The bacterium which reduces the fertility of soil is
A) Nitrosomonas
B) Bacillus denitrificans
C) Azotobacter sp.
D) Nitrobacter

Answer Verified Verified
Hint: The ability of soil to promote soil growth is called soil fertility. Important factors that make soil fertile are the presence of nutrients like nitrogenous compounds and microorganisms.

Complete Answer:
To solve this answer we must know the types of bacteria present in the soil.
Fertile soil is the prerequisite for good crop yield. A fertile soil has rich deposits of all the essential minerals like nitrogenous compounds like nitrates and nitrites. It should maintain an adequate pH of 5.5-7. It should also have useful microorganisms which help in making nitrogen available to the plants.

The types of bacteria present in soil are-
A) Nitrosomonas- These are the bacterias which oxidises ammonia into nitrites by the process called nitrification. This organism makes the soil fertile.
B) Denitrifying bacteria- This bacterium carries out denitrification of soil. They convert the nitrates present into soil to free atmospheric nitrogen. Plants cannot utilise free nitrogen. Thus making the soil infertile.
C) Azotobacter- This bacterium has the ability to fix atmospheric nitrogen non-symbiotically. The free nitrogen is converted into nitrates, nitrites and ammonium compounds. As a result of which soil is made fertile.
D) Nitrobacter- This bacterium is responsible for oxidizing nitrites in the soil to nitrates. This nitrate enhances the soil fertility.

Thus the correct answer is (B) The bacterium which reduces the soil fertility is Bacillus denitrificans.

Note: Nitrogen is the important molecule in the living organisms. Free nitrogen in the atmosphere cannot be utilised by the plants. It is converted into utilisable form like nitrates, nitrites and ammonium compounds by microorganisms which are taken up by the plants to carry out metabolic activities.