List any three criteria for determining the essentiality of any element.

Criteria of Essentiality (Arnon and Stout Criteria):

1. Absolute Necessity: The element must be absolutely necessary for supporting normal growth and reproduction. Plants cannot complete their life cycle without it.
2. Direct Metabolic Role: The element must be directly involved in plant metabolism—either as a constituent of essential molecules or as an enzyme activator.
3. Specific and Non-Replaceable: The requirement must be specific to that element and cannot be replaced by another element, even if chemically similar.