Exchange of Gases between Soil and Atmosphere?
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Exchange of Gases between Soil and Atmosphere?

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The exchange of gases between the soil and the atmosphere is facilitated by

two mechanisms

1. Mass flow: With every rain or irrigation, a part of the soil air moves out into the

atmosphere as it is displaced by the incoming water. As and when moisture is lost

by evaporation and transpiration, the atmospheric air enters the soil pores. The

variations in soil temperature cause changes in the temperature of soil air. As the

soil air gets heated during the day, it expands and the expanded air moves out into

the atmosphere. On the other hand, when the soil begins to cool, the soil air

contracts and the atmospheric air is drawn in.

2. Diffusion: Most of the gaseous interchange in soils occurs by diffusion.

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. If the partial pressure on one of the gases (i.e. carbon dioxide) is greater in

the soil air than in the atmospheric air, it (CO2

) moves out into the atmosphere.

Hence, the concentration of CO2

is more in soil air.

On the other hand, partial pressure of oxygen is low in the soil air, as oxygen

present in soil air is consumed as a result of biological activities. The oxygen

present in the atmospheric air (partial pressure of O2

is greater) therefore, diffuses

into the soil air till equilibrium is established. Thus, diffusion allows extensive

movement and continual change of gases between the soil air and the atmospheric

air. Oxygen and carbon dioxide are the two important gases that take in diffusion

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