Terra preta as a super soil

Amazonian ecosystems exhibit low fertility levels in highly weathered soils and therefore, it has been assumed for many years that environmental limitations prevent intensive agriculture in this region (e.g. Meggers, 1954). Within this ecosystem, however, patches of dark-coloured soils have been found; these patches exhibit stark differences in soil properties compared to the predominant soils in Amazonia, with the dark patches being characterised by high organic matter and nutrient levels (Glaser, 2007). These dark soils are locally called Terra Preta de Indio, which translates to Amazonian Dark Earths.

Terra preta is typically defined as a soil characterised by a several decimetre-thick topsoil horizon, which exhibits high levels of soil organic matter, biochar, and nutrients in addition to containing archaeological artefacts of pre-Colombian origin (Glaser and Birk, 2012). A typical terra preta soil profile exhibits a number of varying qualities that differ to a ferralsol, the region’s predominant soil. Ferralsols tend to have a shallow surface horizon that is light brown in colour. This horizon overlays a several metre thick subsoil horizon that has a loamy or sandy texture with a structure dominated by micro aggregates (Glaser et al. 2001). Terra preta on the other hand, has a dark grey or black coloured topsoil that can reach a depth of up to one metre. Within this horizon, potsherds, small bone, and charcoal particles are commonly found in addition to signs of bioturbation and aggregates of biogenic material. Beneath this horizon, there is a light coloured layer that tends to show patches of different brown, grey, and black colour. This layer shows clear signs of mixing of topsoil and subsoil material. Significantly, the subsoil horizons are identical to the subsoil horizons found in the adjacent soils in the area (Glaser et al. 2001).

Terra preta soils have historically presented a conundrum regarding their formation; previous theories have included allochthonous inputs, including volcanic deposits from the Andes or from sediment deposits from ancient lakes (Glaser and Birk, 2012). However, pedological and archaeological investigations have confirmed that this soil has an anthropogenic origin, formed from waste and charcoal deposits made by pre-Colombian Indians from 500 to 2500 years B.P. and abandoned after the invasion of Europeans (Neves et al. 2003).

It is these charcoal deposits that have made terra preta soils so unique; the porosity of the bio-char facilities the formation of organic chemical compounds, resulting in the soil to become rich in carbon, as well as other essential nutrients, such as NPK, which in turn increases the fertility of the soil (Glaser and Knorr, 2008). Similarly, increased fertility is also facilitated by the high organic matter content; it increases nutrient recycling and results in the soil to have a high cation exchange capacity, which limits leaching (Glaser, 2007). Furthermore, the presence of pottery shards acts as a mulch layer, limiting soil moisture loss while the uniquely evolved microbes present in the soil contribute to soil richness and nutrient recycling (Woods and McCann, 1999).

Unsurprisingly, terra preta could act as a model for sustainable agriculture in the humid tropics and other soils characterised by a low nutrient holding capacity. The techniques that led to the formation of terra preta have the capability to decrease land degradation from recently utilised intensive agriculture in the region in addition to reclaim the degraded areas (Glaser, 2007). Furthermore, the high stability of soil organic matter in terra preta coupled with experimental data (e.g. Kuzyakov et al. 2009) has shown that carbon can be sequestered for millennia within this type of soils. Consequently, terra preta has the ability to combine sustainable farming practices with long-term carbon dioxide sequestration. It would not be wrong to speculate the potential this black soil has in transforming agriculture around the world. 

References 

Denevan, W.M. (1996) ‘A bluff model of riverine settlement in prehistoric Amazonia’, Annals of the Association of American Geographers, 86, 654-681. 

Glaser, B. (2007) ‘Prehistorically modified soils of Central Amazonia: a model for sustainable agriculture in the 21st century?’, Philosophical Transactions of the Royal Society B: Biological Sciences, 362, 187–196, 

Glaser, B. and J.J. Birk (2012) ‘State of the scientific knowledge on properties and genesis of Anthropogenic Dark Earths in Central Amazonia (terra preta de Índio)’, Geochimica et Cosmochimica Acta, 82, 39-51. 

Kern, D.C., G. D’Aquino, T.E. Rodriques, F.J.L Franzao, W. Sombroek, T.P. Myers and E.G. Neves (2003) ‘Distribution of Amazonian Dark Earths (terra preta) in the Brazilian Amazon’ in J.Lehmann et al. (eds.) Amazonian Dark Earths: Origin, Properties, Management. Kluwer Academic Publishers: Dodrecht, 105-124. 

Kuzyakov. Y, I. Subbotina, I. Bogomolova, and L. Xu X. (2009) 'Black carbon decomposition and incorporation into soil microbial biomass estimated by 14C labeling', Soil Biology and Biochemistry, 41, 210-219. 

McCann, J.M., W.I. Woods and D.W. Meyer (2001) ‘Organic matter and Anthrosols in Amazonia: interpreting the amerindian legacy’. In: Sustainable Management of Soil Organic Matter (eds. Rees et al.), CABI Publishing: Wallingford, 181-189.

Meggers, B.J. (1954) ‘Environmental limitation on the development of culture’, American Anthropologist, 56, 801-824. 

Neves, E.G., J.B. Peterson, R.N. Bartone and C.A. Da Silva (2003) ‘Historical and socio-cultural origins of Amazonian Dark Earths’ in J. Lehman et al. (eds.) Amazonian Dark Earths: Origin, Properties, Management. Kluwer Academic Publishers, Dodrect, 29-50. 

Sombroek, W. D. Kern, T. Rodriguez, M. Cravo, W. Woods and B. Glaser (2002) ‘Terra preta and Terra mulata: pre-Columbian Amazon kitchen middens and agricultural fields, their sustainability and their replication’. In: 17th World Congress of Soil Science.