Essential Elements: An essential element is the one which has a specific structural or physiological role and without which plants cannot complete their life cycle.
The criteria for the essentiality of an element are:
(i) It is indispensable for the growth of plant.
(ii) The element is directly involved in the nutrition of plants. It becomes a component of either a structural or functional molecule. The element may additionally have a corrective effect of mineral balance and other soil conditions.
(iii) A plant is unable to complete its vegetative or reproductive phase in the absence of the element.
(iv) The element cannot be replaced by any other element.
(v) The absence or deficiency of the element produces disorders that are a direct result of the lack or deficiency of the element.
17 elements have been found to be essential. They are C, H, O, N, P, K,S, Mg, Ca, Fe, B, Mn, Cu, Zn, Mo, Cl and Ni