Soil composition
Soil composition includes:
- solid phase;
- liquid phase, or soil solution;
- gas (gaseous) phase, or soil air.
Soil is an independent natural-historical organomineral natural body formed on the Earth's surface as a result of long-term influence of biotic, abiotic, and anthropogenic factors, consisting of solid mineral and organic particles, water, and air and possessing specific genetic-morphological features and properties that create suitable conditions for plant growth and development. Soil is a complex self-regulating multicomponent bio-inert unified system.
Soil is a homogeneous system consisting of three phases: solid, liquid, and gaseous.
The solid phase consists of mineral and organic parts and represents the soil skeleton. It includes solid particles, between which there are free voids—pores filled with water or air.
The ratio of solid, liquid, and gaseous phases determines the regime of plant provision with terrestrial life factors. For different soil types it is different, and its change makes it possible to regulate plant living conditions. The ratio 2:1:1 is considered optimal, i.e., solid phase—50%, liquid and gaseous—25% each.
The creation and maintenance of an optimal ratio of soil phase volumes is achieved by a number of methods of soil tillage, amelioration, application of fertilizers, owing to which the water, thermal, air, and nutrient regimes improve, thereby creating favorable conditions for plant growth and development.
Gas phase
Gas phase is the result of interaction between atmospheric air and gases formed in the soil. In its composition, a higher content of carbon dioxide compared with atmospheric air is noted—0.3–1%, sometimes up to 2–3% or more—and a lower oxygen content. The gas phase is characterized by high mobility, which depends on many conditions: organic matter content, weather conditions, type of vegetation, etc.
Sufficient oxygen content in the soil creates favorable conditions for the activity of aerobic microorganisms. Conversely, when it is deficient, conditions develop for anaerobic bacteria, which are often pathogenic for plants.
The volume of soil air is in dynamic equilibrium with the liquid phase: the more water, the less air. Gas exchange processes in the soil occur continuously as a result of decomposition of organic substances, respiration of plant roots and soil organisms, and also certain chemical reactions. As a result of gas exchange, the above-soil air is enriched with carbon dioxide, improving photosynthesis conditions. When carbon dioxide interacts with water of the liquid phase, weak acidification of the soil solution occurs according to the reaction:
Acidification promotes the transition of certain mineral substances of the solid phase, for example, phosphates and calcium sulfate, into a plant-available form. At the same time, an excess of carbon dioxide leads to oxygen deficiency and the creation of anaerobic conditions, which is observed during waterlogging and overcompaction of soils. Oxygen deficiency in the gas phase inhibits the growth and development of microorganisms and plants, impedes nutrient uptake, and intensifies reduction processes in the liquid and solid phases.
Soil air is concentrated in non-capillary pores, that is, in large soil interstices. When all pores are filled with water, soil air is displaced; conversely, if the soil is dry, air fills all pores—capillary and non-capillary.
The most optimal water-to-air ratio develops in loose, structured, cultivated, and tilled soils. Regulation of the water and air regimes of soils by appropriate treatments in combination with the use of fertilizers and ameliorants improves root and air nutrition of plants, thereby increases the quantity and quality of products, and promotes the development of soil biota.
Liquid phase
Liquid phase, or soil solution, is a solution of mineral and gaseous substances soluble in water. It is the most active and dynamic phase of the soil, from which plants assimilate nutrients and at the same time interaction of plants with fertilizers, ameliorants, solid and gaseous phases occurs.
The soil solution includes cations (Ca2+, Mg2+, H+, Na+, K+, NH4+, etc.), anions (HCO3-, OH-, Cl-, NO3-, SO4-, H2PO4-, etc.), water-soluble organic compounds and soluble gases CO2, O2, NH3, etc. The entry of ions into the soil solution occurs from the solid and gaseous phases, fertilizers and ameliorants, soil biota excretions, atmospheric precipitation, and groundwater. Thus, the composition and concentration, acidity, buffering capacity, and osmotic pressure of the soil solution are dynamic and determined by soil-climatic conditions and anthropogenic impact.
The concentration of salts in the soil solution depends on properties, mineral composition, soil type, natural conditions, degree of salinization and migration of salts along the soil profile, anthropogenic impact, etc. The concentration of various salts can vary from thousandths to hundredths of a percent (10–200 mg/L) in low-fertility soils to 1% or more (>10,000 mg/L) in strongly saline soils (solonchaks), and in medium-fertility soils—about 500 mg/L. An excess of salts above 2000 mg/L usually negatively affects agricultural crops, especially in the first 2–4 weeks after seed germination. With age, resistance to high concentrations increases.
The properties of the liquid phase as a whole are determined by the soil water regime.
Solid phase
The solid phase of soil consists of:
- mineral part, whose share is from 90% to 99.5%;
- organic part, or soil organic matter, which accounts for from 0.5% to 10%.
The organic part, or soil organic matter, is the remains of plant and animal organisms and products of their decomposition and neosynthesis, among which humus predominates.
Oxygen, silicon, aluminum, and iron account for almost 93% of the solid phase, carbon, potassium, and calcium—4.6%, and 2.5% for all remaining elements. Carbon, oxygen, hydrogen, phosphorus, and sulfur are contained in the mineral and organic parts, whereas nitrogen—almost entirely in the organic part.
Table. Average chemical composition of the solid phase of soil (% by mass) according to A. P. Vinogradov.
| Element | Content | Element | Content | Element | Content |
|---|---|---|---|---|---|
| Oxygen | 49,0 | Barium | 0,05 | Gallium | 0,001 |
| Silicon | 33,0 | Strontium | 0,03 | Tin | 0,001 |
| Aluminum | 7,1 | Zirconium | 0,03 | Cobalt | 8·10-4 |
| Iron | 3,7 | Fluorine | 0,02 | Thorium | 6·10-4 |
| Carbon | 2,0 | Chromium | 0,02 | Arsenic | 5·10-4 |
| Calcium | 1,3 | Chlorine | 0,01 | Iodine | 5·10-4 |
| Potassium | 1,3 | Vanadium | 0,01 | Cesium | 5·10-4 |
| Sodium | 0,6 | Rubidium | 0,006 | Molybdenum | 3·10-4 |
| Magnesium | 0,6 | Zinc | 0,005 | Uranium | 1·10-4 |
| Hydrogen | 0,5 | Cerium | 0,005 | Beryllium | 1·10-4 |
| Titanium | 0,46 | Nickel | 0,004 | Germanium | 1·10-4 |
| Nitrogen | 0,10 | Lithium | 0,003 | Cadmium | 5·10-5 |
| Phosphorus | 0,08 | Copper | 0,002 | Selenium | 1·10-6 |
| Sulfur | 0,08 | Boron | 0,001 | Mercury | 1·10-6 |
| Manganese | 0,08 | Lead | 0,001 | Radium | 8·10-11 |
Mineral part of soil is the main component of soils; it consists of fragments and particles of primary rocks and minerals, secondary, i.e., newly formed minerals, oxides, salts, and other compounds formed in the process of weathering and soil formation. The mineral part includes all ash substances and 1–3% of nitrogen of the total amount.
The mineral part of soil originated 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 the lithosphere and soils. The soil cover formed under the combined influence on mineral nature of physical and chemical factors, as well as living organisms, primarily plants and microorganisms.
The geochemical composition of soil is inherited from soil-forming rocks. Thus, a high content of silicon oxide determines its high content in the soil. On carbonate rocks, soils enriched in alkaline earth elements are formed.
Silicates and aluminosilicates
According to chemical structure, minerals are divided into silicates and aluminosilicates. Among silicates, for all soil types in the sand and silt fractions, quartz predominates—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.
Groups of silicon tetrahedron linkages.
The basis of the mineral part of soils consists of silica-oxygen compounds. The most common soil mineral is quartz, or silicon oxide. Aluminum and iron are mainly part of aluminosilicate and ferrosilicate minerals. Silicon and oxygen atoms form strongly bonded SiO4 groups having a tetrahedral structure. Due to the tetravalency of silicon, SiO4 groups can form various complex combinations with each other.
In the structures of minerals of finely dispersed soil fractions, silica-oxygen tetrahedra can connect into layers, chains, or individual groups of SiO4 tetrahedra. The total oxidation state of these groups is negative. In complex combinations of silica-oxygen tetrahedra, some silicon atoms can be replaced by aluminum atoms.
Groups of silicon tetrahedron linkages.
In the crystal lattice of quartz, SiO4 tetrahedra are connected to each other through oxygen atoms with four other SiO4 tetrahedra. The general formula of quartz is (SiO2))n. In the crystal structure of feldspars, some silicon atoms are replaced by aluminum. To compensate for the arising negative charge of the silica-alumina-oxygen framework, atoms of sodium, calcium, and others are included in their composition, embedding in the "cavities" of the lattice. Thus, the feldspar albite has the formula Na[SiAlO8].
Aluminum in tetrahedral coordination with oxygen ions or the 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 (hydrargillite).
The structure of such minerals can be represented as follows:
The formula reflects the chemical composition of the layer (packet), and the dots—interpacket spaces.
The mineral part of soils consists of primary and secondary minerals. In sandy and sandy loam soils, primary minerals mainly predominate; loamy soils consist of primary and secondary minerals, while clayey soils—mainly of secondary minerals with an admixture of quartz. The division of minerals into primary, i.e., with a particle size greater than 0.001 mm, and secondary, less than 0.001 mm, is conventional, since the latter are products of physicochemical weathering of primary minerals and the formation of hydrates of sesquioxides, silica, and other compounds.
During weathering, hydrolysis of feldspar and mica leads to replacement of metal cations in the crystal lattices of minerals by hydrogen ions:
Physicochemical weathering is inseparable from the biological transformation of rocks and minerals under the influence of living organisms and their metabolic products.
Primary soil minerals
Primary soil minerals are minerals that passed from the Earth's crust into the soil without changing their structure. These include minerals of the soil skeleton:
- quartz and its varieties;
- feldspars: orthoclases, plagioclases, micas, hornblendes, augite, tourmaline, magnetite, calcite, dolomite, etc.
Primary soil minerals are part of parent soil-forming rocks formed as a result of weathering and destruction of rocks. In soils, they are present as sand particles ranging in size from 0.05 to 1.0 mm and silt particles ranging in size from 0.001 to 0.05 mm. In small amounts, they are present as clay particles less than 1 µm in size and colloidal particles less than 0.25 µm in size.
From primary minerals, under the influence of physicochemical processes such as hydration, hydrolysis, oxidation, and the activity of soil organisms, hydrates of sesquioxides and silicas, mineral salts, and also secondary minerals are formed.
During the destruction of feldspars and micas, potassium, calcium, magnesium, iron, and some other nutrients for plants are released.
Secondary soil minerals
Secondary minerals, or clay minerals, are kaolinite, montmorillonite, hydromicas, etc. They are mainly represented as clay and colloidal particles, less often as silt particles.
In the crystal lattices of aluminosilicate minerals of the finely dispersed soil fraction, combinations of silica-oxygen tetrahedral and aluminum-hydroxyl octahedral layers lie.
The crystal lattice of kaolinite is formed by packets of two layers connected to each other by oxygen atoms: a tetrahedral silica-oxygen layer and an octahedral aluminum-hydroxyl layer:
The crystal lattices of montmorillonite and hydromicas are formed by one aluminum-hydroxyl layer and two silica-oxygen layers attached to it:
The bond between packets in minerals of the kaolinite group is stronger, and the interpacket spaces are small. Therefore, the interaction of microcrystalline particles with water occurs only on the outer surface.
In minerals of the montmorillonite group, the interpacket spaces are larger, and the bond between packets is weaker, so water molecules can penetrate into the interpacket spaces. In cation exchange with the soil solution of minerals of this group, cations located on the surface of particles and in the interpacket spaces participate. This explains the high exchange absorption capacity of montmorillonite-group minerals and the presence of non-exchange cation absorption. This group is characterized by high dispersity, swelling, stickiness, and viscosity.
Soil clay minerals are divided into:
- montmorillonite (montmorillonite — Al2Si4O10(OH)2·nH2O, beidellite — Al3Si3O9(OH)3·nH2O, nontronite, saponite, sauconite, etc.).
- kaolinite (kaolinite — Al2Si2O5(OH)4 and halloysite Al2Si2O5(OH)4·2H2O),
- hydromicas (hydromuscovite (illite) (K,H3O)Al2(OH)2[Al,Si]4·nH2O, hydrobiotite, vermiculite),
- sesquioxide minerals (hematite, boehmite, hydrargillite, goethite, etc.).
Montmorillonite minerals have the greatest absorption capacity, kaolinite the least. Thus, the absorption capacity of kaolinite is 8–15 times less than that of montmorillonite. This feature is important in fertilizer absorption.
Montmorillonite — Mg3(OH)4[Si4O8(OH)2]·H2O — is characterized by high dispersity: 40–50% colloidal (size less than 0.0001 mm) and 60–80% clay (size less than 0.001 mm) particles. It predominates in chernozems. Due to its high dispersity, the absorption capacity reaches 120 mg·eq/100 g; it swells when moistened. Cations (K+, NH4+, Na+, Ca2+, etc.) can penetrate into the interlayer space of the crystal structure; upon dehydration (drying) of the soil, they are fixed and become unavailable to plants until the next moisture saturation.
Secondary aluminosilicate minerals are present in the soil as finely dispersed crystals and are characterized by high absorption capacity.
The kaolinite group is less dispersed, has slight swelling and stickiness, absorption capacity—no more than 25 mg·eq/100 g of soil, particle size less than 0.001 mm, and good water permeability.
In sod-podzolic and chernozem soils formed on mantle loams, montmorillonite and hydromicas predominate in the composition of highly dispersed fractions. In red soils, yellow soils, and sod-podzolic soils formed on products of ancient humid weathering of granite, the content of kaolinite-group minerals is significantly higher.
Hydromicas are formed from micas, have an inconsistent chemical composition, and in physical properties occupy an intermediate position between montmorillonite and kaolinite. Hydromicas are present in all soils in the clay and colloidal fractions. Due to their high dispersity, they have a large surface area and absorption capacity.
Micas determine the agrochemical and agrophysical properties of soil. They are a source of potassium plant nutrition; their composition includes up to 5–7% potassium. The energy of colloidal absorption of potassium is high, as a result of which the absorbing complex contains 0.510 mmol/100 g of soil. Red soils and laterites, due to the small content of micas and hydromicas and an excess of kaolinite-group minerals with low potassium content, are characterized by potassium deficiency.
Weakly crystallized minerals that significantly affect soil absorption capacity include allophane, free silicic acid, various acids, and their salts. The mineral part of soil includes amorphous substances: hydrates of aluminum oxides Al2O3 ⋅ nH2O, iron Fe2O3 ⋅ nH2O, and silicon SiO2 ⋅ nH2O. Their highest content is noted in red soils and yellow soils. At isoelectric points, these substances form amorphous precipitates, which, as they age, form new minerals:
Mineral salts
Soil contains mineral salts: carbonates, sulfates, nitrates, chlorides, phosphates of calcium, magnesium, potassium, sodium, iron, aluminum, manganese. All nitrates and chlorides, as well as potassium and sodium salts, are highly soluble in water, but their content in soils (except saline soils) is relatively small. Slightly soluble salts (calcium and magnesium carbonates and calcium sulfate) occur in the solid phase in some soils in significant amounts, while insoluble ones—phosphates of calcium, magnesium, iron, and aluminum—occur in all soils.
Table. Content of trace elements in soil and lithosphere, wt. %.
| Element | Content | Element | Content | ||
|---|---|---|---|---|---|
| in soil | in lithosphere | in soil | in lithosphere | ||
| Manganese | 0,085 | 0,09 | Copper | 0,002 | 0,01 |
| Fluorine | 0,02 | 0,027 | Zinc | 0,005 | 0,005 |
| Tungsten | 0,01 | 0,015 | Cobalt | 8·10-4 | 0,003 |
| Boron | 0,001 | 3·10-4 | Molybdenum | 3·10-4 | 3·10-4 |
| Nickel | 0,004 | 0,008 | Iodine | 5·10-4 | 3·10-5 |
The main source of trace elements in soil is soil-forming rocks. Thus, soils formed on products of weathering of acidic rocks, i.e., granites, liparites, granite porphyries, are poor in nickel, cobalt, and copper. Soils formed on products of weathering of basic rocks (basalts, gabbro), on the contrary, are enriched in these elements.
Some trace elements, for example, iodine, boron, fluorine, selenium, arsenic, can enter the soil from atmospheric air, volcanic eruptions, and precipitation. For iodine and fluorine, these sources are the main ones.
Biological factor of soil formation
Due to the activity of living organisms, in soil compared with the Earth's crust, the carbon content increased 20-fold, and nitrogen—10-fold.
Soil formation under natural conditions proceeds very slowly. The use of fertilizers and agrotechnical practices makes it possible to accelerate this process. Thus, fertilizer application enhances the vital activity of plants and soil microflora, which leads to accumulation of organic substances and biologically important elements.
References
- Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: D. N. Pryanishnikov VNIIA Publishing House.
- Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.
- Niklyaev V. S. (2000). Fundamentals of Agricultural Production Technology. Farming and Crop Production. Moscow: Bylina.
- Bazdyrev G. I., Loshakov V. G., Puponin A. I. et al. (2000). Farming. Textbook for Universities. Moscow: Kolos Publishing House.
