Question

Pressure plate apparatus is used for measurement of soil moisture tension upto

• A. 4.2 bars
• B. 0.33 bar
• C. 30.0 bars
• D. 15.0 bars

Hint:

Tensiometer = $0$ to $0.8$ bars (Wet range).
Pressure Plate = $0$ to $15$ bars (Full available range).
Remember 15 bars = PWP = The point of no return for most plants.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the maximum soil moisture tension (suction) value that a standard pressure plate apparatus is typically used to measure in soil physics laboratories.
Detailed Explanation:

• What is Pressure Plate Apparatus?
It is a laboratory instrument used to determine the soil moisture characteristic curve (pF curve).
Saturated soil samples are placed on a porous ceramic plate inside a pressure chamber.
Compressed air is applied to the chamber. This air pressure forces water out of the soil pores through the plate until the soil moisture tension equals the applied air pressure.

• Measurement Range:
While tensiometers work only in the low suction range ($0$ to $0.85$ bars), pressure plate extractors are designed for higher tensions.
The most common use of the pressure plate is to find the moisture content at:
Field Capacity: $0.33$ bar suction.
Permanent Wilting Point (PWP): $15.0$ bars suction.

• Significance of 15 Bars:
The Permanent Wilting Point is the lower limit of "Available Water" for most agricultural crops.
Standard pressure plate extractors are built to withstand and accurately maintain pressures up to 15 bars because that covers the entire biologically relevant range for plant growth.

• Relationship to pF:
A tension of 15 bars corresponds to a pF value of approximately 4.2 ($log_{10} 15000$ cm water).

Step 2: Final Answer:
The pressure plate apparatus is standardly used to measure soil moisture tension up to 15.0 bars, corresponding to the permanent wilting point.