Yellowing of bottom fan leaves.

Hi guys. I've ran into a problem. No matter what I do I can't seem to get the bottom fan leaves from Turing yellow. They spread all the way up after awhile. I feed them yesterday with biobizz. Maybe use a bit more? How often should I feed them. I've not had this problem before.

In flower?

and how far up are we talking? Do you allow your soil to dry out totally before you water again?

Sounds like either low nitrogen or rootbound. But i am guessing in the dark. Need some more details.

Well it could be overwatering, over feeding, under feeding, pH, Temp, root bound...I mean the list goes on and on and on...but you start with what is most common and work your way in...