Question

Given below are two statements, one is labeled as Assertion (A), and the other one is labeled as Reason (R).
Assertion (A): Within the same mineralogical composition, soils containing higher humus showed higher CEC.
Reason (R): Humic substances have different types of negatively charged functional groups with huge numbers.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Humus has pH-dependent charges.
As soil pH rises, carboxyl and phenolic groups deprotonate, significantly increasing the net negative charge and CEC of the organic matter.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Cation Exchange Capacity (CEC) represents the total capacity of a soil to hold exchangeable cations.
Both clay minerals and organic matter (humus) provide the negative surface charges responsible for CEC.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion (A): "Within the same mineralogical composition, soils containing higher humus showed higher CEC."
Humus has a exceptionally high cation exchange capacity, typically ranging from \( 150 - 300 \, \text{cmol(p}^+)/kg \), which is significantly higher than that of most clay minerals (e.g., kaolinite is \( 3 - 15 \), montmorillonite is \( 80 - 120 \)).
Therefore, if two soils have the same clay mineral composition, the one with more humus will have a much higher overall CEC.
Thus, Assertion (A) is true.
- Reason (R): "Humic substances have different types of negatively charged functional groups with huge numbers."
Humus consists of complex humic substances (humic acid, fulvic acid, humin) that contain a very high density of oxygen-containing functional groups, primarily carboxyl (-COOH) and phenolic hydroxyl (-OH) groups.
At normal soil pH levels, these groups deprotonate (lose hydrogen ions), leaving a large number of negative charges on the colloid surface:
\[ \text{R-COOH} \rightarrow \text{R-COO}^- + \text{H}^+ \]
These negative charges attract and exchange cations, creating the extremely high CEC of humus.
Therefore, Reason (R) is true and is the direct, correct scientific explanation of Assertion (A).

Step 3: Final Answer:
Both (A) and (R) are true and (R) is the correct explanation of (A).