Perlite vs. Vermiculite
Jim Fry
Northport, New York
Reprinted from the New York Chapter newsletter, November 2000
I've often wondered what the difference between perlite and vermiculite was and was even told by someone in a local nursery that they were interchangeable. Recently I came across a description in Horticulture magazine. Vermiculite is an aluminosilicate clay mineral that is mined and heated to expand the particles. It's sterile, soaks up to three to four times its volume in water, and attracts nutrients such as calcium, magnesium, potassium, and phosphorous.
Perlite, on the other hand, is a silicon-rich volcanic rock. It's also mined and heated to expand the particles. It will soak up some water but is mainly used to aerate and improve drainage in potting mixes.
I asked one of our local plant meisters why I couldn't use vermiculite instead of perlite for my potted rhodies. After all, those nutrients that are attracted by the vermiculite are good for rhodies, aren't they?
He explained that the vermiculite attracts nutrients by remaining wet. For other plants that might be okay, but for rhodies it's a death sentence usually carried out by our old nemesis, phytophthora.