The lushest patches of some jungles are rooted in enigmatic black soil – with unexpected origins
TO FIND it, you have to go digging in rainforests. And to the untrained eye, it does not seem special at all – just a thick layer of dark earth that would not look out of place in many gardens. But these fertile dark soils are in fact very special, because despite the lushness of tropical rainforests, the soils beneath them are usually very poor and thin. Even more surprising is where this dark soil comes from.
“You might expect this precious fertile resource to be found in the deep jungle, far from human settlements or farmers,” says James Fraser, who has been hunting for it in Africa’s rainforests. “But I go looking for dark earth round the edge of villages and ancient towns, and in traditionally farmed areas. It’s usually there. And the older and larger the settlement, the more dark earth there is.”
Advertisement
Such findings are overturning some long-held ideas. Jungle farmers are usually blamed not just for cutting down trees but also for exhausting the soils. And yet the discovery of these rich soils – first in South America and now in Africa too – suggests that, whether by chance or design, many people living in rainforests farmed in a way that enhanced rather than destroyed the soils. In fact, it is becoming clear that part of what we think of as verdant virgin rainforest is actually long-abandoned farmland, enriched by the waste created by ancient humans.
Enduring mystery
The story starts in the Amazon. Most of the rainforest soils there are poor in nutrients because any organic matter decomposes rapidly and the nutrients get leached away by rainwater. The soils have a characteristic red colour because they contain iron oxides, and so are known as oxisols. But it has long been known that there are patches of dark, fertile soil. The richest type is known as “terra preta”, from the Portuguese for “dark earth”. While dark earths underlie less than 1 per cent of the Amazon rainforest, individual patches can be as large as several square kilometres and more than a metre thick.
In recent decades, it has become clear that far from being natural, these soils were created by humans. The evidence is overwhelming. They are full of pottery and the charred remains of burnt wood from fires set by humans, along with organic waste from crop residues, and animal and fish bones. The black carbon from charcoal is thought to be a key ingredient of dark earths. It can persist in soils for more than a millennium, and its porous structure is thought to trap nutrients.
Some archaeologists argue that such “anthropogenic dark earths” solve an enduring mystery – how the large populations that existed in parts of the Amazon in pre-Colombian times grew enough food in poor jungle soils. Now it seems that people in Africa created dark earths, too.
After spending just four months in Liberia in west Africa last year Fraser, an anthropologist at University of Sussex in the UK, located more than 150 sites. The areas of dark earth form distinctive rings around existing and abandoned settlements. Close in are soils that form at active middens created by dumped waste, which are often used as kitchen gardens. Further out are old midden soils, which are planted with crops such as cocoa and kola. The dark soils are very different from the rust-red oxisols beneath the surrounding forests, says Fraser, who has returned to Liberia to continue his research.
Dark soils are turning up elsewhere in Africa, too. Kojo Amanor, a land-use researcher at the University of Ghana in Legon, has mapped dark earths around abandoned villages in central Ghana. Similar findings have been reported in Guinea, Chad, Cameroon, Malawi, Congo-Brazzaville and Ethiopia. It remains to be seen if the same is also true of rainforests in Asia, where soils are generally of a different, more fertile type.
In the Amazon, the knowledge of how to create dark earths died 500 years ago when European settlers decimated the native population. But Fraser, who studied dark soils in the Amazon before moving to Africa, says that in Africa the practice is much more recent and better understood by villagers.
Explorers in the 19th century reported that farmers in some parts of Africa burned wood and other vegetation under a covering of soil, and then distributed the resulting ash across their fields. That was clearly deliberate, but in some cases the creation of dark soils may simply have happened by chance. “We really don’t know if this was done on purpose,” says Fraser. “I suspect it was originally just a side effect of a way of living.”
Whether they were deliberately created or not, villagers know all about dark soils. “They are understood as an inevitable and fortuitous consequence of settled life – and one that produces a valuable resource,” Fraser says.
So understanding exactly how dark earths are created could be extremely useful. There may be some simple things that farmers in Africa and elsewhere can do to improve soils and boost yields. As a bonus, the creation of dark earths also locks away carbon that would otherwise end up in the atmosphere.
Indeed, some environmentalists dream of creating dark soils on an industrial scale, by making large volumes of charcoal, or biochar as they call it. Critics, though, say to make a big difference, to grow material for making biochar.
Even if biochar can make only a small contribution to reducing climate change, it is clear that the creation of dark soils has lasting consequences. Andre Junqueira, an ethnobotanist now at Wageningen University in the Netherlands reported last year that in the Amazon, forests that have regrown on dark soils are distinct from those that have regrown on normal soils, with more vines, palms, nut and fruit trees, and plants with spines (). In addition, the canopy is lower and the undergrowth denser than in a natural forest, and the biodiversity is at least as great.
These differences are not just a result of the different soil determining which plants thrive. They are also partly due to farmers in the past planting desirable species like fruit trees. What’s more, these ancient farmers may have selected strains that do especially well in dark soils, as some modern farmers do.
Ancient gardens
The findings also tie in neatly with controversial claims made 25 years ago by anthropologist Darrell Posey. He pointed out that the Amazon rainforest and surrounding grasslands are dotted with “islands” that are rich in fruit trees, medicinal plants and Brazil nuts. These are, Powell argued, the vestiges of farm gardens created when the jungle was more densely populated with people. The discovery that these islands are often on patches of black earth lends more credence to the idea that some areas of forests are relics of human-made landscapes.
It has long been clear that the notion that ancient people lived in harmony with the environment was hopelessly romantic. Wherever people live, they change things. Now it appears that we do occasionally change things for the better.
Making Islands
Our waste can do more than enrich soils. The raised tree islands within Florida’s Everglades, which provide a refuge for many animals, were thought to exist where there are bumps in the underlying bedrock. Recent excavations, however, have revealed charcoal, bones and human artefacts beneath the islands. Gail Chmura of McGill University in Montreal, Canada, thinks ancient trash piles provided the foundations on which the tree islands grew.
