By far the most common type of orchid bark you'll see in potting mixes is fir bark. It comes in three grades: fine, medium, and coarse. The finer it is, the more slowly it dries but the less airflow it permits. So for orchids that don't like to dry out, such as Paphiopedilum, use a fine grade. For orchids that like a lot of airflow and want to dry quickly, such as Cattleyas, use a coarse mix. You'll also see slabs of bark used for mounting orchids, which allows even more airflow to the roots.
There are two main reasons orchids are often potted in fir bark:
Most orchids are epiphytes that naturally grow on tree bark in the wild
Fir bark decomposes more slowly than many other types of bark, which helps reduce the frequency of repotting, which is generally stressful to orchids. Finer bark will break down more quickly than coarser bark; for plants grown in fine mixes, they should be repotted every year or two, while coarse bark mixes may seldom need repotting unless the plant has outgrown its pot.
Orchids with numerous thin roots, such as Gongora, tend to like finer potting mixes. Plants with thicker roots, such as Angraecum, tend to like a coarser potting medium. This isn't always an important consideration, but many plants seem to like a potting mix that matches the scale of their roots.
Bark, like other organic materials, tends to make a potting mix slightly acidic. Most orchids like this, though some lithophytes expect to be growing on limestone! By varying other components of the potting mix, you can alter the pH somewhat in either direction. Sphagnum moss is more acidic than bark, for example.
You'll sometimes hear that bark robs nitrogen from orchids; this is a myth. There is no need to use a high-nitrogen fertilizer for plants grown in bark.
Coconut husk chips are a common substitute for fine or medium fir bark. By comparison, they retain more moisture and break down more slowly.
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