Of all the essential nutrients, nitrogen is required by plants in the largest quantity and is most frequently the limiting factor in crop productivity.
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.|