Why do plants absorb nitrogen from the ground and not the air?

Question

Why would the plant need nitrogen from the ground when the air is 78% nitrogen?

In: Biology

Answer 1

There actually are some plants that can absorb nitrogen from the air. Legumes for example do this, and it’s a process called nitrogen-fixing. However, plants didn’t evolve the ability to do it themselves. Instead, they evolved to form symbiotic relationships with nitrogen-fixing bacteria, that they incorporate into their own structure similar to the gut bacteria in humans. It’s much easier for plants to evolve this symbiotic relationship than to directly evolve nitrogen fixation mechanisms, so this is the thing that evolves.