Cation Exchange in Soils

Cation Exchange in Soils

	Cation Exchange in Soils The soil humus consists of a negatively charged anion called a micelle. Each negative charge on the humic micelle attracts a univalent cation. The humic micelle is made up of carbon, hydrogen and oxygen. Sources of negative charges on the humic micelle are a partially dissociated carboxylic group (-COOH), enolic hydroxyl group (-OH) and phenolic hydroxyl groups (-OH). The negative charge on a humic micelle depends on the pH. When the pH increases, more H+ ions dissociate from the groups.
Consequently the negative charge on the humic micelle increases. Clay matter of the soil is a mixture of sand and humus. There are cations surrounding the clay micelle which are called exchangeable cations because they can be replaced by other cations. When ammonium sulphate is added to the soil, the ammonium ions gradually replace other cations especially calcium ions. This process is reversible and is called cation exchange.
Cation exchange capacity (CEC) is the total number of negative charges per unit weight of the soil. In other words, it is the total number of cation adsorption sites per unit weight of soils. The factors affecting cation exchange capacity include the following: Soil texture The CEC of soils increases when their percentage of clay increases i.e. when their texture becomes finer.
Soil humus content Since a negatively charged humic micelle attracts cations and holds them, the CEC increases when the percentage of humus increases. Nature of clay The CEC depends on the nature of clay minerals present in it since each mineral has its own capacity to exchange and hold cations. The CEC of a soil dominated by vermiculite is much higher than the CEC of another soil dominated by kaolinite. Soil reaction When the pH of soil increases, more H + ions dissociate from the clay minerals especially kaolinite, thereby the CEC of soil dominated by kaolinite also increases.

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