Chemical Properties of Soil
Cation Exchange Reactions
Adsorbed cations on colloids can be replaced by other cations in solution. Similarly, one anion can replace another anion. This phenomenon of exchange of ions between the colloidal surfaces and solution is known as ion exchange where ions can move from the adsorption sites on colloids into the soil water solution (and vice versa) where they are available for root uptake and are also subject to leaching. The most commonly adsorbed cations on soil colloids are hydrogen (H⁺), aluminum (Al3⁺), iron (Fe3⁺, Fe2⁺), calcium (Ca2⁺), magnesium (Mg2⁺), potassium (K⁺), and sodium (Na⁺), while the most common adsorbed anions are chloride (Cl¯), sulfate (SO42¯), nitrate (NO3¯), dihydrogen phosphate (H2PO4−), and bicarbonate (HCO3¯).
Principles Governing Cation Exchange Reactions
To better understand cation exchange reactions and the roles they play, it is important to be familiar with the principles that govern how these reactions take place.
Reversibility
Cation exchange reactions, the swapping of two cations from soil solution to being bound to a colloid is a reversible reaction.
Charge Equivalence
A basic principle of cation exchange reactions is that the exchange takes place on a charge-for-charge basis. For example, one H⁺ ion is exchanged with one Na⁺ ion in a reaction, it would require two singly charged H⁺ ions to exchange with one divalent Ca2⁺ ion.
Ratio Law
Exchanges between more than one similarly charged cations between solution and colloid, will eventually even out creating an equilibrium. This means the ratio of cations in solution will be the same as those on colloids.
Anion Effects on Mass Action
Mass action means that the greater the number of an ion in the soil, the more exchange sites it will occupy. As an example, in high-lime (calcium carbonate) soils, most exchange sites are occupied by calcium. In another example, high-sodium soils are often treated with gypsum (calcium sulfate).
Cation Selectivity
Some types of ions are held more tightly to the colloid than others and thus they won’t all replace each other equally; it will actually vary based on the strength of the charge holding ions to colloids. In general, the higher the charge and the smaller the hydrated radius of the cation, the more strongly it will adsorb to the colloid.
Complementary Cations
In soils, colloids are always surrounded by many different adsorbed cation species. The likelihood that a given adsorbed cation will be displaced from a colloid is influenced by how strongly its neighboring cations (sometimes called complementary ions) are adsorbed to the colloid surface as discussed above in Cation Selectivity. For example, consider an adsorbed Mg2⁺ ion.
Saturation Percentage
In a given soil, the proportion of the cation exchange capacity satisfied by a particular cation is termed the saturation percentage for that cation. Thus, if 50 percent of the CEC is satisfied by Ca2⁺ ions, the exchange complex is said to have a calcium saturation percentage of 50.
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