Question

Nitrogen is an essential nutrient for plant growth. But farmers who cultivate pulse crops like green gram, Bengal gram, black gram, etc, do not apply nitrogenous fertilizers during cultivation. Why?

Answer 1

Hint: Plants like pulses have special bacteria present on their root nodules that are capable of fixing the atmospheric nitrogen. Some bacteria are also free living and are found independently too like Anabaena, Nostoc etc.

Complete answer:
Rhizobium is a bacterial genus which is associated with the formation of plant root nodules. Rhizobium enters root legumes through cracks and tears or root hairs. Rhizobium bacteria fix nitrogen from the atmosphere and turn it into ammonia which acts as a natural fertilizer. The Rhizobium bacteria colonize the plant cells of the root nodules. They use these plant cells to convert nitrogen from the atmosphere to ammonia with the help of the enzyme nitrogenase. The Rhizobium bacteria then provide organic compounds like glutamine to the plant. The relationship of Rhizobium and plants is a mutualistic relationship as the Rhizobium provides the plant with nitrogen and the plant provides the Rhizobium with organic compounds derived from photosynthesis.
Nitrogen is the most scarce nutrient in most soil. Hence it is the most used fertilizer nutrient. The Rhizobium helps plants overcome the scarcity of nitrogen in soil by providing them with nitrogen. Leguminous plants like green gram, Bengal gram and black gram have enough root hairs and root legumes for the optimal growth of the Rhizobium bacteria. Hence, farmers do not apply nitrogenous fertilizers during cultivation.

Note:
Rhizobium was first isolated by Martinus Beijerinck in 1888 from the nodules of legumes.
Rhizobium is a diazotrophic bacterium.
Certain Rhizobium species are pathogenic to plants. They include:
-Rhizobium rhizogenes
-Rhizobium radiobacter