Peat moss is decomposing parts of Sphagnum moss and is usually found deep in a bog. It is rich in organic matter and also is acidic so can be used to lower the pH in soil. Growing in bogs around the world peat moss is deposited at a rate of about about 1 mm a year, so a metre deep patch would take around a thousand years to grow.
Peat can be bought in bags from horticultural suppliers. The blacker the peat, known as sedge peat(the residue from sedges and grasses) is older and less stable than the younger, browner sphagnum moss peat
(derived from sphagnum moss). Sedge peat decomposes rapidly and can become a soggy airless mass if allowed to get very wet. It is best not use it as a potting mixture and only in the garden if nothing else is available.
Sphagnum peat can make an excellent, if expensive, soil conditioner . It is widely available and well worth its price. There is a popular misconception that the darker the peat the, the more fertile it is. Nothing could be further from the truth. In fact, peats contain no worthwhile amounts of nutrients. On the other hand, peat has the ability to retain up to fifteen times its weight in water and it will, of course, hold plant nutrients in solution as well. It opens up heavy soil and makes a fine medium for root growth. Peat is expensive, so has to be used sparingly. Make use of it around roots when planting in heavy soils. Use it to lighten up the top two and a half to five centimetres of heavy soil to make a seed bed.
A word of warning though. Peat is very difficult to rewet once it has dried out. So it is important to wet it thoroughly before use and to ensure that it is never allowed to dry out. Before using, leave the bag open to allowing the rain to wet it before use.