Why can’t I use potting soil for my Phalaenopsis?
by Anne-Marie Blancquaert
On first view, orchids have the same structures as all other plants: roots, stems, leaves and flowers. However, because of their different growth habitats, the orchid parts have adapted and do not require the same types of materials around their roots as more traditional plants.
Most orchids, and Phalaenopsis is no exception, are epiphytes that in nature will use their roots to attach themselves to trees or even rocks to take maximum advantage of light and air movement.
The thick roots that develop on your Phalaenopsis, some of which prefer to stay outside the pot, are composed of two structures: an inner thin thread-like central core and a spongy material (called the velamen) which covers the central core. The tips are green and take up the moisture, directing it to the plant. The spongy material has a silvery look, protects and stores extra water. Storing of water is limited, however, so they will not tolerate prolonged dry periods. On the other hand, prolonged exposure of the roots to water will cause rot, which will stress your plants. So, Phalaenopsis need to dry somewhat between waterings.
Knowing this, you can understand that if you would plant your Phalaenopsis in potting soil the roots would not have any opportunity to dry out, as there would be no air movement around them. Root rot and slow death will result.
Phalaenopsis have very thick roots as do Vandas. Many other orchids have thin roots, such as members of the Oncidium family. The thickness of the roots helps in selecting the potting medium.
Orchids with thick roots require a more open medium (medium to large bark mix) while thin rooted orchids prefer a smaller bark mix.
Charles Wilson, in a webinar for AOS he gave on Bulbophyllum, provided the following formula for making media for the majority of your orchids:
For almost all orchids follow a 7/1/1 ratio meaning 7 parts bark, 1-part charcoal and 1-part perlite.
—Use small 7 parts fine bark, 1 part medium charcoal, and 1 part medium perlite for fine rooted orchids
—Use 7 parts medium bark, 1 parts medium charcoal, and 1 part medium perlite for thick rooted orchids
—Add equal parts of the appropriate mix above (depending upon root size) and potting soil for use with terrestrial orchids.
I have collected different media formulas over the years which I put in a separate document which will be published in the Beginners Corner as well.
Happy orchid growing!