CDS (I) 2020 General Knowledge · Question 69 of 120

Correct — B, Histosol. Histosols are the organic soils of the USDA soil taxonomy — soils made up mainly of organic material rather than of mineral grains. The formal test is a thickness one: 40 cm or more of organic soil material beginning within 40 cm of the surface, with an organic carbon content of 12 per cent or more by weight under the USDA scheme. They form where water stands long enough to keep oxygen out, so dead plant matter accumulates faster than decay can remove it — bogs, fens, marshes and waterlogged uplands. The three grades of that material give the everyday names: peat where the plant tissue is still recognisable, mucky peat where it is half broken down, and muck where decomposition has gone furthest. None of the other three orders is defined by organic content: they are mineral soils named for a clay behaviour, a frozen layer and a subsurface accumulation.

• (a)Vertisol — A mineral soil defined by clay behaviour, not by organic matter. Vertisols are rich in swelling clays, so they crack deeply when dry and close up when wet — India's black cotton soil over the Deccan Trap is the familiar example.
• (c)Gelisol — Defined by permafrost within two metres of the surface, not by organic content. Some Gelisols are indeed organic, but the diagnostic feature is the frozen layer, and in the taxonomy Gelisols take precedence over Histosols where both apply.
• (d)Spodosol — A mineral soil of cool, moist coniferous country, defined by a bleached, leached upper horizon over a subsurface layer where iron, aluminium and humus have accumulated. Organic matter concentrates in a horizon here; it does not make up the soil.

The USDA soil taxonomy divides soils into twelve orders, each defined by a diagnostic property rather than by a region. Histosols are the organic order; Gelisols the permafrost order; Vertisols the shrink-swell clay order; Spodosols the order with a leached upper horizon over an accumulation of iron, aluminium and humus. Naming the diagnostic property is usually enough to answer any question about which order fits a description.

The stem gives one property — very high organic matter — so the task is to recall which order that property defines. The Greek and Latin roots help more than rote memory: histos for tissue in Histosol, gelare to freeze in Gelisol, vertere to turn in Vertisol (the churning of a cracking clay). Gelisol is the trap that catches careful candidates, because tundra peat really is organic-rich; the point is that its defining feature is the permafrost, and the taxonomy classifies it as a Gelisol for that reason. Histosols matter well beyond the exam: they are a very large store of soil carbon, and draining them for cultivation releases it.

• A Histosol is defined by 40 cm or more of organic soil material starting within 40 cm of the surface, with 12 per cent or more organic carbon by weight in the USDA scheme.
• Its three material grades are peat (fibric, slightly decomposed), mucky peat (hemic) and muck (sapric, highly decomposed).
• Histosols form under waterlogging, where the lack of oxygen slows decay below the rate at which plant matter accumulates.
• Vertisols are shrink-swell clay soils — India's black cotton soil formed on the Deccan basalt is the standard example.
• Gelisols are defined by permafrost within two metres of the surface and take precedence over Histosols where both criteria are met.

• Choosing Gelisol because tundra soils are peaty — the permafrost, not the peat, is what defines that order.
• Confusing a soil that accumulates humus in one horizon (Spodosol) with a soil made of organic material throughout (Histosol).
• Assuming Vertisols are organic because they are dark; their colour comes from the clay minerals of the parent basalt.

Either as a property-to-order identification like this one, or as a matching set pairing soil orders with their salient characteristics.

• practice — not a real PYQ

  Which one of the following soil orders is defined by the presence of permafrost within two metres of the surface?

  • (a)Histosol
  • (b)Gelisol
  • (c)Aridisol
  • (d)Oxisol

  Answer(b) Gelisol — the frozen layer is the diagnostic feature, even where the soil above it is rich in organic matter.

• practice — not a real PYQ

  The black cotton soil of peninsular India belongs to which one of the following soil orders?

  • (a)Spodosol
  • (b)Histosol
  • (c)Vertisol
  • (d)Ultisol

  Answer(c) Vertisol — a shrink-swell clay soil that cracks deeply on drying, developed on the basalt of the Deccan Trap.