Terra preta is characterized by the presence of low-temperature charcoal in high concentrations; of high quantities of pottery sherds; of organic matter such as plant residues, animal faeces, fish and animal bones and other material; and of nutrients such as nitrogen (N), phosphorus (P), calcium (Ca), zinc (Zn), manganese (Mn). It also shows high levels of microorganic activities and other specific characteristics within its particular ecosystem. It is less prone to leaching than surrounding soils. Terra preta zones are generally surrounded by terra comum, or "common soil"; these are infertile soils, mainly acrisols, but also ferralsols and arenosols.
Terra preta soils are of pre-Columbian nature and were created by man between 7000 BP (ca. 5000 BC) and 500 BP (ca. 1450 AD) ("Before Present"). The soil's depth can reach 2 metres (6 feet). Thousands of years after its creation it is reputedly known as self-regenerating at the rate of 1 centimetre per year by the local farmers and caboclos in Brazil's Amazonian basin, and they seek it out for use and for sale as valuable compost.
However, because of their elevated charcoal content and the common presence of pottery remains, it is now widely accepted that these soils are a product of indigenous soil management involving a labor intensive technique termed slash-and-char. The technique is differentiated from slash and burn by a lower temperature burn (thus producing more charcoal than ashes) and in being a tool for soil improvement.
This type of soil appeared between 450 BC and AD 950 at sites throughout the Amazon Basin.
The Spanish explorer Francisco de Orellana, the 16th C explorer who was the first European to transverse the Amazon River, reported densely populated regions running hundreds of kilometers along the river, suggesting population levels exceeding even those of today. These populations left no lasting monuments because they used local wood as their construction material, which unfortunately rotted in the humid climate (stone was unavailable). While it is possible Orellana may have exaggerated the level of development among the Amazonians, their semi-nomadic descendants have the odd distinction among tribal indigenous societies of a hereditary, yet landless, aristocracy, a historical anomaly for a society without a sedentary, agrarian culture. This suggests they once were more settled and agrarian but after the demographic collapse of the 16th and 17th century due to European-introduced diseases they reverted to less complex modes of existence but maintained certain traditions. Moreover, many indigenous people were forced to adapt to a more mobile lifestyle in order to protect themselves against colonialism. This might have made the benefits of terra preta, such as its self-renewing capacity, less attractive — farmers would not have been able to enjoy the use of renewed soil because they would have been forced to move for safety. Slash-and-burn might have been an adaptation to these conditions.
For 350 years after the European arrival by Vicente Yáñez Pinzón, the Portuguese portion of the basin remained an untended former food gathering and planned agricultural landscape occupied by the Indigenous peoples who survived the arrival of European diseases. There is ample evidence for complex large-scale, pre-Columbian social formations, including chiefdoms, in many areas of Amazonia (particularly the inter-fluvial regions) and even large towns and cities. For instance the pre-Columbian culture on the island of Marajo may have developed Social stratification and supported a population of 100,000 people. The Native Americans of the Amazon rain forest may have used Terra preta to make the land suitable for the large scale agriculture needed to support large populations and complex social formations such as chiefdoms.
Terra Preta is defined as a type of latosol, having a carbon content ranging from high to very high (more than 13-14% organic matter) in its A horizon, but without hydromorphic characteristics. The composition of Terra preta presents important variants. For instance, the gardens close to dwellings received more nutrients than fields farther away. The variations in Amazonian dark earths prevent from establishing a clear separation line on whether they were intentionally created for soil improvement or whether the lightest variants are a by-product of habitation. The varied features of the dark earths throughout the Amazon Basin suggest the existence of an extensive ancient native civilization dating back 500 to 2500 years bp.
Terra preta's capacity to increase its own volume – thus to sequester more carbon – was 'discovered' by pedologist William I. Woods of Southern Illinois University Edwardsville. This central mystery of Terra preta, is actively studied by many researchers from various disciplines.
The processes responsible for the formation of Terra preta soils are:
The transformation of biomass into charcoal produces a series of charcoal derivates covered under the name of pyrogenic or black carbon, the composition of which varies; from lightly charred organic matter, up to soot particles rich in graphite formed by recomposition of free radicals (Hedges et al. 2000). Here, all types of charbonated materials are called charcoal. By convention, charcoal is considered to be any natural organic matter thermically transformed with an O/C percentage less than 0.6 (smaller values have been suggested). Because of possible interactions with minerals and organic matter from the soil, it is almost impossible to identify charcoal with any certainty by determining only the proportion of O/C. The H/C percentage or molecular markers such as benzenepolycarboxylic acid, are therefore used as second level of identification.
Charcoal was added to poor soils, as wood charcoal processed at low temperature and with a limited supply of oxygen (i.e., with smothered fires). William Woods (University of Kansas, Lawrence), expert on (ancient) abandoned living sites, has measured in Terra preta up to 9% black carbon (against 0.5% in surrounding soils). B. Glaser et al have found up to 70 times more carbon than in surrounding Ferralsols, with approximative average values of 50 Mg ha-1 m-1.
Amending the soil with low temperature charcoal produced from a mix of wood and leafy biomass (termed biochar) has been observed to increase the activity of arbuscular mycorrhizal fungi. Finnish researcher Janna Pietikäinen has tested high porosity materials such as zeolite, activated carbon and charcoal; these tests show – contrary to her expectations - that microbial growth is substantially improved with charcoal. It may be so that these small pieces of charcoal tend to migrate within the soil, providing a habitat for bacteria that decompose the biomass in the surface ground cover. It is theorized that this process may have an essential role in Terra preta soils' self-propagation; a virtuous cycle would be established as the fungus spreads from the charcoal, fixing additional carbon, stabilizing the soil with glomalin, and increasing nutrient availability for nearby plants. Many other agents contribute, from earthworms to humans and the charring process.
The chemical structure of charcoal in Terra preta soils is characterized with poly-condensed aromatic groups, providing prolonged biological and chemical stability that sustains the fight against microbial degradation; it also provides, after partial oxydation, the highest nutrients retention. Wood charcoal (but not that from grasses or high cellulose made at low temperature), thus has an internal layer of biological oil condensates that the bacteria consume, and that is similar to cellulose in its effects on microbial growth (Christoph Steiner, EACU 2004). Charring at high temperature loses that layer and brings little increase in the soil fertility. Glaser et al. (1998 and 2003) and Brodowski et al. (2005) have proved that the formation of condensed aromatic structures depends on the manufacture of charcoal. It is the slow oxidation of charcoal that creates carboxylic groups; these increase the cations' exchange capacity in the soil. Lehmann et al have studied the nucleus of black carbon particles produced by the biomass. They have found it highly aromatic even after thousands of years in the soil and presenting spectral characteristics of fresh charcoal. Around that nucleus and on the surface of the black carbon particles, there were higher proportions of forms of carboxylic and phenolic Cs spatially and structurally distinct from the particle's nucleus. Analysis of the groups of molecules provides evidences both for the oxydation of the black carbon particle itself, as well as for the adsorption of non-black carbon.
This charcoal is thus decisive for the sustainability aspect of Terra preta soils. Amendements of Ferrasol with wood charcoal greatly increases vegetal productivity. Note that agricultural lands have lost in average 50% of their carbon due to the practice of intensive cultivation and other degradations of human origin.
It is important to note that the fresh charcoal must first be “charged” before it can function as a biotope. Several experiments demonstrate that uncharged charcoal can bring a provisional depletion of available nutrients when first put into the soil - until its pores fill up with nutrients. This is overcome by soaking the charcoal for a few weeks (2 to 4 weeks) in any liquid nutrient (urine, plant tea. ...).
Charcoal's porosity brings a better retention of organic matter, of water and of dissolved organic nutrients, as well as of pollutants such as pesticides and aromatic poly-cyclic hydrocarbons. Organic matter
The high absorption potential of organic molecules (and of water) is due to the porous structure of charcoal. The Terra preta soils, containing these great quantities of charcoal, are equally characterized by a high concentration of organic matter (on average three times more than in the surrounding poor soils), up to 150 g/kg. Organic matter can be found at 1 to 2 metre deep.
Gerhard Bechtold proposes to call "Terra Preta" the soils that show, at 50 cm depth, a minimum proportion of organic matter superior to 2.0 or 2.5%. The accumulation of organic matter in moist tropical soils is a paradox, because of optimum conditions for degradation. It is remarkable that these anthrosols regenerate in spite of these tropical conditions' prevalence and the fast mineralisation rates. It has been demonstrated that the stability of organic matter is mainly due to the biomass being only partially consumed. Nutrients
Terra preta soils also show higher quantities of nutrients, and a better retention of these nutrients, than the surrounding infertile soils. The proportion of P reaches 200-400 mg/kg. The quantity of N is also higher in anthrosol, but that nutrient is immobilized because of the high proportion of C over N in the soil.
The anthrosol's availability of P, Ca, Mn, and Zn is clearly higher than the neighbouring Ferrasol. The absorption of P, K, Ca, Zn, and Cu by the plants increases when the quantity of available charcoal increases. The production of biomass for two crops (rice and Vigna unguiculata (L.) Walp.) increased by 38-45% without fertilization (P < 0.05), compared to crops on fertilized Ferrasol.
Amending with pieces of charcoal approximately 20 mm in diameter, instead of ground charcoal, did not change the results of experience except for manganese (Mn), for which absorption considerably increased.
Nutrient drainage is minimal in this anthrosol, despite their abundant availability, resulting in high fertility. When inorganic nutrients are applied to the soil, however, the nutrients' drainage in anthrosol exceeds that in fertilized Ferralsol.
As potential sources of nutrients, only C (via photosynthesis) and N (from biological ﬁxation) can be produced in situ. All the other elements (P, K, Ca, Mg, a.s.o.) must be present in the soil. In Amazonia the approvisionning in nutrients from composting in situ is excluded for natural soils heavily washed-out (Ferralsols, Acrisols, Lixisols, Arenosols, Uxisols, ...) that do not contain these elements in high concentration. In the case of Terra preta, the only possible nutrient sources are primary and secondary. The following components have been found:
Saturation in pH and in base is more important than in the surrounding soils (Sombroek, 1966; Smith, 1980; Kern and Kämpf, 1989; Sombroek et al., 1993; Glaser et al., 2000; Lehmann et al., 2003; Liang et al., 2006).
Bacteria and fungi (myco-organisms) live and die within the porous media, thus increasing its carbon content. Johannes Lehman and W. Zech, Bruno Glaser à l'Universite de Bayreuth (Allemagne), Embrapa (Manaus, Brazil) and many others, are studying these phenomena.
Until now there is no scientiﬁc evidence for a particular micro-organism to be responsible for the formation of Terra Preta, but a significant production of biological black carbon has recently been identiﬁed, especially under moist tropical conditions. It is possible that the fungus Aspergillus niger is mainly responsible for it. Topoliantz and Ponge's work, summarized in a synthetic article in “Soil Biology & Biochemistry”, shows that the peregrine earthworm Pontoscolex corethrurus (Oligochaeta: Glossoscolecidae), widespread in all Amazonia and notably in clearings after burning processes thanks to its high tolerance of a low content of organic matter in the soil, has been shown to ingest pieces of charcoal and to mix them in a finely ground form with the mineral soil. The authors, who experimentally verified this process, point at this as an essential element in the generation of Terra preta soils, associated with agronomic knowledge involving layering the charcoal in thin regular layers favourable to its burying by Pontoscolex corethrurus. Some ants are repelled from fresh Terra Preta soils, their density of appearance is found to be low after about 10 days as compared to control soils – see Terra Preta Experiments