Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: In highly acidic soil (pH < 5) most of the aluminium present in aluminium hydroxide cations form or bound by organic matter.
Reason R: Exchangeable acidity in soil mainly contributed by adsorbed hydrogen (H+) and aluminium ions on soil colloids.
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 the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

Soil acidity:
- Active acidity: H+ in soil solution.
- Exchangeable acidity: H+ and Al3+ on colloids.
- Al toxicity: Al3+ in acidic soils (pH < 5).
Liming neutralizes acidity and precipitates Al.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This question tests knowledge of soil acidity and aluminium toxicity.

Step 2: Detailed Explanation:
Assertion A:
In highly acidic soil (pH < 5), most of the aluminium is present in aluminium hydroxide cations (Al(OH)2+, Al(OH)2+) or bound by organic matter.
Aluminium exists in various forms in acidic soils.
At low pH, aluminium is solubilized and can be toxic to plants.
This statement is true.
Reason R:
Exchangeable acidity in soil is mainly contributed by adsorbed hydrogen (H+) and aluminium (Al3+) ions on soil colloids.
These ions displace calcium, magnesium, and potassium.
This contributes to soil acidity.
This statement is true.
Relationship between A and R:
Both statements are true but they describe different aspects.
A describes the form of aluminium in acidic soils.
R describes the contribution to exchangeable acidity.
R does not explain A (aluminium forms in acidic soil).
Thus, R is NOT the correct explanation of A.

Step 3: Final Answer:
Thus, both A and R are correct but R is NOT the correct explanation of A.
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