Here in Ohio, we have heavy clay soil that tends to compact easily. When we get steady rain – as we often do – clay soil gets waterlogged, and once it dries out a bit, it turns as hard as a brick. None of these conditions are good for typical garden plants.
And so, we use soil amendments such as vermiculite and perlite to prevent compaction, provide aeration (so plant roots can get air) and manage the movement of water in the soil.
While both vermiculite and perlite are used for these reasons, they are not interchangeable, and actually quite different in their actions.
Perlite is super-heated volcanic glass and looks like small, white balls of styrofoam. It is commonly added to potting mix and has a near-neutral pH and low buffering capacity in soil.
It provides excellent aeration and while it does absorb water, it drains it away rapidly. And so, perlite is best for plants that prefer dry, quick-draining soil.
Vermiculite is super-heated mica and looks like shiny flakes of golden to dark brown when dry. When wet, it swells greatly into worm-like shapes. Vermiculite has a near-neutral pH and high buffering capacity in the soil.
While vermiculite provides moderate aeration, it can hold 3-4 times its volume in water. Therefore, vermiculite is best used for plants that prefer moist or boggy soil.
Feel free to use the graphic as a reminder of when to use perlite or vermiculite. If you want to know how to make your own potting mix, knowing the properties of these two substances is a great place to start. Your plants will thank you!