Elements of common growing media (or soil contents)
If you've shopped around for soil mixes, you've probably noticed several ingredients that are listed in their contents. Here's the scoop on a few of the most common:
Sphagnum/Peat: Peat is organic soil that consists of rotted plants, inorganic minerals, water and air. It differs from muck (Yes, "muck" is a technical term used by soil scientists) because in muck the organic material is decomposed beyond recognition. Sphagnum comes from bogs in Canada, and it's usually over 90% organic. And it's real aerated, eh? It's a bit light weight, so usually bark or sand is added to avoid the "topple factor," the technical term for when your plant tips over. When it's wet, sphagnum absorbs water, but when it dries out, it can actually repel water, which is the kiss of death. Peat must always be mixed with wetting agents, and is usually mixed with some of the items listed below.
Composted Bark: Put bark in a big pile and allow it to rot for 4 months (when it's 150 degrees in the middle on the pile, it's done) and you've got yourself some composted bark. Composted bark is actually much heavier than peat. If composted correctly, bark can resist disease. On the other hand, new bark and fine particles can cause problems. It helps to have properly sized chunks of bark, which will help to aid in porosity.
Vermiculite: Vermiculite is a silica rock, kind of like mica, that expands into little hollow kernels when heated quickly at a very high temperature. It's usually added to potting mixes because it's light, it absorbs several times its weight in water, and it aids porosity. If you want to find out more about vermiculite's chemical properties and how it's made, check out the Vermiculite Association web page at www.vermiculite.org. (Really.)
Perlite: Perlite is a volcanic ore that, when heated, pops open like popcorn. Perlite doesn't absorb water (it holds it on its surface) so it hurts your mix's total water retention. What it does do, however, is make more air-filled pores, so you can be sure roots are getting enough oxygen. It's light. It's nice. And you can use it as mulch `cause it's pretty (in a puffy, white ball kind of way).
Coir: Coir is a waste product of the coconut processing industry - it is a shredded form of the outside of the coconut shell. It's often used as a substitute for peat in potting mixes for plants that need moist soil, because of it's superior water holding ability. Coir is usually sold in a compressed brick form that expands with water. One drawback - it can be high in salts because of the processing methods used to make it. So it should be soaked in water and washed before being added to the mix. Coir is currently at the center of the "Great Peat Debate" - being hailed as the environmentally preferable alternative to harvesting natural peat reserves. (Hence the claim "peat-free" in most ads for coir.) But some have argued that importing organics to the temperate zone from the tropics is also environmentally sketchy. So, the debate continues.