NDA (II) 2018 General Ability Test · Question 105 of 146

Correct — B, Ammonium sulphide. All four options begin with the ammonium ion, so nitrogen is not what separates them; the partner ion is. Nitrate, phosphate and sulphate are all forms in which plants can actually take up a nutrient from the soil, and all three are sold in bulk as fertilisers. Sulphide is not. Ammonium sulphide is unstable in solution and readily converts to ammonia and hydrogen sulphide, two gases with a powerful stench, so the compound would lose its nitrogen to the air almost as fast as it was spread. Its recorded uses are industrial rather than agricultural — photographic developing, applying a patina to bronze, and textile manufacture.

• (a)Ammonium nitrate — This is one of the world's important nitrogenous fertilisers, with an NPK rating of 34-0-0. It carries nitrogen in both the ammonium and the nitrate form, and holds on to it better than urea does.
• (c)Ammonium phosphate — This is a complex fertiliser supplying two of the three primary nutrients at once, nitrogen and phosphorus. Di-ammonium phosphate is among the most widely used fertilisers in Indian agriculture.
• (d)Ammonium sulphate — A long-established nitrogenous fertiliser that also supplies sulphur, a secondary nutrient. It is especially useful on alkaline soils because it has an acidifying effect.

Fertilisers are classified by the nutrients they carry. Straight fertilisers supply one primary nutrient — urea and ammonium sulphate for nitrogen, single superphosphate for phosphorus, muriate of potash for potassium. Complex or mixed fertilisers such as di-ammonium phosphate and the NPK grades supply two or more together. A compound qualifies as a fertiliser only if the plant can absorb the nutrient in the form supplied and the compound is stable enough to survive storage and spreading.

The item is built so that recall alone will not settle it — a candidate who only remembers 'ammonium compounds are nitrogen fertilisers' has no way to choose. The discriminating question is what the anion does. Ask of each option whether the negative ion is itself a plant nutrient in a usable form, and sulphide is the one that fails: sulphur reaches plants as sulphate, not sulphide, and sulphide in the root zone is associated with waterlogged, oxygen-starved soils rather than with fertility. The instability clinches it. A fertiliser that decomposes into two escaping gases would deliver almost nothing to the crop.

• Ammonium sulphide solutions easily convert to ammonia and hydrogen sulphide, both foul-smelling gases.
• Ammonium nitrate has an NPK rating of 34-0-0 and is more stable in the field than urea, whose rating is 46-0-0.
• Ammonium phosphate supplies nitrogen and phosphorus together and so counts as a complex fertiliser.
• Ammonium sulphate supplies both nitrogen and sulphur, and leaves an acidic residue that suits alkaline soils.

• Choosing by the ammonium ion alone, which is common to all four options and therefore decides nothing.
• Assuming any sulphur compound feeds a crop; plants take sulphur up as sulphate, and sulphide is a marker of waterlogged, oxygen-poor soil.
• Confusing ammonium sulphate, a real and common fertiliser, with ammonium sulphide, which is not.

Asked as a negative one-from-four where three options share an obvious feature and the fourth is separated by a single ion.

• practice — not a real PYQ

  Which one of the following fertilisers has the highest nitrogen content?

  • (a)Ammonium sulphate
  • (b)Urea
  • (c)Ammonium nitrate
  • (d)Calcium nitrate

  Answer(b) Urea — its NPK rating is 46-0-0, ahead of ammonium nitrate at 34-0-0 and ammonium sulphate at about 21% nitrogen.

• practice — not a real PYQ

  Di-ammonium phosphate is classified as a complex fertiliser because it

  • (a)is manufactured by a complex industrial process
  • (b)supplies more than one primary plant nutrient
  • (c)dissolves only in hot water
  • (d)must be mixed with farmyard manure before use

  Answer(b) supplies more than one primary plant nutrient — it carries both nitrogen and phosphorus, unlike a straight fertiliser such as urea.