I'd say magnesium. Folar feed with epsom salt. 1 tsp/quart. Keep plant out of strong light until droplets dry.
Also looks a little like N def. I'd be concerned about your soil ph. Measure runoff ph (try to consider how long the water sits in the soil until it is displaced into runoff).
Do you let the soil dry between waterings? Container should feel very light when it's ready to watering, just before leaves start to droop from being too dry. If you're overwatering it will hold the soil ph lower, not letting the roots experience higher ph as the soil dries.