The largest reservoir of nitrogen is in the Earth’s atmosphere, where it makes up 78% of the air by volume. However, this nitrogen is not usable in that form because it is a gas at atmospheric pressure and temperature. In order to use nitrogen for agricultural or industrial purposes, it must be “fixed” into a more usable form such as ammonia (NH3) or nitrate (NO3-). This process can be done chemically or biologically, but most commercial fertilizer is produced via the Haber-Bosch process, which uses high temperatures and pressures to combine nitrogen and hydrogen gases to produce ammonia.