Air regime of soils is a set of processes of interaction of plants with gases contained in soil.
The air contained in the soil, its composition and gas exchange with the surface layer of the atmosphere refer to the terrestrial factors of plant life.
The importance of air in plant life
In the process of life, plants, in contrast to photosynthesis, breathe by consuming oxygen and releasing carbon dioxide. Through respiration, oxidative reactions take place in plants, in which energy is released for growth and development.
Vernadsky noted that soil without gases is not soil. Speaking of the importance of biochemical processes, the importance of soil in the biosphere, we thus point to the predominant role of gases in soil processes.
Air oxygen is necessary for germination of seeds. Seeds placed at the bottom of a vessel and covered with a layer of water swell but do not produce germination. When seeds come in contact with air, they germinate amicably.
The above-ground part of the plant is better supplied with oxygen than the underground. But sometimes, in the practice of agriculture, it happens that plants die from the lack of it in the surface layer of air. For example, in winter crops, when a lot of snow falls on the unfrozen soil, plants continue to vegetate, quickly using up the oxygen supply under the snow. Since new portions of oxygen are not supplied, this leads to suffocation of winter crops, which results in rooting of winter crops. A similar situation occurs when an ice crust is formed in winter crops.
The root system also needs oxygen. The attitude of cultivated plants to the lack of soil air is different. Legumes, oilseeds, root and tuber crops are the most demanding in this respect, cereals are less sensitive due to partial supply of roots with oxygen through air-bearing cavities of stems. The cavities are especially strongly developed in corn and rice.
Air oxygen plays an important role for soil microorganisms that decompose plant residues in the soil. Nitrogen-fixing bacteria also need nitrogen to function normally.
Composition of soil air
The gaseous phase of soil consists of soil air and vaporous water. Share of gaseous phase strongly depends on type and structure of soil, its physical and mechanical properties.
Soil air occupies all soil pores free from water. At that, the more porosity and less humidity is, the more air is accumulated. The most favorable for plant development is air content in large pores, and water in small and medium pores.
For grain crops the optimal air content in arable layer is 15-20% of total porosity, for row crops – 20-30%, for perennial grasses – 17-21%.
Soil air contains less oxygen and more carbon dioxide compared to atmospheric air (oxygen 20.90%, carbon dioxide 0.03%). Although their ratio can vary considerably. The optimum content of oxygen in soil air is 7-12% and carbon dioxide about 1%.
Oxygen concentration in the upper soil layers with good aeration is close to atmospheric. While on heavy with poor gas exchange, it can decrease by tens or hundreds of times, to tenths or hundredths of a percent. Carbon dioxide content, on the contrary, in soils with poor aeration increases by hundreds of times compared with atmospheric air and reaches 20% or more.
When the carbon dioxide content in the soil is higher than 3-5% and oxygen less than 10%, plant oppression occurs.
Soil gas exchange
Soil gas exchange occurs continuously by means of air-permeable pores communicating with each other and the atmosphere. The speed of this process depends largely on a number of factors: variations in atmospheric pressure, temperature, soil moisture, wind, vegetation, the structure of the arable layer.
Temperature and pressure fluctuations cause air to expand or contract, causing it to move from higher pressure zones to lower pressure zones.
In loose and well-structured soils with many large pores between aggregates gas exchange rate is high, on the contrary, in swollen (overcrust), non-structured soils covered with crust gas exchange rate is low.
Water entering the soil with precipitation, irrigation, or rising groundwater causes soil air to be pushed into the atmosphere, and conversely, when moisture is reduced, atmospheric air is drawn in. Rainfall provides up to 6-8% of all gas exchange.
Wind affects gas exchange depending on speed, macro- and microrelief features, soil structure and tillage technique. On porous soils in the absence of vegetation, wind plays the greatest role in gas exchange.
Aeration is the process of gas exchange between soil air and atmospheric air.
Diffusion, the process of movement of gases in the direction of the gradient of change in their partial pressure, is the main and continuous factor of gas exchange of the air regime. Due to the fact that the concentration of oxygen in the soil is less and carbon dioxide more than in the atmospheric air, conditions of diffusion are created, i.e. penetration of oxygen into the soil from the atmosphere, and carbon dioxide – from the soil into the atmosphere.
Indicators of soil air regime
Air capacity is the volume of soil occupied by air at a given moisture content. Because of constant variations in moisture and porosity of the soil, the air capacity is also subject to fluctuations.
Air permeability is the ability of the soil to pass air and is the main condition of gas exchange between soil and atmospheric air.
Regulation of the air regime of soils
The main method of air regime regulation is loosening.
A water-stable cloddy soil structure is a condition for creating an optimal air regime.
Application of organic fertilizers (manure, peat, green fertilizers) changes the composition of soil air, increasing the amount of carbon dioxide in the arable layer. For example, 20 tons of manure applied to 1 hectare increases the content of CO2 in the soil by 70-140 kg.
In zones of high moisture, especially in the spring period, ridging is used for sowing potatoes, soybeans and other crops.
Fundamentals of agricultural production technology. Farming and crop production. Edited by V.S. Niklyaev. — Moscow: «Bylina», 2000. — 555 с.
Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. — Moscow: Publishing House «Kolos», 2000. — 551 с.