Beagle, Seriously, the best northiana I've ever seen was the one grown next to the auto propagation misters in the Miami greenhouse. It was potted in the same mix he uses for rafflesiana, bicalcarata, alata, in other words, nothing special. It is very well drained, as all Neps should be grown. We are finding the shade and humidity issue is critical. Northiana really does not like humidity drops, even if slight. Our plants are looking so much better since being placed in those big clear plastic storage containers, and then putting those under the benches. Maybe you should try that, and then bring the whole thing in when temps fall below 65F.
Trent