Those “crunchy” little white pebbles in your soil mix might look like Styrofoam, but they’re not. They are actually chunks of perlite, which comes from a type of volcanic glass.
So, what is perlite used for? Perlite is used in seed starting mix, potting soil, and succulent/cacti mix to provide aeration. Perlite is also used to root plant cuttings without soil. Perlite can serve as a soilless growing medium for hydroponics, aquaponics, or aeroponics.
Perlite is a popular additive in many different soil mix products. There are alternatives available (such as vermiculite).
In this article, we’ll talk about what perlite is used for. We’ll also answer some common questions about perlite.
Let’s get started.
What Is Perlite Used For? (When To Use Perlite)
Perlite is often used to help with starting new plants (either from seed or from cuttings). It is used in traditional growing with soil and also in soilless systems (like hydroponics).
For example, perlite is used for:
- Seed Starting Mixes
- Potting Soil Mixes
- Succulent/Cacti Soil Mixes
- Medium For Rooting Plant Cuttings
- Soilless Growing Medium (Hydroponics, Aquaponics, Aeroponics)
Perlite as part of a seed starting mix is one of the more common uses, so we’ll start there.
Perlite In Seed Starting Mix
Seed starting mix is used to germinate seeds (usually indoors, in trays or small containers). If you have ever bought seed starting mix from the store, you know that it usually has small white chunks of “popcorn” in it.
Those little white chunks crush easily and look like Styrofoam, but they are tiny pieces of perlite. Perlite holds both air and water (but not too much water).
This is why perlite is added to seed starting mixes. It helps to maintain air and water levels in soil – and seeds need both air and water to germinate.
Normally, seed starting mix has only a small portion of perlite (25% to 30% or less).
Below are three seed starting mix recipes that use perlite: a lighter one, a medium one, and a denser one. You can experiment a bit with the ratios to see what works best for you.
Peat-Perlite: basic mix with only two ingredients; a fairly light mix. (75% peat moss and 25% perlite)
- 3 parts peat moss
- 1 part perlite
Image courtesy of user:
Ragesoss via:
Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:Schultz_Sphagnum_
Peat_Moss.jpg
Sand-Peat-Perlite: equal parts perlite & sand; a little more dense than the first mix. (75% peat moss, 12.5% perlite, 12.5% sand).
- 6 parts peat moss
- 1 part perlite
- 1 part sand
Compost-Peat-Perlite: put some of your compost to good use! Just make sure the compost is sterilized first. A little denser due to the compost. (1/3 compost, 1/3 peat moss, 1/3 perlite)
- 1 part sterilized compost
- 1 part peat moss
- 1 part perlite
*Note that coconut fiber (coco coir) is sometimes used with (or instead of) peat moss. Vermiculite is also sometimes used with (or instead of) perlite – but vermiculite holds more water than perlite.
Perlite In Potting Soil
Potting soil is used as a growing medium for houseplants (among other uses). If you have ever bought a potted plant from the store, it will often have those little white chunks of perlite in the soil mix.
Perlite is added to potting soil to help prevent over or under watering. Beginner plant parents are prone to both of these mistakes!
Perlite counters this tendency by retaining air (which plant roots need to breathe) or releasing water as the soil dries out.
The potting soil mix you use will depend on what you are trying to grow. However, since your potted plants will be past the seed/seedling stage, they will need nutrients to grow.
So, all potting soil mixes will need some nutrients added. Here is a potting soil recipe for houseplants from Cornell University:
- ½ bushel sphagnum peat moss
- ¼ bushel vermiculite
- ¼ bushel perlite
- 8 tbsp. ground dolomitic lime
- 2 tbsp. superphosphate
- 3 tbsp. 10-10-10 fertilizer
- 1 tbsp. iron sulfate
- 1 tbsp. potassium nitrate
If you prefer something simpler, Clemson University suggests 2 parts pine bark, 1 part peat moss, and 1 part sand. Remember to use liquid fertilizer for your plants, since this mix has little nutritional value.
If you want a potting soil mix for succulents or cacti, you will need more sand than you would otherwise. The University of Florida suggests:
- 2 parts sterilized garden soil
- 1 part peat moss
- 1 part perlite
- 1 part coarse sand
Sterilized garden soil is heated, which kills off weed seeds, fungal spores, and bacterial pathogens. To sterilize soil, fill a sheet pan with moist garden soil and put it in the oven at 250 degrees Fahrenheit.
Use a food thermometer to make sure the soil temperature reaches 180 degrees Fahrenheit, and cook for 30 minutes.
You can learn a lot more about making homemade potting soil (as well as other recipes) here.
Perlite To Ensure Drainage For Succulents & Cacti
Succulents and cacti don’t need much water. In fact, it is easy to over water them. So, it makes sense to add perlite to their soil mixes to prevent over watering.
There is a succulent/cacti recipe above, but Iowa State University Extension also has one, suggesting 1 part organic material (such as sterilized compost or coconut fiber) mixed with 2 parts mineral material (such as perlite or coarse sand).
Even if you buy a succulent or cactus soil mix from the store, you can still add your own perlite to make it drain better.
Perlite To Root Plant Cuttings
If you are propagating plants by cuttings, you can use perlite as part of your growing medium to help the new plants get going.
The Purdue University Extension suggests a mixture of 50% perlite and 50% vermiculite. This will hold enough water and air to support root growth without drowning the roots or drying them out.
Missouri State University Extension suggests that you can also use a 50% perlite and 50% peat moss mixture for propagating plants by cuttings. Vermiculite by itself is not a good idea, since it holds too much water and tends to get compacted.
You may need to water the perlite to keep it wet enough for the cutting and its roots until it becomes established.
Perlite As A Soilless Growing Medium (For Hydroponics, Aquaponics, & Aeroponics)
Hydroponics, aquaponics, and aeroponics are methods to grow plants without soil. Instead, they use a soilless growing medium (such as perlite) combined with water (or a nutrient mist, in the case of aeroponics).
Perlite has excellent oxygen retention, and it can hold 3 to 4 times its weight in water. It is also sterile when purchased new, so it reduces the chances of seed/seedling pathogens (root rot, damping off, etc.)
Other materials for soilless growing medium include mineral wool, grow stone, and sand.
What Is Perlite Made Of?
Perlite is made of volcanic rock, which is heated to 1,800 degrees Fahrenheit so that it expands. This level of heat also ensures sterility, since pathogens cannot survive such heat.
Perlite contains a large amount of silicon, as well as aluminum, sodium, potassium, and some other elements in lesser amounts (iron, magnesium, calcium).
Can Perlite Be Used Instead Of Sand?
Perlite can be used instead of sand to improve drainage in a soil mix. Perlite is lighter than sand, so you will end up with a less dense soil mix.
You can also opt to use add both sand and perlite to your soil mix. This will give you a “middle ground” for the density of your mix.
Can Perlite Be Reused?
You can reuse perlite, since it doesn’t readily break down (it does not biodegrade). However, there are some risks involved in reusing perlite.
For one thing, perlite may retain salt from fertilizer or soil, which it may then release later. Perlite can also harbor the eggs of pests, which can make their way into another crop planted in the soil with reused perlite.
Perlite used for growing is no longer guaranteed to be sterile. As such, it may contain pathogens (bacteria, viruses, fungi) that can harm plants.
Is Perlite Biodegradable?
Perlite is not biodegradable. It is a rock (volcanic glass), so it takes a long time to get ground down into finer particles (and even then, it still has the same chemical formula).
Can Perlite Go Bad?
Perlite does not really “go bad” or expire. It is a volcanic rock, so it doesn’t biodegrade (like a banana peel or grass clippings would).
Still, perlite used as a growing medium can pick up pathogens and then spread them to other plants if it is reused. You could certainly heat up the perlite (by itself or as part of a soil mix) to sterilize it and kill off any pathogens.
Where Does Perlite Come From?
The perlite that we see and use in soil mixes is not found that way in nature! Perlite is a type of volcanic glass, which is first mined and collected.
It is then crushed and ground into smaller pieces (the grade tells you how big the chunks are). The perlite chunks are then heated up to high temperatures (1,800 degrees Fahrenheit), which sterilizes them and causes them to expand (up to 13 times their original size!).
Is Perlite Fertilizer? (Does Perlite Have Nutrients?)
Pure perlite by itself does not qualify as fertilizer. Although it does contain some nutrients, they are not readily available to plants.
Perlite is a rock, so it does not biodegrade in any meaningful way. It would take a long time for perlite to break down enough for its nutrients to become available to plants.
However, perlite could be saturated with nutrients, which would then be released to plants over time. In that case, perlite would act as a slow-release fertilizer delivery method, rather than being fertilizer itself.
Why Is My Perlite Turning Yellow, Green, Or Brown?
Perlite is naturally white, so it will become stained and discolored over time. For example:
- Perlite can turn yellow if it is holding onto minerals or nutrients from soil or fertilizer (salts).
- Perlite will often turn green as the result of algae that grows on the surface of wet soil. Algae will grow in warm, wet, bright environments with humid air.
- Perlite might turn brown if it gets stained by soil (sometimes particles get dried on or “baked-in” to the perlite by sunlight and heat).
Conclusion
Now you know what perlite is used for and why it is useful. You also know the answers to some common questions about perlite.
I hope you found this article helpful. If so, please share it with someone who can use the information.
To find books, courses, seeds, gardening supplies, and more, check out The Shop at Greenupside!
Hey – you can get our weekly gardening newsletter here!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy!
~Jonathon