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, appeared on the surface of the Earth as a result of long-term influence of biotic, abiotic and anthropogenic factors, consisting of solid mineral and organic particles, water and air and having specific genetic and morphological features, properties, which create appropriate conditions for growth and development of plants. Soil is a complex self-regulating multicomponent biocomponent unified system.
The gas phase is the result of interaction between atmospheric air and gases formed in the soil. In its composition there is a higher content of carbon dioxide compared to atmospheric air – 0.3-1%, sometimes up to 2-3% or more and a lower content of oxygen. Gas phase is characterized by high mobility, which depends on many conditions: the content of organic matter, weather conditions, the nature of vegetation, etc.
Sufficient oxygen content in the soil creates favorable conditions for the activity of aerobic microorganisms. On the contrary, with its deficiency there are conditions for the development of anaerobic bacteria, which are often pathogenic to plants.
The volume of soil air is in dynamic equilibrium with the liquid phase: the more water, the less air. Gas exchange processes in soil occur constantly as a result of decomposition of organic matter, respiration of plant roots and soil organisms, as well as some chemical reactions. As a result of gas exchange, the above-ground air is enriched with carbon dioxide, improving conditions for photosynthesis. When carbon dioxide interacts with water in the liquid phase, the soil solution is slightly acidified by the reaction:
CO2 + H2O ⇔ H+ + HCO3–.
Acidification promotes transition of some minerals of a solid phase, for example, phosphates and calcium sulfate, in the form accessible to plants. At the same time, an excess of carbon dioxide leads to a lack of oxygen and the creation of anaerobic conditions, which is observed when soils are over-watered and over-consolidated. Lack of oxygen in gas phase inhibits growth and development of microorganisms and plants, prevents assimilation of nutrients, increases reduction processes in liquid and solid phases.
Soil air concentrates in non-capillary pores, i.e. in large spaces of soil. If all pores are filled with water, the soil air is displaced, on the contrary, if the soil is dry, the air fills all pores – capillary and non-capillary.
The most optimal ratio of water and air is formed on loose structural cultivated and cultivated soils. Regulation of water and air regimes of soils by appropriate treatments in combination with application of fertilizers and ameliorants improves root and air nutrition of plants, thereby increasing quantity and quality of production, contributes to development of soil biota.
The 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 soil, from which plants assimilate nutrients and simultaneously the interaction of plants with fertilizers, ameliorants, solid and gaseous phases takes place.
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. Input of ions into the soil solution comes from solid and gaseous phases, fertilizers and ameliorants, excreta of soil biota, atmospheric precipitation and groundwater. Thus, composition and concentration, acidity, buffer and osmotic pressure of soil solution are dynamic and are determined by soil-climatic conditions and anthropogenic impact.
The concentration of salts in the soil solution depends on the properties, mineral composition, soil type, natural conditions, degree of salinity 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 fertile soils to 1 and more percent (> 10,000 mg/l) in highly saline (solonchaks), in medium fertile soils – about 500 mg/l. Excess salts over 2,000 mg/L usually adversely affect crops, especially in the first 2 to 4 weeks after seed germination. Tolerance to high concentrations increases with age.
The properties of the liquid phase are generally determined by the water regime of the soil.
The solid phase of soil consists of:
- mineral part, the share of which ranges from 90% to 99.5%;
- organic, or organic matter of the soil, which accounts for 0.5% to 10%.
The mineral part is the debris 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, 1-3% of nitrogen from the total amount.
Oxygen, silicon, aluminum, and iron account for nearly 93% of the solid phase, carbon, potassium, and calcium for 4.6%, and 2.5% for all the remaining elements. Carbon, oxygen, hydrogen, phosphorus, and sulfur are in the mineral and organic parts, while nitrogen is almost entirely in the organic part.
The organic part, or organic matter of the soil, is the remains of plant and animal organisms and products of their decomposition and neosynthesis, among which humus prevails.
Table. Average chemical composition of the solid phase of soil (% of mass) by A.P. Vinogradov
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.