Botton leaves are turning yellow then fading away

needs more nitrogen or senescence.. if it continues to progress upward, you need more nitrogen. if it is isolated to shielded, lower leaves, then you can ignore it. This looks more like a nitrogen deficiency, because i see paling leaves creeping upward on the plant. pH could be causing it, but in those cases you usually see more than 1 symptom displayed. This looks like N-def 95%. check you pH. Make sure it is somewhere 5.8-6.8 range. "They" often say a plantn does not need N in bloom, but "they" are wrong. Every cell produced needs N as a building block even in bloom. i've never needed to drop N more than 5-10% from vege to bloom and i track ppm mixed of every molecules every feed. I'm faily familiar with safe ranges that work for all of them, relative to my hard water, of which npk is not augmented by hard water like Ca/Mg/S is, so the safe zones of npk are translateble to other gardens. So, maybe take the fertilizer component with the most N with least impact on other things and bump it 5-10%, then wait and see how that goes. Don't have a major reaction... slow and steady wins the race.