Correct Option
The correct option is (c) 1 and 3 only.
Explanation
Soil fertility is significantly enhanced by the presence of specific microorganisms capable of converting atmospheric nitrogen into organic forms usable by plants. This process is known as biological nitrogen fixation, and the organisms involved serve as biofertilizers.
Statement-wise Analysis
- Statement 1 is Correct: Certain bacteria (such as Rhizobium, Azotobacter, and Azospirillum) and blue-green algae (cyanobacteria like Anabaena and Nostoc) possess the ability to fix atmospheric nitrogen. They convert inert nitrogen gas ($N_2$) into ammonia ($NH_3$) or related nitrogenous compounds.
- Statement 2 is Incorrect: These microbes do not deplete the soil of nitrogen; rather, they enrich the soil with nitrogenous compounds. By fixing atmospheric nitrogen, they increase the nutrient content of the soil, thereby enhancing its fertility and supporting plant growth.
- Statement 3 is Correct: Microorganisms that fix atmospheric nitrogen are technically referred to as biological nitrogen fixers. They play a crucial role in sustainable agriculture by reducing dependence on synthetic chemical fertilizers.
Key Takeaway: Biological nitrogen fixers, including specific bacteria and cyanobacteria, enrich soil fertility by converting atmospheric nitrogen into usable forms, acting as natural biofertilizers.