What we now know is that the Amazon did, indeed, support large communities of aboriginal peoples, just as Oreallana had said, which seem to have mysteriously vanished into the hunter-gatherer tribes seen by later European explorers. But we also know, from archaeological evidence, that they may (and probably did) survive into post-Columbian times, only to quickly disappear. And it was apparently the disease brought by Orellana and his men that caused the sudden decline, as European contact did in so many other places. When the cities were swept by disease, they were abandoned, and the jungle quickly swallowed up the sites, leaving nothing behind for explorers to find eighty years later. So, as we now know, the legend of the rich cities of the Amazon did have a basis in fact. And it was the terra preta, the black soil, which sustained that vast culture, that was the real gold of El Dorado.

***

Apparently, the indigenous farmers of the region had taken to carbonizing their farm waste, grinding the charcoal to a fine powder, and adding it to the soil. The richest soil samples, those with the greatest fertility, were between nine and forty percent charcoal by volume, and the charcoal was powdered to a fine powder – a few hundred microns was the average particle size. There are few bits of charcoal any larger than a quarter of an inch in size. The charcoal was produced in a low-temperature process, not heating it too excessively. It contained within its molecular structure plant resins that had been heat stabilized by the pyrolization process.

Because nobody had ever bothered to investigate powdered charcoal’s effects on soil fertility carefully, soil scientists had simply always assumed that charcoal when added to the soil, was inert and its effects primarily mechanical. Chemically, it is very stable at ambient temperature – even on geological time scales – and does not participate in chemical reactions, so it was simply assumed that any nutrients it trapped were simply unavailable to plants. Close investigation of the terra preta situation proved this to not be the case. Not at all.

What the soil scientists, working with microbiologists, discovered was that a community of bacteria exists in symbiosis with the root hairs of plants in terra preta soils. The bacteria produce enzymes that release the mineral ions trapped by the heat stabilized plant resins in the charcoal and make it available to the root hairs of the plant as nutrients. In return, the plants secrete nourishment for the bacteria. Not only that, but the resins within the charcoal act like an ion exchange resin, adsorbing traces of mineral ions onto the charcoal particle surfaces from the rain water, and trapping it within the charcoal’s molecular structure, where it can be held for centuries – until the soil bacteria associated with a root hair come along and secrete the enzymes necessary for it to be released once again. So the trace minerals always present in rainwater actually act as a fertilizer – providing the nutrients needed by the crops, year after year. The secret of the soil fertility of the terra preta was finally understood. And it was understood how the indigenous farmers were able to produce bumper crops year after year, decade after decade without a single application of chemical fertilizer and without wearing out the soil.

more