Hello Aad, I have not yet been fortunate enough to find Worsleya at a price I can afford, but I do grow a lot of epiphytic and lithophytic orchids in inorganic media like pumice and scoria. I would assume their requirements are not entirely unlike those of Worsleya. The general rule of thumb for fertilizing plants in inert media is "weakly, weekly." That is, you should use a dilute solution of fertilizer on a regular schedule when the plant is in active growth. If you use a concentrated mix, you will have problems with salt buildup in the pumice as the fertilizer solution evaporates. If you don't fertilize regularly, you'll have nutritional deficiencies. Try to find a fertilizer that contains micronutrients, not just N-P-K. For epiphytes and lithophytes in inorganic mix, I dilute my fertilizer to 1/4 the recommended concentration (or less) and try to remember to fertilize at every second watering. On those days that the plants are watered without fertilizer, flush the pot thoroughly with lots of water. Ideally, use water with low mineral content. Hope this helps. Nick -- Nicholas Plummer nickplummer@gmail.com Durham, North Carolina, Zone 7