Of all the 16 essential plant nutrient elements needed for plant growth, development and reproduction, nitrogen is the most important and most easily limiting or deficient throughout the world particularly in the tropics.
Animals and man depend on the protein manufactured by plants from nitrogen which could be regarded as the key nutrient in plant growth.
This unit examines the forms and functions of nitrogen in plant, the gains and losses in soil-plant system and those factors that govern nitrogen transformations in soil.
Forms and Functions of Nitrogen in Plants
Nitrogen gas, N 2 ; which accounts for about 78% of atmospheric gas has to be converted into two forms utilizable by plants before it can be regarded as useful to the plants. These two forms are the cation form, ammonium ions (NH + ) and the anion form, nitrate (NO – ).
It is not clear yet which of the two forms is preferred by plants. If the nitrate form, NO – 3 is absorbed by plants, it has to be reduced in plant tissue to amino form (-NH 2 ) which is utilized directly to form organic nitrogen compounds such as glutamic acid (an amino acid).
More metabolic energy (ATP) is expended by plants fed with nitrate than those supplied with ammonium. Plants can also take up nitrogen in form of amino acids.
Plants can utilize NO – 3 because they possess a special enzyme system (not found in animals) for reduction of NO – 3 to (NH + 4 ) a process often referred to as assimilatory nitrate reduction.
Although, nitrate appears to be more available than ammonium, NO – 3 uptake is a wasteful way of taking up nitrogen. It has been found that urea as well as amino-acids could be taken up just as the uptake of NH + and NO – 3 .
Read Also: Time and Methods of Fertilizer Application on Crops
Most normal agricultural plants contain 1 to 6% nitrogen which is an important structural component of many compounds necessary for plant growth and development.
Nitrogen is a major constituent of chlorophyll (for example, chlorophyll a with molecular formula C 55 H 22 0 5 N 4 M g ), protein and related amino acids, various enzymes, nucleic acids and many other compounds in the cell of plants.
Other functions of nitrogen in plants include carbohydrate utilization, root growth and development, above ground vegetative growth and stimulation of uptake and utilization of other nutrient elements such as potassium, phosphorus and sulphur.
Deficiency Symptoms
Nitrogen is a mobile element and therefore deficiency symptom shows up first in the older parts (leaves or stem) of the plants and later extends to younger portions of the plants.
When nitrogen becomes insufficient in supply, the nitrogen already absorbed into the plant moves and translocate to the younger leaves and stem parts from the older leaves hence nitrogen is described as being mobile.
Nitrogen insufficiency results in the yellowing of lower (older) leaves beginning at tips extending backwards along inner margin (midrib) as deficiency becomes more severe.
The yellowing is due to lack of chlorophyll since nitrogen is a component of chlorophyll.
The whole plant may be stunted in growth and develop restricted yield if any.
A peculiarity of nitrogen is that deficiency symptom disappears within few days if N- fertilizer is restored.
Excess Nitrogen:
Over-supply of nitrogen to crops may make them develop excessive vegetative growth at the expense of reproductive development.
The crop has dark green luxuriant lanky growth and abnormal cells due to insufficiency of other elements such as K, Ca and Mg. Lack of Ca results in thin cell wall rendering the plant susceptible to lodging and attack by pests and diseases.
