BBC’s Science Focus is digging in the dirt to get inside the mystery of “dark earth,” which seems to make the Amazon as fertile as it is… though no one knows how it got there:
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.
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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.
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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.
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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.”