Problem from root damage?

Yeah, i think this is just teh case of a large plant relative to the pot it is in. Plants get more difficult to maintain their health -- a solo cup grow would be a parallel of what's happening here, though more extreme. Might be getting rootbound? fold the pot down to inspect the sides and if the root has formed a thick outer wall around it, that would be contributing to the problem. I've never had to root trim before... probably some videos out there, though. does look like it needs more N, and should be safe to do that. If you start seeing a bunch of coinciding or confusing symptoms that don't quite match normal porgresssion etc, it's related to the roots of a large plant in a small volume or medium. If you can find a spot you can let it sit and allow the roots to grow into the ground, that may be helpful to your cause. Root damage can obviously impact nutrient intake, but the damage itself doesn't cause what nutrients the plant has to 'fall out' so, this was likely coincidental to the n-deficiency symptoms.