Soil matrix

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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]

Contents

Gravel, sand and silt

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 clays

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 clays

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]

Crystalline chain clays

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 clays

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 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 clays

silica-sesquioxide San Joaquin soil profile.png
silica-sesquioxide

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 colloids

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 preta

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 also

Related Research Articles

<span class="mw-page-title-main">Kaolinite</span> Phyllosilicate clay mineral

Kaolinite ( KAY-ə-lə-nyte, -⁠lih-; also called kaolin) is a clay mineral, with the chemical composition: Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica (SiO4) linked through oxygen atoms to one octahedral sheet of alumina (AlO6).

<span class="mw-page-title-main">Mineral</span> Crystalline chemical element or compound formed by geologic processes

In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.

<span class="mw-page-title-main">Clay</span> Fine grained soil

Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impurities, such as a reddish or brownish colour from small amounts of iron oxide.

<span class="mw-page-title-main">Soil</span> Mixture of organic matter, minerals, gases, liquids, and organisms that together support life

Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.

<span class="mw-page-title-main">Shale</span> Fine-grained, clastic sedimentary rock

Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

<span class="mw-page-title-main">Weathering</span> Deterioration of rocks and minerals through exposure to the elements

Weathering is the deterioration of rocks, soils and minerals through contact with water, atmospheric gases, sunlight, and biological organisms. Weathering occurs in situ, and so is distinct from erosion, which involves the transport of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity.

<span class="mw-page-title-main">Gibbsite</span> Form of aluminium hydroxide, mineral

Gibbsite, Al(OH)3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al(OH)3 (but sometimes as α-Al(OH)3). It is also sometimes called hydrargillite (or hydrargyllite).

<span class="mw-page-title-main">Bentonite</span> Rock type or absorbent swelling clay

Bentonite is an absorbent swelling clay consisting mostly of montmorillonite which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

<span class="mw-page-title-main">Clay mineral</span> Fine-grained aluminium phyllosilicates

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2Si2O5(OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

<span class="mw-page-title-main">Chlorite group</span> Type of mineral

The chlorites are the group of phyllosilicate minerals common in low-grade metamorphic rocks and in altered igneous rocks. Greenschist, formed by metamorphism of basalt or other low-silica volcanic rock, typically contains significant amounts of chlorite.

<span class="mw-page-title-main">Montmorillonite</span> Phyllosilicate group of minerals

Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, is a 2:1 clay, meaning that it has two tetrahedral sheets of silica sandwiching a central octahedral sheet of alumina. The particles are plate-shaped with an average diameter around 1 μm and a thickness of 0.96 nm; magnification of about 25,000 times, using an electron microscope, is required to resolve individual clay particles. Members of this group include saponite, nontronite, beidellite, and hectorite.

<span class="mw-page-title-main">Illite</span> Group of non-expanding clay minerals

Illite, also called hydromica or hydromuscovite, is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina octahedron (O) – silica tetrahedron (T) layers. The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling. Structurally, illite is quite similar to muscovite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer potassium. The chemical formula is given as (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)], but there is considerable ion (isomorphic) substitution. It occurs as aggregates of small monoclinic grey to white crystals. Due to the small size, positive identification usually requires x-ray diffraction or SEM-EDS analysis. Illite occurs as an altered product of muscovite and feldspar in weathering and hydrothermal environments; it may be a component of sericite. It is common in sediments, soils, and argillaceous sedimentary rocks as well as in some low grade metamorphic rocks. The iron-rich member of the illite group, glauconite, in sediments can be differentiated by x-ray analysis.

<span class="mw-page-title-main">Smectite</span> Mineral mixture of phyllosilicates

A smectite is a mineral mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 (TOT) structure and belong to the clay minerals. Smectites mainly consist of montmorillonite, but can often contain secondary minerals such as quartz and calcite.

<span class="mw-page-title-main">Halloysite</span> Aluminosilicate clay mineral

Halloysite is an aluminosilicate clay mineral with the empirical formula Al2Si2O5(OH)4. Its main constituents are oxygen (55.78%), silicon (21.76%), aluminium (20.90%), and hydrogen (1.56%). It is a member of the kaolinite group. Halloysite typically forms by hydrothermal alteration of alumino-silicate minerals. It can occur intermixed with dickite, kaolinite, montmorillonite and other clay minerals. X-ray diffraction studies are required for positive identification. It was first described in 1826, and subsequently named after, the Belgian geologist Omalius d'Halloy.

Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.

<span class="mw-page-title-main">Chamosite</span> Phyllosilicate mineral member of the chlorite group

Chamosite is the Fe2+end member of the chlorite group. A hydrous aluminium silicate of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore. Like other chlorites, it is a product of the hydrothermal alteration of pyroxenes, amphiboles and biotite in igneous rock. The composition of chlorite is often related to that of the original igneous mineral so that more Fe-rich chlorites are commonly found as replacements of the Fe-rich ferromagnesian minerals (Deer et al., 1992).

<span class="mw-page-title-main">Soil aggregate stability</span> Ability of soil aggregates to resist breaking apart when exposed to external forces such as erosion

Soil aggregate stability is a measure of the ability of soil aggregates—soil particles that bind together—to resist breaking apart when exposed to external forces such as water erosion and wind erosion, shrinking and swelling processes, and tillage. Soil aggregate stability is a measure of soil structure and can be affected by soil management.

<span class="mw-page-title-main">Aluminoceladonite</span> Mineral

Aluminoceladonite is a low-temperature potassium dioctahedral mica mineral which is an end-member in the illite-aluminoceladonite solid solution series. The chemical formula for aluminoceladonite is K(Mg,Fe2+)Al(Si4O10)(OH)2.

The physical properties of soil, in order of decreasing importance for ecosystem services such as crop production, are texture, structure, bulk density, porosity, consistency, temperature, colour and resistivity. Soil texture is determined by the relative proportion of the three kinds of soil mineral particles, called soil separates: sand, silt, and clay. At the next larger scale, soil structures called peds or more commonly soil aggregates are created from the soil separates when iron oxides, carbonates, clay, silica and humus, coat particles and cause them to adhere into larger, relatively stable secondary structures. Soil bulk density, when determined at standardized moisture conditions, is an estimate of soil compaction. Soil porosity consists of the void part of the soil volume and is occupied by gases or water. Soil consistency is the ability of soil materials to stick together. Soil temperature and colour are self-defining. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil. These properties vary through the depth of a soil profile, i.e. through soil horizons. Most of these properties determine the aeration of the soil and the ability of water to infiltrate and to be held within the soil.

Seventeen elements or nutrients are essential for plant growth and reproduction. They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl). Nutrients required for plants to complete their life cycle are considered essential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential, although some of them, such as silicon (Si), have been shown to improve nutrent availability, hence the use of stinging nettle and horsetail macerations in Biodynamic agriculture. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation, the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant. A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.

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