Like all organisms, plants need nutrients to grow and carry out basic metabolic processes. Plants draw their nutrients from the soil, so fertile soil is important to their health. Nitrogen is among the most important nutrients plants take from the soil, and many gardeners choose to replenish it with the application of liquid nitrogen fertilizer.
Nitrogen is abundant in the atmosphere but occurs in a highly stable form that plants can't break down to get at the individual molecules. As such, nitrogen is one of the most important nutrients in soil and supports myriad life processes in plants. Nitrogen is an essential component of proteins, enzymes and DNA and is instrumental in processes requiring the transfer of energy. It helps to form chlorophyll, a substance that helps plants convert light into sugar.
According to the Purdue University Cooperative Extension, liquid nitrogen fertilizers are available in three forms. Nitrates are highly water-soluble and come in solids or dissolved in water. Although ammonia exists as a gas, it is condensed under pressure into a liquid. Because it is hazardous and must be injected into the soil, however, ammonia is used for agricultural operations rather than home gardens. Urea is a third form that also dissolves in water to form a liquid fertilizer.
Nitrogen can significantly increase crop yields. Before the discovery of a process by which atmospheric nitrogen could be synthesized into usable agricultural nitrogen, farmers relied on the natural replenishment of soil nitrogen through decomposition, fixation by legume root nodules and a small amount contributed by lightning. According to the U.S. Department of Agriculture, since the introduction of nitrogen fertilizers, U.S. farmers are able to feed 10 times as many people.
Because nitrogen fertilizers tend to dissolve easily in water, they also dissolve easily in rainwater and wash away, leaving soils depleted and local water systems polluted. One form of nitrogen, ammonium, which forms from ammonia or urea, holds a slight charge that causes it to stick to soil particles, but other forms are easily washed away. Always follow the instructions on the packaging to reduce leaching and runoff.
The same quality that causes nitrogen compounds to dissolve easily in water also draws water from plants and, if used in excess, can cause fertilizer burn. Fertilizer burn is actually dehydration, caused when nitrogen fertilizers pull water from plants. Overuse of nitrogen fertilizers also has dire consequences for local ecosystems. Throughout the world, aquatic dead zones can be attributed to nitrogen fertilizer runoff that causes aquatic plants to grow out of control, choking out all other life.