Leguminous crops are incorporated into crop rotation in order to

Step 1: Analyze the question The question asks why legumes (beans, cowpea, groundnut, soybean) are included in crop rotation.

Step 2: Explain the concept Legumes have root nodules that house Rhizobium bacteria. These bacteria live in a mutualistic (symbiotic) relationship with the plant. They convert atmospheric nitrogen (N₂) into nitrogen compounds such as ammonia, which the plant can use. This process is nitrogen fixation.

When the legume dies or its residues are ploughed back into the soil, the fixed nitrogen is released and enriches the soil. The next crop, such as maize or yam, which uses up a lot of nitrogen, then benefits.

Step 3: Evaluate the options

• A. Improve aeration: Aeration is improved by tillage and by loosening the soil, not mainly by growing legumes.
• B. Promote nitrogen fixation: Correct. Root nodule bacteria fix nitrogen and restore soil fertility.
• C. Increase the rate of soil formation: Soil formation depends on weathering of rocks and decay of organic matter, and crop rotation does not mainly affect it.
• D. Improve water holding capacity: This is mostly improved by humus and organic matter, and it is not the main reason for using legumes.

Step 4: Answer The correct option is **B.

Exam tip: Whenever you see "legume," think "root nodules, Rhizobium, nitrogen fixation, soil fertility." A common memory line is: Legumes Leave nitrogen.