Nitrogen
Of all the essential nutrients, nitrogen is required by plants in the largest quantity and is most frequently the limiting factor in crop productivity.
- In plant tissue, the nitrogen content ranges from 1 and 6%.
- Proper management of nitrogen is important because it is often the most limiting nutrient in crop production and easily lost from the soil system.
Nitrogen Forms and Function
Forms of nitrogen available for plant uptake
- Ammonium
- Nitrate
Functions of nitrogen in plants
- Nitrogen is an essential element of all amino acids. Amino acids are the building blocks of proteins.
- Nitrogen is also a component of nucleic acids, which form the DNA of all living things and holds the genetic code.
- Nitrogen is a component of chlorophyll, which is the site of carbohydrate formation (photosynthesis). Chlorophyll is also the substance that gives plants their green color.
- Photosynthesis occurs at high rates when there is sufficient nitrogen.
- A plant receiving sufficient nitrogen will typically exhibit vigorous plant growth. Leaves will also develop a dark green color
Gains of Nitrogen to the Soil
- Biological and Atmospheric Fixation: Conversion of atmospheric nitrogen to ammonium which is subsequently available for plant uptake
- Direct additions of commercial and organic fertilizers
Transformations in the Soil
- Mineralization: Conversion of organic nitrogen to ammonium
- Nitrification: Conversion of ammonium to nitrate
Losses of Nitrogen from the Soil
- Denitrification: Conversion of nitrate to atmospheric forms of nitrogen
- Volatilization: Loss of gaseous ammonia to the atmosphere
- Run-off
- Leaching
- Consumption by plants and other organisms
Nitrogen is a very dynamic element. It not only exists on Earth in many forms, but also undergoes many transformations in and out of the soil. The sum of these transformations is known as the nitrogen cycle.
| Form of Nitrogen | Formula | Availability for plant uptake |
| Ammonia | NH3 | Ammonia is a gas. Ammonium can escape from the surface of the soil under certain conditions and is harmful to plants in high quantities. Ammonium is the basic building block of commercial nitrogen fertilizers. |
| Ammonium | NH4+ | Soil particles attract and retain ammonium on cation exchange complexes. This form may be directly taken up by plants. |
| Nitrate | NO3– | Nitrate is the second form of nitrogen which is available for plant uptake. In most soils, nitrate is highly mobile. However, in the highly weathered soils of Hawaii, nitrate is stored in soils with ‘anion exchange capacity’ and becomes less mobile. |
| Nitrate | NO2– | Nitrite is an intermediate product in the conversion of ammonium to nitrate (nitrification). It is usually present in low quantities, but is toxic to plants. |
| Organic Nitrogen | Various compounds | Organic nitrogen must be converted to ammonium before it is used by plants. This conversion occurs with time and is known as mineralization. |