Which of the following clays contributes most to CEC if the same amount of each clay is present?

Montmorillonite is the clay that contributes the most to cation exchange capacity (CEC) among the options presented. This is primarily due to its structure and properties. Montmorillonite is part of the smectite group of clays, which have a high surface area and a layered structure that allows for significant interlayer spacing. This characteristic enables montmorillonite to adsorb and hold a large quantity of cations, such as potassium, calcium, and magnesium, thereby increasing soil fertility and nutrient availability for plants.

The high cation exchange capacity of montmorillonite is also influenced by its ability to swell when it takes up water, further enhancing its ability to retain nutrients in the soil. As a result, soils with a higher proportion of montmorillonite generally exhibit better nutritional profiles due to its superior CEC compared to other clay types present in the options.

In contrast, kaolinite and illite, while they also contribute to CEC, possess lower cation exchange capacities due to their more stable structural characteristics and less surface area. Sandy soils, which are predominantly composed of sand particles, have minimal CEC because sand does not retain water or nutrients effectively, failing to contribute significantly to CEC compared to clays.