Some scientists Hylmo (1955) and Kramen (1956) think that when water is absorbed at a large rate during respiration, mineral salt ions are also absorbed with water. Lopushinsky observed in 1964 that the rate of ion absorption increased when the rate of transmission increased.

Proponents of ion exchange theory are Devlin (1969) and Pandey and Sinha (1972). Plant root cells contain H+ and OH-. H+ ions are released and K+ ions from the outside solution enter the cell. Similarly exchange between OH- ions and Cl- ions takes place. In this method anion exchange is least and cation exchange is most. Hence it is called cation exchange method. However, cations and anions are not adsorbed simultaneously.

Diffusion theory was introduced by Hope (1953) and Heilmo (1955). Cells of absorptive organs of plants have lower concentrations of ions and higher concentrations of ions in soil solutions. Therefore, few ions enter the cell during the diffusion process. Absorbed ions enter various cells from the cortex. The absorbed ions are then used for metabolic purposes in various cells.

Donnan introduced this theory in 1911-1914. According to this theory, there is a fixed anion or anion charge inside the cell membrane that cannot be absorbed. To neutralize this fixed negative charge, the cation or cationic charge moves in from the outside of the cell membrane in the diffusion process. This process continues until there is an equilibrium of cations and anions inside the cell. Donnan calls this phenomenon equilibrium. In Donnan equilibrium, the ratio of positive charges inside and outside the cell is equal to the ratio of negative charges inside and outside the cell.

Absorption processes that do not require metabolic energy are called passive absorption. Very small amounts of mineral salts are passively absorbed. The principles of passive absorption are:
1. Donnan equilibrium
Donnan introduced this theory in 1911-1914. According to this theory, there is a fixed anion or anion charge inside the cell membrane that cannot be absorbed. To neutralize this fixed negative charge, the cation or cationic charge moves in from the outside of the cell membrane in the diffusion process. This process continues until there is an equilibrium of cations and anions inside the cell. Donnan calls this phenomenon equilibrium. In Donnan equilibrium, the ratio of positive charges inside and outside the cell is equal to the ratio of negative charges inside and outside the cell.
2. Diffusion theory
Diffusion theory was introduced by Hope (1953) and Heilmo (1955). Cells of absorptive organs of plants have lower concentrations of ions and higher concentrations of ions in soil solutions. Therefore, few ions enter the cell during the diffusion process. Absorbed ions enter various cells from the cortex. The absorbed ions are then used for metabolic purposes in various cells.
3. Ion exchange theory
Proponents of ion exchange theory are Devlin (1969) and Pandey and Sinha (1972). Plant root cells contain H+ and OH-. H+ ions are released and K+ ions from the outside solution enter the cell. Similarly exchange between OH- ions and Cl- ions takes place. In this method anion exchange is least and cation exchange is most. Hence it is called cation exchange method. However, cations and anions are not adsorbed simultaneously.
4. Mass flow theory
Some scientists Hylmo (1955) and Kramen (1956) think that when water is absorbed at a large rate during respiration, mineral salt ions are also absorbed with water. Lopushinsky observed in 1964 that the rate of ion absorption increased when the rate of transmission increased.

The process by which mineral salts enter the cell as ions from an external source is called mineral salt absorption. The process of adsorption of inorganic salts is called ‘ion adsorption’. Mineral salt absorption process is of two types. Passive absorption and active absorption.

The source of plant mineral salt is soil water. Mineral salts are dissolved in water. However, water and mineral salt absorption are completely different processes. When mineral salts dissolve in water, they dissociate into cations (+) and anions (-). NaCl dissolves in water and dissociates into Na+ (cation) and Cl- (anion). Plants absorb mineral salts as cations and anions. Cations present in ground water are K+, Na+, Mg++, Fe+++, Zn++, Cu++, Mn++, Ca++ etc. On the other hand, anions present in ground water are NO-3, PO4—, Cl-, BO4—, PO4—, SO4– etc.
It is assumed that K+ and NO3- are absorbed most rapidly and Ca2+ and SO42 most slowly. Some ions inhibit the absorption of other ions. The presence of Ca++ and Mg++ inhibits the absorption of K+.

The mineral salt absorption organ of aquatic plants is not known exactly. However, submerged aquatic plants are thought to absorb mineral salts through their organs. Land plants absorb mineral salts through the young cells of rhizomes and roots. Bryophytes absorb mineral salts with rhizoids and lichens with rhizines.

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.

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.