I have very healthy blueberry plants growing in the reverse. Most of the roots are undoubtedly in the top 12 inches where pH is in the low 6’s but the pH drops to about 5.6 below. It doesn’t seem to matter where the plants get their iron as long as its in some part of their rhizophere. That’s my anecdotal take, anyway. Seems to be an inadequately researched issue although Whitcomb has done experiments with surface apps of sulfur and cured iron deficiency chlorosis in pinoaks before pH readings showed an improvement. Free iron was apparently obtained from narrow sliver of soil on surface.