Adding perlite to soil

Should You Add Perlite to Your Soil?

by Nebula Haze

A common soil amendment for cannabis plant is a substance called perlite. This can be added to soil or coco coir to improve its air-holding capabilities and increase overall drainage ability. It’s not a requirement for cannabis growth, but it’s so useful that nearly all recommended potting mixes contain at least a little perlite.

Horticultural Perlite – A great amendment for soil or coco when growing cannabis. Perlite looks like little white rocks, but the pieces feel oddly light and airy, almost like popcorn.

Nearly All High-Quality Cannabis Soil Mixes Contain At Least a Little Perlite

A 50/50 Potting Mix of Coco & Perlite – Perlite provides more oxygen to the roots, resulting in faster growth. It also prevents nutrient buildup. This perfectly complements the ability of coco coir to hold onto a ton of water. Learn how to mix up your own coco/perlite mix!

Perlite for Growing Marijuana

  • Perlite is one of the most common soil amendments. It is highly recommended for any cannabis soil or coco mix that doesn’t contain some already.

  • Perlite appears as very light, airy white “rocks” that feel almost like popcorn.

  • Adding perlite increases the overall drainage ability in a potting mix, helping prevent overwatering.

  • Perlite helps prevent nutrient buildup which makes it a good choice when growers are giving nutrients and supplements in the water

  • More oxygen in your soil or coco results in faster growth. Roots love oxygen! Perlite increases the amount of oxygen available to the plant roots because it does not retain water. As a result, air pockets form around the perlite even when the growing medium is wet.

How Much Perlite to Add?

It’s recommended to add perlite so it makes up around 10-50% of the total volume of potting mix.

Add 10-20% perlite if you want better water retention and don’t plan on using a lot of extra nutrients. This is because a lot of extra perlite can cause the nutrients leach out faster from the soil as water drains through easily.

Add 30-50% perlite if you plan to use a lot of added nutrients or supplements and are looking to get the fastest growth from your plants.

I have used many types of perlite including Epsoma, Black Gold, Shultz and even Miracle-Gro perlite. Any 8-quart bag of perlite will work. Perlite can often be found in garden stores or the garden section of home improvement stores like Home Depot and Lowes. I normally advise against all things Miracle-Gro, but their 8-quart bag of perlite is okay if you can’t find anything else.

Get a Bag of Perlite on Amazon

What is Perlite?

The light, almost fluffy perlite we use for gardening does not occur naturally. It is actually manufactured from expanded volcanic rock.

It all starts with lava (like the kind from a volcano!) which cools and sometimes turns into obsidian, a shiny black glass that can be mined from the ground.

Example of obsidian, which is formed out of lava

As centuries pass, obsidian absorbs water from the air. This hydrated obsidian is mined and crushed into small pieces. The pieces are then expanded by adding huge amounts of heat. Due to the high amount of water contained inside, the heat causes the perlite pieces to pop like popcorn. That is why perlite feels so light – it is made up mostly of air!

Production of Horticultural Perlite

Amazing picture from 911metalugist.com

Get to know your potting mix: Vermiculite and perlite

Look up the word “vermiculite” in Henry Beard and Roy McKie’s hilarious “A Gardener’s Dictionary” and you’ll see it defined as an “obscure order of nuns devoted to gardening.” In fact (as opposed to in fun), it is a silicate material similar to mica that is sometimes found in the potting mixes gardeners use to start seeds in spring. The word, from Latin, means “breeds worms.”

I think I can explain that. When my sisters and I were little, we would take a handful of our parents’ vermiculite, pour water on it and watch in awe as the dry, compressed flakes expanded to form wormlike columns. This is what vermiculite does in potting soil: Because it is spongy and absorptive, it holds water, so you don’t have to water a container so often. This is especially important with plug trays or soil blocks into which seeds are sown, where the mix can dry out quickly and put tiny seedlings at risk. Organic matter plays a similar role in soil, but vermiculite, mineral by nature, is sterile and inert, thus protecting the seedlings against a fungus that causes sudden collapse — damping-off — and other ills.

Gardeners don’t use vermiculite quite as much as they used to, in part because of an environmental disaster. Vermiculite mined at a plant in Libby, Mont., once the main source of the material, was found to be contaminated with asbestos fibers. Though the plant was closed and the industry reformed, people have, to some degree, made the switch to perlite.

Perlite is made from a mined volcanic glass of the same name. As a raw material it contains water, trapped by the rapid cooling of lava. The moisture vaporizes explosively when heat is applied. The result is a much-expanded mineral popcorn, white in color thanks to light reflecting off tiny bubbles on the surface of its particles. It has a texture that retains water on that surface (though not in the volume that vermiculite does) but retains air in the spaces between. That lightens your potting mix considerably, as well as providing valuable oxygen for plant roots, along with better drainage than vermiculite. Most potting mixes contain at least 25 percent perlite, which is why they look as if a takeout container had been chopped into bits and stirred in. But it’s a harmless mineral and, like vermiculite, sterile and inert.

Each material has its uses. For seed-starting, I go with a vermiculite mix for my germination but a perlite mix for growing in pots. A mix containing both can also be valuable. It is easy to mix your own, but make sure you buy horticultural-grade vermiculite and perlite. Both are available from places that sell gardening supplies, bricks-and-mortar and online. If you are using either in quantity — to lighten soil in large containers or even in small garden beds — you’ll want to shop for prices.

Some gardeners use them to store dahlias or bulbs over the winter. Or, as vermiculture mavens consistently do, employ vermiculite as a medium for, yes, breeding worms.

Damrosch’s latest book is “The Four Season Farm Gardener’s Cookbook.”

Tip of the week:

Final call to prune rosebushes: Use thick gloves and lopping shears to remove dead, weak and conflicting canes. Keep about half a dozen canes around an open center and cut them back to about 18 inches, just above an outward facing bud. Remove any suckering canes emerging from the soil.

— Adrian Higgin

How to Use Perlite for Lightweight, Well-Draining Container Gardens

Mary Jane is a writer, engineer, and organic gardener from small-town Canada. She blogs about organic gardening and the garden lifestyle at Home for the Harvest.

Container gardening is great for growing herbs, vegetables, and flowers. To grow healthy container plants it’s important to pay special attention to the potting soil used in the container. Adding perlite to potting soil is a good way to ensure the container garden drains well while also creating a light, fluffy soil for your plants.

Container plants should be planted in a light, well-draining, nutritious soil mix.

Why Not Use Backyard Garden Soil in Container Gardens?

Existing garden soil is not the best choice for container gardens. It can often have too much clay, too little organic material, or other issues such as contamination or weed seeds. Container gardens require a light, well-draining, nutritious growing medium.
Rather than using garden soil, use a high-quality potting soil that contains perlite. Perlite will trap air in the soil, ensuring that roots can perform their primary functions. High-quality potting soil often already contains perlite, along with a mix of peat moss with lime, coconut coir, high-quality compost, and sometimes added nutrients (which can be organic or synthetic). Always check the ingredients of potting soil before purchasing it.

Perlite is an expanded volcanic glass that your container plants will love!

Perlite is a mined volcanic glass which is “popped” in ovens (like popcorn) to create an expanded lightweight growing medium. Perlite is safe for organic gardening and is listed as an allowed soil amendment by the Organic Materials Review Institute. It’s low density makes it perfect for trapping air in soil mixes. Perlite keeps soil light, provides plant roots with air, and promotes drainage.

If you’ve seen store-bought potting soil that contains little white pieces that look like tiny bits of styrofoam, chances are you’ve seen perlite! Perlite can be dusty, so be sure to wear gloves and a dust mask when working with it. It can also be moistened prior to mixing to keep dust down.

Perlite is a key component of a good container gardening mix.

Using Perlite for Lightweight, Well-Draining Container Gardens

Aeration is very important for healthy plant roots. The cells in plant roots require oxygen from the surrounding soil environment to release the energy needed for healthy growth and nutrient uptake.

Overwatering can cause a plant to “drown”, because the roots become saturated with water and don’t have access to adequate oxygen. Adding perlite to potting mix will trap air into the soil and allow water to drain out. It can be added to custom soil mixes or to pre-mixed potting soil to lighten it up (even if it already contains a bit of perlite already). Ensuring your container soil has enough oxygen will also help the beneficial micro-organisms in the soil thrive, as they also require oxygen. For more information on how to mix your own DIY potting soil using perlite (including printable soil mix recipes) check out this tutorial.

This mint would grow in a shallow container, but was planted in a 2” deep planter to keep it within arm’s reach of the kitchen counter!

Beyond Aeration

In addition to trapping air to provide oxygen to plant roots, perlite is also perfect for adding lightweight volume to oversize plant containers. It’s always nice to use a suitably-sized pot for the plant, but sometimes an oversize pot is used for decor or accessibility. It can be expensive and heavy to fill up an oversize container with high-quality potting soil, making perlite a desirable option.

Even in a large pot, many plants will want to send their roots down as low as possible. Some gardeners fill the bottom of large containers with styrofoam, which is also lightweight but is a petrochemical product. Other gardeners use pebbles or cobbles to fill up the bottom, but that makes for a very heavy container.

Instead of using styrofoam or rocks, use perlite in the bottom of the pot to fill the extra volume without adding weight or chemicals to the soil. The perlite can be mixed with a bit of potting soil to keep tall containers from becoming top-heavy, or the perlite can be used by itself as a bottom layer in wide oversize pots. Roots will grow down into the perlite layer whether or not soil is mixed into it.

Perlite was used as an upper layer in this mini cactus container garden. The perlite drains well, looks sharp, and was a cheaper alternative than purchasing decorative sand or pebbles.

Another way that perlite can be used in container gardens is to add a lightweight decorative mulch to the top of an ornamental container plant. Placing fresh perlite on top of the soil around the plant or using it for a lightweight upper layer of growing medium creates a fresh, modern container arrangement.

Perlite is Perfect for Container Gardening

Perlite is the perfect soil amendment to keep your container garden soil light, airy, and full of life. Try a mix that contains perlite or whip up your own custom soil mix! Your container plants will thank you.

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

Our Top Choice Espoma PR8 8-Quart Organic Perlite
  • Aerates soil and promotes root growth
  • Helps loosen heavy soils
  • Prevents compaction
Check on Amazon →
Hoffman 16504 Horticultural Perlite, 18 Quarts
  • Lightweight soil conditioner
  • Loosens clay soil
  • Improves drainage and aeration
Check on Amazon →
PVP Industries, Inc. Pro4CU105408 Horticultural Coarse Perlite – 4 Cubic Feet
  • Helps plants thrive
  • Improves soil structure
  • Keeps soil workable for years
Check on Amazon →

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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?

A mound of perlite! source

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

Perlite as a part of a 3 part DIY potting mix. source

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

Rooting ficus cuttings in a perlite mixture. source

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

  • Espoma Organic Perlite, 8-Quarts
  • Hoffman Horticultural Perlite, 18-Quarts
  • PVP Industries Horticultural Coarse Perlite, 4 Cubic Feet

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!

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Are Vermiculite and Perlite the Same?

It seems that many people believe that vermiculite and perlite are similar products used for similar purposes. This is not true.

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I was “visiting” the garden center of my local Lowe’s store the other day (Lowe’s is my second home!). I wanted to purchase a large bag of coarse perlite. I couldn’t find any and asked one of the workers whether they had some stored somewhere. He checked his inventory and said the perlite was on back-order but that he had vermiculite. He assured me that vermiculite and perlite were similar products and that I could just substitute one product (vermiculite) for the other (perlite). In one regard, he was telling me the truth, but in another, he was totally off-base. Not knowing the difference could ruin your project.

Both vermiculite and perlite are inorganic products, both are relatively sterile, and both are often used as soil additives. Vermiculite is a soft,spongy material made from super-heating mica. Perlite is a hard, highly porous material made by super-heating volcanic glass. Typically, vermiculite will be tan/brown in color and perlite will be white. Whereas vermiculite absorbs water in its plate-like structure, perlite traps water in its very porous, undulated structure. Vermiculite is close to a neutral pH whereas perlite has a slightly more alkaline pH. Both are very light-weight.
Though both products do in fact aerate the potting soil, the fact that vermiculite holds moisture longer than perlite makes their usage different.

Vermiculite retains more water and retains it for longer periods than perlite. Vermiculite affords slightly less aeration than perlite. For plants that thrive growing in a more-wet soil, vermiculite would do well as a potting soil additive. Many plants are more sensitive to alkaline conditions, so in that case, vermiculite would be a better choice than perlite. Since many foliage plants appreciate more water-retentive potting soil, they do well in a vermiculite-added potting soil. Also, simply because of its water retention/nutrient retention, vermiculite makes a good seed-starter medium.

Perlite only traps water on its large surface area, consisting of nooks and crevices, and thus releases its water more quickly. In doing so, it can help raise the humidity around plants. While vermiculite would be a better choice for starting seeds, perlite would be a better choice when rooting cuttings. Cuttings would tend to rot more easily in vermiculite. For plants that need a quickly draining soil, a soil that does not retain much moisture, a soil that is extremely well aerated, and a soil that could have a higher pH, perlite would be the best additive. Cacti and non-cacti succulents would be such plants. Also, because most epiphytes like to have their roots dry out quickly between watering and also appreciate higher humidity, perlite would be the best choice.
Thus, although vermiculite and perlite have some similar properties, they are not the same. When plants need more water-retentive soil or for seed-starting, use vermiculite. For plants that prefer quick-drying soil or for starting cuttings, use perlite.

Using Perlite In Potting Soil

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  • Perlite in Potting Soil
  • Other Uses For Perlite
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A Good Balance Of Perlite In Potting Mix

Perlite in Potting Soil

Perlite is the snowy white granular particles that look like small pieces of Styrofoam you will see in many potting mediums.

Perlite is formed when volcanic mineral rock is heated quickly causing it to expand and explode. The result is a very lightweight, porous material that is hard and does not break apart easily; it is sterile, has a neutral pH and can hold 3-4 times its weight in water yet will not rot or become soggy.

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Medium grade perlite is best to use in potting mixes for seeds and seedlings.

The principal value of perlite in any potting medium is providing aeration and improving water drainage.

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The surface area of perlite particles are covered with tiny cavities that hold moisture while the particles themselves create tunnels in the mixture that allow air and water to flow freely to the roots.

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Perlite can also be used on top of potting mix to cover newly sowing seeds in trays or pots.

Germinating seeds have no trouble pushing their way to the surface through this layer.

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The most important aspect of growing seedlings is healthy roots. Perlite in potting soil provides that all important environment.

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Perlite is one of the best media for growing plants, it is possible to grow most plants in perlite alone and is just as successful as traditional peat mixes.

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However there are no nutrients in perlite. So fertilizer must be added as soon as the first true leaves appear.

other uses for Perlite

Perlite has many application, including being one of the major components in horticultural growing mediums of many kinds.

In the garden you can use perlite to open up air ways in heavy soils by sprinkling the granules over the surface of your garden before you start planting. Wet it down to moisten it and work the perlite into the top 4 – 6 inches (10-15 cm) of soil.

Some hydroponic systems use perlite as the sole growing medium with only water and nutrients added.

Roots grow unencumbered in perlite making it excellent for rooting cuttings, place cuttings in the perlite to a depth just past the lowest leaf nodules. Seal in a plastic bag and place in indirect light for two to three weeks. Plant cuttings in potting soil when roots reach 1/2 to 1 inch in length.

related pages

Home Page – – Top of Perlite in Potting Soil Page – Potting Mix Ingredients Page – Return to Potting Soil Page

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