Are There Different Grades of Vermiculite?

Vermiculite

• The United States Geological Survey lists vermiculite as a magnesium-aluminum-iron silicate mineral. The word vermiculite has its Latin origins in “vermiculare,” which means to breed worms. In its natural mineral state, vermiculite has shiny flakes that look similar to mica, but when it is heated, the flakes expand into particles that look like tiny accordions, giving them a worm-like appearance. The convoluted folds of each particle trap air and the layers absorb water. When used as a soil amendment or potting mix component, this gives vermiculite the dual ability to keep plant roots moist while also aerating them.

Grades

• Typically, horticultural vermiculite is available in four sizes, from 1, which is the coarsest, to 4, which is the finest. Sometimes, the grades are labeled Large (Grade 1), Medium (Grade 2), Fine (Grade 3) and Super Fine (Grade 4). Particle sizes range from 5/16 inch for Grade 1 to 0.04 inch for Grade 4. Vermiculite may expand up to 20 times its original size and hold water from 220 to 325 percent of its weight, depending on grade.

Uses

• If your garden soil is heavy or compacted, you can incorporate vermiculite to loosen it so plant roots can penetrate more easily. You can add up to 50 percent by volume of vermiculite to your garden soil. The heavier your soil is, use a larger grade of vermiculite to make it more porous. For example, if you have heavy clay soil, use the large (Grade 1) vermiculite. If you want to root cuttings from your favorite plants, you can use vermiculite right out of the bag as a cutting medium. Water it first so it is able to keep the cuttings moist, and keep it moist after you stick your cuttings into it. As long as your container has good drainage, it’s nearly impossible to overwater the vermiculite and rot the cuttings. Medium (Grade 2) vermiculite works for most cuttings, but the coarsest vermiculite (Grade 1) is best for larger and heavier stems. If you start your own seeds, use the finest vermiculite (Grades 3 or 4) to cover the seeds, with Grade 4 as the best choice for the tiniest seeds. To store bulbs over the winter, pack them in any size vermiculite to absorb any excess moisture to better withstand rotting and to insulate them from cold temperatures.

Distinction

• Another naturally occurring mineral sometimes mistaken for vermiculite is perlite, which is a by-product of volcanic activity. However, its appearance and uses are slightly different than those of vermiculite. Perlite looks like white flecks of popcorn, and vermiculite is tan with shiny flecks. Although they both loosen heavy soil, vermiculite absorbs water into its particles while perlite only retains water around the outside of particles without absorbing water inside. You can use vermiculite when your plants need a more moisture-retentive potting mix and perlite for plants that need sharper drainage.