Most of the mineral elements present in the soil enter plants through roots but all of these may not be essential for their survival. Some are absorbed and accumulated by plant only because they are present in excess amount. For example plants growing near nuclear test sites take up strontium, even though it is not required by them. Thus, an essential element is that which is necessary for supporting normal growth and reproduction, its requirement must be specific i.e. its deficiency cannot be met by supplying other element and it must be directly involved in the metabolism of plant.