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

Explanation

An element is essential to plants if it is necessary for supporting its normal growth and reproduction. The requirement of this element must be specific and is not replaceable by any another element in the soil. They must be directly involved in the metabolism of the plant.
Criteria for Essentiality
An element can not be considered as essential merely on the basis of its presence in the plant. It is considered essential on the basis of the following criteria
(i) The plant is unable to grow normally and complete its life cycle in the absence of the element.
(ii) The element is specific and can not be replaced by another element.
(iii) The element plays a direct role in the metabolism of the plants.
The essential elements are further classified into two categories
(a) Macroelements These are the elements required by plants in larger quantities. These are C, H, O, N, P, K, Mg, Ca and S.
(b) Microelements (Trace elements) These are required by plants in low quantities (often less than 1 ppm). These include B, Zn, Mn, Cu, Mo, Cl, Fe and Ni.