How do most plants obtain the nitrogen they need for growth?

• A. By directly fixing atmospheric nitrogen (\(N_2\)) through their roots
• B. By absorbing nitrate and ammonium ions from the soil
• C. By absorbing nitrogen gas (\(N_2\)) directly from the air through their leaves
• D. By producing nitrogen through photosynthesis

Answer: (B)

Verified step by step guidance

1. Understand that nitrogen is an essential nutrient for plants, as it is a key component of amino acids, proteins, and nucleic acids.
2. Recognize that most plants cannot directly use atmospheric nitrogen (\(N_2\)) because it is highly stable and requires conversion into a usable form.
3. Learn that nitrogen fixation is primarily carried out by certain bacteria (e.g., Rhizobium) in the soil or in symbiotic relationships with plants, converting \(N_2\) into ammonia (\(NH_3\)).
4. Understand that plants absorb nitrogen primarily in the form of nitrate (\(NO_3^-\)) and ammonium (\(NH_4^+\)) ions from the soil through their roots.
5. Note that photosynthesis does not produce nitrogen; it is a process that converts light energy into chemical energy stored in glucose, and nitrogen must be obtained externally from the soil.