Cation exchange capacity measures the ability of the soil to attract, hold onto, and exchange positively charged particles. These positively charged particles are called cations, as opposed to negatively charged particles that are called anions.
Clayey and organic matter-rich soils tend to be negatively charged. Their negative charges attract and hold the oppositely, positively charged cations. Consequently, clayey and organic matter-rich soils generate higher cation exchange capacities. Their higher capacities are indicative of their fertility. For the strong attraction of the oppositely charged particles resists the loss of minerals through leaching, or being washed away in soil runoff.
At the same time, the higher cation exchange capacities of clayey and organic matter-rich soils are accompanied by higher buffering capacities. For the strong attraction of the oppositely charged particles likewise resists changes in soil pH, which may be acidic, neutral or alkaline.