Terra preta, also known as Amazonian Dark Earth (ADE), is a really interesting phenomenon. ADE is an anthropogenic soil, mostly found in Brazilian Amazonia, characterized by its dark colour. The ADE’s colour derives from the high amount of charcoal and organic matter contained. It is actually the high quantity of charcoal and organic matter that “defines” terra preta. The incomplete combustion of wood produces charcoal with highly aromatic humic substances, which, once oxidized, a process that takes centuries, increase the soil’s ability to hold nutrients and hence its fertility. Terra preta is a very good soil for agriculture because it is rich in nitrogen, phosphorus, potassium and calcium. The major source of the incomplete combusted charcoal is cooking fire (Glaser et al. 2001)
From an archaeological point of view, should be quite clear what terra preta is: the result of a prolonged human occupation of a site where people have cooked and dumped wastes for hundreds of years. But, many archaeologists do not agree with this interpretation, for them terra preta was “created specifically for permanent farming” (Erickson 2008). It is the evidence of intensive pre-Columbian agriculture! I have read several papers on this topic: while many of them give for granted the “intensive agriculture” interpretation none of them provide archaeological evidence to support it. Because of all the speculations that have been done on the terra preta phenomenon, everybody now is looking for terra preta in its study site!! We have reached the extreme of scholars who said “With the exception of low levels of organic matter and charcoal, Johannes Lehmann (personal communication, 2004) favorably compares the Mound Inventory sample [in the Llanos de Moxos – Bolivian Amazon] to those of anthropogenic ADE found in Brazil” (Erickson & Balee 2006:200-201). How is that possible that you can call something “terra preta” if there is no charcoal and no organic matter??
The reason for this attachment to the productive aspect of terra preta is simply because it is sexy. We can learn from the past; we can establish development projects with indigenous communities and they can re-discover their own ancient agriculture; thanks to terra preta, intensive agriculture in the Amazon can be sustainable etc.
Yes, we have to admit that the terra preta rhetoric is cool. But, should science be driven by the coolness of its conclusions?
Erickson, C.L. (2008): Amazonia: the historical ecology of a domesticated landscape. - In: Silverman, H. & W. Isbell (eds.): Handbook of South American Archaeology. - New York: Springer: 157-183.
Erickson, C.L. & W. Balée (2006): The historical ecology of a complex landscape in Bolivia. - In: Balee, W. & C.L. Erickson (eds.): Time and complexity in historical ecology. New York: Columbia University press: 188-233.
Glaser, B., Haumaier, L., Guggenberger, G. & W. Zech (2001): The “Terra Preta” phenomenon: a model for sustainable agriculture in the humid tropics. - In: Naturwissenschaften 88(1): 37-41.