Tuesday, 22 March 2011

Terra preta

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.


  1. Good point; science should be driven by its unanswered questions!
    For me the big unanswered question is "how was it made"? There's too much terra preta land area for the answer to be accidental waste heaps, but it also implies a vast undertaking by the Amazonian peoples.
    An emerging line of inquiry in Germany (Dotterweich, Pieplow) suggests a sophisticated urban sanitation + agriculture system as the core process. It involves organised solid waste collection for lacto-fermentation in sealed vessels. Charcoal (and possibly other stuff) is added on top of each "contribution" to form an efficient odour barrier before the fermentation does its work, rather like composting toilets today.
    At some point the not-unpleasant fermented product is transported to fields and gardens, maybe with intermediate composting, maybe not. This fits the pottery and other wastes in terra preta - if a pot breaks, leave it there.
    This can be hygienic (seals, lactic acid) and we could do it today. It makes sense of a lot of the content of terra preta, and of the first explorer's reports of a large urban population.
    But there's still a missing step. I can't find an article which presents the evidence for this being the ancient process, rather than these guys' inference.

  2. Hi Malcolm,
    I agree with you, "the" question is "how was it made"? There is a good paper from Neves et al: HISTORICAL AND SOCIO-CULTURAL ORIGINS OF AMAZONIAN DARK EARTHS which sums up quite well the state of the art. Have a look at it if you haven't yet. I think that the "waste" model explains quite well the process. If you have settlements where the houses are built and abandoned frequently (as it is in current indigenous communities) during a long period of time, let's say 5-6 hundred years, you will end up with patches of terra preta of a few hectares, even if the population size never was of more than 20-30 families. Neves et al report that terra preta is found at the back of the houses, not in front of them, suggesting that people kept their "front garden" clean :-)
    Have you seen the more recent post about coprostanol in terra preta?

  3. In "eenmans-esgrond" a soil made by a single family of farmers 3 to 4 generations in Drenthe (the Netherlands) the presence of ceramic debris shows which parts of the field where receiving organic material first (1700) and which received it until 19th century. As pottery is part of terra preta a chronology of pottery is the only key towards its development. Can pottery be traced in history based on isotope differences among ages ? I assume pottery styles would remain in practice over centennia.