Soil Organic Matter.pptx

Soil Organic Matter
and
its role in soil fertility

What is Soil Organic Matter (SOM)?
● Organic matter serves as a reservoir of nutrients and water in
the soil, aids in reducing compaction and surface crusting, and
increases water infiltration into the soil.
● Of all the components of soil, organic matter is probably the
most important and most misunderstood.
● Yet it's often ignored and neglected.
● Let's examine the contributions of soil organic matter and talk
about how to maintain or increase it.
● Many times we think of organic matter as the plant and animal
residues we incorporate into the soil.
● We see a pile of leaves, manure, or plant parts and think,
“Wow! I'm adding a lot of organic matter to the soil.”

● What's the difference between organic material and OM?
● Organic material is anything that was alive and is now
in or on the soil. For it to become OM, it must be decomposed
into humus. Humus is organic material that has been converted
by microorganisms to a resistant state of decomposition.
● Organic material is unstable in the soil, changing form and mass
readily as it decomposes. As much as 90 percent of it disappears
quickly because of decomposition.
● Organic matter is stable in the soil. It has been decomposed until
it is resistant to further decomposition. Usually, only about 5
percent of it mineralizes yearly.
● That rate increases if temperature, oxygen, and moisture
conditions become favourable for decomposition, which often
occurs with excessive tillage. It is the stable organic matter that
is analyzed in the soil test.

1. Soil Organic Matter (SOM)
● SOM, in its widest sense, covers all the living and dead
organism contained within the soil.
● However, when soil scientists use the term they are usually
referring to the remains of plants and animals. These are the
residues, either recent or may be many years old additions to
the soils.
● Soil organic matter is that portion of the soil that consists of
biological residues, from plant to animal to microorganism.
● Organic residues supply not only readily available nutrient
sources but also the building blocks of humus.

1.1 Definition SOM
 SOM refers to the sum of total of all organic carbon containing
substances in soils
- Plant and animal residues in various stages of decomposition
- microbiologically synthesized substances
- organisms that live in soil
 Therefore,
● All organic substances, by definition, contain carbon.
● OM in the world’s soils contains about 3 x as much carbon as
is found in all the world’s vegetation.
● SOM, therefore, plays a critical role in the “Global carbon
balance” that is thought to be major factor affecting “Global
Warming, or Greenhouse effect”.

● Although, OM comprises only a small fraction of the total mass
of most soils, this dynamic soil component exert a dominant
influence on many soil physical, chemical and biological prop.
● SOM is a complex and varied mixture of organic substances.
● It provides much of the cation exchange and water holding
capacities of surface soils.
● Certain component SOM are largely responsible for the formation
and stabilization of soil aggregates.
● SOM also contains large quantities of plant nutrients and acts as a
slow-release nutrient storehouse, especially for nitrogen.
● Furthermore, OM supplies energy and body building constituents
for most of the micro-organism.
● For all these, the quantity and quality of SOM, is a central factor
in determining Soil quality.

 The term SOM is generally used to represent the organic constituents
in the soil, including undecided plant and animal tissues, their partial
decomposition products, and the soil biomass.
 Thus, this term includes: i) identifiable, high-molecular-weight organic
materials such as polysaccharides and proteins, ii) simpler substances
such as sugars, amino acids, and other small molecules,
iii. humic substances.
● It is likely that SOM contains most if not all of the organic compounds
synthesized byliving organisms.
 SOM is frequently said to consist of humic substances and non-humic
substances.
● Non-humic substances are all those materials that can be placed in one
of the categories of discrete compounds such as sugars, amino acids, fats
and so on.
● Humic substances are the other, unidentifiable components. Even this
apparently simple distinction, however, is not as clear cut as it might
appear. Distribution SOM is shown below:

 Organic compounds of soil - live organisms and their
undecomposed, partly decomposed and completely decomposed
remains as well as products of their transformation.
 Living organisms alive - edaphon.
 Soil organic matter - non-living components which are a
heterogeneous mixture composed largely of products resulting from
microbial and chemical transformations of organic debris. Soil
organic matter can exist in different morphological patterns, which
are the bases of the classification of so called forms and types of
humus
 Unaltered materials - fresh and non-transformed components of
older debris.
 Transformed products - (humus) - bearing no morphological
resemblance to the structures from which they were derived.These
transformed components are reffered to as the humification
processproducts.

 Humic substances- a series of relatively high-molecular-weight,
brown to black colored substances formed by secondary synthesis
reactions. The term is used as a generic name to describe to colored
material or its fractions obtained on the basis of solubility
characteristics:
● Humic acids (HA)
● Fulvic acids (FA)
● Humins
 Non-humic substances- compounds belonging to known classes of
bichemistry, such as :
● Carbohydrates
● Lipids
● Amino acids
 The chemical and colloidal properties of SOM can be studied only in
the free state, that is, when freed of inorganic soil components. Thus the
first task of the researcher is to separate organic matter from the
inorganic matrix of sand, silt, and clay.

 Properties of humic substances
● Humic acids - the fraction of humic substances that is not
soluble in water under acidic conditions (pH < 2) but is soluble at
higher pH values. They can be extracted from soil by various
reagents and which is insoluble in dilute acid. Humic acids are the
major extractable component of soil humic substances. They are
dark brown to black in color.
● Fulvic acids - the fraction of humic substances that is soluble
in water under all pH conditions. They remains in solution after
removal of humic acid by acidification. Fulvic acids are light
yellow to yellow-brown in color.
● Humin - the fraction of humic substances that is not soluble
in water at any pH value and in alkali. Humins are black in color.

Elemental composition of humic substances and several
plant material (by Kononova)
Substances
% dry ash-free basis
C H O N
Fulvic
acids
44 - 49 3.5 – 5.0 44 - 49 2.0 – 4.0
Humic
acids
52 - 62 3.0 – 5.5 30 - 33 3.5 – 5.0
Proteins 50 - 55 6.5 – 7.3 19 - 24 15.0 – 19.0
Lignin 62 - 69 5.0 – 6.5 26 - 33 -

2.1 SOM - its role in soil fertility
 Soil organic matters are sources of plant nutrients which are
liberated in available forms during mineralization.
 The physical and chemical properties of soil depend on soil
organic matter.
 Activities of soil micro-organisms, most of the CEC and
aggregate stability of soils and moisture retention capacity
depends on soil organic matter.
 The main source of soil organic matter is plant tissues. The
root and shoot system of trees, shrubs, grasses and other
plants supply large quantities of organic residues every year.
 Animals which feed on plant material leave their own bodies
in the soil after life cycles.

2.2 SOM - role in soil fertility
 SOM consists of a whole series of products undecayed plants
and animal tissues to fairly amorphous brown to black
materials bearing no terrace of the anatomical structure is
normally defined as soil humus.
Humus can be considered to be a store-house of various
nutrients essential to plant growth. During the slow microbial
decomposition of the soil humus, there is a gradual release, with
subsequent mineralization of C, N, S, P and other elements.
 It improves soil structure, its drainage and aeration, water
holding capacity, buffer and exchange capacitates, influences
the solubility of minerals and serves as a source of energy for
the development of micro-organisms, 95% N and 33% P of soil
are obtained from O.M.
 It has a capacity to control soil temperature.

 SOM consists of a variety of components.
 These include, in varying proportions and many intermediate
stages: raw plant residues and microorganisms (1 to 10 per cent)
"active" organic fraction (10 to 40 per cent), resistant or stable
organic matter (40 to 60 per cent) also referred to as humus.
 Raw plant residues, on the surface, help reduce surface wind
speed and water runoff. Removal, incorporation or burning of
residues predisposes the soil to serious erosion.
 The "active" and some of the resistant soil organic components,
together with microorganisms (especially fungi) are involved in
binding small soil particles into larger aggregates.
 Aggregation is important for good soil structure, aeration, water
infiltration and resistance to erosion and crusting.

 The resistant or stable fraction of soil organic matter
contributes mainly to nutrient holding capacity (cation
exchange capacity) and soil color.
 This fraction of organic matter decomposes very slowly and
therefore has less influence on soil fertility than the "active"
organic fraction.
 Organic matter in soil serves several functions. From a
practical agricultural standpoint, it is important for two main
reasons.
 First as a "revolving nutrient bank account"; and second, as an
agent to improve soil structure, maintain tilth, and minimize
erosion.

 As a revolving nutrient bank account, OM serves two main functions:
● Since SOM is derived mainly from plant residues, it contains all of the
essential plant nutrients. Accumulated OM, therefore, is a storehouse
of plant nutrients.
● Upon decomposition, the nutrients are released in a plant-available
form. The stable organic fraction (humus) adsorbs and holds nutrients
in a plant available form.
 In order to maintain this nutrient cycling system, the rate of addition
from crop residues and manure must equal the rate of decomposition.
 If the rate of addition is less than the rate of decomposition, soil organic
matter will decline and, conversely if the rate of addition is greater than
the rate of decomposition, soil organic matter will increase.
 The term steady state has been used to describe a condition where the
rate of addition is equal to the rate of decomposition.
 Fertilizer can contribute to the maintenance of this revolving nutrient
bank account by increasing crop yields and consequently the amount of
residues returned to the soil.

2.3.1 Roles of soil micro flora and soil fauna in the
decomposition of OM
 The soil micro flora (bacteria, actinomycetes, fungi and algae)
have some characteristics for secrete enzymes that digest OM
outside the cell, accumulate nutrient against a concentration
gradient, most are aerobic, compete with vascular plants for the
soil growth factors and the bacteria & fungi are the major
decomposers of soil organic matter.
 Bacteria, actinomycetes and fungi all are heterotrophic and the
fungi are tolerate wide range of the pH, flourish under acid
conditions, invade and penetrate organic materials and
decompose lignin.

2.4.1 Roles of soil micro flora and soil fauna in the
decomposition of OM
 While soil fauna (protozoa, nematode, collembola, mites and
other associated mesofauna) are different types of functional
role for decomposition of soil organic matter.
 It is play an important role in breaking up plant remains such
as leaves, mixing the surface organic matter into the soil.
 Material, which has passed through the gut of soil fauna, is
more readily attacked by the micro flora and the rate of
mineralization is increased.
 Most of protozoa feed on bacteria by capturing, ingesting and
digesting solid particles, a form of nutrition known as
Phagotrophic.

2.5.1 Composition of Organic Matter
 On analysis, however, organic matter yields following chemical
constituents:
● Carbohydrates (Sugars, Starch and Cellulose)
● Lignin
● Protein
● Fats, Oils and Waxes
● Resins
● Pigments
● Minerals e.g. Ca, P, S, Fe, Mg and K.
 The gross average chemical composition of soil organic matter
are carbohydrates - 10%, N- components (e.g. Proteins) - 10%,
fatty acids, resins, waxes - 15% and Humic substances 65%.

2.6.1 Soil organic matter is made up of a range of compounds.
Why are some of these compounds considered to be
important for nutrient release while others are important
in the physical structure of soil ?
 Some soil organic matter compounds considered to be
important for nutrient release while others are important in
the physical structure because of the compounds of SOM is
evident or bond to act as ion exchanger and store house for
nutrients like, nitrogen, phosphorus and sulphur.
 Other side also it promotes soil color, good structure,
improving tilth, aeration and moisture movement & retention
in the physical properties of soils.
 Even the SOM provides C- as energy source to N-fixing
bacteria.

2. 7 SOM - role in soil fertility
2.7.1 What Are the Benefits of Organic Matter?
 Nutrient Supply
● Organic matter is a reservoir of nutrients that can be released to
the soil. Each percent of organic matter in the soil releases 20 to
30 pounds of nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3
pounds of sulfur per year.
● The nutrient release occurs predominantly in the spring and
summer, so summer crops benefit more from organic-matter
mineralization than winter crops.
 Water-Holding Capacity
● Organic matter behaves somewhat like a sponge, with the ability
to absorb and hold up to 90 percent of its weight in water.
● A great advantage of the water-holding capacity of organic matter
is that the matter will release most of the water that it absorbs to
plants. In contrast, clay holds great quantities of water, but much
of it is unavailable to plants.

2.7.2 What Are the Benefits of Organic Matter?
 Soil Structure Aggregation
● Organic matter causes soil to clump and form soil
aggregates, which improves soil structure. With better soil
structure, permeability (infiltration of water through the
soil) improves, in turn improving the soil's ability to take up
and hold water.
 Erosion Prevention
● This property of organic matter is not widely known. Data
used in the universal soil loss equation indicate that
increasing soil organic matter from 1 to 3 percent can reduce
erosion 20 to 33 percent because of increased water
infiltration and stable soil aggregate formation caused by
organic matter.

2.8.1 How Much Organic Matter Is in the Soil?
 An acre of soil measured to a depth of 6 inches weighs
approximately 2,000,000 pounds, which means that 1 percent
organic matter in the soil would weigh about 20,000 pounds per
acre.
 Remember that it takes at least 10 pounds of organic material to
decompose to 1 pound of organic matter, so it takes at least
200,000 pounds (100 tons) of organic material applied or
returned to the soil to add 1 percent stable organic matter under
favorable conditions.
 In soils that formed under prairie vegetation, organic-matter
levels are generally comparatively high because organic material
was supplied from both the top growth and the roots.

2.8.1 How Much Organic Matter is in the Soil?
 We don't usually think of roots as supplying organic material, but a
study in the Upper Great Plains showed that a mixed prairie had an
above - ground (shoot) yield of 1.4 tons of organic material per acre,
while the root yield was about 4 tons per acre. The plants were
producing roots that were more than twice the weight of the shoots.
 Soils that have developed under forest vegetation usually have
comparably low organic-matter levels. There are at least two reasons
for these levels: i) trees produce a much smaller root mass per acre than
grass plants, and ii) trees do not die back and decompose every year.
 Instead, much of the organic material in a forest is tied up in the tree
instead of being returned to the soil.
 Soils that formed under prairie vegetation usually have native organic
matter levels at least twice as high as those formed under forest
vegetation.

3.1 Source of SOM
● The sources of organic matter in the soil include living or dead
from of animals, plants and other forms of life.
● The organic matter represents certain stages in an old less
turnover of Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus &
Sulphur between living organism and the mineral matter.
● Grasses, trees and tree remains, bacteria, fungi soil animals
contribute towards the level of organic matter in the soil to
a large extent.
- When fresh organic matter decomposes, due to the action of
various types of microorganisms, humus is produced which
resists further decomposition and forms a complex with the soil
clay.
- Humus is left over after decomposition has ended, and it is
extremely important in increasing and maintaining soil fertility.

3.2 Source of SOM
How Can I Maintain or Improve Soil Organic Matter Levels?
Building soil organic matter is a long-term process but can be
beneficial. Here are a few ways to do it.
 Reduce or Eliminate Tillage:
►Tillage improves the aeration of the soil and causes a flush of
microbial action that speeds up the decomposition of organic
matter. Tillage also often increases erosion. No-till practices
can help build organic matter.
 Reduce Erosion:
► Most soil organic matter is in the topsoil. When soil erodes,
organic matter goes with it. Saving soil and soil organic
matter go hand in hand.

3.3 Source of SOM
 Soil-Test and Fertilize Properly
● You may not have considered this one. Proper fertilization
encourages growth of plants, which increases root growth.
Increased root growth can help build or maintain soil organic
matter, even if you are removing much of the top growth.
 Cover Crops
● Growing cover crops can help build or maintain soil organic
matter. However, best results are achieved if growing cover crops
is combined with tillage reduction and erosion control measures.
A good supply of soil organic matter is beneficial in crop or
forage production. Consider the benefits of this valuable resource
and how you can manage your operation to build, or at least
maintain, the organic matter in your soil.

3.4 Composition of SOM
● Generally the composition of organic matter are carbohydrates,
range in complexity from simple sugars to the cellulose, the fats
and oils are glycerides of fatty acids such as butyric, stearic and
oleic associated with resins of many kinds, lignins and the crude
proteins are among the more complicated. They contain not only
carbon, hydrogen and oxygen, but also nitrogen and smaller
amounts of such elements as sulphur, iron and phosphorus.
● To find the exact composition of the organic matter has been an
extremely difficult task since humus forms complex with the
mineral matter of the soil.
● However, some gross chemical composition of SOM are to be
calculated and presented the following chemical constituents on
analysis.

Gross Chemical Composition of SOM
Average % composition of SOM
Carbohydrates 10%
N – components ( e.g. Proteins) 10%
Fatty acids, resins, waxes 15%
Humic substances 65%

3.5 Decomposition of SOM
Freshly added OM decomposes in 2 phases
1) Initial rapid phase
2) Slower 2nd phase
Environmental factors influencing decomposition:
Are the factors governing microbial activity
● temperature
● pH
● moisture
● O2 availability
● inorganic nutrients
● accessibility
all influence decomposition of both freshly added and
humified OM

3.5.1 Decomposition of SOM
● List conditions, favor rapid decomposition of OM in soil, are
temperature and moisture. As one moves from a warmer to a cooler
climate, the OM and N of soils tends to increase. At the same time, the
C/N ratio widens somewhat. In general, the decomposition of OM is
accelerated in warm climates, a lower loss in cool regions. Soil moisture
also exerts a positive control upon the decomposition of OM in soils.
● Ordinarily, under comparable conditions the nitrogen and OM
increase, as the effective moisture becomes greater. At the same time, the
C/N ratio becomes wider. Therefore, the temperature and precipitation
(rainfall) favor the rapid decomposition OM.
● Some SOM compounds considered to be important for nutrient release
while others are important in the physical structure of soil, because the
compounds of SOM is evident or bond to act as ion exchanger and store
house for nutrients like, nitrogen, phosphorus and sulphur.
● Other side also it promotes soil color, good structure, improving tilth,
aeration and moisture movement & retention in the physical properties of
soils.

3.5.2 Decomposition of SOM
 Plant material is transformed from one organic compound to
another mainly by organisms in the soil.
 Organisms create by-products, wastes, and cell tissue
compounds released as waste by one organism can often be
used as food by another
 SOM is labile* - it can decline rapidly if the soil environment
changes and renewable – it can be replenished by inputs of
organic material to the soil.
* Labile = Constantly or readily undergoing chemical, physical, or
biological change or breakdown; unstable.

3.5.3 Factors affecting SOM decomp.
Major factors affecting SOM decomposition are moisture,
temperature, aeration and C:N ratio.
3.1 Moisture: When large quantities of organic matter are applied
as manure in arid regions, the slender moisture in the soil may
be largely used for the decomposition of organic matter and
the crop following may well suffer from lack of moisture.
3.2 Temperature: The soil organisms are most active at 24 - 35 0C.
Organic matter of Indian soils is low because of the high rate
of decomposition under tropical and subtropical climate.
Except in a few localized areas in the hilly and high altitudes
regions, the organic matter in most of the cultivated soils
rarely exceeds 1%, O.M. content in Indian soil is
generally 0.5%.
3. 3 Aeration: In clayey soils, decomposition is less rapid.

3.5.4 Factors affecting SOM decomposition
3.4 C : N ratio: Nitrogenous amendments increase CO2
evolution and greater loss of cellulose, hemicelluloses and
other plant polysaccharides. Low N-content or wide C : N
ratio slows down decaying process. Therefore, C: N ratio is
used for predicting the rate of decomposition.
 It is important in controlling the available N, total organic
matter, rate of organic decay and in developing sound soil
management schemes. The C: N ratio of arable soil is 8:1 - 15:1
lower in arid soil and subsoil.
 C: N ratio in plant: 20 - 30:1 in legumes and farm manures and
as high as 400 : 1 in sawdust. In microbes, it is 4:1 – 9:1.
 The C: N value for soil is in between plant and microbes. It is
lower in arid soils than that of humid soils when annual
temp is about the same. It is also lower in warmer regions
than in cooler ones if the rainfalls are about equal.

4.0 Humus
 HUMUS : is a complex and rather resistant mixture of
brown or dark brown amorphous and colloidal substances
modified from the original tissues or synthesized by the
various soil organisms.
 Fulvic acid, humic acid and humin all come under humus.
 In humus, 40-45% lignin and 30-33% proteins and rests
are fats, waxes and residual materials. Lignin and proteins
constitute about 70-80% hence humus is called Lignin-Protein
complex.
 In humus the C/N ratio is 10:1. In most of the Indian soil,
C: N ratio is average 14:1.

4.1 Nature of SOM - Humus
Extraction of SOM
SOM = a range of products
The major portion of SOM is humus,
● a stable black material
● no resemblance to material from which derived
Physically SOM can be subdivided into 2 groups
● A fraction readily separated from mineral particles
● A fraction dispersed only by chemical treatment (Humified fraction)
Chemically SOM is comprised of a range of compounds
ranging from
- freshly added litter
- partly decomposed litter
- chemically stabilized humic material

4.2 Extraction of SOM - Humus
● The major constituents of SOM cannot be identified in precise
chemical terms
Further subdivision of humus:
● A traditional and empirical separation (Achard 1786) is to extract
the soil with NaOH followed by treatment with HCL to pH 2.
● Humic Acid (HA) – extracted by alkali and ppt by acid
● Fulvic Acid (FA) – extracted by alkali and not ppt by acid
● Humin – not extracted by dilute alkali or acid

4.3 Characteristics of The Humic Fraction
 The composition of Humus contains 50% C, 35% O, 5% N and
5% H.
 Further the humus characterized into :
1. Humic Substances,
2. Non-humic Substances.
 Humic substances materials produced in the soil are either
yellow or brown to black, acidic, polydisperse substance of
high molecular weight.
 On the basis of solubility, humic substances are divided into
three classes:
1. Fulvic acid: Lowest molecular weight and both acid and alkali
soluble

2. Humic acid: Medium mol. Wt. and alkali soluble but acid
insoluble.
3. Humin: High mol-wt. and both acid and alkali insoluble
except under the most drastic conditions.
 Fulvic acid is most susceptible to microbial attack where as
humin is most resistant.
 Non-Humic Substances: include all those classes of
compounds occurring in plants and microorganisms that
appear to have relatively definite characteristics e.g.
carbohydrates, proteins, fats, waxes, resins, pigments and low
mol. wt. compounds. Most of these could be relatively easily
attacked by soil microorganism and has a rapid turnover in the
soil.

(HA, FA, Humin):
Humic Acid
● colloidal size
● MW around 200,000
● core derived from lignin
● N present in pyridine rings
● amino acids present
Fulvic Acid
● similar to HA but generally lower MW molecules
Humins
● HA- type compounds adsorbed on mineral surfaces

4.6 Typical element analysis of humus:
Elements Percentage (%)
C 44 – 53
H 3.6 – 5.4
N 1.8 – 3.6
S 0.3 – 1.0
O 40 – 47

4.7 Significance of C : N ratio
 Two major significance of C : N ratio i.e.
1. Keen competition among microorganisms for available N
results when residues having a high C: N ratio are added to
soils.
2. Because this ratio is relatively constant in soil, the maintenance
of 'C' and hence O.M. in soil depends largely on the soil
Nitrogen level.
When decay occurs, the C: N ratio of the remaining plant
material decreases since 'C' is being lost as CO2 and N is
conserved. The older plants, the larger will be C: N ratio and
longer will be the period of Nitrate suppression. The practical
significance of this relatively constant ratio is that a soil's O.M.
content cannot be increased without simultaneously increasing its
Organic Nitrogen content and vice-versa.

4.8 Amounts of SOM
SOM measured by
1) Loss on ignition
2) Oxidation of SOM with strong acid
Top 15 cm soil OM ranges:
► < 1% in desert soils
► almost 100% in organic soils
► 1 - 5% in agricultural soils
► 1 -10% in forest soils

4.8.1 Estimation of Organic Carbon
 Soil organic carbon (SOC) will be estimated by standard
WALKLEY and BLACK METHOD (1947), i.e. called Walkley
and Black rapid titration method.
 In this method, the soil is digested with chromic acid and
sulfuric acid making use of the heat of dilution of sulfuric acid.
The soil organic carbon is, thus oxidized to CO2.
 The highest temperature attained by the heat of dilution of
sulfuric acid is approximately 120 oC, which is sufficient to
oxidize the active forms of the soil organic carbon, but not the
mere inert form of carbon in soil.
 The excess of chromic acid, not reduced by the organic matter
of the soil, is determined by titration with standard ferrous
sulphate solution in presence of NaF or phosphoric acid and
diphenylamine indicator.

4.8.1 Estimation of Organic Carbon ……
 The organic matter (humus) in the soil gets oxidized by chromic
acid K2Cr2O7 + conc. H2SO4) utilizing heat of dilution of
sulfuric acid. The untreated chromate is determined by back
titration with ferrous ammonium sulphate (FAS) ( redox
titration)
 APPARATUS REQUIRD:
● 250ml conical flasks, Burette, pipette measuring cylinder etc.
 REAGENTS:
● 1 N potassium dichromate ( 49.04gm K2Cr2O7 dissolve in 1 litter
distilled water).
● 0.5 N (approx.) FAS, Fe(NH4)2(SO4).6H2O (392.14gm dissolve
in 2 litter distilled water).
● Diphenylamine indicator: 0.5g diphenylamine dissolve in a
mixture of 20ml of water and100ml of Conc. H2SO4.
● Orthophosphoric acid (H3PO4) (85%) or sodium fluoride (pure).

4.8.1 Estimation of Organic Carbon ……
PROCEDURE:
1) Weight 5gm soil in a 500ml conical flask.
2) Add 10 ml of 1N potassium dichromate soln.
3) Add 20ml of Conc. Sulfuric acid through sides of the flask.
4) Shake gently to mix and stand for 30 minutes.
5) Add 20ml distilled water and 10ml of H3PO4 or 0.5g of NaF.
6) Add 1ml of diphenylamine indicator. Titrate with 0.5 N FAS.
7) End point color changes from violet through blue to green.
CALCULATION:
% Organic carbon in soil = (Blank reading – sample reading) x 0.2
% Organic matter = (%Organic carbon x 1.724)

4.9 Measurement of SOM
► almost all figures for SOM have been obtained destructively:
● by igniting a sample of soil or measured in the laboratory as OC
● and weighing the CO2 evolved
● assuming that carbon makes up 58% of soil organic matter, the
amount of CO2 evolved is multiplied by 1.724 to convert to
“organic matter”
i.e., CO2 x 1.724 = organic matter OR (% total C x 1.724)
 Therefore, the practical significance of constant ratio is that of
soil's O.M. cannot be increased without increasing its
Organic Nitrogen content and vice-versa.
58 g C present in = 100 g O.M.
1 g C present in 100/58 g. = 1.724 g O.M.
Therefore, C : O.M. = 1: 1.724
1.724 is called Bemlen Factor

4.10 Measurement of SOM
How is SOM measured in lab at present?
 SOM is usually measured in the laboratory as organic carbon,
Soil organic matter is estimated to contain 58% organic
carbon (varies from 40 to 58%) with the rest of the SOM
comprising of other elements (eg., 5%N, 0.5% P and 0.5% S).
 A conversion to SOM from a given organic carbon analysis
requires that the organic carbon content be multiplied by a
factor of 1.724 (100/58 g) of Bemlen Factor.
 Thus, 2% SOM is about 1.2% organic carbon.

4.11 Nature and Characteristics of Humus
 Tiny colloidal humus particles (micelles) are composed of C, H and O.
 Surface area of humus colloids is very high, generally exceeds that of
silicate clays.
 Negatively charged, the sources of charge being carboxylic (- COOH)
or phenolic (C6H5OH) groups. The extent of the negative charge is pH
dependent (i.e. high at high pH).
 Water holding capacity: 4 - 5 times that of silicate clays.
 At high pH, CEC: 150 - 300 C mol/kg soil.
 Low plasticity and cohesion, thus favorable effect on aggregate
formation and stability.
 Cellulose is not readily available for the use of bacteria but this is first
acted upon by fungi and changed into similar substances and made
available for the use of bacteria.
 Wood is decomposed by actinomycetes.
 Organic matter content of Indian soils is low because of the high rate
of decomposition under tropical and sub-tropical climate.

4.12 Nature and amount of SOM
 The litter layer which consists of dead plants residues at the uppermost
layer of the soil.
 Partly decomposed plant residues formed by the action of soil fauna
and microorganism on the litter.
 Biological organic molecules which are components of plants and
animals tissue e.g. proteins, carbohydrates, lignins, lipids, peptides etc.
 Humic substance which are the stable end products of decomposition of
plants and animals residues.
 Temperate soils have high SOM than tropical soils and this is due to
the factors of soil formation and are represented as Organic matter
= f (climate, time, vegetation, parent material, topography,…..).
 It has been observed that SOM contents decrease 2 to 3 times for
every 100 C rise in mean temperature.
 Humic substances are considered as the most important constituents
of soils.
 They form the largest fraction of SOM and play the most dominating
role in improving soil productivity.

4.13 Functions of SOM
SOM influences plant growth through its effect on soil:
a) Physical properties
● SOM promotes good structure, improving;
- tilth
- aeration
- moisture movement & retention
b) Chemical properties
● SOM acts as;
- Ion exchanger
- N, P, and S storehouse
c) Biological properties
● SOM provides C as energy source to N-fixing bacteria
► Therefore, maintenance of SOM is important in forest
and agricultural cropping, especially on sandy soils.

4.13.1 Functions of SOM
9.1 Coarse organic matter on the surface reduces the impact of the falling
rain drop. Surface runoff and erosion and thus reduced.
9.2 The addition of easily decomposable organic residues causes synthesis
of complex organic substances i.e. soil particles into structural units
called aggregates.
9.3 Live roots decay and provide channels down through which new
plants root grow more luxuriantly. The same root part of which is
stored for future use by plants.
9.4 Organic matter increases the water holding capacity of the soil. Thus,
the permanent wilting percentage is increased.
9.5 Serves as a reservoir of chemical elements that are essential for plant
growth. Most of the soil -N occurs in organic combination. Also a
considerable quantity of phosphorus and sulphur exist in organic
forms.
9.6 Organic matter upon decomposition produces organic acids and
carbon dioxide which helps to dissolve minerals such as potassium.
9.7 Organic matter helps to buffer soils against rapid changes in pH due
to the addition of lime and fertilizers.

4.13.2 Functions of SOM
9.8 Humus provides a storehouse for the exchangeable and available
cations e.g. potassium, calcium and magnesium.
9.9 Serves as a source of energy for the growth of soil microorganisms.
9.10 Fresh organic matter supplies food for such soil like as earthworms.
9.11 Evaporation losses of water are reduced by organic mulches.
9.12 Trashy, coarse organic matter on the surface of soil reduces losses of
soil by wind erosion.
9.13 Surface mulches lower soil temperatures in the sun and keep the soil
warmer in winter.
9.14 Fresh organic matter has a special function in mal soil phosphorus
more readily available in acid so upon decomposition, organic matter
release citratoxalates, tartarates and lactates which combine iron and
aluminium more readily than does phosphorus.
9.15 Organic acids released from decomposing organic matter help to
reduce alkali in soils.

5.0 Importance of SOM
 It is the food source for soil microorganism and soil fauna.
 It is the store house for nitrogen supply to higher plants.
 It is the store house of P and S and there contributes significantly to
the supply of these nutrients to higher plants.
 It improves various other chemical properties of soil eg. the
increased CEC and enhanced ligancy help in trapping nutrients
cations like K, Ca, Mg ,Zn, Cu, Fe etc.
 It improves soil buffering capacity.
 SOM contributes to nutrients release from soil minerals by
weathering reactions.
 Plant growth and development are benefited by the physiological
action of some organic materials that is directly taken up by plants.
 Improvement in soil physical properties like soil structure,
porosity, water holding capacity, soil drainage is direct
consequence of organic matter in soil.
 The organic substance influence various soil processes leading to soil
formation.

5.1 Active Fraction
 10 to 30% of the soil organic matter (active fraction) is
responsible for maintaining soil microorganisms.
 The active fraction of organic matter is most susceptible to soil
management practices. (Inactive = humus)
5.2 Adding fresh OM
 In a soil which at first has no readily decomposable materials,
adding fresh tissues under favorable conditions:
1. Immediately starts rapid multiplication of bacteria, fungi, and
actinomycetes,
1. Which are soon actively decomposing the fresh tissue.
 As most readily available energy sources are used up,
microorganisms again become relatively inactive,
 Leaving behind a dark mixture usually referred to as humus –
a stable organic compound

5.3 Stable Organic Matter – Humus
 Thus, soil organic compounds become stabilized and resistant
to further changes by microorganisms
 Stabilized organic matter acts like a sponge and can absorb six
times its weight in water.
 Newly formed humus
1. Combination of resistant materials from the original plant
tissue,
2. Compounds synthesized as part of the microorganisms
tissue which remain as the organisms die. ( Fulvic and Humic
Acid)
 Humus is resistant to further microbial attack- N and P is
protected from ready solubility.

5.4 Management of SOM
 All organic substances, by definition, contain carbon. OM in the
world’s soils contains about three times as much carbon as is
found in all the world’s vegetation.
 SOM, therefore, plays a critical role in the Global Carbon balance
that is thought to be the major factor affecting Global warming, or
the Greenhouse effect.
 Although OM comprises only a small fraction of the total mass of
most soils, this dynamic soil component exerts a dominant
influence on many soil physical, chemical, and biological
properties.
 In addition to enhancing plant growth through the just mentioned
effects, certain organic compounds found in soils have direct
growths- stimulating effects on plants.
 For all the reasons, the quantity and quality of SOM is a central
factor in determining soil quality.

5.5 SOM = SOIL HEALTH
 Measuring SOM is one step in assessing overall soil quality or
soil health-
 Measuring various key attributes of soil organic matter
quantity and quality will give an indication of the health of the
soil.
 Or look at the state of the soil organisms in the soil.
 Or look at how well the soil "Holds Together".

5.6 SOIL QUALITY
 Soil quality is the capacity of soils within landscapes to sustain
biological productivity, maintain environmental quality, and
promote plant and animal health.
 Protecting soil quality like protecting air quality and water
quality should be fundamental goal of our Nation's
Environmental Policy
 Soil Organic matter encompasses all organic components of a
soil:
► Fresh residues
► Decomposing organic matter
► Stable organic matter
► Living organisms

5.7 SOIL HEALTH
 Soil health is the change in soil quality over time due to
human use and management or to natural events.
 Descriptive terms for Soil Health:
♣ Organic Matter – high
♣ Crop appearance = green, healthy, lush
♣ Erosion – Soil will not erode
♣ Earthworms – numerous
♣ Infiltration – fast, no ponding
♣ Compaction - minimal

5. 8 Effects of Soil Management on SOM
Management influences the quantity of SOM in
2 ways
1. by altering rate of OM decay
2. by altering annual input of OM from dead plants
and animals

5.9 Managing Soil Organic Matter
 There have been vast changes in the nature of agricultural
production. In the past, farms were small, and much of what was
produced was consumed on the farm and system allowed limited
removal of soil nutrients since there was an opportunity to return
most of the nutrients back to the land.
But now the time has changed which the migration of peoples
from rural to urban areas, decreased farm land and production
have resulted decreased with plant nutrients from the soil and less
opportunity for nutrient cycling.
Maintenance of organic matter for the sake of maintenance alone
is not a practical approach to farming. It is more realistic to use a
management system that will give sustained production.
 Ultimately, soil organic matter must be maintained at a level
necessary to maintain soil tilth.

 5.9 Managing Soil Organic Matter ........
 Increased levels of organic matter and associated soil fauna lead to
greater pore space with the immediate result that water infiltrates more
readily and can be held in the soil. The improved pore space is a
consequence of the bioturbating activities of earthworms and other
macro-organisms and channels left in the soil by decayed plant roots.
 On a site in Brazil, rainwater infiltration increased from 20 mm/h
under conventional tillage to 45 mm/h under no tillage. Over a long
period, improved organic matter promoted good soil structure and
macroporosity. Water infiltrates easily, similar to forest soils (Figure 2).
 The consequence of increased water infiltration combined with a
higher organic matter content is increased soil storage of water (Figure3).
 Moreover, organic matter intimately mixed with mineral soil
materials has a considerable influence in increasing moisture holding
capacity. Especially in the topsoil, where the organic matter content is
greater, more water can be stored.

FIGURE 1
Effect of amount of soil cover on rainwater runoff and infiltration

FIGURE 2
Water infiltration under different types of management

FIGURE 3
Quantity of water stored in the soil under conventional tillage
and conservation agriculture

5. 10 Example of Management Effects
Rothamstead (Australia) experimental plot example
● reversion of arable land to woodland
 C and N contents more than double in less that 100 years
 more plant debris enters the soil under woodland than
under arable cultivation
● conversion of arable land to grassland
 N and C contents increase over a 200 year period, about 25
years being needed to move halfway to the final equilibm.
content
● different system of manuring
 with no manuring OM declines over 25 years with little
change after that
 OM has trebled over 125 years with 3 t C ha/year addition
Source: Russells Soil Conditions and Plant Growth (11th ed)
♠ Arable (adj.) of land suitable for growing crops

5.11 Effects of Clay on Decomposition
● decomposition is greatly influenced by soil texture, clays soils
retain much more OM than sandy soils
● clays stabilize OM in soils
- in montmorillonitic clays the OM may penetrate b/n the
crystals and become inaccessible to most organisms
- there is some evidence to show that bacteria within soil
aggregates exist only in pores x 3 their own diameter
 Therefore the greater the clay contents the greater is
the pore space inaccessible to bacteria

5.12 Effects of Temperature and waterlogging
● low temperature
● waterlogging
Either or both of these conditions can lead to:
 the formation of peats with turnover times exceeding 2000
years
 the formation of tundra soils with turnover times exceeding
100 years
● the shortest turnover times, 4 years, apply to equatorial
forests
 these have maximum net primary production
 rapid decomposition does not allow accumulation of OM in
soils of equatorial rainforests

5.13 Ecosystem Litter Return
Land use or vegetation type Organic C (t/ha)
Arable farming 1 – 2
Temperate annual grain crops 0.5 – 2
(roots and stubble)
Temperate grassland 2 – 4
Temperate perennial pastures 2+
(mainly as roots)
Temperate forests 1 – 3
(as litter + unknown as root detritus)
Alpine and Artic forest 0.1 – 0.4
Coniferous forest 1.5 – 3
Deciduous forest 1.5 – 4
Tropical rainforest (Colombia) 4 – 5
Tropical rainforest ( West Africa) 4 – 5
Source: White, R.E. (1987). Introduction to the Principles and Practice of Soil Science, Second Edition.
Blackwell Scientific Publications.
Richards, B.N.. (1987). The Microbiology of Terrestrial Ecosystems. Longman Scientific an Technival,
New York

5.14 Mineralization of C in soils
Retarding factors
● assimilate distributed below ground
● assimilate deficient in nutrients
● assimilate rich in lignin and waxes
● waterlogging – anaerobiosis
● low temperatures
● clay textures
Accelerating factors
● assimilate added in litter
● assimilate rich in nutrients
● assimilate rich in carbohydrate
● aeration in highly porous systems (but not arid)
● high temperatures
● sandy textures
Source: Oades, J. M. (1988): The retention of organic matter in soils.
Biogeochemistry 5: 35 – 70.

5.15 FORESTS Vs GRASSLAND
● within the same climatic zone OM contents in forests are less
than grasslands
● forests produce x 2 OM and perhaps x 2 the debris reaches the
soil surface
However
● soil OM under Savannah has twice the turnover time of that
under tropical seasonal forests
● similar differences occur between forests and grasslands in
temperate regions

5.16 FORESTS Vs GRASSLAND CONTD…
this is due to
● the place of addition of the OM
- mostly added to the soil surface in forests where it is rapidly
mineralized
- accessible to decompose organisms
- mostly added to the soils as root systems in grasslands and is
well distributed in the top meter
- thoroughly mixed with soil before mineralization
- this slows the C flow thru the soil
● Rapid minzn surface OM of root systems is a major factor
involved in the differences in retention of C under grasses and
forests

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