Soil Air

Soil Air the part of ground air that is in the soil and is similar to the air of the atmosphere but depleted in oxygen content and enriched in carbon dioxide. Alternatively, the gaseous phase of soil is called soil air. In a completely dry soil most of the pores are filled with air. As the soil water content increases the amount of air in the soil decreases.

The composition of air in an well-aerated soil is close to the composition of atmospheric air, as the oxygen consumed in the soil by plants and micro-organisms is readily replaced from the atmosphere. However, in a poorly aerated soil, the composition of air differs from the atmospheric air.

Two important gases in soil air are carbon dioxide and oxygen. Carbon dioxide is produced as a by-product of plant root respiration and biological activity. Oxygen is consumed in the soil by the same processes, and plant roots and aerobic microorganisms require oxygen to function normally. For most plant species, transaction of oxygen from the leaves to the roots is not adequate to supply oxygen at the required rate. Hence, plant roots must supplement oxygen supply from soil air. Oxygen is depleted from soil air due to consumption by organisms and restricted exchange fair with the atmosphere. Oxygen content in soil air is replenished by oxygen from the atmosphere above the soil surface. This transport occurs primarily by gaseous diffusion.

In a surface layer of a well-aerated soil, the oxygen content is between 18 and 21 percent, while at a greater soil depth and specially in soils that are wet for a long period it can be very much lower. The carbon dioxide content of soil air is usually around 0.36% and can reach up to 21 percent if the anaerobic condition exists due to waterlogging and can reach nearly 20 per cent. Under reducing conditions soil air may contain methane, hydrogen sulphide, and ammonia.

The low noncapillary porosity (air capacity) of poorly drained soils is probably caused by seasonal flooding or waterlogging which reduces biotic activity and due to clogging of pores and cracks by the downward flow of dispersed topsoil material and possibly by relatively large flakes of mica. The relatively high porosity of Old Himalayan Piedmont Plain soils (about 18%) is probably caused by their low content of fine silt in relation to fine and medium sand which leaves relatively large spaces between the sand grains. Air capacity of topsoils is greatly reduced by ploughing or puddling. [Md Akhter Hossain Khan]