Question

Given below are two statements, one is labeled as Assertion (A), and the other one labeled as Reason (R).
Assertion (A): Drainage increases the soil temperature.
Reason (R): Drainage decreases the heat capacity.
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:

Draining excess water also reduces evaporative cooling, which helps the soil surface retain more absorbed solar heat.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Soil thermal properties depend on the relative proportions of soil solids, water, and air.
Water has a much higher specific heat capacity than soil minerals and air.

Step 2: Detailed Explanation:
- Assertion (A) is correct: Wet soils are cold soils. Removing excess water via drainage allows the soil to warm up more rapidly.
- Reason (R) is correct: The specific heat capacity of water is $18 \text{ J/g}\cdot\text{K}$, whereas that of dry soil minerals is only about $0.8 \text{ J/g}\cdot\text{K}$.
When soil is drained, water is replaced by air, which has a very low heat capacity.
This exchange reduces the overall volumetric heat capacity of the soil, meaning less solar radiation is required to raise the soil temperature.
Therefore, the decrease in heat capacity directly explains why drainage increases soil temperature.

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