^I agree, the soil looks compact, and lacking drainage. I really doubt the soil has been depleted, it looks like an aeration/overwatering problem. Those, will both lead to low PH, which could also be a problem. Magnesium will be one of the first deficiencies to show, due to low PH, which futher evidences this theory. Sadly, the best way to increase aeration, is to transplant to a soil rich in perlite(or similar), which is a bit of a pain in the ass, at this point. Would be well worth the effort though, IMO. Also, you can flush with something PH'ed to around 6.6, to bring your PH up to spec,(flush until the runoff comes out the same as you're pouring in) before the transplant(or afterwards, but before, is easier), and it should hold there, as long as you've knocked off a bunch of the old soil.