Yeah alltatup,
That's almost exactly what I'm getting at. Lets use nitrogen deficiency. It is a yellowing in older leafs. They turn yellow because the plant is puling that nitrogen out of them and diverting it towards new bud formation. That's what makes new growth always look healthier than the other parts of the plant.
pH is how nutrient toxicities and lockout happen.
Plants use different processes (Diffusion, Bulk Flow, and Osmosis) to move nutrients around. Each process handles concentrations differently.
I personally like to think of it in terms of a person.
Eating too much cake and candy is bad for you. Starving as bad for you. Overdosing on vitamins is bad for you.
If your pH is off it would be like your digestion slowing down and your stomach starts to hurt. For example acid reflex.
If you get to many vitamins you start to puke and your body will start to reject them.