Is it a Nitrogen or Potassium Deficiency (or something else...)

I have this problem with yellowing leaves every year- usually after flowering when I switch nutes but this year it started early.

Conditions:
Medium- Promix BX (General Purpose)
Nute- Botanicare Pure Blend Pro Grow 3-2-4
Feeding Cycle: Every 2 weeks
Soil pH: 6.5 (tested today)

Pictures:


Thanks for the input