Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Drainage increase 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. Only A is correct
• D. Only R is correct

Hint:

Wet soils are cold soils because water has a high specific heat. Draining soil removes water, decreases its heat capacity, and prevents evaporative cooling, allowing the soil to warm up much faster.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Soil thermal properties are heavily influenced by soil water content.
Water has a much higher specific heat capacity than dry soil minerals.
Removing excess water from the soil profile via artificial drainage alters the soil's thermal properties, allowing it to warm up faster.

Step 2: Detailed Explanation:
Let us analyze both statements:
Reason R Analysis: The specific heat of water is $1.0\text{ cal/g}^\circ\text{C}$, whereas that of dry soil solid minerals is only about $0.2\text{ cal/g}^\circ\text{C}$.
When wet soil is drained, excess water is replaced by air (which has an extremely low heat capacity).
This leads to a significant decrease in the soil's overall volumetric heat capacity. Thus, Reason R is correct.
Assertion A Analysis: Because the heat capacity of the soil decreases after drainage, a much smaller amount of solar energy is required to raise the temperature of a drained soil by $1^\circ\text{C}$ compared to a wet soil.
Furthermore, drainage reduces the amount of water lost through evaporation, which is a highly cooling process (evaporative cooling).
Consequently, drained soils warm up much faster in the spring, increasing the average soil temperature. Thus, Assertion A is correct.
Because the decrease in heat capacity directly explains why the temperature increases, R is the correct explanation of A.

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