How do plants obtain usable nitrogen from soil?

Plants cannot directly utilize the abundant nitrogen gas found in the atmosphere. Instead, they obtain usable nitrogen from the soil primarily in the form of nitrate ions ($N{O}_3^{-}$) or ammonium ions ($N{H}_4^{+}$).12

Plants acquire these essential nutrients through several processes:

• Nitrogen Fixation: Certain microorganisms, such as nitrogen-fixing bacteria (e.g., Azotobacter and Rhizobium) and cyanobacteria, convert atmospheric nitrogen into ammonia, which can then be transformed into forms plants can absorb.32
• Decomposition: Microorganisms break down the remains of dead plants, animals, and waste products through ammonification, releasing ammonia and ammonium into the soil.42
• Nitrification: Nitrifying bacteria further transform soil ammonia into nitrates, which are readily incorporated into plant tissues.4
• Fertilization: In agricultural settings, humans supplement soil nitrogen using fertilizers, which provide nitrogen in forms like ammonium salts, urea, or nitrates.2
• Specialized Adaptations: Carnivorous plants obtain nitrogen by digesting the proteins and nucleic acids of the prey they capture.1