How to Make a Terrarium Soil Mix

From years of experimenting and trial‑and‑error, I’ve learned that a balanced, thriving terrarium leans on two pillars: the soil mix and the tiny critters that keep it in check. This post focuses on the soil. A good terrarium mix should be well aerated, drain easily, and stay rich in nutrients for the long haul. I’ve been keeping terrariums since 2019, and these are the ingredients and proportions that work for me.

Why I Mix Instead of Layer

Terrarium soil mix is typically a combination of potting soil, charcoal, sphagnum moss, sand and sometimes bark. I know that some people put these ingredients in layers, but I prefer to mix everything together so the moss and charcoal are present throughout the entire substrate. Each ingredient plays a different role that helps balance the system.

Ingredients and Their Roles

• Potting soil: This is the base of the mix. It holds water and nutrients for plants, provides structure for root growth and acts as the biological base where microbes live. I use FoxFarm Ocean Forest potting soil. It’s pricier than what you find at big-box stores, but cheap soil is lower in nutrients, so it's less optimal for the long haul, and it often harbors fungus gnat eggs. Once those hatch, your terrarium will be full of tiny flies, and serenity goes out the window.

• Bark: Acts as a long‑term stabilizer and slow‑release fertilizer. It improves aeration by creating micro‑voids and prevents compaction. I don’t add extra bark anymore because FoxFarm soil already contains it, but if your soil is uniform, add some natural (untreated) bark to avoid a homogenous texture.

• Sand: A small amount keeps the soil light and well drained.

• Charcoal: An absolute must‑have. It absorbs toxins, keeps bacterial growth in check, maintains a stable pH and keeps water fresh in a closed system. I crush it into smaller pieces and mix it throughout the soil.

• Sphagnum moss: Use dead, dried sphagnum moss. It retains water without becoming soggy and keeps moisture in the soil rather than on the glass walls. Excess moisture on the glass encourages calcifying algae that eventually leave hard‑to‑remove stains. I cut the moss up and mix it into the soil instead of using it as a divider; a separate mesh divides the drainage layer from the soil. It also helps keeping the soil light.

• Springtails: Tiny bugs that feed on fungi and mold. I bought a colony back in 2019 after everything rotted in one of my terrariums and have been raising them on charcoal ever since. You can buy springtails online.

My Ready‑Made Mix (Proportions)

If you’d rather mix your own soil, these are the proportions I use:

• 1½ parts potting soil

• ½ part bark

• 1 part sand

• 1 part charcoal

• 2 parts sphagnum moss

Mix everything thoroughly so the ingredients are evenly distributed.

You can buy a ready-made terrariums soil mix on my website, that is made according the the above recipe