Nitrogen is the most abundant element in the atmosphere, forming 78 percent of the air that people breathe. Nitrogen is also one of the essential elements required by living organisms, required for the formation of protein and DNA. Nitrogen is one of the major ingredients in both organic and synthetic fertilizers. Nitrogen can take different forms in the soil, each of which plays its own role in soil fertility and plant health.
Many people use the terms "ammonia" and "ammonium" interchangeably when they refer to different chemical compounds. Ammonia consists of one nitrogen atom and three hydrogen atoms, and it is not a charged particular, meaning the number of positive and negative particles balance each other. Ammonium, on the other hand, contains one nitrogen atom and four hydrogen atoms, giving the molecule a slightly positive charge. Both forms are available in soil. Nitrate--also available in soil--consists of one nitrogen atom and three oxygen atoms, giving the molecule a slightly negative charge.
Although nitrogen is abundant in the air around them, plants cannot use it. Atmospheric nitrogen occurs in pairs, making a very stable molecule that is not easily broken. Making nitrogen available to plants requires a process called fixation, where nitrogen forms a molecule that is more easily broken, freeing the nitrogen for use by the plant. Both ammonia and nitrate provide nitrogen in fixed form, meaning both can be used by plants.
Although both ammonia and nitrate can be used by plants in the soil, they play different roles in the process. Ammonia generally remains only temporarily in the soil and is quickly converted to other nitrogen-containing compounds, include nitrates. Ammonia is produced through plant and animal decay and through the actions of nitrogen-fixing bacteria in the roots of legume plants. Through the process of nitrification, ammonia is converted into nitrates. Nitrates are formed directly when atmospheric nitrogen combines with oxygen during lightning storms.
Ammonia is applied directly as a fertilizer, forced into a liquid state under high pressure in tanks and injected into the soil, where it combines with water to form nitrates. Because ammonia exerts such a powerful attraction to water, it can cause burns, both to plants and people, if mishandled. Raw manure, which is high in ammonia, can leach water from plants, causing them to dehydrate. Nitrates are not applied in pure form but derive from other nitrogen-containing compounds, including ammonia.
Both ammonia and nitrates are part of the nitrogen cycle that sustains all life. However, overuse of synthetically produced fertilizers has dire consequences for the environment. Because nitrates have a slightly negative charge, they do not attach easily to soil. Instead, if used in excess, they run off into groundwater and waterways, contributing to pollution and aquatic dead zones. Because ammonia converts into nitrates, it has a similar impact on the environment.