Soil gas

Soil gases are the gases found in the air space between soil components. The primary natural soil gases include, and. The oxygen is critical because it allows for respiration of both plant roots and soil organisms. Other natural soil gases are atmospheric methane and. Some environmental contaminants below ground produce gas which diffuses through the soil such as from landfill wastes, mining activities, and contamination by petroleum hydrocarbons which produce volatile organic compounds. Soil gases can diffuse into buildings, the chief concerns among these pollutants are radon which is radioactive and causes cancer and methane which can be flammable at only 4.4% concentration.

Gases fill soil pores in the soil structure 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:
 * Soil Air: 79.2% Atmosphere: 79.0%
 * Soil Air: 20.6% Atmosphere: 20.9%
 * 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.