Orchids seem to thrive in a porous material. In Asia, most orchids are sold in coconut husks filled with charcoal. Orchid growers swear by it and list the following reasons:
- To keep the "mix" sweet.
- To filter impurities.
Laboratory tests show that charcoal possesses a remarkable ability to:
- Keep nutrients in the soil, increasing soil fertility.
- Prevent decay and can endure in soil for thousands of years.
- It is inert.
- Allow the flow of air through the potting mix.
- Retain moisture and help in good drainage.
- Retains (adsorbs) excess nutrients (fertilizer) and releases them later when needed.
- Helps to increase the soil pH.
Source: Charcoal and Orchids. Myth, Magic I know I would never grow an Orchid – Australian Orchid Nursery (australianorchids.com.au)