Microbial breakdown of soil organic residues is reduced under poor aeration and hence organic matter is accumulated. In well-aerated soil, aerobic microorganisms are active and they convert simple sugars to CO2 and water using oxygen. In submerged or reduced soil, anaerobic microorganisms are active and they convert sugars to less CO2 and more CH4 (methane) which is an atmospheric pollutant. This process also gives out some organic acid, ethylene gas etc., which are toxic to plant roots and some microbes. The aerobic decomposition will be faster than anaerobic decomposition.
Oxidation and reduction of inorganic elements
The oxidized state of nitrogen and sulphur are easily utilized by the plants. Reduced forms of some of the elements are toxic. Though solubility of iron and manganese increases, they become toxic to the plants.
Soil colour is also altered by aeration. Well-aerated soils have red, yellow and reddish brown colours. Reduced soils have grey and blue colours. The discolouration of soil in patches is called mottling.
Plant and root growth
Soil aeration is important factor for the normal growth of plants. Roots absorb oxygen for their respiration and release CO2. The supply of oxygen to roots in adequate quantities and the removal of CO2 from the soil air are very essential for healthy plant growth. When the supply of oxygen is inadequate, the plant growth either slows or ceases completely as the accumulated CO2 hampers the growth of plant roots.
The abnormal effect of insufficient aeration on root development is most noticeable on the root crops. Abnormally shaped roots of these plants are common on the compact and poorly aerated soils. The penetration and development of root are poor. Such undeveloped root system cannot absorb sufficient moisture and nutrients from the soil.
Microorganism population and activity
The microorganisms living in the soil also require oxygen for respiration and metabolism. Some of the important microbial activities such as the decomposition of organic matter, nitrification, sulphur oxidation etc., depend upon the amount of oxygen present in the soil air.
The deficiency of oxygen in soil air slows down the rate of microbial activity. The decomposition of organic matter is retarded and nitrification arrested. The microorganism population is also drastically affected by poor aeration.
Formation of toxic material
Poor aeration results in the development of toxins and other injurious substances such as ferrous oxide, H2S gas, CO2 gas, ethylene, organic acids, etc., in the soil.
Water and nutrient absorption
A deficiency of oxygen has been found to check the nutrient and water absorption by the plants. The energy of respiration is utilized in absorption of water and nutrients. Under poor aeration in water-logged soils, plants exhibit water and nutrient deficiency.
Development of plant diseases
Insufficient aeration of soil leads to the development of some diseases like, wilt of gram and dieback of citrus and peach.