Terra preta is a black soil found in patches across the Amazon. It was
made by people, before Europeans arrived, and it is still fertile today.
The soil around it is poor. That contrast is why it matters.
The name is Portuguese and means “black earth”. Farmers call it
terra preta de índio. Soil scientists call it an anthroposol,
which just means a soil made by humans.
Where it comes from
Terra preta is an Amazonian soil. The patches follow the rivers, from
eastern Amazonia to the middle of the basin. They sit on high ground above
the floodplain, close to the water but out of reach of floods. That is
where people lived, which is the whole explanation for the map.
It is not only in Brazil. Sites have been recorded in Colombia, Ecuador,
Peru and Guyana. Similar man-made dark soils exist in West Africa, in
Benin and Liberia, and in southern Africa.
A patch covers anywhere from 20 to 350 hectares. The black layer can be
two metres deep.
How it was found
In 1541, Francisco de Orellana travelled down the Amazon and reported
large populations along the banks, with real agriculture. Nobody believed
him for a long time, because later travellers found nothing like that.
The explanation came much later: European diseases had spread ahead of the
colonists and killed most of those people.
In 1870, James Orton described Amazonian soil that was “black and very
fertile”. In 1966, a Dutch soil scientist called Wim Sombroek published
the first serious study of it. His work is where the modern interest in
biochar starts.
In the 1990s, Michael Heckenberger and others found large pre-Columbian
settlements along the Rio Negro. The fertile patches turned out to be the
remains of towns.
Why the surrounding soil is poor
Most of the Amazon sits on ferralsols. These are very old soils, and warm
rain has been washing through them for a very long time. Anything soluble
left long ago. What is left is acidic and holds almost no nutrients. Add
fertiliser and most of it drains straight past the roots.
The forest still grows because it does not depend on the soil. Leaves
fall, rot within weeks, and the nutrients are taken straight back up by
shallow roots and fungi. The fertility is in the plants, not in the
ground. Cut the forest and that loop stops. A cleared plot gives two or
three harvests, then needs eight to ten years of fallow.
On terra preta, that fallow gets much shorter. Some reports put it at a
few months.
What is in it
Terra preta is household waste, piled up over centuries and mixed into
the ground. When you analyse it you find:
-
Charcoal. Up to 9% of the soil is black carbon, against
about 0.5% in the soil next to it. It came from cooking fires and burnt
rubbish, so it burned slowly and at low temperature. -
Organic matter, about three times what the neighbouring
soil holds. -
Bones, mostly fish bones and turtle shell. This is why
phosphorus runs at 200 to 400 mg/kg when the surrounding soil has
almost none. -
Ash, which brings potassium, calcium and magnesium, and
raises the pH. -
Broken pottery. Sometimes there is so much of it that
surveyors used it to find the patches. It also lets archaeologists date
the soil.
When it was made
Most terra preta was formed between 800 BCE and 500 CE. The oldest sites
go back to around 2800 BCE. It stops at European contact, when the
societies that made it collapsed.
So the soil has been sitting there for five hundred years with nobody
looking after it, under heavy tropical rain, and it is still fertile.
Terra preta and terra mulata
There are two soils, not one.
Terra preta proper is the very dark soil found where people actually
lived. It is full of pottery and bones. You do not need to assume any
intention here: live in one place for centuries and throw your waste in
the same spot, and this is what you get.
Terra mulata is lighter brown. It has charcoal in it, but much less
pottery and bone. It forms a ring around the dark patches and covers a
larger area. It looks like farmland rather than a village: ground that was
improved on purpose with charcoal carried out from the settlement. If that
is right, part of this was deliberate soil management.
Why it lasted
Normal organic matter added to tropical soil disappears within a few
years. The charcoal is what makes the difference. Burning without much air
turns the carbon in wood into flat, linked rings, and soil microbes have
no enzyme that breaks those down easily. Some of it does decompose. The
rest stays for centuries.
But charcoal on its own would only give you black, empty soil. What makes
it fertile is what happened to it afterwards. Its surfaces oxidised over
time and became able to hold nutrients instead of letting them wash
through. It bound to clay and organic matter. Its pores hold water. The pH
and the nutrient levels stayed high, and the nutrients stayed put.
So the charcoal works more like a container than a fertiliser. The ash,
bones and food waste filled it.
Worms and microbes
The earthworm Pontoscolex corethrurus seems to have done a lot of
the work. It eats charcoal fragments and carries them down into the
mineral soil, which mixes the profile without anyone digging.
The microbes are less clear. Terra preta holds a different microbial
community from the soil around it, and the fungus Aspergillus niger
has been suggested as an important one. No single organism has been shown
to be responsible.
What it produces
Crops grown on terra preta yield two to three times what the poor soil
next to it gives. Trials using terra-preta-style amendments have reported
biomass increases of up to 280%, and yield gains of 20 to 50% in several
countries.
Some plots have been farmed for more than forty years with no fertiliser
at all. Local farmers also say the black earth grows back after it is dug
out and sold. That has been reported often enough to take seriously, but
nobody has explained how it works.
What it means for biochar
Terra preta is the reason people started putting charcoal in soil again.
It shows three things that no short trial can show: charcoal in soil can
last a very long time, it can improve fertility and not just store carbon,
and it keeps working with no maintenance.
There are limits to what it proves, though.
Terra preta is not charcoal. It is charcoal plus bones, ash, manure, food
waste and pottery, added over centuries, mixed in by worms, then left for
five hundred years. Spreading char on a field copies one ingredient out of
several.
The comparison is also flattering. Those yield multiples are measured
against some of the poorest soil on the planet. On good farmland there is
much less to fix, and the measured gains are far smaller.
And it took centuries. Nobody was in a hurry. A project that has to
deliver results this year is doing something different.
What is still unknown
- How much of it was deliberate, and how much was just waste piling up.
- How much of it exists. Estimates vary by a factor of ten, because the patches are under forest and surveys have followed the rivers.
- Whether it really regenerates, and if so, how.
- How to make it faster. Nobody has managed to reproduce mature terra preta from scratch.
Why it is worth knowing about
Before terra preta, the idea that you could put carbon in the ground and
have it stay there for a thousand years was a model. Terra preta is the
experiment, already done, by people who were not running an experiment.
The results are still in the ground and anyone can go and take a sample.
They were not trying to store carbon. They were living somewhere and
dealing with their rubbish. We can still measure it two thousand years
later.