Perlite vs Vermiculite: Differences (and When To Use Them)


Photo from PerliteVermiculite

Do you want to learn the difference between perlite and vermiculite? This article has you covered with science facts, expert gardeners’ opinions on what they are, and, more importantly, when and how you should (and should not) use them.

Hence, are vermiculite and perlite the same? Despite being both originate through heating of mineral rocks, they differ for three essential aspects:

  1. Aspect: color and shape are different
  2. Water holding capability: significant for vermiculite, limited for perlite
  3. Use: perlite to increase aeration, vermiculite to increase water retention

Want to know more about those differences and how they can make the difference in how and why you should (and should not) use them? Just keep reading.

Vermiculite vs Perlite: The Differences (with Photos)

I used to confuse them for a long while. However, they are not only different for how they look, but also on how you can use them. Use vermiculite instead of perlite, and your herbs will suffer. Let’s dive in!

Origins: From Magma to Clay

Vermiculite is a clay-based mineral (clay is a very fine and smooth medium very common on soil, here for more) forms underground and mined mainly in Russia and Africa. This mineral, to become the vermiculite you buy, is heated up in industrial furnaces to release the water in it.

When this happens, the mineral literally pops (like popcorn) due to the water becoming a (larger in volume) gas and leaving the mineral. This will create empty pockets inside the material, making it very lightweight. When it cools down, it keeps a vermin shape (here the name).

Perlite, on the other hand, is a specific type of solidified magma (high silica content), also called volcanic glass. Such raw material is turned in the perlite you find at the gardening shop by heating it up at more than 1500F (815C). This, in the same way of vermiculite, will cause the (relatively high) water content in this rock to evaporate, leaving empty pockets inside perlite when it evaporates. The material will also expand many times its original size becoming very lightweight.

Important to remember that are both pH neutral (so do not affect the soil and the absorption of the nutrients as detailed here).

Let’s dive in the main differences you need to know that will make you save time and money

Aspect: Colour, Shape, and Weight

Perlite and vermiculite are relatively easy to differentiate.

Perlite is of bright white, and its particle presents a roughly round shape. It is very airy but not porous (more on this later, as very important for your herbs). Perlite is sold in different sizes: extra coarse, coarse, medium, fine, and extra fine. The perlite you can find in the shops has a very low density (it uses a lot of space but still very light) of around 40-120 kg/m3 (just as a comparison, water is around 1000kg/m3). Hence, perlite is lighter than water (as it tends to float on top of it, more on this later).

Perlite close look-up – Source

Vermiculite, on the other hand, presents a yellow towards gold-brownish particles. These have cylindrical shapes, they are very airy and lightweight. It has cavities as perlite, but more importantly, is porous (more on this on a particular section). It is sold in 3 different sizes: fine, medium, and large (from 1mm to 12mm). It has a higher density than perlite (from 170 to 220kg/m3 depending on the particle size). Nonetheless, vermiculite is still lighter than water, but, due to its water-holding capability, it will not float. This is important for your use of vermiculite.

Vermiculite particle, close look up (a few mm each) – Source

Their Chemical composition, as discussed by Professor Mueena Samar and Minerals Dupre, is shown in the table below. They are both inert (do not release any substance in the soil), and their pH is generally close to neutral (very good news for the majority of plants and herbs).

ElementPerlite (Percentage)Vermiculite (Percentage)
Silicon dioxide72.0839.6
Alumina Oxide12.929.2
Titanium dioxide0.91
Iron oxide1.57.9
Magnesium oxide0.6323.5
Quick lime0.882.1
Caustic soda3.760
Potassium oxide4.336.1
Bound water30
Fluorine00.6

The major difference, in terms of their composition, is the high percentage of Silicon in Perlite, as opposed to the high amount of magnesium in vermiculite.

Water Holding Capability and Oxygenation: Ignore Them And You Will Kill Your Herbs

This is something significant if you want to be successful in using perlite and vermiculite in your potting soil. Indeed, if you remember what we said before, perlite is airy but not porous, while vermiculite is airy and porous.

Regarding water-holding capability perlite will retain a small amount of water. This is because the water will stick to the surface and its irregularity. This explains why perlite will float on water.

However, vermiculite will absorb water like a sponge does through its pores. This will make the vermiculite expanding up to 4 times and becoming muddy (if too much water is poured in) due to its “clay” origins.

Knowing this different behavior will make the difference between thriving or perishing herbs.

Oxygenation is the capability of the material to let oxygen passing through.

Perlite is ideal for making the amount of air reaching the roots of your herbs. However, this is not the case for vermiculite. Indeed, when vermiculite is watered tends to collapse, making their oxygenation capability lower than perlite as it will get more compact.

I have to say than talking with several gardeners and reading around the opinion are quite diverse and sometimes contrasting. However, something is very clear. Perlite and vermiculite cannot be interchanged due to their radically different water capability.

Let’s dive in!

When To Use Perlite

Perlite should be used to increase the aeration of your potting soil. Hence, you can use perlite for:

  1. Old potting soil that lost its airy structure: perhaps (as in my case), you have a bag of potting soil hanging around that you did not manage to use for a while. In such a case, you might notice that it lost a bit of its original structure and airiness. As a rule of thumb I do suggest to add a third of the soil volume in perlite as discussed in the best potting soil mix recipe;
  2. In case you are creating the potting soil yourself: as detailed in potting soil mix recipe perlite is a great addition to give structure to the soil and increase drainage capability;

If, for any reason, you have a very clay-based potting soil (never happened to me), do not use perlite. Indeed, at first, it might sound like a good idea to use perlite to break the compact clay-heavy soil. However, after a while, the cracks originated by the perlite will get full of water (remember clay retains water quite well), and the perlite will go pushed straight on top. Once on top, it can fly very quickly all over the place (as very light, like popcorn), making the situation indoors, especially quite messy.

How To Use Perlite In Pots?

Perlite is a great medium that can help with aeration and drainage. However, it is very light, and so, when water tends to float. This explains why having perlite on the surface of the soil is useless. It is not providing aeration or structure improvement as just sitting on top of your potting soil, not inside it. Moreover, it’s quite bright and white color, makes the potting soil looks unnatural.

When you create your potting mix with perlite, as a side suggestion, just add perlite (in a third ratio) to half of the soil, so you have the other half perlite-free. Perlite should be uniformly distributed in the soil (just mix them with a spatula, here more). Then you can use such soil with perlite fill the first bottom half of your container and the remaining perlite-free soil to fill the top part of the soil. This will provide excellent aeration for the roots of your herbs and, at the same time, will keep the perlite away from the surface. Use fine or extra-fine perlite to avoid affecting the soil structure.

Here you can have a look at Amazon’s good quality perlite.

If you are using a standard potting mix for herbs (usually perlite-free), perlite is not necessary. If you really want, add potting mix to half of the container, add 2 tablespoons of perlite, add the remaining perlite-free soil.

Any problem with using perlite? Yes, perlite does release dust that you do not want to breathe. Hence, use some kind of breath protection when handling it (even a scarf is more than enough covering your nose and mouth). It is not a problem when wet. Moreover, good quality perlite such as this one on Amazon, for instance, has been pre-washed, so removing even the minimal risk of inhalation.

When To Use Vermiculite?

As strange as it might sound, I do not recommend vermiculite for grown potted herbs (or general plants). Indeed, vermiculite makes watering extremely challenging. Indeed, the claimed advantage by many of providing aeration does not stand. Indeed, once wet, vermiculite loses its structure, collapse, and become muddy and compact. Also, 30 years expert gardeners (here for more) agree with the same point of view.

You can actually squeeze, as shown in this video, like a muddy sponge.

You might have read that vermiculite can add valuable plant minerals like potassium, magnesium, and calcium. Indeed, as also detailed here, vermiculite is given by such minerals. However, these minerals are not going to be released into the soil for your herbs. Indeed, do not forget that vermiculite is a rock essentially, and such minerals are not released into the soil if not in a minimal amount.

Hence, why do vermiculite exist? Well, it has lots of uses out of potting gardening, and it is excellent for those plants really thirsty that otherwise would need frequent watering. It is also used in hydroponic systems (here the soil is replaced with an inert medium like a combination of perlite and vermiculite).

The only use I see for vermiculite for more traditional indoor gardening (hydroponics is a kind of recent approach that I will experiment in the future and let you know), is for seedlings. How can you use it effectively? Keep reading to learn how to create the best seedlings conditions ever.

Seedlings: Perlite or Vermiculite? Both!

Seeds need air and quite often a consistently moist environment. Expert gardeners suggest half perlite and half vermiculite. Why do you need both? Because perlite will provide the aeration roots need to develop, while vermiculite will give the long-lasting moisture (you water once and last several days), seedlings need to develop.

Moreover, this light mixture (thanks to the perlite) allows the first weak roots of the seedlings to make their way through the medium easily (compared to potential compact soil). However, here the catch! This light-medium must be replaced with good quality potting soil (here my best pick) once the herbs start emerging from the medium.

Seedlings in a medium of perlite/vermiculite is ideal –

Pro tip: many gardeners advise to fertilize seedlings. From my modest point of view, I do not agree. Indeed, seedlings have all the nutrients they need to sprout (otherwise, they will never manage to develop if every time a fertilized soil was required). This explains why a 50/50 mix of perlite and vermiculite is more than enough. Indeed, both materials do not have any organic trace on them (due to their origins, and the heating process they undergo).

If you have seedlings but do not want to bother with vermiculite and perlite why you do not give a try to Jeffy pellets, here you can find a detailed guide. Simple to use, you will not get your hand dirty and container, soil all in once for simple and effortless transplant.

Perlite and Vermiculite: Alternatives – Do It for The Environment

Why should you look for alternatives? When looking for perlite or vermiculite, should you look for other options? If you want really environmentally friendly then yes. Perlite and vermiculite are finite resources. Indeed these are naturally produced material at a very slow timescale compared to the speed by which we (humans) mine and use them. Moreover, high carbon-intensive processes are needed to produce them (heating at high temperatures).

However, the good news is that perlite and vermiculite consumption is quite low compared to the resources available. Nonetheless, if you want to be more environmentally friendly, then you should opt for something different.

Perlite can be replaced with pumice. It presents the same drainage (no water retention if not for the water that sticks on the surface) and oxygenation capability. The significant difference is the weight. Pumice is heavier. However, for me, it is an advantage. Indeed, it does not float on the soil and also helps keep the pot more stable (case of wind when you open the window).

Vermiculite, considering its high water capability, retention can be replaced with coir. This is a natural renewable material produced by the coconut manufacturing industry as a waste product (so totally green), as mentioned in this scientific publication for the curious. The main disadvantage compared to vermiculite is that, as any organic material, it tends to decompose over (relatively long) time. Some also suggest sawdust. However, I do not recommend this. Not only in the relatively dangerous in handy it (for the dust) but more importantly because you might need to add extra nitrogen to compensate for the fact that the sawdust will use it from the soil to decompose. This will make your life more complicated, so avoid it.

How To Create the Perfect Potting Soil?

Perlite is quite often a key ingredient in the perfect potting medium mix. Not only me but also many gardeners use it on a regular base.

What do they do? Well, in general around a third of the potting mix they create is perlite, no vermiculite for herbs and not thirsty plants. Other ingredients might be peat moss, compost, and fertilizer.

For more have a look at this DIY detailed guide below (click image). If you want an easy solution you can also have a look at my best choice for indoor potting soil in this detailed guide.

Related Questions

Can I reuse perlite? Yes, perlite can be cleaned and reused. Perlite is a non-organic material (rock) that hence does not decompose. It is suggested to sterilize perlite by microwaving, oven, or boiling in water.

Is vermiculite safe? Vermiculite is harmless in regular use, and no security produced is necessary when handling it according to the manufacturer’s guidance. For more detailed information check this article

Further Readings

Sand, Silt, And Clay

5 Easy Ways To Sterilize Potting Soil (with Infographics)

Water Absorbing Crystal For Indoor Herbs: Another way to increase soil water retention

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Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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Differences in color, watering capability and use of perlite and vermiculite