Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Aluminium toxicity in the soil cause poor root development
Reason (R): Calcium in the gypsum can displace excess Aluminum
In light of the above statements, choose the most appropriate answer from the options given below.

• 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 correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

- Gypsum is highly soluble compared to lime (\(\text{CaCO}_3\)), allowing it to leach deep into the subsoil to alleviate subsoil aluminum toxicity, making it a critical amendment for deep-rooted crops in acid soils.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Soil acidity leads to chemical constraints, particularly aluminum (\(\text{Al}^{3+}\)) toxicity, which severely limits crop productivity in acid soils.
Reclaiming these soils involves applying amendments to displace or neutralize toxic aluminum species.

Step 2: Detailed Explanation:
Let us evaluate both statements and their logical relationship:
- Assertion (A):
In highly acidic soils (\(\text{pH} < 5.0\)), aluminum becomes highly soluble as toxic \(\text{Al}^{3+}\) ions.
These ions interfere with root cell division, disrupt cell wall expansion, and inhibit the uptake of water and essential nutrients (like calcium and phosphorus).
This directly causes stunted, thick, poorly branched root systems (poor root development).
Therefore, Assertion (A) is correct.
- Reason (R):
Gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) is a common amendment used to mitigate subsoil aluminum toxicity.
The calcium (\(\text{Ca}^{2+}\)) supplied by gypsum competes with and physically displaces \(\text{Al}^{3+}\) from soil colloid exchange sites, pushing it into the soil solution where sulphate (\(\text{SO}_4^{2-}\)) binds with it to form non-toxic, leachable complexes like \(\text{AlSO}_4^+\).
Therefore, Reason (R) is correct.
- Relationship between (A) and (R):
While both statements are factually correct, the mechanism of why aluminum toxicity causes poor root development (inhibiting cell division and blocking calcium channels in root tips) is not explained by the fact that gypsum can be used to reclaim it.
Thus, (R) is not the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct but (R) is not the correct explanation of (A) (Option B).