Plants use nitrogen in the form of nitrate (NO₃^-) and ammonium (NH₄⁺) to make protein.

Here's why:

* Nitrogen is a key component of amino acids, which are the building blocks of proteins.

* Plants cannot directly use atmospheric nitrogen (N₂), which makes up about 78% of the air. They need it in a usable form.

* Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia (NH₃) through a process called nitrogen fixation.

* Ammonia can be directly absorbed by plants, but it is often converted to ammonium (NH₄⁺) in the soil.

* Nitrifying bacteria then convert ammonium into nitrite (NO₂^-) and then into nitrate (NO₃^-).

* Nitrate is the preferred form of nitrogen for most plants, and it is readily absorbed through their roots.

Once inside the plant, both nitrate and ammonium are used to synthesize amino acids, which are then used to build proteins.