Anyone new to gardening, terrarium building, or growing plants in general, would probably be surprised by the number of options they have when it comes to choosing their soil. It’s just dirt, right? That’s what I thought at least.
Turns out, it’s not that simple.
The most common options for terrarium soil include potting mix, coir, and moss. For best results, use your own custom soil mix, but off-the-shelf potting mix usually works just fine for most terrariums. Healthy soil provides sufficient drainage, water retention, and enough nutrients for your plants.
So now that you’ve been introduced to a few different soil options, let’s take a closer look at what each one of those is, why they work, and how to figure out what soil will work best for your terrarium.
Table of Contents
Recommended Terrarium Soils
Let’s start out by going over some of the basic options there are for terrarium soil. And by soil, I mean something you can use as your terrarium substrate as-is and expect good results.
Potting mix
If you’re looking for just one thing you can grab from your local plant nursery or gardening store and use as your terrarium soil, pre-made, commercial potting mix is a great option. It’s a solid choice if you want a single, well-rounded option that “just works” for most plants without needing to do too much research.
Usually, potting mix consists of peat, maybe some shredded bark, and aeration components like vermiculite or perlite.
The combination of these components give potting mix aeration, drainage, and moisture retention, all things you are going to need for your terrarium soil.
The only downside of using potting mix is you don’t have total control over what exactly is “mixed” in the potting mix.
So there can be some variation in the results you’ll get depending on exactly which potting mix you choose. You can mitigate this by checking the back of the bag to see what ingredients/soil amendments are actually included in your potting mix.
As long as you’re getting good drainage, no compacting, and decent moisture retention, you should be fine. If you’re not exactly sure how to tell based on the ingredients, I’ll cover some common soil amendments in the rest of this article so that you’ll have an idea of what to look for.
Note: don’t confuse potting mix with potting soil, which compacts easily and is not recommended for your terrarium if used alone. Potting mix often comes with aeration components/soil amendments that help prevent compaction that you might see with potting soil.
Natural soil (for natural plants)
Natural soil you’ve found yourself is another good choice but only if you are growing plants that you’ve harvested from the wild yourself from the area you’re gathering your natural soil.
Here’s a photo of one of my older terrariums that uses natural soil:
The idea behind using natural soil is simple: you want to replicate the environment from which you’ve found your plant as closely as you can. After all, if you can see plants growing from this soil, how bad can it be?
I don’t recommend using natural soil for plants you’ve purchased or obtained otherwise. The risk of using natural soil is that it’s probably not sterile.
It can introduce unwanted fungi and pests to your terrarium. Your plant also could have adapted to the soil it came with and may not respond well to the new soil when repotted/transplanted.
One way to mitigate the risk of using natural soil is to sterilize it yourself by baking the soil at 200°F (93°C) for at least 30 minutes.
On the other hand, if you’re just running a fun little experiment, you could just add natural soil to your terrarium as is and see what life grows from it.
DIY Mixes
If you want to optimize the soil composition for your terrarium, creating your own soil mix is the way to go. It’s the only way to know for sure what exactly is going into your terrarium soil.
It also makes it easier to troubleshoot, experiment, and make improvements to your terrarium soil. As you get more experience using different mixes for your terrariums, you will get a better idea of what works best for you.
When creating your own terrarium soil mix, you will want to keep 3 objectives in mind:
- Aeration/drainage
- Water retention
- Nutrition
Having all three of those characteristics will ensure your plants will have plenty of oxygen circulating their roots, adequate water levels in the soil, and nutrients to support growth and development. Depending on what types of plants you have in your terrarium, you can vary how much of each characteristic you have in your mix.
Without getting into too many details about which ingredients you should use and why, here’s a few recipes you can use for your soil mix:
| base | aeration/drainage | nutrition/other |
|---|---|---|
| Potting soil (2 parts) + sphagnum moss (2 parts) + sand (1 part) | Orchid bark (1 part) + perlite (1 part) | Comes from potting soil |
| Potting soil (1 part) + sphagnum moss (2 parts) | Orchid bark (1 part) + sand (1/2 parts) | Worm castings (1/4 parts) |
| Coco coir (2 parts) + sphagnum moss (1 part) | Sand (1 part) + Orchid bark (1 part) | Aqua soil (1/2 part) + activated charcoal (1/2 part) |
Don’t get too caught up in the exact proportions and measurements. As long as it’s roughly correct, you should be good to go.
You can always add more or less of each component based on your (or your plant’s) preference. You can even mix and match different components depending on what you have available.
For the most part, the majority of the soil mix should come from the base. Most recipes I’ve seen devote 50-70% of the mix to soil base. It’s essentially the meat of your soil mix. This is what gives your soil water retention, volume, and a little bit of nutrients depending on the base.
Aeration components take up the next largest piece of the composition. Most recipes devote somewhere around 20-30% of the mix to aeration components. These serve as a counter-balance to the base by preventing soil compaction/waterlogging and creating small pockets of air for your plant roots to receive oxygen. They also add a little bit of texture and character to your soil.
The remainder of the composition goes to nutrition. Terrarium plants don’t need a significant supply of nutrients because you generally don’t want them to grow too large such that they outgrow the size of their container. However, they will need at least a little bit to sustain themselves. Although it only takes up a small portion of the mix and could be considered optional, nutrition components will be critical if your base is entirely devoid of any nutrients.
From looking at the example recipes, you might be wondering, why multiple ingredients for each of the base/aeration/nutrition? The idea behind that is diversification. By adding multiple different components, the mix you end up with is essentially an average of all of them. It helps to mitigate your risk of having too much/not enough of some soil characteristic by not putting all your eggs in one basket.
Since we’ve been talking about DIY mixes, let’s get into a little more detail about the ingredients that actually go into these mixes.
Recommended Terrarium Soil Components for Custom Mixes
Peat Moss
Peat moss is the classic choice for soil amendment. It’s essentially a dead, fibrous material made from dead moss and other organic material found at the bottom of bogs that spent centuries decomposing in an anaerobic environment.
Adding peat moss to your soil will increase water retention while also reducing soil compaction.
You would usually want to mix this with some nutritional components because peat moss alone doesn’t have much nutritional value.
Peat moss is slightly acidic with a pH between 3.0-4.0, so it’s a good way to reduce the overall acidity of your soil if mixed properly. That’s good news if you’re growing plants that like a little acidity like succulents which prefer pH levels between 5.5 and 7.0.
One thing you might want to note: peat moss arguably might not be the most environmentally friendly soil amendment choice out there. It takes centuries for all those dead plants and insects to decompose before it’s mined from the bottom of bogs and wetlands. While it’s great for soil use, you could classify it as a non-renewable resource.
On the upside, peat moss is probably the cheapest choice compared to coir and sphagnum moss, which I’ll cover below.
Coco Coir/Coco Peat
Coco coir (also called coco peat) is basically shredded coconut fibers. Sometimes it’s also included in potting mix, but if you decide to buy pure coco coir, it usually comes in the shape of a brick.
One important note about coconut coir is that it does not have any nutrients. I would describe it as more of a soil conditioner, something that you would add to soil to change or improve its physical qualities.
Coir is a great addition to increase soil’s water retention. With a pH of 5.5-6.5, it can also be added to slightly increase the acidity of your soil.
So it’s definitely not something you would use as the sole ingredient for your soil. If you got yourself a block of pure coir, make sure to incorporate some nutritional additives like worm castings.
Compared to peat moss, coco coir does not hold onto water as long. On the upside, the pH level is not as acidic as peat moss. That means you probably won’t have to second guess your pH levels as much as you would with peat moss because most plants will be fine with this pH range.
Some are a proponent of coir for its environmental friendliness in comparison to peat moss. While peat moss is harvested from wetlands rates that most argue are unsustainable, coco coir is essentially a waste byproduct of coconut harvests.
The only piece about coco coir sustainability that might raise some eyebrows is that harvesting the husk uses a lot of water, which is a limited resource in India and Sri Lanka where the majority of coco coir is harvested.
Sphagnum moss
Sphagnum moss is a light, stringy, fibrous material that is great at retaining water (without getting soggy).
Like peat moss and coir, sphagnum moss is a soil amendment, something you would add to soil to improve its physical properties. It’s probably not something you would use alone because it doesn’t have any nutrients.
Like coir, but unlike peat moss, sphagnum moss is environmentally friendly. It grows in the same regions as peat moss, but is harvested from the surface rather than mined from the bottom. It’s ready for harvest in less than 10 years unlike the centuries it takes to develop peat moss.
One property of sphagnum moss that differs from coir and peat moss is its neutral pH. So it’s a great choice for plants that prefer a more neutral soil pH level, like polka dot plants or creeping figs.
Sand
Sand is probably not the first thing that comes to mind when you hear the word “soil”. However, sand can be a good choice to add to your soil mix if you want to increase the aeration and drainage of your soil.
The physical composition of sand essentially boils down to tiny grains of rock and minerals sized about 0.05-0.2 milimeters in diameter.
As you might imagine, rocks don’t really absorb water that well. Sand particles have low surface area and low charge that inhibits water from sticking to each grain of sand.
What this means is that sand is not going to retain water as well as something like peat, whose composition is about 50% organic matter. But it does mean it’s a great component to boost the drainage and aeration qualities of your soil. With it’s neutral pH, it also shouldn’t impact the pH levels of your mix by much.
Sand also doesn’t provide many nutrients for your plants. So it’s unlikely you would want to use sand on its own as your soil. It’s best used in combination with other nutrient-providing and water-retaining ingredients to create well-draining, aerated, and compaction-resistant soil.
Perlite/Vermiculite
Perlite is made of tiny, porous chunks of crushed and heated volcanic glass that have a solid structure.
Since it’s made of natural ingredients, it’s completely non-toxic. Perlite has a neutral pH, which means it’s unlikely that adding perlite to your soil mix is going to throw off the pH levels.
Perlite is similar to sand in that its main purpose is to increase the drainage and aeration of your soil. Unlike sand, perlite is lighter due to its porous structure. It’s also a little better at holding onto water.
One drawback to using perlite is that it can cause fluoride burn in your plants if used in excess. Although, this will probably be mitigated as water cycles through the terrarium over time. It’s also a little bit dusty, which probably isn’t too good for your lungs.
Vermiculite is also a similar aeration component to perlite that can be used in place of perlite if you happen to have it on hand. The only main difference is vermiculite will hold onto moisture a little better than perlite.
If given the choice, I would prefer perlite over vermiculite because the idea of adding aeration components is to facilitate the drainage of water and provide oxygen to your plant roots. For this purpose, I wouldn’t really want something that holds onto water especially in the context of terrarium soil, where moisture should already be abundant.
Orchid Bark
Orchid bark is basically large chunks of bark sourced from a variety of coniferous trees. Each chunk usually ranges from ¼-½ inches in size (6-13 mm).
These large chunks create pockets of air in the soil which allow water to flow through your soil and give your plant roots access to oxygen.
Orchid bark is probably the most aesthetically pleasing aeration component you can add to your soil mix. It’s made of all-natural materials whose reddish, brownish colors blend in nicely with the rest of your soil.
The only downside of using orchid bark is that it will decompose over time, increasing the acidity of your soil. This process happens over the course of two years, which could be a problem if your plants prefer more neutral soil.
So if you want something that will last for decades, sand or perlite would probably be a better choice.
Aqua soil
Aqua soil is a clay-based, nutrient-rich substrate you would typically find used in aquariums, however it can also be used in your terrarium soil mix.
What distinguishes aqua soil from peat moss or coco coir is that it’s rich in nutrients and organic acids. Because of this, it can be used as a standalone substrate without needing to wonder if your plants are getting enough nutrients.
The only downside of using aqua soil is it’s going to be more pricey compared to peat moss or coco coir. If you want to cut down on costs, you can mix in aqua soil as a nutrient-providing component along with the cheaper soil-base materials, coco coir and peat moss.
Worm Castings
Worm castings (also known as vermicast) are basically worm poop. While it sounds icky, it can be really beneficial for your plants.
It’s an all-natural nutrient-providing component that you would mix in with base materials to increase the nutrient content of your mix. It’s even been shown to outperform commercial plant medium.
Worm castings have higher nutritional content and reduced contaminant levels than the organic matter the worms consumed to begin with. It also contains some worm mucus, which helps prevent nutrients from washing away from the soil.
It’s also rich in microbial life, which assists in providing nutrients for your plants by converting existing nutrients into plant-available forms.
What this also means is that the nutrients will be slow to release. Slow release nutrients can be a good thing in a closed terrarium, where you don’t want your plants to grow too quickly and outgrow your container.
Activated Charcoal
Activated charcoal (also called activated carbon) is a form of carbon that’s been exposed to temperatures above 800°C in an inert atmosphere.
The idea behind using activated charcoal is that it can purify your terrarium by absorbing toxins and chemicals that accumulate over time, which could cut down on odors and potentially be healthier for your plants.
Often, you’ll see this recommended to use as a separate layer in a terrarium. But you can also just mix it in with your soil.
I actually am not a huge fan of adding activated charcoal to your terrarium because I’m not entirely convinced of it’s effectiveness.
While it does work for absorbing chemicals and toxins, it’s not entirely clear how beneficial that would be in the context of a terrarium. It’s also not going to last forever.
If you want to learn more, I have an article that gets deeper into the controversy behind activated charcoal in more detail that you can read here.
So what is the best soil for your terrarium?
To summarize, the best soil is one that you’ve custom mixed yourself for your specific plants growing in your terrarium.
A good mix would typically have a base material, aeration components, and nutrient-providing additives. The mix should be able to hold onto water, provide drainage and aeration, and provide at least some nutrients for your plants to sustain themselves.
If you don’t want to spend too much time or effort, you can certainly get by with commercial, off the shelf potting mixes.