Terra preta (Portuguese pronunciation:[ˈtɛʁɐˈpɾetɐ], literally "black earth" inPortuguese), also known asAmazonian dark earth orIndian black earth, is a type of very dark, fertileanthropogenicsoil (anthrosol) found in theAmazon Basin. In Portuguese its full name isterra preta do índio orterra preta de índio ("black soil of theIndian", "Indians' black earth").Terra mulata ("mulatto earth") is lighter or brownish in color.[1]
Terra preta owes its characteristic black color to its weatheredcharcoal content,[2] and was made by adding a mixture of charcoal, bones, broken pottery, compost and manure to the low fertility Amazonian soil. A product of indigenousAmazoniansoil management andslash-and-char agriculture,[3] the charcoal is stable and remains in the soil for thousands of years, binding and retaining minerals and nutrients.[4][5]
Terra preta is characterized by the presence of low-temperature charcoal residues in high concentrations;[2] of high quantities oftiny pottery shards; of organic matter such as plant residues, animalfeces, fish and animal bones, and other material; and ofnutrients such asnitrogen,phosphorus,calcium,zinc andmanganese.[6] Fertile soils such asterra preta show high levels ofmicroorganic activities and other specific characteristics within particularecosystems.
Terra preta zones are generally surrounded byterra comum ([ˈtɛʁɐkoˈmũ,ku-]), or "common soil"; these are infertile soils, mainlyacrisols,[6] but alsoferralsols andarenosols.[7] Deforested arable soils in the Amazon are productive for a short period of time before their nutrients are consumed or leached away by rain or flooding. This forces farmers to migrate to an unburned area and clear it (by fire).[8][9]Terra preta is less prone tonutrient leaching because of its high concentration of charcoal, microbial life and organic matter. The combination accumulates nutrients, minerals and microorganisms and withstands leaching.
Terra preta soils were created by farming communities between 450 BCE and 950 CE.[10][11][12] Soil depths can reach 2 meters (6.6 ft). It is reported to regenerate itself at the rate of 1 centimeter (0.4 in) per year.[13]
The origins of the Amazoniandark earths were not immediately clear to later settlers. One idea was that they resulted from ashfall from volcanoes in theAndes, since they occur more frequently on the brows of higher terraces. Another theory considered its formation to be a result ofsedimentation intertiary lakes or in recent ponds.[citation needed]
Soils with elevatedcharcoal content and a common presence ofpottery remains can accrete accidentally near living quarters as residues from food preparation, cooking fires, animal and fishbones, broken pottery, etc., accumulated. Manyterra preta soil structures are now thought to have formed under kitchenmiddens, as well as being manufactured intentionally on larger scales.[14][15]Farmed areas around living areas are referred to asterra mulata.Terra mulata soils are more fertile than surrounding soils but less fertile thanterra preta, and were most likely intentionally improved using charcoal.[citation needed]
This type of soil appeared between 450 BCE and 950 CE at sites throughout theAmazon Basin.[12] Recent research has reported thatterra preta may be of natural origin, suggesting that pre-Columbian people intentionally utilized and improved existing areas of soil fertility scattered among areas of lower fertility.[16]
Amazonians formed complex, large-scale social formations, includingchiefdoms (particularly in the inter-fluvial regions) and even large towns and cities.[17] For instance, the culture on the island ofMarajó may have developedsocial stratification and supported a population of 100,000. Amazonians may have usedterra preta to make the land suitable for large-scale agriculture.[18]
Spanish explorerFrancisco de Orellana was the first European to traverse theAmazon River in the 16th century. He reported densely populated regions extending hundreds of kilometres along the river, suggesting population levels exceeding even those of today. Orellana may have exaggerated the level of development, although that is disputed. The evidence to support his claim comes from the discovery ofgeoglyphs dating between 0–1250 CE and fromterra preta.[19][20] Beyond the geoglyphs, these populations left no lasting monuments, possibly because they built with wood, which would have rotted in the humid climate, as stone was unavailable.[citation needed]
Whatever its extent, this civilization vanished after thedemographic collapse of the 16th and 17th century, due to European-introduced diseases such as smallpox[20] andbandeirante slave-raiding.[21] The settled agrarians again became nomads, while still maintaining specific traditions of their settled forebears. Their semi-nomadic descendants have the distinction among tribal indigenous societies of a hereditary, yet landless,aristocracy, a historical anomaly for a society without a sedentary, agrarian culture.[citation needed]
Moreover, manyindigenous peoples adapted to a more mobile lifestyle to escapecolonialism. This might have made the benefits ofterra preta, such as its self-renewing capacity, less attractive: farmers would not have been able to cultivate the renewed soil as they migrated.Slash-and-char agriculture may have been an adaptation to these conditions. For 350 years after the European arrival, the Portuguese portion of the basin remained untended.[citation needed]
Terra preta soils are found mainly in theBrazilian Amazon, where Sombroeket al.[22] estimate that they cover at least 0.1–0.3%, or 6,300 to 18,900 square kilometres (2,400 to 7,300 sq mi) of low forested Amazonia;[1] but others estimate this surface at 10.0% or more (twice the area ofGreat Britain).[13][23] Recent model-based predictions suggest that the extent ofterra preta soils may be of 3.2% of the forest.[24]
Terra preta exists in small plots averaging 20 hectares (49 acres), but areas of almost 360 hectares (890 acres) have also been reported. They are found among various climatic,geological, andtopographical situations.[1] Their distributions either follow main water courses, from East Amazonia to the central basin,[25] or are located on interfluvial sites (mainly of circular orlenticular shape) and of a smaller size averaging some 1.4 hectares (3.5 acres) (see distribution map ofterra preta sites in Amazon basin).[26] The spreads oftropical forest between thesavannas could be mainly anthropogenic—a notion with dramatic implications worldwide foragriculture andconservation.[27]
Terra preta sites are also known in theLlanos de Moxos of Bolivia,Ecuador,Peru andFrench Guiana,[28][29] and on the African continent inBenin,Liberia, and theSouth Africansavannas.[6]
In the international soil classification systemWorld Reference Base for Soil Resources (WRB)Terra preta is called PreticAnthrosol. The most common original soil before transformed into a terra preta is theFerralsol.Terra preta has acarbon content ranging from high to very high (more than 13–14% organic matter) in its A horizon, but without hydromorphic characteristics.[30]Terra preta presents important variants. For instance, gardens close to dwellings received more nutrients than fields farther away.[31] The variations in Amazonian dark earths prevent clearly determining whether all of them were intentionally created for soil improvement or whether the lightest variants are a by-product of habitation.[citation needed]
Terra preta's capacity to increase its own volume—thus to sequester more carbon—was first documented bypedologist William I. Woods of the University of Kansas.[13] This remains the central mystery of terra preta.[citation needed]
The processes responsible for the formation ofterra preta soils are:[7]
The transformation of biomass into charcoal produces a series of charcoal derivatives known aspyrogenic orblack carbon, the composition of which varies from lightly charred organic matter, tosoot particles rich ingraphite formed by recomposition offree radicals.[32][33] All types of carbonized materials are called charcoal. By convention, charcoal is considered to be any natural organic matter transformed thermally or by adehydration reaction with an oxygen/carbon (O/C) ratio less than 60;[32] smaller values have been suggested.[34] Because of possible interactions with minerals and organic matter from the soil, it is almost impossible to identify charcoal by determining only the proportion of O/C. The hydrogen/carbon percentage[35] or molecular markers such as benzenepolycarboxylic acid,[36] are used as a second level of identification.[7]
Indigenous people added low temperature charcoal to poor soils. Up to 9% black carbon has been measured in someterra preta (against 0.5% in surrounding soils).[37] Other measurements found carbon levels 70 times greater than in surroundingferralsols,[7] with approximate average values of 50 Mg/ha/m.[38]
The chemical structure of charcoal interra preta soils is characterized by poly-condensedaromatic groups that provide prolonged biological and chemical stability against microbial degradation; it also provides, after partial oxidation, the highest nutrient retention.[7][38] Low temperature charcoal (but not that from grasses or highcellulose materials) has an internal layer of biologicalpetroleum condensates that the bacteria consume, and is similar to cellulose in its effects on microbial growth.[39] Charring at high temperature consumes that layer and brings little increase in soil fertility.[13] The formation of condensed aromatic structures depends on the method of manufacture of charcoal.[36][40][41] The slowoxidation of charcoal createscarboxylic groups; these increase thecation exchange capacity of the soil.[42][43] The nucleus of black carbon particles produced by the biomass remains aromatic even after thousands of years and presents the spectral characteristics of fresh charcoal. Around that nucleus and on the surface of the black carbon particles are higher proportions of forms ofcarboxylic andphenolic carbons spatially and structurally distinct from the particle's nucleus. Analysis of the groups of molecules provides evidences both for the oxidation of the black carbon particle itself, as well as for the adsorption of non-black carbon.[44]
This charcoal is thus decisive for thesustainability ofterra preta.[42][45]Amendingferralsol with wood charcoal greatly increases productivity.[25] Globally, agricultural lands have lost on average 50% of their carbon due to intensive cultivation and other damage of human origin.[13]
Fresh charcoal must be "charged" before it can function as abiotope.[46] Several experiments demonstrate that uncharged charcoal can bring a temporary depletion of available nutrients when first put into the soil, that is until its pores fill with nutrients. This is overcome by soaking the charcoal for two to four weeks in any liquid nutrient (urine, plant tea, worm tea, etc.).[47]
Charcoal's porosity brings better retention of organic matter, of water and of dissolved nutrients,[42][48] as well as of pollutants such aspesticides and aromatic poly-cyclic hydrocarbons.[49]
Charcoal's high absorption potential of organic molecules (and of water) is due to its porous structure.[7]Terra preta's high concentration of charcoal supports a high concentration of organic matter (on average three times more than in the surrounding poor soils),[7][38][43][50] up to 150 g/kg.[25] Organic matter can be found at 1 to 2 metres (3 ft 3 in to 6 ft 7 in) deep.[30]
Bechtold proposes to useterra preta for soils that show, at 50 centimeters (20 in) depth, a minimum proportion of organic matter over 2.0–2.5%. The accumulation of organic matter in moist tropical soils is a paradox, because of optimum conditions for organic matter degradation.[38] It is remarkable thatanthrosols regenerate in spite of these tropical conditions' prevalence and their fast mineralisation rates.[25] The stability of organic matter is mainly because the biomass is only partially consumed.[38]
Terra preta soils also show higher quantities of nutrients, and a better retention of these nutrients, than surrounding infertile soils.[38] The proportion ofP reaches 200–400 mg/kg.[51] The quantity ofN is also higher in anthrosol, but that nutrient is immobilized because of the high proportion ofC overN in the soil.[25]
Anthrosol's availability ofP,Ca,Mn andZn is higher than ferrasol. The absorption of P,K, Ca,Zn, andCu by the plants increases when the quantity of available charcoal increases. The production of biomass for two crops (rice andVigna unguiculata) increased by 38–45% without fertilization (P < 0.05), compared to crops on fertilized ferralsol.[25]
Amending with charcoal pieces approximately 20 millimeters (0.79 in) in diameter, instead of ground charcoal, did not change the results except formanganese (Mn), for which absorption considerably increased.[25]
Nutrient leaching is minimal in this anthrosol, despite their abundance, resulting in high fertility. When inorganic nutrients are applied to the soil, however, the nutrients' drainage in anthrosol exceeds that in fertilized ferralsol.[25]
As potential sources of nutrients, only C (viaphotosynthesis) and N (from biological fixation) can be producedin situ. All the other elements (P, K, Ca, Mg, etc.) must be present in the soil. In Amazonia, the provisioning of nutrients from the decomposition of naturally available organic matter fails as the heavy rainfalls wash away the released nutrients and the natural soils (ferralsols, acrisols, lixisols, arenosols, uxisols, etc.) lack the mineral matter to provide those nutrients. The clay matter that exists in those soils is capable of holding only a small fraction of the nutrients made available from decomposition. In the case ofterra preta, the only possible nutrient sources are primary and secondary. The following components have been found:[38]
Saturation inpH and in base is more important than in the surrounding soils.[51][52]
The peregrineearthwormPontoscolex corethrurus (Oligochaeta:Glossoscolecidae) ingests charcoal and mixes it into a finely ground form with the mineral soil.P. corethrurus is widespread in Amazonia and notably in clearings afterburning processes thanks to its tolerance of a low content of organic matter in the soil.[53] This as an essential element in the generation ofterra preta, associated with agronomic knowledge involving layering the charcoal in thin regular layers favorable to its burying byP. corethrurus.[citation needed]
Someants are repelled from freshterra preta; their density is found to be low about 10 days after production compared to that in control soils.[54]
A newly coined term is 'syntheticterra preta'.[55][56] STP is a fertilizer consisting of materials thought to replicate the original materials, including crushed clay, blood and bone meal, manure andbiochar[55] is of particulate nature and capable of moving down the soil profile and improving soil fertility and carbon in the current soilpeds and aggregates over a viable time frame.[57] Such a mixture provides multiple soil improvements reaching at least the quality ofterra mulata. Blood, bone meal and chicken manure are useful for short term organic manure addition.[58] Perhaps the most important and unique part of the improvement of soil fertility is carbon, thought to have been gradually incorporated 4 to 10 thousand years ago.[59] Biochar is capable of decreasing soilacidity and if soaked in nutrient rich liquid can slowly release nutrients and provide habitat formicrobes in soil due to its highporosity surface area.[2]
The goal is an economically viable process that could be included in modern agriculture. Average poor tropical soils are easily enrichable toterra preta nova by the addition of charcoal and condensed smoke.[60]Terra preta may be an important avenue of futurecarbon sequestration while reversing the current worldwide decline insoil fertility and associateddesertification. Whether this is possible on a larger scale has yet to be proven. Tree Lucerne (tagasaste orCytisus proliferus) is one type offertilizer tree used to maketerra preta. Efforts to recreate these soils are underway by companies such asEmbrapa and other organizations in Brazil.[61] A growing number of startups offer to scale biochar production in the Global South via the sale of carbon credits. Examples include Tanzanian startup Dark Earth Carbon (DEC), which is supported through the UK Foreign, Commonwealth & Development Office’s Green Growth Facility (International Climate Finance).[62][63] A growing number of active initiatives around the world are marketing biochar projects and carbon credits.[64]
Syntheticterra preta is produced at the Sachamama Center for Biocultural Regeneration in High Amazon, Peru. This area has manyterra preta soil zones, demonstrating that this anthrosol was created not only in the Amazon basin, but also at higher elevations.[65]
A syntheticterra preta process was developed by Alfons-Eduard Krieger to produce a high humus, nutrient-rich, water-adsorbing soil.[66]
Terra preta sanitation (TPS) systems have been studied as an alternativesanitation option by using the effects of lactic-aid conditions inurine-diverting dry toilets and a subsequent treatment byvermicomposting.[67]
Terra Preta soils consist predominantly of char residues composed of ~6 fused aromatic rings
Much of what has been considered natural forest in Amazonia is probably the result of hundreds of years of human use and management.
