The document discusses the significance of organic matter in soil, explaining its role in improving soil structure, water retention, and nutrient availability. It details the composition and classification of organic residues, the process of decomposition, and various factors that affect soil organic matter and decomposition rates. The carbon cycle and C:N ratio are also outlined, emphasizing the importance of organic matter for microbial activity and overall soil health.

1. ORGANIC MATTER AND ITS DECOMPOSITION
Dr. K. VANANGAMUDI
Formerly Dean (Agriculture), AC & RI, Coimbatore
Dean, Adhiparashakthi Agricultural College, Kalavai
Professor and Head - Seed Science and Technology,
Tamil Nadu Agricultural University,
Coimbatore 641 04, Tamil Nadu, India.
1. Organic matter
 In sandy soil, the sticky and slimy organic materials cement the sand particles together
to form aggregates.
 In clayey soil, it modifies the properties of clay by reducing its cohesiveness and
making clay more crumby.
2. 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%).
3. Composition of plant tissues
20-50%
10-30%
1-5%
1-15%
 Celluloses
 Hemicellulose
 Starch, sugar
 Proteins
 Fats, waxes, tannins 1-10%
 Lignins 10-30%
4. Organic matter classification
1. Based on solubility
a) Water insolubles:
 Proteins, peptides.
 Nitrogenous peptones and S containing materials.

2. b) Water soluble:
 NO3, NH4 compounds.
 Non nitrogenous carbohydrates (celluloses, hemicellulose, starch, sugar etc.,).
c) Ether soluble:
 Fats, oils, waxes, resins etc., lignins.
2. Based on rate of decomposition
a) Rapidly decomposed: Sugars, starches, proteins etc.
b) Less rapidly decomposed: Hemicelluloses, celluloses etc.
c) Very slowly decomposed: Fats, waxes, resins, lignins etc.
5. Decomposition of soil organic matter
 Enzymatic oxidation of the bulk with the release of CO2, water, energy and heat.
 Essential elements are released (N, P, S etc.,) and immobilized by a series of reactions.
 Formation of compounds which are resistant to microbial action.
 Aerobic condition:
When glucose is decomposed under aerobic conditions, the reaction is as under:
Sugar + Oxygen CO2 + H2O
CO2, NH4, NO3 , H2PO4, SO4, H2O and essential plant nutrients like Ca, Mg, Fe, Cu,
Zn etc.,
 Anaerobic conditions:
Sugar + Oxygen Aliphatic acids (Acetic, formic etc.,) or hydroxy acids (Citric,
lactic etc.,) or alcohols (Ethyl alcohol etc.,)
C6H12O6 + 2O2
2C6H12O6 + 3O2
C6H12O6 + 2O2
2CH3COOH + 2CO2 + 2H2O
C6H8O7 + 4H2O
2C2H5OH + 2CO2
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
i) Ammonification:
Organic N – Polypeptides - Amino acids – NH3 or NH4
 Transformation of organic nitrogenous compounds (amino acids, amides, ammonium
compounds, nitrates etc.,) into ammonia.
 Hydrolytic and oxidative enzymatic reaction under aerobic conditions by heterotrophic
microbes like Bacillus, Clostridium, Proteus, Pseudomonas and Streptomyces.

3. ii) Nitrification:
 Conversion of ammonia to nitrites (NO2) and then to nitrate (NO3)
 Aerobic process by autotrophic bacteria
NH4 + O2 NO2 + 2H+ + H2O + energy
NO2 + O2 NO3 + energy
Nitrosomonas Nitrobactor
NH4 NO2
Ammonia Nitrite
NO3
Nitrate
iii) Denitrification:
 Conversion of nitrates into gaseous nitrogen or nitrous oxide.
Pseudomonas / Bacillus
Nitrate Nitrogen gas
Hydrolysis
Proteins Peptones
Amenization
Ammonia peptides Amides
b). Decomposition of insoluble substances
i) Breakdown of protein: Proteins are first hydrolyzed to a number of intermediate products.
Aminization: Conversion of proteins to amino acids.
Ammonification: Conversion of amino acids and amides to ammonia.
ii) Breakdown of cellulose: Decomposition of the most abundant carbohydrates.
Hydrolysis Hydrolysis Oxidation
Organic acids CO2 +
Cellulose
H2O
Cellobiose Glucose
(Cellulase) (Cellobiase)
iii) Breakdown of hemicelluloses: Hydrolyzed in to sugars and uronic acids.

 Sugars and are converted to organic acids, alcohols, carbon dioxide and water by
microbes.
Uronic acids are broken down to pentose and CO2.
iv) Breakdown of starch: First hydrolyzed to maltose by the action of amylases.
 Maltose is next converted to glucose by maltase.
(C6H10O5)n + nH2O n(C6H12O6)
c) Decomposition of ether soluble substance
Fats Glycerol + Fatty acids
Glycerol CO2
+ Water
d)Decomposition of lignin: Complete oxidation gives rise to CO2 and H2O.

4. e)Sulphur containing organic compounds converted to SO4
-2 + H+ + energy by sulphur
oxidizing bacteria
f) P containing organic compounds
Mineralisation: Biological conversion of organic forms of C, N, P and S to inorganic or
mineral forms
Immobilization: Conversion of inorganic forms of C, N, P and S by the soil organism into
organic forms
6. Carbon cycle
 Carbon is a constituent of all organic matter
 Fixed as organic form by photosynthesis.
 Fixed carbon becomes unavailable for use in the generation of new plant life.
 Carbon containing materials need to be decomposed and returned to the atmosphere for
the survival of the higher organisms.
 Carbon cycle: revolves about CO2 and its fixation and regeneration.
 Final decomposition and production of CO2 from humus and the rotting tissues.
 Involves two processes namely immobilization and mineralization.
 Immobilization: conversion of inorganic forms of nutrients to organic forms.
 Mineralization: conversion of organic forms of C, N, P and S into inorganic or mineral
forms.
1. C: N ratio
 Refers to ratio between the nitrogen content in the microbes and organic residues, and
to the carbon content.
 When fresh plant residues are added to soil, they are rich in carbon and poor in N.
Results in wider C: N ratio (40:1).
 Decomposition of organic matter in soil changes to humus resulting in narrow C: N
ratio (10:1).
 When materials with high carbon are added to soil, microbial population increased due
to plentiful supply of food materials.
 During this process the microorganisms utilize the soil N for their body build up and
there is a temporary block of N.
 When decomposition of fresh organic residues takes place, C: N ratio is 20:1 due to
increase in the availability of N.
 C: N ratio of cultivated soils ranges from 8:1 to 15:1, with an average of 10:1 to 12:1.

5.  Legumes and farm manures: 20:1 – 30:1.
 Straw: 100:1
 Saw Dust: 400:1
 In microorganisms: 4:1 to 9:1
 Humus: 10:1
 C: N ratio is lower in soils of arid regions than humid regions.
 C: N ratio is smaller in subsoils.
7. Role of organic matter
 Creates a granular condition of soil which favours aeration and permeability.
 Water holding capacity of soil is increased by reducing surface runoff, erosion etc., and
results in good infiltration.
 Surface mulching with coarse organic matter lowers wind erosion and soil temperatures
in the summer and keeps the soil warmer in winter.
 Organic matter serves as a source of energy for the microbes
 Reservoir of nutrients, hormones and antibiotics that is essential for plant growth.
 Supplies food for earthworms, ants and rodents.
 Makes soil P readily available in acid soils.
 Organic acids released from decomposing organic matter help to reduce alkalinity in
soils
 Humus (a highly decomposed organic matter) provides a storehouse for the
exchangeable and available cations.
 Acts as a buffering agent which checks rapid chemical changes in pH and soil reaction.
8. Factors affecting soil organic matter
1. Climate:
a.Temperature: Decomposition of organic matter is accelerated in warm climates as
compared to cooler climates.
b. Rainfall: Increase in organic matter with an increase in rainfall.
2.Natural vegetation: 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.

6. 5.Cropping and tillage: Cropped lands have much low N and organic matter than comparable
virgin soils.
6.Rotations, residues and plant nutrients: Crop rotations of cereals with legumes results in
higher soil organic matter.
9. Factors affecting decomposition
1. Temperature: 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.
3. Nutrients: Lack of N slows down decomposition.
4.Soil pH: Microbes grow best at pH 6 to 8. Severely inhibited below pH 4.5 and above pH
8.5.