Question

An organism used as a biofertilizer for raising soybean crop is
A. Nostoc
B. Azotobacter
C. Azospirillum
D. Rhizobium

Answer 1

Hint: The word biofertilizer is made up of two words, ‘Bio’ meaning natural and organic, and ‘Fertilizer’. Biofertilizers are natural substances and organisms that are used to increase the productivity of crop plants by enhancing and enriching the nutrient quality of the soil.

Complete answer: Soybean is a leguminous plant. Soybean belongs to the pea family of the Fabaceae family. The leguminous plants have nodules in their roots which house the symbiotic bacteria that help them to grow and provide the plants with nitrogenous nutrition. Rhizobium is a bacteria that live inside the root nodules of the leguminous plants. They form a symbiotic association with the plant. They fix the atmospheric nitrogen for the plants. They are used in soybean and pea plant fields as biofertilizers.
a) Nostoc – It is a cyanobacterium which is also known as the Blue-green algae or BGA. They are the autotrophic microbes. These cyanobacteria can fix atmospheric nitrogen into the nitrogenous compounds using their special cells known as the heterocysts. They are used as biofertilizers in the paddy fields. They reduce the alkalinity of the soil.
b) Azotobacte – It is a free-living bacterium that fixes the atmospheric nitrogen into the nitrogenous compounds while living freely in the soil. These bacteria enrich the nitrogen content of the soil. They are used in the tea and coffee fields as biofertilizers.
c) Azospirillum – It is a free-living bacterium that fixes the atmospheric nitrogen into the nitrogenous compounds while living freely in the soil. These bacteria enrich the nitrogen content of the soil. They are used in the tea and coffee fields as biofertilizers.
Hence, the correct answer is option D.

Note: Apart from Nostoc, some other Cyanobacteria like Anabaena, Aulosira, and Oscillatoria are also used as biofertilizers. Anabaena azollae is found in the leaves of the Azolla plant where it fixes the nitrogen. Due to this reason, it is used in the rice fields. Anabaena cycadae is found in the coralloid roots of gymnosperm Cycas. Aulosira is found in the rice fields in India.