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UPSC Prelims 2018 · Agriculture · Soils
With reference to agricultural soils, consider the following statements:
- A high content of organic matter in soil drastically reduces its water holding capacity.
- Soil does not play any role in the sulphur cycle.
- Irrigation over a period of time can contribute to the salinization of some agricultural lands.
- (a) 1 and 2 only
- (b) 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Show the answer and explanation
Answer: (b) 3 only
Irrigation water evaporates and leaves its salt behind — the one statement that matches how soils actually change.
Statement by statement
1. ✗ WRONG — it increases it Humus can hold several times its own weight of water. More organic matter = MORE water-holding capacity.
2. ✗ WRONG — soil is central to it Soil microbes turn organic sulphur into sulphate that plants absorb; soil is a major sulphur reservoir.
3. ✓ CORRECT Irrigation water evaporates and leaves its salts behind, especially where drainage is poor — as in parts of Punjab, Haryana and the Indira Gandhi Canal area.
Why the others are wrong
(a) Accepts both — organic matter RAISES water-holding, and soil microbes run the sulphur cycle.
(c) Organic matter raises water-holding capacity.
(d) Accepts all three — the absolute “does not play ANY role” should have warned you.
The trap
Statement 2 uses “does not play ANY role” — an absolute that is almost never true in nutrient cycles.
🔒 Organic matter → holds more water · soil drives the sulphur cycle · long irrigation without drainage → salinity.