Soil gas

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Soil gas is the air space in soil containing oxygen, and the oxygen allows for respiration of both plant roots and soil organisms. This air space could also contain carbon dioxide as a product of respiration of plant roots and soil organisms.

Air can fill soil pores as water drains or is removed from a soil pore by evaporation or root absorption. The network of pores within the soil aerates, or ventilates, the soil. This aeration network becomes blocked when water enters soil pores. Not only are both soil air and soil water very dynamic parts of soil, but both are often inversely related.

Composition of air in soil and atmosphere:[1]

  • Nitrogen: Soil Air: 79.2% Atmosphere: 79.0%
  • Oxygen: Soil Air: 20.6% Atmosphere: 20.9%
  • Carbon Dioxide: Soil Air: 0.25% Atmosphere: 0.04%

Gas molecules in soil are in continuous thermal motion according to the kinetic theory of gases, there is also collision between molecules - a random walk.

In soil, a concentration gradient causes net movement of molecules from high concentration to low concentration, this gives the movement of gas by diffusion.

Numerically, it is explained by Fick's law of diffusion.

Soil gas includes air, water vapour and the pollutants that might be picked up from the soil underneath a building and carried by air leakage into the building. The chief concerns among these pollutants are radon and methane gases.[original research?]


  1. ^ Russell, E. J.; Appleyard, A. . (1915). "The Atmosphere of the Soil: Its Composition and the Causes of Variation". The Journal of Agricultural Science 7: 1. doi:10.1017/S0021859600002410.  edit