Which soil component is most associated with high cation exchange capacity?

• A. Quartz
• B. Clay minerals
• C. Fine sand
• D. Calcium carbonate

Answer: (B)

Cation exchange capacity is the soil’s ability to hold and exchange positively charged ions on the surfaces of soil particles. This capacity mainly comes from the negative charges on those surfaces. Clay minerals provide the most of these charges because their crystal structures have permanent negative charges from isomorphic substitution, especially in the common 2:1 clays like smectite and illite. The large surface area of clay minerals gives many sites for cations to occupy and swap.

In contrast, quartz and fine sand are largely inert silica with very little surface charge and thus contribute little to CEC. Calcium carbonate grains have some surface interactions, but they offer far fewer exchange sites compared with clays, so their impact on CEC is much smaller. Organic matter can also contribute a high CEC, but it isn’t part of the choices here.

So, soils rich in clay minerals have the highest CEC, making clay minerals the best answer.