Peat moss, which grows in peat bogs and is a deposit of partially decayed vegetation, has been used for many years as a soil additive to help loosen up soil, add nutrient and to help soil retain water. 
The problem is that a peat bog can take anywhere from a few hundred to a few thousand years to form.  In the UK, most peatlands have now been destroyed and the government is taking action to preserve what remains. Peat deposits in southeast Asia could be destroyed within the next few decades. (http://www.greenlivingtips.com/articles/277/1/For-peats-sake-.html) 
The amount of peat harvested for various uses, including gardening, each year is estimated to take 2000 years to reform!
Coconut coir is a sustainably produced product that is even more effective at retaining water as peat.  And unlike peat it is pH neutral (the pH of peat can vary) and is 100% environmentally friendly.  Coir is a natural byproduct of the coconut industry which was previously considered waste.  When a coconut is picked, 1/3 of the outer "husk" is coir fibers.  These fibers are an excellent alternative to non-renewable peat!