Soil air in agriculture: a comprehensive overview

What is soil air?

“The air, which is found in the spaces (empty pores) between the soil particles, is called soil air”. Roots of plants use this air for respiration. Soil air has great importance in plant growth and biological activities of soil.

Nitrogen gas, despite being in the highest concentration in the air components of the soil, does not directly contribute to plant growth due to being inactive, but Carbon-dioxide and Oxygen, being helpful in photosynthesis and respiration, control soil biological activities.

The lack of Co₂ in green plants and around them in the air reduces photosynthesis, while excessive Co₂ and O₂ deficiency adversely affect the growth of plants and soil bacteria.

Soil air capacity

It is the amount of soil air that is found in the soil according to the water holding capacity of the soil. It is equal to the non-capillary stomata in the soil. Soil organization, texture, organic matter, transpiration, ploughing, soil cracking etc. affect the soil air capacity. As the soil depth increases, the amount of air decreases. Light, friable and granular and organic soils have more air.

Composition of soil air

The proportion of carbon dioxide in a well-drained soil is less than 1%, while in soil with poor drainage, the proportion of carbon dioxide increases from 5% to 20%. Soil air composition is as follow-

Air	N2	CO2	O2
Atmosphere	78.80%	0.03%	20.95%
Surface Soil	78.00%	0.15%	18.20%
Organic Soil	78.00%	1.85%	18.23%
Clay	78.00%	0.66%	19.69%
Loam	78.00%	0.62%	19.20%
Sandy soil	78.00%	0.03%	19.95%

Factors affecting soil air composition

Factors affecting soil air composition are as follows-

• Soil Properties- The main influence of soil properties is multiple with the factors which increase the soil air permeability and soil air retentiveness. Those factors are soil texture, moisture, structure, and organic matter content. In water-free granular soils, the amount of CO2 is low and the amount of O2 is high. With increasing in soil depth, the amount of Carbon-Dioxide increases and the amount of Oxygen decreases.

• Organic matter and biological activity- Organic soils contain more Carbon-dioxide than mineral soils. Bacterial decomposition is an important factor in the production of CO2, so the addition of organic matter increases the amount of carbon dioxide in the soil.

• Changes in season- The amount of CO₂ in surface soil air is less in winter season and more in summer because the activity of bacteria is less at low temperature.

Soil aeration problem in the field

Aeration problem arises in two conditions of moisture:

1. Excessive humidity.
2. Exchange of gases with the atmosphere.

Excessive humidity

In this condition the problem of waterlogging arises which is harmful for the growth of plants. Waterlogging occurs due to lack of good drainage and fine texture of the soil. Such soils do not have proper aeration.

Exchange of gases with the atmosphere

The mutual exchange of soil air and atmospheric gases depends on two things:

1. Speed of biochemical reaction.
2. The actual speed of each gas with which it moves in and out of the soil.

If the use of O₂ in the soil and removal of CO₂ happens quickly, then the exchange of atmospheric gases also takes place quickly. By increasing the speed of biochemical reactions, the use of oxygen and the removal of carbon dioxide are accelerated.

The exchange between soil air and atmospheric gases takes place by two methods:

1. Mass flow
2. Diffusion

Mass flow

The flow of gases occurs due to the difference in pressure. Gases flow from a place of high pressure to a place of low pressure. When rain or irrigation water enters into the soil, it mixes with the air and a balance is established between water and air.

Diffusion

Most of the exchange of gases occurs by diffusion. Gases in a gaseous mixture diffuse due to their partial pressure difference. This diffusion is possible only when the soil pore-space are freely open and have direct contact with the atmosphere. The gaseous flow rate is very less in dense and heavy soils.

Soil Aeration and Plant Growth

Plants grow properly only in the proper circulation of soil air. If the air circulation is not good in the soil, then the growth of plants is affected as follows:

• Plant growth is reduced- Due to lack of air, the roots of tuber and root crops like potato, carrot, radish, beet, turnip, etc., become abnormal in shape. Due to lack of air, underdeveloped roots are unable to absorb the required amount of moisture and nutrients; Hence the growth of plants stops.
• Nutrient absorption by plants is reduced- Due to lack of Oxygen in the soil air, plants are unable to absorb nutrients due to lack or No respiration in the roots, resultant plants growth stop.
• Water absorption by plants is reduced- Due to slowing of respiration and unbalanced amount of CO2, the permeability of the root cells decreases, due to which the plants are not able to absorb the proper amount of water and their growth stops.
• The growth of plants slows down due to the formation of Toxic organic matter- In anaerobic decomposition, toxic substances and organic acids are produced and their excess is harmful to the plants. Fe and Mn are more soluble in the reduced state than in the oxidized state and their high amounts are toxic to the plants.

Air requirements of plants

Cereal crops are often more resistant to low aeration than leguminous and oily seed crops. Different plant roots require different amounts of oxygen. When the oxygen content in the soil is less than 10%, the growth of roots is slow and stops completely at 5%.

Control of Soil aeration

Soil air can be controlled to a certain extent by improving soil structure and drainage. The development of stable spherical or crumby structures in heavy structure soils creates good aeration conditions due to rapid drainage.

Light plowing and weeding and hoeing in the soil break the crust on the upper surface, due to which aeration starts well.

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