🧫 Soil organic matter

Soil organic matter is central to soil fertility because it influences nutrient cycling, structure, and biological activity. This lesson explains its composition and the key processes governing decomposition and nutrient release.

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Soil organic matter – composition, decomposition, mineralization and immobilization

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.

Soil organic matter plays an important role in deciding / maintaining soil

physical conditions. It also influences soil chemical properties especially cation

exchange capacity. Organic matters supply the energy sources for soil micro

organisms. Soil development is another aspect which is influenced by the soil

organic matter.

Plant tissue is the major source. Animals are considered as the secondary

sources. They attack original plant tissues, contribute waste products and leave

their own bodies after death.

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.

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  C 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.

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.

Composition of organic residues:

Plant residues contain 75% moisture and 25% dry matter. This 25% is

made up of Carbon (10-12%), Oxygen (9-10%), Hydrogen (1.5-2.5%), N(1-2%)

and mineral matter (1-3%).

Composition of plant tissues:

Carbohydrates

Celluloses 20-50%

Hemicellulose 10-30%

Starch, Sugar 1-5%

Proteins 1-15%

Fats, waxes, tannins 1-10%

Lignins 10-30%

Inorganic residues (mineral matter)

1. Water insolubles

Proteins, Peptides

Nitrogenous Peptones and S containing

materials

2. Water solubles

(No3, NH4 compounds)

Soil organic residues

Non Nitrogenous Carbohydrates (celluloses

Hemicellulose,,Starch, Sugar etc)

Ether solubles (Fats, oils, waxes,

resins etc) Lignins

The organic matter is also classified on the basis of their rate of decomposition

1. Rapidly decomposed : Sugars, starches, proteins etc.

2. Less rapidly decomposed : Hemicelluloses, celluloses etc.

3. Very slowly decomposed : Fats, waxes, resins, lignins etc

Decomposition of soil organic matter:

Different organic residues contain different organic compounds. There is

great variation in the rate of decomposition of organic residues. Sugars, starches

and simple proteins are very rapidly decomposed. On the other hand Fats, waxes

and lignins are very slowly decomposed. Hemicellulose, celluloses and

protein are intermediate. Even though the composition may vary the end products

are more or less the same. The general reactions taking place during

decomposition are

1. Enzymatic oxidation of the bulk with the release of Co2, water, energy and

heat

2. Essential elements are released (N, P, S etc) and immobilized by a series of

reactions.

3. Formation of compounds which are resistant to microbial action.

Molecules very resistant to microbial action is formed either through

modification of compounds or by microbial synthesis

Under aerobic conditions the products formed are

Co2, NH4, NO3, H2PO4, SO4, H2O and essential plant nutrients like Ca, Mg,

Fe, Cu, Zn etc.

Under anaerobic conditions

CH4, organic acids like lactic, propionic, butyric, NH4, various amine

residues (R-NH2) H2S, ethylene (CH2=CH2) and humic substances.

A. Decomposition of soluble substances : When glucose is decomposed under

aerobic conditions the reaction is as under:

Sugar + Oxygen  CO2 + H2O

Under partially oxidized conditions,

Sugar + Oxygen  Aliphatic acids (Acetic, formic etc .) or Hydroxy acids

(Citric, lactic etc.) or Alcohols (ethyl alcohol etc.)

Some of the reactions involved may be represented as under:

C6H12O6 + 2O2  2CH3.COOH + 2CO2 + 2H2O

2C6H12O6 + 3O2  C6H8O7 + 4H2O

C6H12O6 + 2O2  2C2H5OH + 2CO2

Ammonification – organic N - Polypeptides – Peptides – amonoacids – NH3 or

NH4

i ) Ammonification: The transformation of organic nitrogenous compounds

(amino acids, amides, ammonium compounds, nitrates etc.) into ammonia is called

ammonification. This process occurs as a result of hydrolytic and oxidative

enzymatic reaction under aerobic conditions by heterotrophic microbes.

ii)Nitrification: The process of conversion of ammonia to nitrites (NO2) and then

to nitrate (NO3

bacteria.

-) is known as nitrification. It is an aerobic process by autotrophic

NH 3  Nitrosomon  as  NO 2  Nitrobacte  r  NO 3

Ammonia Nitrite Nitrate

The net reactions are as follows:

NH4 + O2  NO2 + 2H [+] + H2O + energy

NO2 + O2  NO3

- + energy

iii) Denitrification: The process, which involves conversion of soil nitrate into

gaseous nitrogen or nitrous oxide, is called Denitrification. Water logging and high

pH will increase N loss by Denitrification.

Nitrate  Pseudomona  s  / Bacillus  N 2 gas

2. Under anaerobic conditions:

C6H12 O6 (Glucose) - Lactic acid, butyric acid Ethyl alcohol are formed

Protein and other N compounds are converted into elemental N.

B. Decomposition of Insoluble Substances

i) Breakdown of Protein : During the course of decomposition of plant materials,

the proteins are first hydrolyzed to a number of intermediate products.

Aminization: The process of conversion of proteins to aminoacids.

Ammonification: The process of conversion of aminoacids and amides to

ammonia.

ii) Breakdown of cellulose : The decomposition of the most abundant carbohydrates.

Hydrolysis hydrolysis oxidation

Cellulose  Cellobiose  Glucose  Organic acids  CO2 + H2O

(cellulase) (cellobiase)

This reaction proceeds more slowly in acid soils than in neutral and alkaline soils. It

is quite rapid in well aerated soils and comparatively slow in poorly aerated soils.

iii) Breakdown of Hemicellulose : Decompose faster than cellulose and are first

hydrolyzed to their components sugars and uronic acids. Sugars are attacked by

microbes and are converted to organic acids, alcohols, carbon dioxide and water.

The uronic acids are broken down to pentose and CO2. The newly synthesized

hemicelluloses thus form a part of the humus.

iv) Breakdown of Starch : It is chemically a glucose polymer and is first

hydrolyzed to maltose by the action of amylases. Maltose is next converted to

glucose by maltase. The process is represented as under:

(C6H10O5)n +nH2O  n (C6H12O6)

C. Decomposition of ether soluble substances:

Fats  glycerol + fatty acids

Glycerol  CO + water

2

D. Decomposition of lignin : Lignin decomposes slowly, much slower than

cellulose. Complete oxidation gives rise to CO2 and H2O.

Sulphur containing organic compounds:

Converted to SO4

-2 + H+ + energy by sulphur oxidizing bacteria.

P containing organic compounds:

Various micro organisms mineralize phospholipids and other organic P compounds

in the presence of phosphates enzymets H2PO4 and HPO4

-2 depending on soil PH.

Mineralisation: The biological conversion of organic forms of C, N, P and S to

inorganic or mineral forms is called mineralization.

Immobilization: The conversion of inorganic forms of C, N, P and S by the soil

organism into organic forms is called Immobilization.

Factors affecting decomposition

1. Temperature : Cold periods retard plant growth and organic matter

decomposition. Warm summers may permit plant growth and humus accumulation.

2. Soil moisture : Extremes of both arid and anaerobic conditions reduce plant

growth and microbial decomposition. Near or slightly wetter than field capacity

moisture conditions are most favorable for both processes.

3. Nutrients : Lack of nutrients particularly N slows decomposition.

4. Soil pH : Most of the microbes grow best at pH 6 to 8, but are severely inhibited

below pH 4.5 and above pH 8.5.

5. Soil Texture : Soils higher in clays tend to retain larger amounts of humus.

6. Other Factors : Toxic levels of elements (Al, Mn, B, Se, Cl), excessive soluble

salts, shade and organic phytotoxins in plant materials.

Role of organic matter

1. Organic matte creates a granular condition of soil which maintains favorable

condition of aeration and permeability.

2. Water holding capacity of soil is increased and surface runoff, erosion etc., are

reduced as there is good infiltration due to the addition of organic matter.

3. Surface mulching with coarse organic matter lowers wind erosion and lowers

soil temperatures in the summer and keeps the soil warmer in winter.

4. Organic matter serves as a source of energy for the microbes and as a reservoir

of nutrients that are essential for plant growth and also hormones, antibiotics.

5. Fresh Organic matter supplies food for earthworms, ants and rodents and makes

soil P readily available in acid soils.

6. Organic acids released from decomposing organic matter help to reduce

alkalinity in soils; organic acids along with released CO2 dissolve minerals and

make them more available.

7. Humus (a highly decomposed organic matter) provides a storehouse for the

exchangeable and available cations.

8. It acts as a buffering agent which checks rapid chemical changes in pH and soil

reaction.

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Summary Cheat Sheet

Quick Recall Points

• SOM improves structure, water holding, and nutrient buffering.
• Decomposition links carbon inputs to nutrient mineralization.
• Stable organic fractions contribute to long-term soil resilience.

Exam Traps

• High residue input does not guarantee immediate nutrient release.
• SOM quality (C:N ratio, composition) affects decomposition rate.

References

2 sources • [1] [2]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

FAO Soils Portal

Website