35. Describe the carbonic acid exchange theory for absorption of mineral salts by plants.

According to this theory, the CO₂ released during respiration of root cells combines with water to form carbonic acid (H₂CO₃). Carbonic acid dissociates into H+ and an anion HCO₃– in soil solution. These H+ ions may be exchanged for cations adsorbed on clay particles.

The cations thus released into the soil solution from the clay particles, may be adsorbed on root cells in exchange for H+ ions or as ion pairs with bicarbonate. Thus, soil solution plays an important role in carbonic acid exchange theory.