Perlite: What It Is And How To Use It In Your Garden

160 Shares

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

Listen to this post on the Epic Gardening Podcast

Subscribe to the Epic Gardening Podcast on iTunes

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?

A mound of perlite!
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 in potting mix
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

Using perlite in hydroponics to root cuttings
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

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!


The Green Thumbs Behind This Article:


Kevin Espiritu
Founder

What are those little white specks in your potting mix? Perlite, that's what! Learn what perlite is and how to incorporate it into your gardening here.
Did this article help you? Yes No
× How can we improve it?
× Thanks for your feedback!

We're always looking to improve our articles to help you become an even better gardener.

While you're here, why not follow us on Facebook and YouTube? Facebook YouTube

Last update on 2018-12-11 / Affiliate links / Images from Amazon Product Advertising API

160 Shares

11 thoughts on “Perlite: What It Is And How To Use It In Your Garden”

    • Mix ratios depend on how it’s being used, what it’s being used for, and what other materials it’s being blended with.

      For instance, there’s a number of people who root cuttings in straight perlite with nothing else. This is great to get the plants started, but won’t work well for the long-term.

      Orchids require a fairly dry potting mix. Those can use more perlite and wood chip than soil, possibly as high as 50% perlite!

      It also depends on the other components of your potting mix. If you’re looking for a good standardized potting mix that can be put to use in most kitchen gardens or with a wide variety of plants or flowers, Kevin has a great video on YouTube showing the one which Steven Cornett of Nature’s Always Right uses, which is about 30% perlite. You can find that video at this link.

      In my raised beds, I recently blended in roughly about 5% perlite into the upper layers just to add a little extra aeration and to reduce compaction. As I add more composted material to the soil, there’s also some larger particulate matter in there which will provide good drainage and reduce compaction too.

      As you can see, there’s no single mix ratio that’s considered “right” — it’s all relative to what you’re planting and how you’re using the perlite!

  1. Hello,
    Completely new to gardening in containers as I live in a condo. I had a question concerning Perlite, as I am very unfamiliar with this white substance. But I came across a company that sells a lot of landscaping products. I asked for Perlite in Course grade because people were mentioning about it the last time I was looking at plants. Well I got a bag that said it was Premium Grade Perlite. As I got home I took notice when I opened the bag it had White and Grey Perlite. The bag itself said it contained Perlite and Crystalline Silica 1%.
    Would you know if I got the correct product for my container gardening or should I return it to that landscaping company.
    Please advise.

    • Crystalline silica will not harm your plants in any way. It’s a naturally-forming mineral deposit found in most rocks, and in fact plants probably like it. So it’s not harmful to your plants.

      However, breathing in the dust of crystalline silica can cause very severe lung damage in humans, and is recognized as a carcinogen.

      Now, keep in mind that not all crystalline silica is dangerous… quartz crystals are a form of crystalline silica that many people have around. But the dust itself is dangerous to us, and in a bag of perlite, there will be both perlite dust and silica dust.

      If you do decide to use it, be sure to wear a respirator mask until all the dust has subsided in the area where you’re using it, and don’t work with it indoors. Once the granules and dust have been mixed with the soil, you shouldn’t have a problem, as the dust just becomes part of the soil structure.

      Perlite itself is pretty dusty stuff, so it doesn’t hurt to have that respirator mask anyway! It also helps when spreading diatomaceous earth (another silica form, if not a crystalline one — and much less risky to humans).

      I personally would return it because I think that a “premium” perlite should be 100% perlite and not perlite blended with silica. But that’s a personal preference.

  2. If you live in San Diego, you can also check out Eastman Soil. You can pick it up or they’ll deliver to you. 4 cubic feet for $13. I’m about to order some and garden soil but was looking up uses for perlite before I did. Seems like a great thing to have, thanks for the wonderful info!

  3. Kevin- you did it again. Explained a pretty darn complicated topic at an expert teacher level. What I’m not so happy about is the sharply rising cost of Perlite– even those products you mentioned in your post. I cannot afford Perlite purchased in quarts. I need 40 metric cubes of the stuff. However, if there is enough interest (I do not have any following), it sure would save so many of us buckets if we could somehow do a cooperative purchase from Alibaba or something. 15 cubic meters- https://www.alibaba.com/product-detail/horticultural-expanded-perlite-as-hydroponics-growing_60476492434.html?spm=a2700.details.maylikever.9.Jj6UIJ.
    300 bag minimum. What say you fellow gardeners. Willing to buy half a box car? If Kevin is not up for it, maybe I could swing it. – Dan

    • Here in San Diego Debra Lee Baldwin has recommended use of Dry Stall. This is perlite made for equine uses. At the local tack shop, it’s $16 for a 40-pound bag. I have used a lot of it for improving the drainage of my sandy soil that tends to compact like concrete. (But 40 m^3 is a LOT of bags!)

Leave a Comment