Which soil property describes the capacity to hold and exchange positively charged ions?

• A. Cation exchange capacity
• B. Anion exchange capacity
• C. Capillary rise
• D. Permeability

Answer: (A)

Cation exchange capacity describes the soil’s ability to hold and exchange positively charged ions. Soil particles, especially clay minerals and organic matter, carry negative charges on their surfaces. These negative sites attract and hold positively charged nutrient ions such as calcium, magnesium, potassium, and ammonium. The total number of exchangeable cations a soil can hold is the cation exchange capacity, usually expressed in cmol(+)/kg. Soils with high CEC have more sites to retain nutrients and can supply them gradually to plant roots, helping prevent leaching and improving fertility. This capacity depends on the amount and type of clay minerals and the amount of organic matter: clays like smectite or vermiculite and high organic matter boost CEC, while sandy soils with little clay or OM have lower CEC. Anion exchange capacity, capillary rise, and permeability describe other soil properties—anions exchange, water movement through capillary action, and how easily water or air move through soil, respectively—and do not capture the soil’s ability to hold and exchange positively charged nutrients.