The white material in commercial potting soil is perlite or vermiculite, substances that retain water and add texture to the soil to keep it from becoming compacted. Most commercial potting soil mixtures are made up of at least 25 percent perlite.
Perlite and vermiculite act as a water source for container plants, which can dry out quickly. Both substances hold significant amounts of water, allowing plant roots to draw moisture from the white pellets when the soil becomes dry. Vermiculite can become compacted if it is compressed, which interferes with drainage. Homemade potting soil can contain up to 20 percent each of perlite and vermiculite.