1. Base your plants and how many off the container you use
2. Generally, no fertilizer should be used with your soil medium
3. And provide proper drainage
Keeping these ideas in mind will keep your plants healthy throughout their lifetime.
Note: In most cases, fertilizer or compost should not be used unless you want your plants to outgrow and/or overshadow smaller plants in your terrarium. While fertilizer is great with individual potted plants, little ecosystems like terrariums balance themselves out on their own with minimal care.
Step 1: Gather your supplies
Any size container (preferably glass to be able to see your substrate)
House plants ( or native plants, but they can be picky)
You don't have to use activated charcoal unless you have a sealed container for your Terrarium. If you do, then it's a must. I am using it in this instructable for demonstration purposes. The charcoal helps keep the water clear of buildup of microorganisms that might grow in your substrate, such as algae and keeps the air clean for your plant to breathe. Besides, who wants a smelly plant in their house?
The type of plants you use can be almost anything, but it ALL depends on the container. If you have a plant that will outgrow your container size, or the environment is not one it can grow in (given time, a cactus will rot in a moist closed container), then you'll be kicking yourself later. Do research on what your using beforehand and your plants will be all the healthier for it.
In this instructable I used African Violet, Spike Moss, and dwarf Sword Fern.
For a list of suitable Terrarium plants, follow this link: www.thegardenhelper.com/terrarium~plants.html