What is Perlite? What is its use in the garden?
Have you ever used a bag of commercial potting soil? If so, you may have noticed small white objects that look like styrofoam balls in the mix.
These little balls are a type of mineral product called perlite. Every ingredient in those mixes have a beneficial effect on plants, and perlite is no exception.
If you want to become good at gardening or hydroponics, perlite can be your best friend. Seasoned gardeners swear by this mineral, and use it extensively in their horticultural endeavors.
What is so special about these queer, innocuous-looking balls? Learn more in our in-depth guide to all things perlite.
Don’t know where to find a good perlite product on the market? Here’s our favorite brand.
PR8 8-Quart Organic Perlite
Natural, clean perlite that is great for horticulture or hydroponic use.
What is Perlite?
Perlite is the name of a naturally occurring mineral. In nature, it exists as a type of volcanic glass, created when the volcanic obsidian glass gets saturated with water over a long time.
And since fertile volcanic areas have been settled since biblical times (due to fertile soils), humans have been aware of perlite at least since Third Century BC.
Natural perlite dark black or grey colored amorphous glass. Amorphous means that it doesn’t have any definite shape or structure, unlike a crystal.
What is Perlite made of?
Like all other volcanic rocks, perlite is also pretty heavy and dense in its natural form. Perlite typically contains the following ingredients:
- 70-75% silicon dioxide
- Aluminum oxide
- Sodium oxide
- Potassium oxide
- Iron oxide
- Magnesium oxide
- Calcium oxide
- 3-5% Water
Since it is a naturally occurring mined mineral, perlite is a nonrenewable resource. The major producers are Greece, US, Turkey, and Japan.
It is a relatively cheap mineral and is often used for industrial purposes like construction and in the manufacture of plasters, masonry, and ceiling tiles.
But of special interest to us here is the use of perlite in gardening and hydroponics.
And for that, the hard mineral glass needs to be processed into the light, white colored, plasticky stuff that resembles styrofoam, confusing many a rookie gardener about its origin and purpose!
Lets’ look in detail at the processes that transform perlite glass into “perlite foam” in the next section.
How is Perlite Made?
The processed perlite that we see in gardening mixes is basically “volcanic popcorn.” That is a very literal description.
Since perlite glass is rich in water, it pops when heated to very high temperatures, exactly like popcorn. So the processed perlite balls are created by crushing natural perlite glass and then baking them in industrial ovens.
To complete the transformation, crushed perlite needs to be heated quickly to 900 degrees Celsius (around 1650 degrees Fahrenheit). The mineral structure is softened by the heat, allowing the water trapped inside to expand into steam in a bid to escape.
The process leads to expansion of the crushed pieces of the mineral. It is not usual for perlite pieces to expand between 7 and 16 times their original size and volume, creating those lightweight faux-styrofoam balls.
The foamy balls have a lot of porous openings inside them and are clean, sterile and generally stable. It can hold its shape with ease in the soil without crumbling.
Significance of Perlite for Gardening
There are several reasons why perlite is such a useful additive to gardens and hydroponic setups. They mainly stem from its unique physical and chemical properties:
- Perlite is physically stable and retains its shape even when pressed into the soil.
- It has a neutral pH level
- It contains no toxic chemicals and is made from naturally occurring compounds found in soil
- It is incredibly porous and contains pockets of space inside for air
- It can retain some amount of water while allowing the rest to drain away
These properties allow perlite to facilitate two critical processes in soil/hydroponics, which are essential for plant growth:
All plant cells need oxygen, even those that are underground. The green parts up top are capable of creating it during photosynthesis.
But down below, the root system has to absorb it from the soil. Aerating the soil allows little pockets of air to remain, which helps with the growth of strong root systems.
Without water, no living thing can survive. But when it comes to plants, excess water in the soil can lead to drowning.
In this situation, the root system is starved of oxygen, causing eventual death. Proper drainage is crucial to allow empty air spaces to remain in the soil.
Adding perlite to the soil improves its drainage capabilities, as it has excellent filtering and water draining capabilities. The presence of all those pores allows most of the excess water to drain off.
And those air pockets also mean that perlite is great for root systems as well. When the soil gets packed down, the air pockets are lost. But since perlite is a harder mineral, it retains its shape, keeping those air pockets around for the roots.
How To Use Perlite In The Garden
Perlite has several uses in regular gardens:
In Soil Mixes: you can make your own homemade soil mixes using a combination of perlite, loam, and peat moss in equal measures. In pots, it keeps everything loose, aerated and well draining.
On the surface: perlite can be scattered on the surface of the soil as well, where it acts as a wicking agent. It will gradually work down into the soil, improving drainage.
For root cuttings: it encourages root growth much better than just plain water. You can place your starting seeds or cuttings in an air-filled Ziploc bag contained moistened perlite for several weeks.
How To Use Perlite In Hydroponics
Perlite is equally useful in hydroponics and soil-less horticulture:
Propagation of plant cuttings: Perlite stimulates root growth, and prevents drowning by helping drain excess water away from the cuttings. It can be used with rooting compounds.
Standalone Growing Media: Perlite is a decent option in some instances as a hydroponic medium. But it is not suitable for high water settings, like deep water culture, or ebb and flow systems.
In mixture with other growing media. Perlite is commonly mixed with vermiculite in equal amounts (50-50). This greatly solves the water-retaining issue of Perlite while improving the water-holding capacity of vermiculite, making it able to use in the water-rich systems stated above.
Are there different types of Perlite?
Perlite manufactured for gardening and horticulture purposes are usually graded into three different categories depending on the size of the individual particles:
This has the highest porosity and draining capabilities. It is best suited for succulent plants and orchids. It is also least affected by winds! But it doesn’t work its way up to the topsoil very easily.
Medium Grade Perlite
This straddles the middle ground regarding aeration and draining. It is best suited for potted seeds and seedlings.
This is the lightest grade, best suited for starting seeds and root cuttings. Fine particles of perlite can also be scattered lightly on top of the soil in your gardens and lawns.
Is Perlite Organic?
There are two ways to look at this:
From a chemistry perspective, organic compounds are those that contain carbon. Perlite does not contain carbon, so it is an inorganic mineral.
But in the context of growing stuff, like organic farming, the meaning or the word “organic” is different. It means something that is naturally extracted from the earth and doesn’t undergo significant chemical processing.
Perlite is a mined mineral that undergoes some physical processing. It is actually allowed by the National Organic Standards Board for use in certified organic agriculture.
So if you are planning to do some organic farming or horticulture, yes, perlite is a safe “organic” additive.
How Does Perlite Compare to Some Other Mineral Additives
Perlite vs. Vermiculite
Perlite is directly comparable to another mineral additive called Vermiculite. Both have overlapping functions and help with soil aeration and seed starting.
Vermiculite also comes from some kind of rocks and expands in the same popcorn fashion like perlite. But vermiculite has a stronger expansion potential.
Perlite has more air porosity than vermiculite, and better drainage effects as well. Vermiculite, on the other hand, retains water much better than perlite.
Perlite is better suited for succulent plants, while vermiculite is better for tropical plants that need more moisture retained in the soil.
They both have their uses, and many experts tend to combine these two minerals in their soil mixes.
Perlite vs. Diatomaceous Earth
Diatomaceous earth is also a mineral additive, available in a fine powder form. It is commonly referred to as DE.
DE is used more for pest control than anything else in gardening. It also has high water retention abilities. But since it is a powder, it doesn’t help much with aeration.
DE is not really a contender against perlite in any conceivable way. Both additives can be used together, for their respective benefits to the soil.
Pros and Cons of Perlite
- Excellent for aeration of roots
- Very stable and inert structure
- Helps improve drainage
- Cheap and easily available
- Useful for hydroponics and gardening
- Finer grades are affected by airflow/winds
- Does not retain water
- Contains no nutrients
- Tends to float in excess water
- Give off dust. So wear a mask to protect your exhalation when working with perlite
Where can you buy Perlite?
You can get perlite in significant amounts and many varieties at Home Depot, Lowes, your local nurseries, any hydro shops. Or order online at Amazon, eBay.
I often buy in bulk to save it for later uses since Perlite is an effective, safe growing medium that can last long.
I always choose a 100% made perlite package and mix it later with soil or other growing media. But that’s my liking you can select perlite as a mix of soil, soilless growing media, or fertilizer. Just pay attention to the product’s component to know that.
Rookie gardeners tend to forget the importance of oxygen supply to the roots of growing plants and seedlings. Perlite is a key additive that can really improve the growth of seeds, saplings, rootings, and adult plants. It can be used as a standalone growth medium, or with other additives.
If you want to become good at gardening or hydroponics, perlite can be your best friend. What is so special about these queer, innocuous-looking balls? Learn more in our in-depth guide to all things perlite.
Perlite: What It Is And How To Use It In Your Garden
When you open up a bag of commercial potting mix, you expect to see little white specks in it without really questioning why they’re there. But what is perlite, really? What is perlite made of? What does it do for the soil, and is there a reason to add more?
Here, we’ll explore the world of horticultural perlite, and shed some light on the best ways to put it to use for you.
My Favorite Perlite Brands
- Aerates soil and promotes root growth
- Helps loosen heavy soils
- Prevents compaction
- Lightweight soil conditioner
- Loosens clay soil
- Improves drainage and aeration
- Helps plants thrive
- Improves soil structure
- Keeps soil workable for years
Listen to this post on the Epic Gardening Podcast
What is Perlite?
Perlite is a form of amorphous volcanic glass, although it’s often confused by new gardeners as being some lightweight material like styrofoam. It’s occasionally called expanded pyrite and has the nickname “volcanic popcorn”, and I’ll get into why in the next segment. If you looked at a piece of horticultural perlite under a microscope, you would see that it’s quite porous. The cavities in perlite help store nutrients and some moisture that the plant might need, but drain excess water away. It is non-toxic, clean, disease-free, and extremely lightweight and easy to work with..
Perlite is often used in industrial settings as well as in the garden. It’s commonly mixed into such products as lightweight plasters, ceiling tiles, or masonry for stability or as an insulator. It’s also popular as a filtration agent, often used for filtering spent grain or other solids out of beer or in the biochemical industry.
There’s many other uses, but to gardeners, it’s an essential ingredient in their garden.
How is Perlite Made?
Perlite begins as a naturally-forming volcanic glass, a special variety which is created when obsidian makes contact with water. This type of volcanic glass has a much higher H2O content than other varieties. Like most other materials from volcanic origin, it’s in the grey to black range with some color variation, and is very dense and heavy. So why does the stuff we use in gardening appear to be white and lightweight?
Expanded perlite is formed when normal pyrite is heated. Heating perlite to a range of 1,560-1,650 °F (850-900 °C) causes the mineral to soften. As it does, the water that’s trapped in the volcanic glass vaporizes and tries to escape. This causes the glass to expand to 7-16 times its original volume, and remaining trapped air changes the color from dark to a brilliant white due to the reflectivity of the remaining water inside the glass.
This newly-created material is much lighter in weight than its previous form and has numerous crevices and cavities. It can easily be crushed with moderate pressure, but does not crumble under the light pressure exerted on it by other soils, and it doesn’t decay or shrink. It is clean and sterile.
The typical chemical composition of perlite varies slightly, as most volcanic glass does. However, perlite which is optimal for the expanding process typically consists of 70-75% silicon dioxide. Other chemicals include:
- aluminum oxide (12-15%)
- sodium oxide (3-4%)
- potassium oxide (3-5%)
- iron oxide (0.5-2%)
- magnesium oxide (0.2-0.7%)
- and calcium oxide (0.5-1.5%)
All of these are natural minerals, and are often part of other soil blends. It has a pH of 6.6 to 7.5.
Using Perlite In Your Garden
As mentioned earlier, perlite offers a lot of benefits to your garden.
The most important one is drainage. Perlite is a natural filtration system, allowing excess water to easily drain away while retaining a little moisture and catching nutrients that plants need to grow. This is especially true in raised beds and container gardens, but also in the ground as well.
Airflow in the soil is greatly improved in a bed amended with perlite, and that’s necessary both for your plant’s roots to breathe and for any worms, beneficial nematodes, and other good natural garden inhabitants. Because it’s a mineral glass and thus harder than the soil around it, it also helps to slow down compaction, and keeps your soil fluffy and lightweight.
What Type of Perlite to Use
People often ask whether you should use coarse perlite as opposed to medium or fine-grade. Coarse perlite has the highest air porosity, so it offers the most drainage capability and ensures the roots of your plants can breathe well. It’s popular among people who grow orchids and succulents, and also people who do a lot of container gardening, as it provides excellent drainage, but the coarser bits don’t work their way to the surface of the soil blend as much as fine perlite does. Larger perlite is also less prone to being caught by a breeze and blown away!
The finer stuff is useful as well, but it’s used for in quality seed-starting mixes or rooting cuttings as the drainage provided encourages rapid root production. Fine perlite can also be lightly scattered across your lawn’s surface, where over time it’ll work down into the soil and improve drainage.
If you’re making your own potting soil, perlite is one of the most used components in the industry for the above reasons. It’s cheap, lightweight, and easy to blend into peat or other water-retaining ingredients! But there’s other additives like diatomaceous earth and vermiculite. Why shouldn’t you use those instead?
Again, it comes back to drainage. Diatomaceous earth, or DE as it’s also referred to, is more moisture-retentive than perlite is. It’s usually available as a powder rather than a granule, so it doesn’t reduce soil compaction in the same way, and it tends to clump when wet, which doesn’t allow as good airflow. There are many other uses for diatomaceous earth in the garden including pest control, and you can use it in conjunction with your perlite, but not to replace it.
When comparing perlite vs. vermiculite, vermiculite is very moisture retentive. It’ll absorb water and nutrients and keep them in the soil, which makes it perfect for seed starting blends or for plants that prefer lots of water. In conjunction with perlite, the vermiculite will absorb water and nutrients to feed your plants, while the perlite will help drain the excess water away. So both have their own place in your garden, even in the same container or bed, but they’re not interchangeable.
Using Perlite In Hydroponics
Perlite has its place in soil, but it is extremely useful in hydroponic gardening as well. One of the most popular ways to use it in hydroponics is in propagating plants by cuttings. As roots grow in response to the plant searching for a water source, a well-draining media like coarse perlite tends to provoke them to grow rapidly as they search for the tiny pockets of nutrients and moisture hidden within the mineral base. Ensuring that your cuttings are well-drained also prevents root rot. It helps if you use a rooting compound like Clonex to further stimulate root growth, too.
To take care of your cuttings better, see my article on caring for your plant cuttings.
Even after your cuttings are started, perlite can be a standalone hydroponic growing media. However, it can be problematic in higher-water settings, such as ebb-and-flow systems or deep water culture. The lightweight nature of perlite, and its high air content, means it tends to float… and you don’t want your media to wash away!
Where To Buy Perlite
One place to buy bulk perlite is at a big box store like Home Depot. Most stores have a reasonable selection, although you may wish to closely look at the label to make sure that it is 100% perlite rather than a soil or fertilizer blend. You can also find it at a good hydroponics store as well. But I like to order it online where I can easily find a perlite to suit my personal preferences. There’s a wide variety, but a few types which I’d recommend are
Perlite is truly a multipurpose additive to your plant media, providing lots of benefit with relatively few drawbacks. Whether you grow in containers on your patio, or are starting cuttings indoors under grow lights, you will find it to be a useful addition to your garden shed. It offers superior drainage at a low price and won’t break down. This volcanic popcorn really works!
Perlite is an amazing soil amendment. After reading this in-depth guide you'll know exactly how to use perlite in your garden effectively.