If you are an experimental gardener like me, then I am sure you have already heard of perlite—a hundred percent! Something that I admire about perlite is it has climbed its way to the top of the gardening world from just being a volcanic byproduct. Today, you will learn more about its journey!
Generally, perlite is a glassy, silicon-rich volcanic rock that is odorless, pH neutral, heat-resistant, and insoluble. Its capillary action ability makes it an excellent growing medium that can retain water and enhance soil structure. It is available in three grades: coarse, medium, and fine.
How does perlite interact with water? How about with heat? How much does it cost? You might need to slam that brake for now because all of these questions will be answered in every stopover along this article. Fuel up and let us drive through!
Table of Contents
- 1 The Origin of Perlite: Where Does It Come From?
- 2 Physical and Chemical Properties of Perlite
- 3 Which Perlite Grade is Best for You? (3 Grades)
- 4 Is It Possible to Make DIY Perlite?
- 5 FAQs
- 6 Summary of Perlite Facts
- 7 Sources
Perlite is a rock product from volcanoes that is high in silicon content. It is glassy and expands when exposed to hot temperatures (900-1100°C). Approximately 70% of the world’s perlite deposits are found across Turkey’s Aegean coast—where there is an abundance of volcanoes.
Our first stopover is perlite’s origin!
Perlite comes from volcanoes and is considered an amorphous glass with high moisture content. When we say something is amorphous, it means it has an indefinite shape. Interestingly, perlite is created when water seeps into the lava crystal named obsidian.
Now we have gone through the origin of perlite, let us drive further to know what is produced after that hydration of obsidian.
Perlite is classified as a natural rock that has the unique property of massively expanding once sufficiently heated. Its color ranges from white to gray. It has no odor, is non-flammable, non-explosive, and cannot be compacted. It exists naturally and is inert chemically, with a pH range of 6.5 to 7.5.
|Appearance||Dry, white, glassy|
Simply put, perlite is odorless and neutral—no nutrients, no buffer effect, and no cation exchange capacity. However, it became a prominent gardening product because it’s lightweight.
How can this information help you as a gardener?
As someone sensitive to dust, I need to use a growing medium that will not produce chalk-like dust.
If you are someone who can relate to that, this growing medium might be for you! With perlite’s glassy and dry appearance, it is suitable for a gardener who has certain dust allergies.
Another factor I considered when I used perlite is its odorless nature. This characteristic of perlite makes it suitable as an indoor plant. You can use perlite for the plants in your kitchen, bathroom, living room, and even on your dining table because it does not produce any off-odors.
Now knowing that perlite has no boiling point, is not soluble, and pH neutral, we can say that physical and chemical changes are not an issue. Plus, it is compatible with most gardening environments! May it be indoors, outdoors, or even hydroponics, you can use perlite.
Perlite is a well-known heat-resistant gardening material. This means that subjecting it to high temperatures will not affect its horticultural performance.
However, when exposed to extreme temperatures exceeding 850-900°C, it will begin to melt. Then again, it is very unlikely for you to come across such a situation while you’re gardening.
Perlite floats and holds water well. However, it does not absorb water because it operates through capillary action. It can hold 3 to 4 times its weight in water.
1. Does perlite float?
Perlite is a lightweight growing medium, thus, it tends to float when watering. Now, maybe you are asking, how can you control this?
The best way to balance this is by mixing perlite with other growing mediums.
Pro Tip: An optimum mix would be a 1:1 ratio of perlite and peat. This mix must be placed halfway through the pot, while the remaining space must be filled with peat.
This would be helpful for plants that have a daily watering schedule. If you are growing a cactus, which is not watered frequently, then the floating issue might not be a big deal. But for hydroponic systems which are submerged in water for so long, this might be a concern.
2. Does perlite hold water?
Perlite has a promising water-holding capacity which is brought by capillary action. If you are not yet familiar with this term, here is a real-life example.
When you are painting using a brush, you can notice paint droplets staying in between each brush hair. The force that holds the paint is our friend, capillary action. It works by defying gravity!
This is the same principle that happens with perlite. The air space in this medium is trapping water on its surface. Thus, it can retain water well.
The question now is…
3. Does perlite absorb water?
The quick answer is no. Because of its physical characteristics such as being a volcanic rock that is glassy in appearance and texture, the material itself cannot absorb water like a sponge.
Instead, the concave spaces and holes in perlite are the ones that hold water via capillary action, as discussed a while back.
Hydrofluoric acid, which is commonly found in rust removers, must not come in contact with perlite as this chemical reaction can produce a toxic gas— silicon tetra-fluoride.
What to do when you inhale this toxic gas?
As a first aid, one must move to a place where there is fresh air and rest in a breathing-friendly position.
Worst case scenario: If one experiences difficulty in breathing, artificial respiration must be provided. The best thing to do at this point is to call a doctor.
Perlite comes in different grades and sizes such as coarse perlite (1-3 mm), medium perlite (1 mm), and fine perlite (<0.5-1 mm).
At this point, you might be asking, which one is the best? Each grade is suitable for a certain purpose.
1. Fine Perlite
The fine grade, commonly called #2 perlite, is the least dense grade of perlite available. Using this is appropriate for root cuttings and when starting small seedlings. This grade can improve water retention and drainage in your flower beds and lawn.
Here is a fine grade perlite available on Amazon priced at 15 dollars!
2. Medium Perlite
This perlite grade falls somewhere between fine and coarse with a commercial name of #3 or #4. Because it is a combination of two grades, it is an excellent choice for potting seedlings to aerate the growing medium. This type is best for hydroponics, succulents, and orchids.
The product below on Amazon costs 14 bucks and is best for potting mixes.
If you are planning to build a hydroponic system, head to our article on how to choose a hydroponic growing medium.
3. Coarse Perlite
Coarse perlite, usually #8 grade offers enhanced drainage to your soil and has the highest pore structure compared to the three forms. Because of its porous nature, it is an excellent choice for water retention and aeration in outdoor gardens (vegetables and herbs) and for houseplants.
A chunky perlite grade also costs approximately 14 USD on Amazon. This one is best for succulents and potting mix.
If you want to compare perlite with other growing mediums, head to our article perlite vs vermiculite.
Do-it-yourself perlite alternatives are possible through the use of bricks, shells, and pots. This DIY perlite substitute can also provide enough space for air and water circulation and enhance the soil structure for plants.
To create a growing medium that works like perlite, you’ll need the following:
- either bricks, shells, or broken pots;
- sieve/screen; and
To do this, do the following steps:
- Crush the bricks/shells/pots using a hammer.
- Sieve the broken pieces using a screen with hole sizes of your desired particle size.
- Collect the sieved particles and sanitize them using food-grade hydrogen peroxide or diluted bleach.
After that, they are ready for use!
Pro-tip: To use food-grade hydrogen peroxide, dilute 2-3 teaspoons per gallon of water. On the other hand, 7.5 teaspoons of bleach must be diluted with a gallon of water.
Learn more on how to use these disinfectants in our article about hydrogen peroxide vs bleach.
Does organic perlite exist?
Organic perlite is often used in the gardening context, since producing it does not use any synthetic processes. However, from a chemistry standpoint, perlite is classified as an inorganic material because it does not contain carbon.
What is the diameter of #2, #3 #4, and #8 perlite?
The particle sizes of the perlite vary across commercial grades. The diameter of #2 perlite is a mix of particles ranging from 0.06 to 0.62 inches. For #3 perlite, particles range from 0.12 to 0.62 inches; while it is 0.33 to 0.75 inches for #4 perlite; and 0.75 to 1.33 inches for #8 perlite.
Is perlite safe in the oven?
Perlite is a non-toxic mineral and can survive heat up to 870 degrees Celsius, which is way beyond the thermal capacities of commercially-available ovens. Above this temperature, perlite can expand 4-20 times from its original size and can pop like popcorn!
Does perlite expire?
Perlite has no expiration date because it is an inorganic material. In other words, perlite does not decompose. This means that perlite can be used and reused again and again—some even last for years!
Perlite is a silicon-rich rock from volcanoes. It is glassy, odorless, pH neutral, non-explosive, non-flammable, heat-resistant, and non-soluble. It has an excellent water-holding capacity due to capillary action. However, the material itself cannot absorb water.
Commercially, perlite is available in a variety of grades. Coarse perlite (1-3 mm) works best with vegetables, herbs, and houseplants. Because of its mixed particle size, medium perlite (1 mm) is recommended for hydroponics, succulents, and orchids. While using fine perlite (0.5-1 mm) is optimum for root cuttings and when starting seedlings.
Alternatives for perlite can also be created from bricks, shells, or broken pots. The crafting process is done by crushing these materials into pieces, sieving them, and sanitizing them for safe garden use.
- “Inorganic and Synthetic Organic Components of Soilless Culture and Potting Mixes” by Papadopoulos, A.P. et al. in Soilless Culture
- “The Water-Holding Capacity of Perlite” by Perlite Institute in Hess Perlite
- “Perlite Gradation and Peat/Perlite Mixtures” by Matkin, O.A. in The Schundler Company
- “Effect of Particle Size Distribution of Perlite and its Mixture with Organic Substrates on Cucumber in Hydroponics System” by Samadi, A. in Journal of Agricultural Science and Technology
- “New Brewing Technologies: Setting the Scene” by Bamforth, C.W. in New Technologies