Question 83 from UPSC CSE 2018 - Environment

Question

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 sulphur 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?

IASCODEX · Accepted Answer

Correct Option: B. 3 only

Strategy: - Pinpoint each statement by examining its scientific linkage.
- Statement 1 contradicts basic soil science: organic matter improves, not reduces, water holding capacity.
- Statement 2 overlooks that soil is critical to the sulfur cycle.
- Statement 3 aligns with established soil science: improper irrigation can lead to salinization.
- Select option B, as only Statement 3 withstands scrutiny.

Key Info: Organic matter is a fundamental component of soil, affecting its physical and chemical characteristics. High organic content increases the water holding capacity of soils due to enhanced aggregation, more micropores, and the sponge-like properties of decomposed plant and animal material. It improves infiltration, reduces erosion, and supports soil organisms, enhancing sustained plant growth.

The sulfur cycle describes the movement of sulfur through geological and biological systems. Sulfur enters the soil by weathering of rocks, decomposition of organic matter, and deposition from the atmosphere. In the soil, microorganisms transform sulfur compounds, making sulfur available to plants. Plants take up sulfur as sulfate ions (SO4^2-), incorporating sulfur into amino acids and proteins. Decomposition returns sulfur to the soil and atmosphere, making soil a vital medium in the sulfur cycle.

Salinization of soils is the accumulation of soluble salts in the soil profile. Primary salinization may occur naturally, especially in arid regions. Secondary or irrigation-induced salinization results from repeated irrigation, particularly with inadequate drainage, causing dissolved salts in irrigation water to accumulate in the root zone. This adversely affects crop growth due to osmotic stress and ion toxicity. Remedial measures include proper drainage, leaching, and use of suitable water sources.

### Key Related Terms
- **Water holding capacity** -- The amount of water a soil can retain for use by plants.
- **Sodification** -- Accumulation of excess sodium in soils, further worsening soil structure and permeability.
- **Micronutrient availability** -- Saline soils often render certain trace nutrients unavailable to crop plants, affecting productivity.

IASCODEX · Answer

Option A: 1 and 2 only

IASCODEX · Answer

Option C: 1 and 3 only

IASCODEX · Answer

Option D: 1, 2, and 3