Gas diffusion in soil
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The air space in soil contains oxygen to provide for respiration of plant roots and soil organisms. This air space could also contain carbon dioxide as a product of respiration of plant roots and soil organisms.
Composition of air in soil and atmosphere:
- Nitrogen: Soil Air: 79.2% Atmosphere: 79.0%
- Oxygen: Soil Air: 20.6% Atmosphere: 20.9%
- Carbon Dioxide: Soil Air: 0.25% Atmosphere: 0.03%
Source: E.J. Russell and A. Appleyard. 1915. The atmosphere of the soil, its composition and causes of variation. Journal of Agricultural Science 7, 1-48.
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.
