Whether you’re customizing your own potting mix, exploring different soil amendments, or purchasing a soilless growing medium, the vermiculite vs perlite question eventually comes up. Which one is best? (Or should you be using both?) As it happens, vermiculite and perlite are ubiquitous in potting soils and seed-starting mixes, but there are some important differences between the two. This article explains where vermiculite and perlite come from and the many ways each of them can be used.
Aside from their uses in horticulture, these two products have also been used widely in construction and manufacturing. Perlite, in particular, is sometimes added to cement or plaster, used in insulation, or incorporated into filtration systems. Vermiculite, historically, has been used in packing liners, insulation, fire protection products, and more. So, what exactly are vermiculite and perlite anyway? What do they actually do? And how did they make the leap from the construction and manufacturing sectors to our local gardening centers?
The reasons perlite and vermiculite are used in horticulture
Besides their many applications in other industries, vermiculite and perlite have been used in horticulture for decades. Turns out, each of these mined resources has specific physical characteristics which provide growers with distinct benefits. For instance, perlite is prized for its ability to help aerate growing media and facilitate drainage. As for vermiculite, it expertly stabilizes soil moisture levels.
Both of these lightweight materials also come in a variety of different grades, ranging in size from very fine to extra coarse. This gives gardeners and horticulturists greater flexibility—and more uses for these valuable minerals.
What is vermiculite?
Long before processed vermiculite lands in a bag of seed-starting mix, it begins as a volcanic rock deposit, extracted from deep underground. Flakes of this mined product contain minerals like magnesium, aluminum, iron, and silicon. Arranged in thin layers, vermiculite’s crystalline structure also includes molecules of water which play an important part in its ultimate transformation. According to the American Geological Institute, “Vermiculite flakes are heated to more than 1600° F (900° C) or higher, causing the water within the flakes to flash steam and expand.” The resulting particles swell “eight to 20 times larger” than their original size.
Up close, this expanded material looks like glittering, folded bellows or, perhaps, tiny accordions, but someone along the way must’ve thought the heat-treated particles looked more wormlike. (The word “vermiculite” comes from the Latin word “vermiculari,” which means to “be full of worms.”)
It’s important to note that some of the questions surrounding the vermiculite vs perlite debate have to do with asbestos. Vermiculite-containing products like old insulation may include potentially harmful asbestos. That’s because from the 1920s through 1990, the lion’s share of U.S.-sourced vermiculite came from one asbestos-contaminated mine outside of Libby, Montana. (the mine has since been closed.)
Of course, it is possible that some currently available vermiculite products may also contain trace amounts of asbestos. Still, these levels are so minute that they’re unlikely to present serious health risks.
What is perlite?
Like vermiculite, perlite also comes from underground. (Nearly all of the world’s perlite comes from mines in the U.S., Greece, China, Japan, Turkey, and Iran.) Perlite is derived from volcanic glass which forms when a glassy-looking rock called obsidian comes into contact with water. As with vermiculite, this mined raw material is then heated—this time to temperatures between 1400° and 1800° F.
As the temperatures climb, the crude perlite product expands and pops—not unlike popcorn—resulting in the airy, sphere-like particles we know as perlite. Perlite is typically bright white, with the look and feel of Styrofoam. However, closer inspection reveals a texture closer to pumice. If you were to magnify an individual piece of perlite, you’d see that the surface is pocked and fissured. Water (and any water-soluble nutrients) can settle into these surface nooks and crannies at least temporarily, providing plants with a little boost between watering sessions.
The main benefits of each
Which factors matter most when deciding on vermiculite vs perlite? If moisture retention is most important, reach for vermiculite. (The material is so absorbent that researchers have studied applications for its use in cleaning up oil spills and mitigating heavy metal pollution!) But if root-zone oxygenation and drainage matter most? Perlite really delivers on these.
Nutritional content of perlite and vermiculite
In thinking about vermiculite vs perlite, you should also consider the nutritional content of each, as well as their capacity to hold onto any nutrients that may be present in your growing medium. This capacity—formally called the cation exchange capacity (CEC)—is just a measure of a material’s potential to grab onto available molecules. (In soil, many of those molecules happen to be nutrients essential for healthy plant growth.)
From your garden soil to the potting mix you use in containers, the composition of various soil types influences CEC. (To better understand how CEC works, keep in mind that cations are molecules that are positively charged. That means they’re naturally attracted to negatively charged molecules.) Heavy clay soil has a high CEC. In other words, it’s more able to attract nearby free nutrient molecules than materials with a low CEC.
Organic matter such as coconut coir, compost, and peat moss also have relatively high CEC measures. The CEC values for vermiculite and perlite, on the other hand, are low. When used by themselves, neither vermiculite nor perlite hang onto potentially available nutrients very well.
Incidentally, while perlite doesn’t contain any essential plant nutrients, vermiculite does contain a little potassium and magnesium. But that doesn’t mean you should rely on it to deliver these nutrients to your plants.
The main differences between vermiculite vs perlite
The differences at-a-glance:
- flaky and spongelike material
- holds and releases a lot of water and nutrients
- more easily compacted
- less reliable than perlite for optimal drainage
- pH may vary; calcium often added to mixes
to nudge pH levels
- sphere-like, porous particles
- holds and releases small amounts of moisture
- retains firm shape
- facilitates good drainage and root zone aeration
- neutral pH
Similarities of perlite and vermiculite
Aside from being mined products, both perlite and vermiculite can be very dusty to handle. If you work with them frequently or have special health conditions, exposure to these dust particles could be risky. To protect yourself from inhaling potentially harmful irritants, wear a mask. Wetting down perlite and vermiculite before handling them can also reduce dust levels.
Should you add either vermiculite or perlite to your garden’s soil?
Although it might be tempting, adding perlite or vermiculite to outdoor garden soil isn’t a great idea. First, neither one will biodegrade. Also, they don’t improve soil fertility—nor do they stay where you put them. (Perlite, especially, tends to separate out of the soil, forming layers of tiny, floating particles after a good soaking.) Compost, worm castings, or aged manure are much better choices.
Which is better to use in potting soil?
When it comes to vermiculite vs perlite in potting soil, making the best choice really depends on the kinds of plants you want to grow. Because perlite’s physical structure allows for good drainage and aeration, it’s regularly included in potting soils designed for cacti, succulents, and other plants which thrive in well-draining conditions. And, since vermiculite does such a good job of soaking up water, it’s often included in specialty potting soils made to support moisture-lovers like African violets.
That said, however, many potting soils do contain both perlite and vermiculite, and the pair of ingredients works perfectly well together. Our article on DIY potting mixes includes recipes that use both products.
Vermiculite vs perlite for plant propagation
When deciding on vermiculite vs perlite for plant propagation, carefully consider exactly what you want to accomplish. For example, if you want to start seeds, vermiculite can be used all by itself for maximum moisture retention. (You’ll also find vermiculite frequently included with peat moss in lightweight, seed-starting mixes.)
Planning to propagate plants from cuttings instead? In that event, a perlite-heavy, soilless mix might serve your needs better as developing roots will be able to access plenty of oxygen. (This helps to make them less susceptible to root rot.) Thanks to perlite’s lightweight bulk, new roots can more easily push through the growing medium, too.
This video shows you the differences between perlite and vermiculite in action:
Other uses of vermiculite and perlite
- Hydroponics—Because it creates so much space for plants’ roots, extra coarse-sized perlite can be used as a standalone medium in some types of hydroponics systems.
- Pots and planters—Large chunks of perlite are still remarkably lightweight, making them a great stand-in for the empty plastic bottles or soda cans you might include in the bottom of a large pot to make it lighter. Best of all, when you use perlite for this purpose, your lighter weight planter still drains well.
- Specialty seed germination—When starting very small seeds, covering them with a fine layer of vermiculite can help to keep them protected and moist. What’s more, since vermiculite is so lightweight, very delicate seedlings can more easily poke through it when they’re ready.
- Seed “bombs”—Equal parts vermiculite, compost, and either clay or paper pulp make an excellent base for homemade seed balls or “bombs.” Simply moisten the ingredients, mix well, add the seeds of your choice, and shape into small balls. Once they’ve dried, you can share them with friends or plant them yourself.
Now that you know where perlite and vermiculite come from, along with their physical differences and the benefits each can provide, making the vermiculite vs perlite choice should come easier. Remember, if you’re looking for something airy and lightweight, perlite is perfect. The mined, heat-treated material promotes aeration, helping to prevent root rot and combat soil compaction in potted plants.
Need good water retention for potted plants which prefer “wet feet”? Then use vermiculite as an ingredient when mixing your own potting soil or choose a product containing some vermiculite. Also mined and heat-treated, vermiculite acts like a tiny sponge—or in this case, a whole bunch of sponges—expanding when wet. And, because seeds need consistently moist soil to germinate, vermiculite is also particularly useful for seed starting.
For more information on soil amendments and care, please visit the following articles:
- The best soil amendments
- How to make your own compost
- Garden soil vs potting soil: What’s the difference?
- The best soil for a raised bed
Pin this article to your Gardening board for future reference!
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