Question

The pH of acid sulfate soils.

• A. Less than 5.0
• B. 5.0 - 6.0
• C. 6.0 - 7.0
• D. Greater than 7.0

Hint:

Active acid sulfate soils have an extremely low pH, typically \( \le 4.0 \).
Remember that "catclays" are synonymous with active acid sulfate soils and represent some of the most acidic natural soils on Earth.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Acid sulfate soils are naturally occurring soils containing iron sulfide minerals (primarily pyrite, \(\text{FeS}_2\)).
When these soils are drained, excavated, or exposed to oxygen, the sulfides oxidize to form sulfuric acid (\(\text{H}_2\text{SO}_4\)), causing extreme soil acidity.

Step 2: Detailed Explanation:
In their waterlogged, undisturbed state, acid sulfate soils are called "potential acid sulfate soils" and may have a near-neutral pH.
However, once exposed to oxygen (active acid sulfate soils), the oxidation of pyrite by chemical and microbial processes (mediated by Acidithiobacillus ferrooxidans) generates significant quantities of sulfuric acid:
\[ 2\text{FeS}_2 + 7\text{O}_2 + 2\text{H}_2\text{O} \rightarrow 2\text{Fe}^{2+} + 4\text{SO}_4^{2-} + 4\text{H}^+ \]
This production of hydrogen ions causes the soil pH to drop drastically, typically reaching values less than 4.0 (or even below \( 3.5 \)).
These soils are colloquially referred to as "catclays" and are highly toxic to most agricultural plants due to the high availability of aluminum and iron, which can reach toxic levels.
Among the given choices, the category "Less than 5.0" is the only correct option that represents this extreme acidity.

Step 3: Final Answer:
The pH of active acid sulfate soils is Less than 5.0.