Age-old African soil improvement method is climate friendly and sustainable

For centuries, villagers in West Africa have converted nutrient-poor rainforest soil into fertile farmland by adding charcoal and kitchen waste. Researchers have now for the first time analysed the soil and studied the method, which could possibly provide a partial solution to food challenges in the tropics. The results were obtained in an unusual collaboration between anthropologists and geologists.

2016.04.19 | Christina Troelsen

The indigenous population of the rainforest in West Africa has been converting nutrient-poor rainforest soil into fertile farmland for centuries by adding charcoal and kitchen waste. The method can potentially be used as a climate-friendly and sustainable alternative to other soil improvement methods. Photo: Victoria Frausin

Agriculture all over the world is facing the challenge of providing food for growing populations. At the same time, there are increasing demands that agriculture should be climate friendly and sustainable. Introducing economically and environmentally sustainable cultivation methods that increase productivity is particularly challenging for regions such as sub-Saharan Africa, where the food supply is insecure.

An international team of researchers with Danish participation studied a previously overlooked soil improvement system used by the indigenous population of the rainforest in West Africa. For centuries, the villagers have added charcoal and kitchen waste to the nutrient-poor red rainforest soil, thereby turning it into extremely fertile black carbonaceous soil, which provides a considerably larger yield of crops than the ordinary soil.

The method of adding charcoal to the soil could possibly prove to be a sustainable and environmentally responsible alternative for farming in resource-poor areas with low food security.

The results of the study have been published in the scientific journal Frontiers in Ecology and the Environment.

‘Black soil’ multiplies yield
Due to wars and conflicts, it has not previously been possible to visit the rainforest areas in West Africa. A few years ago, however, British anthropologists carried out a study in which they interviewed a number of villagers, and they observed how they made the particularly nutrient-rich black soil.

The villagers told them that the black soil – which was found in some places as cultivated patches in the rainforest, and in other places as small vegetable gardens around the houses – produced up to a quarter of the crops the villagers grew for their own use or to sell, even though the special soil only accounted for a small proportion of the village’s total cultivated area. In addition, the yield was steady and thus helped to ensure food supplies in the villages.

Enriching the soil with charcoal is a well-known practice
The anthropologists sent soil samples to a laboratory in the USA, and not surprisingly it proved to contain large amounts of nutrients, especially phosphorus and nitrogen. The soil also turned out to contain 200–300% more organic carbon compared with the normal red rainforest soil. The carbon comes from charcoal residue from the fireplaces in the villages.

The charcoal content particularly attracted the attention of the researchers. They found examples of nutrient-rich and black carbonaceous soil in a number of cultures throughout the world, especially in the Amazonian rainforests, where pre-Columbian Indian tribes presumably improved the tropical soil in the same way by adding charcoal. However, the system came to a complete stop when the Spaniards colonised the area, so we do not know today whether they improved the soil deliberately or simply by chance.

The question is whether this soil improvement method came to West Africa from South America, or whether it occurred on both continents and in different cultures independently of each other. The answer was provided by carbon-14 dating carried out by the Danish geologist and co-author of the scientific article, Associate Professor Søren Munch Kristiansen, Department of Geoscience, Aarhus University.

“It turned out that the black soil in West Africa was up to 700 years old, depending on how deep you dug for samples – the layers could be up to 1.8 metres deep – and therefore dated from long before Columbus sailed to America. The West African ‘discovery’ of black soil thus had nothing at all to do with the Amazon Indian ‘discovery’,” says Associate Professor Kristiansen.

In contrast to the finds of black soil from past generations of Indian tribes in the Amazon, a significant advantage of the discovery in West Africa is that present-day populations can pass on their knowledge of the method. It is the women in the rainforest population in West Africa who determine the most important land in the village and remember the cultivation methods generation after generation. The researchers were provided with information from these tribes about how they mix kitchen waste with charcoal in a special ratio, and other details of the method.

Possible solution to three problems
For the rainforest villagers in West Africa, the black soil helps maintain stable food supplies. If the method could spread to other poor areas, however, it would not only solve problems connected with food supply, but also provide more sustainable agriculture, where the farmland would not be impoverished to the same extent by over-exploitation. The method is also an environmentally responsible alternative to other cultivation methods because it binds large amounts of nitrogen in the soil, thus preventing it from entering the atmosphere as carbon dioxide (C02).

“It would be fantastic of the method could be spread to other regions of Africa via initiatives such as development projects. This method actually has the clear benefit that it originated in Africa and that the Africans can learn about the method from other Africans, which makes it much more credible in their eyes,” says Associate Professor Kristiansen.

There is nevertheless a small ‘but’ associated with the method, and this is a challenge. When a new soil improvement project using charcoal is launched in the tropics, the pH value in the soil can be too high for decades. This has an impact on the plants, which do not necessarily begin to provide higher yields immediately. This can be a long time for a poor family to wait when they need as much produce as possible here and now.

For more information, please contact

Associate Professor Søren Munch Kristiansen
Department of Geoscience
Aarhus University
+45 2338 2424


FACTS: Why does charcoal improve the soil?
Charcoal has a number of beneficial effects in otherwise nutrient-poor soil. It increases the soil’s water-holding capacity, which means more water for the crops, and it provides a better soil structure so that more oxygen can reach the roots of the plants. Charcoal binds nutrients in the soil, making it easier for the plants to utilise them. In addition, the nutrients from charcoal are released slowly by means of fungi living in the ground in symbiosis with the plants. The effect of soil improvement with charcoal also lasts for centuries compared with a few years of positive impact of burning forests, where the charcoal is not mixed into the soil.

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