SOIL SCIENCE

2. Nitrogen (N)
No
.
Functions Deficiency symptoms Diseases
1 constituent of organic
compounds ( protein,
nucleic acid, hormone etc.)
Stunted growth Starvation
2 Integral part of chlorophyll Chlorosis (yellowing) Buttoning
3 It imparts green color to
the plant
Protein & starch content
reduced

3. Sources of nitrogen
⚫Organic source :- protein , amino sugar, nucleic acid
⚫Inorganic sources :- fertilizers , mineralization of organic
compounds
⚫Lightening
⚫Microbes

4. Forms of Nitrogen in soil
Forms
Organic Inorganic
•Proteins
•Free amino
acids
•Amino sugar
•Other complexes
Ionic
•NH4
+
•NO3-
•NO2−
Gaseous
N2
2
N O
NO
NO2
NH3

5. Forms of N absorbed by plants
⚫NO3- :- predominated in warm & well- aerated soil. Hence , plants
grown in well aerated soil absorb nitrate ion.
⚫NH4
+ :- urea CO(NH2)2, urea is absorbed through leaf epidermis,
when urea solution is spread over the foliage , & usually not absorbed
by root.
⚫NO2− :- plant absorb nitrite ions , they are toxic. However they
accumulate in soil in small quantities.
⚫Amino acid & nucleic acid :- absorbed by higher plants ,
accumulated in soil in small quantities.

6. Mineralization
⚫ Mineralization is the conversion of organic form of the
element to inorganic form or mineral form
• In case of nitrogen NH4
+ ,NO3- ,NO2− the inorganic forms
⚫ It takes place in three steps as follows :-
1. Aminization
2. Ammonification
3. Nitrification

7. Aminization
⚫ Proteins breaks down to yield amines , amino acid , carbon dioxide ,
energy & other reaction products .
⚫ Protein R-NH2 +CO2 + Energy + other reaction products
⚫ This process is brought about by heterotrophic soil micro organisms
( Bacteria , Fungi , Actinomycetes).
⚫ They cause this process for their1 own interest I.e., for obtaining
energy.

8. Ammonification
⚫ This step starts after completion of aminization. In this step amines &
amino acid released by aminization process are converted ammonia as
follow :
⚫ R - NH2 + H OH NH3 + R- OH + Energy
⚫ Caused by another group of heterotrophic soil micro organisms
⚫ NH3 released is converted to NH4+

9. ⚫ Two reactions may be suggested to demonstrate the conversion of
ammonia to ammonium ion
1. NH3 + H2O
NH4OH
NH4OH
NH4
+
+ OH-
2.
•
2NH3 + H2CO3
NH3 + HNO3
= (NH4)2CO3 ⇌
= NH4NO3 ⇌
2NH4+ + CO3-
NH4
+ + NO3-
• In both reaction the common product is ammonium ions . Hence this step is
termed as ammonification

10. Nitrification
⚫ This step follows ammonification process.
• In this step , a part of ammonium ion ( NH4
+)released by
ammonification is converted to nitrate ion ( NO3-)
+ Energy (first step)
• It is a two-step process.
• 2NH4
+ + 3O2 2NO2- + 2H2O + 2H +
• 2NO2
- + O2 2NO3- + Energy ( second step )
•

11. Role of enzyme
⚫ In the first step ,ammonium ion are dehydrogenated by an enzyme
dehydrogenase , while the nitrogen released by dehydrogenation is
oxygenated by another enzyme, oxidase.
⚫ In the second step the oxidation of nitrite ions to nitrate ions takes
place by dehydrogenation reaction.
⚫ The enzymes causing nitrification process are secreted largely by
obligate autotrophic bacteria.
⚫ They obtain energy from oxidation of inorganic salts and carbon from
co2.
⚫ The genera of bacteria which oxidize ammonium ion are :
nitrosomonas , nitrosococcus, nitrosocystis, nitrosospira, nitrosogloea.

12. Immobilization
⚫ Immobilization of nitrogen is the reverse of mineralization where available form of
an element is fixed as immobile form,
⚫ It occurs when large quantities of low nitrogen crop residues with wide C:N ratio
(>30:1) .
⚫ Nitrogen in soil :- 1. As bound amino acid = 20-40 %
2. As combined hexose amines = 5- 10 %
3. Mineralized = 1-3 %
⚫ When C:N ratio exceed 30 , immobilization occurs
C:N > 30 = Immobilization
Immobilization &
Mineralization both
C:N = 15-30
20-30
C:N < 20 = Mineralization ( release of NH3 & NO3- )

13. ⚫ coconut coir or cereal straw begin decomposing in soil, the
high amounts of carbohydrates in such residues cause the
microbial population to build up quickly.
⚫ As new cells are formed, nitrogen and other essential
elements from soil are used to build protoplasm. This leads
to a decrease in the levels of inorganic nitrogen for crops.
⚫ A shortage of nitrogen can be avoided in such situations by
supplying enough fertilizer nitrogen to compensate for
immobilization and to meet the crop requirements

14. CONCLUSION
Among all the essential nutrients, nitrogen is the extensively
studied one and still receiving much
attention.
Plants absorb most of the N in the NH4+ and NO3-
forms. The process of mineralization and immobilization leads to
various transformation of nitrogen in soil.
Fate of released ion in soil is directly utilized by the
higher plants, utilized by heterotrophic organisms in further
decomposing organic carbon residues, fixed in biologically
unavailable form, in the lattice of certain expanding type clay
minerals or is released back into atmosphere through
volatilization.

15. Thank you
A.Swaroopa rani