Single leaf yellowing?

[Roberta]

The roots in the pot look good, and the bark is large enough for plenty of air. I'd suggest removing the sphagnum. A really good way to get a feel for the "drying rate" is to weigh the pot right after watering (postal scale or kitchen scale) and note the weight. Then weigh it on subsequent days. When the rate of losing weight slows down (getting pretty dry) it's time to water again. Again, better to water thoroughly (like under the faucet), let it run through the pot, rather than soaking.