Correct Option
Statement 2 is correct: Excessive application of nitrogenous fertilizers, such as urea and ammonium sulphate, can lead to an increase in soil acidity. This occurs primarily through the nitrification process, where ammonium (NH₄⁺) is converted to nitrate (NO₃⁻) by soil microbes, releasing hydrogen ions (H⁺). Additionally, the preferential uptake of ammonium by plants can also contribute to soil acidification.
Statement 3 is correct: Nitrogen, particularly in its nitrate (NO₃⁻) form, is highly soluble in water and carries a negative charge. Soil particles often have a net negative charge, leading to weak adsorption of nitrates. Consequently, nitrates are easily leached downwards through the soil profile with percolating water, contaminating groundwater sources.
Incorrect Options
Statement 1 is incorrect: Excessive availability of inorganic nitrogen from fertilizers generally inhibits the activity and proliferation of nitrogen-fixing microorganisms (both free-living and symbiotic). These microorganisms expend energy to fix atmospheric nitrogen, a process that becomes energetically unfavorable and unnecessary when sufficient fixed nitrogen is already present in the soil.
Option (a) is incorrect because it includes statement 1, which is false.
Option (b) is incorrect because it omits statement 3, which is a significant impact of excessive nitrogenous fertilizer use.
Option (d) is incorrect because it includes statement 1, which is false.