Crops that increase the natural fertility of the soil through nitrogen fixation are known as:

Legumes are plants of the pea family (Fabaceae or Leguminosae), such as gram, peas, beans, lentils, groundnut, soybean, berseem and clover. They improve soil fertility by fixing atmospheric nitrogen into forms that plants can use.

How legumes fix nitrogen:

• The roots of legumes have small swellings called root nodules.
• These nodules contain symbiotic bacteria, mainly of the genus Rhizobium.

The bacteria convert nitrogen gas (N2) from the air into ammonia, using an enzyme called nitrogenase. This process is called biological nitrogen fixation.

The plant supplies the bacteria with food (carbohydrates) and a suitable environment, and in return receives nitrogen compounds. This is a mutually beneficial (symbiotic) relationship.

The pink colour inside active nodules comes from leghaemoglobin, which protects the nitrogenase enzyme from oxygen.

How this improves the soil:

• When legume roots, leaves and stubble decay, the nitrogen stored in them is released into the soil, benefiting the next crop.
• Legumes can add a significant amount of nitrogen per hectare, reducing the need for chemical nitrogen fertilisers.

Examples of legume crops:

• Pulses: gram (chickpea), tur (pigeon pea), moong, urad, lentil, peas.
• Oilseed legumes: groundnut and soybean.
• Fodder legumes: berseem, lucerne (alfalfa), cowpea.
• Green manure crops: sunn hemp (sanai) and dhaincha, which are grown and ploughed into the soil while still green.

Biofertilisers:

• Rhizobium cultures are applied to legume seeds as biofertilisers to improve nodulation and nitrogen fixation.
• Other biofertilisers include Azotobacter and Azospirillum (free-living or associative nitrogen fixers for non-legumes), blue-green algae (cyanobacteria) and Azolla for rice, and phosphate-solubilising bacteria.

Why not the other options?

• ✗Cereal crops (rice, wheat, maize) take up large amounts of nitrogen and do not fix it.
• ✗Fibre crops such as cotton and jute do not fix nitrogen.
• ✗Oilseed crops include some legumes, like groundnut, but most oilseeds, such as mustard and sunflower, are not nitrogen fixers.