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Amazonian Dark Earths: Wim Sombroek’s Vision
William I. Woods (Editor), Wenceslau G. Teixeira (Editor), Johannes Lehmann (Editor), Christoph Steiner (Editor), Antoinette M.G.A. WinklerPrins (Editor), Lilian Rebellato (Editor). 2009. Amazonian Dark Earths: Wim Sombroek’s Vision. Springer; 1st edition (December 1, 2008). 504 pages.
Amazon.com listing [here].
Review by Mike Dubrasich
A landmark book has been published on terra preta, Amazonian dark earths, the carbon-rich soils developed by ancient civilizations in what was once thought to be a pristine wilderness. Dedicated to Dutch soil scientist Wim Sombroek (1934-2003) who was the first modern investigator of terra preta, Amazonian Dark Earths: Wim Sombroek’s Vision is a compilation of the latest, cutting-edge studies in this fascinating and important multi-disciplinary field.
Amazonian soils are predominantly laterites [here], deeply weathered red clays lacking in most soil nutrients. Millions of years of rainfall have leached out everything but iron (hence the red color), silica, and aluminum. Essential plant nutrients such as calcium, phosphorus, and potassium are nearly missing. Amazon vegetation subsists on itself, the thin humus of decaying plant matter being the only source of key metallic oxides.
Except where there is terra preta, or it’s close cousin terra mulata. Terra preta is deep, rich, fertile, black soil that occurs in patches on bluffs along the Amazon and its tributaries. Terra preta is filled with charcoal, ash, mulch, bones, and pottery shards! It is potting soil made from organic matter transported to the sites in pots! These anomalous soils are anthropogenic: people made them.
[Dr. William] Denevan (2001:116–119) has argued that in pre-Columbian times the use of stone axes made long-fallow shifting cultivation very inefficient, and as result probably uncommon until the European introduction of metal axes. Previously, soil fertility must have been maintained and improved by frequent composting, mulching, and in-field burning, making semi-permanent cultivation possible with only brief fallowing. Over time these activities could have produced fertile, self-sustaining dark earths.
Dark earths may occupy 0.1% to 0.3%, or 6,000 to 18,000 km2, of forested lowland Amazonia (Sombroek and Carvalho 2002:130). Because their densities vary greatly within subregions and almost no systematic survey has been accomplished within Amazonia, variations in density projections of an order of magnitude are to be expected. The dark earths occur in a variety of climatic, geologic, and topographic situations, both along river bluffs and in the interior, with depths sometimes exceeding 2.0 m. Individual patches range from 1 ha or so to several hundred hectares. — from Chapter 1, Amazonian Dark Earths: The First Century of Reports by William I. Woods and William M. Denevan
Rather than a pristine, untrammeled, unoccupied wilderness, Amazonia has been home to people for thousands of years. The indigenous residents were agriculturalists who modified soils in order to grow corn (maize), squash, beans, fruiting palms, gourds, pineapples, cotton, arrowroot, and many other cultivated fruits, nuts, tubers, and fibers.
Terra mulata is brownish soil that generally surrounds patches of terra preta. It is not quite as rich and has fewer artifacts, and is even more widespread than terra preta. In theory, terra mulata is the accidentally improved soil adjacent to the deliberately improved soils, or else it is terra preta in the making. In either case, anthropogenically altered soils are in strong contrast to the unaltered laterites, and cover a combined area the size of France.
One of the key elements of terra preta is charcoal, lately termed “biochar”.
Vegetation actively withdraws carbon from the atmosphere and stores it as organic matter. Biochar is created when organic matter is heated without oxygen and it contains twice the carbon content of ordinary biomass (Lehmann 2007). Biochar is much more resistant to decay and can store carbon for centennial timescales (Lehmann et al. 2006). The addition of biochar to the soil was part of the creation of ADE [Amazon dark earths] (Neves et al. 2003). This has lead some to speculate on the viability of a biochar carbon sequestration industry which would reduce atmospheric greenhouse gases (Marris 2006; Sombroek et al. 2002) and improve soil fertility (Lehmann et al. 2003; Glaser and Woods 2004). — from Chapter 14, Locating Amazonian Dark Earths (ADE) Using Satellite Remote Sensing – A Possible Approach by J Thayn, KP Price, and WI Woods.
Biochar is touted as a “solution” to the global warming “problem.” I demure. But biochar is definitely a valuable soil amendment because carbon binds to and stores the metallic oxide nutrients essential to plant growth. The addition of charcoal as well as organic detritus helped to create and sustain terra preta over centuries.
Amazonian Dark Earths: Wim Sombroek’s Vision is a wonderful account of the history and science of anthropogenic soils. The book is as rich as terra preta in literary as well as scientific writing.
