How does soil organic matter influence cation exchange capacity (CEC)?

• A. Organic matter has no effect on CEC
• B. Organic matter decreases CEC
• C. Organic matter increases CEC, improving nutrient-holding capacity and buffering soil pH
• D. Organic matter reduces soil buffering capacity

Answer: (C)

Soil organic matter adds negatively charged sites that can attract and hold positively charged nutrients, increasing the cation exchange capacity. Humus contains functional groups like carboxyl and phenolic groups that ionize and provide exchangeable sites as pH varies, boosting the soil’s ability to exchange cations. In addition, organic matter increases the soil’s surface area and forms organo-mineral complexes, further expanding where cations can be held. This combination not only raises CEC but also improves nutrient-holding capacity and helps buffer soil pH, making nutrients more available and less prone to leaching. The other ideas—no effect, a decrease in CEC, or reduced buffering capacity—don’t align with how organic matter contributes exchange sites and buffering through its chemistry and structure.