Criteria for Essentiality

Three Tests for Essentiality

A mineral element is only called essential to a plant if it passes three separate tests together — meeting just one or two is not enough.

• Absolute requirement: the plant must be unable to complete its life cycle — to grow normally and to produce viable seed — without that element. If the plant can still grow and reproduce fine in its absence, the element is not essential, no matter how commonly it turns up in the plant's tissue.
• Specificity: the requirement for that particular element must be specific to it — no other element can be substituted to fix a deficiency of it. If supplying some other, chemically similar element relieves the deficiency symptom, then the original element was not truly essential in that specific role. …