Consider the following statements regarding soil fertility: 1. Some bacteria and blue-green algae are able to fix nitrogen from the atmosphere. 2. These microbes deplete the soil of nitrogen and reduce its fertility. 3. These microbes are commonly called biological nitrogen fixers. Which of the statements given above is/are correct?

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.

NCERT Class 8 Science — Chapter 2: Microorganisms : friend and foe

mcqeasy

Consider the following statements regarding soil fertility:

Some bacteria and blue-green algae are able to fix nitrogen from the atmosphere.
These microbes deplete the soil of nitrogen and reduce its fertility.
These microbes are commonly called biological nitrogen fixers.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 and 2 only
B2 only
C1 and 3 only
D1, 2 and 3

Did you get this right?

Solution

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.

Your note

More from Chapter 2: Microorganisms : friend and foe

With reference to microorganisms, consider the following statements: 1. They are classified into four major groups: bacteria, fungi, protozoa, and algae. 2. They can survive only in moderate climates and die in ice-cold or hot spring environments. 3. Some microorganisms are unicellular, while others are multicellular. Which of the statements given above is/are correct?

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Which one of the following characteristics distinguishes viruses from other microorganisms?

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Consider the following statements regarding the habitat of microorganisms: 1. They are found in air, water, and soil but not inside the bodies of animals. 2. They can exist freely as well as grow on other organisms. 3. They are capable of surviving in deserts and marshy lands. Which of the statements given above is/are correct?

medium

Which one of the following human diseases is caused by a protozoan pathogen?

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Consider the following pairs of microorganisms and their classification: 1. Aspergillus : Algae 2. Paramecium : Protozoa 3. Penicillium : Fungi 4. Spirogyra : Bacteria How many of the above pairs are correctly matched?

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Correct Option

The correct option is (c) 1 and 3 only.

Explanation

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.