Tear open a bag of commercial potting soil and you immediately see a mixture of rich black soil and white flecks. That white stuff isn't there by accident — it's likely perlite, and it's an essential addition that makes the soil more suitable for plant growth. Perlite promotes soil drainage, so plant roots don't become water-logged.
"It increases pore space in growing media, which is critical for aeration [perforating the soil with pockets of air], water drainage and root growth," emails Jon Traunfeld, director of the University of Maryland Extension Home & Garden Information Center. "It doesn't change the pH of growing media. Plus, it is sterile and lightweight, making it easier to handle and cheaper to ship."
If you're not an ardent gardener, you might not have a clue what perlite is or where it comes from. You might also be surprised to hear that perlite's often called "popcorn for your soil."
It's definitely not popcorn, though.
What Is Perlite?
Perlite is a type of form of amorphous volcanic glass that contains roughly 2 to 5 percent water. It's a lot like pumice, but denser. Both types of glass are formed as molten lava slowly cools. Composition varies a bit, but perlite is made up mostly of silicon dioxide (70-75 percent), aluminum oxide (10-15 percent), plus smaller amounts of sodium oxide, potassium oxide and other minerals.
Miners extract perlite from the earth using explosives and/or machinery. There are large mines in countries like Armenia, China, Greece, Japan, Turkey and America. The U.S. imports roughly one-third of its processed perlite while getting the rest from domestic mines in Arizona, California, Idaho, Nevada, New Mexico and Oregon.
Once perlite is taken from the ground, processing plants crush the material into smaller pieces that are more suitable for a variety of purposes. Then, the magic begins. Workers heat the perlite to roughly 1,600 degrees Fahrenheit (871 degrees Celsius), causing the water within to expand into vapor. This "popping" reaction is where the "popcorn for your soil" moniker comes into play, and it's the same principle that causes the kernels of popcorn in your microwave to expand into a delicious treat.
The end result is what's called expanded perlite, and this process results in a material that's 20 times its original volume, while being 40 times less dense. Before it is popped, perlite is usually gray, but it can be brown, green, blue or red. Once it's expanded, perlite is generally light gray to white.
Structurally, expanded perlite is light and airy, with tiny bubbles and holes throughout. Its primary selling point is its low density and low price, which make it handy for a variety of applications.
Uses of Perlite
In expanded form, perlite's used for lightweight plasters, insulation, ceiling tiles and concrete. It's perfect for noise insulation, too, and the fact that it's non-flammable makes it safe for use in many types of construction. Perlite is also widely used in the beverage industry as a filtration agent, particularly for beer. In its puffed form, it assists filtration by fighting clogs that would otherwise shorten a filter's life.
For most people, perlite's unique properties are most apparent when it comes to gardening, and about 10 percent of the world's perlite use goes toward horticulture every year. "Perlite is sterile, lightweight and contains lots of tiny air spaces, increasing air in the soil and improving drainage," says Melinda Myers, a horticulturist and gardening expert, via email.
"When making my own potting mix I used one part perlite, one part compost and one part peat moss," she says. "You may need to try a few mixtures to see what best fits your gardening and watering style. Like any recipe, the proportions may be adjusted for the way the gardener likes to water and care for plants and the type of plants they are growing."
Traunfeld recommends making perlite 10-33 percent of your potting mix, depending on the intended use and other ingredients you have. "For example, higher percentages are helpful in seed starting mixes and for growing plants with root rot issues," he says.
Root rot is directly related to water drainage. Aeration is another — and it's critical, because roughly 98 percent of the oxygen that plants absorb from their environment happens through their root system.
That's why a lot of savvy homeowners also scatter perlite over their lawns. Perlite naturally works its way through the soil, preventing root rot and increasing aeration, resulting in healthier, more resilient grass growth.
Perlite isn't the only way you can improve your soil quality. Pumice, rice hulls, vermiculite, and pumice have similar qualities, but they have characteristics that vary depending on the issues you're wrestling with. Perlite, for example, is best for situations where you want very good aeration and drainage, as well as good water retention.
Depending on your needs, you can blend perlite with a variety of substrates to create soils that are perfect for different plant species. Myers says that peat moss, pine bark and coir (coconut fiber) are all commonly paired with perlite.