CDS (II) 2021 General Knowledge · Question 27 of 120

Correct — A, Iron. Fertilizers are commercially produced plant nutrients, and what they supply is nitrogen, phosphorus and potassium — the three that plants need in the largest quantities and that soils run short of first. Three of the four options are on that list; iron is not. Iron is certainly essential to plants, taking part in chlorophyll synthesis and in the electron carriers of photosynthesis, but it belongs to the micronutrients, the group required only in small quantities. The standard table of soil nutrients puts nitrogen, phosphorus, potassium, calcium, magnesium and sulphur among the macronutrients, and iron, manganese, boron, zinc, copper, molybdenum and chlorine among the micronutrients. Ordinary bagged fertilizer targets the first list, which is why the shorthand for it is N-P-K. Micronutrient shortages, when they occur, are usually corrected by manure, by a dedicated micronutrient mixture or by a foliar spray, not by the general-purpose fertilizer the stem has in mind.

• (b)Potassium — One of the three nutrients fertilizers are made to supply — the K of N-P-K, delivered as muriate of potash or sulphate of potash.
• (c)Nitrogen — The nutrient fertilizers supply most of. Urea alone accounts for a large share of India's fertilizer consumption and is a nitrogen carrier.
• (d)Phosphorus — The P of N-P-K, supplied through single superphosphate and diammonium phosphate among others. It is a macronutrient and a routine part of fertilizer formulations.

Plants draw sixteen essential nutrients: carbon and oxygen from air, hydrogen and oxygen from water, and the remaining thirteen from the soil. Those thirteen split by quantity needed. Macronutrients — nitrogen, phosphorus, potassium, calcium, magnesium and sulphur — are required in large amounts. Micronutrients — iron, manganese, boron, zinc, copper, molybdenum and chlorine — are required in small ones. Deficiency of any of them affects reproduction, growth and susceptibility to disease, and the soil is enriched by adding manure or fertilizer.

The word 'usually' in the stem is doing careful work and should not be read as a hedge to be ignored. Micronutrient fertilizers do exist and are sold, so the claim is not that iron can never be applied; it is that the fertilizers a farmer ordinarily buys are built around nitrogen, phosphorus and potassium. Sorting the four options by the macro-and-micro split settles the question in a line. It is worth knowing the contrast with manure, which the same chapter draws: manure carries large quantities of organic matter and small quantities of nutrients, improves soil structure and water-holding capacity, and returns micronutrients to the soil, while fertilizer delivers a concentrated dose of a few nutrients and, used continuously without organic matter, can degrade soil fertility and pollute water through run-off.

• Fertilizers are commercially produced plant nutrients and supply nitrogen, phosphorus and potassium.
• Soil macronutrients are nitrogen, phosphorus, potassium, calcium, magnesium and sulphur; soil micronutrients are iron, manganese, boron, zinc, copper, molybdenum and chlorine.
• Air supplies carbon and oxygen, and water supplies hydrogen and oxygen; the remaining thirteen essential nutrients come from the soil.
• Manure supplies small quantities of nutrients but large quantities of organic matter, improving soil structure and water-holding capacity.
• Continuous use of fertilizers without organic matter can destroy soil fertility, and excess fertilizer washed away by irrigation causes water pollution.

• Reading 'not usually available in fertilizers' as 'not needed by plants'; iron is essential, just needed in small amounts.
• Forgetting that carbon, hydrogen and oxygen come from air and water rather than from the soil.
• Assuming manure and fertilizer supply the same things; manure brings organic matter and a broader but weaker nutrient mix.

As an odd-one-out among nutrients, as a macro-versus-micronutrient sorting item, or as a comparison of manure with chemical fertilizer.

• practice — not a real PYQ

  Which one of the following pairs of nutrients does a plant obtain from air rather than from the soil?

  • (a)Nitrogen and phosphorus
  • (b)Carbon and oxygen
  • (c)Potassium and calcium
  • (d)Iron and zinc

  Answer(b) Carbon and oxygen — air supplies these two, water supplies hydrogen and oxygen, and the remaining thirteen essential nutrients come from the soil.

• practice — not a real PYQ

  An advantage of using manure over chemical fertilizer is that manure

  • (a)supplies nutrients in a more concentrated form
  • (b)adds organic matter and improves soil structure
  • (c)acts faster on the standing crop
  • (d)contains only nitrogen, phosphorus and potassium

  Answer(b) adds organic matter and improves soil structure — it raises water-holding capacity in sandy soils and helps drainage in clayey ones.