A recent article in NewScientist magazine (registration needed) helps explain the mystery of dark earths known as Terra Preta, which have been found near to ancient human settlements in the Amazon jungle. These are fertile, dark soils found on top of the generally thin and nutrient-poor soils which are associated with tropical rainforests.
The existence of theses soils conflicts with the traditionally-held view that jungle farmers created clearings for agriculture and then moved on to new areas after a few years when the soil was depleted. But in fact, these earths suggest that there was active improvement of the soils by farmers – and similar dark earths have also been found in West Africa too. 19th century explorers in Africa reported that farmers burned wood and other vegetation beneath a layer of soil and then scattered the resulting ash over their fields.
The article states that much of what we think of as virgin and verdant rainforest is actually long-abandoned farmland, enriched by the waste created by ancient humans.
The dark soils were clearly created by humans: They are reportedly 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. Charcoal within the dark earth is a key ingredient, as it can store nutrients and help to improve moisture retention and soil drainage – the same reasons why biochar is being investigated so much today.
Read the full article here.