maxillaria rufescens with yellowing leaves

[heathersgoinnuts]

I have a maxillaria rufescens with yellowing leaves. Could this be a lack of calcium and magnesium or something else? I grow it under grow lights. I have it potted in bark and sphagnum moss. I water when it is almost dry with reverse osmosis water and 1/4 strength MSU fertilizer. Fourth watering I flush with plain reverse osmosis water. I have about 50-65% humidity in my grow space and 72-77 degrees during the day and 64-70 degrees at night.