CDS (I) 2021 General Knowledge · Question 57 of 120

Correct — D, rich in potash and poor in phosphorus. The whole question, stem and answer together, is a single sentence lifted from the school geography chapter on soils, which reads: 'The alluvial soils vary in nature from sandy loam to clay. They are generally rich in potash but poor in phosphorous.' The paper has cut the sentence in half, put the first part in the stem and made the second part the answer. That is worth noticing as a technique, because the four options are then simply the four ways of pairing rich and poor with the two nutrients, and only one of them restores the original line. Alluvial soils are India's most extensive and most productive group, covering the northern plains and the coastal deltas, and their fertility comes from constant renewal by river deposition rather than from any one nutrient.

• (a)poor in potash and rich in phosphorus. — The exact reversal of the textbook line, and the option a candidate lands on if he remembers that one of the two is deficient but not which.
• (b)poor in both potash and phosphorus. — Describes laterite soils far better than alluvial ones — laterites are poor in organic matter, nitrogen, phosphate and calcium, though even there potash is in excess.
• (c)rich in both potash and phosphorus. — Overstates the case. Alluvial soils are fertile overall, which makes this feel right, but the textbook singles out phosphorus, along with nitrogen and humus, as what they lack.

Alluvial soils are transported soils, laid down by rivers, and they cover the Indo-Gangetic and Brahmaputra plains and the deltas of the eastern coast. Their texture ranges from sandy loam to clay, and in the Upper and Middle Ganga plain they divide into khadar, the new alluvium renewed by floods every year and enriched by fresh silt, and bhangar, the older alluvium of the higher terraces, often containing calcareous concretions called kankar. Because they are constantly renewed rather than developed in place from a parent rock, they carry little profile development and their fertility depends on what the river brings down.

The efficient way to hold Indian soils is one nutrient signature per group, and the signatures are distinctive enough that they rarely collide. Alluvial soils are rich in potash and poor in phosphorus. Black or regur soils are rich in lime, iron, magnesia and alumina and contain potash, but lack phosphorus, nitrogen and organic matter. Laterite soils are poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess. Arid soils lack moisture and humus, and are short of nitrogen. It is worth flagging an apparent inconsistency a careful student will run into: the middle-school geography chapter describes alluvial soils as containing an adequate proportion of potash, phosphoric acid and lime, which reads as though phosphorus were plentiful. The two statements come from different books at different levels, and the senior chapter — the one this question is drawn from almost verbatim — is the one that governs here. In practice, alluvial soils are so widely cultivated that phosphatic fertiliser use in the northern plains is heavy, which is consistent with the senior text.

• The alluvial soils vary in nature from sandy loam to clay, and are generally rich in potash but poor in phosphorous.
• In the Upper and Middle Ganga plain they divide into khadar, the new alluvium deposited by annual floods, and bhangar, the older alluvium.
• Alluvial soils are the most extensive soil group of India, covering the northern plains and the eastern coastal deltas.
• Black soils are rich in lime, iron, magnesia and alumina and contain potash, but lack phosphorous, nitrogen and organic matter.
• Laterite soils are poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess.

• Assuming a fertile soil must be rich in everything; alluvial soils are highly productive yet short of phosphorus, nitrogen and humus.
• Reversing the pair — remembering that one nutrient is deficient but attaching the deficiency to potash.
• Carrying a middle-school phrasing into a senior-level question; the two texts describe the phosphorus status differently, and this item follows the senior one.

Asked as a completion of a textbook sentence, or as an identify-the-soil item built from texture, colour and nutrient clues.

• practice — not a real PYQ

  In the Upper and Middle Ganga plain, the newer alluvium deposited by floods every year is known as

  • (a)Bhangar
  • (b)Khadar
  • (c)Bhabar
  • (d)Terai

  Answer(b) Khadar — the new alluvium enriched by fresh silt each year; bhangar is the older alluvium of the higher terraces.

• practice — not a real PYQ

  Which Indian soil group is described as rich in lime, iron, magnesia and alumina but lacking in phosphorous, nitrogen and organic matter?

  • (a)Alluvial soil
  • (b)Black soil
  • (c)Laterite soil
  • (d)Arid soil

  Answer(b) Black soil — the regur of the Deccan lava country, valued for its moisture retention and for cotton.