Here's why:

* Nitrogen gas (N₂) is abundant in the atmosphere, but most organisms can't directly use it. They need nitrogen in a more readily usable form.

* Nitrogen fixation is the process by which nitrogen gas is converted into usable forms, primarily by certain bacteria.

* Nitrate and ammonium are the most common forms of nitrogen that plants can absorb through their roots.

* Animals get their nitrogen by consuming plants or other animals that have obtained nitrogen from plants.

Let me know if you'd like to delve deeper into the nitrogen cycle!