Soil Air

​Soil air is a continuation of the atmospheric air. It is in constant motion from the soil pores into the atmosphere and from the atmosphere into the pore space. The exchange of air and renewal of oxygen and carbon dioxide between soil and aerial atmosphere is known as soil aeration. Soil aeration is essential for the respiration survival, and metabolic processes of soil organisms and plant roots.

Composition of Soil Air

Soil air contains gases like nitrogen, oxygen, carbon dioxide, water vapour and others. The composition of soil air is different from atmospheric air. Soil air contains more carbon dioxide and less oxygen than atmospheric air. It also contains more water vapour than atmospheric air. The nitrogen content of soil air is almost equal to atmospheric air.

Oxidation - Reduction (Redox) potential (Eh)

§  It indicates the oxidation and reduction states of soil system. In oxidized soil, ferric (Fe3+), Manganic (Mn4+), Nitrate (NO3-) and sulphate (SO42-) ions dominate.

§  In reduced soil, ferrous (Fe2+), manganous (Mn2+), ammonium (NH4+) and sulphides (S2-) are present.

§   The redox potential is denoted by the symbol Eh and measured using platinum electrodes and expressed in millivolts.

§   A positive Eh value indicate oxidized state and a negative Eh value indicate reduced state.

Importance of Soil Aeration

Decomposition of Soil organic matter

¨      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 plants. Reduced forms of some of the elements are toxic. Though solubility of iron and manganese increases, they become toxic to plants.

Soil colour

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 dis-colouration of soil in patches is called mottling.

Plant and root growth

§  Soil aeration is important factor for 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 atmosphere are very essential for healthy plant growth.

§  When the supply of oxygen is inadequate, the plant growth either retards 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 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 materials

§  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 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.

Measurement of soil aeration

      Oxygen Diffusion Rate (ODR) - The rate at which O2 in soil air is replenished is called ODR.  It is one of the criteria generally used to determine the O2 concentration in the soil pore space. The ODR characterizes the soil O2 conditions. There are various factors that can influence the ODR namely depth of the soil, temperature, moisture content, soil texture, etc. The ODR decreases with the depth of the soil indicating that with an increase in depth, the concentration of O2 decreases. The ODR decreases as the moisture content increases because of filling pore spaces with moisture resulting in no spaces for gaseous exchange. The ODR influences the plant growth by affecting various physical and chemical properties of the soil. It has been found that the root growth stopped when the ODR dropped to about 20 g x 10-8 /cm2 / min.