Soil Air: Composition and Factors | Soil Science

In this article we will discuss about:- 1. Composition of Soil Air 2. Factors Affecting the Composition of Soil Air 3. Gaseous Exchange between Soil and Atmosphere 4. Soil Aeration.

The volume of the soil that is not occupied by soil particles is known as ‘Pore Space’. The pore space is usually occupied by air and water. The air filled pores constitute the gaseous phase of the soil system. Oxygen is essential for all biological reactions occurring in soil. Soil air differs from atmospheric air by having a higher carbon dioxide and lower oxygen content.

Composition of Soil Air:

The Soil air contains a number of gases. Out of which oxygen, carbon-dioxide, nitrogen, and water vapour are the most important. The composition of Soil air is more or less similar to that of the atmospheric air except the content of carbon dioxide which is several times higher. The soil fauna and flora consume oxygen and gives out carbon dioxide. Organic matter addition and cropping increases the level of Carbon dioxide in the soil. Carbon dioxide in soil air constantly moves from the soil pores into atmosphere and vice-versa. The nitrogen content in soil air is almost the same as in atmosphere.

Soil air is virtually in continuation of atmospheric air and there is continuous exchange of gases between atmosphere and soil air. The circulation of air in the soil mass is known as ‘soil aeration’.

Factors Affecting the Composition of Soil Air:

There are some factors affecting the composition of Soil air as follows:

I. Nature of the Soil – Sandy Soils have macropore, as a result of which, aeration is very good in that soil. The soils that are water-logged contain small amount of oxygen as the pore space is filled with water immediately after a heavy rains or irrigation. The surface soil contains more macropore than the sub-soil. As a result, gaseous exchange is found to be more in surface soil than the sub soil. The oxygen percentage of Soil air varies with the depth of the soil and this is true in case of carbon dioxide also.

II. Soil organic matter – Soil organic matter is decomposed by microorganism present in the soil. Microbiological decomposition leads to the production of carbon dioxide and its content increases in the soil air. Hence soil rich in organic matter contains higher percentage of carbon dioxide.

III. Season – Season and temperature also influences the carbon dioxide content of the soil. The activity of soil micro-organism increases at high temperature during summer month which results in higher production of carbon dioxide. The composition of soil air shows marked seasonal variation, the intensity of which is affected by the texture of the soil and position of water table.

IV. Soil moisture – The oxygen content of a soil decreases when the macropores are filled with water. But when the soil is artificially drained again, the macropores are filled with air and the oxygen content of soil increases.

V. Vegetation – Soils on which crops are grown contain more carbon dioxide than fallow land as a result of respiration of plant roots. The plant takes the soil oxygen and releases carbon dioxide. As a result, the carbon dioxide content of the cropped land increases near the root zone of the plant.

Gaseous Exchange between Soil and Atmosphere:

Gaseous exchange between soil and atmosphere takes place by diffusion and mass flow as follows:

I. Mas Flow:

Mas flow of air is apparently due to pressure differences between the atmosphere and the soil air and is relatively unimportant in determining the total exchange that occurs. The pressure difference arises from such meteorological factors, as changes in soil temperature, barometric pressure, wind movements and also rainfall and irrigation. The variation in soil temperature causes changes in the temperature of Soil air. When the soil air gets heated during the day, it expand and move out of the soil porespace into the atmosphere.

When the soils begins to cool, the soil air contracts and the atmospheric gases enter into the soil. When the barometric pressure increases, the volume of soil air decreases which facilitate the entry of atmospheric air to the soil and decrease in barometric pressure makes the reverse flow from soil to atmosphere. The soil pores are filled with water of rainfall or irrigation and causes poor aeration. But when drainage occurs, the pores again filled with air from the atmosphere.

II. Diffusion:

Most of gaseous interchange in soils occurs by diffusion. It is the process by which each gas tends to move in the space occupied by another as determined by the partial pressure of each gas. The partial pressure of a gas in a mixture is simply the pressure the gas would exert if it was present alone in the volume which has been occupied by the mixture of gases. Atmospheric and soil air contains a number of gases such as nitrogen, oxygen, carbon dioxide etc. each of which exerts its own partial pressure in proportion to its concentration.

The movement of each gas is regulated by the partial pressure under which it exists. The atmosphere contains more oxygen than soil pore space and soil contains more carbon dioxide than the atmosphere. So the partial pressure of oxygen is higher in the atmosphere than in the soil porespace and the partial pressure of carbon dioxide is higher in soil pore space than in atmosphere even though the total pressure in the atmosphere and soil pore spaces may be the same. Due to which oxygen moves in the soil and carbon dioxide moves out of the soil.

Soil Aeration:

Soil aeration is a mechanism of rapid exchange of oxygen and Carbon dioxide between the Soil pore space and the atmosphere in order to prevent the deficiency of oxygen and toxicity of Carbon dioxide in the Soil air.

Causes of Poor Aeration:

The causes of Poor aeration are as follows:

(i) Compaction of Soil – Compact of finer texture suffers from poor aeration.

(ii) Water logging condition – Water logging condition hampers the aeration.

(iii) Addition of decomposable organic matter – Soil microorganisms decompose the organic matter. They need oxygen for their respiration and release carbon dioxide. This condition hampers the soil aeration.

Factors Affecting Soil Aeration:

The factors affecting Soil aeration are as follows:

(i) Layer of Soil – The top Soil contains much more macro-pore space than the sub-soil. For this, gaseous exchange of top soil contains more oxygen than that of sub-soil.

(ii) Soil moisture – The macropores are filled with water immediately after rain fall and irrigation. This condition hampers soil aeration and plant root suffers more for the deficiency of oxygen. Drainage improves soil aeration to much extent.

(iii) Soil organic matter – Soil microorganisms decompose the soil organic matter and they liberate carbon dioxide in the soil. This condition hampers the soil aeration.

Soil Aeration and Plant Growth:

Soil aeration influences the plant growth. Oxygen is most important for respiration of plant roots and soil microbes. Aerobic microorganism functions properly if oxygen is present sufficiently in the soil. Soil micro-organism decomposes the organic matter in aerated soil and nutrients are liberated for the growth of the plants. The symbiotic (e.g. Rhizobium, Clostridium etc.) and non-symbiotic bacteria (e.g. Azotobactor, etc.) function well in a soil having adequate aeration and they fix the atmospheric nitrogen and enrich the soil with nitrogen.

The nitrifying bacteria (e.g. nitrosomonas, nitrobactor etc.) works under aerobic conditions and oxidizes ammonia to nitrites and/or nitrates, the important form of nitrogen at which most of the plant takes nitrogen from the soil. Under anaerobic condition, the denitrifying bacteria (Thiobacillus denitrificans) works and converts the nitrites or nitrates into ammonia or elemental nitrogen, and thus causes the loss of soil nitrogen.

Aeration promotes root growth and absorption of water and nutrients by plants. The absorption of nutrients by roots is one of the most important physiological functions of living plants and this is dependent on the proper aeration of soil. Absorption of water by plants roots takes place at a greatly retarded rate in soil deficient in the supply of oxygen. In poorly aerated soil, the carbon dioxide (CO₂) gets accumulated which affects water absorption process of the plant.

Soil air is also useful in increasing the nutrient availability of the soil:

(i) By breaking down the insoluble minerals into soluble salts,

(ii) By decomposing plants and animals remains and

(iii) By bringing about nitrifying and nitrogen fixing process of bacteria.

The order of reduction in nutrient absorption due to poor aeration was found by Lawton (1945) to be K > Ca > Mg > N > P in corn in Clay silt loam. Oxygen deficiency disturbs the metabolic process in plants and causes poor growth of the plant.

Under anaerobic conditions, gaseous carbon compounds other than carbon dioxide are evolved which may be toxic to plant except paddy. The paddy plants are well adapted to grow under submerged condition where the external sources of oxygen for the root is particularly absent as this plant has large internal space by which oxygen transportation takes place for the respiration of root.