Soil matrix

The soil matrix is the solid phase of soils, and comprise the solid particles that make up soils. Soil particles can be classified by their chemical composition (mineralogy) as well as their size. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential,[1] but the mineralogy of those particles can strongly modify those properties. The mineralogy of the finest soil particles, clay, is especially important.[2]

Gravel, sand and siltEdit

Gravel, sand and silt are the larger soil particles, and their mineralogy is often inherited from the parent material of the soil, but may include products of weathering (such as concretions of calcium carbonate or iron oxide), or residues of plant and animal life (such as silica phytoliths).[3][4] Quartz is the most common mineral in the sand or silt fraction as it is resistant to chemical weathering, except under hot climate;[5] other common minerals are feldspars, micas and ferromagnesian minerals such as pyroxenes, amphiboles and olivines, which are dissolved or transformed in clay under the combined influence of physico-chemical and biological processes.[3][6]

Mineral colloids; soil claysEdit

Due to its high specific surface area and its unbalanced negative electric charges, clay is the most active mineral component of soil.[7][8] It is a colloidal and most often a crystalline material.[9] In soils, clay is a soil textural class and is defined in a physical sense as any mineral particle less than 2 μm (8×10−5 in) in effective diameter. Many soil minerals, such as gypsum, carbonates, or quartz, are small enough to be classified as clay based on their physical size, but chemically they do not afford the same utility as do mineralogically-defined clay minerals.[10] Chemically, clay minerals are a range of phyllosilicate minerals with certain reactive properties.[11]

Before the advent of X-ray diffraction clay was thought to be very small particles of quartz, feldspar, mica, hornblende or augite, but it is now known to be (with the exception of mica-based clays) a precipitate with a mineralogical composition that is dependent on but different from its parent materials and is classed as a secondary mineral.[12] The type of clay that is formed is a function of the parent material and the composition of the minerals in solution.[13] Clay minerals continue to be formed as long as the soil exists.[14] Mica-based clays result from a modification of the primary mica mineral in such a way that it behaves and is classed as a clay.[15] Most clays are crystalline, but some clays or some parts of clay minerals are amorphous.[16] The clays of a soil are a mixture of the various types of clay, but one type predominates.[17]

Typically there are four main groups of clay minerals: kaolinite, montmorillonite-smectite, illite, and chlorite.[18] Most clays are crystalline and most are made up of three or four planes of oxygen held together by planes of aluminium and silicon by way of ionic bonds that together form a single layer of clay. The spatial arrangement of the oxygen atoms determines clay's structure.[19] Half of the weight of clay is oxygen, but on a volume basis oxygen is ninety percent.[20] The layers of clay are sometimes held together through hydrogen bonds, sodium or potassium bridges and as a result will swell less in the presence of water.[21] Clays such as montmorillonite have layers that are loosely attached and will swell greatly when water intervenes between the layers.[22]

In a wider sense clays can be classified as:

  1. Layer Crystalline alumino-silica clays: montmorillonite, illite, vermiculite, chlorite, kaolinite.
  2. Crystalline Chain carbonate and sulfate minerals: calcite (CaCO3), dolomite (CaMg(CO3)2) and gypsum (CaSO4·2H2O).
  3. Amorphous clays: young mixtures of silica (SiO2-OH) and alumina (Al(OH)3) which have not had time to form regular crystals.
  4. Sesquioxide clays: old, highly leached clays which result in oxides of iron, aluminium and titanium.[23]

Alumino-silica claysEdit

Alumino-silica clays or aluminosilicate clays are characterized by their regular crystalline or quasi-crystalline structure.[24] Oxygen in ionic bonds with silicon forms a tetrahedral coordination (silicon at the center) which in turn forms sheets of silica. Two sheets of silica are bonded together by a plane of aluminium which forms an octahedral coordination, called alumina, with the oxygens of the silica sheet above and that below it.[25] Hydroxyl ions (OH) sometimes substitute for oxygen. During the clay formation process, Al3+ may substitute for Si4+ in the silica layer, and as much as one fourth of the aluminium Al3+ may be substituted by Zn2+, Mg2+ or Fe2+ in the alumina layer. The substitution of lower-valence cations for higher-valence cations (isomorphous substitution) gives clay a local negative charge on an oxygen atom[25] that attracts and holds water and positively charged soil cations, some of which are of value for plant growth.[26] Isomorphous substitution occurs during the clay's formation and does not change with time.[27][28]

  • Montmorillonite clay is made of four planes of oxygen with two silicon and one central aluminium plane intervening. The alumino-silicate montmorillonite clay is thus said to have a 2:1 ratio of silicon to aluminium, in short it is called a 2:1 clay mineral.[29] The seven planes together form a single crystal of montmorillonite. The crystals are weakly held together and water may intervene, causing the clay to swell up to ten times its dry volume.[30] It occurs in soils which have had little leaching, hence it is found in arid regions, although it may also occur in humid climates, depending on its mineralogical origin.[31] As the crystals are not bonded face to face, the entire surface is exposed and available for surface reactions, hence it has a high cation exchange capacity (CEC).[32][33][34]
  • Illite is a 2:1 clay similar in structure to montmorillonite but has potassium bridges between the faces of the clay crystals and the degree of swelling depends on the degree of weathering of potassium-feldspar.[35] The active surface area is reduced due to the potassium bonds. Illite originates from the modification of mica, a primary mineral. It is often found together with montmorillonite and its primary minerals. It has moderate CEC.[36][33][37][38][39]
  • Vermiculite is a mica-based clay similar to illite, but the crystals of clay are held together more loosely by hydrated magnesium and it will swell, but not as much as does montmorillonite.[40] It has very high CEC.[41][42][38][39]
  • Chlorite is similar to vermiculite, but the loose bonding by occasional hydrated magnesium, as in vermiculite, is replaced by a hydrated magnesium sheet, that firmly bonds the planes above and below it. It has two planes of silicon, one of aluminium and one of magnesium; hence it is a 2:2 clay.[43] Chlorite does not swell and it has low CEC.[41][44]
  • Kaolinite is very common, highly weathered clay, and more common than montmorillonite in acid soils.[45] It has one silica and one alumina plane per crystal; hence it is a 1:1 type clay. One plane of silica of montmorillonite is dissolved and is replaced with hydroxyls, which produces strong hydrogen bonds to the oxygen in the next crystal of clay.[46] As a result, kaolinite does not swell in water and has a low specific surface area, and as almost no isomorphous substitution has occurred it has a low CEC.[47] Where rainfall is high, acid soils selectively leach more silica than alumina from the original clays, leaving kaolinite.[48] Even heavier weathering results in sesquioxide clays.[49][20][34][37][50][51]

Crystalline chain claysEdit

The carbonate and sulfate clay minerals are much more soluble and hence are found primarily in desert soils where leaching is less active.[52]

Amorphous claysEdit

Amorphous clays are young, and commonly found in recent volcanic ash deposits such as tephra.[53] They are mixtures of alumina and silica which have not formed the ordered crystal shape of alumino-silica clays which time would provide. The majority of their negative charges originates from hydroxyl ions, which can gain or lose a hydrogen ion (H+) in response to soil pH, in such way was as to buffer the soil pH. They may have either a negative charge provided by the attached hydroxyl ion (OH), which can attract a cation, or lose the hydrogen of the hydroxyl to solution and display a positive charge which can attract anions. As a result, they may display either high CEC in an acid soil solution, or high anion exchange capacity in a basic soil solution.[49]

Sesquioxide claysEdit


Sesquioxide clays are a product of heavy rainfall that has leached most of the silica from alumino-silica clay, leaving the less soluble oxides iron hematite (Fe2O3), iron hydroxide (Fe(OH)3), aluminium hydroxide gibbsite (Al(OH)3), hydrated manganese birnessite (MnO2), as can be observed in most lateritic weathering profiles of tropical soils.[54] It takes hundreds of thousands of years of leaching to create sesquioxide clays.[55] Sesqui is Latin for "one and one-half": there are three parts oxygen to two parts iron or aluminium; hence the ratio is one and one-half (not true for all). They are hydrated and act as either amorphous or crystalline. They are not sticky and do not swell, and soils high in them behave much like sand and can rapidly pass water. They are able to hold large quantities of phosphates, a sorptive process which can at least partly be inhibited in the presence of decomposed (humified) organic matter.[56] Sesquioxides have low CEC but these variable-charge minerals are able to hold anions as well as cations.[57] Such soils range from yellow to red in colour. Such clays tend to hold phosphorus so tightly that it is unavailable for absorption by plants.[58][59][60]

Organic colloidsEdit

Humus is one of the two final stages of decomposition of organic matter. It remains in the soil as the organic component of the soil matrix while the other stage, carbon dioxide, is freely liberated in the atmosphere or reacts with calcium to form the soluble calcium bicarbonate. While humus may linger for a thousand years,[61] on the larger scale of the age of the mineral soil components, it is temporary, being finally released as CO2. It is composed of the very stable lignins (30%) and complex sugars (polyuronides, 30%), proteins (30%), waxes, and fats that are resistant to breakdown by microbes and can form complexes with metals, facilitating their downward migration (podzolization).[62] However, although originating for its main part from dead plant organs (wood, bark, foliage, roots), a large part of humus comes from organic compounds excreted by soil organisms (roots, microbes, animals) and from their decomposition upon death.[63] Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the CEC of humus is many times greater than that of clay.[64][65][66]

Humus plays a major role in the regulation of atmospheric carbon, through carbon sequestration in the soil profile, more especially in deeper horizons with reduced biological activity.[67] Stocking and destocking of soil carbon are under strong climate influence.[68] They are normally balanced through an equilibrium between production and mineralization of organic matter, but the balance is in favour of destocking under present-day climate warming,[69] and more especially in permafrost.[70]

Carbon and terra pretaEdit

In the extreme environment of high temperatures and the leaching caused by the heavy rain of tropical rain forests, the clay and organic colloids are largely destroyed. The heavy rains wash the alumino-silicate clays from the soil leaving only sesquioxide clays of low CEC. The high temperatures and humidity allow bacteria and fungi to virtually decay any organic matter on the rain-forest floor overnight and much of the nutrients are volatilized or leached from the soil and lost,[71] leaving only a thin root mat lying directly on the mineral soil.[72] However, carbon in the form of finely divided charcoal, also known as black carbon, is far more stable than soil colloids and is capable of performing many of the functions of the soil colloids of sub-tropical soils.[73] Soil containing substantial quantities of charcoal, of an anthropogenic origin, is called terra preta. In Amazonia it testifies for the agronomic knowledge of past Amerindian civilizations.[74] The pantropical peregrine earthworm Pontoscolex corethrurus has been suspected to contribute to the fine division of charcoal and its mixing to the mineral soil in the frame of present-day slash-and-burn or shifting cultivation still practiced by Amerindian tribes.[75] Research into terra preta is still young but is promising. Fallow periods "on the Amazonian Dark Earths can be as short as 6 months, whereas fallow periods on oxisols are usually 8 to 10 years long"[76] The incorporation of charcoal to agricultural soil for improving water and nutrient retention has been called biochar, being extended to other charred or carbon-rich by-products, and is now increasingly used in sustainable tropical agriculture.[77] Biochar also allows the irreversible sorption of pesticides and other pollutants, a mechanism by which their mobility, and thus their environmental risk, decreases.[78] It has also been argued as a mean of sequestering more carbon in the soil, thereby mitigating the so-called greenhouse effect.[79] However, the use of biochar is limited by the availability of wood or other products of pyrolysis and by risks caused by concomitent deforestation.[80]

See alsoEdit


  1. ^ Saxton, Keith E. & Rawls, Walter J. (2006). "Soil water characteristic estimates by texture and organic matter for hydrologic solutions" (PDF). Soil Science Society of America Journal. 70 (5): 1569–78. Bibcode:2006SSASJ..70.1569S. CiteSeerX doi:10.2136/sssaj2005.0117. Retrieved 2 September 2018.
  2. ^ College of Tropical Agriculture and Human Resources. "Soil Mineralogy". University of Hawai‘i at Mānoa. Retrieved 2 September 2018.
  3. ^ a b Russell, E. Walter (1973). Soil conditions and plant growth (10th ed.). London: Longman. pp. 67–70. ISBN 978-0-582-44048-7.
  4. ^ Mercader, Julio; Bennett, Tim; Esselmont, Chris; Simpson, Steven & Walde, Dale (2011). "Soil phytoliths from miombo woodlands in Mozambique" (PDF). Quaternary Research. 75 (1): 138–50. Bibcode:2011QuRes..75..138M. doi:10.1016/j.yqres.2010.09.008. Retrieved 9 September 2018.
  5. ^ Sleep, Norman H. & Hessler, Angela M. (2006). "Weathering of quartz as an Archean climatic indicator" (PDF). Earth and Planetary Science Letters. 241 (3–4): 594–602. Bibcode:2006E&PSL.241..594S. doi:10.1016/j.epsl.2005.11.020. Retrieved 9 September 2018.
  6. ^ Banfield, Jillian F.; Barker, William W.; Welch, Susan A. & Taunton, Anne (1999). "Biological impact on mineral dissolution: application of the lichen model to understanding mineral weathering in the rhizosphere" (PDF). Proceedings of the National Academy of Sciences of the United States of America. 96 (7): 3404–11. Bibcode:1999PNAS...96.3404B. doi:10.1073/pnas.96.7.3404. PMC 34281. PMID 10097050. Retrieved 9 September 2018.
  7. ^ Santamarina, J. Carlos; Klein, Katherine A.; Wang, Yu-Hsing & Prencke, E. (2002). "Specific surface: determination and relevance" (PDF). Canadian Geotechnical Journal. 39 (1): 233–41. doi:10.1139/t01-077. Archived from the original (PDF) on 30 September 2018. Retrieved 30 September 2018.
  8. ^ Tombácz, Etelka & Szekeres, Márta (2006). "Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite" (PDF). Applied Clay Science. 34 (1–4): 105–24. doi:10.1016/j.clay.2006.05.009. Retrieved 30 September 2018.
  9. ^ Brown, George (1984). "Crystal structures of clay minerals and related phyllosilicates" (PDF). Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 311 (1517): 221–40. Bibcode:1984RSPTA.311..221B. doi:10.1098/rsta.1984.0025. Retrieved 30 September 2018.
  10. ^ Hillier, Stephen (1978). "Clay mineralogy" (PDF). In Middleton, Gerard V.; Church, Michael J.; Coniglio, Mario; Hardie, Lawrence A.; Longstaffe, Frederick J. (eds.). Encyclopedia of sediments and Sedimentary rocks. Encyclopedia of Earth Science. Dordrecht, The Netherlands: Springer Science+Business Media B.V. pp. 139–42. doi:10.1007/3-540-31079-7_47. ISBN 978-0-87933-152-8. Retrieved 30 September 2018.
  11. ^ Donahue, Miller & Shickluna 1977, pp. 101–02.
  12. ^ Bergaya, Faïza; Beneke, Klaus; Lagaly, Gerhard. "History and perspectives of clay science" (PDF). University of Kiel. Retrieved 20 October 2018.
  13. ^ Wilson, M. Jeff (1999). "The origin and formation of clay minerals in soils: past, present and future perspectives" (PDF). Clay Minerals. 34 (1): 7–25. Bibcode:1999ClMin..34....7W. doi:10.1180/000985599545957. Archived from the original (PDF) on 29 March 2018. Retrieved 20 October 2018.
  14. ^ Simonson 1957, p. 19.
  15. ^ Churchman, G. Jock (1980). "Clay minerals formed from micas and chlorites in some New Zealand soils" (PDF). Clay Minerals. 15 (1): 59–76. Bibcode:1980ClMin..15...59C. doi:10.1180/claymin.1980.015.1.05. Retrieved 20 October 2018.
  16. ^ Wada, Koji; Greenland, Dennis J. (1970). "Selective dissolution and differential infrared spectroscopy for characterization of 'amorphous' constituents in soil clays". Clay Minerals. 8 (3): 241–54. Bibcode:1970ClMin...8..241W. CiteSeerX doi:10.1180/claymin.1970.008.3.02.
  17. ^ Donahue, Miller & Shickluna 1977, p. 102.
  18. ^ "The clay mineral group" (PDF). Amethyst Galleries, Inc. Retrieved 28 October 2018.
  19. ^ Schulze, Darrell G. (2005). "Clay minerals" (PDF). In Hillel, Daniel (ed.). Encyclopedia of soils in the environment. Amsterdam: Academic Press. pp. 246–54. doi:10.1016/b0-12-348530-4/00189-2. ISBN 9780123485304. Retrieved 28 October 2018.
  20. ^ a b Russell 1957, p. 33.
  21. ^ Tambach, Tim J.; Bolhuis, Peter G.; Hensen, Emiel J.M.; Smit, Berend (2006). "Hysteresis in clay swelling induced by hydrogen bonding: accurate prediction of swelling states" (PDF). Langmuir. 22 (3): 1223–34. doi:10.1021/la051367q. PMID 16430287. Retrieved 3 November 2018.
  22. ^ Donahue, Miller & Shickluna 1977, pp. 102–07.
  23. ^ Donahue, Miller & Shickluna 1977, pp. 101–07.
  24. ^ Aylmore, L.A. Graham & Quirk, James P. (1971). "Domains and quasicrystalline regions in clay systems" (PDF). Soil Science Society of America Journal. 35 (4): 652–54. Bibcode:1971SSASJ..35..652Q. doi:10.2136/sssaj1971.03615995003500040046x. Retrieved 18 November 2018.
  25. ^ a b Barton, Christopher D.; Karathanasis, Anastasios D. (2002). "Clay minerals" (PDF). In Lal, Rattan (ed.). Encyclopedia of Soil Science. New York: Marcel Dekker. pp. 187–92. Retrieved 3 November 2018.
  26. ^ Schoonheydt, Robert A.; Johnston, Cliff T. (2011). "The surface properties of clay minerals" (PDF). In Brigatti, Maria Franca; Mottana, Annibale (eds.). Layered mineral structures and their application in advanced technologies. Twickenham, UK: Mineralogical Society of Great Britain & Ireland. pp. 337–73. Retrieved 2 December 2018.
  27. ^ Donahue, Miller & Shickluna 1977, p. 107.
  28. ^ Simonson 1957, pp. 20–21.
  29. ^ Lagaly, Gerhard (1979). "The "layer charge" of regular interstratified 2:1 clay minerals". Clays and Clay Minerals. 27 (1): 1–10. Bibcode:1979CCM....27....1L. doi:10.1346/CCMN.1979.0270101.
  30. ^ Eirish, M. V.; Tret'yakova, L. I. (1970). "The role of sorptive layers in the formation and change of the crystal structure of montmorillonite" (PDF). Clay Minerals. 8 (3): 255–66. Bibcode:1970ClMin...8..255E. doi:10.1180/claymin.1970.008.3.03. Archived from the original (PDF) on 19 July 2018. Retrieved 2 December 2018.
  31. ^ Tardy, Yves; Bocquier, Gérard; Paquet, Hélène; Millot, Georges (1973). "Formation of clay from granite and its distribution in relation to climate and topography" (PDF). Geoderma. 10 (4): 271–84. Bibcode:1973Geode..10..271T. doi:10.1016/0016-7061(73)90002-5. Retrieved 15 December 2018.
  32. ^ Donahue, Miller & Shickluna 1977, p. 108.
  33. ^ a b Russell 1957, pp. 33–34.
  34. ^ a b Coleman & Mehlich 1957, p. 74.
  35. ^ Meunier, Alain; Velde, Bruce (2004). "The geology of illite" (PDF). Illite: origins, evolution and metamorphism. Berlin: Springer. pp. 63–143. Retrieved 15 December 2018.
  36. ^ Donahue, Miller & Shickluna 1977, pp. 108–10.
  37. ^ a b Dean 1957, p. 82.
  38. ^ a b Allison 1957, p. 90.
  39. ^ a b Reitemeier 1957, p. 103.
  40. ^ Norrish, Keith; Rausell-Colom, José Antonio (1961). "Low-angle X-ray diffraction studies of the swelling of montmorillonite and vermiculite". Clays and Clay Minerals. 10 (1): 123–49. Bibcode:1961CCM....10..123N. doi:10.1346/CCMN.1961.0100112.
  41. ^ a b Donahue, Miller & Shickluna 1977, p. 110.
  42. ^ Coleman & Mehlich 1957, p. 73.
  43. ^ Moore, Duane M.; Reynolds, Robert C. Jr (1997). X-ray diffraction and the identification and analysis of clay minerals (PDF). Oxford: Oxford University Press. Retrieved 16 December 2018.
  44. ^ Holmes & Brown 1957, p. 112.
  45. ^ Karathanasis, Anastasios D.; Hajek, Benjamin F. (1983). "Transformation of smectite to kaolinite in naturally acid soil systems: structural and thermodynamic considerations". Soil Science Society of America Journal. 47 (1): 158–63. Bibcode:1983SSASJ..47..158K. doi:10.2136/sssaj1983.03615995004700010031x.
  46. ^ Tombácz, Etelka; Szekeres, Márta (2006). "Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite" (PDF). Applied Clay Science. 34 (1–4): 105–24. doi:10.1016/j.clay.2006.05.009. Retrieved 16 February 2019.
  47. ^ Coles, Cynthia A.; Yong, Raymond N. (2002). "Aspects of kaolinite characterization and retention of Pb and Cd" (PDF). Applied Clay Science. 22 (1–2): 39–45. CiteSeerX doi:10.1016/S0169-1317(02)00110-2. Retrieved 24 February 2019.
  48. ^ Fisher, G. Burch; Ryan, Peter C. (2006). "The smectite-to-disordered kaolinite transition in a tropical soil chronosequence, Pacific coast, Costa Rica" (PDF). Clays and Clay Minerals. 54 (5): 571–86. Bibcode:2006CCM....54..571F. doi:10.1346/CCMN.2006.0540504. Retrieved 24 February 2019.
  49. ^ a b Donahue, Miller & Shickluna 1977, p. 111.
  50. ^ Olsen & Fried 1957, p. 96.
  51. ^ Reitemeier 1957, p. 101.
  52. ^ Hamdi-Aïssa, Belhadj; Vallès, Vincent; Aventurier, Alain; Ribolzi, Olivier (2004). "Soils and brine geochemistry and mineralogy of hyperarid Desert Playa, Ouargla Basin, Algerian Sahara" (PDF). Arid Land Research and Management. 18 (2): 103–26. doi:10.1080/1532480490279656. Retrieved 24 February 2019.
  53. ^ Shoji, Sadao; Saigusa, Masahiko (1977). "Amorphous clay materials of Towada Ando soils". Soil Science and Plant Nutrition. 23 (4): 437–55. doi:10.1080/00380768.1977.10433063.
  54. ^ Tardy, Yves; Nahon, Daniel (1985). "Geochemistry of laterites, stability of Al-goethite, Al-hematite, and Fe3+-kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion formation" (PDF). American Journal of Science. 285 (10): 865–903. doi:10.2475/ajs.285.10.865. Retrieved 10 March 2019.
  55. ^ Nieuwenhuyse, André; Verburg, Paul S.J.; Jongmans, Antoine G. (2000). "Mineralogy of a soil chronosequence on andesitic lava in humid tropical Costa Rica" (PDF). Geoderma. 98 (1–2): 61–82. Bibcode:2000Geode..98...61N. doi:10.1016/S0016-7061(00)00052-5. Retrieved 10 March 2019.
  56. ^ Hunt, James F.; Ohno, Tsutomu; He, Zhongqi; Honeycutt, C. Wayne; Dail, D. Bryan (2007). "Inhibition of phosphorus sorption to goethite, gibbsite, and kaolin by fresh and decomposed organic matter" (PDF). Biology and Fertility of Soils. 44 (2): 277–88. doi:10.1007/s00374-007-0202-1. Retrieved 10 March 2019.
  57. ^ Shamshuddin, Jusop; Anda, Markus (2008). "Charge properties of soils in Malaysia dominated by kaolinite, gibbsite, goethite and hematite" (PDF). Bulletin of the Geological Society of Malaysia. 54: 27–31. doi:10.7186/bgsm54200805. Retrieved 10 March 2019.
  58. ^ Donahue, Miller & Shickluna 1977, pp. 103–12.
  59. ^ Simonson 1957, pp. 18, 21–24, 29.
  60. ^ Russell 1957, pp. 32, 35.
  61. ^ Paul, Eldor A.; Campbell, Colin A.; Rennie, David A. & McCallum, Kenneth J. (1964). "Investigations of the dynamics of soil humus utilizing carbon dating techniques" (PDF). Transactions of the 8th International Congress of Soil Science, Bucharest, Romania, 1964. Bucharest, Romania: Publishing House of the Academy of the Socialist Republic of Romania. pp. 201–08. Retrieved 16 March 2019.
  62. ^ Bin, Gao; Cao, Xinde; Dong, Yan; Luo, Yongming & Ma, Lena Q. (2011). "Colloid deposition and release in soils and their association with heavy metals" (PDF). Critical Reviews in Environmental Science and Technology. 41 (4): 336–72. doi:10.1080/10643380902871464. Retrieved 24 March 2019.
  63. ^ Six, Johan; Frey, Serita D.; Thiet, Rachel K. & Batten, Katherine M. (2006). "Bacterial and fungal contributions to carbon sequestration in agroecosystems" (PDF). Soil Science Society of America Journal. 70 (2): 555–69. Bibcode:2006SSASJ..70..555S. CiteSeerX doi:10.2136/sssaj2004.0347. Retrieved 16 March 2019.
  64. ^ Donahue, Miller & Shickluna 1977, p. 112.
  65. ^ Russell 1957, p. 35.
  66. ^ Allaway 1957, p. 69.
  67. ^ Thornton, Peter E.; Doney, Scott C.; Lindsay, Konkel; Moore, J. Keith; Mahowald, Natalie; Randerson, James T.; Fung, Inez; Lamarque, Jean-François; Feddema, Johannes J. & Lee, Y. Hanna (2009). "Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model". Biogeosciences. 6 (10): 2099–120. Bibcode:2009BGeo....6.2099T. doi:10.5194/bg-6-2099-2009.
  68. ^ Morgan, Jack A.; Follett, Ronald F.; Allen Jr, Leon Hartwell; Del Grosso, Stephen; Derner, Justin D.; Dijkstra, Feike; Franzluebbers, Alan; Fry, Robert; Paustian, Keith & Schoeneberger, Michele M. (2010). "Carbon sequestration in agricultural lands of the United States" (PDF). Journal of Soil and Water Conservation. 65 (1): 6A–13A. doi:10.2489/jswc.65.1.6A. Retrieved 24 March 2019.
  69. ^ Parton, Willam J.; Scurlock, Jonathan M. O.; Ojima, Dennis S.; Schimel, David; Hall, David O. & The SCOPEGRAM Group (1995). "Impact of climate change on grassland production and soil carbon worldwide" (PDF). Global Change Biology. 1 (1): 13–22. Bibcode:1995GCBio...1...13P. doi:10.1111/j.1365-2486.1995.tb00002.x. Retrieved 24 March 2019.
  70. ^ Schuur, Edward A. G.; Vogel, Jason G.; Crummer, Kathryn G.; Lee, Hanna; Sickman, James O. & Osterkamp, T. E. (2009). "The effect of permafrost thaw on old carbon release and net carbon exchange from tundra" (PDF). Nature. 459 (7246): 556–59. Bibcode:2009Natur.459..556S. doi:10.1038/nature08031. PMID 19478781. Retrieved 24 March 2019.
  71. ^ Wieder, William R.; Cleveland, Cory C. & Townsend, Alan R. (2009). "Controls over leaf litter decomposition in wet tropical forests" (PDF). Ecology. 90 (12): 3333–41. doi:10.1890/08-2294.1. PMID 20120803. Retrieved 31 March 2019.
  72. ^ Stark, Nellie M. & Lordan, Carl F. (1978). "Nutrient retention by the root mat of an Amazonian rain forest" (PDF). Ecology. 59 (3): 434–37. doi:10.2307/1936571. JSTOR 1936571. Archived from the original (PDF) on 31 March 2019. Retrieved 31 March 2019.
  73. ^ Liang, Biqing; Lehmann, Johannes; Solomon, Dawit; Kinyangi, James; Grossman, Julie; O'Neill, Brendan; Skjemstad, Jan O.; Thies, Janice; Luizaõ, Flávio J.; Petersen, Julie & Neves, Eduardo G. (2006). "Black carbon increases cation exchange capacity in soils" (PDF). Soil Science Society of America Journal. 70 (5): 1719–30. Bibcode:2006SSASJ..70.1719L. doi:10.2136/sssaj2005.0383. Retrieved 30 March 2019.
  74. ^ Neves, Eduardo G.; Petersen, James B.; Bartone, Robert N.; da Silva, Carlos Augusto (2003). "Historical and socio-cultural origins of Amazonian Dark Earth" (PDF). In Lehmann, Johannes; Kern, Dirse C.; Glaser, Bruno; Woods, William I. (eds.). Amazonian Dark Earths: origin, properties, management. Berlin, Germany: Springer Science & Business Media. pp. 29–50. Retrieved 7 April 2019.
  75. ^ Ponge, Jean-François; Topoliantz, Stéphanie; Ballof, Sylvain; Rossi, Jean-Pierre; Lavelle, Patrick; Betsch, Jean-Marie & Gaucher, Philippe (2006). "Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus: a potential for tropical soil fertility" (PDF). Soil Biology and Biochemistry. 38 (7): 2008–09. doi:10.1016/j.soilbio.2005.12.024. Retrieved 7 April 2019.
  76. ^ Lehmann, Johannes. "Terra Preta de Indio". University of Cornell, Department of Crop and Soil Sciences. Archived from the original on 24 April 2013. Retrieved 7 April 2019.
  77. ^ Lehmann, Johannes; Rondon, Marco (2006). "Bio-char soil management on highly weathered soils in the humid tropics" (PDF). In Uphoff, Norman; Ball, Andrew S.; Fernandes, Erick; Herren, Hans; Husson, Olivier; Laing, Mark; Palm, Cheryl; Pretty, Jules; Sánchez, Pedro; Sanginga, Nteranya; Thies, Janice (eds.). Biological approaches to sustainable soil systems. Boca Raton, Florid: CRC Press. pp. 517–30. Retrieved 14 April 2019.
  78. ^ Yu, Xiangyang; Pan, Ligang; Ying, Guangguo & Kookana, Rai S. (2010). "Enhanced and irreversible sorption of pesticide pyrimethanil by soil amended with biochars" (PDF). Journal of Environmental Sciences. 22 (4): 615–20. doi:10.1016/S1001-0742(09)60153-4. PMID 20617740. Retrieved 14 April 2019.[permanent dead link]
  79. ^ Whitman, Thea & Lehmann, Johannes (2009). "Biochar: one way forward for soil carbon in offset mechanisms in Africa?" (PDF). Environmental Science and Policy. 12 (7): 1024–27. doi:10.1016/j.envsci.2009.07.013. Retrieved 14 April 2019.
  80. ^ Mwampamba, Tuyeni Heita (2007). "Has the woodfuel crisis returned? Urban charcoal consumption in Tanzania and its implications to present and future forest availability" (PDF). Energy Policy. 35 (8): 4221–34. doi:10.1016/j.enpol.2007.02.010. Retrieved 14 April 2019.