Answer:
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) Macro elements
These are the elements required by plants in larger quantities. These are C,H,0,N,RK,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.
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