Biochar gets talked about today like a brand-new climate technology. It is not. The oldest and most convincing proof that it works is more than two thousand years old, and it is sitting in the ground in the Amazon rainforest.
Scattered across the Amazon Basin are patches of dark, unusually fertile soil. The surrounding rainforest soil is acidic, heavily leached by rain, and low in the carbon that makes soil productive. But these dark patches — known as terra preta do índio, "the dark earth of the Indians" — are the opposite. They hold roughly double the organic carbon of the soil right next to them, and they have stayed fertile for two thousand years.
They did not happen by accident. People made them.
Soil that humans built on purpose
Indigenous Amazonians built terra preta mostly between 500 BCE and 950 CE. Today these dark soils are estimated to cover something on the order of 150,000 square kilometres — close to 3% of the entire Amazon basin. That is not a few garden plots. That is a region-sized achievement.
The method most researchers point to is "slash-and-char" rather than the more familiar "slash-and-burn." Instead of burning plant material all the way to ash, people cut it, lit it, and then smothered the fire before it fully combusted. Smothering it early starved the fire of oxygen and produced charcoal instead of ash.
That distinction is the whole story. Burning to ash sends the carbon up as CO₂ and leaves very little behind. Charring locks most of that carbon into a stable, porous black solid that stays in the soil. That solid is what we now call biochar.
Why the carbon is still there 2,000 years later
The reason terra preta has lasted is in the structure of the carbon itself. Charring biomass rearranges it into tough, ring-shaped molecules that soil microbes struggle to break down. Ordinary plant matter — leaves, compost, manure — gets eaten by microbes and returns to the air as CO₂ within months or a few years. Charred carbon does not. Conservative estimates say most of it is still in the soil after a century, and radiocarbon dating suggests some persists far longer than that.
Burning sends carbon into the sky. Charring buries it in the ground — and keeps it there for centuries.
That single property is why biochar sits at the centre of modern carbon-removal markets. It is one of the few ways to take carbon that plants pulled out of the air and park it in the soil long enough to count.
It was never just charcoal
Here is the part the modern hype usually skips. Terra preta was not made from charcoal alone. The same soils are full of pottery shards, fish and animal bones, and everyday organic refuse — the leftovers of people living in one place for generations. The charcoal held everything in place; the bones and food waste supplied the nutrients.
Researchers even find a lighter "sister" soil, terra mulata, ringing the darker patches — most likely the cultivated fields around the settlements. The picture that emerges is an integrated system: dense living areas at the core, intensive farming around the edges, and soil that got richer over time instead of poorer.
That is the real lesson of terra preta, and it is an honest one. Charcoal was the durable backbone of the system. But it earned its place as one ingredient in a larger recipe — not as a magic powder sprinkled on by itself.
What this has to do with West Africa
Now look at where biochar came from: hot, wet, acidic, heavily leached tropical soil that struggles to hold onto nutrients. That description fits much of the farmland across Nigeria and West Africa almost exactly. Many soils in Cross River State, for example, are acidic Ultisols sitting around pH 5.2 to 5.7 — the same kind of challenging ground where charcoal first proved its worth.
In other words, biochar's origin story is not a temperate-climate story or a laboratory story. It is a tropical-soil story. The conditions where it was first shown to build fertility two thousand years ago are the conditions millions of West African farmers work in today.
That is exactly why it matters here, and why we treat it the way the Amazonians did — as part of a system. At BIG, the carbon comes from agricultural residues that would otherwise be burned or dumped: palm kernel shells, cashew shells, and sawdust. The biochar those residues become goes back into local soil, paired with nutrients, and the carbon it locks away earns verified carbon revenue on top.
The history tells you biochar works. The next question is the practical one: how does it actually work, where does the evidence hold up, and where does it disappoint? That is what the companion piece, "How Biochar Actually Works," is about.