A portion of these pores is occupied by air. Plant roots require oxygen for growth so adequate aeration of the medium is necessary. As roots take up oxygen they also give off carbon dioxide. This exchange of gases is primarily by diffusion through the pores of the growing medium. Although total pore space is a measure of a growing medium's ability to hold air and water, pore size determines the rate of drainage and gas exchange.
Large pores permit air to reenter the medium following irrigation.
Since growing medium in containers holds a relatively large quantity of water, the percentage of pore space filled with air is reduced. Therefore, an adequate distribution of large and small pores is essential. Water is also held in the pore space of a growing medium. The availability of this water for plant growth is largely determined by how tightly it is held by the solid component of the medium. The closer a water molecule is to a solid, the more tightly it is held through the forces of adhesion and cohesion.