1. Nitrogen is one of the major plant nutrients and of great importance in the soil and plant and crop nutrition. Apart from water, Nitrogen is the most important factor in growing good crops.
  • Nitrogen stimulates the whole plant to grow bigger and faster increasing the development of stems and leaves together with seed and fruit production and increases the amount of transpiration from the leaves which is also increased.
  • It gives plants a rich dark green colour which is the part of chlorophyll and is also responsible for photosynthesis. It is a necessary part of all proteins, enzymes and metabolic processes.
  • It also causes better tilling

Lack of Nitrogen causes poor and stunted growth because the amount of other nutrients taken from the soil depends on the amount of Nitrogen available. Lack of Nitrogen causes yellowing or pale green leaves because they are unable to make enough chlorophyll. The first symptoms are generally noticed on the lower or older leaves because the nitrogen is pushed to the newer more important leaves.

Nitrogen deficiency can occur when there is an abundance of organic matter with a high carbon level. Nitrogen is used by soil organisms to break down this matter (carbon) making nitrogen unavailable for plant use. This is known as robbing the soil or as negative nitrogen. All plants are susceptible to this deficiency apart from nitrogen fixing legumes.

Figures 1 and 2: Show nitrogen deficiency. Figure 1 (left) Nitrogen deficiency, notice the lighter colour of most of the leaves with one dark green leaf which is nitrogen rich.

Source: ipni                                                                          Source: ipni

Figure 3: Nitrogen deficient radish showing the older leaves dying off first.

Source: aquaticplantcentral

  • Causes a soft sappy growth and leads to lodging in cereals which means the stem becomes soft and the plant falls over, making harvesting difficult.
    • It makes the plant more susceptible to pests and diseases and lowers resistance to frost in winter crops.
    • It retards ripening in cereals and slows down the formation of seeds.
    • Too much Nitrogen increases the Protein content of cereal grains which is bad in malting barley but good in milling wheat.
    • Where Nitrogen and Potassium are supplied together, the Nitrogen increases the area of the plant leaf while Potassium increases the rate of Photosynthesis of the leaf.

Table 1: Nitrogen appears in the soil in three forms.

Ammonia Gas ( NH3 )This either escapes from the soil or is converted into Nitrate.
Ammonium Ions ( NH + ) 4These are held on the clay particles or are in the soil solution.
Nitrates ( NO3 )The most common form of nitrate is Calcium Nitrate. Ca(NO3)2

Nitrogen is taken up by the plant in the form of Nitrates (NO3) or as Ammonium Ions (NH4+). Both these forms are in the soil water making Nitrogen readily available to the plant.

Table 2: Nitrogen is lost from the soil by:

LeachingNitrates and Ammonium Ions being in solution are easily washed out of the soil.
Escape of Ammonia GasBeing a gas is easily lost into the air.
  De-nitrificationThis is the conversion of Nitrates back to Ammonia Gas in the soil: See diagramme of the Nitrogen Cycle.

There are two main sources of Nitrogen, natural supplies and artificial fertilisers.

      Anaerobic: relating to or requiring an absence of free oxygen. NATURAL SUPPLIES
  • The breakdown of organic matter supplies Nitrogen to the soil. Proteins and other Nitrogenous compounds in dead plants and crop residues are broken down to Amino Acids and then to Ammonia Gas by bacteria in the soil. The Ammonia Gas is then built up into Nitrates by bacteria. The Nitrates are taken up by the next generation of plants; see diagramme overleaf. This process is known as the Nitrogen Cycle.
  • Nitrogen from the air can be converted into nitrates that can be used by the plant by Bacteria which live freely in the soil. The anaerobic bacteria called Clostridium Pasteurianum can do this in conditions of low pH (acid soils) and a lack of oxygen. The Azotobacteria can also do this under conditions of plenty of oxygen and where there is lime present.
  • The Rhizobium Bacteria live in nodules in the root systems of Legumes. Legumes are the plants which produce their seeds in pods. Ege.g soybeans, peas, groundnuts, lucerne, clovers and many of the bushes and trees that grow in the veld. The bacteria penetrate the root hairs of the young plant and form a thread into the cells causing the formation of a Nodule. These nodules can easily be seen on the roots of soybeans or groundnuts. The plant then supplies nutrients to the bacteria and the bacteria supply nitrates to the plant which have

been converted from the nitrogen in the soil air. This is an example of symbiosis. Each type of plant has its own strain of Rhyzobium and the seeds of soybeans and groundnuts have to be inoculated with the correct strain of bacteria before planting. In a mixed pasture of grass and clovers in temperate areas, or New Zealand, the clover can add up to 400kg per hectare of Nitrogen from the air to the soil and this Nitrogen is used up by the grass in the pasture.

Figure 4: Shows nodules in the root system of a leguminous plant where the rhizobium bacteria live.

Source: jpkc.nwu.edu

  • Sulphate of Ammonia is manufactured in two ways. Firstly it is produced as a by-product in the making of gas and is also made by a process based on the conversion of nitrogen in the air into Ammonium Sulphate. It is sold as a crystal, and contains 21% of Nitrogen.
  • When applied to the soil the Ammonia is converted into Nitrates which are available to the plant and easily washed out from the soil. The use of Sulphate of Ammonia leads to a loss of Lime from the soil, so that it is best used on calcareous, brak soils or soils with a high pH  such as soils being irrigated with alkaline water. Used on an alkaline soil 100kg of Sulphate of Ammonia will carry 100kg of lime out of the soil.
  • Ammonium Nitrate cannot be used in its pure form as it is liable to explode therefore, it is mixed with Lime (Calcium Carbonate) and sold as a granular fertiliser and contains 34.5% Nitrogen. Although the Ammonium Nitrate takes lime out of the soil the lime in the granules compensates for this. Ammonium Nitrate is highly soluble and readily available to the plant.
  • Nitrate of Soda is found in natural deposits in the ground in Chile which has a very dry climate and it contains 16% Nitrogen. It is very soluble and quick acting and very easily washed out of the soil. It is best not used on heavy clay soils as the Sodium replaces the calcium on the clay particles causing the formation of a sticky sodium clay.
  • Urea is an organic compound containing 46% Nitrogen and is sold as small granules. When applied to the soil the Nitrogen is converted to Nitrates by bacteria as in the Nitrogen Cycle. It is not suitable as fertiliser, and very expensive.
  • Nitrogen is applied both in the seedbed for the young plant and a top-dressing to the growing crop. It is quick acting, and, when applied as a top dressing the leaves of the plant go dark green in about two weeks. There is no residual value with Nitrogen Fertilisers, any excess being washed out of the soil.

Figure 5: The Nitrogen Cycle.

The Nitrogen Cycle only operates in conditions where there is plenty of air, some water, but not waterlogging and a pH around 5 to 7 in the soil.

De-nitrification, the conversion of nitrates back to ammonia gas occurs when the soil is waterlogged and there is a lack of oxygen (air).

Volatilisation is the gaseous loss of a substance into the atmosphere.