Question

Find out the value of pF when there is a tension head of 100 cm of water:

• A. 10
• B. 0.01
• C. 2
• D. 1

Hint:

To quickly find the pF value for any multiple of 10:
Simply count the number of zeros in the water column height in centimeters.
- \( 10 \text{ cm} = 1 \text{ zero} \rightarrow \text{pF } 1 \)
- \( 100 \text{ cm} = 2 \text{ zeros} \rightarrow \text{pF } 2 \)
- \( 1000 \text{ cm} = 3 \text{ zeros} \rightarrow \text{pF } 3 \)

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Soil water tension (or suction) can be expressed as the height of a water column in centimeters.
Because soil moisture tension varies across several orders of magnitude (from 1 cm at saturation to 10 million cm at oven dryness), Schofield (1935) introduced the "pF" scale.
The pF scale simplifies these values by expressing the suction head on a logarithmic scale.
Key Formula or Approach:
The pF value is calculated using the following formula:
\[ \text{pF} = \log_{10} (h) \]
where:
- \( h \) is the soil moisture tension head expressed in centimeters of water.

Step 2: Detailed Explanation:
In this problem, the soil water tension head is given as:
\[ h = 100 \text{ cm} \]
Substituting this value into the pF formula:
\[ \text{pF} = \log_{10} (100) \]
Since \( 100 = 10^2 \):
\[ \text{pF} = \log_{10} (10^2) \]
\[ \text{pF} = 2 \]
Thus, a soil moisture tension head of 100 cm of water corresponds to a pF value of 2.
This tension represents a relatively wet soil, near field capacity (which typically occurs around pF 2.5).

Step 3: Final Answer:
The value of pF is 2.