Plants and animals must have the element nitrogen because it is an important component of nucleic acids and proteins.

How do these organisms obtain nitrogen?

A
Plants and animals get nitrogen directly from the air the same as
they do oxygen.
B
Bacteria get nitrogen from the air, and plants and animals get
nitrogen by eating the bacteria.
C
Nitrogen fixing bacteria convert nitrogen from the air into ammonia,
and animals ingest the ammonia.
D
Bacteria in the soil convert nitrogen from the air into a form that
plants can use, and animals get nitrogen by eating the plants.