Mineral salts remain dissolved in the soil solution and some cations adhere to the colloidal grains. The ions attached to the colloidal granules enter the plant body through the ion exchange process. There are two theories about ion exchange. These are-
1. Carbonic acid exchange theory: According to this theory, CO2 is produced in the process of respiration in plant roots. CO2 reacts with water to form carbonic acid. Carbonic acid breaks down to produce hydrogen ions (H+) and bicarbonate ions (HCO3-). Exchange occurs between root H+ and soil cations and between root HCO3- and soil anions.
2. Cation exchange theory: Proponents of this theory, Jenny & Overstreet (1939), believe that the root shell ions and the colloidal grain shell ions are never fixed. Both ions are always vibrating. Cation exchange occurs when the ions face each other during movement.