Potting soil is a combination of several ingredients that are chosen to provide the best drainage, support and nutrition to plants. Commercial potting soils and homemade mixes contain similar ingredients. They are usually a mix of three substances. On assists in holding moisture, a different ingredient provides support for the plant so it stays upright, and the third provides drainage and aeration. Different combinations are used, depending on the specific needs of the plant.
Compost is the nutrient-rich byproduct of decomposing organic material. Use a vegetative compost from a home compost pile for most homemade potting soil mixes. Commercial potting soils often have a combination of vegetative compost and manure compost. Manure compost commonly uses the manure of chickens, though worm castings are common for home composters. Compost provides nutrition to the plants as well as helps retain moisture. Most commercial and homemade potting mixes contain one part compost.
Peat moss is another additive that adds nutrition, though not in the same quantities as compost, and helps in moisture retention. Peat moss is a mixture of sphagnum moss and peat, a special type of decomposed soil. Peat moss releases moisture slowly into the rest of the potting mixture. It is also lighter than the compost, allowing more aeration to the plants' roots. Peat moss is used in addition to, or as a replacement for, compost in potting mixes.
Perlite is a volcanic rock and resembles small pieces of white Styrofoam in potting mixes. Water inside the rock expands when heated at high enough temperatures, which cause the structure of the rock to break down and become light and airy. Perlite aids with drainage to prevent the potting soil from becoming too wet. It also provides air pockets in the soil for the roots.
Vermiculite is another type of rock. When heated, the vermiculite rocks expand and break apart, resembling worms in the process. Vermiculite aids in the drainage and aeration of the soil, allowing the roots to freely grow. It is often used with peat moss in compost-less potting mixes.
Common sand is usually added in small quantities to assist in drainage and to support the plant so it remains upright. Sand must be clean and sterilized when used in potting mixes. Small pieces of gravel are substituted in mixes for drought-tolerant plants so the roots do not become too wet.
Bark, commonly pine bark, is added to potting mixes to aid in water retention and soil retention. The bark breaks down in the soil over time, releasing nutrients as it does so. It often comes mixed into commercial bags of compost.