Posted on 05/07/2026 7:33:29 PM PDT by Red Badger
Dark earth, the strange patches of black soil rich in nutrients that cause plants to grow at accelerated rates, while also capturing unusually high amounts of carbon from the air, is one of the Amazon rainforest’s greatest mysteries.
Since these patches of dark earth were first discovered by European colonizers in the 1880s, debate has raged over their origins, with ideas ranging from the natural to the artificial.
Variants of this dark, nutrient-rich soil have been found in a range of locations around the world, and are most often associated with the accumulation of materials in soil after long periods of human settlement. In the Amazon, the dark earth variant is locally known as terra preta do índio, “black soil of the Indian,” and bears a characteristically dark coloration that many scientists attribute to the presence of charcoal.
Now, in the face of soil degradation and a carbon-fueled climate crisis, researchers are looking to this ancient soil for solutions to modern problems.
The Effects of Dark Earth
It may come as a surprise that the majority of the soil in the Amazon rainforest is of relatively low quality. Thin and sandy, much of the Amazonian soil allows heavy rains to remove most of its nutrients. However, in the small patches of area known as “dark earth”—usually located near rivers and comprising just a few acres, with estimated ages of more than two millennia—it is vastly more fertile. As one of the world’s richest soils, dark earth holds a relatively high pH value and contains large amounts of nitrogen, potassium, and phosphorus.
Incredibly, trees growing in these dark earth patches are six times the size of those grown in other Amazonian soils, with even their root systems growing far larger than normal. In one experiment conducted in Brazil, University of São Paulo researcher Dr. Anderson Santos de Freitas found that adding just 20% of this dark earth to existing soil doubled crop growth in a struggling region.
Dark earth can also store more than five times the carbon of surrounding soil, accounting for 9 million metric tons, despite making up only 3 to 4 percent of Amazonian soil.
Since its initial discovery in the Amazon, soil reminiscent of dark earth has been identified in other locations globally, on every continent except Antarctica. Yet these forms of dark earth are dated to much more recent periods and have somewhat different chemical makeups.
Indigenous Practices and Dark Earth
The earliest speculations regarding the soil’s origins included volcanic fallout or perhaps the final remains of ancient bodies of water. As the use of scientific instrumentation advanced, analysis of dark earth revealed that it was more similar to compost, consisting of decomposed organic matter such as food waste, manure, and ash, likely representing the accumulated remnants of indigenous life in the region.
However, since most of the region’s indigenous communities have disappeared over the last half millennium due to colonization, determining much about the soil’s origins has remained a challenge.
Of those few groups remaining, the agricultural practices of the Kuikuro people of central Brazil caught the attention of archaeologist Dr. Morgan Schmidt. The Kuikuro people have several practices that could account for dark earth, including peeling their staple crop, manioc, and leaving the peel in the field, burning manioc to make charcoal, and spreading their decaying food scraps around the village.
All of this results in nutrient-rich, carbon-sequestering soil around the village, which the Kuiluro say is intentional. Intriguingly, the composition and positioning of the soil are very reminiscent of dark earth; however, additional future surveys will be required to confirm that dark earth is indeed human-made.
A Distant Origin
However, other research challenges the assertion that dark earth originates entirely from an ancient human presence in areas where it is found today. University of Oregon soil research Professor Lucas Silva conducted a 2021 study that found traces of plant material in the soil that were not native to rainforests but instead came from savannahs.
As his team continued to study dark earth, they discovered something novel: minerals that were not indigenous to the region. Based on this, Professor Silva suspects that rivers transported nutrients from further distant lands, allowing local people to harvest from the nutrient-rich riverbanks and use them in their agriculture.
According to Professor Silva, no one has been able to recreate the dark earth because they have not solved the mystery of its origin. Among the issues with recreating dark earth is that small elements, such as microbial content, play a significant role in the soil’s unique properties.
Dr. Santos de Freitas, who conducted the Brazilian experiment in which 20% of the mysterious soil was placed in a struggling area, has been working in his lab to understand and recreate the delicate microbial balance found in dark earth, each with its own essential role. The goal of that work is to eventually recreate a similar substance in the lab.
“I’m currently enrolled in a postdoc position aiming to isolate the microbial effect of (dark earth) ADE and test different approaches, Dr. Santos de Freitas told The Debrief. “One of them includes biochar to mimic part of the ADE’s chemical composition, but the study is in its beginning.”
While these attempts at reverse-engineering dark earth have yet to fully recreate the variety found in the Amazon, Dr. Santos de Freitas says the current work is a meaningful step in that direction.
“[I’m] not sure if we can call it dark earth, but considering we’ll have an ADE-like product in the future, we’ll have a broad-spectrum fertilizer with both chemical and microbial characteristics that improve plant growth,” Dr. Santos de Freitas told The Debrief, adding that “it could be used for either trees in ecological restoration or crops.”
“Our target is driven by sustainable development,” he said.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.
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Garden Ping!.......................
Older than Dirt Ping!..............
GFY
I called it. When I read the headline I thought that with all the rain washing out the nutrients from the thin soil it must be getting caught in lowland areas. I was thinking localized pits rather than near rivers.
It is human waste. They found where a population took a dump or emptied out the house pots.
I read years ago this came from low pyrolysis of wood, basically like making charcoal but not as carbonized.
This creates an amphipathic (water loving/lipid loving) sponge that holds on to both water and nutrients but doesn’t quickly decompose.
There was a YouTube video about it.
The rest of the terra preta keyword, sorted:
Ah, the good Earth, this time for sure. [blush]
They need to analyze it, recreate it and sell it by the boxcar load. It could replace all the petroleum based fertilizers in the earth..
My nearly Wiccan friend swears by biochar.
Near rivers and “contains large amounts of nitrogen, potassium, and phosphorus.…”
I bet the same could be said about the soil below heron rookeries.
In North America there used to be huge amounts of guano fertilizer under miles-long groves of passenger pigeon nesting trees.
But when that was used up the passenger pigeons were extinct so it hadn’t been replaced.
In these villages there would be lots of organic material brought in from open areas to thatch the big communal house and all the other buildings every so many years. Plus human waste, peeled bark, cordage making, food processing, etc.
What is biochar, what does it do for plants, and is it related to “dark earth” in the Amazon?
ChatGPT:
Biochar is a stable, carbon-rich material made by heating organic matter (wood chips, crop waste, manure, nutshells, etc.) in a low-oxygen environment. That process is called pyrolysis. Instead of the material fully burning into ash and CO₂, much of its carbon becomes a porous charcoal-like structure.
It’s used mainly as a soil amendment and for carbon sequestration.
What biochar does for plants
Biochar itself is not really a fertilizer. Its main value is in how it changes soil structure and chemistry.
Common benefits include:
Improved water retention
Its porous structure acts like a sponge, helping sandy soils hold moisture longer.
Better nutrient retention
Biochar can hold onto nutrients such as nitrogen, potassium, calcium, and magnesium, reducing leaching.
Habitat for soil microbes
The pores create protected surfaces where beneficial bacteria and fungi can live.
Reduced soil acidity in some cases
Many biochars are somewhat alkaline and can raise pH slightly.
Improved soil aeration and structure
Especially useful in compacted or degraded soils.
Long-term carbon storage
Because biochar decomposes very slowly, it can lock carbon in soils for hundreds to thousands of years.
Results vary a lot depending on:
the feedstock used,
pyrolysis temperature,
soil type,
climate,
and whether the biochar is “charged” first.
“Charging” biochar matters
Fresh biochar can initially absorb nutrients from the soil, temporarily reducing plant availability. Many growers therefore “charge” or inoculate it before use by soaking or composting it with:
compost,
manure,
worm castings,
compost tea,
or liquid fertilizers.
That loads the pores with nutrients and microbes before application.
Is biochar related to Amazonian “dark earth”?
Yes — very closely.
The famous Amazonian dark soils are called Terra preta (“black earth” in Portuguese). These are unusually fertile soils created by Indigenous peoples in the Amazon over centuries or millennia.
Researchers discovered that these soils contain:
large amounts of charcoal-like carbon,
pottery fragments,
organic waste,
composted materials,
and rich microbial communities.
The charcoal component in Terra preta behaves much like modern biochar.
Why Terra preta is important
Most Amazon rainforest soils are naturally:
acidic,
heavily weathered,
and low in nutrients.
Yet Terra preta areas remain highly fertile even after centuries. That persistence led scientists to study the charcoal fraction as a model for modern biochar systems.
So modern biochar research was strongly inspired by Terra preta, though they are not identical:
Terra preta formed through long-term human habitation and waste management.
Modern biochar is usually industrially or intentionally produced and added to soil in a more controlled way.
Important caveat
Biochar is not universally beneficial. In some soils or at high application rates, it can:
immobilize nutrients,
alter pH too much,
or reduce growth temporarily.
It tends to work best when combined with organic matter and integrated into broader soil management practices rather than used alone.
In practice, many gardeners and farmers use relatively modest amounts mixed with compost.
Diana; I haven’t read this, but you might be interested! (Perhaps the remains of “middens” of giant sloths exterminated by the gentle “first peoples” ! Who knows!)
It is possible that it might be forgotten knowledge on how to enrich the earth. The Aztecs and Mayans were quite the agriculturalists.
I had a model of the Seaview!.....................
Photo....
Dog: Can’t I get little privacy? Now I’m on the highly popular Free Republic. Sheesh. If they all had indoor toilets they’d be safe from video cameras but not all of them do.
I didn’t.
What a show. The popular culture books say the producer had to give up being serious in order to appeal to youngest viewers with a “monster of the week.”
Funniest putdown was from Time Magazine for the full length movie pilot. “It devotes fully 30% of the film to views of Barbara Eden running in a sausage tight skirt and high heels.”
He said that like it was a bad thing.
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