Expanded clay balls (pellets)
I've had a lot of success recently with expanded clay and thought I would share. I have been using it mainly with Dendrobium and Cattleya but can see how it could easily be used as a substrate for (semi)terrestrials such as Zygopetalum, Paphs and Cymbidium. The principal advantage of expanded clay pellets is that they hold a lot of moisture but also dry out quickly. This combination of wet and dry is what a lot of orchids like. The balls themselves are quite absorbent but there is a lot of air space between them so there is little danger of overwatering. Also, the clay has a lot of nutrients esp. micronutrients like manganese, which is very helpful.
The main disadvantage is that when dry they are quite light in weight, which means there is danger of plants overbalancing if grown in pots. This can be overcome very simply, by putting a few heavy stones in the base of the pot or using clay pots. I am in my early stages with Zygos, Paphs and Cymbidiums. In addition to the clay I have added some crushed bark plus a little ordinary garden dirt (mostly fine clay), and in the case of the Paphs a few limestone chips. The mix looks good but only time will tell. If you give it a try do let us know.