Answer: Vermiculite, and its substitute Perlite, are both naturally occuring minerals that are heat treated and expand to 20 times their original volume. They are both inert and help soil-less mixes retain water and aerate the mix. The only real difference is that vermiculite breaks down a little sooner than perlite. The following information is found in an industry publication:
"Perlite and vermiculite have been used for years to amend professional potting soils made from peat moss (called "soilless" mixes or artificial soils because they literally contain no soil). They also have been used in outdoor mixes, in turfgrass and outdoor plantings, for gardens, for special 100% perlite or vermiculite growing applications, and increasingly for commercial and amateur hydroponic growing and water conservation (especially in landscaping and gardening).
Essentially perlite and vermiculite are used in the horticultural industry because they both provide aeration and drainage, they can retain and hold substantial amounts of water and later release it as needed, they are sterile and free from diseases, they have a fairly neutral pH (especially perlite which is neutral), and they are readily available, non-toxic, safe to use, and relatively inexpensive.
As a rule of thumb, perlite tends to last longer, has a more neutral pH, and functions much better in hydroponics, outdoor applications, lawns and gardens (in part because it is stronger). Nevertheless, for decades they both have been used by professionals, dedicated amateurs and gardeners."
Hope this information helps!
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