Question

Direct method for the determination of soil moisture is

• A. Gravimetric method
• B. Gypsum block method
• C. Tensiometer method
• D. Pressure plate apparatus method

Hint:

Direct = Weighing the water (Gravimetric).
Indirect = Measuring a proxy (Tension, Resistance, Scattering).
Formula for Gravimetric moisture ($\%$):
\[ \frac{\text{Wet Weight} - \text{Dry Weight}}{\text{Dry Weight}} \times 100 \]

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify which method directly measures the amount of water in the soil, rather than measuring a property that relates to moisture (like tension or electrical resistance).
Detailed Explanation:

• Gravimetric Method (Direct):
This is the "standard" or "reference" method. It involves taking a physical soil sample, weighing it (wet weight), drying it in an oven at $105^{\circ}$C for 24 hours until constant weight, and weighing it again (dry weight).
The difference in weight is the actual mass of water lost.
It is direct because we are weighing the water itself.

• Indirect Methods:
Tensiometer: Measures the suction or matric potential of water. We then use a calibration curve to estimate moisture.
Gypsum Block: Measures electrical resistance, which changes with soil moisture content.
Pressure Plate: Measures the moisture content at a specifically applied suction to create a pF curve.

• Advantages and Disadvantages of Gravimetric Method:
Pro: Highly accurate and does not require complex calibration. It is used to calibrate all other indirect instruments.
Con: It is destructive (you must dig up a sample), labor-intensive, and time-consuming (takes 24 hours). You cannot monitor the same spot continuously.

Step 2: Final Answer:
The gravimetric method is the only direct method for determining soil moisture by mass.