Question

Assertion (A): Equilibrium water content in soil at a certain matric potential is higher when the soil is under the drying process (desorption) compared to when the soil is under the wetting process (sorption).
Reason (R): The contact angle between water and the soil solid phase is greater during the imbibition of water than during drainage.

• 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:

Hysteresis causes moisture content at same potential to differ between drying and wetting paths.

Answer 1

The Correct Option is A

This phenomenon is known as hysteresis in soil moisture. It occurs due to differences in contact angle during wetting and drying. During imbibition (wetting), a greater contact angle reduces water retention at same matric potential, validating the assertion and its reason.

Answer 2

Explaining through pore geometry and contact angle: Soil pores are irregular, with narrow necks connecting wider bodies, often called the ink-bottle effect.
During drying (desorption), water already fills the pores and drains out gradually as suction increases, so more water is retained at a given matric potential.
During wetting (sorption), the advancing water front meets a larger contact angle against the still-dry solid surface, which resists entry into the finer pores, so less water is held at the same matric potential.
This difference in contact angle between the wetting and draining fronts is exactly what Reason (R) states, and it correctly explains why desorption retains more water than sorption at the same suction, confirming both (A) and (R) with (R) explaining (A).