Which soil would MOST likely have the highest CEC?

• A. Clay-rich soil with organic matter
• B. Loamy soil without organic matter
• C. Sandy soil
• D. Gravelly soil

Answer: (A)

Cation exchange capacity is a soil’s ability to hold and exchange positively charged nutrients on the surfaces of soil particles. This capacity is highest when there is a lot of surface area and a strong negative charge on the surfaces, which is found in clay minerals and in organic matter. Clay minerals provide many tiny surfaces with permanent negative charges, and organic matter brings a large amount of charge through its functional groups and high surface area. A soil that is rich in both clay and organic matter has many sites to hold exchangeable cations like calcium, magnesium, and potassium, so it shows the highest CEC.

In contrast, a loamy soil without organic matter has some surface area and charge from its mineral components but lacks the additional charge contributed by organic matter, so its CEC is lower. Sandy soils have relatively little surface area and few sites for exchange, resulting in low CEC. Gravelly soils are even coarser with minimal surface area and charge, giving they very low CEC. This is why soils with substantial clay and organic matter retain nutrients better and buffer pH changes more effectively.