▷ A Guide to Gardening with Perlite (And a Bit About Vermiculite)
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Gardening with Perlite and Vermiculite: Soil Enhancements

Written by Alan Ray / August 25, 2018
All You Need to Know About Perlite and Some About Vermiculite

Perlite & Vermiculite

If you garden regularly or just putter around with plants and flowers you're probably familiar with perlite and/or vermiculite. Most plant friendly people are. Even people who don't garden when asked, knew the pair had something to do with dirt and plants although few could tell us specifically what that was when asked. In fact, they returned more questions than we asked. They wanted to know the following.

What are perlite and vermiculite? How are they used? What do they consist of? Are they the same thing? Serve the same purpose? Are they natural, chemical or man-made? You'll learn the answer to these questions and even a few perlites of wisdom in this informative gardening soil article.

At Organic Daily Post we've taken an in-depth look into those two gardening companion favorites, perlite and vermiculite, and want to share our findings with our readers. While we covered both, our focus here was perlite. We addressed vermiculite in a more general sense to reveal the differences between the two.

Perlite

Perlite is a natural silicate rock meaning it contains a large amount of silica (SI) in its composition according to the Mineral Information Institute. Perlite is also a mineral which retains a water content of between 2 and 5 percent which is fairly high. That moisture plays a critical role in transforming mineral from rock into the light weight perlite found in garden centers and nurseries. 

Perlite begins its life as obsidian or volcanic glass. Obsidian is created by the rapid cooling of lava before crystallization is formed. To become perlite the obsidian must come in contact with and be hydrated by water. It is this naturally occurring process of moisturizing that gives perlite its unique properties when processed.

The Process

The final bagged product of perlite we pick up at our favorite garden store is a far cry from its original structure when first mined. Earth moving behemoths dig the perlite from open-pit mines where it eventually finds its way into a rock crusher and afterward sorted according to grade.

After extraction, this volcanic glass is transferred to a facility where large furnaces await. The perlite is loaded into the furnace then heated to a temperature of 1600 degrees Fahrenheit or 871 degrees Celsius (or centigrade). At this temperature a transubstantiation occurs within the rock.

The moisture content within the glass vaporizes causing it to suddenly burst like a kernel of popcorn. This mini explosion greatly expands the perlite causing it to swell up to fifteen times its normal size and become a soft, spongy material. It is this finished product that is used as a soil amendment due to its water retention capabilities and ability to aerate soil.

Moreover, the finished product is extremely lightweight, weighing only 5 to 8 pounds per cubic foot. That's pretty astonishing considering by contrast, a cubic foot of dry, loose soil weighs between 75 and 110 pounds. Additionally, the newly formed perlite is now comprised of tiny compartments of air with the exterior being covered in moisture absorbing cells. Its whitish look and spongy feel often leads it to be mistaken for Styrofoam balls in a plant's pot.

While wonderful in many ways for the gardener, there are multiple uses for perlite in construction as well as industrial implementations. Perlite is used as roofing insulation, in masonry as a block filler to reduce noise levels and in numerous other ways. For this article, we'll concern ourselves with perlites horticultural applications.  

As a Soil Supplement

In the garden, perlite's best known use is being added to soil or as a stand alone soiless medium. Perlite particles when mixed into dirt or moss create little clefts and air pockets that allow water and oxygen to proceed unimpeded to the plant's roots. This unique characteristic encourages the dispensation of nutrients to the roots and by creating space within the dirt, promotes better drainage of the soil.

These properties are particularly important in that a plant absorbs over 95% of its oxygen through its roots. Poorly draining soils do more than just keep a plant's roots wet, which can lead to root rot, they also block out oxygen which causes the roots to stop pumping, suffocating the plant and eventually, killing it.

Additionally, due to the unique shape of each perlite particle, and it permanency, moisture and nutrients hold fast to the tiny crevices on the surface until the plant has need of them. The granular quality of perlite allows for good drainage of any excess moisture that may have accumulated. This is paramount to growing a healthy plant that doesn't like wet soil. It is estimated that about one-half of all plant fatalities are due directly to over watering.

Helping Houseplants Receive More Light

This extraordinary little supplement also possesses important light-reflecting capability that can be very beneficial to houseplants during periods of lower light as on overcast days or during the short-sunned winter months. Winter can be a very dry time for house plants because heating the house during these periods can significantly dry out the air causing the tips of the plants to become brown and brittle.

By placing a tray of water near your plants with some perlite in it or by sprinkling perlite on the surface of the potting mix containing the plant, you can greatly increase the value of the sunlight it receives through the window as the perlite acts as a reflector that directs light back to the plants. This can make a huge difference in the overall health of the plant and promote good plant vigor.

TIP: By keeping the perlite moistened you can increase the humidity surrounding your plants, allowing them to take advantage of the extra moisture. While on the subject, vermiculite retains much more water due to its larger surface area which creates even more humidity than perlite as it evaporates. More on vermiculite later.

Mix & Match

As mentioned, perlite makes an excellent amendment to straight soil but can also be used as an exclusive grow medium. It works really well when mixed with equal parts of sphagnum moss or peat moss. For best results, the moss's should be shredded before being introduced to the mixture.

This combination of perlite and moss make an all natural grow medium that covers all the bases of good soil requirements. It retains moisture while at the same time allowing for good drainage and oxygenation, critical for a healthy plant. It aerates the soil due to each particle of perlite having its own irregular shape. Additionally, nutrients and moisture cling to the cracks and fissures on the surface and are absorbed by the plant as needed.

Plants started from seed will appreciate the easier path they are afforded when grown in perlite as its light weight and lack of density permits their tiny shoots to push through to the surface with less resistance. The roots enjoy this perk as well.

Water and Perlite

When it comes to watering indoor plants they can be a bit finicky. Plants in general like a lightly but continuously moistened soil. Some plants, such as succulents, don't require watering as often because they retain moisture in their broad, glossy leaves. Succulents prefer to go a bit dry before refueling. It can be tricky knowing when to water these plants and when to let them dry out. This is where incorporating perlite into the soil can help.

If an indoor plant like say, a Jade or other succulent, dries out too much, which happens because people feel they don't need to water them, this can cause the dirt to shrink up to where it separates from it's container. When this occurs, any water you add will simply run off the sides and not be absorbed by the remaining soil containing the root ball. Hard, dry dirt doesn't absorb water very readily or flash floods wouldn't occur. That principle remains the same even with a potted plant.

If you do encounter this problem, simply take the pot containing the plant and and let it soak in a pan of water until it re-hydrates completely. Add some additional soil mixed with perlite afterward to fill in the gaps. Fill it up to near the top of the pot and re-water lightly.

Perlite works to help the plant receive and maintain the correct amount of water and nutrients. Its scrobiculate surface retains excess moisture which can then be extracted by the plant when it begins to dry out. These characteristics make perlite so useful to the gardener and advantageous to the plants.

This unique ability is especially good for those of us who aren't quite sure when to water succulents and for those who occasionally forget.

Perlite & Root Cutting

Perlite makes an excellent starter medium for cuttings or clones, as they are now called. By itself or mixed, cuttings and roots seem to thrive in perlite but there two things you can do to help expedite the growing process. You want to keep the perlite moist and keep the cuttings out of direct sunlight. Warmer temps will produce roots and new plants faster than when grown in cooler temperatures. Too much direct sunlight can toast new shoots.

Got it - Now What About That Vermiculite?

Vermiculite is non-organic soil and is similar in certain ways to perlite but there are differences. Both are lightweight, water retentive, non-toxic and fireproof, making them alike in that sense, but industrial usage is different for both. One major difference lies in the fact perlite is made of volcanic glass whereas, vermiculite is comprised of mica, a sheet silicate mineral. Both are heavily mined. Vermiculite mines exist in South Africa, China, Russia and Brazil. Like perlite, vermiculite also has many industrial uses.

While both possess similar water retaining properties, vermiculite has a larger surface area and acts more like a sponge, absorbing and holding water to the point of saturation then releasing it slowly over time. It also creates a soil environment that retains water. If you are growing cactus or other plants that have low water needs, vermiculite is not the correct amendment for those type plants. You want quicker draining like that of a sandy soil. Perlite is your best bet in this instance.

Switch plant types, however, and things change. If you were growing mushrooms for example, vermiculite would be an excellent choice for an additive to the substrate (the surface from which they grow) because of its moisture retentive properties. Vermiculite is also effective at aerating (expanding) the soil.   

That's a Wrap

So there you have it. All you need to know about perlite and vermiculite and how to choose the one best suited for your plant's particular needs. Knowledge is power and power feels good.

Now the next time you hear someone discussing perlite or vermiculite, you'll not only know what they're talking about...you'll know what you're talking about too.

Happy Planting!

About the author

    Alan Ray

    Alan Ray is an independent writer who has experienced an eclectic career. He co-created and was the Head Writer for a reality television series as well as composing and producing the Theme song. He has written radio comedy, specials for National Public Radio, hundreds of blogs and no end of gardening and how-to articles spanning a galaxy of topics from aquaponics to the history of the flush toilet. He has also worked tirelessly on countless projects that went nowhere. Additionally, Alan spent years in recording studios in Nashville, Tennessee, where he worked as a staff songwriter for various music publishing companies. He is both a multiple BMI Award recipient and ASCAP award-winning songwriter, respectively. He has also written 5 books and is a New York Times best selling author (Non-Fiction/Humor). While touring the country doing book-signings, he did over 700 live radio interviews with every major radio station in the nation as well as the BBC and Radio Free Europe. Currently, he is a regular contributor to this site in addition to two of the top gardening magazines in Canada including a Medical Marijuana magazine and also a B2B mag. He enjoys small town life in southern Tennessee where his time is spent writing, messing with plants and laughing a lot with his lovely and quick-witted wife.

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