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RAS question

Which of the following CORRECTLY describes the typical soil profile of Aridisols in western Rajasthan, from surface to depth?

Correct answer: (B) Thin sandy A-horizon (low OC) - Sandy/loamy B-horizon - Calcic Bk-horizon (Kankar) - C-horizon (parent material).

In western Rajasthan, the typical Aridisol profile runs from a thin, low-organic-carbon sandy A-horizon to a sandy or loamy B-horizon, then a calcic Bk kankar horizon, and finally the C-horizon of parent material.

  1. (A)

    Organic-rich topsoil - Leached sandy horizon - Kankar pan - Parent material

  2. (B)

    Thin sandy A-horizon (low OC) - Sandy/loamy B-horizon - Calcic Bk-horizon (Kankar) - C-horizon (parent material)

  3. (C)

    Clay-rich E-horizon - Cemented hardpan - Deep sandy subsoil - Bedrock

  4. (D)

    Humic A-horizon - Argic B-horizon (clay accumulation) - Gypsic horizon - Bedrock

Explanation

Option B gives the correct surface-to-depth sequence for an Aridisol in western Rajasthan. The profile begins with a thin, light-coloured sandy A-horizon with very low organic carbon, followed by a sandy or loamy B-horizon with slight structural development. Its key diagnostic layer is the Bk or calcic horizon, where calcium carbonate accumulates as kankar at the maximum wetting depth under limited rainfall, before the profile grades into the C-horizon of parent material, often aeolian sand or older alluvium. Kendriya Vidyalaya Sangathan, Study Material: Social Science Class 10, follows the same core desert-soil logic: arid soil is found in western Rajasthan, lacks humus because of dry climate and sparse vegetation, and has kankar in the lower horizon as calcium content increases downward.

Why the other options are wrong

  • (A) It starts with organic-rich topsoil and a leached sandy horizon, but western Rajasthan arid soils are low in organic matter, with kankar lower down rather than a strongly leached surface profile.
  • (C) A clay-rich E-horizon and cemented hardpan do not match the arid profile, which is sandy to loamy near the surface and marked by a lower calcic kankar horizon.
  • (D) A humic A-horizon and argic clay-accumulation B-horizon imply much more organic matter and clay illuviation than the western Rajasthan Aridisol profile permits.

Concept

This tests soil-profile interpretation within Rajasthan physical geography, especially how aridity shapes horizon development. RAS repeats this concept because western Rajasthan soils are a standard link between climate, vegetation, calcium carbonate accumulation and agricultural limitations.

Source

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