Perlite is a processed volcanic material most often used as an additive to other potting mediums. Perlite is low-cost, holds water well and is decay resistant, thus it is a popular additive to fir bark. Sphagnum Moss is a soft springy khaki-colored plant that grows on the surface of bogs. It is capable of retaining up to 10 times its weight in water. Sphagnum moss also contains an antiseptic which inhibits the growth of some fungi.
Sphagnum moss is available in the form of long or short fibers, alive or dried.
Live sphagnum moss is the best for orchids. If not over watered or over fertilized, it will continue to grow after being placed in the orchid pot. It is most often used as an additive to other potting mediums, though it is sometimes used alone. Crushed Cork is another mixture additive. It should not be used alone as it tends to break down quickly.