Terrarium soil and substrates are clearly the least exciting part of a terrarium build (it takes a committed botanist to get excited about dirt) but it’s still a critical part of your terrarium.
Get it wrong and you’re at risk of stunted plant growth, rot, bug infestations and more.
Sure, you could just grab a handful of soil from your backyard and hope for the best, but it’s a good idea to at least check what you’re putting in.
In this article, we’re going to cover all the basic questions about terrarium substrates, so you know exactly what you should be putting in your terrarium and where to find it.
Potential Terrarium Substrate Bases
Potting Soil / Potting Mix
Regular packaged potting soil is a universally available option.
It’s as basic as it comes, and though it’s made for outdoor use it should work for a variety of typical houseplant varieties. It’s real weakness is its poor drainage and tendency to compact too much.
We all know what happens to wet soil, it just turns to a thick mud.
Not ideal growing conditions for any plant, and tropical plants simply won’t tolerate it.
It’s worth noting that “potting mix” can mean a variety of things. It probably includes some of the other materials on this list (which isn’t necessarily a bad thing) but I’d definitely stay away from the likes of Miracle-Gro and other artificially fertilised mixes. Always check the label!
- It’s affordable and easily available.
- It comes full of nutrients for your plants.
- It’s sterilised, so you can be sure you’re not adding any bacteria or creepy crawlies to your mix that could pose a risk to your terrarium.
- Potting mix really doesn’t drain well so you terrarium runs a higher risk of rot.
- Not good for tropical plants which don’t tend to like soggy soil.
Aquarium soil is regular soil that has been baked to produce dry pellets.
It’s great because it has all the minerals and positive growth aspects of regular potting soil – but with much better drainage.
That’s because the pellets are round, so they don’t pack together in a uniform shape. They’re also pretty tough, so they’ll resist compaction for years.
Right now this is my go-to choice for substrate. It’s super convenient (no need to mix customs substrates) and very effective at growing plants in terrariums.
Grab a bag on Amazon right here and try it yourself!
Coir (Pronounced Kwoy·uh… Apparently)
Coir is a fibrous material made from coconut husks.
As you might imagine, it’s dry as hell when you get it in brick form, but once it’s hydrated it can retain a ridiculous amount of moisture.
It’s advantages for terrariums are due to it being a very stable and reliable material. You’re able to better control moisture and drainage versus using potting soil, and because it’s not found in nature (in this form) it’s not susceptible to pests.
However, it contains no nutrients whatsoever. So unless you’re mixing it with another substrate or adding fertiliser, your plants are going to have a hard time growing in it on its own.
It’s definitely worth a consideration as part of a substrate blend. You can grab a block on Amazon here for pretty cheap.
- High water retention
- Neutral pH and resistant to decay
- Readily available in brick form.
- Natural, sustainable product.
- Pest resistant
- Contains no nutrients, so not for use in isolation.
- Prone to compact over time if used on its own.
If you’re making a native terrarium (i.e. using local ingredients) then soil taken from the local area is one option.
Without knowing it’s composition, it’s a bit of a gamble. Native terrariums bring in a lot of unknown factors and the soil may carry things like rot, disease or pests. But if the plants are thriving outside, in theory they should thrive in a similar enclosed environment.
SerpaDesign made a great video on how to source materials for native terrarium building if you’re interested in going this route.
Useful Terrarium Soil/Substrate Supplements
Perlite is a type of white volcanic glass, that kind of looks like styrofoam balls. It’s light, porous and has a variety of uses in terrariums.
If you’re building a tropical terrarium, perlite can be a useful addition to your soil mix.
Its unique structure allows it to hold on to water on it’s outer surface, without absorbing it internally – making it a great solution to provide consistent water to plants without creating a soggy mix. Grab a bag here.
- Perlite will not rot, degrade or break – making it a reliable long-term soil addition.
- Because it’s so porous, perlite is great at aerating soil and providing better drainage (perfect for tropical plants).
- White styrofoam ball lookalikes don’t look particularly natural in a terrarium. I don’t like the look of them at all.
- Due to being so light, I have heard of them “floating” towards the top of a growth medium. I doubt you’ll wake up one day to find them all on top of your plants, but it’s something to bear in mind.
Vermiculite is very similar to perlite in function, but has a different appearance and structure. It’s actually a mineral (aluminum-iron magnesium silicate) and looks like brown flaky rocks.
Here’s a relatively fine vermiculite that should be great for terrariums and other smaller pots.
- Unlike perlite, vermiculite actually retains and provides nutrients for growth.
- Looks more natural than perlite (in my humble opinion).
- Provides the highest possible water retention of terrarium soil additives, making it the best solution for water loving plants.
- Doesn’t provide as much aeration/drainage as perlite does.
Orchid bark is a more natural way to add granularity and aeration to a soil mix. It’s a chunky mix of bark shavings (named because it’s often used for orchids, it doesn’t come from them) that helps to provide structure and spaces in a soil medium.
Finding a mix that’s small enough for terrariums is the tricky part, it’s often super chunky.
Here’s a bag of highly rated orchid bark on Amazon that should fit the bill.
- All natural, sustainable material.
- Actually looks natural!
- Great for root/soil aeration.
- Doesn’t retain water as well as perlite or vermiculite.
- More susceptible to rot, seeing as it’s a natural product.
Sand can be an easy addition to your growth medium to increase its aeration and drainage. Coarser horticultural sand will be best, but any sand will do (barring sand from the beach).
Obviously, if you’re going for a desert terrarium, you’ll have a much higher proportion of sand. Same goes for any cactus or succulent mix.
Personally, I prefer to use black sand where possible. That way it’s less visible in the substrate and the darker substrate contrasts better with green plants.
If you’re running with pure coir as your soil base, you’re going to need to add some nutrients to your growth medium. Be wary of adding any homemade compost that may contain any rotting material, I’d definitely opt for something packaged where possible.
Earthworm castings are a great readily available solution.
A nice way of saying “worm poop”, earthworm castings are compost produced through the feeding actions of earthworms.
It’s all natural fertiliser, which is also well-draining and has a high water-retention capacity. Here’s a big, inexpensive bag.
Make Your Own DIY Terrarium Soil/Substrate Mix
Even if you use the same plants and materials as someone else, every terrarium is unique.
Where you live, where you place your terrarium, how you care for it – they all require a slightly different soil solution.
A custom soil mix is often the best approach as no one size fits all, but it can be a little daunting to figure out yourself.
My 4 Step Soil Solution
I see custom soil mixes as a gradient. You start off with a base then add differing amounts of soil supplements depending on your circumstances.
- Select a base material according to the type of plants you want to grow e.g. coir, potting soil (or sand if you’re making a desert terrarium).
- Add compost as necessary if using a base that doesn’t provide nutrients. About 10% of your overall mixture should be sufficient.
- Supplement with moisture retaining materials such as vermiculite or perlite depending on how water-loving your choice of plants are.
- Add additional aerating elements where necessary e.g. sand or orchid bark.
Now, Over to You
What’s your preferred terrarium soil mix?
I’d love to hear some specific examples of what mixes you use for different plants. After all, the more we know, the better we can look after our plants.
Share them in the comments below.