What is Terra Preta?

Biochar is widely associated with Terra Preta, the famously fertile and persistent anthropogenic "black soils" of Amazonia. I had understood Terra Preta soils to be the result of deliberate "slash and char" practices by native Amazonians, as opposed to the much more destructive "slash and burn" agriculture introduced by European settlers, which leaves soils degraded and depleted after only a few growing seasons.

As reported in Charles Mann's book 1491, an extraordinary account of civilizations in the New World prior to contact (everything they taught us in school was wrong!), native peoples lacked the steel tools that would have been required to slash much of anything. He even recounts an experiment wherein rainforest locals were hired to chop down trees using the relatively primitive tools that were known to have existed prior to contact. Slashing your way into burning a forest plot for a brief agricultural fling, without steel tools, just doesn't fit into anyone's energy budget, sustainable or otherwise!

When we look at the agricultural practices of the contemporary descendents of Terra Preta's creators, a different story emerges. An episode of the BBC documentary series Around the World in 80 Gardens tells of Terra Preta soils made by smolder-burning rotting wood and then mixing with ashes, shards of unfired clay pottery, spoiled food, and other organic wastes; as demonstrated in this six-minute video clip:

For a more extensive recounting of the Terra Preta phenomenon, check out the BBC special: The Secret of El Dorado. This more complex view of Terra Preta as a fertilizer that was deliberated produced from char, minerals, and organic debris is consistent with lab analysis of Terra Preta soil particles, which appear to be accretions of organo-mineral nutrients around a char core.

When you think about the way plants acquire nutrients from the soil, and the long timescale of human cultural evolution, it just makes sense that, once the benefits of char as a soil amendment were observed, it would be embellished and improved upon by astute gardeners whose very livelihoods depended on successful experimentation and innovation. The question now is, can we likewise improve on basic biochar by learning a lesson from this ancient agricultural wisdom; only do it on a much grander scale through the judicious application of technology?