Agronomy Degree Practice Exam

Which nutrient do legumes primarily fix from the atmosphere?

1. Nitrogen

2. Oxygen

3. Carbon dioxide

4. Potassium

The correct answer is: Nitrogen

Legumes primarily fix nitrogen from the atmosphere through a symbiotic relationship with nitrogen-fixing bacteria, such as Rhizobium, found in their root nodules. This process is a vital aspect of the nitrogen cycle, allowing legumes to convert atmospheric nitrogen gas (N₂) into ammonia (NH₃), which can then be utilized by the plant for growth and development. The ability to fix nitrogen enables legumes to enrich the soil with this essential nutrient, making them valuable in crop rotations and as cover crops. This is particularly important for sustainable agriculture, as it reduces the need for synthetic nitrogen fertilizers and promotes soil health. In contrast, oxygen is not fixed by legumes; rather, it is produced during photosynthesis. Carbon dioxide, while fundamental for plant growth, is absorbed during photosynthesis rather than being fixed in the same way nitrogen is. Potassium is a macronutrient important for various plant functions but is not an element that legumes or any other plants fix from the atmosphere.