With reference to agricultural soils, consider the following statements:A high content of organic ma

Q: What is UPSC Prelims?

UPSC Prelims is a competitive examination conducted in India. This question is from the 2018 UPSC Prelims Environment & Ecology paper.

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To prepare for UPSC Prelims Environment & Ecology, practice previous year questions (PYQs), understand concepts thoroughly, and solve mock tests regularly. This 2018 question covers Soil & Land Degradation – Desertification & Remediation topic.

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This UPSC Prelims 2018 Environment & Ecology question has a medium difficulty level and is marked as an important question.

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Question

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.Which of the statements given above is/are correct?

Accepted Answer

Correct Answer: B — 3 onlyStep-by-step Solution:Statement 1 — High organic matter drastically reduces soil water holding capacity: This is incorrect. The opposite is true. High organic matter content in soil increases its water holding capacity, not reduces it. Organic matter (humus) improves soil structure, creates more pore spaces, and is hygroscopic (absorbs and holds water). Soils rich in organic matter — like black soils (regur) or loamy soils — can hold water for longer, benefiting plant growth during dry spells. ✗Statement 2 — Soil plays no role in the sulphur cycle: This is incorrect. Soil plays a very important role in the sulphur cycle. Sulphur is primarily stored in sedimentary rocks and soil organic matter. When rocks weather, sulphur is released into the soil. Soil bacteria (like Desulfovibrio and Thiobacillus) transform sulphur between different chemical forms. Plants absorb sulphate (SO₄²⁻) from the soil — making soil a critical reservoir and transformation site in the global sulphur cycle. ✗Statement 3 — Irrigation can contribute to salinization of agricultural lands: This is correct. Secondary salinization is a well-documented problem caused by irrigation, particularly in arid and semi-arid regions. When irrigation water evaporates, it leaves behind dissolved salts in the soil. Over time, with inadequate drainage, these salts accumulate in the root zone, making the soil saline and eventually unproductive. This is a major problem in irrigated areas of Punjab, Haryana, Rajasthan, and the Indus plains globally. ✓Why other options are incorrect:Option A (1 and 2 only): Both statements 1 and 2 are factually wrong.Option C (1 and 3 only): Statement 1 is incorrect — organic matter increases, not reduces, water holding capacity.Option D (1, 2 and 3): Statements 1 and 2 are both incorrect.Key Concept: Soil organic matter (SOM) benefits: improves soil structure (aggregation); increases water holding capacity; provides slow-release nutrients; supports microbial biodiversity; reduces erosion; sequesters carbon. Salinization: primary (natural, from salt-rich parent material or saline water table); secondary (human-caused, from irrigation without adequate drainage). The sulphur cycle involves: volcanic emissions → atmospheric SO₂ → acid rain → soil sulphate → plant uptake → decomposition → soil again. Soil is a critical biogeochemical reactor for S, N, C, P cycles.

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