Plants use Nitrogen in the form of

Step 1: Analyze the question The question asks which form of nitrogen plants mainly absorb from the soil. Plants can't use the free nitrogen gas (N₂) in the air, so it must first be converted into a usable compound.

Step 2: The concept: the nitrogen cycle

• Nitrogen fixation: Bacteria such as Rhizobium (in legume root nodules) and Azotobacter convert N₂ into ammonia/ammonium.
• Nitrification: In the soil, Nitrosomonas converts ammonium into nitrites. Then Nitrobacter converts nitrites into nitrates.
• Absorption: Plant roots take up nitrogen mainly as nitrate ions (NO₃⁻). This is the most abundant and most readily absorbed form in well-aerated farmland soil.
• Use in the plant: The nitrates are converted into amino acids and then proteins.

Step 3: Evaluate the options

• A. Ammonia: it is toxic to plants in large amounts and is not the main form absorbed.
• B. Ammonium: plants can absorb some of it, but it is quickly converted to nitrate by nitrifying bacteria. It is not the main form.
• C. Nitrates: the principal form plants absorb.
• D. Nitrites: these are toxic to plants and are only an intermediate product.
• E. Urea: it must first be converted by soil bacteria (ureases) into ammonia and then into nitrates.

Answer: C. Nitrates

Tip: Remember the sequence N₂ → Ammonia/Ammonium → Nitrite → Nitrate → Plant protein. The nitrate is the last stage before the plant takes it up. In exams, whenever you see "form of nitrogen absorbed by plants," think nitrates.