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Terra preta: Unlocking ancient agricultural goldmine

Many soil scientists insist an ancient Amerindian agrarian society will soon make a huge impact on the modern world. They say once the intricacies and formulation of the society's “terra preta” (dark earth) is unlocked, the benefits will help stop environmental degradation and bring fertility to depleted soils. Developing and developed nations will benefit.


The story goes that in 1542, while exploring the Amazon Basin near Ecuador in search of El Dorado, Spanish conquistador Francisco de Orellana began checking the area around one of the Amazon's largest rivers, the Rio Negro. While he never found the legendary City of Gold, upon his return to Spain, Orellana reported the jungle area held an ancient civilization — a farming people, many villages and even massive, walled cities.

Later explorers and missionaries were unable to confirm Orellana's reports. They said the cities weren't there and only hunter-gatherer tribes roamed the jungles. Orellana's claims were dismissed as myth.

Scientists who later considered Orellana's claims agreed with the negative assessments. The key problem, they said, was large societies need much food, something Amazonia's poor soils are simply incapable of producing. And without agriculture, large groups of people are unable to escape a nomadic existence, much less build cities.

Dark earth

More recently, though, Orellana's supposed myths have evolved into distinct possibilities. The key part of the puzzle has to do with terra preta.

It turns out that vast patches of the mysterious, richly fertile, man-made soil can be found throughout Amazonia. Through plot work, researchers claim terra preta can increase yields 350 percent over adjacent, nutrient-leached soils.

Many well-respected researchers now say terra preta, most of it still hidden under jungle canopy, could have sustained large, agronomic societies throughout Brazil and neighboring countries.

Amazing properties

The properties of terra preta are amazing. Even thousands of years after creation, the soil remains fertile without need for any added fertilizer. For those living in Amazonia, terra preta is increasingly sought out as a commodity. Truckloads of the dark earth are often carted off and sold like potting soil.

Chock-full of charcoal, the soil is often several meters deep. It holds nutrients extremely well and seems to contain a microbial mix especially suited to agriculture.

Thus far, despite great effort, scientists have been unable to duplicate production of the soil. If researchers can ever uncover the Amerindians' terra preta cocktail recipe, it will help stop the environmentally devastating practice of slash-and-burn agriculture in the Amazon jungle. Terra preta's benefits will also be exported across the globe.

However, even without unlocking all of the soil's secrets, things learned in the study of it are already being brought to row-crop fields.

Among researchers studying terra preta is Johannes Lehmann, a soil fertility management expert and soil biogeochemistry professor at Cornell University. Lehmann, who recently spoke with Delta Farm Press, says things learned from terra preta will help farmers with agricultural run-off, sustained fertility and input costs. Among his comments:

On how Lehmann came to terra preta research…

“I spent three years living and working in degraded Amazonia field sites. Inevitably, if you work in the central Amazon, you come across terra preta.

“The visual impact of these soils is amazing. Usually, the soils there are yellow-whitish colored with very little humus. But the terra preta is often 1 or 2 meters deep with rich, dark color. It's unmistakable. We know terra preta are preferentially cropped.”

On the various properties of terra preta and its modes of action…

“There are a few factors that contribute to this fertility — sustainable fertility. Remember, these are soils that were created 1,000 to 5,000 years ago and were abandoned hundreds or thousands of years ago. Yet, over all those hundreds of years, the soils retain their high fertility in an environment with high decomposition, humidity and temperatures. In this environment, according to text books, this soil shouldn't exist.

“That alone is fascinating for us.

“Among the most important properties are high nutrient concentrations (especially for calcium and phosphorus). Most likely, this is linked to a unique utilization of agricultural and fishery waste products.

“We believe that fish residues are an important portion of the high phosphorus concentrations. Phosphorus is really the number one limiting nutrient in the central Amazon.

“Another interesting aspect of terra preta's high fertility is the char (charcoal) content of the soil. This was deliberately put into the soil by the Indians and doesn't only create a higher organic matter — and therefore higher fertility through better nutrient-retention capacity — but this special type of carbon is more efficient in creating these properties.

“You can have the same amount of carbon in terra preta and adjacent soils and the infertile soil won't change. Terra preta's abilities don't just rely on more carbon, but the fact that its char and humus is somehow more efficient in creating beneficial properties. That's the truly unique aspect.”

Having lived in the Amazon and studied it, how much terra preta does Lehmann believe there is?

“There are no precise numbers of how much terra preta there is (in Amazonia). No one has done any large-scale investigation of that. It's very difficult to find out in the Amazon's jungle environment. Suitable remote-sensing techniques haven't yet been used.

“So (the 10 percent) estimates sometimes cited are crude extrapolations from the few areas we're familiar with. But we know that in familiar areas there are huge patches of terra preta. These are hundreds of hectares large. When there have been maps produced of areas containing terra preta — say an area around a stream — patches are everywhere.

“It is also true that terra preta is widespread. Almost anywhere in the central Amazon, you can step out of the car and ask a local ‘Is there any terra preta around?’ and they'll show you. It's everywhere.”

What were the Indians growing? Tree crops? Row crops?

“There has been some pollen analysis. It suggests manioc and maize were being grown 2,000 to 3,000 years ago. In the pollen bank, these crops didn't pop up sporadically but in large numbers.

“But all kinds of crops were grown by the Indians. Palm trees, under-story fruit trees, Brazil nut trees — all were very important.”

On the differences between slash-and-burn and slash-and-char agriculture…

“We have very good indications that the Amerindian populations couldn't have practiced slash-and-burn and created these soils.

“It's also highly unlikely that a population relying on stone axes would have practiced slash-and-burn anyway. The normal soils are so poor that with a single slash-and-burn event, you can only crop without fertilizer for two years at most. Then the soil has to be left fallow again.

“Primary forest trees have a diameter of 2 or 3 meters. If all you had was a stone ax in your hand, you'd find a different way to deal with agriculture than felling these huge trees every two years.

“The difference between (the two systems) is the slash-and-char wouldn't burn in an open fire. Charcoal would be produced under partial exclusion of oxygen. We envision that happening by natives covering up piled up logs with dirt and straw. These charcoal-making systems are still being used around the world.”

How close are researchers to duplicating terra preta?

“We're working intensively. We don't need to take any terra preta anywhere. What we want to do is become knowledgeable about how terra preta was created and then create it elsewhere with local resources.

“Research on this is ongoing in Columbia, in Kenya. I have research colleagues in Japan and Indonesia also working on this. At the moment, there is a lot of excitement but there's a lot of work to do.”

How terra preta could help industrialized countries…

“We envision systems based on some of the principles of terra preta. And this isn't just for tropical agriculture. This could be very important for U.S. agriculture.

“Terra Preta could mean a reduction in environmental pollution. What works as a retaining mechanism in Amazonia could work in the United States where there are concerns of phosphates and nitrates entering groundwater and streams. We have only begun to realize the potential of how this could reduce pollution in industrialized countries.

“Luckily the principles of creating bio-char soils will be very similar no matter what area of the world you're in. Results obtained in Brazil will be pertinent for the United States.

“In terms of widespread adoption, it's still some way away. There are still knowledge gaps. For instance, we know there are important differences in the effects of bio-char on soil fertility depending on what material you use and what temperature and under what conditions the char is produced. That's something we should be able to resolve within a year or two. Once that's done, we can take the systems to Extension Services around the world and make larger scale, on-farm research plots.

“We're already working with dozens of Kenyan farmers on this. The project only began this year. By next year, we hope to have a better idea of how this works on farms.”

Where will the bio-char come from?

“Perhaps agricultural and forestry waste products could be the answer.

“Something else that gets us very excited is a link to energy production systems (utilizing) pyrolysis…

“Really, pyrolysis is a just a complicated word for making charcoal. Prototypes of this system for commercial power plants have been developed. These create bio-oil, hydrogen and other co-products — including bio-char — from the production of charcoal.

“We want to gain a better understanding of what effects this bio-char has on soil functions. It should be quite similar to a bio-char produced in a kiln or field. Such a system will be an entry point for large-scale production and use.

“There are competing uses for the power plant byproducts. Currently, power plants either use the byproducts for their own energy needs or they sell it to be used as charcoal briquettes.

“It could become profitable as soon as some of the environmental effects — currently external — are internalized. For instance, cleaner streams, cleaner groundwater, carbon sequestration and other things.”

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