It is well known that restricted soil aeration adversely affect root development, processes of respiration and other essintial biological processess. It is therfore, important to know the content of the soil air and its composition. It has already been mentioned that, depending on its texture, the soil may have pore space of 30-60 percent.The pore space not fill by water is occupied by air. Under moist field conditions, the non-capillary pore space (macropores) generally constitutes the air space, the capillary pore space(micropores) being occupied by water.Ordinarily, the occupation of nearly one-third of the pore space in the soil by air and two-thirds of it by water constitutes the most favourable condition for the plant growth.

The composition of soil air reflects a dynamic balance between two competing processes. The consumption of oxygen and the liberation of carbon dioxide by plant roots and the soil organisms tend to increase the difference in the composition of the soil air and the atmosphere above the soil surface.gaseous diffusion tents to reduce the difference in composition.

Like atmospheric air, the soil air is composed largely of nitrogen and oxygen, but differs from the former in containing more moisture, more carbon dioxide and a little less oxygen. The actual content of carbon dioxide varies greatly during the crop season because of the higher rates crop growth and the biological activities in the soil.

The composition of soil air under different condition is given in Table 3

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