Perlite and vermiculite are similar in many respects for gardeners. Gardeners use both to aerate soil, aid in moisture retention and root seedlings. There are differences between the two, however, that may affect the way you garden. In some instances, you will get the best results by experimenting with both under your specific growing conditions.
Perlite is a form of natural glass. Heat perlite to the right temperature and it will expand four to 20 times its original volume. This is an unusual property for volcanic glass. The expansion process leaves a structure with very low density. According to Idaho Minerals, a perlite mining company "expanded perlite exhibits very low thermal conductivity, low sound transmission, high fire resistance, a large surface are and low moisture retention." Vermiculite is a mined ore, specifically, hydrated magnesium aluminum silicate. Expose vermiculite to extremely high temperatures briefly and it will exfoliate, or expand, into elongated particles, up to 20 times its original size. Exfoliated vermiculite is "lightweight, incombustible, compressible, highly absorbent, non-reactive yet compatible as a functional filler," according to the Vermiculite Association.
In its natural state, vermiculite resembles mica. When processors expose it to heat, it expands and looks more like small worms. Perlite has a relatively round shape, consistent with the notion that it pops like popcorn. The expansion process turns perlite white in color, transitioning from its transparent light gray to the glossy black color of its natural state. Vermiculite tends to be a brown or gold color.
High cation exchange capacity (CEC) means the medium has the ability to absorb and release large amounts of cations from the soil solution. Cations are positively charged elements. Plants benefit from cation exchange by being able to take in normalized concentrations of available nutrients. Vermiculite has a medium CEC. Perlite has a low CEC, bordering on zero.
Perlite retains water around the granules rather than in them. Vermiculite granules retain water in air spaces. Vermiculite may at times hold too much water for your plant and you run the risk of over watering the plant. There is less of a chance of over watering with perlite because of the way water runs around it. This difference also means that perlite is more likely than vermiculite to float to the surface of your container each time you water.
A great majority of home improvement stores and gardening centers sell horticultural perlite throughout the year. Gardeners have a much more difficult time locating bags of vermiculite on store shelves. The leading vermiculite processor in the United States closed its mine and 20 processing plants in 1990. This reduced the supply of vermiculite in the United States. In addition, demand for vermiculite fell after reports surfaced linking it to asbestos, according to manufacturer The Schundler Company.
The Virginia Vermiculite company uses the open pit method to mine vermiculite. They separate the vermiculite from other minerals, dry it and screen it to produce four different sizes. To mine perlite, the Cornerstone Industrial Minerals Corporation in Oregon removes topsoil and organic matter, and drills and blasts to get the ore. They replace the organic material later. They crush the ore and separate it into various grades.