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.