Substrate

The choice of substrate will significantly effect the sustainability of your enclosure. On one hand, you want your substrate to be long lasting and hold its porosity. On the other hand, you want the substrate to support a thriving community of microfauna and to be conducive to plant growth. Whichever substrate you choose will also need a drainage layer. Before starting work on your drainage layer you need to first decide whether or not you will want a water feature and a water area.

Drainage Layer

A drainage layer is an inert layer along the bottom on the terrarium, generally separated from the soil with a layer of window screen or other fine mesh. It acts as a basin which allows water drain from the substrate without waterlogging. If water remains in the substrate, it can turn you whole substrate into a swamp, causing your plants roots to rot and killing off your microfauna. It is possible to set up a terrarium with no drainage layer, but it is not advised. Not only does the drainage layer prevent roots from becoming water-logged, but the water in the reservoir also provides humidity during periods of infrequent misting and may also help to stabilize temperatures. One disadvantage of a drainage layer is that is elinates some vertical space which might be better used to give the animals additional climbing room.

You will want to provide access to the drainage layer, in order to remove excess water if necessary. This can be accomplished by drilling your tank and installing a drainage valve (a somewhat involved process), by inserting a tube all the way to the bottom of the tank and thus allowing for siphoning at a later date, or creating an exposed pool, which you can siphon directly from.

The simplest way to set up a drainage layer is to use a layer of pebbles or LECA (light expanded clay aggregates). LECA is a form of expanded clay used in hydroponics, analogous to coco puffs, but made from clay instead of corn, and can generally be bought at a local pet store or online for much cheaper. It is superior to pebbles because it is lighter, allowing for a lighter tank overall (a bonus if you are a college student who will be moving every few years). Pebbles on the other hand are cheap (you can by a large bag for very cheap from a home and garden store) but weigh significantly more. Pour a layer of LECA into the bottom of your vivarium to create a layer about 1.5" thick. Insert a tube for siphoning if necessary. Next, cover the layer with window screen to prevent your soil from falling into the basin. You are now ready to proceed with adding your soil.

The False Bottom

The false bottom is an alternative drainage layer. A false bottom is platform raised up from the bottom of the tank, which the soil sits on top of. This creates an empty basin which can hold more water than one filled with pebbles and wieghs significantly less when drained. Additionally, it completely separates the soil from the water reservoir, preventing the wicking of water up into the substrate. To make a false bottom, you will need the following items: PVC pipe, egg crate (this is found in the lighting section of a home improvement store and sold as suspended ceiling light diffuser), zip ties, and window screen. The PVC pipe shoulf have an outer diameter equivalent to the height of your false bottom. Cut the egg crate using wire cutters (and wearing safety glass - there will be stray pieces of sharp plastic) so that it fits snugly into the bottom of the empty terrarium. Next, cut the pvc into pieces which are as long as they are tall (a hacksaw works well for cutting PVC). Use the zip-ties to fasten these to the eggcrate. Now you can set the eggcrate into you terrarium. Simply cover the egg crate in window screen, and voila! You have a false bottom. See, that wasn't so hard, was it?

Soil

Which soil should you use? An exciting recent development in self-sustaining terrariums is the idea that minerals in the substrate, especially calcium, can nutritionally supplement your animals. There is not any clear evidence for the proposed benefit of calcium enriched soil in vivariums, but it is something worth checking out if you are attempting a completely supplement-free picosystem. Another proposed merit to clay based soils in there ability to hold up for extended amounts of time without breaking down. For a very involved discussion on the merits of calcium enriched clay soils read the dendroboard thread, The Ultimate Clay Based Substrate Thread.

The simplest soil to use in a terrarium is standard potting soil. Potting soil is generally peat based, with fertilizer added, as well as particles to prevent compaction over time. Compacted soil is bad news for a thriving picosystem. As the soil becomes dense and waterlogged, the plant roots have trouble thriving. Furthermore, the lack of pores in the soil limit microfauna to the surface, dramatically decreasing the microfauna colony size. A frequent additive used to prevent compaction in potting soil is perilite, which looks like little styrofoam balls. While perilite works great to prevent soil compaction, there is anecdotal evidence that frogs can accidentally ingest it, where it can cause severe intestinal problems. Its light weight also makes it prone to stick to the frogs skin, causing irritation. For these reasons it is best to avoid perilite containing soils in the terrarium. If you are set on using perilite, make sure to cover the soil in a thick layer of leaf litter. Standard potting soil works well for about a year, after which time it becomes depleted, broken down and compacted. For these reasons, you might want to consider a terrarium soil mix, such as Atlanta Botanical Gardens (ABG) mix.

Atlanta Botanical Gardens soil mix (ABG mix) is a mixture of peat, sphagnum moss, charcoal and tree fern fiber. It is very loose, which allows it to drain well and also to support a large community of microfauna. It also holds up well for an extended period of time (5-10 years). ABG mix, as the name implies, was developed at the Atlanta Botanical Gardens, for their terrariums. There are several recipes floating around, but a standard recipe is:

1 part fine peat
1 part milled sphagnum moss
1 part fine charcoal
2 parts fine tree fern fiber
2 parts fine orchid bark.

Whichever soil you decide to use (and I do suggest you try ABG for your first vivarium), make sure to top it with a nice layer of leaf litter.