Perlite or Vermiculite – which is best?

Perlite or vermiculite – which is best? There you are, standing in the garden centre or gazing at an online shop wondering which you need, which one is best for the gardening tasks you need to do.

And indeed, is there something else you could use that would be better? Or even, do you need either of them at all?

The focus of this blog is obviously on perlite and vermiculite, but we will consider other options too. Firstly, though, we had better have a closer look these two materials, discover what they’re made of, their attributes and why one is often better than the other for the gardening task in hand.

Perlite or Vermiculite – What is the Difference?

Perlite and vermiculite are both what can be termed soil additives. Or, just as valid a description, one of the ingredients in a bought or homemade potting compost. In order to understand the more critical differences and similarities, ie those which you need to know when gazing at the relevant bags in the store, this table may help.

	Perlite	Vermiculite

Colour	white	pale golden brown
pH	pH 6.6 – 7.5	pH 7
weight	Lighter weight than vermiculite
Water retention	some	yes
Improves drainage	yes	some – keeps soil moist
Soil aeration	yes	not really
Retains nutrients	no	yes
Use as covering for seeds	yes	yes
Use in seed & potting compost mix	yes	yes

And in more detail –

Perlite

Perlite begins life as volcanic glass; a particular type formed when obsidian contacts water. This creates a volcanic glass with a high water content. At this stage, we might not recognise it as perlite.
Obsidian itself is volcanic glass formed when lava extruded from a volcano cools rapidly. It is dark, translucent and has been used for tools, arrowheads and so on since Neolithic times.

Perlite then goes through a manufacturing process, turning it into what I always think of as gardeners’ popcorn. Although it’s inedible for humans, I hasten to add! What happens is that during the manufacturing process, the heat puffs up the perlite into little white balls, which always reminds me of popping corn at home.

Vermiculite

By contrast, vermiculite is a magnesium-aluminum-iron silicate which is mined and then processed. Like perlite, vermiculite is also processed with high heat which makes it puff up, although it has a flaky texture rather than the powder-like perlite. It is available to gardeners in four grades, or sizes, the smallest of which is the one we use with seeds and seedlings.

Perlite or Vermiculite- which is best?

The answer to that question depends on what you would like this soil additive to do.
Both are excellent for adding to custom potting compost mixes. If you plan to add them to a bought compost, check it first, as many include perlite already.

Sowing seeds –
Both are good, so its personal choice. They can be used both in the soil mix and also as a top covering instead of soil for seeds which need more light to aid germination.

Transplanting seedlings, rooting cuttings –
Using either is fine. I like to keep the soil mix free draining, but too much perlite can mean these young plants get too dry too quickly. Don’t be afraid to repot carefully if this happens.

Houseplants –
For plants which need their soil to dry out between waterings, such as cacti and succulents, then perlite is better.
If the plants need more moisture, then use vermiculite.

Perlite or Vermiculite or Both? Garden Teacher Tip

Although it may seem tempting to add both perlite and vermiculite to a custom potting compost, its not the best idea. If you’re after a mix which is both free draining and yet still retains water long enough for the young plants to use it, using a good proportion of well-rotted compost is better.

Are Perlite and Vermiculite okay to use in Organic Gardens?

Basically, yes, as they are both organic as in naturally occurring, rather than synthetic materials. However, chemically speaking they are classed as inorganic.

The rule for domestic gardeners according to Garden Organic, is that they are acceptable to use but not on a regular basis. (As is horticultural grit also pictured) They are on the permitted substances list from the Soil Association, which certifies organic farms and commercial operations.

There may some queries over whether the production processes are always fully organic. If this really bother you then check out different companies. Some brands have the organic symbol.

Do perlite and vermiculite decompose over time?

Not really, certainly not in the short term. However, unlike plastic carrier bags, they don’t cause problems to the environment or to animal and plant life. This would seem to suggest that their use is beneficial. And arguably improve a heavy garden soil long after the gardener has moved on to pastures new.

However, the environmental, or sustainability, question is more about the energy used in processing. And whether it is good practice to dig up minerals which do not replenish in the way that other soil additives such as corn husks or coir do. A tricky one, and one which I’ll leave with you to research more if you’re so inclined. There’s not the time to get into that discussion here.

and finally

Hopefully I’ve helped you understand the difference between perlite and vermiculite; and where are the best places and tasks to use each one in your garden. If the blog (and those below) have left you hungry to find out more about gardening, you may like to consider a Plews Gardening Course. This is where we enjoy 1-2-1 practical sessions in your own garden. A mix of practical and theory (the “why we do things”) bespoke to you, your garden and what you want to learn. And we can transplant some of the sessions into 1-2-1 zoom calls.

For further gardening advice and inspiration, check out Plews Potting Shed blogs, including the selection below, and our monthly Tipsheet – You could come and find us on Instagram Pinterest and Facebook too.