1. Nitrate (NO₃⁻): This is the most common form of nitrogen used by plants. It is readily absorbed by plant roots and used to synthesize amino acids, nucleic acids, and other nitrogen-containing molecules.

2. Ammonium (NH₄⁺): While less common than nitrate, ammonium is also readily absorbed by plants. It is often produced by nitrogen-fixing bacteria and can be directly used by plants for growth.

These two forms are essential for plant growth and, consequently, the entire food chain.