The History and Future Promises of Biochar, Terras Pretas, and African Dark Earths Soils

By Catherine Price

Biochar is a material that has increasingly attracted scientific attention over recent years. This attention has occurred because biochar acts as a carbon sink, it can act as a soil conditioner, it increases fertility, and can increase the water holding capacity of soil. Biochar is a carbon-rich substance produced by a thermochemical process called pyrolysis. Pyrolysis is the thermal decomposition of biomass which occurs at a very high temperature and under oxygen-deprived conditions. Three by-products are produced: biochar (solid), bio-oil (liquid) and syngas (gas).

Photo by Bruno Pereira on Unsplash

Biochar affects soil physically (density, aeration and colour), chemically (soil pH), and biologically (macrobial biomass and community composition). The changes biochar makes to soil conditions can impact global carbon, water and nutrient cycles. Biochar interacts with the carbon cycle by sequestering CO2 from the atmosphere during the growth of vegetation. Carbon is also stored in soil.

The term biochar was first used in 2000, and it became the topic of discussion at the 2006 Pennsylvania Soil Conference. Whilst biochar is depicted as a new technology which can sequester carbon, increase soil fertility, and assist with waste management, it has a long and complex history. Terras pretas (Portuguese for black earths) were formed between 2500 and 500 years ago and are found in the Amazon. Terras pretas, otherwise known as Amazonian Dark Earths, are small areas (2 to 20 hectares) in the lowland regions of Amazonia which contain large amounts of black carbon from the incomplete combustion of organic materials. These terras pretas are not natural but were formed through human intervention. The indigenous people burnt wood in hearths, and the remains of this wood were applied to the soil. This enabled the terras pretas to form. However, it is unclear if the terras pretas were produced intentionally or unintentionally.

Similar to the Amazonia terra preta soils are the African Dark Earths. Both of these soils are extremely fertile. This fertility is enabling the African Dark Earths soils to continue to be used today. The fertility is maintained because the African Dark Earths are still being formed. The production and use of African Dark Earths rely on women’s work which is growing vegetables, cooking, producing palm oil and potash. Through these everyday activities, carbon is added to the soil through small-scale biochar production. The soil becomes fertile where women deposit rubbish, ash, char and organic waste. After a period of cultivation with garden crops that women grow (peppers, sweet potato, onions, cassava, careless greens and aubergines), these gardens are moved and men plant tree crops. These tree crops include cocoa, oil palm and silk cotton, and are cash crops which do not grow well in red upland soils. By planting in the African Dark Earths, the tree crops grow better and increased yields are produced.

It is indigenous knowledge which guided the production of terra preta and which continues to do so with the African Dark Earths. Humans are working with nature to produce food and other products in a symbiotic relationship. The formation of these soils is a long term process which relies on indigenous knowledge.

When biochar is put forward as a technology that can potentially help address climate change and improve soil fertility, it is often articulated as being routed in techno-scientific knowledge and has the ability to achieve quick results. This stands in complete contrast to the knowledge and time associated with terra preta and African Dark Earths. The binary distinction of humans and non-humans, and nature and culture only exist in European thought, and do not feature in many other cultures. The binary between nature and culture means that scientific knowledge dominates and traditional knowledges are swept aside. Perhaps the way to move forward is to not think of terra preta or African Dark Earths and biochar as distinct entities, but instead to consider how these can come together. There is a need to recognise indigenous knowledge alongside scientific knowledge. Maybe then some of the problems associated with the Anthropocene can begin to be solved. However, it is also important to ensure the indigenous communities still continue to benefit from terra preta or African Dark Earths. It should not be scientists or industrialised countries who only see the benefits from biochar research.

Finally, if biochar is to be used by industrialised countries to assist with the climate crisis and to help increase soil fertility, it is important to acknowledge the roots of its existence. It is not a new technology that has been developed by scientists, but it is a material that has been used for hundreds of years. We should acknowledge indigenous communities for the material they initially produced.

Dr Catherine Price is a Research Fellow in the School of Geography, University of Nottingham. Her research interests include relationships between humans and the more-than-human world, the environment, climate change, and the social and ethical impacts of agricultural technologies. She is currently working on the interdisciplinary Biochar Demonstrator project which is addressing the key deployment barriers of biochar for carbon sequestration. Details of her research can be found at .

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