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Montmorillonite is the type of clay that contributes the most to cation exchange capacity (CEC) when equal amounts are present. This is due to its unique structure and composition. Montmorillonite is a type of smectite clay that has a high surface area and a greater negative charge due to its expansive crystalline structure, allowing it to hold and exchange a larger number of cations (positively charged ions) compared to other clay types.

The high CEC of montmorillonite makes it especially effective in retaining nutrients and enhancing soil fertility, as it can attract and hold essential cations necessary for plant growth, including calcium, magnesium, and potassium. This property is crucial for improving soil health and productivity.

In contrast, while other clay types like illite and kaolinite do contribute to CEC, they do not exhibit the same level of cation retention capacity as montmorillonite. Illite possesses moderate CEC due to its fixed structure, which limits its ability to swell and take up additional nutrients extensively. Kaolinite, on the other hand, has a lower CEC because it consists of more stable alumina and silica layer arrangements, providing less surface area for cation retention compared to montmorillonite.