The process by which soil air is replaced by atmospheric air is called soil aeration.
Soil air has higher concentrations of carbon dioxide and water vapor but less oxygen than atmospheric air due to the consumption of oxygen and production of carbon dioxide by soil organisms. The magnitude of difference depends on the rate of gaseous exchange between the atmosphere and the soil. Aeration is brought about by processes of diffusion and mass flow.
Diffusion is the movement of gases through air-filled pores from regions of higher to lower gas concentration and is proportional to air-filled porosity. Diffusion is low in compacted soils because of reduced pore size and number and the discontinuity of soil pores. Similarly, diffusion is low in wet soils because of the absence or reduction of air-filled pores and the extremely low diffusion rate of air in water. Mass flow occurs as a result of the following:
- Expansion and contraction of soil gases die to temperature and barometric pressure changes.
- Soil air removal through precipitation and irrigation, and replacement as water is removed by drainage, plant use, and evaporation.
- Wind action causing air to be forced into the soil at some locations and pulled out at others. Relative to diffusion, mass flow is considered to have a minor influence on soil aeration.
Poorly aerated soils are often deficient in oxygen. Oxygen is used by plant roots and soil organisms for respiration, which produces carbon dioxide. Without an adequate exchange of gases between the atmosphere and the soil, oxygen levels decline and carbon dioxide levels increase in soil air. This can result in reduced absorption of nutrients and water by plant roots, since they must have sufficient oxygen for respiration to generate the energy necessary for this process. Microbial decomposition of organic matter is also inhibited in oxygen-deficient soils, as is the bacterial oxidation of ammonia to nitrate nitrogen. Denitrification, the conversion of nitrate to N2 to N2O gases, occurs in persistently wet soils, resulting in a loss of soil nitrogen to the atmosphere.
Turfgrass communities growing in compacted or persistently wet soils are often invaded by various weed species. This reflects, in part, the differential adaptation of plants to poorly aerated soils. Some weed species that typically grow under these conditions may have the capacity to transmit foliar-absorbed oxygen to their toots to satisfy respiratory requirements. Thus, specific weeds may have an advantage over many turfgrasses through their ability to survive persistently wet conditions.
Source: Turfgrass Management 7th Edition
