In order to grow lush, healthy plants, a growing medium should have several qualities. A good growing medium must provide air circulation to the roots of the plants. It must also provide good drainage while retaining enough moisture and nutrients to support the plant without becoming hard and compacted. Most commercial potting mixes are a combination of several different materials.
Peat moss is a light, fluffy material and a primary ingredient in many commercial growing mediums. Peat moss is the result of dead mosses that have fallen to the bottom of damp, boggy areas where the plants eventually decomposed. The quality of peat moss will vary depending on the species of decomposed plant. Although peat moss remains a popular growing medium, its use is no longer encouraged due to the destruction of bog habitats caused by the harvest and milling of peat.
Sphagnum moss is also the result of decomposed bog plants, but sphagnum moss is harvested while the plant is still alive and floating on top of the water. Sphagnum moss is a highly desirable growing medium with excellent drainage and aeration. Although sphagnum moss is most commonly used dried, living sphagnum moss is often used for plants that require a consistently damp growing medium.
Most forms of bark used in growing media are byproducts of the timber industry and tend to be less expensive than peat moss. However, some barks can be very expensive. Similar to peat moss, bark provides excellent drainage but tends to become dry faster than peat moss.
Perlite, which looks like small popcorn, is the result of white volcanic rock that has been heated to a very high temperature, causing the rock to expand. Once expanded, the rock is crushed, leaving a light, puffy substance. Perlite, an odorless, sterile material, is often added to other growing media to improve drainage and air circulation.
Vermiculite, often combined with other growing media because it retains nutrients while improving drainage and aeration, is a puffy, gray material that results from the heating of chips of mica. Although vermiculite is similar to perlite, it is less durable.
Although sand is an effective growing medium, it can be very heavy, so it is often combined with perlite or vermiculite. While fine sand can become compacted and can decrease drainage, coarse sand is useful, especially when added to potting soils formulated for cactus and succulents.