Recent Studies on Soil Organic Matter, Nature, Composition, functions of Soil Organic Matter

1. Further Investigations during 20th century
C52H46O10(OCH3)COOH(OH)4CO + H2NRCOOH
C52H46O10(OCH3)COOH(OH)4 C=NRCOOH + H2O
According to Waksman;
1. HS not specific cpds but a mixture of cpds non specific
nature like cellulose, hemicellulose, fats, waxes etc.
2. Artificial cpds formed by the action of alkali solutions on
the soil
Waksman in his book “Humus” (1937, page 157)
There is no doubt now that this complex originates
mainly from the lignin constituents of plant residues, by
various biological and chemical modifications; this
fraction may, therefore, be included in the lignin group

2. Further Investigations during 20th century
Tyurim (1937) – Book on The Organic Matter of
Soils – HS specific cpds – methods adopted by
Waksman to determine humus composition not
correct and unsuitable
Springer (1934 – 35) also criticized Waksman and
supported that HS exist as cpds of specific
nature – Lignoprotein cpds do not acquire new
properties like increased exchange reactions
Later many scientist showed:
1. Non lignin cpds can form HS in the soil
2. Microbial activity & metabolism play a major role
3. Aromatic cpds formed from aliphatic cpds
including carbohydrates

3. Present Stage in Soil Humus Study
HS Structure, Composition and Properties
Studied: X ray analysis, Electron Microscopy,
Infrared Spectroscopy, Chromatography in
conjunction with chemical methods
HS formation complex transformation – organic &
biochemical – Role microbes & animal kingdom
Role of SOM in weathering of rocks and minerals,
Soil Forming Processes, etc being studied
Interaction between SOM and Minerals
Participation in Physiological and Biochemical
Processes in the plant

4. Books for Reading
1. Soil Organic Matter, its Nature, its Role in Soil
formation and in Soil fertility. By kononova, M.
M. 1966
2. Soil Components Vol.I. Organic Components By
John E. Gieseking, 1975.
3. Soil Organic Matter By Schnitzer and Khan,
1978.
4. Soil Organic Matter and its Role in Crop
Production By Allison
5. Chemistry of the Soil By F. E. Bear
6. Humus By S. A. Waksman

5. Books for Reading
7. Introduction to Soil Microbiology By Martin
Alexander
8. Soil Biochemistry By A. D. Mclaren and G. H.
Peterson, 1967
9. Study work on Soil Organic Matter and Soil
Fertility By G. H. Peterson
10. Agrosphere – Nutrient Dynamics, Ecology and
Productivity By K. R. Krishna, 2003.
11. Humus Chemistry: Genesis, Composition and
Reactions. By Stevenson, F. J., 1982.
12. Cycles of Soil: C, N, P, S and Micronutrients By
Stevenson, F. J., 1985.

6. Soil Organic Matter
1. General Types of Organic
Materials
2. Specific Types of SOM
Substances and Pools
3. SOM Decomposition
4. Soils and Soil Forming
Processes
5. Factors and Processes that
affect SOM Content

7. Soil Components
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10. Organic Matter (OM) versus Soil Organic
Matter (SOM)
Organic Matter – (OM) the material of which
organisms are composed. The material may
be alive or dead, and may be found in the
atmosphere, in organisms, or in the water
and soil.
OM may be from plant or animal tissues that
contain C. There are many types of organic
matter, and the C content varies widely.

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13. SOIL ORGANIC MATTER
Fraction of the soil composed of anything that once lived
The term "Soil organic matter" (SOM) has been used in different
ways to describe the organic constituents of soil.
SOM as defined by Baldock and Skjemstad (1999) is "all organic
materials found in soils irrespective of origin or state of
decomposition".
Organic matter .is the vast array of carbon compounds in soil.
Originally created by plants, microbes, and other organisms, these
compounds play a variety of roles in nutrient, water, and biological
cycles

14. SOIL ORGANIC MATTER
Soil Organic Matter - (SOM) includes all OM fixed in
place on the soil, or already incorporated into the soil.
SOM inputs to the soil are from decomposing plants,
vertebrates, invertebrates and microbes. Leachates from
leaves (through rainfall) and exudates from plants and
animals also added to SOM.
SOM exists in some stage of decomposition.
Decomposition begins as soon as a plant part dies or a
leachate or exudate is released into the soil. In fact, some
leaching of substances from plant parts begin before plant
death, and some roots decompose while the plant is still
alive (grasses).

15. SOIL ORGANIC MATTER
Organic matter is the vast array of carbon compounds in soil.
Originally created by plants, microbes, and other organisms, these
compounds play a variety of roles in nutrient, water, and biological
cycles.
Consists of:
plant and animal remains in variable stages of decomposition
Metabolic Products of microbes utilizing organic residues as a
source of energy – root and microbial exudates
Products of secondary synthesis in the form of bacterial plasma
Humus – HS consisting of HA, FA, Humin, Hymatomelanic Acid

16. SOIL ORGANIC MATTER
The first three categories form a group – Consist of N and
non N cpds – CHOs, Proteins, Fats, Waxes, Organic Acids,
Resins etc – Constitute 10 – 15% of SOM
Cpds that did not come under any existing organic
molecules – Nature, Origin and Properties not fully known –
Humic Substances – Constitute 85 – 90% of SOM
Thus SOM has two types of cpds:
1. Organic Substances of non specific nature
2. Natural Humic Substances of specific nature

17. Humus
 well-decomposed organic material
dark brown, amorphous
stable
poorly characterized chemically
~ 1-5 % of mineral soils

18. Distribution of humus forms in the soils of four great soil
groups

19. Humus - Structure
•colloids of polymeric nature
•structure: aromatic ring of di- or tri- hydroxy
phenol type
•bridges: -O, -CH2-, -NH-, -N=, -S-

20. SOIL ORGANIC MATTER
Proximate Constituents of SOM:
1. Carbohydrates – Mono and Di Sachharides – Cellulose,
Hemi cellulose, Pectins, Pentosans, Mannans,
Polyuronides
2. Fats and Related cpds
3. Proteins and their derivatives –albumins, amino acids,
amides, Purine, Pyridine and Pyrimidine cpds, Org cpds
with N
4. Lignin and their derivatives – precursors of lignin –
Decomposition products
5. Tannins in simple and condensed form
6. Resins and terpenes
7. Organic acids, aromatic cpds, hydrocarbons, alcohol and
related cpds

21. SOIL ORGANIC MATTER
Proximate Constituents of SOM: Shmuk, 1930
1. Carbohydrates – Pentoses, Pentosans, Hexoses, cellulose
& early decomposition products
2. Hydrocarbons - Paraffin
3. Organic acids – Oxalic acid, Succinic acid etc
4. Alcohols – Mannitol
5. Esters – Glycerides of Caproic and Oleic acid
6. Aldehydes – Vannins, Salicylaldehyde
7. Resins
8. N containing cpds – Argenine, Choline, Creatinine,
Histidine etc

22. SOIL ORGANIC MATTER
Other Proximate Constituents identified:
1. Allantonins – Shorey, 1938
2. Polyuronides – Shorey & Martin, 1932; Waksman &
Reuszer, 1932; Norman & Bartholomew, 1943
3. Uronic acids – Rudakov and Birkal, 1949
4. Methylglyoxal (CH3COCOH) – Enders, Hibbert, 1942 –
Primary Structural Element of Protolignins
5. Polysaccharides of Bacterial Origin – Fuller, 1947;
Forsyth, 1950
6. Amino sugars Glucosamine, Galactosamine – Bremner
and coworkers, 1954, 58
7. Carbohydrates and Organic acids of low molecular wejght
– Nagar, 1953 and Martin, 1954

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24. Methylglyoxal
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25. What constitutes Soil Organic Matter?
Litter: Macro organic matter (e.g. crop residues) that lies
on the soil surface
Light fraction: Plant residues and their partial
decomposition products that.
reside within the soil proper
Microbial biomass: Cells of living microorganisms, notably
bacteria,
actinomycetes, and fungi
Faunal biomass: Tissues of animals (primarily
invertebrates)

26. What constitutes Soil Organic Matter?
Below ground plant constituents: Primarily roots with
lesser amounts of dead roots and exudates
Water-soluble organics: Organic substances dissolved in
the soil solution
Stable humus: Humified remains of plant and animal tissues
that have become stabilized by microbial and chemical
transformations and/or by association with inorganic soil
components

27. Organic Constituents of Soil
•Non humic substances
Lipids (1-6%)
oSoluble in moderately hydrophobic solvents
Carbohydrates (5-25%)
oLow concentrations of free sugars in the soil solution
oComplex carbohydrates that can be extracted and
separated from other organic constituents
oPolymers of various sizes and shapes that are so strongly
attached to clay and/or humic colloids that they cannot be
easily isolated, purified, or identified

28. Organic Constituents of Soil
Proteins/peptides/amino acids (9-16%)
Low concentrations of free amino acids in the soil solution
oAmino acids, peptides, and proteins bound to clay minerals and
humic colloids
oMucopeptides and teichoic acids originating from bacterial cell
walls
Other (trace)
•Humic substances (up to 80%)
Most active fraction of humus
Series of highly acidic, yellow-to black-colored, polyeloctrolytes call
humic and fulvic acids

29. Components of Soil Organic Matter
Soils with high organic matter content are the most productive, store
more water and contribute to a better environment.
The Soil Biology Primer (Chapter 1): By Elaine R. Ingham and Small Organisms in the Soil Affect the Environment in
Big Ways: www.iaswcd.org 29

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31. Composition of Soil Organic Matter

32. Hyphae Associated with Organic matter

33. Soil Organic Matter Content of Indian Soils
Soil Type Range Representative
Values
Black Soil 0.7 – 4.31 1.39
Red Soil 1.3 – 4.12 1.14
Laterite Soil 0.23 – 6.29 1.88
Alluvial Soil 0.28 – 3.18 0.95

34. Soil Organic Matter Content of Mineral Soils
Alfisol 0.8 – 6.5 3.0
Aridisol 0.2 – 1.7 1.0
Histosol 20 – 98 80.0
Mollisol 1.5 – 6.5 4.0
Oxisol 1.5 – 5.0 3.0
Spodosol 1.5 – 5.0 3.5
Vertisol 1.5 – 3.0 2.0

36. SOM versus Soil Organic Carbon (SOC)
1. OM in not made up entirely of OC, there are
other substances that make up about 40% of the
weight.
2. % SOC x 1.724 = % SOM
3. % SOM x 0.58 = % SOC
4. The Van Bemmelen conversion factor is an
average: The actual range in nature is between
0.2 and 3.0.

37. Total Soil C versus SOC
 Total Soil C = Soil Inorganic C + Soil Organic Carbon
 Soil inorganic carbon is most common in semiarid and arid
regions, but is also found in the eastern USA.
 The main sources of inorganic C is from CaCO3 (a.k.a. calcite
or lime) as calcareous dust fall, from weathering of
limestone and concrete, agricultural application,
construction in cities, traffic on gravel roads, exposure of
unweathered calcareous rock in mine spoil and pits, and
from marl formation in wetland soils by periphyton algae
in/under calcium-rich waters.
 We directly measure total C by combustion at 550 C, then
subtract the content of inorganic C measured by CO2
evolution to get the organic carbon portion. 37