What does Cation Exchange Capacity describe?

• A. The soil's capacity to hold and exchange positively charged nutrients (cations) on clay and organic matter.
• B. The soil's capacity to hold water for plant use.
• C. The soil's ability to exchange nutrients of any charge regardless of type.
• D. The rate at which nutrients are mineralized.

Answer: (A)

Cation Exchange Capacity describes how many positively charged ion sites the soil can hold and exchange with nutrients in the soil solution. This ability comes from negative charges on clay minerals and organic matter, which attract and hold cations such as calcium, magnesium, potassium, and ammonium. Plants access these nutrients by displacing the bound cations with ones from the soil solution, so CEC is about capacity to hold and exchange, not the current amount present.

This concept is influenced by soil texture and organic matter content—clay-rich and high-organic-matter soils have higher CEC, while sandy soils have lower CEC. It also matters for fertility management, because soils with higher CEC can hold more nutrient cations and release them as plants need them.

It’s not about how much water the soil can hold, nor about exchanging charged nutrients regardless of their type (that would ignore that CEC specifically concerns cations, while anions have a separate exchange capacity). It also isn’t a measure of how fast organic matter mineralizes nutrients.