Question

Given below are two statements
Statement I: Higher exchangeable sodium ($>$15%) makes the soil dispersed and favours soil crustation
Statement II: Exchangeable calcium in soil helps in forming stable soil aggregates by its effect of flocculation
In light of the above statements, choose the correct answer from the options given below

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Sodium (Na) = Disperses = Destroys Structure (Bad).
Calcium (Ca) = Flocculates = Forms Structure (Good).
Sodium makes soil "soft" and "muddy" when wet, and "rock hard" when dry.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question examines the contrasting effects of monovalent Sodium (\( Na^+ \)) and divalent Calcium (\( Ca^{2+} \)) ions on the physical properties and structural stability of soil.
Detailed Explanation:

• Evaluating Statement I (True):
Sodium is a monovalent ion with a large hydrated radius. When the Exchangeable Sodium Percentage (ESP) exceeds \( 15\% \), the soil is classified as "Sodic."
High sodium concentrations cause clay particles to repel each other and disperse. When these dispersed clay particles dry at the surface, they settle into a hard, impermeable layer known as a "surface crust." This crust blocks water infiltration and seedling emergence.

• Evaluating Statement II (True):
Calcium is a divalent ion (\( Ca^{2+} \)) that can bridge two negatively charged clay surfaces together. This process is called flocculation.
Flocculation is the first step in the formation of stable soil aggregates. By holding particles together, calcium helps maintain a porous, well-structured soil that resists compaction and erosion.

• Application in Reclamation:
This is why Gypsum (\( CaSO_4 \cdot 2H_2O \)) is used to reclaim sodic soils. The calcium in gypsum replaces the dispersed sodium on the exchange complex, leading to flocculation and improved soil structure.

Step 2: Final Answer:
Both statements are true and describe fundamental principles of soil colloid behavior.