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List the criteria for essentiality of elements.
Answer
Arnon and Stout gave the following criteria for the essentiality of elements :

1. The essential elements should be necessary for growth and reproduction of the plant. It should be indispensable. The plant should not be able to complete its lifecycle in the deficiency of the element. 

2. The element should not be replaceable and its requirement should be specific. The deficiency of the element should not be met by supplying some other element. 

3. There should be direct involvement of the element in the metabolism of the plant.