If you have bought planting medium or soil mix you've probably noticed several ingredients that are listed in their contents. Here is what the ingredients do and what they are.
Peat is an organic material that forms in the waterlogged, sterile, acidic conditions of bogs and fens. These conditions favour the growth of mosses, especially sphagnum. As plants die, they do not decompose. Instead, the organic matter is laid down, and slowly accumulates as peat because of the lack of oxygen in the bog. It is very light so bark or sand is usually added to make it more dense. When it's wet peat water when it dries out however it can actually repel water, not good when you are trying to grow plants. Peat must always be mixed with wetting agents and is usually mixed with some of the items listed below.
Composted bark is used as a soil improver for clay or impoverished soils. The rich humus content increases water retention enabling the soil to hold onto valuable nutrients for sustained plant growth. The increased fibrosity of the soil enables worms to become more active and thus better manage the soil. If you put bark in a big pile and allow it to rot for a few months you then have composted bark. Composted bark is heavier than peat. and if composted correctly can resist disease.
Vermiculite is a silica rock and when heated to high temperatures its layers separate which gives it water absorbtion properties. It's usually added to potting mixes because it gives very light open compost, holds moisture, absorbs nutrients and then slowly releases them.
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 can also be used as used as a mulch.
Wetland ecologists say that peat is being harvested at non-sustainable rates. While the peat industry argues that peatlands can be managed at sustainable levels, it recognizes that alternatives to peat must be developed in order to meet environmental concerns of consumers.
As useful as peat is for horticulture Coir is a good alternative. When coconuts are harvested and husked, the long fibres are removed and used for such things as upholstery stuffing, rope, doormats, and brushes. The short fibres are left over and have found use in horticulture as coconut "peat."
In the past, this material was considered waste and left to accumulate into enormous piles. In countries where coconuts are harvested commercially, some of these piles are thought to be as much as a century old. Not only is coir a renewable resource, its horticultural use helps solve a waste disposal problem.