If you’ve ever purchased potted plants or veggie starts, or opened a bag of commercial potting mix, you may have noticed tiny white lumps in the soil that look like Styrofoam bits.
These lumps are called perlite, and they’re not simply fillers or artificial rocks as some might believe.
What is perlite?
First, let’s go over what perlite isn’t: Perlite is not a type of soil, it’s a soil additive that can also be used as a growing medium.
Perlite is not a fertilizer, and has no nutritional or microbial value to plants or soil. Its benefits are derived solely from keeping the soil structure loose and light.
What perlite is: an inorganic, non-toxic, lightweight soil amendment. But don’t let that dissuade you if you’re an organic gardener! (More on that below.)
Also known as expanded pyrite, perlite is a mined volcanic rock that is mixed into many industrial building products for stability, such as masonry construction, loose-fill insulation, cement, and plaster.
It’s also used as filter media for swimming pool filters, and as a filter aid for beverages (like juices, beer, and wine) and for waste water.
In the gardening world, perlite is used to improve soil structure by providing drainage and aeration. “Air for the soil” is what I like to call it, and it’s the next best thing to compost for circulating air between the roots for strong, healthy plant growth.
Look under a microscope and you’ll find that perlite is filled with many tiny cavities that hold water (like a sponge), making it efficient at delivering moisture to plant roots. All these nooks and crannies enable perlite to hold three to four times its weight in water.
It’s also capable of storing nutrients for a short period when they otherwise might wash away immediately.
At the same time, the cavities mean perlite is quite porous, so it drains excess water more readily than vermiculite and other potting media. This is a good thing, since keeping your soil from becoming waterlogged is the number-one way to prevent root rot and fungal diseases.
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Is perlite organic?
To call something organic depends on your personal beliefs (and here, it’s merely a case of semantics).
From a chemistry perspective, organic compounds contain carbon. Since perlite does not contain carbon, it’s considered an inorganic material.
But from a gardening perspective, the term “organic” refers to something that is naturally produced without synthetic processes or significant chemical changes.
It’s true that perlite has to be “popped” in an industrial process that turns it into the lightweight material we see and know, but it is still a naturally occurring mineral that’s extracted from the earth.
And officially, horticultural perlite is OMRI listed for use in organic agriculture, so you don’t need to worry about what you’re putting in the ground.
How is perlite made?
The perlite we see in potting mixes is often called “volcanic popcorn,” and for good reason.
Straight out of the ground, perlite is a dense, amorphous volcanic glass with very high water content. In its natural state perlite is brown or black in color, as it’s typically formed during the cooling process of lava that produces obsidian.
During processing, raw perlite is superheated very rapidly to temperatures of 1560°F to 1650°F, which causes the material to soften and the moisture inside the glass to turn to steam. As the trapped moisture inside bubbles, trying to escape, it expands the material from 7 to 16 times its original size — much like popcorn popping.
The expanded material turns white due to the reflectiveness of the trapped bubbles, and the resulting perlite is a chemically inert, sterile material that takes on an ultra lightweight, foamy texture.
How to use perlite in the garden
Perlite is commonly used in potting soil and soilless mixes (particularly for indoor seed starting) to keep the soil structure loose and well-draining without risk of compaction over time.
You can also use it to propagate plant cuttings. Rather than just rooting your cutting in water, you can root it in a small jar filled with moistened perlite.
The same goes for seeds: start them in moistened perlite only, or test the germination of older seeds in baggies filled with moistened perlite (as an alternative to the coffee filter method of starting seeds).
In raised beds or in-ground garden beds that have trouble with cakey clay soil, you can improve drainage by raking in a 2-inch layer of perlite into the top 6 to 12 inches of soil, at the same time you amend the soil with compost and other nutrients.
Because it doesn’t break down, a single application of perlite can keep the planting bed light and loose for several years! It was my “secret ingredient” during my years of gardening in Southern California, when I needed something more than just compost to break up all the hard dirt clods in our beds.
Some (but not all) bagged potting and garden soils also benefit from having more perlite added to the mix.
This is especially beneficial for deep-rooted plants that are more productive when the roots don’t have to drive down into dense, compressed soil (think root vegetables like carrots and daikon — you’ll see how my previous clay soil was notorious for turning out silly, topsy-turvy carrots).
I also like adding extra perlite to my garlic planting beds in fall, as the perlite helps keep bulbs from being waterlogged in winter and spring. Perlite also helps the soil dry out in the week or two before your garlic crop is ready for harvest.
Perlite is an essential soil conditioner that I always keep on hand, and I buy several bags every year as I always find a use for it.
Be aware that smaller grades of perlite, and in some cases cheap perlite with little quality control, can get dusty (especially as you get to the bottom of a bag).
If you’re sensitive to fine particles in the air, be sure to wear a dust mask and glasses when you’re handling perlite. (I like to keep both of these items in my gardening toolkit. See my sources linked below for stylish reusable mask options that I own for working with dusty gardening products.)
Grades or sizes of perlite
Perlite is typically available in four grades or granule sizes, which correspond to coarseness levels.
|Type of Perlite||Grade||Granule Size|
|Super coarse perlite||#4||1 inch|
|Coarse perlite||#3||1/2 inch|
|Medium perlite||#2||1/4 inch to 3/8 inch|
|Fine perlite||#1||1/8 inch|
Super coarse and coarse perlite: This type of perlite is often used to amend raised beds and garden beds, or dense soils with high water-holding capacity (clay). Size #4 perlite is a whopper of a particle that should only be used for extremely heavy soil.
Medium perlite: You’ll mostly see medium-grade perlite in commercial potting soil. It’s a good all-around size for potted plants, window boxes, and general garden use.
Fine perlite: These smaller particles are ideal for starting seeds or rooting cuttings. Fine perlite is tough to find in this size as a standalone product, so I tend to go with fine pumice, which is usually labeled for bonsai or succulent planting.
Benefits of using perlite in the garden
Perlite is a highly useful component of gardening for many reasons:
- It is physically stable and retains its shape, even in heavy or saturated soil.
- It doesn’t decompose, so it’s ideal for use in potting mixes for plants that are repotted infrequently (such as succulents and other houseplants).
- It has a neutral pH level, making it suitable for any container or garden bed.
- It contains no toxic chemicals or additives; when you buy a bag labeled as perlite, that’s exactly what you’re getting.
- It’s able to absorb some water while letting the rest of it drain freely.
- It provides excellent aeration. Plants actually absorb 98 percent of their oxygen through the roots, so good aeration is crucial for healthy root development. Proper airflow also supports earthworms, beneficial nematodes, and other good things in the soil food web, which in turn supports plant life.
Because of these properties, perlite is also popular in orchid, cactus, and succulent planting mixes that like to be on the drier side, and in hydroponic setups as a standalone growing medium.
Does perlite cause fluoride burn in plants?
There’s a rumor that perlite is responsible for fluoride burn in houseplants, which appears as brown leaf spots or scorched leaf tips in susceptible plants like dracaena, spider plants, and Easter lilies.
If you’re using a commercial potting soil that contains perlite, however, the chances of that happening are very slim.
Fluorine toxicity can be caused by many things, including fluoridated water, superphosphate fertilizers, low soil pH, and other environmental conditions that have little to do with potting medium that simply has perlite in it.
Where to buy perlite
The most convenient source of perlite is your local independent garden center or big-box plant nursery. When buying perlite, make sure you are buying 100 percent perlite and not a soil or soilless blend.
I’ve also linked my favorite brands of perlite (below) in a variety of sizes, which you can easily buy online.
If you can’t find perlite locally, pumice makes a good substitute as it has similar qualities. Sometimes you can use vermiculite in a pinch (especially as a seed starting medium), but keep in mind it retains more moisture than perlite does.
When it comes down to it, perlite is still the best choice for a soil amendment when you want moderate water retention, excellent aeration and drainage, and long-term benefits.