Introduction
The composition of the air within the soil is only slightly different from that which is found in the atmosphere.
Some typical values are as follows:
| Component | Atmospheric Air | Soil Air |
| (Composition by volume) | ||
| Nitrogen | 79% | 79% |
| Oxygen | 20.96% | 20.3% |
| Carbon Dioxide | 0.035% | 0.15 - 0.65% |
Oxygen Requirements
Grass roots and soil organisms require an appropriate amount of oxygen to achieve optimum growth, however, it is usually the concentration of carbon dioxide and other elements (as well as waterlogging) that restrict and impair root growth and biological activity.
Smooth Stalked Meadow Grass growth has been shown to be unaffected by a soil oxygen content of 2%, whilst Creeping bentgrass is unaffected by a soil oxygen content of 1%.
As soil oxygen levels decrease, however, there is a reduction in the overall respiration rate occurring within the soil and this could result in the soil becoming anaerobic.
Reduced levels of oxygen also slow the breakdown of organic matter because there is less microbial activity.
Anaerobic Soil
'Anaerobic soil conditions occur only when the rate at which oxygen enters the soil from the atmosphere is less than that at which it is utilized in the respiratory process of plant roots, bacteria, fungi or other organisms'. (R. Scott Russell - 1977)
Soil Aeration
The exchange of atmospheric air and soil air is called 'Soil Aeration' and results from a number of methods:
The primary method by which exchange occurs is through diffusion.
(Diffusion is where a higher concentration of gas moves to where there is a lower concentration of the gas. Therefore, as oxygen is of a higher concentration in the atmosphere it will move into the soil. By contrast, there is a higher concentration of carbon dioxide in the soil and so this will move out to the atmosphere).
Compaction as well as surface or sub-surface smearing will reduce the available pore space through which the gases can move.
This can result in there being a barrier between the air in the atmosphere and that in the soil. Anaerobic and toxic conditions can build up due to this barrier to diffusion. It is therefore important to maintain a well aerated rootzone through various maintenance and user management practices.
Other, less significant, methods by which gas exchange occurs include:
- Rainfall and irrigation which displace soil air,
- Wind flow speed,
- Increases or decreases in atmospheric pressure,
- Fluctuations in the daily temperature.
References
Hanson, A.A. & Juska, F.V., (1969), 'Turfgrass Science', American Society of Agronomy, 93-96
Beard, J. (1973), 'Turfgrass Science and Culture', 337-338
Russell, E.W. (1973), 'Soil Conditions and Plant Growth', 10th Edition, 411
Russell, R.Scott (1977), 'Plant Root Systems: Their function and interaction with the soil', 193