If present in soils in equal masses, which of the following, when increased, will increase the soil cation-exchange capacity the most?

The correct choice relates to the role of particle size and charge in soil cation-exchange capacity (CEC). CEC refers to the ability of soil to hold and exchange cations, which are positively charged ions that plants need for growth.

Small, negatively charged particles, such as clay or organic matter, have a high surface area relative to their mass. This increased surface area allows more sites for cation binding, effectively enhancing the soil's capacity to retain and exchange nutrients. The negative charge of these particles attracts and holds onto positively charged cations, improving the overall nutrient availability in the soil.

In contrast, large particles, whether positively or negatively charged, do not provide the same level of surface area and potential for cation adsorption. Large positively charged particles would not effectively hold cations due to their size and lower surface area. Similarly, large negatively charged particles may attract some cations, but the extent of cation-exchange capacity remains limited compared to their smaller counterparts. The efficiency of small, negatively charged particles in enhancing cation-exchange capacity is paramount for nutrient retention in soils.