The mineral part of soil is the main component of soils.
The mineral part of the soil arose during the processes of weathering of rocks and minerals of the upper layers of the lithosphere and their transformations. This is confirmed by the similar chemical composition of lithosphere and soils. Soil was formed under the combined influence of physical and chemical factors on the mineral nature, as well as living organisms, especially plants and microorganisms.
The geochemical composition of the soil is inherited from the soil-forming rocks. Thus, high content of silicon oxide determines its high content in soil. Soils enriched with alkaline earth elements are formed on carbonate rocks.
Biological factors of soil formation
Thanks to the activity of living organisms, the carbon content of soil compared to the Earth’s crust has increased by 20 times and the nitrogen content by 10 times.
Soil formation in natural conditions proceeds very slowly. Application of fertilizers and agricultural techniques allows to accelerate this process. Thus, the application of fertilizers enhances the vital activity of plants and soil microflora, which leads to the accumulation of organic matter and biologically important elements.
Silicates and aluminosilicates
According to chemical structure minerals are divided into silicates and aluminosilicates. From silicates for all types of soils in sand and dust fractions prevails quartz – SiO2 characterized by low absorption capacity and high water permeability. In soils, its content is usually more than 60% in sandy soils – more than 90%. Quartz is chemically inert and has high strength.
The basis of the mineral part of soils is silicon-oxygen compounds. The most common soil mineral is quartz, or silicon oxide. Aluminum and iron are predominantly composed of aluminosilicate and ferrosilicate minerals. Silicon and oxygen atoms form tightly bound SiO4 groups that have a tetrahedral structure. Due to the tetravalence of silicon, SiO4 groups can form various complex combinations of compounds.
In the structures of minerals of finely dispersed soil fractions, silica-oxygen tetrahedrons can be combined into layers, chains, or separate groups of SiO4 tetrahedrons. The total oxidation degree of these groups is negative. In complex combinations of silica-oxygen tetrahedrons, some of the silicon atoms can be replaced by aluminum atoms.
In the crystal lattice of quartz, SiO4 tetrahedrons are connected through oxygen atoms to four other SiO4 tetrahedrons. The general formula for quartz is (SiO2)n. In the crystal structure of feldspars, some of the silicon atoms are replaced by aluminum. To compensate the emerging negative charge of the silica-alumina framework they include atoms of sodium, calcium and others that are embedded in the “cavities” of the lattice. Thus, albite feldspar has the formula Na[SiAlO8].
Aluminum in tetrahedral coordination with oxygen ions or hydroxyl group OH forms octahedral groups, where the aluminum atom is surrounded by six oxygen atoms or hydroxyl groups. The formula of such a compound (layer) [Al(OH)3]-n corresponds to the mineral gibbsite (hydrargyllite).
The structure of such minerals can be represented as follows:
The formula reflects the chemical composition of the layer (package), and the dots represent the inter-package intervals.
The mineral part of soils consists of primary and secondary minerals. In sandy and sandy loam soils mainly primary minerals prevail, loamy soils consist of primary and secondary minerals, and clayey soils mainly consist of secondary minerals with quartz admixture. Division of minerals into primary, i.e. with particle size more than 0,001 mm and secondary less than 0,001 mm is conditional, as the latter are the products of physical and chemical weathering of primary and formation of hydrates of semi-hydrated silica oxides and other compounds at that.
During weathering, hydrolysis of feldspar and mica leads to replacement of metal cations in the crystal lattices of minerals by hydrogen ions:
Physical and chemical weathering is inseparable from the biological transformation of rocks and minerals under the influence of living organisms and their products.
Primary soil minerals
Primary soil minerals are minerals that have passed from the earth’s crust into the soil without changing their structure. They include minerals of soil skeleton:
- quartz and its varieties,
- feldspars: orthoclases, plagioclases, mica, hornblende, augite, tourmaline, magnetite, calcite, dolomite, etc.
Primary soil minerals are part of the parent soil-forming rocks formed as a result of weathering and destruction of rocks. They are present in soils in the form of sandy particles sized 0.05 to 1.0 mm and dusty particles sized 0.001 to 0.05 mm. In small amounts are present in the form of silty particles smaller than 1 μm and colloidal particles smaller than 0.25 μm.
From primary minerals under the influence of physical and chemical processes such as hydration, hydrolysis, oxidation and life activity of soil organisms oxide hydrates such as R2O3 and silicon oxide (silica earths), mineral salts as well as secondary minerals are formed.
Destruction of feldspars and mica releases potassium, calcium, magnesium, iron and some other plant nutrients.
Secondary soil minerals
Secondary minerals, or clay minerals, are kaolinite, montmorillonite, hydromica, etc. Secondary minerals, or clay minerals, such as kaonite, montmorillonite, hydromica, etc., are mostly present as muddy and colloidal particles, less often as dusty particles.
In crystalline lattices of aluminosilicate minerals of fine-dispersed fraction of soils there are combinations of silica-oxygen tetrahedral and alumohydroxyl octahedral layers.
The crystal lattice of kaolinite is formed by packages of two layers bound together by oxygen atoms: a tetrahedral silica-oxygen layer and an octahedral alumohydroxyl layer:
The crystal lattices of montmorillonite and hydromica are formed by one alumohydroxyl layer and two silicic acid layers attached to it:
The bond between the packages in kaolinite group minerals is stronger, and the inter-package spaces are small. Therefore, the interaction of microcrystalline particles with water occurs only on the outer surface.
In minerals of montmorillonite group the interpack spaces are larger, and the bond between the packages is weaker, so water molecules can penetrate into the interpack spaces. Cations located on the surface of the particles and in the interpack spaces take part in cation exchange with the soil solution of minerals of this group. This explains high exchangeable absorptivity of minerals of montmorillonite group and availability of non-exchangeable absorption of cations. This group is characterized by high dispersibility, swellability, stickiness and viscosity.
Soil clay minerals are divided into:
- montmorillonite (montmorillonite – Al2Si4O10(OH)2·nН2O, beidelite – Al3Si3O9(OH)3·nH2O, nontronite, saponite, sonite, etc.).
- kaolinite (kaolinite – Al2Si2O5(OH)4 and halloysite Al2Si2O5(OH)4·2Н2O),
- hydromica (hydromuscovite (illite) (К,Н3O)Аl2(OН)2[Аl,Si]4·nН2O, hydrobiotite, vermiculite),
- minerals of halved oxides (hematite, bemitite, hydrargyllite, goethite, etc.).
The greatest absorption capacity possesses montmorillonite minerals, the lowest – kaolinite. Thus, the absorption capacity of kaolinite is 8-15 times less than that of montmorillonite. This peculiarity has significance in absorption of fertilizers.
The montmorillonite Мg3(OН)4[Si4O8(OН)2]·Н2O is characterized by high dispersion: 40-50 % of colloid (size less than 0,0001 mm) and 60-80 % of silty (size less than 0,001 mm) particles. It prevails in chernozems. Because of high dispersibility its absorption capacity reaches 120 mg-eq/100 g, it swells under humidifying. Cations (K+, NH4+, Na+, Ca2+, etc.) can penetrate into interplanar space of crystal structure, which when soil dehydration (drying) are fixed and become unavailable for plants until the next saturation with moisture.
Secondary aluminosilicate minerals are found in the soil in the form of fine crystals and are characterized by high absorption capacity.
The kaolinite group is less dispersed, has small swelling and stickiness, absorption capacity is no more than 25 mg-eq/100 g of soil, particle size is less than 0.001 mm, water permeability is good.
In sod-podzolic and chernozem soils formed on overlying loams, montmorillonite and hydromica prevail in the composition of highly dispersed fractions. In red soils, yellow soils and sod-podzol soils formed on the products of ancient humid weathering of granite, the content of kaolinite group minerals is much higher.
Hydromica is formed from mica, has unstable chemical composition, and by its physical properties occupies an intermediate position between montmorillonite and kaolinite. Hydromica is present in all soils in muddy and colloidal fractions. Because of high dispersibility they have big surface and absorbing ability.
Mica determine agrochemical and agrophysical features of soil. They are the source of potassium nutrition of plants, their composition includes up to 5-7% of potassium. Energy of colloidal absorption of potassium is high and as a result in absorbing complex it contains 0.510 mmol/100 g of soil. Red soils and laterites due to small content of mica and hydromica and an excess of kaolinite minerals of the low-potassium group are characterized by potassium deficiency.
Weakly crystallized minerals having significant influence on absorbing ability of the soil are allophane, free silicic acid, different acids and their salts. The mineral part of soil includes amorphous substances: hydrates of oxides of aluminum Al2O3 · nH2O, iron Fe2O3 · nH2O and silicon SiO2 · nH2O. Their greatest content is noted in the red earth soils and yellow earth soils. At isoelectric points, these substances form amorphous precipitates, which form new minerals as they age:
Soil contains mineral salts: carbonates, sulfates, nitrates, chlorides, phosphates of calcium, magnesium, potassium, sodium, iron, aluminum and manganese. All nitrates and chlorides, as well as potassium and sodium salts are well soluble in water, but their content in soils (except for saline soils) is relatively small. Low-soluble salts (calcium and magnesium carbonates and calcium sulfate) are found in solid phase in some soils in significant quantities, while insoluble calcium, magnesium, iron and aluminum phosphates are found in all soils.
Table. Content of trace elements in soil and lithosphere, mass %
In addition to macronutrients, there are trace elements in the soil. Soil-forming rocks are the main source of them in the soil. Thus, soils formed on the weathering products of acid rocks, i.e. granites, liparites, granite-porphyries, are poor in nickel, cobalt, copper. Soils formed on the weathering products of basic rocks (basalts, gabbro), on the contrary, are enriched with these elements.
Some trace elements, such as iodine, boron, fluorine, selenium, arsenic can enter the soil from the atmospheric air, volcanic eruptions and precipitation. For iodine and fluorine, these are the main sources.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.