Firstly before we start I just want to point what NOT to use. Compost, top soil, or naturally degraded soils like seasoil should be avoided. Although they’re high in nutrition, they have a tendency to create anaerobic environments because of their tendency to compact and how many organisms can be present in the mix.
I recommend making a terrarium mix based on these three principles.
The base. This will be what makes the majority of the mix. It should hold moisture well. Peat and coco coir usually the go to’s.
The chunks. This helps prevent the soil from compacting and can provide aeration and evaporation. You can use orchid bark, coco-chunks, or sand. Perlite and pumice works as well, but some people don’t like the look of it because of the stark white appearance. There’s nothing wrong with them though.
The cleanser. Charcoal, activated carbon, or biochar are used to help keep smells down and cleanse the soil from impurities especially from tap water. They also add small amounts off nutrition into the mix as well.
By using these 3 ingredients you can help the plants grow exceptionally well and keep them healthy for a long time.
I would recommend using a 3-3-1 ratio of these ingredients. This might look like 3 cups of peat moss, 3 cups of orchid bark, and 1 cup of biochar.
Now for the plants. Some of the most common terrarium plants include ferns, peperomias, and various epiphytes like orchids and tillandsia because of the high humidity. But it’s worth experimenting and trying out different plats. You can also often use various propagations of other plants as well.
When I add plants into a terrarium I like to completely remove the soil leaving behind bare roots. This makes it easier to add into the set up, but also prevents pests from potentially getting in.
As the plants grow you may need to prune them to keep them tidy inside or remove dead leaves. Water only when the standing water has cleared up in the false bottom. It’s easy to see with transparent glass.
Some things to keep in mind while maintaining a terrarium is fungus outbreaks. Because of the higher humidity mycelium or “white fuzzy mold” can appear. Although it’s completely harmless to plants it can be unsightly and there 2 methods to controlling them. 1 is water less. Let the terrarium dry out a bit before watering it next. 2 is springtails. They’re extremely small insects that feast on molds on other fungi and will help keep a terrarium clean for a long time.