Potting soil is also known as potting mix. The main use for potting mix is to give the plant a place for its roots to grow. Many potting mixes contain vermiculute or perlite, which keep the potting mix fluffy so air can reach the plant's roots. Potting mix allows for drainage, meaning it holds on to water long enough for the plant to take some of it, but then the rest of the water flows away so that the roots do not become too wet.