Potting Mix is combination of organic materials (e.g., peat moss, aged bark and pumice) but no soil (dirt) that is used in lieu of potting soil to improve drainage.  Because potting mix has no soil, it tends to be lighter than potting soil and provides a more sterile medium for plants with fewer disease carrying agents.  Potting mix is most commonly recommended for plants in containers, and potting mix is particularly desirable for starting seeds.