What is the difference between nitrogen fixation and nitrification?

To become biologically available, inert nitrogen gas from the atmosphere must be "fixed"—that is, combined with hydrogen to form ions of ammonium (NH ). This transformation, called nitrogen fixation, can be accomplished when air in the top layer of soil comes in contact with various specialized nitrogen-fixing bacteria. Once atmospheric N2 has been converted into ammonium, other specialized bacteria perform a process known as nitrification. In this process, ammonium ions are first converted into nitrite ions (NO ), then (by yet another group of specialized microorganisms) into nitrate ions (NO ).