The criteria for essentially of an element are given below:
1. The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element, the plants do not complete their life cycle or set the seeds.
2. The requirement of the element must be specific and not replaceable by another element. In other words, deficiency of any one element cannot be met by supplying some other element.
3. The element must be directly involved in the metabolism of the plant. All elements that are present in a plant do not fulfill these criteria hence cannot be essential for plant survival.
