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<h2 id="gravel-sand-and-silt">Gravel, sand and silt</h2> <p><a href="/facts/Gravel/IwHG9JfI">Gravel</a>, <a href="/facts/Sand/MdTIt0M8">sand</a> and <a href="/facts/Silt/HAxJkhDP">silt</a> are the larger <a href="/facts/Soil_texture/1PQkqW0u">soil particles</a>, and their mineralogy is often inherited from the <a href="/facts/Parent_material/ywpDLX4Q">parent material</a> of the soil, but may include products of <a href="/facts/Weathering/uSYGlv0H">weathering</a> (such as <a href="/facts/Concretions/zNoJ5Fu3">concretions</a> of <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a> or <a href="/facts/Iron_oxide/KDo8hLWn">iron oxide</a>), or residues of plant and animal life (such as silica <a href="/facts/Phytoliths/ArjDG3EI">phytoliths</a>).<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> <a href="/facts/Quartz/4n86tE2G">Quartz</a> is the most common mineral in the sand or silt fraction as it is resistant to <a href="/facts/Chemical_weathering/uSYGlv0H">chemical weathering</a>, except under hot climate;<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> other common minerals are <a href="/facts/Feldspar/DVvFnfA9">feldspars</a>, <a href="/facts/Micas/ag4cdnsb">micas</a> and <a href="/facts/Ferromagnesian/mVL7Ahuw">ferromagnesian</a> minerals such as <a href="/facts/Pyroxenes/hWMTLF9o">pyroxenes</a>, <a href="/facts/Amphiboles/2Nf6RDRA">amphiboles</a> and <a href="/facts/Olivines/tLLL4rae">olivines</a>, which are dissolved or transformed in clay under the combined influence of physico-chemical and biological processes.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="mineral-colloids-soil-clays">Mineral colloids; soil clays</h2> <p class="note">Further information: <a href="/facts/Clay_minerals/A1Sod7Hp">Clay minerals</a></p> <p>Due to its high <a href="/facts/Specific_surface_area/l8Dhv2S5">specific surface area</a> and its unbalanced negative <a href="/facts/Electric_charges/xxsBfmlB">electric charges</a>, <a href="/facts/Clay/xnwouEkB">clay</a> is the most active mineral component of soil.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> It is a colloidal and most often a crystalline material.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> 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 <a href="/facts/Gypsum/MItYU70n">gypsum</a>, 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 <a href="/facts/Clay_minerals/A1Sod7Hp">clay minerals</a>.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Chemically, clay minerals are a range of <a href="/facts/Phyllosilicate/WclrQW2I">phyllosilicate</a> minerals with certain reactive properties.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p><p>Before the advent of <a href="/facts/X-ray_diffraction/WtI0F5DN">X-ray diffraction</a> clay was thought to be very small particles of <a href="/facts/Quartz/4n86tE2G">quartz</a>, <a href="/facts/Feldspar/DVvFnfA9">feldspar</a>, <a href="/facts/Mica/ag4cdnsb">mica</a>, <a href="/facts/Hornblende/dKHtfvot">hornblende</a> or <a href="/facts/Augite/7nq9ugk9">augite</a>, 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 <a href="/facts/Parent_material/ywpDLX4Q">parent materials</a> and is classed as a <a href="/facts/Secondary_mineral/vLoLTKUF">secondary mineral</a>.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> The type of clay that is formed is a function of the parent material and the composition of the minerals in solution.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Clay minerals continue to be formed as long as the soil exists.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> Mica-based clays result from a modification of the primary <a href="/facts/Mica/ag4cdnsb">mica</a> mineral in such a way that it behaves and is classed as a clay.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> Most clays are <a href="/facts/Crystalline/e2R2mk3E">crystalline</a>, but some clays or some parts of clay minerals are <a href="/facts/Amorphous_solid/YTIzmrR0">amorphous</a>.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> The clays of a soil are a mixture of the various types of clay, but one type predominates.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p><p>Typically there are four main groups of clay minerals: <a href="/facts/Kaolinite/N7D6K65q">kaolinite</a>, <a href="/facts/Montmorillonite/PxKtefgA">montmorillonite</a>-<a href="/facts/Smectite/A1Sod7Hp">smectite</a>, <a href="/facts/Illite/m4u7Ee4X">illite</a>, and <a href="/facts/Chlorite/EYtQI4wC">chlorite</a>.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> 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.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Half of the weight of clay is oxygen, but on a volume basis oxygen is ninety percent.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> The layers of clay are sometimes held together through <a href="/facts/Hydrogen_bonds/85V86IIg">hydrogen bonds</a>, sodium or potassium bridges and as a result will swell less in the presence of water.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> Clays such as <a href="/facts/Montmorillonite/PxKtefgA">montmorillonite</a> have layers that are loosely attached and will swell greatly when water intervenes between the layers.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> </p><p>In a wider sense clays can be classified as: </p> <ol><li>Layer Crystalline alumino-silica clays: <a href="/facts/Montmorillonite/PxKtefgA">montmorillonite</a>, <a href="/facts/Illite/m4u7Ee4X">illite</a>, <a href="/facts/Vermiculite/NDeQ8QQR">vermiculite</a>, <a href="/facts/Chlorite_group/1jzSNrwH">chlorite</a>, <a href="/facts/Kaolinite/N7D6K65q">kaolinite</a>.</li> <li>Crystalline Chain carbonate and sulfate minerals: <a href="/facts/Calcite/gKMHYQJo">calcite</a> (CaCO3), <a href="/facts/Dolomite_(mineral)/tQfyK6AF">dolomite</a> (CaMg(CO3)2) and <a href="/facts/Gypsum/MItYU70n">gypsum</a> (CaSO4·2H2O).</li> <li>Amorphous clays: young mixtures of <a href="/facts/Silica/A3WILp6J">silica</a> (SiO2-OH) and <a href="/facts/Alumina/wXM9P6pI">alumina</a> (Al(OH)3) which have not had time to form regular crystals.</li> <li>Sesquioxide clays: old, highly leached clays which result in oxides of <a href="/facts/Iron/QK1JYy8E">iron</a>, <a href="/facts/Aluminium/7wmR8jYE">aluminium</a> and <a href="/facts/Titanium/53ArnDoy">titanium</a>.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a></li></ol> <h3>Alumino-silica clays</h3> <p>Alumino-silica clays or <a href="/facts/Aluminosilicate/3gclsh2j">aluminosilicate</a> clays are characterized by their regular <a href="/facts/Crystalline/e2R2mk3E">crystalline</a> or quasi-crystalline structure.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> <a href="/facts/Oxygen/acyn6o8r">Oxygen</a> in ionic bonds with <a href="/facts/Silicon/cgWx0hdr">silicon</a> forms a <a href="/facts/Tetrahedral/7mkSrl6j">tetrahedral</a> coordination (silicon at the center) which in turn forms sheets of <a href="/facts/Silica/A3WILp6J">silica</a>. Two sheets of silica are bonded together by a plane of <a href="/facts/Aluminium/7wmR8jYE">aluminium</a> which forms an <a href="/facts/Octahedral/G38q92xW">octahedral</a> coordination, called <a href="/facts/Alumina/wXM9P6pI">alumina</a>, with the oxygens of the silica sheet above and that below it.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a> <a href="/facts/Hydroxyl/1Hs2QAEv">Hydroxyl</a> 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-<a href="/facts/Valence_(chemistry)/ck7cHByj">valence</a> <a href="/facts/Cations/wrbwhF3z">cations</a> for higher-valence cations (<a href="/facts/Isomorphism_(crystallography)/f1XqB5Iq">isomorphous</a> substitution) gives clay a local negative <a href="/facts/Electric_charge/xxsBfmlB">charge</a> on an oxygen atom<a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> that attracts and holds water and positively charged soil cations, some of which are of value for <a href="/facts/Plant_growth/QG8xICDa">plant growth</a>.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> Isomorphous substitution occurs during the clay's formation and does not change with time.<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> </p> <ul><li>Montmorillonite clay is made of four planes of oxygen with two silicon and one central aluminium plane intervening. The <a href="/facts/Aluminosilicate/3gclsh2j">aluminosilicate</a> 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.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> 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.<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> 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.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> As the crystals are not bonded face to face, the entire surface is exposed and available for surface reactions, hence it has a high <a href="/facts/Cation_exchange_capacity/4DoXx9Jk">cation exchange capacity</a> (CEC).<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a><a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a><a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a></li> <li>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-<a href="/facts/Feldspar/DVvFnfA9">feldspar</a>.<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> The active surface area is reduced due to the potassium bonds. Illite originates from the modification of <a href="/facts/Mica/ag4cdnsb">mica</a>, a primary mineral. It is often found together with montmorillonite and its primary minerals. It has moderate CEC.<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a><a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a><a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a><a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a></li> <li>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.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> It has very high CEC.<a class="footnote-ref" id="fnref:44" href="#fn:44"><sup>44</sup></a><a class="footnote-ref" id="fnref:45" href="#fn:45"><sup>45</sup></a><a class="footnote-ref" id="fnref:46" href="#fn:46"><sup>46</sup></a><a class="footnote-ref" id="fnref:47" href="#fn:47"><sup>47</sup></a></li> <li>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.<a class="footnote-ref" id="fnref:48" href="#fn:48"><sup>48</sup></a> Chlorite does not swell and it has low CEC.<a class="footnote-ref" id="fnref:49" href="#fn:49"><sup>49</sup></a><a class="footnote-ref" id="fnref:50" href="#fn:50"><sup>50</sup></a></li> <li>Kaolinite is a very common, highly weathered clay, and more common than montmorillonite in acid soils.<a class="footnote-ref" id="fnref:51" href="#fn:51"><sup>51</sup></a> 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 <a href="/facts/Hydroxy_group/1Hs2QAEv">hydroxyls</a>, which produces strong hydrogen bonds to the oxygen in the next crystal of clay.<a class="footnote-ref" id="fnref:52" href="#fn:52"><sup>52</sup></a> As a result, kaolinite does not swell in water and has a low <a href="/facts/Specific_surface_area/l8Dhv2S5">specific surface area</a>, and as almost no <a href="/facts/Isomorphous_replacement/7E9yPKRA">isomorphous</a> substitution has occurred it has a low CEC.<a class="footnote-ref" id="fnref:53" href="#fn:53"><sup>53</sup></a> Where rainfall is high, acid soils selectively leach more silica than alumina from the original clays, leaving kaolinite.<a class="footnote-ref" id="fnref:54" href="#fn:54"><sup>54</sup></a> Even heavier weathering results in sesquioxide clays.<a class="footnote-ref" id="fnref:55" href="#fn:55"><sup>55</sup></a><a class="footnote-ref" id="fnref:56" href="#fn:56"><sup>56</sup></a><a class="footnote-ref" id="fnref:57" href="#fn:57"><sup>57</sup></a><a class="footnote-ref" id="fnref:58" href="#fn:58"><sup>58</sup></a><a class="footnote-ref" id="fnref:59" href="#fn:59"><sup>59</sup></a><a class="footnote-ref" id="fnref:60" href="#fn:60"><sup>60</sup></a></li></ul> <h3>Crystalline chain clays</h3> <p>The <a href="/facts/Carbonate/FES1QHkN">carbonate</a> and <a href="/facts/Sulfate/yuWVxsUf">sulfate</a> clay minerals are much more soluble and hence are found primarily in desert soils where leaching is less active.<a class="footnote-ref" id="fnref:61" href="#fn:61"><sup>61</sup></a> </p> <h3>Amorphous clays</h3> <p>Amorphous clays are young, and commonly found in recent volcanic ash deposits such as <a href="/facts/Tephra/fYHmdLu1">tephra</a>.<a class="footnote-ref" id="fnref:62" href="#fn:62"><sup>62</sup></a> They are mixtures of <a href="/facts/Aluminium_oxide/wXM9P6pI">alumina</a> and <a href="/facts/Silicon_dioxide/A3WILp6J">silica</a> which have not formed the ordered crystal shape of <a href="/facts/Aluminosilicate/3gclsh2j">alumino-silica</a> 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 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.<a class="footnote-ref" id="fnref:63" href="#fn:63"><sup>63</sup></a> </p> <h3>Sesquioxide clays</h3> <p><a href="/facts/Sesquioxide/KbfWnDuI">Sesquioxide</a> clays are a product of heavy rainfall that has leached most of the silica from alumino-silica clay, leaving the less soluble oxides iron <a href="/facts/Hematite/716YBzGr">hematite</a> (Fe2O3), <a href="/facts/Iron_hydroxide/KDo8hLWn">iron hydroxide</a> (Fe(OH)3), <a href="/facts/Aluminium_hydroxide/e0pXvDGr">aluminium hydroxide</a> <a href="/facts/Gibbsite/MdBYjXPW">gibbsite</a> (Al(OH)3), hydrated manganese <a href="/facts/Birnessite/olIYIBKQ">birnessite</a> (MnO2), as can be observed in most <a href="/facts/Lateritic/3FYFwspo">lateritic</a> <a href="/facts/Weathering/uSYGlv0H">weathering</a> profiles of tropical soils.<a class="footnote-ref" id="fnref:64" href="#fn:64"><sup>64</sup></a> It takes hundreds of thousands of years of leaching to create sesquioxide clays.<a class="footnote-ref" id="fnref:65" href="#fn:65"><sup>65</sup></a> <i>Sesqui</i> 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 <a href="/facts/Sorption/nWDhtFkL">sorptive</a> process which can at least partly be inhibited in the presence of decomposed (<a href="/facts/Humus/vlhws4I0">humified</a>) organic matter.<a class="footnote-ref" id="fnref:66" href="#fn:66"><sup>66</sup></a> Sesquioxides have low <a href="/facts/Cation-exchange_capacity/4DoXx9Jk">CEC</a> but these variable-charge minerals are able to hold anions as well as cations.<a class="footnote-ref" id="fnref:67" href="#fn:67"><sup>67</sup></a> 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.<a class="footnote-ref" id="fnref:68" href="#fn:68"><sup>68</sup></a><a class="footnote-ref" id="fnref:69" href="#fn:69"><sup>69</sup></a><a class="footnote-ref" id="fnref:70" href="#fn:70"><sup>70</sup></a> </p> <h2 id="organic-colloids">Organic colloids</h2> <p><a href="/facts/Humus/vlhws4I0">Humus</a> is one of the two final stages of <a href="/facts/Decomposition/6Bve4pgl">decomposition</a> of organic matter. It remains in the soil as the organic component of the soil matrix while the other stage, <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a>, is freely liberated in the <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a> or reacts with <a href="/facts/Calcium/hf252CC0">calcium</a> to form the soluble <a href="/facts/Calcium_bicarbonate/j7rbUvab">calcium bicarbonate</a>. While humus may linger for a thousand years,<a class="footnote-ref" id="fnref:71" href="#fn:71"><sup>71</sup></a> 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 <a href="/facts/Lignin/CdaxpB3b">lignins</a> (30%) and complex <a href="/facts/Sugars/jb5KXRHT">sugars</a> (polyuronides, 30%), <a href="/facts/Proteins/Jxmagu3E">proteins</a> (30%), <a href="/facts/Waxes/MAXhjzNh">waxes</a>, and <a href="/facts/Fat/87HcP11s">fats</a> that are resistant to breakdown by microbes and can form <a href="/facts/Metal_complexes/1tCyeQUm">complexes with metals</a>, facilitating their downward migration (<a href="/facts/Podzolization/Y2fVnh0r">podzolization</a>).<a class="footnote-ref" id="fnref:72" href="#fn:72"><sup>72</sup></a> 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.<a class="footnote-ref" id="fnref:73" href="#fn:73"><sup>73</sup></a> Its chemical assay is 60% carbon, 5% nitrogen, some oxygen and the remainder hydrogen, sulfur, and phosphorus. On a dry weight basis, the <a href="/facts/Cation-exchange_capacity/4DoXx9Jk">CEC</a> of humus is many times greater than that of clay.<a class="footnote-ref" id="fnref:74" href="#fn:74"><sup>74</sup></a><a class="footnote-ref" id="fnref:75" href="#fn:75"><sup>75</sup></a><a class="footnote-ref" id="fnref:76" href="#fn:76"><sup>76</sup></a> </p><p>Humus plays a major role in the regulation of <a href="/facts/Carbon_dioxide_in_Earth%2527s_atmosphere/h6A4HSzy">atmospheric carbon</a>, through <a href="/facts/Carbon_sequestration/WDBvr9eU">carbon sequestration</a> in the soil profile, more especially in deeper horizons with reduced <a href="/facts/Biological_activity/jJ14jyvc">biological activity</a>.<a class="footnote-ref" id="fnref:77" href="#fn:77"><sup>77</sup></a> Stocking and destocking of <a href="/facts/Soil_carbon/IbEAIAyj">soil carbon</a> are under strong climate influence.<a class="footnote-ref" id="fnref:78" href="#fn:78"><sup>78</sup></a> They are normally balanced through an equilibrium between <a href="/facts/Primary_production/14RzcF5E">production</a> and <a href="/facts/Mineralization_(soil_science)/xPKCqQSu">mineralization</a> of organic matter, but the balance is in favour of destocking under present-day <a href="/facts/Climate_warming/rEN4BDE7">climate warming</a>,<a class="footnote-ref" id="fnref:79" href="#fn:79"><sup>79</sup></a> and more especially in <a href="/facts/Permafrost/SkFC7LMT">permafrost</a>.<a class="footnote-ref" id="fnref:80" href="#fn:80"><sup>80</sup></a> </p> <h2 id="carbon-and-terra-preta">Carbon and terra preta</h2> <p>In the extreme environment of high temperatures and the leaching caused by the heavy rain of <a href="/facts/Tropical_rain_forests/jTGAqqk5">tropical rain forests</a>, the clay and organic colloids are largely destroyed. The heavy rains wash the <a href="/facts/Alumino-silicate/3gclsh2j">alumino-silicate</a> clays from the soil leaving only <a href="/facts/Sesquioxide/KbfWnDuI">sesquioxide</a> clays of low <a href="/facts/Cation-exchange_capacity/4DoXx9Jk">CEC</a>. The high temperatures and humidity allow bacteria and fungi to virtually decay any organic matter on the rain-forest <a href="/facts/Forest_floor/ECKTb8Ig">floor</a> overnight and much of the nutrients are volatilized or leached from the soil and lost,<a class="footnote-ref" id="fnref:81" href="#fn:81"><sup>81</sup></a> leaving only a thin root mat lying directly on the mineral soil.<a class="footnote-ref" id="fnref:82" href="#fn:82"><sup>82</sup></a> However, carbon in the form of finely divided <a href="/facts/Charcoal/tpYGBHTn">charcoal</a>, also known as <a href="/facts/Black_carbon/yBb6hpLy">black carbon</a>, is far more stable than soil colloids and is capable of performing many of the functions of the soil colloids of sub-tropical soils.<a class="footnote-ref" id="fnref:83" href="#fn:83"><sup>83</sup></a> Soil containing substantial quantities of charcoal, of an anthropogenic origin, is called <a href="/facts/Terra_preta/W4ppqnYV">terra preta</a>. In <a href="/facts/Amazonia/MGVatclX">Amazonia</a> it testifies for the agronomic knowledge of past <a href="/facts/Amerindian/LG2r6wXm">Amerindian</a> civilizations.<a class="footnote-ref" id="fnref:84" href="#fn:84"><sup>84</sup></a> The <a href="/facts/Pantropical/29Y2gEhX">pantropical</a> peregrine earthworm <i><a href="/facts/Pontoscolex_corethrurus/sN8KCQEI">Pontoscolex corethrurus</a></i> has been suspected to contribute to the fine division of charcoal and its mixing to the mineral soil in the frame of present-day <a href="/facts/Slash-and-burn/2Bxk7Rjz">slash-and-burn</a> or <a href="/facts/Shifting_cultivation/RJlTae4l">shifting cultivation</a> still practiced by Amerindian tribes.<a class="footnote-ref" id="fnref:85" href="#fn:85"><sup>85</sup></a> Research into terra preta is still young but is promising. <a href="/facts/Fallow/G6t9P5yA">Fallow</a> periods "on the Amazonian Dark Earths can be as short as 6 months, whereas fallow periods on <a href="/facts/Oxisol/yyCvo0z0">oxisols</a> are usually 8 to 10 years long"<a class="footnote-ref" id="fnref:86" href="#fn:86"><sup>86</sup></a> The incorporation of charcoal to agricultural soil for improving water and nutrient retention has been called <a href="/facts/Biochar/z16ymbQ3">biochar</a>, being extended to other charred or carbon-rich by-products, and is now increasingly used in <a href="/facts/Sustainable_agriculture/XLdTEVAC">sustainable</a> <a href="/facts/Tropical_agriculture/9rVv0vL8">tropical agriculture</a>.<a class="footnote-ref" id="fnref:87" href="#fn:87"><sup>87</sup></a> Biochar also allows the irreversible sorption of pesticides and other pollutants, a mechanism by which their mobility, and thus their environmental risk, decreases.<a class="footnote-ref" id="fnref:88" href="#fn:88"><sup>88</sup></a> It has also been argued as a mean of <a href="/facts/Carbon_sequestration/WDBvr9eU">sequestering</a> more carbon in the soil, thereby mitigating the so-called <a href="/facts/Greenhouse_effect/LGdNYYj3">greenhouse effect</a>.<a class="footnote-ref" id="fnref:89" href="#fn:89"><sup>89</sup></a> However, the use of biochar is limited by the availability of wood or other products of <a href="/facts/Pyrolysis/h1cEegI7">pyrolysis</a> and by risks caused by concomitent <a href="/facts/Deforestation/L1TNhN9p">deforestation</a>.<a class="footnote-ref" id="fnref:90" href="#fn:90"><sup>90</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Physical_properties_of_soil/J5L9erg3">Physical properties of soil</a></li></ul> <h2 id="bibliography">Bibliography</h2> <ul><li>Donahue, Roy Luther; Miller, Raymond W.; Shickluna, John C. (1977). <a href="https://archive.org/details/soilsintroductio00dona"><i>Soils: An Introduction to Soils and Plant Growth</i></a>. Prentice-Hall. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0-13-821918-5.</li> <li><a href="http://ag.arizona.edu/pubs/garden/mg/soils/soils.html">"Arizona Master Gardener"</a>. Cooperative Extension, College of Agriculture, University of Arizona. Retrieved 27 May 2013.</li> <li>Stefferud, Alfred, ed. (1957). <a href="http://archive.org/stream/yoa1957#page/n18/mode/1up"><i>Soil: The Yearbook of Agriculture 1957</i></a>. United States Department of Agriculture. <a href="/facts/OCLC_(identifier)/Yqad8waq">OCLC</a> <a href="https://search.worldcat.org/oclc/704186906">704186906</a>. <ul><li>Kellogg. "<a href="//archive.org/stream/yoa1957#page/n17/mode/1up">We Seek; We Learn</a>". In Stefferud (1957).</li> <li>Simonson. "<a href="//archive.org/stream/yoa1957#page/n34/mode/1up">What Soils Are</a>". In Stefferud (1957).</li> <li>Russell. "<a href="//archive.org/stream/yoa1957#page/n49/mode/1up">Physical Properties</a>". In Stefferud (1957).</li> <li>Richards & Richards. "<a href="//archive.org/stream/yoa1957#page/n68/mode/1up">Soil Moisture</a>". In Stefferud (1957).</li> <li>Wadleigh. "<a href="//archive.org/stream/yoa1957#page/n57/mode/1up">Growth of Plants</a>". In Stefferud (1957).</li> <li>Allaway. "<a href="//archive.org/stream/yoa1957#page/n87/mode/1up">pH, Soil Acidity, and Plant Growth</a>". In Stefferud (1957).</li> <li>Coleman & Mehlich. "<a href="//archive.org/stream/yoa1957#page/n92/mode/1up">The Chemistry of Soil pH</a>". In Stefferud (1957).</li> <li>Dean. "<a href="//archive.org/stream/yoa1957#page/n100/mode/1up">Plant Nutrition and Soil Fertility</a>". In Stefferud (1957).</li> <li>Allison. "<a href="//archive.org/stream/yoa1957#page/n105/mode/1up">Nitrogen and Soil Fertility</a>". In Stefferud (1957).</li> <li>Olsen & Fried. "<a href="//archive.org/stream/yoa1957#page/n115/mode/1up">Soil Phosphorus and Fertility</a>". In Stefferud (1957).</li> <li>Reitemeier. "<a href="//archive.org/stream/yoa1957#page/n123/mode/1up">Soil Potassium and Fertility</a>". In Stefferud (1957).</li> <li>Jordan & Reisenauer. "<a href="//archive.org/stream/yoa1957#page/n129/mode/1up">Sulfur and Soil Fertility</a>". In Stefferud (1957).</li> <li>Holmes & Brown. "<a href="//archive.org/stream/yoa1957#page/n133/mode/1up">Iron and Soil Fertility</a>". In Stefferud (1957).</li> <li>Seatz & Jurinak. "<a href="//archive.org/stream/yoa1957#page/n138/mode/1up">Zinc and Soil Fertility</a>". In Stefferud (1957).</li> <li>Russel. "<a href="//archive.org/stream/yoa1957#page/n145/mode/1up">Boron and Soil Fertility</a>". In Stefferud (1957).</li> <li>Reuther. "<a href="//archive.org/stream/yoa1957#page/n154/mode/1up">Copper and Soil Fertility</a>". In Stefferud (1957).</li> <li>Sherman. "<a href="//archive.org/stream/yoa1957#page/n162/mode/1up">Manganese and Soil Fertility</a>". In Stefferud (1957).</li> <li>Stout & Johnson. "<a href="//archive.org/stream/yoa1957#page/n167/mode/1up">Trace Elements</a>". In Stefferud (1957).</li> <li>Broadbent. "<a href="//archive.org/stream/yoa1957#page/n179/mode/1up">Organic Matter</a>". In Stefferud (1957).</li> <li>Clark. "<a href="//archive.org/stream/yoa1957#page/n185/mode/1up">Living Organisms in the Soil</a>". In Stefferud (1957).</li> <li>Flemming. "<a href="//archive.org/stream/yoa1957#page/n367/mode/1up">Soil Management and Insect Control</a>". In Stefferud (1957).</li></ul></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>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. doi:10.2136/sssaj2005.0117. 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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 27 November 2022. <a href="978-0-87933-152-8" target="_blank">978-0-87933-152-8</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Donahue, Miller & Shickluna 1977, pp. 101–02. - Donahue, Roy Luther; Miller, Raymond W.; Shickluna, John C. (1977). Soils: An Introduction to Soils and Plant Growth. Prentice-Hall. ISBN 978-0-13-821918-5. <a href="https://archive.org/details/soilsintroductio00dona" target="_blank">https://archive.org/details/soilsintroductio00dona</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Bergaya, Faïza; Beneke, Klaus; Lagaly, Gerhard. "History and perspectives of clay science" (PDF). University of Kiel. 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(1977). Soils: An Introduction to Soils and Plant Growth. Prentice-Hall. ISBN 978-0-13-821918-5. <a href="https://archive.org/details/soilsintroductio00dona" target="_blank">https://archive.org/details/soilsintroductio00dona</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>Donahue, Miller & Shickluna 1977, pp. 101–07. - Donahue, Roy Luther; Miller, Raymond W.; Shickluna, John C. (1977). Soils: An Introduction to Soils and Plant Growth. Prentice-Hall. ISBN 978-0-13-821918-5. <a href="https://archive.org/details/soilsintroductio00dona" target="_blank">https://archive.org/details/soilsintroductio00dona</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>Aylmore, L.A. Graham; Quirk, James P. (1971). "Domains and quasicrystalline regions in clay systems". Soil Science Society of America Journal. 35 (4): 652–54. Bibcode:1971SSASJ..35..652Q. doi:10.2136/sssaj1971.03615995003500040046x. 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Retrieved 11 December 2022. <a href="https://www.srs.fs.usda.gov/pubs/ja/ja_barton002.pdf" target="_blank">https://www.srs.fs.usda.gov/pubs/ja/ja_barton002.pdf</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>Schoonheydt, Robert A.; Johnston, Cliff T. (2011). "The surface properties of clay minerals". In Brigatti, Maria Franca; Mottana, Annibale (eds.). Layered mineral structures and their application in advanced technologies. Twickenham, United Kingdom: Mineralogical Society of Great Britain & Ireland. pp. 337–73. Retrieved 11 December 2022. <a href="https://www.researchgate.net/publication/280884094" target="_blank">https://www.researchgate.net/publication/280884094</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li> <li id="fn:29"><p>Donahue, Miller & Shickluna 1977, p. 107. - Donahue, Roy Luther; Miller, Raymond W.; Shickluna, John C. (1977). Soils: An Introduction to Soils and Plant Growth. Prentice-Hall. 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