What is Perlite? The Mineral That Rocks Your Plant Substrate

These days, you don’t have to be a geologist to have heard of perlite.

This foamy white rock is a true horticultural staple; found in garden centers, homes, hydroponic setups and outdoor growing spaces worldwide.

It’s able to both improve your soils and substrates, or in some cases completely replace them.

Making perlite a versatile material to say the least!

So, we’re going to break down why it’s so great and how to use it at its best.

Let’s get into it.

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Though perlite looks and feels like Styrofoam, I can assure you it’s not.

The better analogy is actually popcorn, if popcorn were made of rock…

Volcanic rock that’s been subjected to enormous temperatures and pressures to the point where it expands and “pops” into these foamy white amorphous fragments that you see before you. 

The result is a lightweight mineral with a huge porosity and surface area (7-16x it’s original volume), both of which contribute to perlite’s essential functions as a horticultural material – drainage and aeration.

Though it’s actually quite brittle to the touch, in a substrate mix it holds up super well. This means it can resist compaction, providing space for both air and water movement in the medium.

The one thing that it doesn’t do well – and is commonly reported to do – is retain water (more on this later).

So, perlite is an excellent tool to have in your plant kit as long as you know what you’re using it for.

Perlite is typically derived from obsidian, a jet-black volcanic glass that frankly looks nothing like perlite…

The expanding process forms the bright white colouration due to the trapped bubbles reflecting lots of light.

Much like its glassy source, perlite is made up of a variety of different elements and compounds. It’s mostly silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃) with some sodium, potassium, iron, magnesium and calcium to boot.

That being said, perlite is chemically inert and is generally regarded in the literature as having “no buffering capacity and contains no mineral nutrients.”

So, whilst it’s not exactly a feast for your plants, the fact that its inert – and therefore has a neutral pH – is part of what makes it such an easy choice for all kinds of substrates. 

Despite their prominent horticultural adoption (and often interchangeable use), perlite and vermiculite are very different materials.

Both in structure and function.

In reality, one is not a good substitute for the other.

If perlite is popcorn – puffed, rigid and porous – then vermiculite is more like wafer, a highly-absorbent stacked set of sheets.

Simply put, vermiculite really shines in water retention and perlite in drainage.

But there’s far more to perlite than just that, which we’re about to go into.

Benefits of Perlite

1. Drainage

Probably the primary purpose of perlite; it’s able to dramatically boost drainage in a substrate mix through its unique structural qualities.

The rigid composition resists compaction over time – preventing the caking of your finer substrate components – and provides air pockets where water can run freely through.

2. Aeration

Perlite is puffed rock, right? 

All that porosity creates lots of opportunities for air pockets, leading to the each perlite granule usually being around 30% air.

This means when you’re mixing perlite into a substrate it’s able to bring all that air right to where the plants need it (and struggle to get it) – in the plant roots zone.

3. Highly resistant

It’s inorganic nature means perlite absolutely will not rot, and will not degrade or break down inside of a substrate for a very long time.

5+ years at least.

Trust me, you’ll need to refresh your substrate long before perlite becomes the problem.

4. Neutral pH

Though it can’t buffer pH like vermiculite can, the neutral pH does make it a versatile material that can be used in tandem with any other mix of substances.

Honestly, it’s an easy choice.

5. Sterile

As a completely inorganic material, it comes sterile (as long as it’s been stored correctly). Which makes one less horticultural material to worry about bringing any unwanted pests or infections into the mix. 

6. Reusable

Thanks to perlite being neutral and sterile, it’s completely reusable. It’s not going to react or bind with anything, so you can rely on it to perform its function in another setting.

7. Lightweight

Being such a lightweight material makes it easy to handle and use. It’s not going to make your plant pots too heavy for your shelves and it’s not going to crack any terrarium glass either.

Cons of Perlite

1.It doesn’t hold much moisture

Where most people go wrong with perlite is in thinking that it’s good for water retention – it’s not.

We now know thanks to its expanded structure that it’s not able to absorb any water at all. Plus, of the water molecules that perlite can hold on its surface – 80% of that water has been shown to be unavailable to plants.

They simply can’t penetrate those pores to unlock the water with their roots.

2. It delivers no nutrients

The issue with perlite is not so much that it contains no beneficial elements and compounds for plant growth – it does – it’s actually because it’s so inert it is unable to release any of them

In the industry, this is called having a low cation exchange capacity (CEC).

So, not only does it not supply any nutrients of its own, it’s not going to be able to hold or deliver any external nutrients either (e.g. liquid fertilizer).

3. It’s not renewable

The volcanic rock that perlite comes from is mined from the Earth.

So, there is technically a finite amount of it. We cannot produce any more within a reasonable timescale (it’ll take thousands of years to reproduce naturally).

That’s not ideal.

But, due to the nature of expanding rock, we get lots of perlite from the original material. And, it’s reusable and not needed in huge quantities, so thankfully we’re not at risk of ever running out.

4. It’s dusty (and an irritant)

Perlite can be a small pain to handle – literally.

With it being so brittle, it can get a bit dusty, and that dust is considered a mild irritant to the lungs.

If you’re handling large volumes of it, I’d recommend a mask to be safe.

But if you’re just using a small bag at home, try misting it down a little before use to prevent the dust kicking up into the air.

5. It’s so lightweight it can… move

It’s not something I’ve come across in the home, but I have heard of perlite particles floating to the surface of a mix if its often exposed to water (i.e. in the garden).

In an Indoor Plant Mix 

Tropical Plants

Perlite is used a lot in the tropical houseplant world, and (mostly) for good reason.

Tropical plants like loose, well-draining substrate mixes that replicate life on the rainforest floor. Particularly epiphytic species (e.g. the various types of Pothos) that are naturally used to drying out between soakings.

Perlite is a great way to achieve all of the above qualities in a mix for potted plants (but I still worry that people use it for it’s almost non-existent water content).

Somewhere in the region of 10-30% perlite is a balanced range for most mixes.

If your mix needs lots of water retention too, I’d opt for 10% perlite and 10% vermiculite instead.

Arid Plants

Perlite is arguably more suitable for cacti and succulent mixes.

Where drainage is essential and water retention is not desirable.

Here you can go much higher in your ratio, and it’s not unusual to see 50% perlite mixes. 

In a Terrarium Mix

Drainage is super important in a terrarium, but perlite isn’t my favourite material to use in a terrarium substrate mix.

It’ll do the job (and I have used it before), but aesthetics is an important part of a terrarium and white foamy balls doesn’t exact look natural does it?

In my terrarium substrate mixes, I prefer to use orchid bark as my main drainage element. Or, fine lava rock works really well too and it’s dark colour makes it blend in a lot better.

Propagation and Hydroponics

You can use high concentrations of perlite (or even 100% if you like) when growing plants in semi-hydroponics or sprouting seedlings.

Though vermiculute is arguably better for seedlings, perlite can still work.

For me, it’s more useful as a transition medium when taking plants from pure water to soil. It can help to strengthen root growth and ready them for terrestrial planting.

In the Garden

Perlite is commonly used as a soil additive in the garden. 

Though I don’t have a garden myself, it’s my understanding that it can be used in a variety of ways – both in container gardening and seed sowing – to provide aeration and drainage. 

Not all perlite is created equally, and when it comes to drainage bigger is better.

You’ll typically see the range of fine, medium, coarse and super coarse granule size.

For horticultural purposes, the finer powder-like perlite is still going to aerate soil quite effectively, but won’t provide the same level of structure and drainage as a larger, coarser perlite.

I’d reserve the fine perlite mostly for plant propagations, rooting new plants and germinating seeds.

For substrate mixes you’ll have the best success with medium grade perlite

It’s usually what you’ll get when you buy horticultural perlite but it’s always worth checking the label.

• See perlite on Etsy.

The coarse perlite and above can get very chunky, and are typically used more for outdoor use where there’s lots of space and you need a lot of it.

There is a common belief that perlite causes flouride burn, and there may be some truth to it, but the takeaway is quite different.

This study by Tammy L. Everett and Paul V. Nelson shows;

“Perlite sources vary in their soluble levels of fluoride. The initial concentration of fluoride in leachate from the five sources of perlite used in this study ranged from 0.05 to 0.83 ppm. The higher value was potentially toxic but it did not persist. Soluble fluoride rapidly decreased in subsequent leachings to very low concentrations.”

It’s important to note here that they did specifically use perlite with high concentrations of fluoride in this study.

The takeaway? 

Perlite is unlikely to ever cause flouride burn beyond an inital brief exposure, and the easiest way to mitigate the risk is to simply water your plants.

Do you use perlite in your horticultural projects?

Love it or hate it? Let me know in the comments!