Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Equilibrium water content in soil at a certain matric potential is higher when the soil is under drying process (desorption) compared to when the soil is under wetting process (sorption).
Reason R: Contact angle between water and the soil solid phase is greater during imbibition of water than during drainage.
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:

Think of a sponge. It's easier to keep a sponge wet than to get it wet from a dry state.
Hysteresis = History matters.
Key factors: Ink-bottle effect, Trapped air, and Contact angle changes.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question explores the phenomenon of "Soil Water Hysteresis"—the fact that the relationship between soil moisture and matric potential depends on the soil's history (whether it is getting wetter or drier).
Detailed Explanation:

• Assertion A Analysis (True):
Hysteresis means "lagging behind." For a given matric potential (e.g., $100$ cm suction), a soil that is drying out will hold more water than a soil that is currently being wetted.
The desorption curve always lies above the sorption curve on a moisture characteristic graph.

• Reason R Analysis (True):
Contact angle hysteresis is one of the four main causes of soil water hysteresis.
The "advancing" contact angle (during wetting/imbibition) is larger than the "receding" contact angle (during drying/drainage).
Since the radius of curvature of the water meniscus depends on the contact angle (\( r = R \cos \alpha \)), the suction required to empty a pore (drying) is different from the suction required to fill it (wetting).

• Other Causes of Hysteresis:
- Ink-bottle effect: Pores have narrow necks and wide bodies.
- Trapped air: Air bubbles prevent some pores from re-wetting fully.
- Swelling/Shrinking: Soil structure changes as moisture changes.

• Logical Link:
Because the contact angle is different during the two processes, the water behaves differently at the same potential. Thus, Reason R is a valid physical explanation for Assertion A.

Step 2: Final Answer:
Both statements are scientifically correct, and the reason (contact angle hysteresis) is one of the primary drivers of the observed moisture hysteresis.