Question

The most dominant exchangeable cation in the earth's crust

• A. Aluminium
• B. Calcium
• C. Sodium
• D. Magnesium

Hint:

Among basic cations in soil, Ca$^{2+}$ is usually present in the highest quantity.

Answer 1

The Correct Option is B

Calcium is the most dominant exchangeable cation in soils due to its abundance in minerals and its strong ability to hold onto cation exchange sites.

Answer 2

Comparative reasoning: On the exchange complex of most agricultural soils, the typical order of cation dominance is \( \text{Ca}^{2+} > \text{Mg}^{2+} > \text{K}^{+} > \text{Na}^{+} \), because calcium-bearing minerals such as calcite, apatite, and calcic feldspars are abundant in the earth's crust and weather to release large amounts of \( \text{Ca}^{2+} \).
Its divalent charge and moderate hydrated radius also let it bind exchange sites more strongly than the monovalent cations sodium and potassium, following the general lyotropic series of adsorption affinity.
Aluminium becomes a dominant exchangeable cation only in strongly acidic, highly weathered soils, a special rather than general case, so across soils broadly calcium remains the most abundant exchangeable cation.