One of the key stages in making a terrarium is choosing the right soil. Terrarium soil will differ depending on the plants that you have chosen to grow and the type of terrarium that you have decided to build.
What is Terrarium Soil?
When people talk about terrarium soil, it is important to understand that this ‘soil’ is not exactly the same thing as the soil you will find outside, in a natural environment or in a garden. Rather than being referred to as ‘soil’, this is more properly referred to as a soilless growing medium, or soilless potting mix.
This growing medium can be made up of a number of different organic materials in different proportions. Specific mixes can be produced to suit the differing needs of different terrarium plants. The specific mix you choose will of course depend on which plants you want to grow.
The idea is to mimic a natural environment without introducing the elements of a natural environment that could cause issues within the terrarium setting.
The mix of ingredients within the soil in a terrarium will depend on a range of factors including whether you are creating an open or a closed terrarium and on which plants you are growing.
To understand terrarium soil or growing medium, you need to understand the difference between an inert growing medium, and a bioactive soil or substrate for a terrarium.
The former is a sterile material without all the varied life than usually teems in healthy soil. This type of terrarium substrate is common for displays that are for ornamental use, that will change little over time. But will be less resilient as it will not have the biodiversity or number of beneficial interactions as a bioactive terrarium substrate will have.
A bioactive soil is one that contains at least some of the micro-organisms and other life found in a healthy soil. The soil in a terrarium will not be as rich in life as a soil in a garden or natural environment outside, but it can contain life. The benefit of a bioactive substrate is that it will be more resilient over time. But the changes can be greater and the results more unpredictable, and problems can creep in.
Terrarium Growing Medium– Basic Requirements
Choosing or creating a growing medium for terraria is a little different to choosing a growing medium for other containers for houseplants. There are extra considerations, especially for closed terraria, that you need to bear in mind.
As always, when choosing or making a growing medium for plants you are growing indoors, you need to think about:
- Moisture retention/ drainage.
- Aeration, structure, and the avoidance of compaction.
- Fertility/ nutrient content.
- PH level.
When growing in a terrarium you also need to think about the avoidance of fungal pathogens and other contaminants that may be present in soils.
You need to choose a sterile, soilless mix that will not introduce these problems into a closed terrarium system. So you cannot just import soil from a garden setting to a terrarium. You need to purchase or create a special mix.
And you must of course also consider the fact that terraria typically do not allow water to drain away as it can in other containers. So should choose a substrate that can keep roots moist but not waterlogged over time in a closed terrarium, or drier yet still with sufficient moisture in an open one.
Components of the Growing Medium for a Terrarium
There are a number of different ingredients that might be added to growing medium mixes for a terrarium:
- Regular Peat-free potting mix.
- Coconut coir.
- Sphagnum moss.
- Orchid bark.
- Worm castings.
- Activated charcoal.
The precise mix of ingredients, however, can vary significantly depending on the precise details of the terrarium created and on the plants that will be grown within the mix.
Regular Peat-free potting mix
For open terrariums and in some other terraria, you can incorporate a typical peat-free potting mix into your growing medium or substrate. This will sometimes be mixed with one or more of the other ingredients listed below.
It is best for environmental and ethical reasons to choose a peat-free option, and always to avoid the use of peat in all houseplant cultivation and other horticultural pursuits.
Sometimes used as the primary structure ingredient for the substrate in a terrarium, coconut husk or coconut coir is also sometimes an ingredient in the above. This retains water and releases it slowly over time, but contains no fertility for plants so will need to be added to to provide the nutrients that plants need.
Sphagnum moss is not always a sustainable choice. But it is often used to increase moisture storage capacity in the substrate. It is also often used as a kind of mulch to retain moisture when used in its live form to cover the soil surface.
Orchid bark helps to improve soil structure and balance the moisture and drainage of the substrate used. It is a common ingredient in many houseplant mixes, including those used in terraria.
This helps to aerate the soil and prevent compaction, though as a mined material does come at an environmental cost.
Another material whose sustainability leaves something to be desired, vermiculite helps to increase water retention in the substrate.
Like perlite, pumice is another mined material that can help improve aeration and reduce compaction problems.
Worm castings are a useful form or organic matter that can help to give plants in terraria the nutrients they need. They can also improve the structure and water storing capacity of the terrarium soil.
This is often recommended for terraria because it reduces odor and helps to reduce the incidence of fungal issues.
Sand and small pebbles and the like are often introduced below the growing medium in a terrarium. But these are not really required, and can in fact increase the chances of problems like root rot taking hold. They can however if you wish be layered in for aesthetic reasons.
Making Your Own Terrarium Soil Mix
In creating your own terrarium soil mix, you should first of all think very carefully about whether you wish to create a more stable but less resilient inert substrate, or a bioactive substrate.
If you want to create a bioactive substrate, the easiest way to do so is simply to add a small quantity of soil from a garden or a natural setting close to home. This will introduce microorganisms and other beneficial soil life to the mix. Though it can also introduce potential pathogens too so can be a risk.
The easier option is sometimes to choose a soilless mix. In this case, your goal is to select substrate materials from those listed above, that will create conditions suitable for the type of terrarium you have created and for the plants that you have chosen to grow.