Perlite Myths – Should It Be Used in Gardens and Potted Plants?

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Robert Pavlis

Perlite is used a lot in horticulture and can be found in many soilless mixes. There are concerns about fluoride and aluminum toxicity which can be harmful to plants, but how big of a problem is this?

Lots of people recommend the use of perlite in gardens, especially for raised beds. Is that a good idea?

This post will look at the truth about perlite.

Perlite Myths - Should it be Used in Gardens and Potted Plants?
Perlite Myths – Should it be Used in Gardens and Potted Plants?, Credit: Grower Experts

What is Perlite?

Perlite is volcanic rock that has been heated to form small, white pieces of porous material. It’s added to soil and soilless mixes to increase drainage. It is lightweight, odor-free and easy to handle. Some claim it has a pH of 6.5 to 7.5 but reliable sources say it is 7.0–7.5. The exact pH does not matter much because it is chemically inert and will have little effect on the soil solution pH. Perlite has a negligible cation exchange capacity (CEC) which means it does not hold on to nutrients very well.

Horticultural products are available in different sized particles. Larger particles have a lower total pore space and water-holding capacity and higher relative air-filled pore space. Medium perlite has a pore space of 63%, a water-holding capacity of 38% and an air-filled pore space of 25% at container capacity. The bulk density ranges from 0.05 to 0.10 g/mL (3–6 lb/ft3).

Source for coarse perlite.

Different grades of perlite
Different grades of perlite, Credit: Perlite Institute

Horticultural Uses for Perlite

Perlite has similar characteristics to sand, except that it is much lighter. It adds drainage and air to the mix. It is chemically inert and therefore adds few nutrients. It is quite stable, but it is easily crushed, turning into a white powder that offers little value to soil. Unlike sand, it floats and many gardeners hate the fact that it floats to the surface of potted plants.

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Perlite Use in a Soilless Mix

It is a common ingredient in both commercial and DIY soilless mixes. It prevents peat moss from compacting too much and makes the mix a bit drier. Most commercial mixes contain small amounts, probably because of it’s relative high cost. Some succulent mixes may contain higher levels of perlite.

Perlite Use in Propagation

It is a very good media for rooting cuttings, either on its own or mixed 50:50 with peat moss. It provides enough moisture along with a lot of air, which is perfect for rooting.

Perlite in Hydroponics

Perlite is a good media for hydroponics where it can be reused. Compared to other types of hydroponic media it is less expensive.

Perlite Use in the Garden

I see a lot of recommendations for using it in the garden, particularly in raised beds. I don’t recommend it for either.

Perlite floats to the top of soil and then wind blows it around. It is not a natural component of soil and it is not an environmentally sustainable product. The United States imports about 25 to 30 percent of its processed perlite, mainly from Greece. Sand is usually available locally and requires much less processing. Compost and manure will do the same thing, perhaps not as quickly, but they are much better for building soil long term.

There is no problem putting a bit of spent perlite into the garden. However potting soil does not really get old and can be reused for many years.

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Is Perlite Sterile?

I see this claim a lot and fresh material out of an unopened bag may be fairly sterile because it is produced at high temperatures and not much will grow in dry perlite. As soon as the material is used either in a soil mix or as a media for starting cuttings, it is no longer sterile. You can easily see that yourself – algae has no problem growing in it once it is wet.

Some claim that it won’t mold. Maybe not on its own but as an additive to soil, it does not prevent mold from growing in the mix.

Water Holding Capacity

Perlite has large pores inside the material that can hold 3–4 times its weight in water. Coarse material holds less water than fine material. Some of this water is unavailable to plant roots. The available and non-available water in commercial perlite of 0–4 mm diameter was 13.6 and 36.5 per cent of its volume.

The water holding capacity of peat moss, perlite and vermiculite are approximately 76%, 38% and 71%. Mixtures of peat moss and perlite result in a water holding capacity between these limits. The difference between peat and perlite seems large but since most soilless mixes have less than 20% perlite, they have essentially the same water holding capacity as peat moss.

Since peat moss holds more water than perlite, adding perlite makes the mix drier while also improving drainage.

Water retention of perlite
Water retention of perlite, credit: Supreme Perlite

Perlite Retains Moisture and Nutrients

This is a popular claim for perlite and it is partially true.

Perlite does hold water which makes it true, but adding it to a peat based mix actually reduces the amount of water in the whole mix. So it is not correct to say that adding perlite increases the moisture.

What about nutrients? Plant nutrients travel with water as ions and so any material that holds water also holds some nutrients. But perlite has a low CEC so it does not hold on to nutrients very well. If you flush perlite with water you will quickly wash the nutrients out. Peat moss or compost have higher CECs and are better for holding nutrients.

Adding perlite to a soilless mix makes the mix drier which might be a good thing if you tend to overwater your plants.

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Fluoride Toxicity of Perlite

Perlite does contain fluoride and there is some concern about it being toxic since some plants are quite sensitive to it. A level of 1 ppm fluoride is considered safe for plants.

Different sources of perlite contain differing amounts of fluoride. When perlite is flushed with water even the source with the highest level is below the 1 ppm limit and this falls rapidly with a few flushes. When tested with so-called fluoride sensitive plants, there was no fluoride toxicity from perlite.

The idea that some plants may be sensitive to fluoride is probably overstated. This is discussed more in Fluoride Toxicity in Plants – Is Tap Water Harmful?

Aluminum Toxicity of Perlite

Perlite typically contains 13% aluminum oxide and aluminum can be toxic to plants. Perlite is not very soluble in water so only a small amount would be released into the water around plant roots.  As the pH drops, more aluminum solubilizes and becomes available to plants. Provided the pH is maintained above 5.0, aluminum toxicity should not be a problem.

Is Perlite Organic?

In a chemical sense perlite is not organic, it is made from minerals, it’s mined and it’s processed. However, it can be used in organic certified agriculture.

Is it a Natural Product?

Perlite is not a natural product. You can’t find it in nature. The raw material is mined, taken to a factory where it is converted to horticultural perlite. It is man-made.

Is Perlite Dust Harmful?

Perlite is regulated as a “nuisance dust” in most countries. This is dust that contains less than 1 percent quartz which makes it largely harmless to the lungs. The small amount gardeners are exposed to should cause no problems.

If you are concerned about the dust, wet the material before handling it.

Perlite vs Vermiculite

both of these products are used in soilless mixes and both have similar properties. For a detailed comparison have a look at, Perlite vs Vermiculite – Which Soil Additive is Better?

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

24 thoughts on “Perlite Myths – Should It Be Used in Gardens and Potted Plants?”

  1. I bought a bag of perlite from a large nursery store. I opened the bag and it has a lot of powder ( fine crushed perlite) that probably resulted from being stacked at the store. Is it okay to use in my potting mix? Would this make my soil over moist versus the coarse or medium size perlite that it should have been to start with.

    Reply
    • I don’t think crushed perlite will hold more moisture, but it will slow down evaporation of water, so the soil will stay wet longer.

      Reply

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