Introductory Crop Physiology (1+1)

Mechanism of ion exchange

Theories explaining Mechanism of Ion exchange

There are two theories proposed to explain the mechanism of ion exchange:

1. CO2 hypothesis (or) Carbonic Acid Exchange Theory
2. Cation Exchange hypothesis (or) Contact Exchange Theory.
1. CO2 hypothesis
According to this theory, CO2 released by the roots during respiration, combines with water to produce carbonic acid (H2CO3). The carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3). These hydrogen ions may be exchanged for cations adsorbed on clay particles. The cations thus released into the soil solution from the clay particles may be absorbed on root cells in exchange for H+ ions. While the dissociated bicarbonate ions release the adsorbed anions. Thus, both cations and anions are made available to the closeness of the roots of plants. Thus, soil solution plays an important role in carbonic acid exchange theory.



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b. Cation Exchange hypothesis
This theory states that the ions adsorbed on the surface of root cells and clay particles (micelles) are not held tightly but always oscillate within small volume of space. If the roots and clay particles are in close contact with each other, the oscillation volume of ions adsorbed on root-surface may overlap volume of ions adsorbed on clay particles. Then, the ions adsorbed on clay particle may be exchanged with the ions adsorbed on root surface directly without first being dissolved in soil solution.



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