Concept:Crop rotation maintains soil fertility because leguminous crops can restore lost nitrogen to the soil.
Explanation:Continuous cropping reduces soil nutrients, especially nitrogen.
Some crops in the rotation are legumes.
Legumes have root nodules containing nitrogen-fixing bacteria.
These bacteria convert atmospheric nitrogen into a form plants can use.
Thus, the soil is enriched with nitrogen naturally.
After cropping, the remaining roots and nodules also add organic matter when they decompose.
This is why crop rotation helps sustain soil fertility over time.
Answer:B. some of the crops fix nitrogen in the soil