Plants require nitrogen because the element is a chief component of many of their structures and metabolic processes. Among other things, nitrogen is a critical ingredient in chlorophyll, the chemical that enables photosynthesis to occur. Nitrogen is one of the six primary macronutrients that plants need, and many gardeners use fertilizers to ensure that their plants have enough of it.
Nitrogen is a component of many plant structures, but it plays an especially important role in the structure of the roots. When the roots have adequate nitrogen, they perform more efficiently. This allows the plant to draw more water and other nutrients, and produce more growth. When plants do not have enough nitrogen, they languish and die. Small size, poor growth and yellow or red leaves are all symptoms of nitrogen deficiency. Too much nitrogen can also cause problems for plants. This does not happen very often in the natural world, but gardeners can cause nitrogen toxicity if they use too much fertilizer.
As it makes up the bulk of the Earth’s atmosphere, plants are surrounded by gaseous nitrogen. However, they cannot use it in this form; they must absorb it through their roots from a water-based solution in the soil.