A well-aerated soil is one in which gases are available to plant roots and other soil organisms, in sufficient quantities and in proper proportions to support their normal respiratory activities. It is important that oxygen which is necessary for soil biota is continuously renewed to support metabolic activities.
A soil which is considered well aerated must have the following two characteristics. First, sufficient space for air movement should be present. Second, there must be ample opportunity for the ready movement of gases into and out of the soil. Soil aeration is an important factor having far reaching influence on the metabolic activity of plant roots and other soil organisms. Continuous supply of oxygen in the soil is necessary to sustain soil organisms.
One of the most apparent effects of poor soil aeration is the reduction in the rate of organic matter decomposition, which you might recall is an essential process to maintain soil fertility. The reduced rate of break down of organic matter is due to the lack of oxygen rather than from the built up of carbon dioxide. Accumulation of organic matter in swampy areas shows that the lack of oxygen retards the decomposition of organic matter.Aerobic soil organisms are unable to survive in the absence of gaseous oxygen. For example, bacteria that are responsible for the oxidation of nitrogen and sulphur are relatively ineffective in poorly aerated soils. This is also true for the symbiotic nitrogen fixers and some of the free living nitrogen fixers such as azotobacter.
Higher plants are also adversely affected by poor soil aeration because the root growth is curtailed; nutrient and water absorption is adversely affected; and certain toxic elements accumulate in poorly aerated soils.