All at top? Zn, if you've deduced it to these 2 possiblities. Your in soil, so bring that pH up closer to 7. You want it slgihtly acidic, though, so err on that side of 7 in soil. ~6.5-6.8.
It's minimal, so it shouldn't impact harves in a noticeable way to your eye... maybe even within normal variance statistically too.