Nitrogen is one of the essential elements for plant growth, and one of the three most commonly supplemented with fertilizers, the "N" in the N-P-K numbers printed on the label of all fertilizer bags and boxes. It is also part of a complex process, the nitrogen cycle, that converts plant and animal waste into a form that plants can use to build their leaves and other tissues.
The Importance of Nitrogen
Nitrogen is one of the building blocks that all living organisms use to create amino acids and nucleic acids such as DNA and RNA. Though a huge amount of nitrogen exists as a gas in the atmosphere, plants and animals cannot use it directly. Gaseous nitrogen must be converted into compounds such as ammonium (NH4) and nitrate (NO3) before it can be absorbed by plant roots.
The Nitrogen Cycle
Nitrogen enters the soil through rainfall, the decay of plant and animal matter, nitrogen fixation by bacteria and applications of fertilizer. Some is lost through leaching, some is converted by bacteria into forms that can be taken up by plants and some is converted to gaseous nitrogen by microorganisms. Fossil fuel emissions also return nitrogen to the atmosphere.
Forms of Nitrogen
When plant and animal matter decays, bacteria and fungi convert some of the organic compounds into ammonium (NH4), which has a positive charge. It is attracted to clay and humus particles with a negative charge and held there. As it is released over time, other bacteria may convert it to nitrate (NH3) and nitrite (NH2), which are forms that plants can use. These are easily leached from the soil by rainfall, however, and may not remain available to plants for very long.
Nitrogen Uptake By Plants
Both ammonium and nitrate can be taken up by roots and used by plants, but nitrate is the form that is most often absorbed. Some plants, such as legumes, have a partnership with certain bacteria that take nitrogen from the air and convert it into a form that the plant can use, receiving carbohydrates in exchange. These bacteria form nodules on the roots and the nitrogen is absorbed directly from these structures.
Effects of Fertilization
Applying nitrogen in the form of ammonium or nitrate will promote leaf growth in the plant. Because nitrate is soluble, it is often in limited supply in the ground, so fertilization has an immediate and noticeable effect. Long-term effects, however, may be more subtle. The leached nitrate enters the groundwater or runs off into streams and lakes, creating pollution problems and causing excess algae growth. In long-term studies, fertilization seems to be associated with a decrease in carbon in the soil and a decrease in crop yields.