Ion exchange theory:
The mineral elements are absorbed in the form of ions. The cations and anions in the plant cells are replaced with the anions and cations of equal charge from the external medium in which the cells are kept. This method can be described by two theories:
Contact exchange theory:
This was introduced by Overstreet and Jenny in 1939. According to this theory ions are transferred from soil particles to root or vice-versa without passing into solution. The ions are electrostatically absorbed to the surface of root cells or clay particles and held strongly. Such ions oscillate in a small volume of space termed as oscillation volume. Whenever the oscillation volume of two ions of similar charge overlaps, one is replaced for the other. This is termed as contact exchange.
Figure: Contact exchange theory
Carbonic acid exchange theory:
According to this theory the soil solution plays a significant role in exchange of ions by giving a medium. CO2 discharged during respiration joins with water to form carbonic acid (H2CO3) that dissociates as H+ (Hydrogen ions) and HCO3- (bicarbonate ions). A cation absorbed on clay micelle might be replaced with H+ of soil solution and this cation diffuses into the root in exchange for H+ ion.
Figure: Carbonic acid exchange theory
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