Question

Assertion (A): Drainage increases the soil temperature.
Reason (R): Drainage decreases the heat capacity.

• 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:

In assertion-reason questions, ensure that both statements are not only correct but logically connected.

Answer 1

The Correct Option is A

Well-drained soils retain less water, lowering their heat capacity, allowing them to warm up more quickly. Thus, drainage increases soil temperature by reducing heat retention.

Answer 2

Quantitative reasoning:
For a given quantity of absorbed solar heat \( Q \), the rise in soil temperature is given by \[ \Delta T = \frac{Q}{mc} \] where \( m \) is the mass of the soil-water system and \( c \) is its specific heat capacity. Water has a specific heat capacity much larger than dry mineral soil. When a soil is drained, its water content falls, which lowers the effective heat capacity \( C = mc \) of the profile. Since \( \Delta T \) is inversely proportional to \( C \) for the same heat input \( Q \), a lower heat capacity produces a larger temperature rise. This confirms that (R) is true and is indeed the mechanistic reason why (A) is true, so both statements are correct and (R) explains (A).