Question

Soil acidity can be introduced by \(\underline{\hspace{1cm}}\).
A. Mine tailings rich in pyrites
B. Extraction of Al from its ore
C. Use of nitrogen-rich fertilizers
D. Use of limestone

• A. A, B and D only
• B. A and C only
• C. A, B and C only
• D. A, B, C and D

Hint:

To remember causes of soil acidity, think: - Acid Rain: From industrial pollutants (SOx, NOx). - Mining: Oxidation of sulfides like pyrite. - Fertilizers: Nitrification of ammonium. Limestone (lime) is the cure, not the cause.

Answer 1

The Correct Option is B

Step 1: Analyze statement A.
Pyrite (FeS\(_2\)) is a sulfide mineral. When exposed to air and water, it oxidizes to form sulfuric acid (H\(_2\)SO\(_4\)), a strong acid. This process, known as acid mine drainage, drastically increases soil and water acidity. Statement A is a cause of acidity.

Step 2: Analyze statement B.
The primary process for extracting aluminum (Bayer process) involves digesting bauxite ore in a hot solution of sodium hydroxide (a strong base). The waste product, red mud, is highly alkaline, not acidic. Statement B is not a cause of acidity.

Step 3: Analyze statement C.
Ammonium-based nitrogen fertilizers can cause soil acidity through nitrification. Soil bacteria convert ammonium (NH\(_4^+\)) to nitrate (NO\(_3^-\)), releasing hydrogen ions (H\(^+\)) in the process, which lowers the soil pH. Statement C is a cause of acidity.

Step 4: Analyze statement D.
Limestone is calcium carbonate (CaCO\(_3\)). It is an alkaline material used to neutralize soil acidity by reacting with H\(^+\) ions. It is a treatment for acidity, not a cause. Statement D is not a cause of acidity. Conclusion: A and C are causes of soil acidity.