Amazonian Dark Earths (ADE) or "terra preta" refers to organic-rich soils which have been documented in many areas within the Amazon rainforest of South America. A type of "super-midden", ADE is essentially a human-created A horizon, or anthrosol, built on top of existing nutrient-poor soils. Scholars debate, to some extent, whether they are the result of deliberate construction, or "anthropogenic", or merely an indirect benefit of living in villages and sharing a garbage dump, "anthropic". It was most likely a little of both.
Amazonian Dark Earths are found at archaeological remains of villages in association with pottery fragments, stone flakes and animal and plant remains; they are also found at burial mounds and artificially constructed ditches. Radiocarbon dating on charcoal found within ADE patches suggest that they were mostly created between 2500 and 500 years ago. However, one ADE patch on the Jamari River in the Madeira basin of southeastern Amazonia has been dated to between 4,800 and 2,600 years ago. These dates match (and probably represent) data concerning the first permanent agricultural and sedentary lifeways in the region.
Benefits of Amazonian Dark Earths
Despite the fact that the Amazon rainforest contains a high number and diversity of plants and animals, native soils in the Amazon are notoriously infertile, with high levels of iron and aluminum and low pH levels, all of which present an enormous limitation for horticultural activities. The only naturally fertile soils in the Amazon are in the floodplains of whitewater rivers. Amazonian Dark Earths are patches of highly fertile soils, which were created by precolumbian people to allow reasonable levels of crop growing. ADE soils are located on the tops of bluffs overlooking rivers and lakes throughout the Amazon, as well as on the river terraces and levees within the floodplains of the rivers.
Creation of the ADEs allowed precolumbian people to concentrate food and other resources in areas to make it possible to rely on them. In addition, because the construction of ADE involves the ash and charcoal from earth ovens, terra preta provides a large potential for the sequestration of carbon, taking carbon out of the atmosphere and putting it to work to improve the nutrient value of soil. The addition of ash and charcoal to midden deposits is called biochar.
Amazonian Dark Earths Nutrients
ADE patches are not uniform in structure; in this respect, they're like an ordinary midden. At the center is a core of fertile soil, called terra preta; the core is surrounded by an area of soil that gradually fades in fertility as it extends outward. The patches range in size from one or two hectares (2.5-5 acres) to an area of more than 90 hectares (~23 acres). Some ADE patches are as much as 150 centimeters (~4 feet) thick.
Amazonian Dark Earths were created by intensive burning and nutrient deposition, and have been identified in pre-Columbian midden areas, home gardens and outlying fields. The soils are enriched with phosphorous, calcium, manganese, magnesium, and zinc, and have increased nutrient and moisture retention. ADE are made up of the wastes from food preparation and processing, human and animal excrement, organic housing materials, ash and charcoal from fires, and human and animal burials.
Recent studies (Fraser et al. 2011, Major et al 2005) of the fertility of Amazonian Dark Earth have found that its production and use allowed the South American people to domesticate and tend a broad diversity of nutrient-demanding crops, such as citrus and bitter manioc; and today, allow their descendants to more readily grow more marketable species.
The Lost Cities and ADE
In the 16th century, Spanish conquistador Francisco Pizarro sent two men into the Amazon searching for fabled "city of gold". Francisco de Orellana and Friar Gaspar de Carvajal disappeared into the jungle, but eventually returned, claiming that they had seen great walled cities occupied by thousands of people who grew crops. Several explorers went looking for El Dorado and never returned--Percy Fawcett was one famous example.
But the story of lost cities in the Amazon was disbelieved by everybody else, until the first decades of the 21st century, when researchers Michael Heckenberger and Eduardo Goes Neves discovered the descendants and the archaeological remnants of the cities. Amazonian Dark Earths are what made the "Lost Cities" possible.
ADE Outside of Amazonia
Although the combination of soils, climate, and people's needs are not paralleled exactly anywhere outside of the Amazon, nutrient-rich soils which have been improved by biochar addition are also found in Europe where they're called Plaggen soils, and in the Murray River region of Australia (dated 3500-600 years ago), where scholars call it Terra Preta Australis.
Sources
This glossary entry is a part of the About.com guide to Ancient Farming, and the Dictionary of Archaeology.
Birk JJ, Teixeira WG, Neves EG, and Glaser B. 2011. Faeces deposition on Amazonian Anthrosols as assessed from 5[beta]-stanols. Journal of Archaeological Science 38(6):1209-1220.
Downie AE, Van Zwieten L, Smernik RJ, Morris S, and Munroe PR. 2011. Terra Preta Australis: Reassessing the carbon storage capacity of temperate soils. Agriculture, Ecosystems & Environment 140(1-2):137-147.
Fraser J, Junqueira A, Kawa N, Moraes C, and Clement C. 2011. Crop Diversity on Anthropogenic Dark Earths in Central Amazonia. Human Ecology 39(4):395-406.
Lima HN, Schaefer CER, Mello JWV, Gilkes RJ, and Ker JC. 2002. Pedogenesis and pre-Colombian land use of "Terra Preta Anthrosols" ("Indian black earth") of Western Amazonia. Geoderma 110(1-2):1-17.
Major J, Clement C, and DiTommaso A. 2005. Influence of market orientation on food plant diversity of farms located on Amazonian Dark Earth in the region of Manaus, Amazonas, Brazil. Economic Botany 59(1):77-86.
Neves E, Petersen J, Bartone R, and Augusto Da Silva C. 2004. Historical and Socio-cultural Origins of Amazonian Dark Earth. In: Lehmann J, Kern D, Glaser B, and Wodos W, editors. Amazonian Dark Earths: Springer Netherlands. p 29-50.
Schaan DP, Kern DC, and Frazão FJL. 2009. An Assessment of the Cultural Practices Behind the Formation (or Not) of Amazonian Dark Earths in Marajo Island Archaeological Sites. In: Woods WI, Teixeira WG, Lehmann J, Steiner C, WinklerPrins A, and Rebellato L, editors. Amazonian Dark Earths: Wim Sombroek's Vision: Springer Netherlands. p 127-141.
