Feature Articles 


Terra Preta

In the depths of the Amazonian basin, a specific type of soil is found that is known to be of human origin – but which modern science has so far been unable to reproduce. It seems to have been “primitive man’s” attempt to terraform the Amazon into fertility.

Philip Coppens

Since the latter half of the 20th century, two leading thoughts have come to the forefront of humanity: one is the possibility that we can destroy our planet – and whether our industrialised economy is killing the planet; the second is so-called “terraforming” other planets – making them inhabitable and suitable for human habitation. Both “techniques” transform an existing ecosystem and reside in opposite camps – destruction and creation.
Though topical, and for many perhaps theoretical, it is not a purely modern issue, an outcome of Man’s conquest of space, or the science fiction generations that have grown up in the 20th century. During that same century, it has become clear to science that people in the Amazon have created and used similar techniques – two millennia before Mankind went into space.

"Terra Preta de Indio" (Amazonian Dark Earths, earlier also called “Terra Preta do Indio” or Indian Black Earth) is the local name for certain dark earths in the Brazilian Amazon region. These dark earths occur, however, in several countries in South America (Brazil, Ecuador and Peru) and possibly beyond.
As ecologically rich as the rainforest may appear, the soil it stands in is unsuited to farming – largely a result of the incessant rain washing away all nutrients. But those pockets of soil that are Terra Preta, are suitable for farming and thus form an out of place patch of fertility in an otherwise harsh environment.
In fact, it has the ability to maintain nutrient levels over hundreds of years. According to Bruno Glaser, a chemist at the University of Bayreuth, "If you read the textbooks, it shouldn't be there." According to a study led by Dirse Kern of the Museu Goeldi in Belem, Terra Preta is "not associated with a particular parent soil type or environmental condition", suggesting it was not produced by natural processes.

Typical Terra Preta pockets in the Amazon are seldom larger than 2 acres, reaching down to a depth of approx. 50 centimetres, with traces going down to 2-3 metres deep. Terra Preta, in short, is like a small pocket of different soil, stretching over a small area of land, and not going to any depth. Still, when the various pockets are added up, about ten percent of the Amazon landmass is like this (though others argue only 0.3 percent of the basin is covered), a space roughly the size of France – or twice the UK.
As a rule, Terra Preta has more plant-available phosphorus, calcium, sulfur, and nitrogen than is common in the rain forest. The soil is specifically well-suited for “tropical fruits”. Corn, papaya, mango and many other foods grow at three times the rate than in the “normal” tropical soil. Fallows on the Amazonian Dark Earths can be as short as six months, whereas fallow periods on Oxisols are usually eight to ten years long. Only short fallows are presumed to be necessary for restoring fertility on the dark earths. However, precise information is not available, since farmers frequently fallow the land due to an overwhelming weed infestation and not due to declining soil fertility. In 2001, James B. Petersen reported that Amazonian Dark Earths in Açutuba had been under continuous cultivation without fertilization for over forty years.
What’s more: the soil behaves like a living organism; it is self-renewing. It acts more like a super-organism than an inert material. It is even more remarkable when it was discovered that it was most likely created by pre-Columbian Indians, between 500 BC and 1500 AD, and abandoned after the invasion of Europeans (other dating suggests 800 BC to 500 AD). Dating of the soil samples has shown that cultivation stopped in 1500, at the time of the Spanish Conquest. Francisco de Orellana, of the Spanish Conquistadors, reported that as he ventured along the Rio Negro, hunting a hidden city of gold, his expedition found a network of farms, villages and even huge walled cities. When later Spanish settlers arrived, none could find the people of whom the first Conquistadors had spoken. Had they been lured here with a lie? And if the farms did not exist, a “city of gold” seemed to have been an even bigger lie. Later, scientists were sceptical of Orellana’s account, as in their opinion, the Amazonian soil could not support such large farming communities. Of course, these scientists were speaking at a time when Terra Preta was not yet identified.
Wim Sombroek of the International Soil Reference and Information Centre in Wageningen, the Netherlands, has identified one of the biggest patches of Terra Preta near Santarem, where the zone is three miles long and half a mile wide. The plateau has never been carefully excavated, but observations by geographers Woods and Josephn McCann of the New School in New York City indicate that this area would have been able to support about 200,000 to 400,000 people. This makes Orellana's account at least plausible.

Despite some answers, other questions remain unanswered. This should not come as a major surprise, as the first description of Terra Preta soil was given in 1871, when Hartt called it “terra cotta”. Though he identified it, he – and others since – was unable to identify its origin. In 1928, Barbosa de Farias proposed that Terra Preta sites were naturally fertile sites. Camargo (in 1941) speculated that these soils might have formed on fall-out from volcanoes in the Andes, since they were only found on the highest spots in the landscape. Other theories included a formation as a result of sedimentation in Tertiary lakes or in recent ponds.
A natural explanation remained the best-liked flavour until the 1950s, when the camp became divided and more and more began to favour an anthropogenic origin. During the 1960s and 1970s, Terra Preta sites all over the Amazon basin were mapped and investigated with respect to the physical and chemical parameters of the soil, which supported the anthropogenic origin of the soil type. The fact that most of the sites are not too far from navigable waterways, where man would be expected to settle, added to this conviction.
However, was it a by-product of habitation or was it a clear example of terraforming, i.e. intentionally created for soil improvement? That question remained unanswered, though most now argue that people altered the soil with a transforming bacterial change.

How was it made? In the 1980s, it was thought that it was a kind of kitchen-midden, which acquired its specific fertility from dung, household garbage and the refuse of hunting and fishing. The soil was also full of ceramic remains, a clear sign of human intervention. The preferred conclusion was therefore that biological waste products had been gathered and then used as fertiliser, resulting in Terra Preta. However, how the humus gained its stability and special properties remained subject to speculation – could it really be true that this almost magical ability was an advantageous but totally accidental by-product?
At the end of the 1990s, investigations on molecular level showed that Terra Preta contained tremendous amounts of charring residues, which are known to contain high amounts of nutrients and to persist in the environment over centuries. This is a 20th century problem and the global carbon cycle has been brought to wide attention due to its importance for the global climate.
Soil organic carbon is an important pool of carbon in the global biogeochemical cycle (estimates placing it at just over 80%). It is normally considered to be a problem. And as Amazonian dark earths have high carbon contents that are five to eight times higher than the surrounding soil, Terra Preta could, in some theories, be considered as “bad” soil. And if we were to see this as contaminated earth, we should note that the horizons which are enriched in organic matter, are not only 10-20cm deep as in surrounding soils, but may be as deep as 1-2m. Therefore, the total carbon stored in these soils can be one order of magnitude higher than in adjacent soils. But rather than seeing this as the problem, it is actually the solution…

The Amazonian basin is not the only site where Terra Preta has been found. The terrain of the Bolivian Llanos de Mojos is savannah grassland with extreme seasons: floods in the wet, fires in the dry. Crops are hard to grow and few people live there. But back in the 1960s, archaeologist Bill Denevan noted that the landscape was crossed with unnaturally straight lines. Large areas were also covered with striped patterns.
Clark Erickson, a landscape archaeologist, was drawn to the numerous forest islands dotted across the savannah. Down on the ground he found them littered with prehistoric pot sherds, similar to the ceramics found in Terra Preta soil. Some mounds were as much as 18m high and much of the pottery was on a grand scale as well. Erickson and a colleague, William Balée, realised that the entire region must have been linked with agriculture, but they needed evidence for their conviction. They soon found that some of the mounds were still inhabited by indigenous people and that their language provided for words for staple crops like maize, as well as cotton and dye plants.
The straight lines turned out to be canals for irrigation, next to which were found causeways. These canals themselves are a masterwork of engineering; in the bottom of these canals, the ancient engineers had wedged diamond-shaped rocks, so that the canals would remain free from sediment. The water flow itself would clean the canals, thus not requiring a human agent.
As to the mounds, Erickson’s interpretation of the lie of the land is that the mounds were built to offer protection from floodwaters, with the most sacred buildings always at the centre of the mound on the highest level. There is historical evidence for this: a Spanish expedition of 1617 remarked on the extent and high quality of a network of raised causeways connecting villages together. The area is so vast that it could have sustained hundreds of thousands of people. Erickson believes that the Mojos Plains were home to a society which had totally mastered its environment.

But how did they do it? Orellana reported that the indigenous people used fire to clear their fields. We know that the Bolivian savannah has also been the “victim” of fire – though perhaps we should argue that it was “blessed” with fire. Bruno Glaser has found that Terra Preta is rich in charcoal, i.e. incompletely burnt wood. Terra Preta contains up to 64 times more of it than surrounding red earth. He believes that it acts to hold the nutrients in the soil and sustain its fertility from year to year. In experimental plots, adding a combination of charcoal and fertiliser into the rainforest soil boosted yields by 880% compared with fertiliser alone. As such, we have made one important step towards one of the great secrets of the early Amazonians: put the soil on fire, and it will regenerate. Of course, though science may have long forgotten about this technique, in the highlands of Mexico, these techniques can still be seen at night, when local farmers set parts of their field alight. But it is not as simple as that. A simple slash-and-burn technique does not produce enough charcoal to make Terra Preta. Instead, a "slash-and-char" technique must have been used. Named by Christopher Steiner of the University of Bayreuth, this technique does not burn organic matter to ash, but incompletely, whereby the charcoal was then stirred into the soil. Carbon is, as mentioned, a key ingredient in this process. When a tree dies or is cut down, the carbon stored in the trunks, branches and leaves is released; but when plants and trees are "only" reduced to charcoal, the carbon remains in the charcoal, apparently for periods up to 50,000 years, according to research by Makoto Ogawa. And this explains the high levels of carbon in Terra Preta.

Today, we know that the distribution of Terra Preta in the Amazon correlates with the places Orellana reported were centres of farmers. Today, as in the past, Terra Preta holds a great promise for the Amazonian population – as well as other areas of the world facing the same problem. Modern chemicals and techniques have failed to generate significant food from Amazonian soil in a sustainable way. Though some of the secrets of this soil have been discovered and will help in provide great help to many impoverished regions, some ingredients of Terra Preta remain unidentified – or at least difficult to reproduce. In fact, one missing ingredient is how the soil appears to reproduce. Science may not know the answer, but the Amazonian people themselves argue that as long as 20cm of the soil is left undisturbed, the bed will regenerate over a period of about twenty years. A combination of bacteria and fungi are believed to be the transformative agents, but the agents themselves remain elusive from the scientific microscopes.

The secrets of Terra Preta have been lost, both in the Amazon and on the Bolivian plains. The people who created it just disappeared. The communities Orellana saw, were gone some decades later. What became of them? Tragically, Orellana, his and other groups were responsible for their demise. The visitors brought diseases to which the Amerindians had little resistance: smallpox, influenza, measles, etc. So even though perhaps hundreds of thousands of people could survive in the New World for millennia by transforming the land they lived on, they had no protection against the new viruses that were brought in by the European “visitors”. And this is yet another example of the paradox of destruction and creation. But – literally – from their ashes, new knowledge and agricultural techniques are arising.