Perlite and vermiculite are staples in soilless potting mixes. Their –ite suffixes denote their mineral compositions, but that’s where their similarities end. They differ in origin, appearance and uses. Although they are sometimes mistaken for each other, each makes a unique contribution to amending potting mixes and garden soil. This difference depends on the amount of drainage or water retention a plant needs.
Easily identified by its resemblance to tiny bits of popcorn, perlite is not a plant source but a mineral. It begins as a glassy rock that is formed from volcanic flows. The intense heat, followed by cooling during lava formation, traps moisture in the rocks. When the rocks are heated to 1600 degrees Fahrenheit, the volcanic glass expands and “pops” to release this moisture, similar to how popcorn pops. The resulting product, called perlite, is white, fluffy and lightweight.
Perlite’s primary uses are to loosen soil and potting mixes and to improve drainage. It loosens soil by opening small pockets through which water travels freely around plant roots. These pockets also allow oxygen to aerate the roots. Although perlite does not absorb water like a sponge, the individual particles have lots of nooks and crannies, which trap moisture and make it available to plant roots. Perlite has insulating properties that allow it to mitigate extreme fluctuations in soil temperatures.
Vermiculite’s mineral composition is magnesium-aluminum-iron silicate, and in its natural form it looks like the shiny flakes of mica, another mineral. When vermiculite is heated to temperatures of 900 degrees F or higher, the flakes exfoliate and release steam from water molecules that are contained inside the mineral. The flakes expand up to 20 times their volume and form particles that look like worms, or miniaccordions. This wormy resemblance explains its name, which has its Latin derivation in “vermiculare” -- to breed worms.
Unlike perlite, which does not absorb water, the accordionlike folds in vermiculite retain moisture, making it a suitable amendment when you need a water-retentive potting mix or garden soil. Vermiculite also loosens and aerates soil, preventing heavy soils from compacting and allowing roots to grow unrestricted. Its light weight lightens the load in hanging baskets so that they don’t overload their mounting hooks and brackets. Vermiculite is a winter packing material for bulbs and tubers that need overwintering indoors.