To put a finer point on it, same thing happens at low ph — actually it’s worse at low ph; P combines with primarily iron and aluminum at low ph and with calcium at high ph. Another problem with high ph is additional P release from naturally occurring or applied rock phosphates in the soil which can then go on to fixate with the noted nutrients and create sort of a vicious circle. At any ph, P can fixate with plant nutrients (somewhere between 70 and 90% of phosphate fixates just about the moment it hits the ground — the lowest P fixation does occur at ph just below 7.0 but still a very significant percentage) and as a result, hyper-excessive P can cause deficiencies. So that’s sort of where we are with excessive P. Now to turn to ph, if you see iron deficiencies at high ph, it’s likely not caused by P fixation (unless P is ridiculously excessive as noted above). Iron readily oxidizes in high ph soil, converting to the ferric form which is not plant available. Maybe you meant low ph? — P fixates strongly with iron and is basically completely insoluble in very low ph soil (like 4.0).