CDS (II) 2020 General Knowledge · Question 11 of 120

Correct — A, Inceptisols. In the American soil taxonomy used for India's national soil survey, Inceptisols are young soils — more developed than the raw Entisols, but only just, with a weak subsurface horizon and little weathering. That description fits an enormous part of India: the older alluvium of the Indo-Gangetic and Brahmaputra plains, the deltaic tracts, and much of the thin, immature soil on hill and plateau slopes where erosion keeps stripping the profile before it can mature. Since the Northern Plains alone are one of India's largest physical divisions and are floored with alluvium, the order that covers young alluvial and immature soils is bound to lead. The soil survey's usual figure is that Inceptisols cover roughly two-fifths of the country, with Entisols next at somewhat over a quarter, Alfisols third and Vertisols smaller again.

• (b)Entisols — A genuine second, and the closest call in the list. Entisols are the newest soils of all — active floodplains and river banks, sand dunes of the Thar, and skeletal soils on steep rock. Large, but not as large as the more settled young alluvium that classifies as Inceptisols.
• (c)Alfisols — The red soils of the peninsula, which have had time to develop a clay-enriched subsoil. Substantial in the south and east but well behind the two young orders.
• (d)Vertisols — The black cotton soils, clay-rich and self-churning, that crack deeply in the dry season. They are confined mainly to the Deccan lava country of Maharashtra, Madhya Pradesh, Gujarat and adjoining areas, so they cover only a small share of India.

Two classifications of Indian soils run side by side. The older Indian Council of Agricultural Research scheme names soils by appearance and origin — alluvial, black, red and yellow, laterite, arid, saline, peaty and forest soils. The soil taxonomy used for scientific mapping instead sorts soils by the horizons actually present in the profile, into orders such as Entisols, Inceptisols, Alfisols, Vertisols, Aridisols, Ultisols and Mollisols. The two systems overlap without matching: alluvial soil of the plains falls mostly into Inceptisols and Entisols, black soil into Vertisols, and red soil largely into Alfisols.

The way into this question is through physical geography rather than through soil chemistry. Ask which kind of surface covers the most of India, and the answer is river-laid alluvium of the great plains plus thin soils on hills and plateaus. Both of those are young, weakly developed profiles, so the leading order has to be one of the two young orders, which removes Alfisols and Vertisols at once. Between Entisols and Inceptisols, the older and more widespread alluvium has had enough time to develop the faint subsurface horizon that defines an Inceptisol, so Inceptisols lead. One honest limitation: the percentage figures that circulate for each order in India trace back to the national soil survey's mapping, and we were not able to open that publication directly, so this card gives the ranking and the reasoning and treats the exact percentages as approximate.

• Inceptisols are young soils with only weak horizon development — more evolved than Entisols, far less than Alfisols.
• In India they cover the older alluvium of the Indo-Gangetic and Brahmaputra plains, the deltas, and much shallow soil on hill and plateau slopes.
• Entisols are the youngest order of all — active floodplains, sand dunes and skeletal soils on steep rock.
• Vertisols are the black cotton soils formed on the Deccan lavas; their high clay content makes them crack deeply when dry and swell when wet.
• Alfisols correspond broadly to the red soils of the peninsula, with a clay-enriched subsurface horizon.

• Choosing Vertisols because black cotton soil is the most talked-about Indian soil. It is regionally famous and nationally small.
• Assuming the most fertile order must be the most extensive. Extent follows geology and river action, not fertility.
• Mixing the two classifications and looking for alluvial soil in a list of taxonomy orders.

As a largest-area or matching item on soil orders, or as a question pairing an Indian soil type with the region and crop it supports.

• practice — not a real PYQ

  Black cotton soil, which cracks deeply in the dry season, has developed mainly on which one of the following parent materials?

  • (a)Granite and gneiss
  • (b)Deccan lava flows
  • (c)Riverine alluvium
  • (d)Wind-blown sand

  Answer(b) Deccan lava flows — the weathering of basaltic lava produces the clay-rich, moisture-retentive black soil of Maharashtra, Madhya Pradesh and adjoining States.

• practice — not a real PYQ

  In the soil taxonomy used for mapping Indian soils, Entisols are best described as which one of the following?

  • (a)The most highly weathered soils
  • (b)Soils showing little or no horizon development
  • (c)Soils with a thick, dark, base-rich surface horizon
  • (d)Soils permanently saturated with salts

  Answer(b) Soils showing little or no horizon development — active floodplains, sand dunes and thin soils on steep rock all fall into this youngest order.