In 2001 a paper was published about a farmer in Acutuba who had grown crops on terra preta soils for 40 years without needing to add any fertilizer. Astonishing as this seems, these "dark earth" soils possess a remarkable vitality and fertility, and it is speculated that along the Rio Negra the large populations described by Francisco de Orellana in the Chronicles of his 1542 quest to find the mythic city of El Dorado, were sustained by terra pretta de indio - Portuguese for "Indian black earth". The Amazonian soils are notoriously poor in quality, despite the lush forest that grows on them, and in contrast the terra preta is a legacy of the Amazonian civilizations that lived there in the past.
There has been much speculation as to the origins of terra preta soil, in particular whether it was deliberately created to improve the fertility of the region, or whether it was an accident of nature or serendipitous to the way of life among the Amazonian tribes. What seems clear is that the essential component of the soil is a kind of charcoal, which may have been formed either by a kind of composting process or by burning biomass which became added to the soil, either deliberately or by chance. Consensus of view now is that the soils were formed deliberately by local farmers, who knew well the causes of its quality.
A central figure in the investigation of terra pretta is the archaeologist, James Petersen, who was murdered by bandits in a bar on a jungle road near Iranduba in the Brazilian Amazon. Petersen called the soil a "gift from the past" and he believed that understanding its composition and origins might provide a means to improve soil fertility for small farmers today and to eliminate the carbon emissions that arise when slash-and-burn methods are used to clear forest to grow crops. Betty J. Meggers, who worked in the Amazon during the mid 1900s called the region a "counterfeit paradise" since its verdant glory existed only because the plants that grew there were able to suck every drop of water and nutrient from a soil that was fundamentally unsuitable to grow much on.
The slash and burn approach is in line with this view since forests are cut-down and the cover is burned in order to provide mineral and nitrogen rich ash to nourish the soil with. However, the soil is only productive for a few years until it reverts to its original barren state. However, evidence accumulated for an advanced civilization, rather than subsisting stone age savages, whose remains were embedded in vast swathes of black earth.
Johannes Lehmann of Cornell University is of the opinion that the black earth may offer the promise of creating sustainable agriculture, and possibly to averting global warming. The vital ingredient of terra preta is thought to be charcoal - "biochar" - which is able to bind the essential nutrients N, P and K, which impedes dramatically the rate at which they are washed-way by the continual rains. Minute pores are formed in the charcoal over time which can hold more nutrients on its larger surface area and act as "condominiums" for microorganisms to grow in and so increases their density in the soil. The idea is to create "terra preta nova", or artifical terra preta by deliberately adding charcoal to soil in the aim of recreating the properties of natural Amazonian terra preta.
He says, "With a handful of biochar, you can keep many more nutrients in the soil than in a handful of mulch or compost. It is like mopping-up nutrients with a magnet that looks like a sponge - that is, it has high surface area like a spomnge but can attract a thin layer of material like a magnet."
It is likely that to produce a soil with genuine terra preta characteristics will take a number of years of "fermentation" but it has been shown that soil treated with biochar and nutrients can have an immediate effect when added to very poor soils. Lehmann goes further and thinks that pro ducting biochar on a billions of tonnes per year scale could significantly reduce and even reverse carbon emissions and global warming, by burying that carbon in a stable form out of the biological carbon cycle. I think he is overoptimistic here especially if large-scale biochar factories are envisaged.
However, a human collective of small-scale productions could lock-up almost one billion tonnes annually, with positive impacts on soil health, and a reduced demand for freshwater and nutrient supplies, worldwide.
This should, however, be compared with potential complementary methods for regenerative agriculture which depend far less on added N,P and K through growing year-round cover crops, and forest gardens, which once established are largely self-sustaining. In terms of carbon-capture the latter are predicted to meet a capacity of 40% of all human carbon emissions, which would require the creation of a lot of biochar (3.5 billion tonnes per year) if they were to be absorbed in this way.
"Black Gold of the Amazon," By Michael Tennesen, http://discovermagazine.com/2007/apr/black-gold-of-the-amazon