Cant stop N deficiency

Since you're in flower and it's pulling mobile nutes up from the lower leaves, I'd guess you need more bloom nutes. P + K + Mg much more than Nitrogen. A 5.9 pH runoff could be locking out some of the nutes too, I'm thinking Mg specifically (which prefers at least 6.0 or 6.5+). I suggest you try feeding it with a higher pH water to get the runoff into the correct range, and see if that helps.