There is so much environmental destruction wrought by the hands of human cultures that a ecologically minded person could easily despair. This makes the examples of the times humans did alright all the more precious: the chinampas of Mexico, the hedgerows of northwest Europe, and Amazonia’s terra preta.
Terra preta translates to black earth, and this is not a metaphor. In the photo below, the meter stick on the left is in a hole of classic yellow rainforest soil: depleted and fairly lifeless.
The meter stick on the right is in a hole dug into terra preta. There are three things that make this soil absolutely precious:
- It is rich and vibrant with nutrients, such that anything growing in it grows marvelously well.
- It is “anthropogenic”, meaning “humans made this happen”.
- It is alive to the point of self-regeneration.
Growing plants in terra preta
Farmers who plant in the terra preta soils know how lucky they are. These soils are rich in carbon, nitrogen, phosphorus, calcium, magnesium, and zinc, as well as the fungi and critters that make a soil function. Given the baseline of tropical soil nutrient cycles, these soils retaining their color and quality after 500 years of rain (80 inches a year!) is nothing short of a miracle.
Who made the terra preta soils?
The thing that seemed so quirky about terra preta sites when they first came to the attention of soil scientists was their spotty nature. Rather than appear in bands or layers or other familiar soil patterns, terra preta sites tend to be islands, unconnected to each other.
Though at least one of these “islands” is as large as 360 hectares (~900 acres), most are only 20ha (~50 ac). It was only after the scientists capitulated that these must be anthropogenic sites (the sheer volume of pottery shards in the soil was undeniable evidence of human culture) that the “spots” made sense: people gather around campfires, hearths, village plazas, and town squares. Further proof? The sites cluster along the river valleys.
But who? It seems there are a number of lost civilizations in the Amazon river basin. From 500 B.C.E. till the Spanish explorers arrived in the 1500′s C.E., these people lived along the riverways of the Amazon basin in Brazil, Bolivia, and likely in neighboring countries, too.
While most of the studied-by-Western-anthropologists tribes in Amazonia exhibit the social customs and leadership patterns common to hunting and gathering groups through out the world, a few who are currently living hunter-gatherer lifestyles have customs and leadership patterns typically found only in agrarian societies. Inherited leadership (nobility), for example.
It is the ancestors of these groups who are suspected to be the source of the remarkable terra preta soils.
How can soil self-regenerate?
So rich and vital are these soils that farmers have been known to sell the top layer of their soil. What?! What farmer does that?
Well, apparently these soils can increase themselves by as much as 1 centimeter per year. Take a 60 cm deep site, sell the top 10 cm, and in 10 years, you have 60cm of terra preta again. There are 2 crucial pieces to this:
- The larger pieces of carbon in the soil carbon remain, creating habitat for the soil critters, which leads to
- The soil critters remain (including the mycorrhizal fungi), and they go on about their live cycles, adding their poop while alive and their bodies once dead. This, in turn, feeds other microbes. The upshot: healthy soils are alive!
It’s important to note the relationship of form to function here: it is the honeycomb structure of the carbon debris that begets the ability for these soils to sustain life far far differently from any other soil in the region.
As for whether the soils are adding to themselves from above (getting taller) or from below (getting deeper), I don’t know. I’m also not sure if there is an absolute downward limit of their presence, a threshold that the rainwater soaking through the soils imposes on the environment.
That just means there’s more to learn. Like… could this be an alternative to slash and burn? Can we imitate now what those cultures were doing then? Can we do it faster? This is where the hubbub over biochar comes in.
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- Garden Soil Science, except for the bit about the pH
- The bit about pH
- South-South success for urban farms in Caracas, Venezuela