David D. writes
Where a leaf or soil analysis is not helpful (e.g. the lab has no
reference for a particular plant's NPK etc. requirements due to the
plant's being uncommon), and one has to fall back on visual cues to
find out what is missing from a plant's nutritional intake (assuming
otherwise balanced nutrition), how does one visually distinguish
Magnesium from Potassium deficiencies?

Apply a foliar feed of Mg, K and K+Mg and observe the effects.
I would suggest leaving some controls, and adding a multi-element foliar
feed to your little trial.

--
Oz
This post is worth absolutely nothing and is probably fallacious.
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