Perlite is a very common ingredient used in potting soil that helps improve drainage and aeration. It appears as small, round-shaped foam-like pebbles that ‘s mixed with other components to create potting mixes suitable for different plants.
In this article, I’ll explain all the things you need to know about perlite, how it is made, the kinds of perlite available and how to use it in the garden.
What is Perlite and How is it Made?
Perlite is made from volcanic glass that is heated at very high temperatures (1600 degrees Fahrenheit).
In the process, the perlite expands and pops, much like popcorn does in the microwave, allowing it to expand to between 10 to 15 times its previous size. As a result, you get something that looks like small styrofoam pebbles that are very lightweight.
Like popcorn, the heating process produces tiny pockets of air in perlite which enables it to absorb more moisture. This feature makes it very useful as it makes water readily available to plants’ roots.
One of the most important features of perlite is its porous nature. As such, it is able to provide plants not only with moisture retention but simultaneously improve drainage.
This irony of water retention and good drainage is something that many houseplants require since they need water to grow optimally. But, have problems with waterlogging which can eventually can cause fungal problems and root rot.
This dual characteristic is why perlite is regularly used for plants that prefer to be on the dry side or don’t enjoy consistently moist conditions.
It is also one of the reasons why perlite is more commonly used compared to vermiculite.
Is Perlite Organic?
To answer this question it is important to define the term organic first. That’s because its definition can vary depending on who you ask.
For example, from a chemistry standpoint, a material is considered organic if it contains carbon molecules. If this is what you go with, then perlite is not organic as it does not have any carbon in it.
But, if you take it from a gardening or horticultural perspective, organic means something that is produced naturally. That is, there is no artificial process or additives used in creating that material.
Under this definition, perlite is indeed organic as it comes from volcanic glass that’s extracted from the earth.
Of course, you can argue that it has gone through processing whereby it has heated to expand. But, like many other products including fish fertilizer, its original source is still natural.
This is why the Organic Materials Review Institute considered horticultural perlite valid for use in organic agriculture.
Uses of Perlite in the Garden
The most common use for perlite in gardening is to create soilless potting mixes. Much like peat moss, it is not used on its own. Instead, it is mostly mixed with other ingredients to create a custom mix perfect for specific plants.
This allows you to create different potting media for each kind of plant you have. In the process, you’re able to give it the best chance to grow optimally.
The reason why perlite is very valuable is because it provides a light, loose material that is well-draining. Just as importantly, it does not get compacted over time.
Perlite offers a natural filtering ability that allows excess water to drain while still retaining enough moisture to keep the plant hydrated. It also retains some nutrients to help he plant grow.
This is essential especially for plants that don’t like sitting in moist soil. Some examples include succulents, most houseplants which are tropical in nature and cacti.
Perlite also improves aeration allowing air to circulate and reach the roots.
A plant’s roots needs to breathe. And, it order to stay healthy, there needs to be a balance of oxygen and moisture. Thus, combined with its ability to help with drainage it allows plants to grow optimally.
On the other hand, substrates that retain too much moisture will prevent oxygen from reaching the roots. In the process, it increases the risk of root rot and other problems over the long term.
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Types of Perlite Available
When you go check out the array of perlite products in your garden center or online, you’ll notice that there are a few different kinds of perlite.
These are categorized by grade, which divides the kind of perlite based on how fine or coarse the particles are. Here’s a quick breakdown and description of each of the grades.
- Grade #4: Super coarse perlite – Granules are about an inch in size. This is the most porous. As such, it provides the best drainage, which makes it useful for dense soils. You want to use this for plants that like to be on the drier side of things. In non-heavy soils, you want to be careful as it can drain too well causing the plant lack water or forcing you to have to water more often than you should.
- Grade #3: Coarse perlite – the size of coarse perlite is around half an inch per granule. Coarse perlite is very much like the previous grade but not as big or porous. As such, it provides good drainage as well and is often used for garden beds and soils that tend to retain water.
- Grade #2: Medium perlite – Granule size varies from between 1/4 of an inch to 3/8 of an inch. Medium grade perlite is the most versatile in that it is used in most applications including general gardening, container plants and even planter boxed.
- Grade #1: Fine perlite – this is the finest level, with the granules being around 1/8 of an inch in size. Fine perlite is often used for starting seeds as well as propagating cuttings.
In general, the types of perlite vary in how coarse they are. The more coarse the particles, the larger the granules will be. This also means that they are more porous.
So, as a general rule, the coarser grades drain more moisture while the finer ones much less. Thanks to their size, coarser bits are likewise less likely to get blown off by strong winds.
Benefits of Using Perlite in the Garden
Perlite is a very useful ingredient especially when creating potting mixes. That’s because it carries a number of characteristics that plants need to grow properly.
More importantly, the mix of these characteristics into one substrate makes perlite very valuable especially if you want to use a soilless potting mix or need to improve the drainage ability of your garden soil.
Here’s a list of the benefits of perlite.
- Absorbs and retains moisture while improving drainage – this is one of the biggest benefits of perlite. And, it is the reason many houseplant owners use perlite in their potting mixes. Its ability to improve drainage makes it very valuable to cacti, succulents and tropical plants (where most houseplants fall under) because they don’t like their roots sitting in water for long periods or staying in consistently moist conditions (which can result in root rot and other diseases).
- Improves aeration – the porous nature of perlite thanks to its heating and popping process makes it somewhat like popcorn, allowing for better airflow. Since plants’ roots need a balance of oxygen and water, perlite lets the roots get sufficient air circulation.
- Neutral pH – this makes it easy to use perlite along with other ingredients without drastically changing soil pH. The neutral pH also makes it ideal for garden beds and growing container plants.
- Will not decompose – like peat moss, not degrading or decomposing makes perlite useful since one application can last for a long time. This makes it perfect for plants that don’t need or don’t appreciate being repotted often including houseplants and succulents.
- Very stable – this is related to decomposition. Perlite has a stable structure that allows it to keep its shape in different conditions. As such, it won’t change shape or composition in different kinds of soil, making it easy to work with whatever the environment.
- No additives or chemicals included – you don’t need to worry about any chemicals, additives or toxins since none are added to the perlite you’ll find in packages.
Fluoride Burn: Is Fluoride Toxicity Real or Myth?
Something worth knowing when using perlite is that it can cause fluoride toxicity or fluoride burn in some plants. Among the plants that are susceptible include spider plants and dracaena.
Fluoride burn from perlite presents itself through browning of the leaf tips of your plants as well as some brown spots.
That said, studies have shown that the odds of this happening is very rare. And, more likely than not factors may cause this including fluoride in water (if you use tap water), high phosphate fertilizers, low soil pH and others.
That said, research also shows that some perlite will leach fluorides especially initially. The amount varies from 0.05 to 0.83 ppm, with the latter one being potentially toxic to the plants. As such, depending on the perlite you use, it can happen.
The good news is that it has no effects on human health.
One of the biggest downsides of using perlite is health safety.
Perlite dust can float into the air as you mix or handle the substrate causing them to enter your eyes, nose and mouth. When this happens, it can cause irritation as well as aggravate any respiratory issues you ma have.
This is more prevalent in fine grade perlite as well as the cheap products that have little quality control.
While it does not cause long term health problems like cancer, this immediate and short term effects warrants being careful when using it. Unfortunately, some commercially sold perlite may contain small amounts of quartz which is considered a carcinogen.
If you’re just starting out, I highly suggest using safety goggles and a face mask until you get the hang of things.
Where to Buy Perlite
Like peat moss, perlite is very accessible and readily available in local nurseries and garden centers. You can likewise get some online.
If you happen to be using bulk amounts, I highly suggest comparing prices since the more you use the bigger the discrepancy prices can make. This is likewise true for peat moss.
In rare instances that you cannot get hold of perlite, you can use pumice as an alternative. Another options is vermiculite although it is important to note that vermiculite is not as porous as perlite so it will retain more water and not drain as well as perlite. So, you’ll need to make some adjustments.
Overall, perlite is a great choice if you need to improve drainage. It is perfect for mixing into potting soil because of its good water retention and aeration ability.