Proponents of ion exchange theory are Devlin (1969) and Pandey and Sinha (1972). Plant root cells contain H+ and OH-. H+ ions are released and K+ ions from the outside solution enter the cell. Similarly exchange between OH- ions and Cl- ions takes place. In this method anion exchange is least and cation exchange is most. Hence it is called cation exchange method. However, cations and anions are not adsorbed simultaneously.