UPSC Prelims 2015 · Question 11 of 98

UPSC Prelims 2015 question on Nitrogenous Fertilizer Impacts

What can be the impact of excessive/inappropriate use of nitrogenous fertilizers in agriculture?

1. Proliferation of nitrogen-fixing microorganisms in soil can occur.

2. Increase in the acidity of soil can take place.

3. Leaching of nitrate to the ground water can occur.

Select the correct answer using the code given below.

  1. 1 and 3 only
  2. 2 only
  3. 2 and 3 only
  4. 1, 2 and 3
Show answer

Answer: C. 2 and 3 only

Verdict

The answer is 2 and 3 only. Excess nitrogenous fertiliser acidifies soil and leaches nitrate into groundwater, but it does not multiply nitrogen fixing microorganisms.

Analysis

Statement 2 is CORRECT. Nitrification of ammonium releases hydrogen ions, which lowers soil pH over time. Statement 3 is CORRECT. Nitrate is highly soluble and poorly retained by soil particles, so surplus nitrogen moves down into groundwater, a well documented contamination pathway. Statement 1 is INCORRECT. When ammonium is already abundant from fertiliser, plants have no need for the symbiotic association, so nitrogen fixing bacteria are suppressed rather than multiplied.

Source

United Nations Environment Programme Year Book chapter on nitrogen.

How to crack it

Reason from incentive rather than from a pollution list. Biological nitrogen fixation is metabolically expensive, so it happens when nitrogen is scarce and shuts down when nitrogen is plentiful. Any statement claiming that adding a nutrient boosts the organisms that manufacture that same nutrient is backwards. This substitution logic recurs across soil science, so hold it as a rule: abundance suppresses the biological pathway that supplies the same resource.

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