Function and Deficiency Symptoms | Mineral Deficiency | Definition and Patient Education
Higher plants generally utilize the oxidized forms such as nitrate (NO) and nitrite (NO) or the reduced form (N) of nitrogen which is made available by a variety of nitrogen fixers.
Natural or Atmospheric nitrogen fixation: By photochemical and electrochemical reactions, oxygen combines with nitrogen to form oxides of nitrogen. Now they get dissolved in water and combine with other salts to produce nitrates.

Physical nitrogen fixation out of total nitrogen fixed by natural agencies approximately 10% of this occurs due to physical processes such as lightning (i.e.. electric discharge), thunder storms and atmospheric pollution.
Due to lightning and thundering of clouds, N2 and O2 of the air react to form nitric oxide (NO). The nitric oxide is further oxidised with the help of O2 to form nitrogen dioxide (NO2).
N2 + O2 2NO
2NO + O2 2NO2
NO2 combines with H2O to form nitrous acid (HNO2) and nitric acid (HNO3). The acid falls along with rain water. Now it acts with alkaline radicals to form water soluble NO (nitrates) and NO (nitrites)."
2NO2 + H2O → HNO2 + HNO3
HNO3 + Ca or K salts → Ca or K Nitrates
The nitrates are soluble in water and are directly absorbed by the plants.
Asymbiotic biological nitrogen fixation: This is done by many aerobic and anaerobic bacteria, cyanobacteria (blue green algae) and. some fungi: e.g.:
Free living bacteria: Free living N2 fixing bacteria add
10-25 kg of nitrogen /ha/ annum.
Aerobic : Azotobacter
Anerobic : Clostridium
Photosynthetic : Chlorobium
Chemosynthetic : Thiobacillis
Cyanobacteria (blue-green algae) e.g., Anabaena, Nostoc, Tolypothrix cylindrospermum, Calotherix and Aulosira etc. They add 20-30 kg of N2-per hactare of soil and water bodies.
Free living fungi e.g., Yeast cells and Pullularia.
Symbiotic biological nitrogen fixation: Symbiotic bacteria are found in the root nodules of the members of family Legurninosae. The best known nitrogen fixing symbiotic bacterium is Rhizobium leguminosarum (Bacillus radicicola).
Mechanism of Biological Nitrogen Fixation
Several-schemes incorporating such idea have been proposed and Burris (1966) accepts that the total reduction of nitrogen occurs on an enzyme complex (Nitrogenase) without release of intermediates less reduced than ammonia.
The enzyme complex nitrogenase consists of two sub-units
A non-heme iron protein commonly called Fe protein (or dinitrogen reductas, component I).
An iron molybdenum protein called MoFe protein (or dinitrogenase, component II).
According to Burris (1962) hypothesis for nitrogen fixation suggesting the function of ATP and ferredoxin at each step in the reduction of nitrogen, The pretty function of A TP donor is furnished by pyruvate which also acts as electron 'donor for N2 reduction as well.
Pyruvate on one hand acts as ATP donor while on other hand it supplies hydrogen ions and electrons for nitrogen reduction via NADH2 and ferredoxin.
The nitrogenase enzyme require 16 ATP molecules, 8 hydrogen ions and 8 electrons to reduce one molecule of nitrogen to 2NH3 molecules.
N2 + 8e- + 8H+ + 16ATP ® 2NH3 + H2+ 16ADP + 16Pi
Explaining the mechanism of nitrogenase activity, its now believed that electrons are transferred from the reducing agent (Ferredoxin, Flavoprotein or Dithionite) to complex of Mg-ATP and Fe-protein (component II). From here electrons flow to Mo Fe protein (component I) and then to substrate (nitrogen) which is finally reduced (to NH3).
The ammonia formed in biological nitrogen fixation is not liberated. It is highly toxic and is immediately converted into amino acids.
Ammonia + a-ketoglutarate + NADH
Glutamate + NAD+ + H2O.
The amino acids are transported through phloem to other parts of the plant.
Application of fertilizers: Most of the soil usually contain sufficient amounts of essential mineral elements for the better crop production. Some of them are, however, deficient in certain elements. These elements are required to be supplemented externally by adding the appropriate fertilizers. Moreover, constant agricultural cultivation in field may also cause depletion of certain elements which must be replenished in order to improve the fertility of soil. The important elements need to be replenished in crop fields are nitrogen, phosphorus and potassium. These are grouped as nitrogenous ferttlizers, phosphate fertilizers and potash fertilizers. These are abbreviated as NPK. Common sources of NPK are ammonium chloride, ammonium sulphate, ammonium nitrate, bone meal, calcium magnesium phosphate and nitrate of soda.