Question

Assertion (A): Cations adsorbed on soil colloids determine aggregate formation.
Reason (R): Cations form electropositive links between electronegative soil particles.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Divalent cations improve soil aggregation by neutralizing colloid charges.

Answer 1

The Correct Option is A

Cations (like Ca\textsuperscript{2+}, Mg\textsuperscript{2+}) bridge negative charges on colloids and help flocculate soil particles into aggregates.

Answer 2

Alternate approach — Electrostatic bridging mechanism:
Soil colloids (clay and humus particles) carry a net negative surface charge. When multivalent cations such as \( \text{Ca}^{2+} \) or \( \text{Mg}^{2+} \) are adsorbed on these surfaces, each cation's positive charge can simultaneously neutralise negative sites on two neighbouring colloidal particles, physically bridging them together. This bridging pulls individual clay and humus particles into larger flocs, which then bind with silt and sand to form stable soil aggregates.
Since this cation-bridging mechanism is precisely how adsorbed cations control aggregate formation, Reason (R) correctly and directly explains why Assertion (A) is true, confirming option Both (A) and (R) are correct and (R) is the correct explanation of (A).