Which type of soil is dominant in Jaipur, Dausa and Alwar districts ?

Correct — option (4), Alfisols.

All four options are soil orders of the USDA Soil Taxonomy, the system soil surveys use to classify Rajasthan's soils. RPSC's key names Alfisols as the dominant order in Jaipur, Dausa and Alwar.

Alfisols are mature soils with a clay-enriched subsoil. Over a long period, water percolating down carries fine clay out of the surface layer and deposits it lower in the profile. A soil that shows this has moved well beyond the barely formed sands of the west.

Jaipur, Dausa and Alwar lie in the semi-arid north-east of the state. Soil studies of Rajasthan report that the soils of the eastern plain derived from schists and gneisses are dominated by Haplustalfs, a group within the Alfisols.

Tie each soil order to a defining property and a belt of the state. Entisols are the barely formed sands of the west, Vertisols the cracking black clays of Hadoti, and Alfisols the clay-enriched soils of the eastern plain.

Why the others are wrong

• (1)
  Entisol — Entisols are young soils with little or no horizon development, such as wind-blown sand and freshly laid river alluvium.

  In Rajasthan they include the dune sands of the west. The key's answer for Jaipur, Dausa and Alwar is an older soil whose clay has already moved into the subsoil, which is the mark of an Alfisol, not an Entisol.

• (2)
  Inceptisols — Inceptisols are soils at an early stage of horizon development — more developed than Entisols, less than Alfisols.

  Soil studies of Rajasthan report Ustochrepts, a group of Inceptisols, alongside Vertisols in the Chappan plain of the south and on the Hadoti plateau. For Jaipur, Dausa and Alwar, RPSC's key picks the more developed order, Alfisols.

• (3)
  Vertisols — Vertisols are deep, clay-rich soils that swell when wet and crack when dry — the black soils.

  In Rajasthan they are tied to the Hadoti plateau of Kota, Bundi, Baran and Jhalawar, where the parent material comes from Deccan basalts and Vindhyan shales, and to the Chappan plain. Jaipur, Dausa and Alwar lie outside that black-soil belt.

The USDA Soil Taxonomy groups soils into orders by diagnostic layers (horizons) and properties — how far a soil has developed, how dry it is, whether its clay shrinks and cracks — not by colour. Soil surveys classify Rajasthan's soils into five orders: Entisols, Aridisols, Inceptisols, Alfisols and Vertisols.

Entisols show little development. Inceptisols show the first horizons. Aridisols are soils of dry climates. Alfisols have a clay-enriched subsoil. Vertisols are clays that shrink and crack.

In Rajasthan the orders follow climate and parent rock. The sands of the Thar are mostly Entisols and Aridisols. The eastern plain on schists and gneisses carries Haplustalfs, which are Alfisols. The Chappan plain and Hadoti plateau carry Vertisols with Inceptisols.

Key facts

• Rajasthan's soils are classified into five USDA soil orders: Entisols, Aridisols, Inceptisols, Alfisols and Vertisols.
• Alfisols are soils with a clay-enriched subsoil; Haplustalfs are Alfisols of semi-arid to sub-humid climates with a marked dry season.
• Soils on the schists and gneisses of Rajasthan's eastern plain are reported to be dominated by Haplustalfs.
• Vertisols are deep, clay-rich soils that swell when wet and crack when dry; in Rajasthan they occur on the Hadoti plateau.
• Entisols show little or no horizon development and include the wind-blown sands of western Rajasthan.

Common traps

• Soil orders are defined by diagnostic properties, not colour. Entisols are barely formed, Inceptisols are beginning, Alfisols have a clay-rich subsoil and Vertisols are cracking clays.
• "Alluvial soil" in the older groups does not map to one USDA order. Fresh alluvium can be an Entisol, while older alluvial plains can develop into Inceptisols or Alfisols.
• Inceptisols and Alfisols both show horizons. The deciding test is the subsoil: a clear build-up of clay marks an Alfisol.

UnlockIAS compared this question with questions from other RAS Prelims papers and found none similar enough to link.

• practice — not a real PYQ

  The deep, clay-rich soils of the Hadoti plateau of Rajasthan, which swell when wet and develop wide cracks when dry, belong to which soil order ?

  • (a)Aridisols
  • (b)Alfisols
  • (c)Vertisols
  • (d)Entisols
  Answer(3) — Swelling and cracking clay is the defining property of Vertisols, the black soils. Option (1), Aridisols, are soils of dry climates. Option (2), Alfisols, are defined by a clay-enriched subsoil, not by shrink-swell cracking. Option (4), Entisols, show little development at all.
• practice — not a real PYQ

  In the USDA Soil Taxonomy, which soil order shows little or no development of soil horizons, as in wind-blown sand ?

  • (a)Entisols
  • (b)Alfisols
  • (c)Vertisols
  • (d)Inceptisols
  Answer(1) — Entisols are the least developed order. Option (2), Alfisols, have a clay-enriched subsoil. Option (3), Vertisols, are defined by shrinking and cracking clay. Option (4), Inceptisols, already show early horizon development.