Soil organic matter plays an important role in soil physical, chemical, and biological properties. It improves soil structure, increases water holding capacity, and provides nutrients to plants. Soil organic matter is composed of decomposing plant and animal residues and humus. Factors like climate, vegetation, texture and drainage influence soil organic matter levels. Maintaining proper soil organic matter through conservation tillage, crop rotations, cover crops and avoiding compaction benefits soil health and plant growth.

1. Role of Soil Organic Matter in
Soil
Ravi Kumar
PhD Environmental Science
(2021BS16D)

2. Introduction
SOIL ORGANIC MATTER (SOM)-
• Substances containing carbon are organic
matter. Soil organic matter consists of
decomposing plant and animal residues. It also
includes substances of organic origin either
leaving or dead.
• SOM primarily derived from plant residues and
secondarily from the animal residues.
• As soil organic matter is derived mainly from
plant residues, it contains all of the essential
plant nutrients. Therefore, accumulated organic
matter is a storehouse of plant nutrients.
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3. Composition of soil & soil organic matter
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4. What is active soil organic matter?
• Active soil organic matter is ‘fresh meat’ to microbes.
• It is the readily digestible and easily decomposed portion of fresh organic residues.
Functions-
• As it is decomposed by soil organisms it helps stabilize soil aggregates
• It releases nutrients by mineralization
• It provides food for microbial activity, which can lead to suppression of plant diseases
and enhanced plant growth.
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Cons-
The amount of active organic matter in the soil can change quickly, in just a year or two, and it’s
highly influenced by soil management practices. To maintain the same level of active soil organic
matter requires a constant supply of fresh organic materials, usually from growing plants.

5. Stable soil organic matter
• Also known as Humus (33-50%)
• The humus fraction has less influence on soil fertility because it is the final
product of decomposition.
Humic Substances-
a. Fulvic acid (FA)
b. Humic acid (HA)
c. Humin (Polymerized product of a part of the FA and HA fractions)
Functions-
Darkens the soil’s colour
It contributes to soil structure and cation exchange capacity
It has a high ion adsorbing capacity (4-6 times that of clay)
It acts as buffering agent and also as an oxidation reduction buffer
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6. Factors affecting soil organic matter
1. Climate
2. Natural vegetation
3. Texture
4. Drainage
5. Cropping and Tillage
6. Crop rotations, residues and plant nutrients.

7. 1. Climate: Temperature and rainfall exert a dominant influence on the amounts of N and
organic matter found in soils.
a) Temperature: The organic matter and N content of comparable soils tend to increase
if one moves from warmer to cooler areas. The decomposition of organic matter is
accelerated in warm climates as compared to cooler climates. For each 10 degree
Celsius decline in mean annual temperature, the total organic matter and N
increases by two to three times.
b) Rainfall: There is an increase in organic matter with an increase in rainfall. Under
comparable conditions, the N and organic matter increase as the effective moisture
becomes greater.
2.Natural Vegetation: The total organic matter is higher in soils developed under
grasslands than those under forests.
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8. 3.Texture: Fine textured soils are generally higher in organic matter than coarse
textured soils.
4.Drainage: Poorly drained soils because of their high moisture content and
relatively poor aeration are much higher in organic matter and N than well
drained soils.
5.Cropping and Tillage: The cropped lands have much low N and organic matter
than comparable virgin soils. Modern conservation tillage practices helps to
maintain high OM levels as compared to conventional tillage.
6.Rotations, residues and plant nutrients: Crop rotations of cereals with legumes
results in higher soil organic matter. Higher organic matter levels, preferably
where a crop rotation is followed.
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9. Role of soil organic matter
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Physical Soil Properties
 Texture
 Structure
 Bulk Density
 Water Holding
Capacity
Chemical Soil Properties
 Availability of
macronutrients
 Availability of
micronutrients
 Cation exchange
capacity
 Heavy metal toxicity
Biological Soil Properties
 Nitrogen mineralization
bacteria
 Denitrification
 Mycorrhizae fungi
 Microbial biomass

10. Physical Soil Properties
Texture-
Soil texture is the relative proportions of the various soil particles in a soil. The three soil
particles that makes soil texture are sand (2 to 0.02 mm in diameter), silt (0.02 to 0.002
mm), and clay (0.002 mm or less in diameter).
Organic-matter content of the soil is highly related to its clay content. The close
correlation between clay mineral concentration and organic matter in soils may be
accounted by the binding of the organic C to clay minerals.
Texture has an effect on aggregation, and there is evidence that the extent of physical
protection of SOM increases with increasing clay content.
Organic matter plays an important role in soil compaction. The susceptibility to
compaction decreases as SOM content increases.
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11. Structure-
Soil structure is the combination or
arrangement of primary soil particles into
secondary units or peds. The secondary units
are characterized on the basis of size, shape,
and grade.
Soil structure is an important property that
mediates many physical and biological
processes and controls SOM decomposition.
Soil structure affects plant growth through its
influence on infiltration, percolation, and
retention of water, soil aeration, and
mechanical impedance to root growth.
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12. Bulk Density-
• Bulk density of soil is defined as the mass
of dry soil per unit bulk volume. The unit
of soil bulk density is Mg/m3. The bulk
density of most surface soils usually ranges
from 1.0 to 1.6 Mg/m3.
• Bulk density is often used as an index of
assessing soil compaction and
productivity.
• As the organic matter increases, the bulk
density is lessened, and vice versa.
• Soil bulk density significantly influences
physical, chemical, and biological
properties of soil–plant systems and
consequently nutrient uptake.
A & B) Soil sampling by Shelby Tube sampler, C & D) In-situ density
determination by Core Cutter Method

13. Water Holding Capacity-
• Organic matter behaves somewhat like a
sponge. It has the ability to absorb and hold up
to 90 percent of its weight in water. Another
great advantage of organic matter is that it
releases nearly all of the water it holds for use
by plants. In contrast, clay holds great
quantities of water, but much of it is
unavailable to plants.
• The addition of organic matter to the soil
usually increases the water holding capacity of
the soil. This is because the addition of organic
matter increases the number of micropores
and macropores in the soil either by “gluing”
soil particles together or by creating favorable
living conditions for soil organisms
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14. Chemical Soil Properties
Availability of macronutrients-
o Organic matter is a major indigenous source of
available N that it contains as much as 65% of the
total soil P and provides significant amounts of
sulfur (S) and other nutrients essential for plant
growth. Organic N is not readily leached or
denitrified.
o Increases in organic-matter levels resulted in
corresponding increases in exchangeable soil
calcium (Ca), magnesium (Mg), potassium (K),
extractable P, and total N in agricultural soils.
o A linear relationship has been demonstrated
between the content of soil organic matter and
the content of total S in surface soils.

15. Availability of micronutrients-
• Organic matter plays a key role in the
soil micronutrient cycle. Organic-matter
fractions such as fulvic acids can form
chelate structures with some metals.
• These chelates can bind micronutrients
such as Cu, Fe, zinc (Zn), and Mn and
improve their availability to plants.
• Organic matter can stabilize soil Fe in a
form that is more readily available to
the sorghum plant.
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16. Cation exchange capacity-
It is the amount of exchangeable cations
present in the soil.
Organic matter, depending on its level in the
soil, can make a significant contribution to the
soil’s CEC.
Increasing organic-matter level in the soil
increased soil CEC.
Research has been reported that an
incremental 1% increase in SOM on a dry-
weight basis (starting near zero) resulted in a
corresponding increase of 1.7 cmol CEC/kg of
soil.
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17. Biological Soil Properties
Nitrogen mineralization bacteria
• Nitrogen mineralization is the conversion of organic N to inorganic N by microbial
activity.
• Urea and ammonium sulfate are dominant N carriers used for crop production
around the world. The oxidation of the ammonium form of N fertilizers, which form
nitrate (NO3
–), can be explained by the following equation:
NH4+ + 2O2 ⇔ NO3
− + H2O + 2H+
• The oxidation of NH4+ in this equation is known as nitrification, and hetero-trophic
and auto-trophic bacteria can carry it out.
• The most important autotrophic genera of bacteria are Nitrosomonas and
Nitrobacter.
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18. Denitrification-
Denitrification is the reduction of N oxides (usually nitrate and nitrite) to molecular N
or N oxides with a lower oxidation state of N by bacterial activity (denitrification) or by
chemical reactions involving nitrite (chemo-denitrification). Denitrification is one of
the major mechanisms for N loss from the soil. Hauck (1981) reported that
denitrification can cause losses of as much as 30% of the applied N under field
conditions.
NO3
−(nitrate) ⇒ NO2 (nitrite) ⇒ NO (nitric oxide) ⇒ N2O (nitrous oxide) ⇒ N2
(dinitrogen)
Most denitrification is biologically catalyzed and closely linked to bacterial respiratory
metabolism.
Soil organic matter has significant influence on the denitrification process in the soil–
plant system. In addition, soil pH, temperature, nitrate concentration, aeration, and
water status control denitrification rates in the soils.

19. Mycorrhizae Fungi
• Vesicular arbuscular mycorrhizal (VAM)
fungi are present in nearly all natural
soils, and these fungi infect the greater
majority of plants including the major
food crops.
• Mycorrhizae fungi have been shown to
improve the nutrition of the host plants
for nutrients that are diffusion limited,
such as P, Zn, Cu, and Fe.
• Adequate amount of organic matter
improves VAM association of plants
because of improved physical and
chemical soil properties.
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20. Microbial Biomass
The microbial biomass mediates many important functions in soils that include nutrient
mineralization, nutrient cycling, and decomposition and formation of SOM as they are
the main sources of enzymes in soils.
Microorganisms can solubilize and mineralize P from inorganic and organic pools of
total P. Humic substances extracted from manures increases the efficiency of N-fixing
organisms like Rhizobium and Azotobacter.
Organic matter serves as source of energy for both macro and micro-faunal organisms.
Organic-matter content of the soils also influences pathogenic microorganisms.
An adequate supply of organic matter favors the growth of saprophytic organisms
relative to parasitic ones and thereby reduces population of the latter.

21. Organic matter management in soil
Farm practices that help to maintain or increase soil organic matter levels:
Use of conservation tillage practices (for example zone tillage or no-till). Tillage
exposes the organic matter to air and will result in the lowering of stable organic
matter due to increased mineralization rates and erosion losses.
Rotation of annual row crops with perennial grass or legume sods will reduce erosion
and build up organic matter as a result of the decomposition of the rootmass.
Establishment of legume cover crops will enhance organic matter accumulation by
providing the nitrogen (N) needed for decomposition of freshly added organic
materials, especially those with a high C to N ratio.
 Avoiding soil compaction which increases waterlogging.
Maintaining proper pH to enhance microbial activity and decomposition of freshly
added materials.
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22. SYMBOL OF TRUST