What can be the impact of excessive/inappropriate use of nitrogenous fertilizers in agriculture? 1) Proliferation of nitrogen-fixing microorganisms in soil can occur. 2) Increase in the acidity of soil can take place.  3) Leaching of nitrate to the ground-water can occur. Select the correct answer using the codes given below:

The correct answer is C, statements 2 and 3 only. The core idea is that fertilizer adds reactive nitrogen to a system already balanced by soil biology and chemistry. Too much of it disturbs that balance rather than feeding nitrogen-fixers.

• Statement 1 is wrong. Heavy doses of urea and ammonium fertilizers suppress free-living fixers such as Azotobacter and reduce nodulation in legumes, because the plant no longer needs Rhizobium. Biological nitrogen fixation falls, not rises.
• Statement 2 is correct. Ammonium-based fertilizers like urea and ammonium sulphate release H+ ions during nitrification, lowering soil pH over time. This is why acidity builds up in intensively farmed tracts.
• Statement 3 is correct. Nitrate is highly soluble and is not held by negatively charged soil particles, so it moves down with percolating water into groundwater. Nitrate contamination of groundwater has been reported in parts of Punjab, Haryana and Rajasthan.

Mains angle: this links to the input-intensive Green Revolution model, soil health, neem-coated urea, and the Soil Health Card scheme.

Trap: candidates read "nitrogen" and assume more nitrogen means more nitrogen fixation. Fixation is a biological process that excess fertilizer switches off.