Which one of the following crops is most suitable for sowing if a farmer wants to naturally enrich the soil with nitrogen through biological fixation?

Introduction / Context:
Soil fertility is a key factor in successful agriculture. Nitrogen is an essential nutrient for plant growth, and farmers often seek ways to increase nitrogen content in their fields. One eco friendly method involves growing certain crops that can fix atmospheric nitrogen in association with bacteria. This question asks you to identify which crop, when sown, helps enrich soil with nitrogen naturally.

Given Data / Assumptions:

We are choosing a crop to enrich soil with nitrogen, not simply to harvest grain or oil. Options include wheat, mustard, sunflower, gram and rice. We assume knowledge of leguminous plants and their symbiotic relationship with nitrogen fixing bacteria. The focus is on biological nitrogen fixation rather than chemical fertilisers.

Concept / Approach:
Leguminous plants, such as peas, beans, lentils and gram, form root nodules that house symbiotic bacteria (for example, Rhizobium). These bacteria can convert atmospheric nitrogen into forms that plants can use, such as ammonia. This process is called biological nitrogen fixation. When leguminous crops like gram are grown and their residues are left in the field or ploughed back, the soil becomes richer in nitrogen. Non leguminous crops such as wheat, mustard, sunflower and rice do not form these nitrogen fixing nodules and therefore do not significantly enrich the soil with nitrogen in this way.

Step-by-Step Solution:
Step 1: Identify which crops in the list are legumes. Gram is a leguminous pulse crop, while wheat, mustard, sunflower and rice are not legumes. Step 2: Recall that legumes host nitrogen fixing bacteria in root nodules that add nitrogen compounds to the soil. Step 3: Recognise that farmers often use legume crops in crop rotation to improve soil fertility without heavy fertiliser use. Step 4: Confirm that gram, like peas and beans, is known for improving soil nitrogen levels when used in rotations or as green manure. Step 5: Conclude that gram is the best choice among the options for enriching soil with nitrogen.

Verification / Alternative check:
You can verify this by thinking of typical crop rotation patterns taught in agriculture and biology. A common rotation might alternate cereal crops such as wheat or rice with legumes such as gram or peas. The legume phase helps restore nitrogen depleted by the cereal phase. Mustard and sunflower are oilseed crops and are not used primarily to fix nitrogen. The emphasis in textbooks and extension programmes on legumes as nitrogen enrichers confirms that gram, the only legume in the list, is the correct answer.

Why Other Options Are Wrong:
Wheat and rice are cereal crops that mainly take up nitrogen from the soil but do not add it through nitrogen fixation. Mustard and sunflower are non leguminous oilseed crops and also depend on soil nitrogen rather than enriching it. None of these non legume crops form nitrogen fixing root nodules. Therefore, they are not preferred if the primary goal is to improve soil nitrogen content through biological means.

Common Pitfalls:
A common mistake is to choose a widely grown staple like wheat or rice simply because it is familiar, without considering its biological properties. Another pitfall is to focus on high yielding or profitable crops rather than on their effect on soil fertility. To avoid such errors, remember that when the exam question mentions enriching the soil with nitrogen, you should immediately think of leguminous crops such as gram, peas, beans and clover. Recognising this association makes these agricultural biology questions much easier.

Final Answer:
To naturally enrich the soil with nitrogen, a farmer should sow a leguminous crop such as gram (a leguminous pulse crop).