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.
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
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.
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.
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.
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…
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.
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.
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.
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.