Perlite is a volcanic rock that has been heated and expanded to become a lightweight, white material. Perlite is sterile and pH-neutral. When added to a soil mix, perlite can improve air space and water drainage. It is a hard material that does not break apart easily. Perlite pieces create tiny air tunnels, that allow water and air to flow freely to the roots. Perlite will hold from 3 to 4 times its weight in water, yet will not become soggy. Perlite can be used instead of sand to reduce the soil mix weight.