A basic understanding of the factors that mediate this relationship can be valuable in developing sound management practices. Pores are created by the spaces between the solid components of the medium. Therefore, a mix which contains coarse aggregate has fewer but larger pores than one made up of fine aggregate. The size and distribution of pores is one of the most critical factors in developing a growing medium with optimum physical characteristics. 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.