Which essential nutrient is not readily available from air or water and must be supplied to plants?

• A. Nitrogen
• B. Oxygen
• C. Hydrogen
• D. Carbon dioxide

Answer: (A)

Nitrogen is essential, but it’s not readily available to plants from air or water. Although the atmosphere is full of nitrogen gas (N2), that form is very stable and not directly usable by most plants. They need nitrogen in reactive forms such as nitrate (NO3−) or ammonium (NH4+), which come from soil minerals, organic matter breakdown, or manufactured fertilizers. Because those usable forms must be supplied, nitrogen fits the description of an essential nutrient that isn’t readily available from air or water. Oxygen, hydrogen, and carbon dioxide are accessible from air or water—oxygen from air and water, hydrogen from water, and carbon dioxide from air for photosynthesis—so they don’t require the same kind of external supply.