Criteria of Essentiality

Amon and Stout as early as 1939, suggested certain criteria that an element must fulfil in order to be classified as essential. These criteria are listed below.

1. An element is essential if in its absence the plant cannot complete its life cycle and form viable seeds.
2. An element is essential if it forms a part of any molecule or a constituent of the plant that in itself is essential for the plant. For example, nitrogen in protein, magnesium in chlorophyll arid iron in cytochromes.
3. The element must act directly inside the plant, and not enhance or suppress the availability of some other element.