Question

Soil moisture content decreases the order:
(A). Permanent wilting
(B). Hygroscopic
(C). Field Capacity
(D). Oven dry
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (C), (A), (B), (D).
• C. (B), (A), (D), (C).
• D. (C), (B), (D), (A).

Hint:

Moisture tension and moisture content have an inverse relationship.
As soil moisture tension increases (becomes more negative, e.g., from \( -0.1 \) to \( -10,000 \text{ bar} \)), the soil moisture content decreases.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Soil water is held in soil pores with varying degrees of tension, described as soil moisture tension or matric potential.
As soil dries, the water film around soil particles becomes thinner, and the remaining water is held with higher tension.
The different stages of soil moisture are defined by specific matric potentials and contain varying amounts of water.

Step 2: Detailed Explanation:
Let let us list the four soil moisture stages and their typical energy states:
- (C) Field Capacity: This is the amount of water held in the soil after excess gravitational water has drained away.
It typically occurs at a matric potential of \( -0.1 \text{ to } -0.3 \text{ bar} \) (or \( \text{pF } 2.0 \text{ to } 2.5 \)). This stage represents the highest water content among the four options.
- (A) Permanent Wilting Point: This is the soil moisture content at which plants can no longer extract water, causing them to wilt permanently.
It occurs at a matric potential of \( -15 \text{ bar} \) (or \( \text{pF } 4.2 \)). This stage contains significantly less water than field capacity.
- (B) Hygroscopic Coefficient: This is the water held tightly as a thin film by dry soil particles in equilibrium with a saturated atmosphere.
It occurs at a matric potential of \( -31 \text{ bar} \) (or \( \text{pF } 4.5 \)). This water is non-liquid and unavailable to plants.
- (D) Oven Dry Soil: This stage is achieved by heating soil in an oven at \( 105^\circ\text{C} \) until it reaches a constant weight.
The matric potential is extremely low (approaching \( -10,000 \text{ bar} \) or \( \text{pF } 7.0 \)), and virtually all water is removed. This represents the lowest water content.
Therefore, the decreasing order of soil moisture content is:
Field Capacity (C) \( > \) Permanent Wilting (A) \( > \) Hygroscopic (B) \( > \) Oven Dry (D).
This matches the sequence (C), (A), (B), (D).

Step 3: Final Answer:
The correct decreasing order of soil moisture content is (C), (A), (B), (D).