Question:
Which of the following soil horizon of a typical soil profile will have the highest abudance of organic matter?
Which of the following soil horizon of a typical soil profile will have the highest abudance of organic matter?
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Master Soil Horizons from top to bottom:
- O $\rightarrow$ Organic surface litter.
- A $\rightarrow$ Topsoil (maximum humus and biological activity).
- E $\rightarrow$ Zone of eluviation (bleached/leached).
- B $\rightarrow$ Subsoil (zone of illuviation/accumulation).
- C $\rightarrow$ Weathered parent rock.
- R $\rightarrow$ Consolidated bedrock.
- O $\rightarrow$ Organic surface litter.
- A $\rightarrow$ Topsoil (maximum humus and biological activity).
- E $\rightarrow$ Zone of eluviation (bleached/leached).
- B $\rightarrow$ Subsoil (zone of illuviation/accumulation).
- C $\rightarrow$ Weathered parent rock.
- R $\rightarrow$ Consolidated bedrock.
Updated On: Sep 7, 2026
- C horizon
- A horizon
- B horizon
- R horizon
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The Correct Option is B
Solution and Explanation
Concept:
A soil profile is a vertical cross-section of the regolith from the surface down to unweathered bedrock, displaying distinct parallel layers called master horizons (O, A, E, B, C, and R).
These horizons develop over centuries through physical weathering, chemical leaching, and the biological breakdown of plant and animal matter.
Step 1: Structural Differentiation of Soil Horizons:
1. A Horizon (Topsoil): The uppermost mineral horizon, often referred to as the topsoil. It is characterized by an accumulation of humified organic matter thoroughly mixed with fine mineral particles. The decomposition of surface litter by earthworms, fungi, and bacteria enriches this layer with dark, fertile humus, giving it the highest organic matter content among the given options.
2. B Horizon (Subsoil): The zone of illuviation (accumulation), where silicate clays, iron, and aluminium oxides leached from overlying horizons precipitate. It contains significantly less organic matter than the topsoil.
3. C Horizon (Parent Material): A substratum composed of partially disintegrated and weathered parent rock. It is devoid of biological humification and contains negligible organic matter.
4. R Horizon (Bedrock): The bottom layer of consolidated, unweathered solid rock (such as granite, basalt, limestone, or sandstone) with no organic content.
Step 2: Comparison and Conclusion:
Although a dedicated organic O horizon exists in undisturbed forest soils, among the standard mineral horizons listed in the options (A, B, C, and R), the A horizon contains the highest abundance of organic matter.
Final Answer:
The soil horizon with the highest organic matter content among the choices is the A horizon, corresponding to option (B).
A soil profile is a vertical cross-section of the regolith from the surface down to unweathered bedrock, displaying distinct parallel layers called master horizons (O, A, E, B, C, and R).
These horizons develop over centuries through physical weathering, chemical leaching, and the biological breakdown of plant and animal matter.
Step 1: Structural Differentiation of Soil Horizons:
1. A Horizon (Topsoil): The uppermost mineral horizon, often referred to as the topsoil. It is characterized by an accumulation of humified organic matter thoroughly mixed with fine mineral particles. The decomposition of surface litter by earthworms, fungi, and bacteria enriches this layer with dark, fertile humus, giving it the highest organic matter content among the given options.
2. B Horizon (Subsoil): The zone of illuviation (accumulation), where silicate clays, iron, and aluminium oxides leached from overlying horizons precipitate. It contains significantly less organic matter than the topsoil.
3. C Horizon (Parent Material): A substratum composed of partially disintegrated and weathered parent rock. It is devoid of biological humification and contains negligible organic matter.
4. R Horizon (Bedrock): The bottom layer of consolidated, unweathered solid rock (such as granite, basalt, limestone, or sandstone) with no organic content.
Step 2: Comparison and Conclusion:
Although a dedicated organic O horizon exists in undisturbed forest soils, among the standard mineral horizons listed in the options (A, B, C, and R), the A horizon contains the highest abundance of organic matter.
Final Answer:
The soil horizon with the highest organic matter content among the choices is the A horizon, corresponding to option (B).
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