Here's why:

* Nitrogen is essential: Nitrogen is a crucial component of proteins, DNA, and chlorophyll, all of which are vital for plant growth and development.

* Atmospheric nitrogen is unusable: While nitrogen makes up 78% of the atmosphere, plants cannot directly use this form of nitrogen.

* Nitrogen fixation: Nitrogen-fixing bacteria convert atmospheric nitrogen into a usable form, such as ammonia or nitrates, through a process called nitrogen fixation.

* Symbiosis: Many plants have symbiotic relationships with nitrogen-fixing bacteria. These bacteria live in nodules on plant roots and provide the plant with fixed nitrogen in exchange for sugars and other nutrients.

Let me know if you'd like to know more about the nitrogen cycle or plant-bacteria interactions!