Nitrogen-fixing bacteria living in the soil or in root nodules are particularly useful for improving the growth of which type of crop plants?

Introduction / Context:
Nitrogen is an essential nutrient for plants, but atmospheric nitrogen gas cannot be directly used by most plants. Nitrogen-fixing bacteria convert nitrogen gas into usable forms such as ammonia and nitrates. These bacteria are especially important in agriculture because they naturally enrich the soil. This question asks which group of crops benefits most directly from nitrogen-fixing bacteria, particularly those forming root nodules.

Given Data / Assumptions:

We are considering nitrogen-fixing bacteria in the soil and in root nodules. The crops listed include wheat, paddy, legumes, maize and oilseeds. We assume standard knowledge that certain crops form symbiotic relationships with Rhizobium and related bacteria. We focus on crops where biological nitrogen fixation is especially notable.

Concept / Approach:
Legume crops, such as peas, beans, gram and clover, form symbiotic associations with Rhizobium and other nitrogen-fixing bacteria in their root nodules. The bacteria convert atmospheric nitrogen into ammonium, which plants can use to build proteins and nucleic acids. In return, the plant supplies carbohydrates and a protective habitat. This relationship greatly reduces the need for nitrogen fertilizers in legume fields. While cereals like wheat, paddy and maize also benefit indirectly from nitrogen fixed in the soil, they do not typically form such nodules, making legumes the primary answer.

Step-by-Step Solution:
Step 1: Recall that legumes have characteristic root nodules filled with nitrogen-fixing bacteria. Step 2: Recognise that common legumes include pea, bean, groundnut, lentil and clover. Step 3: Understand that wheat, paddy and maize are cereals that usually require external nitrogen fertilizer and do not form nodules with Rhizobium. Step 4: Note that although nitrogen fixed by free-living bacteria can eventually benefit many crops, the direct and strongest partnership is with legumes. Step 5: Therefore, legume crops are the ones most clearly described as benefiting from nitrogen-fixing bacteria in root nodules.

Verification / Alternative check:
A practical check is to think about crop rotation practices. Farmers often rotate cereals like wheat or rice with legume crops to naturally replenish soil nitrogen. When legume plants are ploughed back into the soil, the nitrogen stored in their tissues and nodules becomes available to the next crop. Agricultural textbooks specifically mention Rhizobium-legume symbiosis as a key example of biological nitrogen fixation. These real-world practices confirm that legume crops are the primary beneficiaries in the context of this question.

Why Other Options Are Wrong:
Wheat, paddy and maize are important cereal crops, but they do not typically form root nodules containing nitrogen-fixing bacteria. They depend largely on soil nitrogen or fertilizers. Oilseed crops like mustard may benefit indirectly from increased soil nitrogen but are not classic examples of nitrogen-fixing symbiosis. Therefore, while many crops enjoy the general soil improvement caused by nitrogen fixation, only legume crops have a direct and strong association with nitrogen-fixing bacteria in nodules, making them the correct answer.

Common Pitfalls:
Students sometimes confuse general soil fertility benefits with specific symbiotic relationships. This may lead them to choose a common cereal crop like wheat or rice because of their importance in diets. Another pitfall is to overlook the key word nitrogen-fixing and focus only on crop names. To avoid this, always associate legume crops with root nodules and Rhizobium when you see questions about biological nitrogen fixation in agriculture.

Final Answer:
Nitrogen-fixing bacteria are particularly useful for improving the growth of legume crops such as peas and beans.