What is the primary source of nitrogen for plants? 

Nitrogen is an essential nutrient for plants, and the primary source of nitrogen for plants is nitrogen compounds in the soil. Nitrogen in the form of nitrate (NO₃^-) and ammonium ions (NH₄⁺) can be taken up by plant roots and used to synthesize amino acids, nucleic acids, and other nitrogen-containing compounds that are essential for plant growth and development. These nitrogen compounds are produced by nitrogen-fixing bacteria and other soil organisms, which convert atmospheric nitrogen into forms that can be used by plants. 

Nitrogen gas in the atmosphere cannot be readily used by plants. Nitrogen compounds in water are not a significant source of nitrogen for most plants, and nitrogen compounds in other plants are not a direct source of nitrogen for most plants.