puzzling, persistent substrate dampness
I'm wondering if there is an obvious explanation that I'm just not seeing to my seemingly constantly damp substrate. I think that's the reason for the failure of a Selaginella uncinata and S. kraussiana and the reason my Ficus thunbergii went from establishing well at first to now gradually losing leaves and becoming chlorotic.
Specs: I have an Exo Terra large tall (36x18x24) that's been set up for maybe six or seven months now. I used the Growstone-like medium as a drainage layer, topped with window screening-type separator (not landscape fabric), and then the ABG mix + calcined clay pre-blended by the vendor. I soaked the substrate briefly in distilled water prior to laying it into the tank. I have glass insert covers over the stock screen top to help hold in humidity (they leave about an inch of screen strip to vent) and am using an LED panel from Spectral Designs. I have one multi-speed 2" computer fan to help circulate the air, on "low". The glass never fogs, and the hygrometer (just the cheap-o typical ones the size of a pack of gum that reads temp and humidity) never reads very high humidity except for overnight. It's in an abode with central air, so the room temps stay in the low 70s for the most part and the ambient room humidity in the winter sucks. I must get decent air exchange, though, because the tank's apparent dampness never seems to be very high during the day or fog up the glass. You would think that would help to dry the substrate, but...?
I don't have a water feature, and I mist by hand a couple of times a day, just enough to wet the wood and epiphytes. The misting isn't enough to rain on to the substrate in any meaningful amount. I rarely put water in the drainage layer because it doesn't seem to raise humidity, and when I have, the water level is well below the substrate barrier.
The trouble is, the substrate has never needed moistening since I put it in at the start. As this is a terrarium (no animals), I do use an eyedropper to occasionally dose the root zones of the few terrestrials in there (so far) with orchid or Tillandsia fertilizer. However, this doesn't happen often enough that it would explain the wetness. I don't understand why it's staying damp, and assume that's the reason the plants mentioned above failed. There are a couple of others that are struggling, though the rest are doing ok (not great, but ok).
Was soaking the substrate at the start ill-advised? Is the calcined clay contributing to this issue? (I've been trying to read through the related thread about its use as a substrate or drainage layer but the pictures come through so large on my screen that it's hard to follow.) Is it likely the dampness isn't the issue? I'm going to try more moisture-loving species next time I'm ready to add more terrestrials to see how that pans out. In the meantime, I would like to save the Ficus and prevent other plant failures. I am assuming it's a root issue since root and foliar feeding of the plant hasn't averted progression of the chlorosis. I know with temperate garden plants that poor root health can result in foliage malnourishment.
It's still a work-in-progress with regards to setup/planting, but the picture is fairly recent. I included a couple of the construction process. There are a couple pieces of wood that touch the base (at the time, I was foolishly planning on a water feature and a less terrestrial base). I doubt any wicking they're doing is causing this because the above-ground parts are never damp and you'd think that all this time wicking would have measurably dried things out.
What am I missing?