Question

Given below are four soil moisture constants
(A) Field capacity moisture content
(B) Saturation moisture content
(C) Hygroscopic moisture content
(D) Permanent wilting point
Choose the correct combination from the options given below that is used to calculate the total plant available water :

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

Hint:

Remember:
- Upper Limit of Available Water = Field Capacity
- Lower Limit of Available Water = Permanent Wilting Point
Subtracting these two values gives the plant available water.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Plant Available Water (PAW) is the portion of water in the soil that can be easily extracted by plant roots.
Key Formula or Approach:
The plant available water is calculated as: \[ \text{Plant Available Water (PAW)} = \text{Field Capacity (FC)} - \text{Permanent Wilting Point (PWP)} \]

Step 2: Detailed Explanation:
Let us review the soil moisture constants:
- (B) Saturation moisture content: The soil pore space is completely filled with water, but some of this water drains away rapidly due to gravity (gravitational water) and is not available for plants.
- (A) Field capacity moisture content: The amount of water retained in the soil after excess gravitational water has drained away. This represents the upper limit of plant-available water.
- (D) Permanent wilting point: The soil moisture content at which plants can no longer extract water, leading to permanent wilting. This represents the lower limit of plant-available water.
- (C) Hygroscopic moisture content: Water held extremely tightly by soil particles as a thin film, which is completely unavailable to plants.
Thus, the available water is the water held between Field Capacity (A) and Permanent Wilting Point (D).

Step 3: Final Answer:
The correct combination is (A) and (D) only, which corresponds to Option (C).