Step 1: Understanding the Concept:
Think of the salts in the soil as a fixed quantity dissolved in a changing volume of water. At saturation, the soil holds its maximum water, call this volume \(V_1\), and gives an EC of \(EC_1 = 11\) dS/m. As the soil drains down towards field capacity, the water volume drops to about \(V_2 = 0.5 V_1\), because saturation moisture is roughly double the moisture held at field capacity in most agricultural soils.
Step 2: Key Formula or Approach:
Since the mass of salt does not change while water is removed, we can use a dilution style relation: \[ EC_1 \times V_1 = EC_2 \times V_2 \]
Step 3: Detailed Explanation:
Putting \(V_2 = 0.5V_1\):
\[ EC_2 = EC_1 \times \frac{V_1}{V_2} = 11 \times \frac{V_1}{0.5V_1} = 11 \times 2 \]
\[ EC_2 = 22 \text{ dS/m} \]
This is the EC of the water still present in the soil as it drains, so it stands for the drainage water EC.
Step 4: Final Answer:
The drainage water works out to 22 dS/m, so option 4 is correct.
\[ \boxed{22 \text{ dS/m}} \]