Previous Year Question

1. Alfalfa is a nitrogen-fixing plant.

• The scientific name of alfalfa is Medicago sativa, and it is a leguminous plant belonging to the family Fabaceae.

• Symbiotic bacteria such as Sinorhizobium meliloti are generally found in its root nodules.

• With the help of the nitrogenase enzyme, these bacteria convert atmospheric N₂ into ammonia that can be used by the plant.

• After an alfalfa crop, nitrogen-rich organic material may be added to the field through roots and crop residues.

• Therefore, alfalfa can help improve soil fertility when included in crop rotation.

2. Amaranthus is not a nitrogen-fixing plant.

• Amaranthus belongs to the genus Amaranthus and the family Amaranthaceae.

• Its roots do not form Rhizobium-containing nitrogen-fixing nodules like those of leguminous plants.

• It absorbs nitrate and ammonium already available in the soil.

• Therefore, Amaranthus is not considered a nitrogen-fixing plant.

3. Chickpea is a nitrogen-fixing plant.

• The scientific name of chickpea is Cicer arietinum, and it is a leguminous plant belonging to the family Fabaceae.

• Symbiotic nodules are formed on its roots in association with bacteria belonging to the Mesorhizobium group.

• The bacteria receive carbohydrates and a protected habitat from the plant, while the plant receives fixed nitrogen.

• Due to this symbiotic association, chickpea may require comparatively less external nitrogen fertiliser.

• Chickpea roots and crop residues can contribute to the nitrogen economy of the soil for the succeeding crop.

4. Clover is a nitrogen-fixing plant.

• Clover is a leguminous plant belonging to the genus Trifolium and the family Fabaceae.

• Nitrogen-fixing nodules are formed on its roots in association with Rhizobium bacteria.

• For this reason, clover is used in pastures, as a cover crop and in crop rotation.

• Clover can help increase the available nitrogen and organic matter in the soil.

5. Purslane (Kulfa) is not a nitrogen-fixing plant.

• The scientific name of purslane is Portulaca oleracea.

• It is a succulent plant belonging to the family Portulacaceae and does not form Rhizobium-containing root nodules.

• It does not perform biological fixation of atmospheric nitrogen.

• Therefore, purslane is not considered a nitrogen-fixing plant.

6. Spinach is not a nitrogen-fixing plant.

• The scientific name of spinach is Spinacia oleracea, and it belongs to the family Amaranthaceae.

• Its roots do not form symbiotic nitrogen-fixing nodules.

• Spinach depends upon nitrogen already available in the soil for its growth.

• Therefore, spinach is not a nitrogen-fixing plant.

Important facts

• Technically, the plant itself does not fix N₂; this function is performed by symbiotic bacteria living in its root nodules.

• The nitrogenase enzyme converts atmospheric nitrogen into ammonia.

• This enzyme is sensitive to oxygen; leghemoglobin helps maintain a regulated oxygen environment inside the root nodule.

• Not all nitrogen-fixing plants are legumes; some non-leguminous actinorhizal plants fix nitrogen in association with Frankia bacteria.

• Among the given plants, only alfalfa, chickpea and clover are leguminous and nitrogen-fixing plants.