The Fertilizers is part of Chemistry. Below are notes on 2 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Chemistry of Nitrogenous Fertilizers: Urea, Ammonium Salts, Nitrates

  • Urea [CO(NH₂)₂]: Primary nitrogenous fertilizer with the highest nitrogen content (~46% N) among common solid fertilizers.
  • Urea is synthesized from ammonia (NH₃) and carbon dioxide (CO₂) under high pressure and temperature.
  • Urea is commonly used for foliar application. It does not contain phosphorus.
  • Ammonium Nitrate [NH₄NO₃]: A common nitrogenous fertilizer (~33-34% N).
  • Ammonium Sulphate [(NH₄)₂SO₄]: A common nitrogenous fertilizer (~20.5-21% N).
  • Ammonium Phosphate: A common fertilizer providing both nitrogen and phosphorus (e.g., Diammonium Phosphate - DAP, ~18% N).
  • Potassium Nitrate [KNO₃]: Provides both nitrogen (~13% N) and potassium.
  • Sodium Nitrate [NaNO₃]: A nitrogenous fertilizer (~16% N).
  • Ammonium Sulphide [(NH₄)₂S]: Not used as a fertilizer.
  • Nitrogen Content Order (Decreasing): Urea (~46%) > Ammonium Nitrate (~33-34%) > Ammonium Sulphate (~20.5-21%) > Sodium Nitrate (~16%) > Ammonium Phosphate (e.g., DAP ~18% N) > Potassium Nitrate (~13%).
  • Fertilizers enhance soil fertility.
  • Superphosphate and Bone meal are sources of phosphorus, not nitrogen.
  • Potassium sulfate is a source of potassium, not nitrogen.

Classification and Types of Fertilizers

  • Fertilizers must be soluble in water for plant absorption.
  • Fertilizers must not be poisonous to plants.
  • Fertilizers should not drastically alter soil pH to extreme levels (e.g., pH 10-13 is highly alkaline and detrimental).
  • A complete fertilizer contains all three primary macronutrients: Nitrogen (N), Phosphorus (P), and Potassium (K). These are commonly referred to as NPK fertilizers.
  • Sodium Nitrate (NaNO₃) is an example of a nitrogenous fertilizer. Other common fertilizers include Urea, Ammonium Nitrate, Diammonium Phosphate (DAP), and Potassium Chloride (KCl).
  • Essential plant nutrients supplied by fertilizers are broadly classified as:
    • Primary macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K).
    • Secondary macronutrients: Calcium (Ca), Magnesium (Mg), Sulfur (S).
    • Micronutrients: Boron (B), Chlorine (Cl), Copper (Cu), Iron (Fe), Manganese (Mn), Molybdenum (Mo), Nickel (Ni), Zinc (Zn).
  • Elements like Hydrogen (H), Oxygen (O), and Carbon (C) are essential for plant growth but are primarily obtained from water and air, not typically supplied as dedicated fertilizer elements to address deficiency, even though they are part of many fertilizer compounds.

21. Miscellaneous

Practice MCQs: The Fertilizers

Chemistry of Nitrogenous Fertilizers: Urea, Ammonium Salts, Nitrates

  1. Which one of the following statements about fertilizers is not correct?

    Asked in: UPSC NDA 1 2022 · Clerk GR II Comb. Comp. Exam 2013

    1. Urea is a phosphorus-containing fertilizer.
    2. Application of fertilizer to the soil increases fertility of the soil.
    3. Urea can be prepared by action of ammonia and carbon dioxide under high pressure and at high temperature.
    4. Urea contains more nitrogen than other fertil‐ izers.
    Show answer

    Answer: A. Urea is a phosphorus-containing fertilizer.

    D. Sulphonate salts of long chain carboxylic acids. Ans: D Soaps are salts of long-chain carboxylic acids, not sulphonate salts. Sulphonate salts are characteristic of detergents.

  2. The soap molecule has -

    Asked in: Head Master Praveshika School (Sanskrit Edu.) 2021

    1. A hydrophilic head and a hydrophilic tail
    2. A hydrophobic head and a hydrophilic tail
    3. A hydrophilic head and a hydrophobic tail
    4. A hydrophobic head and a hydrophilic tail
    Show answer

    Answer: C. A hydrophilic head and a hydrophobic tail

    Hydrophilic means "water-loving" and refers to polar or charged groups that dissolve well in water. Hydrophobic means "water-fearing" and refers to non-polar molecules or parts of molecules that do not dissolve well in water. - Fertilizers are substances that provide nutrients to plants, primarily nitrogen (N), phosphorus (P), and potassium (K), which are macronutrients. - Urea has the chemical formula (NH₂)₂CO. It is a rich source of nitrogen, with approximately 46% nitrogen by weight. - Phosphorus-containing fertilizers typically contain compounds like phosphates (e.g., calcium phosphate in superphosphate). - Statement B is correct: Fertilizers add essential nutrients to the soil, thus increasing its fertility and supporting plant growth. - Statement C is correct: Urea is synthesized industrially from ammonia (NH₃) and carbon dioxide (CO₂) under high pressure and temperature (Bosch–Meiser process). - Statement D is generally correct in the context of solid nitrogen fertilizers; urea has one of the highest nitrogen contents among commonly used solid fertilizers.

  3. Which one of the following is not used as fertilizer?

    Asked in: UPSC NDA 2 2018

    1. Ammonium nitrate
    2. Ammonium sulphide
    3. Ammonium phosphate
    4. Ammonium sulphate
    Show answer

    Answer: B. Ammonium sulphide

    Common fertilizers contain essential plant nutrients, primarily nitrogen, phosphorus, and potassium, often in the form of stable salts. Ammonium compounds are often used as sources of nitrogen, but their suitability as fertilizers depends on their stability and chemical properties in soil.

  4. Which one among the following fertilizers is a source of nitrogen ?

    Asked in: UPSC NDA 2 2024

    1. Superphosphate
    2. Potassium sulfate
    3. Bone meal
    4. Urea
    Show answer

    Answer: D. Urea

    Plants require macronutrients like nitrogen (N), phosphorus (P), and potassium (K) for growth. Fertilizers provide these nutrients. Nitrogen fertilizers specifically supply nitrogen compounds to the soil.

  5. Which of the following fertilizers has more nitrogen content?

    Asked in: WBPSC CS Prelims 2019

    1. Urea
    2. Ammonium Nitrate
    3. Potassium Nitrate
    4. Ammonium Phosphate
    Show answer

    Answer: A. Urea

    Urea (CO(NH₂)₂) typically has the highest percentage of nitrogen content among common solid fertilizers.

  6. The most popular fertilizer for foliar application is

    Asked in: UPPSC Upper Mains 2015

    1. Sodium Nitrate
    2. Ammonium Chloride
    3. Urea
    4. D.A.P.
    Show answer

    Answer: C. Urea

    Urea is the most popular fertilizer for foliar application due to its high solubility in water and high nitrogen content (around 46%). Nitrogen applied through foliar spray is rapidly absorbed by the leaves, providing a quick supply of the nutrient to the plant, which can be beneficial during specific growth stages or when soil application is inefficient.

  7. Arrange the following fertilisers according to the decreasing order of their nitrogen content I. Ammonium sulphate II. Ammonium nitrate III. Potassium nitrate IV. Urea

    Asked in: ASSISTANT ENGINEER(ELECTRICAL) 2016

    1. IV, II, III, I
    2. IV, II, I, III
    3. IV, III, II, I
    4. II, IV, III, I
    Show answer

    Answer: A. IV, II, III, I

    To answer this question, you need to know the chemical formulas of these fertilizers and calculate their approximate nitrogen content by mass. - Urea: CO(NH₂)₂ - ~46% N - Ammonium nitrate: NH₄NO₃ - ~34-35% N - Potassium nitrate: KNO₃ - ~13% N - Ammonium sulphate: (NH₄)₂SO₄ - ~21% N

Classification and Types of Fertilizers

  1. Which among the following is/are wrong about a fertilizer? (i) Fertilizer must be insoluble in water (ii) It should maintain the pH of 10 to 13 (iii) It should not be poisonous to plants

    Asked in: TNPSC CS Prelims 2022

    1. Only (iii)
    2. (i) and (ii)
    3. (i), (ii), (iii)
    4. Only (ii)
    Show answer

    Answer: B. (i) and (ii)

    Fertilizers are substances added to soil or plant tissues to supply nutrients essential for plant growth. They must be in a form accessible to plants, typically dissolved in water, and should not be detrimental to plant health or soil properties like pH.

  2. Which of the following known as complete fertilizer?

    Asked in: TNPSC CS Prelims 2014

    1. Nitrogenous fertilizer
    2. Potash fertilizer
    3. NPK fertilizer
    4. NP fertilizer
    Show answer

    Answer: C. NPK fertilizer

    A complete fertilizer is a fertilizer that contains the three primary macronutrients essential for plant growth: Nitrogen (N), Phosphorus (P), and Potassium (K). These are the nutrients plants require in the largest amounts.

  3. Miscellaneous 9. Which one is a fertilizer?

    Asked in: WBPSC CS Prelims 2020

    1. N₂
    2. O₂
    3. P₄
    4. NaNO₃
    Show answer

    Answer: D. NaNO₃

    Fertilizers are substances that provide nutrients essential for plant growth. NaNO₃, or Sodium Nitrate, is a well-known fertilizer that provides nitrogen, a critical nutrient for plants, in a form that can be readily absorbed by their roots.

  4. The element not present in fertilisers is

    Asked in: UPPSC Upper Mains 2007

    1. Nitrogen
    2. Hydrogen
    3. Chlorine
    4. Phosphorus
    Show answer

    Answer: B. Hydrogen

    Fertilizers primarily supply essential plant nutrients like macronutrients (N, P, K), secondary nutrients (Ca, Mg, S), and micronutrients (B, Cu, Fe, Mn, Mo, Ni, Zn, Cl). Hydrogen is not considered one of these nutrient elements typically supplied by fertilizers.