Soil Organic Matter (SOM)
Definition; Soil Organic Matter refers to the organic component of soil, consisting of:
- Undecomposed and partially decomposed plant and animal residues
- Microbial biomass (living and dead)
- Synthesized substances like humus
Composition of SOM
|
Component |
Approx. % in SOM |
|
Carbon (C) |
50% |
|
Oxygen (O) |
35% |
|
Hydrogen (H) |
5% |
|
Nitrogen (N) |
5% |
- Total N in soil: ~1000 kg/ha
- Typical SOM in Indian soils: ~0.5%
- Humus C:N ratio: ~10:1
- C in 100g SOM = 58g → Hence,
- C:OM = 1:1.724 (Bemlen Factor)
Forms of SOM (Soil Organic Matter)
- Humic Substances (Colloidal, Dark, Resistant)
|
Type |
Solubility |
MW |
Resistance to Microbes |
|
Fulvic acid |
Soluble in acid & alkali |
Low |
Least |
|
Humic acid |
Soluble in alkali, insoluble in acid |
Medium |
Moderate |
|
Humin |
Insoluble in both |
High |
Most |
- Non-Humic Substances; Includes sugars, fats, proteins, waxes, resins – easily decomposable.
- Humus
- Final, amorphous, stable product of organic matter decomposition
- High CEC (150–300 cmol/kg)
- Water holding capacity: 4–5× more than clay
- Negatively charged due to –COOH and –OH groups (pH dependent)
- Forms Ligno-protein complex (Lignin 40–45%, Proteins 30–33%)
C:N Ratio
|
Material |
C:N Ratio |
|
Legumes |
20–30:1 |
|
Sawdust/Wood |
>400:1 |
|
SOM (Indian soils) |
~14:1 |
|
Microbial biomass |
4–9:1 |
|
Ideal for Mineralization |
<20:1 |
|
Immobilization occurs |
>30:1 |
Processes in Decomposition
- Aminization; Proteins → Amino acids (by Bacillus, Pseudomonas)
- Ammonification; Amino acids → NH₄⁺ (ammonium). NH₂–R + HOH → NH₄⁺ + ROH
- Nitrification; By autotrophs (Nitrosomonas, Nitrobacter)
- NH₄⁺ → NO₂⁻ (Nitrosomonas)
- NO₂⁻ → NO₃⁻ (Nitrobacter)
Opt. Temp: 30–35°C
Opt. pH: 6.5–7.5
- Denitrification
- By: Pseudomonas denitrificans, Bacillus, Achromobacter
- NO₃⁻ → N₂O, NO → N₂ (under anaerobic conditions)
- Enhanced by waterlogging, low O₂, excess OM
Factors Affecting SOM Decomposition
|
Factor |
Influence |
|
Moisture |
Optimum decomposition in moist soils |
|
Temperature |
24–35°C ideal for microbes |
|
Aeration |
Poor in clayey soils, slows decomposition |
|
C:N ratio |
Wide ratio slows decay (Immobilization) |
|
pH |
High pH (alkaline) causes NH₃ loss |
Nitrogen Loss Mechanisms
|
Type |
Conditions |
|
Leaching |
NO₃⁻ lost in water |
|
Runoff |
NH₄⁺ lost with soil |
|
Volatilization |
NH₃ gas loss in alkaline soils (pH > 8) |
|
Chemical decomposition |
Under acidic pH (<5.5) |
|
Denitrification |
In anaerobic, waterlogged soils |
Availability of N from Organic Sources
|
Source |
N Supply (kg/ha) |
|
Soil Organic Matter |
20–30 |
|
Rainfall |
6–8 |
|
Non-Symbiotic N-fixation |
2–4 |
|
Atmospheric deposition (dust, etc.) |
12–16 |
Subbiah & Asija (1956):
Alkaline KMnO₄ method to estimate soil N
• Low: <250 kg/ha
• Medium: 250–500 kg/ha
• High: >500 kg/ha
Phosphorus in Humus
|
Component |
% in organic form |
|
Phytin |
40–80% |
|
Nucleic Acid |
0–10% |
Humus enhances P availability by:
- Complexing Fe/Al ions (prevents P fixation)
- Forming phospho-humic complexes
- Protecting P via humate coating
C:N:P ratio = 100:10:1
If C:P > 100:1 → Immobilisation of P
Sulfur Mineralisation
|
C:S Ratio |
Outcome |
|
< 200 |
SO₄²⁻ released |
|
> 400 |
SO₄²⁻ immobilised |
|
200–400 |
Equilibrium |
Peat vs Muck Soils
|
Feature |
Peat Soil |
Muck Soil |
|
Decomposition |
Partially decomposed OM |
Highly decomposed OM |
|
pH |
Acidic (pH < 3.9) |
Slightly acidic to neutral |
|
OM Content |
10–40% |
>40% (well mixed with mineral matter) |
|
Best Use |
Paddy under receding water |
High-value crops, nursery beds |
Roles of Organic Matter in Soil
- Source of plant nutrients (N, P, S, micronutrients)
- Improves water-holding capacity
- Enhances aeration and drainage
- Increases CEC and buffering capacity
- Forms complexes with toxins & metals, reducing their bioavailability
- Provides energy source for microbes
- Helps in temperature regulation
Quick Revision – Mineralisation vs Immobilisation
|
Condition |
Mineralisation |
Immobilisation |
|
C:N Ratio |
< 20:1 |
> 30:1 |
|
Nutrient Form |
Available |
Unavailable |
|
Soil N Status |
Rises |
Drops (temp. lock) |
|
Microbial Activity |
Releases N, P, S |
Ties up nutrients |
