The exchange of ions (anions or cations) from within the plant cells to the ions of soil is called ion exchange. By this method, the plant cell can exchange the unwanted ions for the required ones. Ion exchange is of two types. Contact exchange and Carbonic acid exchange. In contact exchange, the ions that oscillate around the root hair overlap with the ions that oscillate around the soil particle and there will be mutual exchange of ions.
According to carbonic acid exchange theory CO2 released during respiration combines with water to form carbonic acid. This acid disassociates into H+ and HCO3 in soil solution. These H+ ions may be exchanged for K+ ions held by the soil particles. Since the exchange of ions here is mediated by carbonic acid, it is called carbonic acid exchange theory.