Essential elements are classified into four broad categories on the basis of their varied functions:
(i) Essential elements that acts as a components of biomolecules and hence, structural elements of cells (e.g., carbon, hydrogen, oxygen and nitrogen).
(ii) Essential elements that are components of energy-related chemical compounds in plants.
(iii) Essential elements that activates or inhibits enzymes.
(iv) Some essential elements can alter the osmotic potential of a cell.