Question

At what value of sodium adsorption ratio, a soil is considered as Alkali soil :

• A. 11 meq \(\text{l}^{-1}\)
• B. 12 meq \(\text{l}^{-1}\)
• C. 13 meq \(\text{l}^{-1}\)
• D. 14 meq \(\text{l}^{-1}\)

Hint:

Sodic/Alkali soils can be reclaimed by applying chemical amendments like Gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) to displace the toxic exchangeable sodium (\(\text{Na}^+\)) with calcium (\(\text{Ca}^{2+}\)), followed by leaching.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Alkali (sodic) soils are characterized by an excessive amount of exchangeable sodium relative to other cations like calcium and magnesium.
This high sodium level causes clay dispersion, leading to poor soil structure, reduced aeration, and a high pH (typically > 8.5).
The Sodium Adsorption Ratio (SAR) is a key diagnostic parameter used to classify these soils.

Step 2: Detailed Explanation:
The Sodium Adsorption Ratio (SAR) of the soil saturation extract is mathematically defined as:
\[ \text{SAR} = \frac{[\text{Na}^+]}{\sqrt{\frac{[\text{Ca}^{2+}] + [\text{Mg}^{2+}]}{2}}} \]
where the concentrations of the cations are expressed in milliequivalents per liter (meq/l).
According to the classification established by the United States Salinity Laboratory (USSL), a soil is classified as sodic (alkali) when:
1. The Sodium Adsorption Ratio (SAR) is equal to or greater than 13 meq \(\text{l}^{-1}\).
2. The Exchangeable Sodium Percentage (ESP) is equal to or greater than 15%.
3. The Electrical Conductivity (\(\text{EC}_e\)) is less than 4 dS/m.
At an SAR value of 13 or above, the excessive sodium causes the swelling and dispersion of clay particles, which clogs soil pores, seals the surface, and severely restricts water infiltration and root growth.

Step 3: Final Answer:
A soil is considered as Alkali soil at a sodium adsorption ratio value of 13 meq \(\text{l}^{-1}\) (Option C).