Plants get the nitrogen they need for amino acids and DNA mainly from the soil, not directly from the air.

Main nitrogen sources for plants

  • Nitrates in the soil:
    The primary source is nitrate ions (NO3−)(NO_3^-)(NO3−​) dissolved in soil water, which roots absorb and then convert into ammonia inside the plant; this ammonia is used to build amino acids and nucleotides for DNA.
  • Ammonium and ammonia:
    Plants can also take up ammonium (NH4+)(NH_4^+)(NH4+​) or ammonia-derived compounds directly from the soil when available, especially in fertile or fertilized soils.

Role of bacteria and the nitrogen cycle

  • Nitrogen‑fixing bacteria:
    Specialized bacteria in the soil and in root nodules of some plants (like legumes) convert atmospheric nitrogen gas (N2)(N_2)(N2​), which plants cannot use directly, into ammonia that becomes available to plants.
  • Organic matter and proteins:
    Decomposers break down dead organisms and organic matter, releasing nitrogen compounds (like amino acids and ammonium) that plants can absorb as part of their overall nitrogen supply.

Why not directly from the air?

  • Problem with N2N_2N2​ gas:
    Although the atmosphere is rich in nitrogen gas, its very strong triple bond makes it chemically “locked,” so most plants cannot break it and therefore cannot use N2N_2N2​ directly through their leaves or stomata.

In short, plants rely on soil nitrogen (nitrates, ammonium, and bacteria‑fixed ammonia) to create the amino acids and DNA they need, even though the original nitrogen ultimately comes from the atmosphere.

Information gathered from public forums or data available on the internet and portrayed here.