CDS (I) 2021 General Knowledge · Question 21 of 120

Correct — D, Black soil. All three clues are the school text's own sentences about black soil, in its own order. On composition it says that chemically the black soils are rich in lime, iron, magnesia and alumina; on texture, that the black soils are generally clayey, deep and impermeable; on distribution, that black soil covers most of the Deccan Plateau, including parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and some parts of Tamil Nadu. Each clue also has a physical explanation, which is the better way to hold it. The soil is formed by the weathering of the Deccan basaltic lava, and basalt is a lime-, iron- and magnesium-rich rock, so the chemistry is inherited from the parent material. The clay content gives it the behaviour it is famous for: it swells and becomes sticky when wet and shrinks and cracks widely when dry, so a kind of self-ploughing occurs, and because it absorbs and loses moisture slowly it holds water long into the dry season — which is why rain-fed cotton does so well on it that it is also called black cotton soil, or regur.

• (a)Laterite soil — Fails on composition and on distribution. Laterite forms under high temperature and high rainfall, where intense leaching carries lime and silica away and leaves iron oxide and aluminium compounds behind — so it is poor in lime, which the first clue requires in abundance. It is also poor in organic matter, nitrogen, phosphate and calcium, and it occurs mainly in Karnataka, Kerala, Tamil Nadu, Madhya Pradesh and the hills of Odisha and Assam.
• (b)Red and yellow soil — Wrong parent rock and wrong texture. Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern Deccan; its colour comes from the wide diffusion of iron in crystalline and metamorphic rocks, and it looks yellow in hydrated form. The fine-grained variety is fertile, but coarse-grained upland versions are poor, and the type is generally low in nitrogen, phosphorous and humus — not the deep, clayey, impermeable profile described.
• (c)Saline soil — A soil defined by salts rather than by basalt chemistry. Also called usara, saline soils hold a larger proportion of sodium, potassium and magnesium, are infertile and support little vegetation, and occur in arid and semi-arid tracts and in waterlogged and swampy areas — widely in western Gujarat, the eastern coastal deltas and the Sundarbans. They lack nitrogen and calcium, which contradicts the first clue.

The great soil groups of India are distinguished by parent rock, climate and the processes that move material within the profile. Black soil is the weathering product of the Deccan traps and takes its properties from basalt. Laterite is a leaching product of hot, wet climates. Red and yellow soils form on crystalline rocks where rainfall is moderate. Saline soils are an accumulation product of dry climates and poor drainage. Recognising which process made a soil is usually faster than recalling a list of states.

The item gives three independent clues, which means a candidate does not need all of them — any one, tested properly, eliminates three options. The chemistry clue alone is decisive, because lime abundance rules out laterite and saline soils immediately. Note that the paper's list of states is shorter than the textbook's, naming Maharashtra, Madhya Pradesh and Gujarat and stopping there; the same passage also includes Andhra Pradesh and parts of Tamil Nadu, and the northern part of the belt runs into the Malwa plateau, so a short list here is not an error but an abbreviation. The practical importance of this soil is the crop it carries: cotton, and with it much of India's cotton belt in Maharashtra, Gujarat and Madhya Pradesh.

• Chemically the black soils are rich in lime, iron, magnesia and alumina, and also contain potash, but lack phosphorous, nitrogen and organic matter.
• They are generally clayey, deep and impermeable, and their colour ranges from deep black to grey.
• They swell and become sticky when wet and develop wide cracks when dry, producing a kind of self-ploughing, and they retain moisture for a long time.
• They cover most of the Deccan Plateau, formed from basaltic lava flows, and are deepest in the upper reaches of the Godavari and Krishna and in the north-western Deccan.
• They are also called regur soil or black cotton soil, and are the mainstay of rain-fed cotton cultivation.

• Choosing laterite because both soils are associated with the peninsula; laterite is leached of lime, which the first clue rules out.
• Reading the shortened list of three states as evidence against black soil, when the textbook list is longer.
• Assuming a dark, fertile-looking soil must be rich in nitrogen and organic matter; black soil is notably poor in both.

As an identify-the-soil item from a set of characteristics, or in reverse — name the soil and ask which crop or which region goes with it.

• practice — not a real PYQ

  The wide cracks that develop in black soil during the dry season are important because they

  • (a)allow rapid drainage of rainwater to the water table
  • (b)produce a kind of self-ploughing that aerates the soil
  • (c)indicate a high content of organic matter
  • (d)prevent the soil from retaining moisture

  Answer(b) produce a kind of self-ploughing that aerates the soil — the clay shrinks as it dries and swells again when wet, and the same slow absorption and loss of moisture is why the soil holds water into the dry season.

• practice — not a real PYQ

  Laterite soils are formed mainly by which one of the following processes?

  • (a)Accumulation of salts under arid conditions
  • (b)Deposition of silt by rivers in the plains
  • (c)Intense leaching under high temperature and high rainfall
  • (d)Weathering of basaltic lava in a semi-arid climate

  Answer(c) intense leaching under high temperature and high rainfall — lime and silica are washed away and iron oxide and aluminium compounds are left behind, which is why laterite is poor in lime.