I'm pretty sure that iron doesn't suddenly run out. It would be next
to impossible to have enough iron one year and have a serious
deficiency the next. It just doesn't leave the area easily.
Nitrogen, on the other hand, does wash away easily and a garden
needs a steady supply from one source or another.

Steve

Pen wrote:
If nutrients' the problem, yellow lower leaves indicates lack of
nitrogen. Yellow top leaves indicates lack of iron. Many other
factors can also cause yellowing. Any chance you can send a picture
of your garden?