The main ingredient of air is nitrogen, which makes up about 78% of the Earth's atmosphere. Nitrogen gas cannot be directly used by most organisms, but it is essential for plant growth as a nutrient. The second most abundant gas in air is oxygen, which makes up about 21% of the atmosphere and is essential for animal respiration. There are also small amounts of other gases in the atmosphere, such as carbon dioxide, argon, neon, helium and hydrogen.