After reading this article you will learn about the cation and anion exchange in soils.
Cation Exchange in Soils:
Cations surrounding the soil colloidal complex i.e. clay and humic micelle are called exchangeable cations because they can be reversibly replaced equivalently by other cations.
For example when ammonium chloride is applied to the soil, then ammonium ions gradually and reversibly replace other cations in equivalent amount as illustrated in the under mentioned equation:
This phenomenon is known as cation exchange which is a reversible reaction because if after some time, sufficient quantities of calcium salt e.g. calcium chloride is added to the soil, then calcium ions will replace other cations from the clay and humic micelles. So the reaction will proceed in the backward direction.
The electro kinetic theory of ion exchange facilitates the explanation of cation exchange in soils. According to this theory, cations which have formed the diffuse layer of cations around the clay/humic micelle, are in a state of oscillation when some of these cations move away from the surface of the clay/ humic micelle and some of the cations of the applied salt e.g. ammonium chloride, NH4Cl slip in between the surface of the clay/humic micelle and the oscillating cation occurring at the outermost part of the diffuse layer of cations. Then the cation i.e. NH4+ ion of the applied salt gets adsorbed by the clay/humic micelle and the cation which was earlier held by the clay/humic micelle, now remains in the solution as the exchanged cation.
Monovalent cations are more easily replaced from the clay/humic micelle surface than the divalent cations which in turn are replaced more easily than the trivalent cations. More highly hydrated cation e.g. Na+ is more easily replaced than less hydrated one e.g. K+ ion because when the cation is hydrated, its effective size is increased and so it is less tightly held on the surface of the clay/ humic micelle.
Anion Exchange in Soils:
Soil Colloids i.e. clay and humux and hydrous oxides of iron and aluminum are capable of taking up anions from the acidic soil solution.
The hydroxyl groups which have been attached to aluminum at the broken edge of 1:1 clay i.e. kaolinite or at amorphous clay i.e. allophane and the amine group of humus accept protons i.e. hydrogen ions form the acidic soil solution to become positively charged as illustrated in the under mentioned equation:
These positively charged sites attract and hold anions from acidic soil solution as illustrated in the under-mentioned equation:
The anions thus held, may subsequently be replaced by another anion as illustrated in the under-mentioned equation:
Some other anions e.g. phosphate H2PO4 and sulphate SO4 ions are held and exchanged in a somewhat complex manner, because specific reactions takes place between them and the soil constituents when they may react with the protonated hydroxyl ion as illustrated in the under mentioned equation:
Thus the positive charge on the soil colloid is reduced on account of this reaction taking place.
Anions also replace hydroxyl ions of hydrous oxides of iron and aluminum, kaolinite and amorphous clays as illustrated in the under mentioned equation:
Anion exchange capacity increases with the decrease in soil pH. Soils with net positively charged colloids adsorb anions like phosphate, sulphate etc. and repel cations like calcium, magnesium etc. which are then lost by leaching. Anion exchange capacity of the soil is usually very low i.e., a few tenth of a mill equivalent per 100gms of a soil.