UPSC Prelims 2013 · GS Paper I
Question 34: Environment and Ecology
Which of the following adds/add nitrogen to the soil?
- 1.Excretion of urea by animals
- 2.Burning of coal by man
- 3.Death of vegetation
Select the correct answer using the codes given below.
AnswerUPSC’s answer is (c).
Urea in animal urine is converted by soil bacteria to ammonia and then nitrates, which adds nitrogen to the soil. When vegetation dies, decomposers break down its proteins and return the nitrogen to the soil. Burning coal puts oxides of carbon, sulphur and nitrogen into the air rather than building up soil nitrogen directly. So 1 and 3 only.
- 1. Urea is converted to ammonia and nitrates in the soil.
- 2. Burning coal releases gases to the air, and is not counted as adding soil nitrogen.
- 3. Decay of dead plants returns nitrogen to the soil.
How you could have got this
- If you were sure statement 2 is not correct, (b) and (d) go: each of them includes 2.
What you needed to know: Burning coal releases gases to the air, and is not counted as adding soil nitrogen.
- Then, if you were sure statement 3 is correct, (a) goes: it leaves 3 out.
What you needed to know: Decay of dead plants returns nitrogen to the soil.
Only (c) is left. You didn’t need statement 1.
Which options go is worked out from UPSC’s key and the options, not by a model.
Why each option is right or wrong
- (a) Wrong. Death of vegetation also returns nitrogen to the soil.
- (b) Wrong. Burning coal is not counted as adding nitrogen to soil, and urea excretion is left out.
- (c) Correct. Urea excretion and the decay of dead plants add nitrogen to the soil.
- (d) Wrong. Burning coal is not taken as adding nitrogen to the soil.
Worth knowing: Burning coal releases nitrogen oxides that can return to the soil in rain, so statement 2 can be argued; UPSC's key does not count it.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2013, General Studies Paper I, Series A. The paper on upsc.gov.in.