Nitrogen is one of the most important nutrients for crop growth, second only to water, and is the major nutrient the producer can control. Nitrogen exists in many different chemical forms and passes around natural and agricultural ecosystems in a cycle. The various forms of nitrogen determine its availability to plants or whether nitrogen escapes and is no longer available to plants. The supply of useable nitrogen and the rate of losses from the soil affect the sustainability of production.
How plants use nitrogen
Nitrogen is one of the main chemical elements required for plant growth and reproduction. Nitrogen is a component of chlorophyll and therefore essential for photosynthesis. It is also the basic element of plant and animal proteins, including the genetic material DNA and RNA, and is important in periods of rapid plant growth.
Plants use nitrogen by absorbing either nitrate or ammonium ions through the roots. Most of the nitrogen is used by the plant to produce protein (in the form of enzymes) and nucleic acids. Nitrogen is readily transported through the plant from older tissue to younger tissues. Therefore, a plant deficient in nitrogen will show yellowing in the older leaves first due to the underdevelopment or destruction of chloroplasts and an absence of the green pigmented chlorophyll.
Chemical forms of nitrogen
- Nitrogen gas (N2) makes 78% of atmosphere. It is not directly available for use by plants but is directly used in nitrogen fixation and industrial fertilizer manufacture.
- Ammonia (NH3) is a gas that contains nitrogen. In this form the nitrogen is unavailable to plants. It is involved in the cycling of nitrogen in the soil.
- Nitrate (NO3-) is an ionic form of nitrogen and is the most common form available to plants. In this form, nitrogen is mobile, leachable and usually the end product of mineralization.
- Ammonium (NH4+) it is also an ionic form of nitrogen available to plants. Plants use less energy for uptake in this form when compared to nitrate. Nitrogen in this form is less likely to be lost from the soil than other forms.
- Nitrate (NO2-) is an intermediate in the conversion of ammonium to nitrate. Nitrogen in this form is not available to plants and is more prone to be lost from the soil than either nitrate or ammonium.
- Nitrous Oxide (N2O) is a greenhouse gas. Soil nitrogen can be lost in this form through the process of denitrification.
- Nitric Oxide (NO) is also a gas and nitrogen in this form is lost through denitrification. It may be harmful to the ozone layer.
All of those above forms of nitrogen are known as inorganic forms of nitrogen.
Organic nitrogen compounds are complex and unavailable to plants. They are the end products of immobilization
Symptoms of nitrogen deficiency
- The chlorophyll content of the plant is reduced which results in pale yellow color. Older leaves turn completely yellow.
- Flowering, fruiting, protein and starch contents are reduced. Reduction in protein results stunted growth and dormant lateral buds.