Soil gases are the gases found in the air space between soil components. The spaces between the solid soil particles, if they do not contain water, are filled with air. The primary soil gases include nitrogen, carbon dioxide and oxygen. The oxygen is critical because it allows for respiration of both plant roots and soil organisms. Other natural soil gases are atmospheric methane and radon. 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:
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.
Fick’s laws of diffusion describe diffusion and were derived by Adolf Fick in 1855. Fick’s first law relates the diffusive flux to the gradient of the concentration. It postulates that the flux goes from regions of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative), or in simplistic terms the concept that a solute will move from a region of high concentration to a region of low concentration across a concentration gradient.
Most fine roots thrive in a soil atmosphere of 20% oxygen (O) content, but O concentrations will decrease if not renewed through gas diffusion and mass flow facilitated by good soil structure. Gas diffusion becomes limiting in compacted soils, which can lead to restricted root growth, respiration, and nutrient and water uptake. Not surprisingly, compaction has been shown to increase soil CO2 concentrations by restricting soil gas exchange with the atmosphere (Conlin and v.d. Driessche, 2000; Ullah et al., 2009). Reduced gas exchange further affects the functioning and composition of soil microorganisms, with a shift to organisms that rely on electron acceptors other than O (e.g., NO3, Fe, etc.).