Lehmann and also a TPN research group at the Universidade Federal de Piracicaba in the state of Sao Paulo, Brasil.

This is an extremely condensed version of how ADEs formed:

Amazonian peoples, intentionally or not, combined charred wood, organic remains including bones and shells, and pottery shards in the soil. Over time areas where these products were combined changed from the normally highly oxic, highly leached, infertile red or yellow clays of the Amazon into very deep, dark, extremely fertile soils. The darkness of these soils reflects the extremely high levels of carbon (C) in the soil; some of this C is the charred material itself and much of the C is the organic material formed off of the charred substrate. It is not entirely clear to what extent each of the varying components is responsible for the altered characteristics of these soils, but what is clear is that these soils can remain fertile for hundreds of years or under continuous agricultural exploitation. Normally, Amazonian soils become depleted and unproductive after as short as 3 years of use due to extremely rapid oxidation (decomposition) of the C-based organic material. The TPN groups are trying to figure out how to reproduce the properties of these soils.

Carbon

Amazonian Dark Earth begins with carbon (C) and carbon is a curious thing. It is the building block from which all life is constructed. It forms complex bonds and structures that allow it to resist decay, to various degrees, and can form the substrate which can allow for soils to become fertile, which promote further growth and re-structuring of the C atom and its molecule derivatives. When trapped in high pressure, low-oxygen environments for millions of years, it can form petroleum, coal and natural gas. Its gaseous form in the atmosphere helps keep the planet warm. Lately, humans have developed technology that can allow for this life-giving atom to threaten our existence on the planet through poisoning (as we are witnessing in the Gulf of Mexico and Niger Delta and Ecuadorian Amazon basin etc etc etc) or potentially through massive climate change. So how can carbon help save us from ourselves?

Capturing Carbon

Anybody who knows gardening knows that if you throw composted matter or manure in the garden you can create dark soils that are teeming with fertility. The secret is partly the nutrients like N, P, K, Ca and others bound in the decayed organic matter, but also the decomposing C itself, which helps build all sorts of biotic communities in the soil. These biotic critters improve the tilth of the soil and increase water-use efficiency. These biotic communities also accumulate carbon in the soil, this is a form of sequestering carbon from the atmosphere into the soil. Anybody who has paid attention to the "global warming" debate knows that one of the catch-words for solving the problem is sequestration of the C in the atmosphere. If we can sequester C in the garden with compost, why not just compost everything in the world and sequester all the carbon and presto-whamo, global warming will be put on the skids? Well, as it turns out organic carbon will eventually decompose and release CO2 back into the atmosphere. So while adding organics to farming soil is undoubtedly a good thing, it is principally from a fertility aspect, and not a long-term C-sequestration aspect. World wide, agriculture is the largest source of C into the atmosphere, but it doesn't have to be that way.

Pyrolized C and Amazonian Dark Earths

When organic material is pyrolized, or charred, as in making charcoal, C forms a peculiar ring which is highly resistant to decay, unlike normal organic C. Carbon-dating has put some pieces of charcoal in ADE sites at 2,000 years old, so we know that pyrolized C can remain undecayed for at least 2,000 years. It could almost be considered "permanent" sequestration considering that the life-span of organic C in tropical soils is up to 10 years. Recent research on pyrolized C has yielded decidedly mixed reviews on this subject, which is why I stated earlier that it is unclear how each of the components of ADE affect the result.

But this is where ADE soils get interesting, it appears that aside from sequestering C in the soil simply by being C-based, charcoal may provide structure for biotic communities within the soil to proliferate in various ways. First, the surface area of the charcoal itself provides lots of nooks and crannies for bacteria and fungi to hang out. Second, and most important because this gets at why these soils become and stay so fertile, is that pyrolized-C acts as something of a "nutrient sponge", holding onto to essential plant nutrients like P, K, N, Ca, Mg and others for easy exchange. Otherwise these minerals would leach out of the bottom of the soil and be lost to the ecosystem, which is why Amazonian soils are so tragically nutrient poor. The soil biota feeds off of these easily exchangeable nutrients captured by the bits of charcoal and create an extremely fertile, C-rich soil environment. It appears that as long as there is abundant charcoal in the soil, there will be an opportunity to create these C-rich soils which could sequester C for long periods of time.

What to do about it

Many agronomists have been testing various methodologies for applying charred organic material to soils with varying amounts of success at fertility enhancement and sequestration potential. One way to reduce the C-input potential of agriculture would be to char organic residues on farms and apply them back to the farming soil. This would provide benefits such as increased soil aeration, increased water efficiency, increased fertility as well as C-sequestration. However, traditional methods of producing charcoal can produce massive quantities of methane and nitrous oxides, which have even greater greenhouse "forcing" effects. So, to prevent the creation of even more powerful greenhouse gases, a machine needs to be used that can capture the gases, which can then be burned for the pyrolizing process itself or used in other ways.

Conclusion

Agricultural practices world-wide are one of the largest driving forces of greenhouse gas emissions through burning (slash-and-burn ag. is often cited as a culprit) and through decomposition of soil-bound C. I remain strongly convinced that applying charred organic material to agricultural soils remains the single most effective mechanism that we have to combat climate change because it can be practiced on an extremely wide scale and pretty much every type of farmer can do it, ie., we don't need to go super high-tech to do it. An ancillary benefit will be enhanced soil fertility, which has the C-positive effect of reducing the demand for new, non-agricultural soils to be brought under the plow. Using char in your garden or farm does not automatically make the resulting food organic, but using char can go a long way towards making organic farming more practical, effective and cost-friendly. Plus, deep, black, soft soil is a beautiful thing to behold, and create.