Your thinking is correct, the yellowing is due to a nutrient
shortage. If your ground is light, like mine, I have found it
necessary to apply a balanced fertiliser (such as Vitax Q4 - as it
also contains trace elements) . The reasonning in my mind is with all
the rain we have had the nutrients have been leached from the soil.

Regards

Clifford
Bawtry, Doncaster, South Yorkshie