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Soil Components:

4. Soil Air

Soil pores, the voids between minerals, organic matter, and living organisms, are filled with air or water. There is a dynamic equilibrium between water and air content within a soil. When water enters the soil, it displaces air from some of the pores.

4.1.Composition of soil air
4.2. Movement of gasses within soil  
4.3. Soil porosity

Source: Lesley Dampier

4.1. Composition of soil air

The composition of soil air is different from that of the atmosphere because it cannot readily mix with air above the soil. The metabolic activity of plant roots, microbes and soil fauna all affect the composition of soil air. For example, the concentration of carbon dioxide (CO2) in soil (between 0.3 and 3%) is often several hundred times higher than the 0.03% found in the atmosphere. In extreme cases oxygen can be as low as 5-10%, compared to 20% in the atmosphere. Soil air has a higher moisture content than the atmosphere, with relative humidity approaching 100% under optimum conditions. (humidity is not as variable in soil as it is in the atmosphere).  The amount and composition of air in soil are dynamic and to a large degree are determined by water content and activity of soil organisms.  

CO2 chambers

CO2chamber is used to measure soil's respiration

Source: UBC Biometeorology Group
Faculty of Land & Food Systems

CO2 chambers

CO2 chamber is used to measure soil's respiration

Source: UBC Biometeorology Group
Faculty of Agricultural Science

4.2. Movement of gasses within soil

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

1)   Mass flow (convection) of air - the moving force is a gradient of total gas pressure, and it results in the entire mass of air streaming from a zone of higher pressure to one of lower pressure. Mass flow of air is much less important than diffusion, except perhaps in layers at or very near the soil surface.

2)   Diffusion - moving force is gradient of partial pressure of any constituent member of air to migrate from a zone of higher to lower pressure, even while air as a whole may remain stationary. In other words, through diffusion each gas moves in a direction determined by its own partial pressure.

The oxygen flux density due to diffusion is proportional to the oxygen concentration gradient along the axis, and the proportionality factor is called the (oxygen) diffusion coefficient (D).  This statement is an example of Fick’s Law of Diffusion, which can be expressed as follows:

where J is the diffusive flux density of the gas (oxygen in this example) (mg/m2/s) along the x-axis, C is oxygen concentration in the soil air (units are g/m3), x is distance along x-axis (m), dC/dx is the oxygen concentration gradient (g/m4), and D is the (oxygen) diffusion coefficient (m2/s). 

The oxygen diffusion coefficient (D) for diffusion in air is about 10,000 times as large as the coefficient for diffusion in water. Thus the oxygen diffusion coefficient (D) of a soil is very strongly influenced by three factors:

1)   air-filled porosity (Va/Vt), which decreases with increasing soil water content

2)   the continuity of air-filled pores, which decreases with increasing soil water content

3)   the tortuosity of air-filled pores, which increases with increasing soil water content.

Growth of most plants and survival of their roots normally requires maintenance of adequate soil oxygen.  This in turn requires maintainance of soil water well below saturation, to enable rapid gas diffusion in the soil. 

4.3. Soil porosity

Soil porosity (f) is the ratio of pore volume (Vf) to total soil volume (Vt)

f = Vf / Vt

It is generally between 30-60%.  Porosity tells us nothing about the relative amounts of large and small pores, and should be interpreted with caution. Generally, high porosity (e.g. 60%) is an indicator of lack of compaction and good soil conditions.

Faculty of Land and Food Systems