The Role of Nitrogen Fertiliser in Agriculture

By Mrs. Farzana Panhwar
November 2004

The Author is President of the Sindh Rural Women´s Up-lift Group in Hyderabad, Sindh - Pakistan. She also manages the Panhwar Fruit Farm, where some 25 new fruit crops have been successfully introduced to Sindh. → See also:

Nitrogen is an important constituent of protein and protoplasm. And essential for the growth of plants. Its shortage leads to chlorosis (yellow of leaves) and stoppage of growth. Its excess adversely effect the quality of fruit. Its presence in moderate doses is essential for plant growth and fruiting. It is usually deficient in soils. Nitrogen given to plants in the forms organic manure and artificial fertilisers. It is also present in the air but plants can not directly avail of it. The lithosphere and atmospheric content of molecular nitrogen are 18 x 1015 and 3.8 x 10 15 tones. In plant nitrogen lost through leaching, erosion or escape of ammonia or elemental N into the atmosphere.

Function of Nitrogen (Ref. 9)

It is a part of protein, important constituent of protoplasm, enzymes, the biological catalytic agents which speed up life processes. Nitrogen is also present as a part of nucleoprotein, amino acids, amines, amino sugar, polypeptides and other organic compounds in plants. In order to prepare a food for plant, plant required chlorophyll, energy of sunlight to form carbohydrates and fats from CO2 and water and nitrogenous compounds. Nitrogen constitutes about 5-6% of soil organic matter by weight. Nitrogen is added to the soil both symbiotic and non-symbiotic fixation from the atmosphere.

Natural Source of N in the Soil (Ref. 3)

Nitrogen increases the area of the leaf, while potassium increases its efficiency. The maximum up-take of nutrients occurs during the period of most active growth. Ammonium sulfate and sodium nitrate their responses of various crops are different. Ammonium salts in the soil dissolves and take part in cation exchange with soil colloid particle. A proportion of ammonium ions displace from the colloids an equivalent amount of calcium, magnesium and potassium ions, which are absorbed by growing plant. If soil conditions are good the nitrifying organisms will oxidize the ammonium to nitric acid, then immediately dissolve calcium to nitric acid, and then immediately dissolve calcium compounds to produce calcium nitrate. is largely not absorbed directly by the plant is largely absorb by the soil colloids then convert to nitrate.

"X" represent negatively charged soil colloid particles
CaX + (NH4)2SO4 cation exchange (NH4)2X + CaSO4
(NH4)2X nitrification 2HNO3 + H2X
2HNO3 + CaX neutralization Ca(NO3)2 + H2X

Ammonium salt are not available to the plant until after nitrification in the soil.

Phosphate increased the effect of ammonium sulfate slightly and sodium nitrate are markedly. Sodium nitrate is better than ammonium sulfate, except in the presence of phosphorus and potassium or sodium.

Form of N Utilized by Plants

In order to N absorbed by plant. The organic N into inorganic NO3-N called minarlization of N. In which organic N present in soil by the help of soil-microrganism converted in to ammonia (ammonification) and organic N converted in to inorganic NO3-N. The plants takes up N from the soil in the form of NO3- and NH4+ ions, urea as amino acid.

The N uptake in plant is as under:
Nitrate (NO3) Nitrate reductase Nitrite (NO2)
Hyponitritie (HNO) Hyponitrite reducatse Hydroxylamine (NH2OH)
Hydroxylamine (NH2OH) Hydroxylamine reductase Ammonia (NH2)

Nitrogen is taken up in both cationic (NH4)+ and anionic (NO3-) forms. Nitrogen is used as ammonium fertilizer. Ammonium up take is best low pH, also carbohydrate status of the plant effect on NH4+ uptake. High carbohydrate plants favor high NH4+ uptake through enhancement of ammonia assimilation. Ammonium ion and ammonia are inter-convertible. NH3(aq) + H+ = NH4 Ammonium. Nitrogen shows toxicity at higher pH. Wheat, sugarbeat, rice, tolerate high NH4-N concentration at acid to neutral pH.
NH3(aq) could freely diffuse in to it and un-couple photosynthetic photosynthetic phosphorylation.
The nitrogen forms which are readily taken up by plants are NH4+ and NO3- ions.
The inert atmospheric N2 must be converted to either NH3-N or NO3-N before utilization by plants. N2 + 3H2= 2NH3 ammonium fertilizer produced. The total world biological N fixation to be the order of 17.2 x 107 tones per annum.

Two types of microorganisms are involved in biological N2 fixation are:
Free living microorganism, microorganism living symbiotically with higher plants.

Free living N-fixers are:

Nitrogen Fixation by Legumes and Under Field Conditions (Ref. 9)

Crops N-fixed, Kg/hec/year
Alfalfa 261
Red clover 163
Sweet clover 182
Peas 51
Soybean 63

Soil bacteria and fungi require nitrogen in their food, most of them utilize the soluble nitrogen compounds like ammonium salts and nitrate present in the soil.

Rhizobum forms colonies or nodules on the roots of leguminous plants. The plant provides the organisms with energy in the form of carbohydrate and receives some of the combined nitrogen manufactured by the organism from the atmosphere. It is this association between nitrogen-fixing bacterial and legumes, which help in maintain the soil fertility. The gain of nitrogen from good crop is about 45 Kgs per acre per annum.

Azotobacter, clostridium, the non symbiotic N fixation under orchard conditions vary from 0-55 Kg per hectare annually.

Azobacter is an organism, is a free-living soil bacteria, they are capable of fixing nitrogen, if they are provided carbohydrate material. The carbohydrate is oxidized and supply energy necessary for the fixation of the atmospheric nitrogen. This nitrogen becomes available to plants after the death and mineralisation of the cells of the free-living organisms and helps to maintain combine nitrogen in the soil.

The N-fixing bacterial are photosynthetic one are heterotropic Azobacter and fix about 90kg/ha/annum.Rhizobium species live in association with legumes are very important N2 fixers. It fixes N around 100-400 Kg/N/ha/year. The amount of N2 fixed depend on host condition soil pH, K, Ca, P, Co, Mo, status.

Plant obtain their N either by uptake as nitrates and ammonium or by reduction of atmospheric disnitrogen in root nodules of legumes, some other plants ammonia is assimilated in the root, where nitrate reduced in root or exported in the xylem to NO3 reduction sites in the shoot. Nitrogen which is assimilated in the root is converted in to amides, amino acids and ureides:

Sources of Nitrogen (Ref. 2)

Ammonium nitrate and urea are the widely used source of nitrogen. The nitrogen application do increase the soil acidity and lime requirement. Low rate of application involveds low quantities of nitrogen required by the tree with large pool of soil nitrogen available.

Nitrogen in plant can be classified in to three groups:

Conversion of nitrate to ammonia is as under:
NO3 NO2 N2O2-2 NH4OH NH3
Nitrate Nitrite Hyponitrite Hydroxylamine ammonium

Nitrate reduction found both in roots and upper plant ..Nitrate reductase activity is higher in young meristematic tissue.

There are 3 forms of inorganic nitrogen in fertilizers:

Different forms of Nitrogen

Nitrogen is present in various forms. These are as under:

Other forms:

All nitrates are water soluble, their action on crop depends upon the other ions in fertilizer salts, this may be potassium, sodium, calcium or ammonium.
Chilean nitrate of soda-contain 16% of nitrate-nitrogen, 26% of sodium. It is water soluble.
Chilean potash nitrate-common fertilizer contains 15% N (all as nitrate), 10%K2O and 20% sodium.
Potassium nitrate (KNO3) contains 13.8% N and 36.5% K (about 44% K2O.
Calcium nitrate (Ca(NO3)2,NH4NO3,10H20) which contain 15.5% N

Nitrogen materials

Nitrogen may supplied to the soil from any one or more of the following sources.

  1. Organic
    1. Natural
    2. Synthetic
  2. Inorganic

1. Organic

  1. Natural Organic Material
    These material include farmyard manure, oil-cake, dried blood, fish manure, green manure, sewage products, castor, neem and other oil cakes, these become N supplier in about week or ten days.
  2. Synthetic Organic Nitrogen
    Material containing synthetic organic N are readily soluble in water. These are:
    1. Calcium Cyanamide
      Nitrogen combine with calcium carbide to produce calcium cyanamide.
      CaC2 + N2 = CaCN2 + C (carbon)
      This is an exothermic reaction produce at ,100C° depending upon the size of unit. Commercial product contains 60% Calcium Cyanamide; the impurities are about 20% lime (CaO), 10% Carbon, which is responsible for dark gray color and small amount of various other substances. It contains 21% N, and is 35% in pure calcium cyanamide.
      The nitrogen of Calcium cyanamide is not available to plants, unless they pass in to a reaction of the carbonic acid in the soil which converts the calcium cyanamide in to cyanamide to calcium carbonate
      CaCN2 + H2O + CO2 = H2CN2 + CaCO3
      The cyanamide is hydrolysed to Ureas.
      H2CN2 + H2O) = CO(NH2)2
      Soil bacteria convert urea rapidly in to ammonium carbonate and latter more slowly in to nitrate. Cyanamide may form a polymer called dicyano diamide, H4C2N4 if hydrolysis to urea is impeded or if the biological conversion to urea is slow. Both cynamide and dicyanodiamide are toxic to young plants and nitrifying organisms. If it is used on soil, it kills most of micro organism and is used as weed killer. Calcium cyanamide contains calcium hydroxide so it should not used with ammonium sulfate.
      Calcium cynamide ------ H2O ------ Urea-Hydrolysis ------ NH4 + CO2
      It is CaCN2 with 21-22% N is hydrolysed in soil to form urea. It is used as weed killer. Cyanamide decomposes in the soil to form ammonium nitrogen. Cynamide contain lime but it whould not make soil acidic.
    2. Urea
      It contains 46% N. Urea is highly soluble in water and readily absorbd through the leaves.
      It has chemical formula CO(NH2)2 it is called carbamide. In soil urea readily converted by an enzyme urease to ammonium carbonate, which is unstable and release free ammonia.
      It is very soluble and until converted to ammonium compounds through hydrolysis by the enzyme urease. It is just as mobile as nitrate. Commercial urea may contain traces of an impurities (biuret) which damages germination of crops. Pure urea contains 1.5% biuret. Urea free from biuret gives good crop, but suffer two disadvantages:
      - Rapid decomposition in the soil or on the soil surface
      - Free ammonia is formed, this reduces the efficiency of urea. This ammonia damages germinating seeds. Urea is more effective when containing nitrate.
    3. Liquid Ammonia
      Liquefied ammonia is new generation of nitrogen fertilizer.
    4. Urea and Urea-Form Compounds
      It is produced when heating a mixture of anhydrous ammonia and carbon dioxide at high pressure. Urea contain 46.7% Nitrogen, and is readily converted in the soil to nitrate, without leaving any harmful effects.
      Biuret (NH2.CO.NH.CO.NH2) which occurs in urea as impurity is toxic to fruit crops. Urea should not contain more than 0.25% Biuret.

2. Inorganic Sources

  1. Chilean Nitrate of Soda or Saltpeter
    Its compositions is:
    NaCO3 + 2HNO3 ------ 2NaNO3 + CO2 +H2O
    Sodium nitrate shows neutral effect on soil, some impurities like saltpetre or the chilean nitrate of soda is present in a granular form.
  2. Ammonium Salts
    All ammonium salts used as fertilizer are water soluble and nitrified quickly in slightly acidic and neutral soil to form nitrate. Their effects are as under.
    Ammonium sulfate - contains 21% Nitrogen. Due to non-hydgroscopic properties, it is make easy to handle, than nitrates or urea.
    Ammonium chloride - contains 26% Nitrogen, a cheap source of by-product hydrochloric acid.
  3. Ammonium Nitrates
    It contains ammonium and nitrate, these are water soluble.
    Ammonium nitrates - contains 35%N.
    Nitro-chalk - contains enough limestone and it does not make soil acidic.
    Ammonium nitrate-sulfate - contains 26%N
    It supplies both nitrate and ammonical N. It is dangerous in case of fire of its explosive nature. This fertilizer contains almost double the quantity of N as contained by sodium nitrate.
  4. Calcium Ammonium Nitrate
    This material is obtained by mixing of ammonium nitrate with precipitated calcium carbonate.
  5. Ammonium and Nitrate
    Nitrate, nitrate application if followed by rain then nitrate leaches out. In different soils both ammonium and nitrate show difference. On calcareous soil nitrates give better results than ammonium salt. Nitrate salts are safer.
  6. Ammonia Nitrate
    Nitric acid is produced by passing ammonia from reaction, mixed with air over platinum catalyst.
    NH3 + 2O2 (oxygen) = H2O + HNO3
    If nitric acid is neutralized by more ammonia, ammonium nitrate is formed.
    HNO3 + NH3 = NH4NO3
    Ammonium nitrate is very attractive form of fertilizer salts it contains 35% nitrogen half as the ammonium and half as the nitrate radical. Ammonium nitrate could be used as nitro-chalk, which contain 16% nitrogen. This product is granular and easy to handle.
  7. Ammonium Phosphate
    It is mono-ammonium phosphate (NH4.H2.PO4) and is completely soluble in water. It contains 12.2% N and 61.7% P2O5. It can be manufactured by adding the correct amount of ammonia to Phosphoric acid or rock phosphate treated with sulfuric acid and ammonium sulfate, like
    Ca3(PO4)2 + (NH4)2 SO4 + 2H2SO4 = 2NH4.H2PO4 + 3CaSO4.
    Calcium sulfate is removed by filtration and solution of ammonium phosphate is concentrated.
    Di-ammonium phosphate (NH4)2 P2O5 is also manufactured and used alone or mixed with fertilizer. It is easily soluble in water, but it has higher N to P2O5 ratio, containing 21.2%N and 53.8%P2O5.
  8. Ammonium sulfate
    By-products are coal containing 1-2% Nitrogen and 0.5-4% sulfur. The sulfur is present mainly sulfide and calcium sulfate, but as a constituents of organic compounds drive from the original plants and are produced by the action of sulfur bacteria.
    When coal burns nitrogen begins to come off as ammonia at a temperature of 300°C to 400°C. A substantial amount of ammonia is obtained during the gasification of cake by air and steam. Ammonia is also by-product of oil-refining and Shale-distillation plants. Crude gas from the coal is used to remove tar, hydrogen sulfide and oil. The aqueous liquor is obtained by the proves of distilled and free ammonia from ammonium salts. The liberated ammonia gas is mixed with sulfuric acid in saturator and crystal of ammonium sulfate are separated in a centrifuge. About 1/5th of nitrogen is used to recovered 22-23 Ib. of ammonium sulfate from 1 ton of coal.
    It is soluble in water and used for mixed fertilizer and top dressing material. Ammonium sulfate continuos use make the soil acidic , so lime is recommended on the soil.
  9. Ammonium Sulfate Nitrate
    It is produced both in crystalline or granular form. It contain ammonical and nitrate.N in 3:1 ratio. Application of this salt produce acidity.
  10. Calcium Nitrate
    It is soluble in water, nitrogen in form of nitrate is immediately available to the plant. The anhydrous salts Ca(NO3)2 contain 17.1% nitrogen. Calcium nitrate is made by dissolving limestone in nitric acid. In the manufacture of soluble phosphate calcium nitrate is obtained as by -product.
    Nitrate has been produced by nitrification of organic matter on higher ground and gradually transferred in solution with other salts to the lower area in height where it has crystallized under the hot dry conditions of the higher plain.
    The sodium nitrate the calcite contains large quantity of Sodium chloride, little sodium iodate and substantial quantities of sulfate of sodium, potassium and magnesium. Sodium nitrate is very soluble. Sodium nitrate contains high percentage of oxygen.
  11. Potassium Nitrate
    Several methods based upon the different solubility of sodium and potassium nitrate are employed to produce Chileans potash nitrate, Chileans potash nitrate contains various composition, it contain 10% K2O and 15% N.

Other Principal sources of inorganic materials used as sources of Nitrogen

Name of Material Compound Formula N%
Anhydrous ammoniaNH382
Ammonia solutionNH4OH20
Ammonium sulfate(NH4)2SO420.5
Ammonium sulfate nitrate(NH4)SO4.NO226.0
Ammonium nitrateNH4NO333.5
Ammonium Phosphate40% (NH4)2H2PO440
Sulfate60% (NH4)2SO416
Sodium nitrateNaNO316
Potassium nitrateKNO313
Calcium ammonium nitrateNH4NO3 + CaCO325
Calcium cynamideCaCN221
Castor Pomace 5.5
Karanj cake 3.9
Mahua cake 2.5
Neem cake 5.2
Farmyard manure 0.5-1.5
Dried blood 12-14
Slaughter house waste 6-10
Meat meal 5-10

Characteristic of commonly available source of Nitrogen (Ref. 2)

Sources Percent Nitrogen Pound per 1 Ib N Acidity or Basicity (IbCaCO3) Ib of N Acidity Acidity or Basicity (IbCaCO3) Ib of N Basicity
Ammonia, anhydrous821.221.8-
Ammonia, aqua205.001.8-
Ammonium nitrate33.52.981.8-
Ammonium polyphosphate128.334.1-
Ammonium sulfate20.54.885.4-
Calcium nitrate15.56.45-1.3
Di-ammonium phosphate16-185.564.1-
Mono-ammonium phosphate119.095.3-
Potassium nitrate137.69-2.0
Sodium nitrate166.25-1.8
Urea 452.221.6-
Nitrogen solutionVariable ---

Nitrogen solutions may consist of mixtures of urea plus ammonium nitrate, aqua ammonia or anhydrous ammonia.

Various Forms of Nitrogen Fertilizers

Comparison of Nitrogen Fertilizer

A kilo gram of nitrogen shows same effect on crop, whether used ammonium sulfate, or ammonium nitrate ,supplied alone or in a mixture with lime or as "Nitrate of soda " or "potassium nitrate.
The choice of material is based in following things:

Soil Acidification

Liquid Nitrogen Fertilizers (Ref. 4)

Solutions of Nitrogen Compounds

Timing of Nitrogen Application (Ref. 11)

N-Deficiency Symptoms

Health of Crop (Ref. 8)


Nitrogen is an important constituent of protein and protoplasm. Essential for growth of plants. Nitrogen in plant can be classified in to three groups:

  1. Inorganic fraction, in form of NO3- and NH4+
  2. Low molecular weight organic fraction-contains amino acids, amides and amines
  3. High molecular weight organic bipolymers. Protein and nucleic acids
  4. Conversion of nitrate to ammonia is as under:
    NO3- ----- NO2- ----- N2O2-2 ----- NH4OH ----- NH3

Inorganic Nitrogen Sources

Calcium cynamide; urea; Chilean nitrate of soda or saltpeters, ammonium nitrate; ammonium phosphate; ammonium sulfate; ammonium sulfate nitrate; calcium nitrate; potassium nitrate.
Ammonia, anhydrous contain 82% N; urea contains 45%N; ammonium nitrate contains 33.5% N. Nitrogen also fixed by legumes like: alfalfa; red clover; sweet clover; peas; soybean. Rhizobium and Azotobacter Clostridium are capable of fixing nitrogen and supplying atmospheric N to the plants.
Nitrogen deficiency effects on photosynthesis, so leaves turn yellow, it also effects on root growth, carbohydrate utilization, fruit firmness, hardness of plants, maturity time, diseases incidence.
In order to remove N deficiency the consideration of soil, pH and time of application is need to be considered.



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