There is no regulation on potting soil. A lot of material sold on the market today is awful. Choose your potting soil with more care than you do your plants; it will be with you longer!
DO NOT USE GARDEN SOIL OR BEACH SAND IN A POTTING MIX. Or at least not one you hope to grow plants in!
Compost Few mixes that say ‘compost’ mean compost as we know it. If it is clearly marked ‘organic’ and is from a known source, then you can be assured that it is compost. Otherwise, assume it is really sewage sludge and don’t buy it.
Forest fines Ground bark and nothing much more really – most mixes that have this hold more water than necessary except for bog plants. Stay away from these mixes.
Peat moss A lot of controversy swirls around the sustainability of harvesting peat, but it is still the number one water holding constituent in potting soils. Tends towards acidic. Coconut 'coir' is the touted replacement, but there is rumor that it is even LESS sustainable than peat. The jury is still out about which is which and which is hype.
Perlite A natural ‘glass’ material baked at high temperatures that is useful for pore space in potting mixtures
Sand An inert material, sand promotes drainage and good pore space. Sand should not be the majority (by volume) of the soil mix, but its presence in large amounts promotes good plant health and forgives the mistakes of beginning or benign gardener.
Vermiculite A mineral that is heated until it expands – a lightweight and porous material, it is common ingredient in potting soils; if crushed while wet, it loses all efficacy.