Question

Given below are two statements
Statement I: Alkali soils turn black when wet because of humic acid fracton of organic matter.
Statement II: A sparingly soluble salt like gypsum is always present in alkali soils.
In light of the above statements, choose the correct answer from the options given below

• A. Both Statement I and Statement II are true
• B. Statement I is true but Statement II is false
• C. Both Statement I and Statement II are false
• D. Statement I is false but Statement II is true

Hint:

Alkali soils are called "black alkali" due to dispersed humic acids at high pH. They are reclaimed by adding gypsum, meaning gypsum is not naturally present in these soils.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Alkali (sodic) soils are characterized by an Exchangeable Sodium Percentage (ESP) greater than \( 15\% \), high pH (typically \( 8.5\text{ to }10 \)), and low concentrations of soluble salts.
Understanding their chemical properties is essential for diagnosis and reclamation.
Detailed Explanation:
Let us analyze both statements:
- Statement I: In alkali soils, the high exchangeable sodium and high pH dissolve and disperse the soil organic matter (specifically the humic and fulvic acid fractions). This dissolved organic matter moves upward via capillary flow and deposits on the soil surface as water evaporates. This forms a dark or black crust, which is why alkali soils are historically referred to as "black alkali." Thus, Statement I is true.
- Statement II: Alkali soils contain high levels of exchangeable sodium and are typically deficient in soluble calcium. Sparingly soluble salts like gypsum (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)) are not naturally present in alkali soils. In fact, adding gypsum is the primary reclamation method for alkali soils, as the calcium ions displace the exchangeable sodium on the clay complex. Thus, Statement II is false.

Step 2: Final Answer:
Statement I is true but Statement II is false, which corresponds to Option (B).