Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A : Rotation with legume crops can increase the nitrogen levels in the soil thus improving the soil fertility
Reason R : Legume crops harbor nitrogen fixing bacteria in the root nodules that can convert inert nitrogen from the atmosphere to plant usable form
In light of the above statements, choose the correct answer from the options given below

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Crop rotation with legumes is often called "green manuring" if the crop is ploughed back into the soil.
It helps reduce the dependence on synthetic nitrogenous fertilizers.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks about the relationship between crop rotation using legumes and soil fertility, focusing on the biological mechanism of nitrogen fixation.
Detailed Explanation:

• Legume-Rhizobium Symbiosis: Leguminous plants (like pulses, beans, clover) have a symbiotic relationship with Rhizobium bacteria. These bacteria reside in specialized structures called root nodules.

• Biological Nitrogen Fixation (BNF): Inside the nodules, the bacteria use the enzyme nitrogenase to convert atmospheric nitrogen ($N_2$), which is inert and unusable by plants, into ammonia ($NH_3$), which is a plant-usable form.

• Impact on Soil Fertility: After the legume crop is harvested, the nitrogen-rich roots and biomass left in the soil decompose, significantly increasing the available nitrogen content for the subsequent crop. This is why crop rotation with legumes is a cornerstone of sustainable agriculture.

• Relationship: Assertion A states that legumes improve soil fertility via nitrogen increase. Reason R provides the exact mechanism (root nodule bacteria fixing atmospheric $N_2$). Thus, R is the correct explanation for A.
Final Answer:
Both the assertion and the reason are true, and the reason correctly explains the assertion. Thus, the answer is option (A).