Of these 16 elements, first three-C, H and O-are derived from air and water. The remaining 13 elements are all derived from the soil-forming minerals, and so also known as mineral nutrient elements or simply mineral nutrients. They are all essential for plants in the sense that they satisfy the criteria of essentiality as laid down by Amon and Stout (1939), which reads as: (1) The plant must be unable to grow normally or complete its life cycle in

the absence of the element. (2) The element is specific and/or cannot be replaced by another element. (3) The element plays a direct role in metabolism.