In 1939, Arnon and Stout gave the following criteria to measure the essentiality of an element in their nutrition.
(a) The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element, the plants would not be able to complete their life or produce seeds.

(b) The requirement of the element must be specific and not replaceable by another element.

(c) The element must be directly involved in the metabolism of the plant.