What are essential elements for plants? Give the criteria of essentiality. How are minerals classified depending upon the amount in which they are needed by the plants?

(N/A) Essential elements are those chemical elements that are absolutely necessary for the growth, development, and completion of the life cycle of a plant.
The criteria for essentiality, as proposed by Arnon and Stout $(1939)$, are:
$1$. The plant must be unable to complete its vegetative or reproductive stage in the absence of the element.
$2$. The requirement of the element must be specific and not replaceable by another element.
$3$. The element must be directly involved in the metabolism of the plant.
Based on the quantity required, minerals are classified into two categories:
$1$. Macronutrients: These are present in plant tissues in large amounts (generally in excess of $10 \text{ mmol kg}^{-1}$ of dry matter). Examples include $C, H, O, N, P, S, K, Ca,$ and $Mg$.
$2$. Micronutrients: These are needed in very small amounts (less than $10 \text{ mmol kg}^{-1}$ of dry matter). Examples include $Fe, Mn, Cu, Mo, Zn, B, Cl,$ and $Ni$.