Perlite is a mineral of volcanic mineral that is processed into a light, sterile growing medium that is often good for starting seeds. Many seeds require constant moisture to germinate. Some potting soils may not retain enough moisture to allow seeds to quickly germinate. In some cases, perlite is added to these soils and sterilized. The soil provides good root supports, while the perlite helps the soil retain water.
Perlite is a hydrated volcanic ore. Perlite contains between 2 and 6 percent bound water. Bound water is water contained within the rock. The mineral composition of perlite is 99.9 percent sodium potassium aluminum silicate. The other trace components of perlite are cristobalite, tridymite and quartz.
Perlite is a volcanic ore that originally was deposited during the Pennsylvanian geologic era, about 300 million years ago. While this layer of volcanic rock was being deposited, small amounts of water were trapped inside as it cooled.
The perlite is crushed, using industrial grade equipment to make small pellets. Once the pellets are at the desired size, the rock is heated to more than 1,400 F. The water in the rock expands, causing the rock to expand. This expansion creates voids that can hold water or nutrients. In its unprocessed form, perlite weights about 70 pounds per cubic foot. After processing, it weighs between 2 and 10 pounds per cubic foot.
Use in Seed Starting
Perlite is often used in seed starting and starting mixtures because it is sterile and does not contain potential plant pathogens. Perlite retains water well and can remain moist longer than potting soils, thus ensuring that the seeds have enough moisture to germinate properly.
Perlite can powder easily, and its dust can be an irritant. If you are going to work around perlite for long periods when starting seeds, wear eye protection and a dust mask.