Nitrogen is essential to plants, and plenty of it exists in the air around us. Because plants can't breathe nitrogen, they must get it from the soil. If the soil is depleted of nitrogen, a plant can't grow successfully. Legumes such as peanuts and potatoes have evolved a mechanism to get the nitrogen they need and leave some behind in the soil. Rhizobium bacteria that grow on the plants' roots receive enough energy from the plant to produce sufficient nitrogen for the plant and the soil around it. This can keep the soil fertile for future planting.
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