When you create a terrarium, you are in essence creating a miniature ecosystem, which mimics the earth. Layers of soil cover the terrarium's floor, each serving a specific purpose. While not everyone chooses to make terrariums in exactly the same way, there are some standard soil configurations used by successful terrarium enthusiasts.
The first layer in a terrarium's soil is an even layer of gravel or small rocks. The purpose of gravel is to assist in drainage. Instead of gravel, crushed terracotta pots, marble chips or other drainage type material is used. Before adding drainage material to the terrarium, sterilize the material to prevent adding unwanted bacteria to the ecosystem.
Activated charcoal or activated carbon absorbs harmful organic chemicals. One common use for activated charcoal is in water purification systems. About a ½ inch of activated charcoal goes over the drainage material in a terrarium. Some people mix the charcoal with the drainage material, or opt not to include the activated charcoal at all. The purpose of the charcoal is to eliminate unpleasant odors as well as eliminate toxic chemicals harmful to plants.
Another name for sphagnum moss is bog moss. When preparing the terrarium soil, a layer of sphagnum moss covers the activated charcoal layer. The purpose of the sphagnum moss is to separate the potting soil from the activated charcoal and gravel, preventing the potting soil from falling into the gravel or rocks, resulting in muddy soil and a potential for mold developing.
The primary layer of the terrarium's soil is the potting mix. The combination of all the soil, from the gravel to the potting mix, should take up no more than 3/4 of the terrarium's volume, yet the potting soil needs to be at least 1 ½ inches deep. While some people use regular houseplant potting soil, commercial African violet mix makes an excellent terrarium soil, according to an article by Cornell University Extension. Instead of buying a pre-made potting soil, make one by combining equal amounts of sterilized soil, vermiculite and peat moss. Vermiculite is a natural mineral that expands when heated, while peat moss is a dried decayed vegetation. Both are sold at gardening centers.