Question

The horizons of the soil profile are given below. Arrange the stages in chronological order.
(A) A1-A2-Bt-C-Bt-C
(B) A1-A2-Bhir-Bir-C
(C) A-B-C
(D) Ap-Bhir-Bir-C
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (C), (A), (B), (D).
• C. (B), (A), (D), (C).
• D. (C), (B), (D), (A).

Hint:

Start with the simplest, least-weathered profile: the three-horizon sequence A-B-C (C) is always the youngest.
This immediately limits the correct options to those starting with (C).
A profile containing a plowed topsoil (Ap) must follow the undisturbed version (B).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil profile development (soil ontogeny) is a chronological weathering process where distinct master horizons develop over time from parent material under the influence of climate, organisms, topography, and time.

Step 2: Detailed Explanation:
Let us analyze the developmental maturity of the listed soil profiles to arrange them in chronological order:
- Stage 1: Simple profile (C): A young, weakly weathered soil profile characterized by a basic master horizon sequence, namely A-B-C.
This represents the earliest stage of soil genesis.
- Stage 2: Differentiated forest soil (B): Over time, under forest vegetation and humid climates, leaching and illuviation lead to distinct horizons: A1-A2-Bhir-Bir-C.
The "h" and "ir" suffixes represent the accumulation of illuviated organic matter (humus) and iron-aluminum sesquioxides, respectively.
This is a mature, undisturbed Spodosol-like profile.
- Stage 3: Cultivated forest soil (D): When the undisturbed soil (B) is brought under agricultural cultivation, tillage mixes the upper organic and eluvial layers into an arable plow layer, denoted as Ap.
The resulting profile is Ap-Bhir-Bir-C, representing a subsequent anthropogenic stage.
- Stage 4: Highly developed, complex weathered profile (A): Over geological timescales, intensive weathering leads to deep clay translocation, forming an argillic horizon (Bt).
The presence of repeating sequences like A1-A2-Bt-C-Bt-C indicates a highly advanced, ancient profile with lithological discontinuities or paleosol development.
Therefore, the chronological developmental sequence is (C) \( \rightarrow \) (B) \( \rightarrow \) (D) \( \rightarrow \) (A).

Step 3: Final Answer:
The chronological sequence of soil profile development is (C), (B), (D), (A).