When we build a terrarium, the soil is the foundation that decides whether our miniature ecosystem thrives or struggles. The best soil for a terrarium is a well-draining mix that balances moisture retention, aeration, and nutrients, often made from materials like peat moss or coconut coir, perlite, sand, and a touch of charcoal. This balance keeps ferns, fittonia, mosses, and other tropical plants healthy inside a glass container.
We can’t just scoop garden soil into a terrarium and hope for the best. Closed terrariums especially need soil that avoids waterlogging while still holding enough moisture for tropical plants and mosses. By choosing the right blend, we create a stable environment where roots stay and plants like air plants or ferns can flourish.
Whether we prefer ready-made mixes or DIY recipes, the key is understanding what each ingredient contributes. Peat moss or coir holds water, perlite prevents compaction, sand improves drainage, and activated charcoal filters and purifies the air. With the right mix, our terrarium becomes a lasting, self-sustaining display of greenery.
The most effective soils balance moisture retention, drainage, and nutrient availability. Each option has unique properties that suit different plant types, from tropical ferns to desert succulents, and choosing the right one helps us avoid issues like root rot, compaction, or nutrient deficiencies.
Coco coir is a fibrous material made from coconut husks. It has become a popular alternative to peat moss because it is renewable and widely available. We often use it as a base in terrarium mixes because it holds water without becoming soggy.
One of its biggest advantages is excellent aeration. Roots can spread easily through the loose fibres, which reduces the risk of compaction. Unlike some organic materials, coco coir also resists pests and mould, making it stable in humid terrarium conditions.
Coco coir has a neutral pH, so it pairs well with a wide range of plants, especially tropical species like ferns, mosses, and peace lilies. It is lightweight, easy to handle, and can be rehydrated from a compressed brick form, which also makes storage simple.
We usually combine coco coir with perlite or pumice for added drainage. This way, we create a soil that holds enough moisture for tropical plants but still prevents stagnant water around roots.
Peat moss is decomposed sphagnum moss harvested from peat bogs. It has a naturally acidic pH, which makes it suitable for acid-loving plants such as carnivorous species or certain ferns.
Its main benefit is high water retention. It can hold several times its weight in water, keeping terrarium environments consistently moist. This makes it useful in closed terrariums where humidity levels stay high.
However, peat moss comes with environmental concerns. It is a non-renewable resource, and harvesting it contributes to habitat loss and carbon release. Because of this, many of us prefer coco coir as a more sustainable substitute.
On its own, peat moss can compact and limit airflow. To prevent this, we usually mix it with perlite or sand to improve drainage and aeration. Used carefully, it remains a reliable choice for moisture-loving terrarium plants.
Houseplant soil, or standard potting soil, is the most accessible option. It is sold in garden centres everywhere and often contains a balanced blend of organic matter, perlite, and sometimes slow-release fertilisers.
For terrariums, we need to be cautious with fertilisers. Many mixes include added nutrients that may be too strong for small, enclosed environments. Excess fertiliser can encourage algae or mould growth inside glass containers.
The texture of houseplant soil is usually good for tropical plants, but it may hold too much water for succulents or cacti. If we use it, we often adjust the mix by adding extra drainage materials like sand, gravel, or pumice.
Its convenience makes it appealing, especially for beginners. But to get the best results, we should always check the label and adapt the soil to the specific plants we’re growing.
Succulents and cacti need a soil that drains quickly. A mix with sand or gravel prevents water from sitting around the roots, which is the main cause of rot in arid-loving plants.
Cactus and succulent soil blends usually combine potting soil with coarse sand, perlite, or small gravel. This creates a gritty texture that mimics desert conditions. Water flows through easily, and the soil dries out faster between waterings.
We find this type of soil works best in open terrariums. Closed terrariums trap too much humidity for succulents, but open containers allow airflow and reduce excess moisture.
When preparing this mix ourselves, we often use a ratio of 2 parts houseplant soil, 1 part coarse sand, and 1 part gravel or perlite. We can also use a ready-made mix designed for succulents and cacti.
This approach makes succulent soil ideal for hardy species like echeveria, haworthia, or small cacti, which thrive in drier, well-drained environments.
As a general rule: coco coir is the most broadly useful base for closed and tropical terrariums; cactus mix is the default for open succulent terrariums. Peat moss should be reserved for specialist applications where its acidity is specifically required.
When we build a closed terrarium, the soil has to balance drainage, moisture, pH, and nutrients. Each factor directly affects plant health, root stability, and the long-term sustainability of the miniature ecosystem.
In a closed terrarium, too much water around the roots quickly leads to rot. Since the container has no natural outlet, we need a soil structure that allows excess water to move away from roots and settle into a drainage layer.
Adding sand, perlite, or pumice to potting soil improves aeration and prevents compaction. These materials create small air pockets that roots need to breathe.
A simple mix could be:
This combination ensures the soil drains well without becoming too dry. For succulents, we may increase the sand content, while for tropical plants, a lighter addition of perlite is usually enough.
Closed terrariums rely on their own water cycle, so the soil must hold enough moisture to keep humidity stable. Without this, plants dry out or struggle to maintain healthy growth.
Materials like coco coir and sphagnum moss are excellent for moisture retention. They absorb water and release it slowly back into the soil, keeping conditions consistent.
We often layer a thin amount of sphagnum moss above the drainage layer to act as a filter and water reservoir. Coco coir, when mixed into soil, prevents it from drying out too quickly.
This balance is especially important for ferns, mosses, and tropical plants that thrive in humid conditions. By combining both retaining and draining materials, we create a soil that stays damp but never soggy.
Most plants in closed terrariums prefer soil that is close to neutral, around pH 6 to 7. If the soil is too acidic or too alkaline, nutrients can become locked away, making them unavailable to roots.
Maintaining a stable pH ensures consistent nutrient uptake, healthier roots, and fewer problems with plant stress inside the closed terrarium.
Plants in closed terrariums depend entirely on the soil for long-term nutrition. A nutrient-rich mix supports growth, leaf colour, and the activity of beneficial microfauna that help recycle organic matter.
We can enrich the soil with:
Unlike outdoor soil, a closed terrarium doesn’t get replenished naturally. By starting with a mix that contains organic matter, we give plants a steady source of nutrients without needing frequent fertilisation.
It’s important not to overdo it, as too much compost can encourage mould. A small proportion, about 10–15% of the total mix, is usually enough to keep the ecosystem balanced.
When we build a terrarium soil mix, we want ingredients that balance drainage, aeration, and nutrient retention. Each material plays a specific role, from holding moisture to filtering toxins, and the right combination creates a stable environment where plants thrive.
Charcoal, specifically horticultural or activated charcoal, acts as a natural filter in terrarium soil. Its porous structure traps impurities, toxins, and odours that can build up in closed environments.
We prefer mixing fine charcoal directly into the substrate rather than creating a separate layer. This way, it distributes evenly and helps maintain freshness throughout the soil. It also improves drainage by adding texture and preventing compaction.
It’s important to note that barbecue charcoal is unsuitable because it contains additives. We always use horticultural-grade charcoal, which is safe for plants and long-lasting. Even a small amount makes a noticeable difference in keeping terrariums clean and balanced.
Coir, also called coconut fibre or coco peat, comes from the husk of coconuts. It’s a lightweight, renewable alternative to peat moss and is widely used in terrariums.
We like coir because it holds water well while still allowing air to move through the soil. This prevents roots from suffocating in dense, compacted material. It also breaks down slowly, so it remains stable for years.
Another advantage is its neutral pH, which makes it suitable for most terrarium plants. Unlike peat, coir is more sustainable and doesn’t contribute to habitat loss. We often use it as the base component of a mix, combining it with bark, perlite, or charcoal to improve structure and drainage.
Sphagnum moss is a long-fibre moss that’s highly absorbent. It can hold up to 20 times its weight in water, making it excellent for maintaining steady humidity inside a closed terrarium.
We use sphagnum moss both mixed into the soil and as a top dressing. When chopped and blended into the substrate, it creates moisture pockets and prevents compaction. On the surface, it helps regulate evaporation and gives a natural look.
It also has mild antifungal properties, which helps reduce mould growth in humid conditions. Unlike sphagnum peat (which is decomposed), this moss remains springy and airy. For tropical plants and ferns, it’s one of the most reliable ingredients to keep roots healthy.
Pumice is a lightweight volcanic rock filled with tiny air pockets. It’s similar to perlite but heavier, so it doesn’t float to the top of the soil when watered.
Adding pumice improves drainage and keeps the mix from compacting. The porous texture also holds small amounts of water and nutrients, slowly releasing them to plant roots. This makes it especially useful in closed terrariums where airflow is limited.
We often use pumice in combination with coir and bark to balance moisture retention with structure. It’s also a good substitute for perlite if we want a more natural look without white specks in the soil.
Lava rock, another volcanic material, is coarser and heavier than pumice. Its porous surface provides both aeration and moisture retention, making it versatile in terrarium soil.
We like to use small pieces of lava rock mixed into the substrate to prevent compaction. It also provides a stable base for roots to anchor. Unlike gravel, lava rock has internal pores that store water, so it doesn’t just improve drainage but also supports hydration.
As terrarium builders, we like to place a thin layer of lava rock at the bottom of a container. It’s durable and won’t break down, so it helps maintain long-term soil structure.
Earthworm castings are a natural fertiliser produced by earthworms. They are rich in nutrients like nitrogen, phosphorus, and potassium, which support steady plant growth.
We add small amounts of worm castings to terrarium soil to boost fertility without overwhelming delicate plants. They also improve soil texture by increasing its ability to hold moisture while still draining well.
Another benefit is the presence of beneficial microbes, which can help break down organic matter and release nutrients slowly. Because terrariums don’t need heavy feeding, a little goes a long way. We usually mix in about half a part castings for every two parts coir.
Sand is a simple but effective way to improve drainage in terrarium soil. By adding coarse horticultural sand, we make the mix grittier and less likely to compact.
We use sand more often in open terrariums with succulents or cacti, where fast-draining soil is essential. In closed terrariums, it can still be useful in small amounts to balance moisture-heavy ingredients like moss or coir.
The key is to avoid fine beach or play sand, which compacts easily and suffocates roots. Instead, sharp horticultural sand or builder’s sand works best. Mixing sand with coir and pumice creates a stable, breathable base for arid plants.
Tree fern fibre is a fibrous material harvested from the trunks of certain ferns. It’s lightweight, airy, and highly resistant to compaction, making it excellent for terrarium use.
We like it because it provides a strong structure that lasts for years without breaking down quickly. The fibres create open spaces in the soil, improving aeration while still retaining enough moisture for tropical plants.
It’s often used in professional terrarium mixes such as the ABG (Atlanta Botanical Garden) mix. Combined with coir, bark, and moss, it creates a substrate that mimics a natural rainforest floor. Although harder to source than other ingredients, it’s one of the most durable components we can add.
Different plants and terrarium styles need soil mixes that balance drainage, aeration, nutrients, and water retention. By adjusting the proportions of natural materials, we can create reliable recipes that support healthy growth and keep the miniature ecosystem stable over time.
The classic terrarium substrate mix is simple, versatile, and works well for most tropical terrariums. It combines organic matter with structural components to provide a balance of drainage and water retention.
Ingredients:
This mix creates a lightweight yet supportive base. The coco coir and moss hold moisture, while the bark and sand improve aeration and prevent compaction. Charcoal helps absorb impurities and reduces odours, which is especially useful in closed terrariums.
We find this recipe particularly effective for ferns, mosses, and tropical houseplants. It’s easy to prepare, cost-effective, and doesn’t require specialist ingredients, making it a good starting point for beginners.
The ABG (Atlanta Botanical Garden) mix is widely regarded as a gold standard for bioactive terrariums. It was designed to mimic the structure of rainforest soil and support both plants and microfauna.
Ingredients:
This mix is ideal for setups with springtails, isopods, or other clean-up crews. The leaf litter layer provides food for microfauna while also helping to regulate humidity.
We like using ABG mix in dart frog vivariums and other bioactive environments because it holds moisture well without becoming waterlogged. However, sourcing sustainable peat moss can be challenging, so some people substitute it with coco coir for a more eco-friendly option.
The ultimate terrarium substrate mix builds on the classic recipe by adding nutrients for long-term plant health. It’s designed for closed terrariums where plants rely on the soil mix for sustained growth.
Ingredients:
The inclusion of earthworm castings provides slow-release nutrients, making this mix more self-sufficient than simpler recipes. The structure remains airy and well-draining, but the added organic matter ensures plants don’t run out of food over time.
We recommend this mix for terrariums intended to last several years without major maintenance. It supports a wide range of tropical plants, from fittonia to miniature philodendrons, while keeping the ecosystem balanced.
Succulents and cacti need a very different substrate to tropical plants. A desert plant mix must prioritise drainage and prevent roots from sitting in moisture.
Ingredients:
This mix is gritty, fast-draining, and low in organic matter. The sand and pumice provide weight and aeration, while perlite ensures excess water drains quickly. A small amount of potting soil adds just enough nutrients without holding too much moisture.
We use this mix in open terrariums where airflow is high. It’s perfect for echeveria, haworthia, or small cacti. Overwatering is still the main risk, so pairing this mix with a shallow container and minimal watering is key.
The terrarium putty mix is less about plant nutrition and more about structure. It’s used to shape landscapes, create vertical planting areas, or secure hardscape elements like rocks and wood.
Ingredients:
The clay provides adhesion, the sand adds strength, and the moss helps retain a little moisture while keeping the mix workable. When pressed into place, it hardens enough to hold form but remains porous enough for roots to penetrate.
We find this especially useful in designs with cliffs, slopes, or tiered planting areas. It doesn’t replace a soil mix but complements it by giving us creative control over the layout of the terrarium.
Start with the drainage layer. Whatever mix goes into the substrate, the foundation underneath determines whether the system stays balanced. A 2–3 cm layer of gravel or LECA at the base, followed by a thin layer of activated charcoal, ensures excess moisture has somewhere to go in a container without drainage holes. Without this, even the best-formulated substrate will eventually waterlog.
In our experience at Miyagi Bonsai, the most common terrarium soil mistake is using unmodified houseplant compost. Standard potting soil retains too much moisture, often contains fertiliser levels that encourage algae and mould in enclosed conditions, and compacts over time in ways that restrict root development. Starting with a purpose-built mix, or at minimum adding perlite and charcoal to a houseplant base, produces significantly better long-term results.
Miyagi Bonsai stocks a carefully chosen range of terrarium substrates, including coco coir, activated charcoal, ABG mix components, sphagnum moss, and complete terrarium soil kits, with next working day UK delivery available on most orders. The products and recommendations in this guide reflect what we stock, use and see working for our customers every day. We are rated 5 stars across over 100 Google reviews, and our team is on hand by email and phone if you have any questions about terrarium soil, substrate layering, or plant selection.
For closed terrariums, we need a well-draining but moisture-retentive mix. A common option is the ABG mix, which includes components like coco fibre or peat moss, orchid bark, charcoal, and sphagnum moss.
This type of blend prevents waterlogging while still holding enough humidity for tropical plants. It also resists compaction, which helps roots breathe in a high-humidity environment.
We can test drainage by lightly watering the soil and observing. If water pools on the surface or the soil feels soggy for more than a day, it’s too dense.
A good mix should feel slightly damp, like a wrung-out sponge, and allow excess water to filter through without leaving roots submerged.
For open terrariums, especially with succulents or cacti, regular potting soil holds too much water. These plants prefer fast-draining mixes.
We should add sand, perlite, or pumice to increase drainage. This prevents root rot and mimics the dry conditions these plants naturally thrive in.
A strong terrarium mix usually combines:
Each ingredient plays a role in keeping the soil light, clean, and balanced.
If the soil stays healthy and smells fresh, we may not need to replace it for years. In bioactive terrariums with springtails or isopods, the soil can last even longer.
We should consider replacing or refreshing the mix if we notice mould, sour odours, or compaction that limits root growth.
Yes, for succulent terrariums, we can use cactus or succulent potting mixes sold in garden centres. These blends are sandy and gritty, designed for maximum drainage.
Another option is to create our own by mixing regular potting soil with 50% sand, perlite, or pumice. This ensures the soil stays dry enough for succulents in an open terrarium.
The best soil for a closed terrarium is a moisture-retentive, well-draining mix that supports the internal water cycle without waterlogging the roots. The Classic Terrarium Substrate Mix, equal parts coco coir, orchid bark, sphagnum moss, sand, and activated charcoal, suits the widest range of closed terrarium plants, including ferns, mosses, and fittonia. For a more nutrient-rich and long-lasting option, the Ultimate Terrarium Substrate Mix adds earthworm castings to the same base, supporting steady plant growth without external feeding. Both mixes avoid the main pitfalls of using unmodified houseplant compost, excessive moisture retention, fertiliser-driven algae, and compaction, which cause most closed terrarium failures.
No, garden soil is not suitable for terrarium use. It is too dense and heavy for container environments, compacts readily in enclosed conditions, and typically carries weed seeds, pests, and pathogens that will create problems in the humid conditions of a closed terrarium. Garden soil also has unpredictable pH and nutrient levels that are difficult to manage in a small enclosed space. Always use a sterile, purpose-formulated substrate or a DIY mix made from clean, known ingredients. If cost is a concern, a basic mix of houseplant compost amended with perlite and activated charcoal is a significantly better starting point than garden soil and is widely available at low cost.
Activated charcoal acts as a natural filter within terrarium soil. Its highly porous structure traps toxins, impurities, and odour-causing compounds that build up in enclosed environments over time, particularly in closed terrariums where waste products cannot escape. It also inhibits the growth of harmful bacteria and moulds, improving long-term substrate health. Additionally, it improves drainage and aeration by adding texture and preventing the soil from compacting. For best results, mix fine horticultural charcoal throughout the substrate rather than using it as a distinct layer, this distributes its filtering effect evenly. Always use horticultural or activated charcoal, not barbecue charcoal, which contains additives that are harmful to plants.
Closed terrariums genuinely require a specialised substrate, standard garden or houseplant soil is not suitable for enclosed environments. The specific requirements of a closed terrarium (no drainage holes, recycled moisture, sustained humidity) mean the soil must balance moisture retention and drainage more precisely than an open pot. DIY mixes using coco coir, perlite, activated charcoal, and sphagnum moss are specifically designed to meet these conditions. Open terrariums are more forgiving, amended houseplant compost or a cactus mix (for succulents) can work well with careful watering. As a general principle, the more enclosed the terrarium, the more important it is to use a purpose-built substrate rather than a general-purpose potting mix.
