Question

The most dominant exchangeable cation in the earth's crust

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

Hint:

Under normal fertile soil conditions, the typical proportion of exchangeable cations follows the order: \(\text{Ca}^{2+} \gt \text{Mg}^{2+} \gt \text{K}^+ \approx \text{Na}^+\).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The cation exchange complex of soils holds positively charged ions (cations) on clay and organic colloid surfaces. The distribution of these exchangeable cations depends on mineral weathering and soil pH.

Step 2: Detailed Explanation:
Although elements like oxygen, silicon, and aluminum are chemically the most abundant elements in the bulk lithosphere, when we analyze the "exchangeable" ionic fraction in neutral-to-alkaline soils, Calcium (\(\text{Ca}^{2+}\)) is overwhelmingly the most dominant exchangeable cation.
It typically occupies \(60\%\) to \(80\%\) of the total cation exchange capacity (CEC) in productive soils, followed by magnesium (\(\text{Mg}^{2+}\)), potassium (\(\text{K}^+\)), and sodium (\(\text{Na}^+\)).
In highly acidic soils, exchangeable aluminum (\(\text{Al}^{3+}\)) and hydrogen (\(\text{H}^+\)) dominate. However, on a global agricultural scale, calcium remains the most dominant exchangeable cation.

Step 3: Final Answer:
Hence, Calcium is the most dominant exchangeable cation.