Poor Aeration in Soil: 2 Reasons

This article throws light upon the two main reasons responsible for poor aeration in soil. The reasons are: 1. Excess Moisture 2. Gaseous Interchange.

Poor Aeration in Soil: Reason # 1. Excess Moisture:

When a soil is subjected to excess moisture, waterlogged condition is developed. This situation is generally found on poorly drained, fine-textured soils having a minimum of macro-pores through which water can move very rapidly. It also occurs in well drained soils if the rate of water supply to the soil surface is sufficiently rapid.

Due to presence of very high amount of soil moisture resulting flooded condition which in turn develops poor soil aeration the most of the plants cannot grow. Such poor aeration can be prevented through the removal of excess water either by drainage or by controlled run off.

Poor Aeration in Soil: Reason # 2. Gaseous Interchange:

The inadequate interchanges of gases between the soil and the free atmosphere depend on two factors:

(a) The rate of biochemical reactions influencing the soil gases, and

(b) The actual rate at which each gas is moving into or out of the soil.

It is obvious that the greater exchange of gases would be required because of rapid consumption of O₂ and release of CO₂ in soil.

The exchange of gases between the soil and the atmosphere is facilitated by two mechanisms:

(a) Mass flow and

(b) Diffusion resulting from the different factors namely soil temperature changes, barometer variations, action of the wind and changes in the amount of pore space occupied by air as a result of the entrance of rain or irrigation water.

(a) Mass Flow:

Mass flow of air is apparently due to pressure differences between the atmosphere and the soil air.

Temperature may influence the renewal of soil air by two ways:

(i) There may be temperature variations within the soil between the different horizons. The contraction and expansion of the air within the pore spaces as well as the tendency for warm air to move upward may cause some exchange between the different layers and with atmosphere.

(ii) The soil and the atmosphere usually have different temperatures. This differential temperature also permits an exchange between the atmosphere and soil air in the immediate surface. Besides the temperature effect, other factors as mentioned earlier also cause mass flow between soil and atmosphere. It is relatively unimportant in determining the total exchange.

(b) Diffusion:

Diffusion is the molecular transfer of gases. The molecules of gases are in a state of movement in all directions. Through this process each gas tends to move in a direction determined by its own partial pressure (Fig. 5.1). Diffusion allows extensive movement from one area to another even though there is no overall pressure gradient.

Thus, even though the total soil-air pressure and that of the atmosphere may be the same, a higher concentration of O₂ in the atmosphere will result in a net movement of this particular gas into the soil. An opposite movement of CO₂ and water vapour is simultaneously taking place since the partial pressures of these two gases are generally higher in the soil air than in the atmosphere.

Besides partial pressure differences, diffusion process seems to be directly related to the volume of pore spaces filled with air.