What kind of soilless mix did you use in the container? Many are lacking in nutrients, plus getting frequently washed out by heavy downpours makes things worse. I always add about 30-50% compost, plus some time release fertilizer too, like osmocote 14-14-14 or Miracle-Gro Shake N Feed for Roses (9-17-9 I think.)

Generally, I would use 1 TBS/gal if foliar spraying, even more if doing a soil drench to replace any magnesium.