Nitrogen is a part of all living cells, and plays an important role in plant energy cycles, chlorophyll production and protein building. It is one of the three macronutrients essential for plant growth and survival. Nitrogen is a major component in most plant foods because plants need so much of it to survive, and there is often not enough present in the soil.
In 1913, two German scientists seeking a new source of nitrogen for fertilizer and explosives developed the Haber-Bosch process. Using high inputs of heat and pressure, they were able to convert nitrogen in the atmosphere into ammonia. Ammonia is a form of nitrogen that plants can readily absorb and use as food. Though the process is heavily dependent on fossil fuel energy, it has enabled mass food production for a growing world population.
On plant food and fertilizer labels, there is a three number sequence separated by dashes. This is the macronutrient content of nitrogen, phosphorus and potassium (in that order). This number indicates the concentration of each macronutrient in the plant food. So a plant food with a label of 20-0-0 contains nitrogen, but no phosphorus or potassium.
Plant food is often a more balanced blend of nutrients than fertilizers, depending on the plant that is being fed. Some plants, however, require much more nitrogen than the other macronutrients, and so the plant food may consist primarily or exclusively of nitrogen. In these cases, the nitrogen may come in the form of ammonia, urea or solutions of ammonia. Ammonium sulfate should be used with care, as it is the most acidifying form of nitrogen plant food.
Adding excessive amounts of nitrogen to plants may cause damage to the leaves and stress to the plant. Always dilute or apply properly. Test the soil before adding plant food to make sure you're not adding nitrogen when you don't need to. Excess nitrogen from overfeeding plants in the ground leeches into groundwater, runs off into bodies of water, and causes algae blooms that decimate fish populations.
Nitrogen can be added to the soil by mulching with grass clippings or other plant material, adding compost, or fertilizing with manure. Plants called legumes (such as peas, alfalfa, clover and lentils) have a mutually beneficial relationship with bacteria that turns atmospheric nitrogen into ammonia. Lightning also fixes atmospheric nitrogen, which then falls to the ground with rain during a storm.