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 |
