Question

If the electrical conductivity of a saturation extract of the soil is 11 dS/m, what will be the electrical conductivity (dS/m) of drainage water?

• A. 0.11
• B. 1.1
• C. 5.5
• D. 22

Hint:

Salt concentration rises as water content falls after saturation, it does not fall.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We are given the EC of the saturation extract (ECe) of a soil as 11 dS/m. We need to find the EC of the drainage water, which is close to the EC of the soil water left in the profile once free gravitational water has drained out.

Step 2: Key Formula or Approach:
The saturation extract is made by adding enough water to the soil to bring it to a soft, saturated paste. The amount of water used for this paste (the saturation percentage) is roughly twice the amount of water the soil holds once it has drained down.
Since the same mass of dissolved salts is now sitting in about half the water, the salt concentration, and so the EC, roughly doubles.
\[ EC_{drainage} \approx 2 \times EC_e \]

Step 3: Detailed Explanation:
Here \( EC_e = 11 \) dS/m.
\[ EC_{drainage} = 2 \times 11 = 22 \text{ dS/m} \]
The other options come from applying the wrong operation. Dividing by 100 gives 0.11, dividing by 10 gives 1.1, and halving gives 5.5. All three assume the water becomes more dilute as it drains, but the opposite happens. As water content falls after saturation, the same salts are packed into less water, so the EC goes up, not down.

Step 4: Final Answer:
The EC of the drainage water works out to 22 dS/m.
\[ \boxed{22 \text{ dS/m}} \]