Terra Preta (Amazonian Dark Earths)

node · earth · Amazon Basin, concentrated on bluffs along major rivers

Patches of deep, dark, exceptionally fertile soil scattered across the Amazon, generally on bluffs above major rivers, in a landscape whose natural soils are among the most weathered and nutrient-poor on Earth. They are not natural. They are the accumulated residue of long-term human settlement - hearth charcoal, bone, fish and turtle remains, food waste and broken pottery - built up over centuries and, critically, still fertile today. The charcoal is the reason: pyrogenic carbon resists decomposition for millennia and provides a structure that holds nutrients against the tropical rain that leaches them out of ordinary soil. Formation is generally placed between roughly two and a half thousand and five hundred years before the present and stops abruptly, which is what one would expect if the populations that made it collapsed at European contact. How much of the basin they cover is genuinely disputed, with published estimates spanning more than an order of magnitude, and the dispute matters because it is a proxy for how densely the pre-Columbian Amazon was populated.