The document discusses humic substances, their formation, nature, and properties. It describes humic substances as highly colloidal and amorphous natural organic matter with a large surface area and adsorptive capacity. Humic substances improve soil properties like structure, water retention and buffering capacity. Their formation is a complex biochemical process carried out by soil microorganisms. Different theories are presented to explain humus formation from plant and animal residues. Clay-humus complexes are also discussed, along with the processes of mineralization and immobilization of nitrogen in soil.

1. K. Maheshwaran, Asst.Prof. (SS&AC)
Sethu Bhaskara Agricultural College &
Research Foundation
Humic substances-
Formation, Nature and
properties, Theories of
Humus formation

2. Humus nature and properties
 Highly colloidal and amorphous nature
 Surface area and adsorptive capacity are
much higher than that of clay colloids
 CEC is very high
 Humus has high water adsorptive capacity
than clay
 Humic acids are weakly dissociated and
determine the buffering capacity of soil

3.  Significantly low plasticity and cohesion
 Humus has more functional group are more
important in soil processes
 Helps to develops good soil structure
 Development of soil colour is influenced by
humus
 Improve water holding capacity, rate of
infiltration, porosity of the soil

4. Colloidal properties
 Soil humus contains modified lignin,
polyuronides and other constituents
 They carry a large number of negative
charges
 Humic micelles composed of C, H, O with
trace amount of N, S, P and other elements
 Exposed functional groups –COOH and –OH
 Humic micelles adsorb some elements like
Ca, H+, Mg, K, Na etc

5. Difference between organic
matter and Humus
Humus Organic matter
1. Complex and
rather resistant
mixture of brown or
dark brown
amorphous
colloidal
substances
2. More stable
1. Easily
decomposed due
to attack by soil
microorganisms
2. Less stable

6. Humus formation
 Biochemical process, rather complicated
 Synthesized by variety of micro organisms
 Humification depends on the their chemical
composition and soil conditions influencing
soil micro organisms
 On reaching the soil, organic residue of
plant and animal origin undergone diverse
changes

7. Diverse changes
1
• Destructive changes
• Physical action of natural factors (Rain fall, wind etc and
man)
2
• Changes in chemical nature of organic residue
• Oxidation of fat by light and aromatic compound by soil
3
• Effect of tissue enzymes
• Forming complex dark coloured products
• Oxidation of tannin, polyphenols and aromatic amino acids

8. Humus from plant and animal
residues
Synthesis of organic
compounds
Decomposition of original
components

9.  Decomposition of original components of
plant and animal tissues and their
conversion by micro organisms into
mineralization
 The synthesis of organic compounds with
the formation of high molecular weight of
humus substances of nature

10. Humus fraction

11. Humified compounds
Humic acid Fulvic acid Humin
Medium molecular
weight
Low molecular
weight
High molecular
weight
Contain less
oxygen, more
carbon and nitrogen
Contain more
oxygen, less carbon
and nitrogen
It contains less N
Soluble in dilute
alkali
Alkali and acid
soluble
Acid and alkali
insoluble
Precipitate in dilute
acid
Extracted by
2Na2CO3
Resistance to
degradation
Susceptible for
microbial attack
Most resistant to
microbial attack

13. Compounds
• Nitrogenous
• Eg. Protenaceous
Humus
• Non nitrogenous
• Eg. CHO
Humus

14. Theories of humus formation
1. Modified lignin theory
2. Lignin theory
3. Polyphenol theory
4. Flaigs humus formation
5. Konnova concept
6. Sugar amino condensation

15. Modified lignin theory

16. Lignin theory

17. Polyphenol theory

18. Sugar amino condensation

19. Flaig’s theory
Lignin freed of its linkage with cellulose during
decomposition of plant residues
Formation of structural unit
(Phenyl propane)
Polyphenol
Quinones
Humic acid substances
Oxidation
Enzymatic Oxidation

20. Konnova concept
1
• Fungi attack simple CHO and parts of protein and cellulose of plant
residues
2
• Cellulose &polyphenolssynthesized
bymyxobacteriaareoxidisedtoquinonesby poly-phenol-
oxidaseenzyme
• Quinones react with N producehumicsubstances
3
• Lignin is decomposed
• Phenols released during decay also serve as source material for
humus synthesis

21. Clay – Humus Complex
 Clay minerals binds with humus through
some linkages called as Clay-Humus
complex
 Being held together by cations like Ca, Fe,
etc.
 Depending upon the nature of binding cation,
two types of Clay – humus complex have
been recognized.
 The colloidal complex bound by Ca ions is
more stable and is responsible for the
favorable physical condition of the soil,
particularly its structure.
 The other type where Fe acts as the binding
agent creates a poor physical condition of the
soils.

22. Clay – Humus Complex
 There are two types of linkages take place
in formation of clay-humus formation
1. Stable linkage: Being held together by
cations like Ca, Fe, Al, Mg and OH, COOH
group
2. Unstable linkage: Humic acid on the
external surface of the clay extracted
easily by alkali

23. Mineralization and immobilization
Mineralization
 Conversion of organic substances in to
inorganic forms by heterotrophic micro
organisms
 Three steps reactions namely
 Aminisation
 Ammonification
 Nitrification

24. Aminisation
 Hydrolytic decomposition of protein from N
compounds
 Release amines, amino acids by
heterotrophic microorganisms
Protein
R-NH2
+
CO2
+
Energy
+
Other additional products
Heterotrophic microorganisms

25. Ammonification
 Reduction of amines and amino acids into
NH4
+ by heterotrophic micro organisms
R-NH2 + H2O NH3 + R-OH + Energy
NH3 + H2O NH4
+ + OH-
Heterotrophic microorganisms

26. Nitrification
 Microbial oxidation of ammoniacal N to
NO3
– Two step process
a) Conversion of ammoniacal to nitrite
nitrogen
b) Conversion of nitrite to nitrate nitrogen

27. Nitrification- Two step process
Ammoniacal to
nitrite nitrogen
Nitrite to nitrate
nitrogen
•Nitrosomonas
•Nitrosococcus
•Nitrospira
•Nitrobacter

28. Immobilization
 Change of inorganic form of nutrients to
organic form by soil microorganisms
 N content of added organic material is low
(<1.2%), the amount of inorganic N content
in the soil immediately decrease
 Soil microorganisms take up soil nitrogen
for their metabolisms and their
multiplication
 However, the C:N ratio of the added organic
materials to the soil mostly determine the
mineralization and immobilization process

29. Carbon: Nitrogen (C:N Ratio)
 Relationship between organic matter and
nitrogen content of the soil
 Ratio of the weight of organic carbon to the
weight of total nitrogen in a soil
 C:N ratio controlling the available N, total
organic matter and rate of decomposition
 C:N ratio of humus: 10:1
 Most of the arable soil: 5:1 – 15:1

30. Relationship of C:N ratio
C:N ratio Process operates
Narrow
(<20:1)
Mineralization
(Release of inorganic N)
Critical
(20:1-30:1)
Neither net mineralization
nor net immobilization
Wider
(>30:1)
Immobilization
(Production of organic N)

31. …Thank you...