Perlite vs Vermiculite: What Are The Differences And Which Is The Best Choice?
Elizabeth is a Permaculture Garden Designer, Sustainability Consultant and Professional Writer, working as an advocate for positive change. She graduated from the University of St. Andrews with an MA in English and Philosophy and obtained a Diploma in Applied Permaculture Design from the Permaculture Association.
Reviewed By COLIN SKELLY
Colin is a Horticulturist and Horticultural Consultant with experience in a range of practical and managerial roles across heritage, commercial and public horticulture. He holds the Royal Horticultural Society’s Master of Horticulture award and has a particular interest in horticultural ecology and naturalistic planting for habitat and climate resilience.
Perlite and vermiculite are both used to improve aeration and drainage in bought and homemade potting mixes.
But what exactly are perlite and vermiculite? What are the differences between them? And is either one really the best choice for your needs?
Read on to find out more about these two materials.
What Is Perlite?
When perlite is heated to temperatures of 860-900°C the water trapped within the structure vaporises and escapes.1Kuntz, C. (2022, November 10). Why Perlite Works. Perlite Institute. Retrieved March 22, 2023, from https://www.perlite.org/why-perlite-works/
This makes the material expand to as much as twenty times its original size.2Perlite. (2017, May 26). Minerals Education Coalition. Retrieved March 22, 2023, from https://mineralseducationcoalition.org/minerals-database/perlite/
It is sometimes used as a medium for hydroponic systems.
Perlite is pale white in appearance, and when you look closely you can see small pockets, cracks and pores in the surface of each piece.
What Is Vermiculite?
Vermiculite is a mineral that forms naturally due to the weathering or alteration of biolite or phlogopite.
This mineral is heated by processing in huge industrial tube furnaces at 1,000°C for a few minutes.3Vermiculite – An Overview. (n.d.). Science Direct. https://www.sciencedirect.com/science/article/pii/B9780444519979500040?via%3Dihub
This causes the structure to expand into long, worm-like strands, an expansion process called exfoliation.
Now, this group of minerals are also commonly used as an amendment for soil or potting mixes.
Vermiculite is a pale brownish or beige colour and if you look closely, you will be able to see that it has thin layers throughout.
Should You Choose Perlite Or Vermiculite?
Both perlite and vermiculite can improve drainage in a growing medium or soil and both can improve aeration.
However, there are differences between these two materials and where they are used:
Best For Drainage
Perlite has a fissured and cratered surface which does hold some water on the outside, but it holds on to little water, mainly allowing it to slide off.
Perlite is therefore very good for drainage, and not particularly beneficial for water retention.
This means it is good for plants like cacti and succulents which require free-draining conditions.
Best For Moisture Retention
Vermiculite, on the other hand, while it can also aid with drainage to a degree, will hold and store water within its layers.
It works a little like a sponge and will absorb water and expand in size.
It will then slowly release the water it has absorbed over time.
Because it can retain moisture well, without causing waterlogging issues, it is often used in seed starting growing media, as well as growing media for cuttings which can be vulnerable to drops in humidity and soil moisture.
Best For Aeration
Perlite has pores inside which are filled with air – in fact, a large percentage of the material is made up of these air pockets.
So this is one of the best materials to improve aeration.
While vermiculite can improve aeration to a degree by breaking up the soil and allowing air to flow through, it is not as good at providing aeration as perlite.
Best For Nutrient Provision
Vermiculite may not be as effective for aeration, but it is better in one other way.
Perlite has no cation exchange capacity.4Horticultural Growing Mediums and Soil Amendments. (n.d.). Retrieved March 22, 2023, from https://www.ndsu.edu/pubweb/chiwonlee/plsc211/student%20papers/articles09/alex%20hill/ahill%202/ahill/ahill.html
In other words, it does not interact with the soil or potting mix to provide nutrients for plants.
Vermiculite, on the other hand, contains nutrients like potassium, magnesium and calcium which it can give to plants.5Vermeulen, J. (2021, March 30). Cation Exchange Capacity in Vermiculite. Dicalite Management Group. Retrieved March 22, 2023, from https://www.dicalite.com/2021/03/cation-exchange-capacity-vermiculite/
Why Neither Is A Sustainable Choice
However, while, as you can see above, both perlite and vermiculite have benefits as soil amendments, neither is a sustainable choice.
- Both perlite and vermiculite are finite resources, not renewable ones.
- Mining these materials is an extractive industry which can do a lot of environmental harm.6Petrovic, K. (2016, November 27). How The Sustainability Movement Impacts Growing Media. Greenhouse Grower. Retrieved March 22, 2023, from https://www.greenhousegrower.com/production/media/how-the-sustainability-movement-impacts-growing-media/
- Processing the materials involves intense heating, usually using fossil fuels. So both materials come at a high carbon cost.
- There is also a carbon cost associated with transporting these materials from extraction points around the world to the UK.
So it is important to consider alternative options if you prefer to avoid harmful materials and garden in a sustainable way.
Alternative Options
If you are trying to garden in a more eco-friendly and sustainable way, then it is a good idea to avoid using materials that come at too great a cost.
Neither perlite or vermiculite is essential for soil or potting mixes.
Creating healthy soil or healthy potting mixes using plenty of organic matter – ideally from your own garden or nearby, is the best way to grow plants sustainability wherever you live.
Drainage
Instead of using perlite, consider using recycled horticultural grit or horticultural sand to improve drainage.
“When I need to increase drainage, I use a horticultural grit that is derived from the waste of local quarrying and rock processing,” says Master Horticulturist Colin Skelly.
“There is still energy used and carbon dioxide emitted in its transport and probably in its grading.
“Increasingly, I’m opting for fine bark products instead, which I’ve been impressed with. Again, there’s transport involved but the product is far lighter.
“There are also possibilities for creating it myself through composting and grading wood chips.”
These do not have precisely the same properties but will work for many applications.
Moisture Retention
Rather than using vermiculite to improve moisture retention, use coconut coir, or, better yet, locally available organic materials like bark, leaf mould and good quality composts.
Aeration
Adding organic matter can improve aeration in heavy soils and potting media.
Outdoors in the garden, adopting a no-dig approach and letting earthworms and other soil biota do their work will also help.
Rice husks are agricultural byproducts that can also be used to improve aeration in containers.
Nutrient Provision
Organic matter will slowly break down and add nutrients to the soil, in addition to keeping soil or potting mixes optimal in other ways.
So for a sustainable, organic garden, if you wish to create the perfect growing conditions, incorporating more organic matter is always always the answer.
