Perlite has been around longer than any other soilfree growing medium. Made from air-puffed glass pellets, and nearly as light as air, perlite has excellent oxygen retention. Its ability to retain oxygen is the main reason it is used as a supplement in soil and soilfree mixes. The main drawback of perlite is its lightweight consistency, which makes it easy to get washed away. This drawback makes perlite an inappropriate medium in flood and flush type hydroponic systems or those that would be subjected to strong wind and rains if situated outdoors.
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