Ion Exchange Capacity

Ion Exchange

It is interchange of ions between colloids and soil solution and/or between the solid phases if these are in a close contact. If the process is between cations; it is called cation exchange and for anions it is termed as anion exchange. These are reversible reactions.

Cations are positively charged ions. Soil colloids have negative charges on their surfaces. Cations are adsorbed at these negatively charged sites. The adsorbed cations can be exchanged by other cations present in soil solution. This change of one positive ion by another is called cation exchange. For example, when an NH₄⁺ containing fertilizer is added to a soil, many of the numerous NH₄⁺ ions replace the other cations that are already adsorbed to the exchange sites. Cations exchange takes place on the surface of clay and humus colloids as well as on the surface of plant roots. The cations mostly present on the cation exchange sites of the soil colloids are Ca⁺, Mg⁺, H⁺, Na⁺, K⁺ and Al^+.

Cation Exchange Capacity (CEC) is the total exchangeable cations that a soil can hold at a specific pH. It is usually reported in centimoles of charge per kilogram dry soil.

There are number of factors which affect the CEC of soil.

The CEC of a soil changes with the change in pH. As the pH rises, pH dependent CEC also increases. Mostly CEC from humus is pH dependent and up to 10-40% of the soil CEC may be from pH dependent charges. CEC of soil also depends upon amount of clay, type of clay and amount of organic matter.

The CEC of soil generally increases with the increase in clay contents. For 1% clay, a CEC value of 0.5 can be established. For example a soil having 40% clay, the CEC contributed by the clay will be 40×0.5 cmol (+) kg^-1 soil.

The soils having 2:1 type of clays (smectite) have a higher CEC than those having 1:1 type (kaolinite) clays because smectite clays have more isomorphic substitution than do the kaolinite clays.

Soils having a large amount of organic matter have a higher CEC than those having the same amount and type of clay but less organic matter. For every 1% well decomposed organic matter (humus) in soil, a CEC value of 2 can be established. For a soil having 40% clay and 2% organic matter, the CEC of that soil will be approximately equal to 24 cmol (+) kg^-1 soil.

Significance of Cation Exchange Capacity of soil

Cation exchange is important because the cations held on the exchange complex are available to plants. These cations supplement the soil solution. Cation exchange is also important as a mechanism in the purification and alteration of the percolating water. Cation exchange sites adsorb many metals like Cd⁺, Ni⁺, Pb⁺ and Zn²⁺ that might be present in the waste water.

By cation exchange, hydrogen ions from the root surface and microorganisms replace nutrient cations from the soil exchange complex. The replaced nutrient cations can go to the soil solution. Soils with high CEC have less leaching losses of nutrient cations, than the soils with low CEC.

It is important reaction in causing and correcting soil acidity and alkalinity. In sodic soils, the application of gypsum, a source of Ca²⁺, to replace Na⁺ from the clay complex is a good example of the cation exchange in improving physical properties of the soils.