Terra Preta – The Extraordinary Potting Soil of the Ancient Amazonians

When you think about ancient technology, how often does potting soil come to mind?

Well, it should. 

The soil of the Amazon is generally poor, acidic, and leaches nutrients like a sieve. Growing food there is a nightmare—unless you happen upon, or go looking for, some of the intensely fertile, beautiful black gold put there by the Amazon’s ancient inhabitants. We call it Terra Preta (which means “black soil” in Portuguese.)

You’ll find Terra Preta near the remnants of ancient Amazonian cities and towns, a layer of rich, soil 12ft deep in places, covering between 2,400 and 150,000 square miles. It’s made from a fine mix of charcoal, bones, broken pottery, and compost. This soil is packed with nutrients and also acts as a “carbon sink”. That is, it absorbs and stores more carbon dioxide from the atmosphere than it releases. Plants use up the carbon in the soil, so having a soil that can produce its own carbon is huge. 

For a long time many scientists theorized that Terra Preta had formed through natural processes. After all, 10-20% of the volume of Terra Preta is broken pottery. So we’re talking 413,000,000 cubic yards of pottery, at the least. That’s enough broken pottery to cover Manhattan island in 17 feet of shards. It’s the volume of 4.5 Great Pyramids of Giza. And that’s using all the lower numbers—it could be a hundred times that amount. 

Oh yes, sounds totally natural.

But they discarded that theory because the whole mess is interspersed with human feces and other evidence of human occupation (besides the pottery, which…didn’t count?).

The next idea was that humans made Terra Preta accidentally, because, you know, people normally break thousands of square miles worth of pottery, mix it thoroughly with charcoal and animal bone fragments, and spread it wildly around—just “fer fun”. By accident. Without intending to enrich the soil at all.

You and I know that people don’t do that. But I have toddlers. Would toddlers do something like that? 

Given the right materials, the answer is yes.

But would they have the work ethic to spread around for hundreds of square miles?

Again, yes. (Cleaning a room for more than five minutes? No. Spreading dirt from location A to B for eons? Oh yes.) 

But as attractive as the idea of generations of busy toddlers building soil sounds, a recent study has concluded that Tera Preta was made on purpose by intelligent farmers who knew exactly what they were doing.

And now we’re racing to figure out how they did it, which is proving more difficult than we expected. Score one for ancient civilizations! 

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