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 code given below:

The correct option is 2 and 3 only.

Let us analyze the impacts of excessive or inappropriate use of nitrogenous fertilizers in agriculture step-by-step:

1. Impact on Nitrogen-fixing Microorganisms (Statement 1):
Nitrogen-fixing microorganisms (such as Rhizobium, Azotobacter, and blue-green algae) naturally convert atmospheric nitrogen into usable forms for plants. When chemical nitrogenous fertilizers are applied excessively, the soil becomes rich in readily available nitrogen. Consequently, the biological necessity for plants to support symbiotic or free-living nitrogen-fixing microbes is drastically reduced, which suppresses the activity and population of these beneficial microorganisms rather than causing them to proliferate. Therefore, statement 1 is incorrect.

2. Soil Acidification (Statement 2):
Many nitrogenous fertilizers contain ammonium (NH₄⁺) or urea. When applied to the soil, these compounds undergo a biological process called nitrification, where ammonium is converted into nitrate (NO₃^-) by soil bacteria. This nitrification reaction releases hydrogen ions (H⁺) into the soil, as shown in the chemical equation below:

$${\mathrm{NH}}_4^{+}+2O_2\Rightarrow {\mathrm{NO}}_3^{-}+H_{2}O+2H^{+}$$

The accumulation of these free hydrogen ions (H⁺) increases soil acidity (lowers pH) over time. Therefore, statement 2 is correct.

3. Nitrate Leaching (Statement 3):
Nitrate ions (NO₃^-) carry a negative charge. Because soil particles also carry negative charges, they do not bind or hold nitrate ions effectively. When nitrogenous fertilizers are applied in excess of what crops can absorb, the surplus nitrate remains dissolved in the soil water. During rainfall or irrigation, this highly soluble nitrate easily moves downward through the soil profile (leaching) and contaminates the underlying groundwater. Therefore, statement 3 is correct.

Conclusion:
Since statements 2 and 3 are correct and statement 1 is incorrect, the correct option is indeed 2 and 3 only.