ABSTRACT
As discussed before, the soil gas and liquid phases are located in the pore spaces or voids. Depending upon the conditions, the total pore space may vary from approximately 25% (Manahan, 1975, 2009) to 50% by volume in loam surface soils optimum for plant growth. At this condition, these voids are lled with 50% air and 50% water (Brady and Weil, 2008). Generally, a dynamic equilibrium exists between the air and water content. When water enters the soil, it displaces air from the pore spaces. When the water evaporates from the soil or is taken up by plant roots, air is moving into the pores. The composition and chemical behavior of the gas and liquid phases are determined by the interaction with the solid phase.
