Q.What are the criteria used for identifying essentiality of elements ?

An element is called 'essential' for plant growth only if it satisfies three strict criteria — indispensability, specificity, and direct involvement in metabolism. Note: this topic belongs to the NCERT chapter 'Mineral Nutrition', which is not present in this subject's current chapter menu on the platform (that chapter was dropped in the rationalised syllabus), so this item is marked out-of-syllabus even though a full, correct answer is given below.

Not every element found in plant tissue is 'essential' — some are absorbed incidentally from the soil. Arnon and Stout (1939) laid down three criteria that an element must satisfy to be classified as essential:

1. Indispensability: The element must be absolutely necessary for supporting normal growth and reproduction. In its absence, the plant is unable to complete its vegetative or reproductive (life) cycle.
2. Specificity: The requirement for the element must be specific to that element — no other element, however similar chemically, can fully substitute for it or fulfil the same role. …

An element is called 'essential' for a plant only if it meets three specific criteria (given by Arnon and Stout) — it is indispensable, non-substitutable, and directly involved in metabolism — which together distinguish a truly essential element from one that merely happens to be beneficial.

Only about 17 of the many elements found in plant tissue are actually classified as essential. To decide whether a given element deserves that label, three criteria are applied:

1. Criterion of indispensability (omission causes failure of the life cycle)

The element must be absolutely necessary for the plant's normal growth and reproduction. If the element is withheld from the plant, the plant must fail to complete its vegetative or reproductive (life) cycle — i.e. deficiency symptoms appear and the plant cannot grow or reproduce normally without it.

2. Criterion of specificity (non-replaceability)

The requirement for the element must be specific to that element and cannot be met or substituted by supplying any other element, even a chemically related one. If another element could take over its role, the original element would not be counted as essential.

3. Criterion of direct involvement in metabolism …

An element is called 'essential' for plant growth only if it meets three criteria laid down by Arnon and Stout: it must be absolutely required (indispensable), its role cannot be substituted by any other element, and it must directly take part in the plant's metabolism.

Not every element found in a plant's tissues is essential — some are merely absorbed incidentally from the soil. To decide whether an element is truly essential, three criteria (given by D.I. Arnon and P.R. Stout, 1939) must all be satisfied:

1. Indispensability: In the absence of the element, the plant must be unable to complete its vegetative or reproductive (life) cycle — i.e., without it, the plant cannot grow and reproduce normally.
2. Specific/non-replaceable requirement: The function of that particular element cannot be fulfilled or substituted by any other element. Each essential element has a specific, non-interchangeable role. …