The Azimuth Project
Terra preta (Rev #4)



Terra preta (“black earth” in Portuguese) is a type of very dark, fertile anthropogenic soil found in the Amazon Basin. Terra preta owes its name to its very high charcoal content, and was indeed made by adding a mixture of charcoal, bone, and manure to the otherwise relatively infertile Amazonian soil over many years.


Terra preta is characterized by the presence of low-temperature charcoal in high concentrations; of high quantities of pottery shards; of organic matter such as plant residues, animal feces, fish and animal bones and other material; and of nutrients such as nitrogen, phosphorus, calcium, zinc and manganese. It also shows high levels of microorganic activities and other specific characteristics within its particular ecosystem. It is less prone to nutrient leaching, which is a major problem in most rain forests. Terra preta zones are generally surrounded by terra comum, or “common soil”; these are infertile soils, mainly acrisols, but also ferralsols and arenosols.

Below at left we see nutrient-poor soil in the Amazon basin. At right, terra preta.

Terra preta soils are of pre-Columbian nature and were created by humans between 450 BC and AD 950. The soil’s depth can reach 2 meters. Thousands of years after its creation it has been reported to regenerate itself at the rate of 1 centimeter per year by farmers in Brazil’s Amazonian basin, who seek it for use and for sale as valuable compost.

In the humid and hot tropics terra preta is easy to produce: just add charcoal dust to soil and get amazing soil productivity boosts. In other climate zones two things need to be taken care of:

  1. Pure charcoal eats up surrounding humus (Wardle’s experiment). Gardeners know a similar phenomenon: fresh carbon rich mulch claims nutrients from surrounding soil (C/N Ratio).

  2. Charcoal is water repellent at first. You need to cook it, e.g. by flushing the fireplace with water.

Some of the above, including the photo, was taken from the Wikipedia article on terra preta (see references below).


Under what circumstances can modern-day biochar be used to create self-regenerating terra preta? This question is touched upon here:

A quote:

The hope is that once the carbon is stored in the soil, it will stay there for many thousands of years. But although terra preta shows that’s possible, it is not known whether all soils will benefit from biochar application, or even how long modern manufactured char will persist. “You can’t assume that modern biochar behaves like terra preta,” says (Rachel) Smolker. Soil scientist David Wardle reported in Science last year that, in Swedish forests at least, charcoal may cause carbon to disappear from the soil much more quickly than expected. Wardle and his team left mesh bags containing either humus, charcoal or a mixture of both on the forest floor and recorded how much mass was lost from each over a ten-year period. They found that the mixtures of humus and charcoal lost more mass than the controls of humus and charcoal alone. Wardle thinks that the charcoal promoted microbial breakdown of the humus, accelerating the release of CO2 back into the atmosphere. It’s also possible that some microbes could degrade biochar directly. Although the black carbon that makes up the bulk of biochar is thought to be biologically unavailable to most microbes, research suggests that some microbes might be able to metabolize it. If so, it would be less stable in soil than currently thought.

Another outstanding issue is to what extent modern-day biochar application will fulfil the promise of terra preta in improving soil fertility. Research by Lehmann suggests that in most cases the addition of charcoal improves soil productivity, and although the reasons for the increased fertility still aren’t entirely understood, several things seem to be going on. First, the biochar itself contains some nutrients such as phosphorus, potassium and zinc. But the biochar also seems to help the soil retain some nutrients that would otherwise leach out, as well as helping it to retain water. In addition, biochar might encourage soil microbes that increase crop productivity. And the productivity gains seem to continue to increase even when very high levels of carbon have been added to the soil — up to 140 tonnes per hectare in sandy, weathered soils, and up to about 50 tonnes per hectare on average.


For starters, try:

For more information, see:

category: carbon