The difficult art of copying

Findings from archaeology to sequester carbon and make cleared jungle soil fertile for longer

It has long been assumed that the writings of Francisco de Orellana, who described the Amazon lowlands along the early 1540s as richly populated with villages, were greatly exaggerated – after all, it was mainly hunters, gatherers and slash-and-burn farmers who later roamed the forest in these areas. It was clay shards that first aroused the interest of archaeologists and provided clues that vast areas of land were populated quite differently before the demographic catastrophe triggered by pathogens than in the centuries that followed.

But in contrast to the cities and sacral buildings made of stone in Mesoamerica and in the Inca Empire, the buildings of the South American lowland Indians made of organic material were already overgrown by the forest a few years after the collapse of the societies and left hardly any visible traces. It was not until a targeted search was carried out that, in addition to shards of clay, a soil was found that was clearly different from the standard yellow-brown soil in the Amazon lowlands – the black terra preta.

Terra Preta has some amazing properties: It not only ensures significantly higher yields when used for agricultural purposes, but also ensures that a plantation on cleared land does not have to be abandoned after five to fifteen years, but can exist permanently. Apparently, coarse-pored charcoal particles, which make up 10 to 40 percent of this soil, contribute significantly to this property. Through them, the soil can not only store water better, but also nutrients.

Even if clay shards and Terra Preta are found together and the idea of a technology that died out with the culture goes down well with European Indian romantics, it is just as possible that the Indians did not make the earth themselves, but chose their settlement sites according to the presence of Terra Preta. However, a natural origin of the soil would not be a fundamental obstacle to imitate the substance.

Reuben growth in comparison: Left in a mixture of artificial Terra Preta and potting soil, right in bare potting soil. Photo by Kelpie Wilson. License: CC-BY 2.0

Attempts to exploit the archaeological findings economically and "Terra Preta Nova" The results were very different, showing that the art of copying can be a difficult one and that it is not enough to simply mix charcoal into the ground. As early as the 1980s, initial experiments revealed that the original soil mixed with the charcoal and the plants grown on it play a decisive role.1 A comparative study near the Amazonian metropolis of Manaus later concluded that a combination of artificial manure and charcoal produces better results than a blob application of artificial manure, but that charcoal alone can even inhibit plant growth after two harvests.2 Because there are no long-term studies yet, critics fear that a premature use of inadequately replicated terra preta may not improve soil, but degrade it.

For some years now, however, the focus has no longer been on improving the quality of soil, but on another property of charcoal-treated soil: it prevents coarse amounts of carbon from combining to form carbon dioxide in a relatively cheap way, which is why people are thinking about using it through "climate farming" killing two birds with one stone. Whether that works also seems to depend on the details: An experiment conducted with coniferous forest soil, for example, concluded that the addition of charcoal led not only to the decomposition of humus but also to the release of carbon dioxide previously stored in the soil. 3 Researchers at Cornell University also note in a paper recently published in the journal Environmental Science & Technology that whether it is suitable as a long-term carbon store depends on factors such as the method of production, soils, climates and land development.

Charcoal is produced in smoldering fires where organic materials are heated but not adequately oxygenated. As it smolders, a large portion of the carbon in it is transformed not into carbon dioxide (as in burning or rotting), but into the porous solid known from barbecueing. In order for the production process to be as energy efficient as possible, the following is produced "Biochar"-Charcoal for soil enrichment is not produced in the traditional way, but in pyrolysis plants, which use the released energy for power generation or for district heating. Not only wood is used as a raw material, but also organic waste – for example from organic waste garbage cans or from livestock farming. Heating these materials under pressure produces not only charcoal and energy, but also fluffy and gaseous fuels that can be sold.

Eprida, a company based in the U.S. State of Georgia, passes the ammonia-containing pyrolysis gases over the hot charcoal, causing the formation of ammonium bicarbonate, a dung agent that is effective over a long period of time, on the charcoal’s surface. Other manufacturers are experimenting with the addition of Azospirillum bacteria, which also produce nitrogen. Genuine Terra Preta, on the other hand, contains a lot of fertilizing phosphate, which otherwise hardly occurs in the primeval forest soil. One theory is that it came from huge quantities of fish burrs that were dumped in the fields over long periods of time by the Indians who settled along river courses.

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