One of the main plant nutrients and used by plants to make all proteins, and therefore as well enzymes, chlorophyll and many other essential parts of plants. The amount of nitrogen available to a plant is often the factor which limits it rate of growth and its behaviour within the soil is a very complex one with the amount available to the plant changing constantly. Like all plant nutrients; nitrogen has to be in a suitable form for the plants to take up, as an element it is a gas making up approximately 80% of the air we breathe but in that form is of no use to plants. The important exception to this last point is the legume crops which have evolved a way around this.