Assertion: `Ca^(++)` cannot replace `H^(+)` adsorbed on clay or humus particles.
Reasoning: The retentive capacity of `Ca^(+2)` is more than that of `H^(+)`.

Options

A. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
B. If both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
C. If Assertion is true, but Reason is false.
D. If both Assertion and Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that `Ca^(++)` cannot replace `H^(+)` adsorbed on clay or humus particles. To evaluate this, we need to understand the nature of cation exchange in soil colloids.

2. **Cation Exchange Process**: Cation exchange is a process where cations (positively charged ions) in the soil can be replaced by other cations. This process is reversible and depends on the strength of adsorption of each cation to the soil colloids.

3. **Strength of Adsorption**: The strength of adsorption of cations varies. The order of strength from strongest to weakest is: Aluminum > Hydrogen > Calcium > Magnesium > Potassium > Sodium. This means that Hydrogen ions can be replaced by Calcium ions under certain conditions.

4. **Evaluating the Assertion**: Since the assertion claims that `Ca^(++)` cannot replace `H^(+)`, we find that this is incorrect. In reality, `Ca^(++)` can replace `H^(+)` when conditions allow for it (e.g., when lime is added to acidic soil).

5. **Understanding the Reasoning**: The reasoning states that the retentive capacity of `Ca^(++)` is more than that of `H^(+)`. This refers to the cation exchange capacity (CEC) of the soil, which indicates how well the soil can hold onto cations. It is true that `Ca^(++)` has a higher retentive capacity than `H^(+)`.

6. **Conclusion**: The assertion is false because `Ca^(++)` can replace `H^(+)`. However, the reasoning is true as it correctly describes the retentive capacity of the cations. Therefore, the final answer is that the assertion is false, but the reason is true.

### Final Answer:
- Assertion: False
- Reason: True