In the nitrogen cycle, which form is most readily assimilated by plants?

• A. Nitrate NO3-
• B. Ammonium NH4+
• C. Molecular nitrogen N2
• D. Nitrite NO2-

Answer: (C)

Plants can’t use atmospheric nitrogen (N2) directly. They rely on fixed forms of nitrogen produced by bacteria and other microbes in the nitrogen cycle. Nitrogen-fixing bacteria convert N2 into ammonia, which becomes ammonium (NH4+). In soils, another step called nitrification makes nitrate (NO3-), and it’s this form that plants typically absorb most readily through their roots.

Nitrate is highly soluble and easily transported with water to the root surface, and plants have efficient transport systems to take it up. Ammonium can also be taken up, but it’s more variable and can acidify the soil and become problematic under certain conditions. Because of these factors, nitrate (NO3-) is usually the form plants assimilate most readily in typical soils.

So the idea to remember: N2 isn’t directly usable by plants; fixation produces usable forms, and nitrate is the form plants most commonly take up.