Question

The characteristics of the Plinthite horizon are
(A) It is rich in humus
(B) It is rich in sesquioxide
(C) Presence of yellowish or greyish mottles
(D) Hardens reversibly due to repeated wetting and drying
Choose the correct answer from the options given below :

• (A) and (D) only.
• (B) and (C) only.
• (A) and (C) only.
• (A) and (B) only.

Hint:

Remember the critical distinction: Plinthite hardens irreversibly into ironstone upon exposure and drying. Because statement (D) says "reversibly," it must be eliminated, leaving (B) and (C) as the correct choice.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Plinthite is a diagnostic soil material commonly found in highly weathered tropical soils (primarily Ultisols and Alfisols).
It represents a mineral mixture of clay, silica, and metallic oxides that undergoes physical transformation upon exposure to environmental factors.

Step 3: Detailed Explanation:
Let us analyze each statement to determine the correct characteristics of plinthite:
- Statement (A) is incorrect: Plinthite is extremely poor in organic matter and humus due to rapid microbial mineralization in warm, humid tropical climates.
- Statement (B) is correct: It is a highly weathered, iron-rich, and aluminum-rich mineral mixture, meaning it is rich in sesquioxides ($\text{Fe}_2\text{O}_3$ and $\text{Al}_2\text{O}_3$).
- Statement (C) is correct: Plinthite usually occurs as red, yellowish, or greyish mottles arranged in a segregating pattern due to fluctuating water tables and redox processes in the soil profile.
- Statement (D) is incorrect: A key defining characteristic of plinthite is that it hardens irreversibly (not reversibly) into ironstone, laterite, or hardpans when exposed to repeated wetting and drying cycles.
Therefore, only statements (B) and (C) are correct.

Step 4: Final Answer:
The characteristics of the Plinthite horizon are (B) and (C) only.