The Amazonian rainforest is one of the most lush, biodiverse places on the planet. But what helps it flourish so vividly might not be a gift of nature alone.
Since the 1880s, Western explorers surveying the forest have stumbled upon mysterious patches of black, nutrient-rich soil that’s different from everything else around it. These blotches in the ground, dubbed ‘dark earth’ because of their gloomy colour, are several times better than normal soil at both nourishing the forest’s plants and trapping polluting carbon deep into the planet’s floor.
The issue is we cannot figure out how this dirt got there. Most scientists think now-defunct, ancient Amazonian communities created dark earth on purpose to sustain their people. It’s a gift from the past. Others think long-forgotten environmental events played a crucial role instead.
As our carbon emissions skyrocket, and our soils degrade beyond repair, scientists today are racing to unearth this thousand-year-old mystery in the hope of saving the planet.
Special properties
Despite its flourishing greenery, the Amazon forest’s soil is surprisingly thin and sandy, pale and reddish with clay. It is no good for nurturing the tall, wide trees that spring out of the earth in that area. When big Amazonian storms rain down on the forest’s soils, their nutrients are leached and washed away with the gushing water.
But there are some small patches where the soil is darker, thicker, and richer than everywhere else. These patches – which are on average over 2,000 years old – are usually just a couple of hectares in size and a couple of metres in depth, and they seem to be scattered around the entire Amazon forest, mainly along the big rivers in the central and eastern regions.
Experts call this ‘terra preta’ in Portuguese, Amazonian dark earth in English, or even ‘black gold’, because scientific analyses show that this tar-coloured soil is one of the most nutrient-rich soils modern-day scientists have ever laid their eyes on. It is exceptionally rich in nitrogen, potassium, and phosphorus, and has a much higher pH value than its surrounding soils.
Trees grow six times taller in dark earth soil than in average Amazon dirt. And when researchers in Brazil added just 20-per-cent terra preta to nearby degraded soil where crops were struggling to grow, the crops they planted doubled in growth during that same time, according to a 2023 experiment. Scientists think this is astonishing.
“And below ground, you have the same effect. The roots are growing bigger,” says study author Anderson Santos de Freitas, a microbiology researcher at the University of São Paulo in Brazil. “So something in dark earth is making the plants become bigger, stronger, and with bigger roots as well.”
The super fertile soil also seems to lock up a lot of carbon. Studies show terra preta stores up to 150g of carbon per kg (2.4oz per lb) compared to 20–30g (0.71–1.1oz) in its surrounding soil.
In a 24,281km2 (6-million-acre) slice of the Amazon forest, scientists calculated that even though just 3–4 per cent of the soil is terra preta, it is likely storing upwards of 9 million metric tonnes (8.9 million imperial tonnes) of carbon.
And since mathematical models from 10 years ago predicted dark earth makes up more than 150,000km2 (37 million acres) of the forest, its impact on the world’s carbon sink is probably even larger than what’s been estimated.
Similar dark earths have also been discovered in North America, Australia, Asia, and Europe – in many cities where the Roman empire used to thrive, and ancient Slavic settlements in Northern Germany. There is a growing body of research on dark earth in Africa too, where they’ve been found around abandoned villages in Guinea, Chad, Cameroon, Malawi, Congo-Brazzaville, and Ethiopia.
But all of these soils have slightly different chemical compositions, and many are more recent in age than terra preta ‘do Indio’ (from the Amazon).
“The question is whether these marks, physical or chemical, will last for a long time,” says agronomist Dr Wenceslau Geraldes Teixeira from Universidade Estadual do Rio de Janeiro, in Brazil. Amazonian dark earth’s fertility has lasted “for centuries or millenniums in one the hardest environments to hold nutrients in the soil,” he says.
It is also more widespread, as it's found in places far and wide across the Amazon, and sometimes found in areas larger than anywhere else – which makes its enigma all the more enticing.
Mysterious origin
When the Europeans first spotted mysterious patches of dark earth in the Amazon in the 1880s, they speculated they must have been remnants of volcanic fallout, or dried-up ancient lakes and ponds.
But later chemical analyses show terra preta is made from a concoction of deposited decaying food scraps, manure, blood, urine, some charcoal and ashes from firewood, shards of pottery, and bits of animal bones and shells.
It’s like a pile of ancient compost that’s been maturing for thousands of years. So the majority of anthropologists and archaeologists now posit terra preta is human-made – that it’s the sediments of the everyday life of the millions of Indigenous people who inhabited the forests throughout history.
Most of the Amazon’s Indigenous communities have been wiped out by Western colonisation in the past 500 years, and they didn’t leave records of their agricultural practices – so it’s hard to confirm this theory and figure out whether terra preta was made by mistake, or on purpose.
That’s why Dr Morgan Schmidt, an archaeologist from the Universidade Federal de Santa Catarina in Brazil, has looked for clues in the lifestyle of the modern Kuikuro people of southeastern Amazonia, in central Brazil. Today, the Kuikuro grow fields of manioc – their staple crop – and peel it straight on the field, letting its scraps decompose back into the soil.
Then, other scraps of manioc are burned in big pyres around the village, making charcoal. The rest of their food scraps, including fish waste and animal bones, or their own faeces, aren’t just thrown in one single big pile, or buried in a single hole, says Schmidt.
“They have a habit of spreading it out,” he says, in small patches of compost called middens, all over the village, between houses, in their backyards, and even inside their homes. As a result, their soil has extremely high levels of nutrients and organic carbon, “even where they're not depositing their organic waste,” says Schmidt.
And the locals say they’re doing all this on purpose. “Charcoal and ash we sweep, gather it up and then throw it where we will plant, to turn into beautiful eegepe. There we can plant sweet potatoes,” says one of the local elders. “When you plant where there is no eegepe, the soil is weak. That is why we throw the ash, manioc peelings, and manioc pulp.”
Indeed, when Schmidt compared the composition of eegepe with that of dark earth from archaeological sites, the soil was very similar. Its positioning is similar too: in the archaeological sites of ancient villages that still remain, dark earth is spread out all over the place in small mounds in a radial pattern around the ancient village’s core – just like modern-day middens.
The Kuikuro’s methods are likely a rendition of the thousand-year-old tradition that first made dark earth, says Schmidt.
“It’s difficult to accept that it could be [created] by chance,” says Prof Lilian Rebellato, a professor of Amazonian archaeology at the Universidade Federal do Oeste do Pará in Brazil, who was not involved in Schmidt’s studies. She feels terra preta is a product of “human ingenuity and intelligence”. T
o confirm that dark earth is human-made, more surveys are needed in other Indigenous communities across the pan-Amazonian area where the ancient super fertile soil is found, says Rebellato.
But there are molecular bits and pieces found in ancient Amazonian dark earth that don’t make sense with this long-held belief that it’s all up to human inventiveness, says Prof Lucas Silva, a soil scientist at the University of Oregon in the US. In a 2021 study, his team found traces of a type of carbon from plants like maize or plants from open savannah territories – not the local wet forests.
But these plants had got there over 7,000 years before the rest of the elements of the dark earth. Then, they spotted more and more minerals that came from somewhere else geographically and could not have been made by humans.
For over nine years, his team double-checked the results, but couldn’t match the data with what the rest of the research was saying. “We’re trying to piece this together, but there are more questions than answers,” says Silva.
What likely happened, Silva thinks, is that some of the key nutrients necessary to make ancient dark earth were transported from gushing rivers and deposited along the banks, and the locals harnessed these elements.
This means the mystery of the formation of dark earth is far from resolved, says Silva, and digging up its origin story is still crucial for its future. “If you're going to try to recreate this,” says Silva, “we really need to understand how this was created to begin with.”
Replicating dark earth
Silva thinks that the mystery behind dark earth is why, to this day, nobody has been able to perfectly recreate it. One project called Terra Preta Nova launched in 2001 in Brazil, and “they did not get anywhere close to it, on the fertility, on the carbon,” says Silva. “It’s completely different in physical nature, chemical nature.”
Yet scientists are hopeful they’ll be able to pull off something similar sooner or later. Or at least learn some lessons from it that can be applied to the modern world – to improve agriculture, and to help combat the climate crisis.
Past projects have likely failed to copy the nutrient-rich soil because there are a lot of ingredients to its formula, says Santos de Freitas, who was behind the research that showed just 20 per cent of dark earth in soil can help double plant growth.
“You have biochar, you have microbes, you have nutrients, you have the position, you have a lot of small aspects that are hard to mimic,” says Santos de Freitas.
He thinks the terra preta secret lies, more than anything, in its microbes. In the lab, he’s isolated a set of bacteria crucial for its fertility, and he’s working to understand what each microbe does. His team’s results are still preliminary and unpublished, but they’ve already identified some new bacteria species responsible for fixing nitrogen, for instance, and others for dissolving nutrients.
“The final product is still a long way ahead, but it will be a formula, a mix of microbes, that we can throw in the soil and make the plant grow better,” says Santos de Freitas – and hopefully allow for it to store more carbon too.
Prof Johannes Lehmann, a soil biogeochemist at Cornell University, agrees. “We cannot of course create ancient dark earths, as they would not be ancient if we create them now,” says Lehmann. And terra preta contains materials that either we do not want in soil or we do not want to emulate today, he says. “Do we want to make pottery and smash it?” he says. “Not really.”
He’s been saying since the 1990s that terra preta’s uniqueness is due to its abundance of charcoal, and that’s what we should be learning from. Charcoal helps soil hold onto its nutrients when there is rainfall instead of the water diluting the terrain.
In soil with charcoal, for instance, “calcium is several-fold higher, and yet the leaching of calcium is several lower,” says Lehmann. “That is a very interesting feature to emulate.”
Burning organic matter to make charcoal releases polluting carbon dioxide in the air, so his team developed a charcoal-like substance called biochar. Biochar is made by warming up organic waste without oxygenating it, like in a high-temperature oven.
When he first spread biochar all over corn fields in Colombia those crops yielded up to 140 per cent more corn per acre than their biochar-less counterparts. “If we can figure out how to increase carbon contents in soils, then we have learned something,” says Lehmann.
Since then, biochar has begun to be used as a natural fertiliser for horticulture and agriculture and a possible climate mitigation strategy by hundreds of commercial companies around the globe, spearheaded by organisations like the International Biochar Initiative.
In the UK, scientists are burying 200 tonnes of biochar in fields to test whether it can successfully remove millions of tonnes of carbon from the atmosphere and help the nation meet its emission goals.
But there’s one big issue Lehmann still hasn’t managed to crack: it's still unclear how modern societies can create enough biochar today to fundamentally change our soils and transform them into dark earth enough to make a difference to our carbon-sinking abilities around the world.
Making heaps and heaps of dark earth shouldn’t be the goal, says Geraldes Teixeira, who is also one of the soil researchers leading the Terra Preta Nova project. Maybe dark earth wasn’t meant to be scaled up.
After all, it was created in small, tight-knit localised communities that practised agriculture in a fundamentally different way from our globalised, aggressive, industrial-grade endeavours, he says.
Most of the terra preta found at archaeological sites in the Amazon today was probably created by giant, vast quantities of organic material and ash and charcoal from burnings over long periods. “It’s not an easy task, and it’s an idea for small farmers,” says Teixeira.
And any terra preta-like soil development would owe its existence to Indigenous knowledge, and turning dark earth into a modern-day commodity has both ethical and practical implications. If we make new terra preta in a lab, is it still terra preta?
If terra preta is commercialised and profitable, who profits from it? If we cannot figure out where terra preta came from, who is credited for its inception? If its secrets are widely shared, how do we stop greedy corporations from destroying ancient archaeological sites and digging up the ancient fertile relics for large-scale economies?
Terra preta cannot work the world over, says Teixeira. Different environments have different types of soils, and we should focus on harnessing those unique characteristics and making small improvements to our soils locally.
If there’s one thing we truly learn from dark earth, says Teixeira, it should be that the systems we’ve put in place now just cannot be sustained in the long run: whether that’s how we grow crops, burn down our forests, or manage our waste. We need to take a step back, he says, and think about how we can restructure our systems.
“What we can learn is, okay, people could create a better soil with only the resources from the environment – and it is long-lasting,” says Teixeira. This should serve as an example that “we can do better.”
About our experts
Anderson Santos de Freitas has both a Bachelor's Degree in Biotechnology and a Master's in Biological Sciences earned at the Federal University of Pampa. He is working with Plant-Soil Feedback in degraded areas of the Central Amazon while earning his PhD at the Center for Nuclear Energy in Agriculture.
Wenceslau Geraldes Teixeira has been a researcher at the Brazilian Agricultural Research Corporation since 1995. He focuses on Agronomy, with a speciality in Physics, Management and Conservation of Soil and Water.
Morgan J. Schmidt is a geographer and archaeologist whose speciality is the Amazon, landscape archaeology, historical ecology, and anthropic soils like the Amazonian Dark soils. He earned his PhD from the Federal University of Santa Catarina.
Lilian Rebellato is a Professor of Archaeology at the Federal University of Western Para. She has published numerously varied and well-cited articles since 2003, including those on Amazonian dark earths.
Lucas Silva was an Associate Professor at the University of Orgean before becoming University of Oregon Philip H Knight Chair of Natural Sciences.
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