Question

Soils having pH $>$ 8.5, EC $<$ 4 dS/m and ESP $>$ 15, are called :

• A. Acid soils
• B. Saline soils
• C. Sodic soils
• D. Calcareous soils

Hint:

To remember this classification easily:
- Saline = High EC ($> 4$), low ESP ($< 15$).
- Sodic = Low EC ($< 4$), high ESP ($> 15$), high pH ($> 8.5$).
- Gypsum ($CaSO_4 \cdot 2H_2O$) is used to reclaim sodic soils by displacing exchangeable $Na^{+}$ with $Ca^{2+}$.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Salt-affected soils are classified by the United States Salinity Laboratory (USSL) based on three key parameters:
1. Soil reaction (pH)
2. Electrical Conductivity (EC) of the saturated extract, reflecting soluble salt concentration.
3. Exchangeable Sodium Percentage (ESP), reflecting the proportion of exchangeable sodium on the soil colloid complex.

Step 2: Detailed Explanation:
Let us review the standard classification criteria for salt-affected soils:
1. Saline Soils:
- $\text{EC} > 4.0 \text{ dS/m}$
- $\text{ESP} < 15$
- $\text{pH} < 8.5$
2. Sodic (Alkali) Soils:
- $\text{EC} < 4.0 \text{ dS/m}$
- $\text{ESP} > 15$
- $\text{pH} > 8.5$ (often reaching up to 10.0 due to sodium carbonate and bicarbonate hydrolysis)
3. Saline-Sodic Soils:
- $\text{EC} > 4.0 \text{ dS/m}$
- $\text{ESP} > 15$
- $\text{pH} \approx 8.5$
The given parameters ($\text{pH} > 8.5$, $\text{EC} < 4 \text{ dS/m}$, and $\text{ESP} > 15$) match the diagnostic criteria for Sodic soils.
High exchangeable sodium leads to clay dispersion, breaking down the soil structure and causing extremely poor aeration and hydraulic conductivity.

Step 3: Final Answer:
Thus, these soils are classified as Sodic soils.