ABSTRACT
The exchange of considerable amounts of gases, particularly oxygen and carbon dioxide, between the soil and the atmosphere takes place under the influence of both the pressure gradient and the concentration gradient. Both these kinds of flow may take place in the soil pores and also in the tissue of plants. Soil temperature variations, particularly in the day-night cycle, result in a cyclic expansion and contraction of the soil air and thus its partial exchange with the atmosphere. The factor causing the mass flow of gases in the soil is variation in the atmospheric pressure resulting in similar variation in the pressure of the soil air. The primary mechanism of gas exchange in a soil medium is, under normal field conditions, concentration diffusion. In relation to this there is quite an extensive literature concerning the mathematical modelling of the processes of gas diffusion in the soil, mainly in relation to oxygen.
