Question

Soil water matric potential for a wide range of soil wetness can be measured by

• A. Resistance blocks
• B. Tensiometer
• C. Time domain reflectometer
• D. Pressure plate/membrane apparatus

Hint:

To measure matric potential across the entire agricultural moisture range (0 to -15 bar), the Pressure Plate Apparatus is the laboratory standard.
Tensiometers are strictly limited to wet ranges (\( < 0.8 \) bar) because of water column breakage.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil water matric potential (\(\psi_m\)) represents the safety and strength with which water is held by soil clay and organic surfaces.
Measuring matric potential across the entire range of soil moisture (from saturation to permanent wilting point) is essential to construct soil moisture characteristic curves.

Step 2: Detailed Explanation:
Let us analyze the measurement ranges of the listed methods:
- Tensiometer (B): Measures matric potential directly in the field, but it is limited to the wet range, functioning only from \( 0 \) to \( -0.8 \, \text{bar} \) (beyond which cavitation occurs).
- Resistance blocks (A): Work best in the dry range, typically from \( -1.0 \) to \( -15.0 \, \text{bar} \), and are insensitive in wet soils.
- Time Domain Reflectometer (C): Measures volumetric water content directly, not matric potential.
- Pressure plate/membrane apparatus (D): This laboratory instrument is designed to measure soil water matric potential over an extremely wide range of soil wetness.
By applying positive gas pressure to a soil sample placed on a porous ceramic plate, water is forced out until the soil matric suction equals the applied pressure.
This apparatus can measure matric potentials spanning from \( 0 \) to \( -15.0 \, \text{bar} \) (and even up to \( -100 \, \text{bar} \) using specialized membranes), covering the entire range of agricultural soil moisture from saturation to the permanent wilting point.

Step 3: Final Answer:
Soil water matric potential across a wide range of soil wetness is measured using a Pressure plate/membrane apparatus.