Soil Science Agricultural Chemistry

Step 1: Define the master soil horizons.
Soil scientists classify soil into layers or horizons. The main (master) horizons are designated by letters.

Step 2: List the horizons in order from the surface downward. 1. O horizon (D): The surface layer, rich in organic matter (leaf litter, humus). 2. A horizon (A): Topsoil, a mix of minerals and humus. Zone of leaching. 3. E horizon (not in this specific list, but often present below A): Zone of eluviation (leaching). 4. B horizon (B): Subsoil, zone of accumulation (illuviation) where minerals and clays from above are deposited. 5. C horizon (E): Parent material, weathered bedrock from which the soil is formed. 6. R horizon (C): Bedrock, the unweathered parent rock.

Step 3: Form the sequence based on the given letters. The correct order from top to bottom is D (O horizon), A (A horizon), B (B horizon), E (C horizon), C (R horizon).

Step 1: Analyze statement A.
Pyrite (FeS ) is a sulfide mineral. When exposed to air and water, it oxidizes to form sulfuric acid (H SO ), 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 ) to nitrate (NO ), 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 ). 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.