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.
Amon and Stout proposed criteria for knowing the essentiality of an element. They are :
(a) It is indispensable for the growth of plant.
(b) 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.
(c) A plant is unable to complete its vegetative or reproductive phase in the absence of the element, (d) The element cannot be replaced by any other element.
(e) The absence or deficiency of the element produces
disorders that are a direct result of the lack or deficiency of the element.
(f) The element alone can correct the disorders produced by its absence or deficiency.
16 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. Nickel has been made the seventeenth essential element.