After reading this article you will learn about the cation exchange capacity (C.E.C) of soil and factors affecting it.
The total number of cation adsorption sites per unit weight of soils is called the cation exchange capacity of soils. The cation exchange capacity of the soil may also be defined as the total number of negative charges per unit weight of the soil. It is expressed in Milliequivalents per 100gms of soil.
Factors Affecting Cation Exchange Capacity of Soils:
(i) Soil Texture:
Since the negatively charged clay micelle attracts positively charged cations, and holds them, the cation exchange capacity of soils increases when their percentage of clay increases i.e. their texture becomes finer.
(ii) Soil Humus Content:
Since a negatively charged humic micelle attracts cations and holds them, the cation exchange capacity of soils increases when their percentage of humus increases. The cation exchange capacity of humus varies from 200 to 400 mill equivalent per 100gms. So C.E.C. of soils also depends on the nature of humus they contain.
(iii) Nature of Clay:
The cation exchange capacity of soil also depends on the nature of clay minerals which constitute them.
The cation exchange capacities of different clay minerals are as follows:
Hence the cation exchange capacity of a soil dominated by Montmorillonite or vermiculite is much higher than the cation exchange capacity of another soil dominated by Illite or Kaolinite.
(iv) Soil Reaction:
When the pH of soils increases, more hydrogen ions dissociate from the hydroxyl group located on the broken edge of clay minerals especially kaolinite exposing more hydroxyl groups. Hence the cation exchange capacity of soils dominated especially by Kaolinite increased when the pH of the soil increased.