Nitrogen makes up about 78% of the earth’s atmosphere however the atmosphere of Mars contains less than 3% nitrogen. The element seemed so inert that Lavoisier named it azote, meaning "without life". However, its compounds are vital components of foods, fertilizers, and explosives. Nitrogen gas is colourless, odourless, and generally inert. As a liquid it is also colourless and odourless. Nitrogen is "fixed" from the atmosphere by bacteria in the roots of certain plants such as clover, hence the usefulness of clover in crop rotation.