UPSC Prelims 2018 · Question 82 of 99
UPSC Prelims 2018 question on Agricultural Soils Organic Matter
- ExamUPSC CSE
- Year2018
- PaperGeneral Studies Paper I
- SubjectGeography
- TopicAgriculture Geography
- DifficultyMedium
- TypeStatement
With reference to agricultural soils, consider the following statements:
1. A high content of organic matter in soil drastically reduces its water holding capacity.
2. Soil does not play any role in the sulfur cycle.
3. Irrigation over a period of time can contribute to the salinization of some agricultural lands.
Which of the statements given above is/are correct?
Show answer
Answer: B. 3 only
Verdict
Only statement 3 is correct, so the answer is (b) 3 only.
Statement by statement
Statement 1 is INCORRECT, and it inverts the effect. Organic matter influences soil structure and moisture holding capacity, and increased levels of organic matter with associated soil fauna lead to greater pore space, so water infiltrates more readily and is held in the soil. The improved pore space comes from the burrowing of earthworms and other macro-organisms and from channels left by decayed roots. Organic matter therefore raises water holding capacity.
Statement 2 is INCORRECT. Sulfur is one of the nutrients cycled between soil, plant matter and the atmosphere. It is released by the weathering of rocks and minerals, becomes sulfate on exposure to air, is taken up by plants and microbes into organic compounds, and is released again when organisms decompose. Soil is central throughout.
Statement 3 is CORRECT. Irrigation over a period of time can contribute to the salinisation of some agricultural lands.
Source
FAO documents on soil organic matter, and material on the sulfur cycle in crop nutrition.
How to crack it
Two absolutes are planted here, drastically reduces and does not play any role. Nutrient cycles almost always pass through the soil, so any statement excluding soil from a nutrient cycle is safe to reject on principle. Test the extreme words first and the question resolves without specialist knowledge.