Perlite is a volcanic glass that is mined and then heated in big ovens until it 'pops' into a round, white material. That white material is used in seed-starting and potting mixtures to lighten the soil, allow more air around the roots of plants. It helps to retain water and improve drainage in the potting mix by providing aeration. The principal value of perlite in any potting medium is providing aeration and improving water drainage. The surface area of perlite particles contains tiny cavities that hold moisture while the particles themselves create tunnels in the mixture that allow air and water to flow freely to the roots. These properties make it helpful for gardening.