Which one of the following is not a nitrogen fertilizer?

Correct — C, N₂. The other three formulas are three of the commonest nitrogen fertilizers on any dealer's shelf, but N₂ is the nitrogen gas that makes up about four-fifths of the air — and that is precisely the nitrogen a crop cannot use. The two nitrogen atoms are held by a triple bond, one of the strongest bonds in chemistry, and a root hair has no means of breaking it. Plants take nitrogen up only as ions in soil water, chiefly nitrate and ammonium. Getting atmospheric nitrogen into that form takes either a living catalyst or an industrial one: the nitrogen-fixing bacteria that live in legume root nodules and in free soil, or the Haber-Bosch process, which combines nitrogen with hydrogen at high temperature and pressure to give ammonia, from which every fertilizer in the other three options is then built. So the item's logic is that a fertilizer must supply nitrogen in a form roots can absorb; spreading nitrogen gas over a field would change nothing, because the field is already sitting under an atmosphere of it.

• (a)(NH₄)₂SO₄ — Ammonium sulphate is a genuine nitrogen fertilizer, and one of the oldest. It supplies nitrogen as the ammonium ion and sulphur as sulphate, and is favoured on alkaline soils because it acidifies as the ammonium is nitrified.
• (b)NH₄NO₃ — Ammonium nitrate is a nitrogen fertilizer that delivers both of the ions a plant can absorb — ammonium for slow uptake and nitrate for immediate use. Its other, dangerous property is that it is a powerful oxidiser, which is why its storage is regulated.
• (d)(NH₂)₂CO — Read the formula out: (NH₂)₂CO is urea, the world's most widely used nitrogen fertilizer and the one with the highest nitrogen content of any solid nitrogen fertilizer in common use. More than nine-tenths of industrial urea production goes to fertilizer.

Nitrogen is the nutrient crops need in the largest quantity, because it goes into every amino acid, protein and chlorophyll molecule. The air is about 78 per cent nitrogen, yet plants cannot touch it: the N triple bond N molecule is chemically inert at ordinary temperatures. Roots absorb nitrogen only as nitrate or ammonium ions dissolved in soil water. Nature bridges the gap through nitrogen-fixing bacteria — Rhizobium in the root nodules of legumes, Azotobacter and cyanobacteria free in soil and water — and industry bridges it through the Haber-Bosch synthesis of ammonia, from which urea, ammonium sulphate, ammonium nitrate and the rest are manufactured.

Three of the four options are formulas you can decode into named fertilizers, and the fourth is an element. That asymmetry is the quickest route in. If you can read (NH₂)₂CO as urea the question is over, but even without that the reasoning holds: a fertilizer is something added to soil to supply a nutrient in an absorbable form, and free nitrogen gas is neither added nor absorbable. It is worth being clear that N₂ is not useless to agriculture — it is the raw material at the head of the whole chain, both in the root nodule and in the ammonia plant. It is simply not itself a fertilizer. UPSC has tested the industrial end of that same chain, asking whether the ammonia that goes into urea is made from natural gas.

• About 78 per cent of the atmosphere is N₂, but the triple bond makes it unavailable to plants, which absorb nitrogen as nitrate or ammonium ions.
• Urea, (NH₂)₂CO, has the highest nitrogen content of any solid nitrogenous fertilizer in common use, and more than 90 per cent of world urea production is used as fertilizer.
• Urea is manufactured from ammonia and carbon dioxide; the ammonia itself comes from the Haber-Bosch reaction of nitrogen with hydrogen.
• Ammonium sulphate supplies nitrogen plus sulphur; ammonium nitrate supplies both ammonium and nitrate nitrogen and is also a strong oxidiser.
• Biological nitrogen fixation is carried out by Rhizobium in legume root nodules and by free-living organisms such as Azotobacter and cyanobacteria.

• Thinking that because the air is mostly nitrogen, plants have all they need; the triple bond puts it out of reach.
• Failing to read (NH₂)₂CO as urea and dismissing it because it does not look like a nitrogen compound.
• Confusing nitrogen fixation with nitrification; fixation turns N₂ into ammonia, nitrification turns ammonium into nitrate.

Usually as an odd-one-out among fertilizer formulas, or as a statement item on which nutrient a named fertilizer supplies, or on how urea and ammonia are manufactured.

• practice — not a real PYQ

  Plants absorb nitrogen from the soil mainly in which one of the following forms?

  • (a)Dinitrogen gas dissolved in soil water
  • (b)Nitrate and ammonium ions
  • (c)Nitrogen dioxide
  • (d)Elemental nitrogen adsorbed on clay

  Answer(b) Nitrate and ammonium ions — these are the forms roots can take up; the N triple bond N molecule is inert and must first be fixed by bacteria or by industry.

• practice — not a real PYQ

  Which one of the following solid fertilizers has the highest nitrogen content?

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

  Answer(c) Urea — it has the highest nitrogen content of any solid nitrogenous fertilizer in common use. Single superphosphate is a phosphorus fertilizer and carries no nitrogen at all.