🧫 Humus
Covers humus formation, fractions, and their contribution to soil fertility and buffering.
Humus is the stable organic fraction of soil formed through decomposition and synthesis, and it plays a central role in nutrient retention, structure, and fertility.
Humus is a complex and rather resistant mixture of brown or dark brown
amorphous and colloidal organic substance that results from microbial
decomposition and synthesis and has chemical and physical properties of great
significance to soils and plants.
Humus Formation
The humus compounds have resulted from two general types of biochemical
reactions: Decomposition and Synthesis.
1. Decomposition: a) Chemicals in the plant residues are broken down by soil
microbes including lignin. b) Other simpler organic compounds that result from the
breakdown take part immediately in the second of the humus-forming processes,
biochemical synthesis. c) These simpler chemicals are metabolized into new
compounds in the body tissue of soil microbes. d) The new compounds are subject
to further modification and synthesis as the microbial tissue is subsequently
attacked by other soil microbes.
2. Synthesis: Involve such breakdown products of lignin as the phenols and
quinones. a) These monomers undergo polymerization by which polyphenols and
polyquinones are formed. b) These high molecular weight compounds interact with
N-containing amino compounds and forms a significant component of resistant
humus. c) Colloidal clays encourage formation of these polymers. d) Generally two
groups of compounds that collectively make up humus, the humic group and the
nonhumic group.
Soil organic matter fractions
Humic matter
Non humic matter
When soil is extracted with alkali the humic substances go into solution.
The insoluble portion forms the non humic matter.
Humic group
This group makes up about 60-80% of the soil organic matter.
They are most complex. They are most resistant to microbial attack.
Humic substances have aromatic ring type structures.
These include polyphenols and poly quinones.
These are formed by decomposition, synthesis and polymerization.
The humic substances are classified based on resistance to degradation and
solubility in acids and alkalis into
Humic acid
Fulvic acid
Humin
Non humic group
This group makes upto 20-30% of the organic matter in soil.
These are less complex and less resistant to microbial attack as compared to
humic substances.
They are polysaccharides, polymers having sugar like structures and
polyuronides.
These include proteins, carbohydrates, lignins, fats, waxes, resins, tannins
and some compounds of low molecular weight.
Theories on humus formation
Lignin theory: Proposed by Waksman (1936).
According to this theory humic substances are formed due to the incomplete
degradation of lignin
Kononovas theory
According to this theory humic substances are formed by cellulose
decomposing mycobacteria earlier to lignin decomposition
Polyphenol theory: (Flaig and Sochtig (1964)
As per this theory the humic substances are formed by the condensation of
phenolic materials.
The polyphenols of lignin are oxidized to quinones.
These quinones are condensed with low molecular weight microbial
products to form humic molecules.
The microbial products are amino acids, nucleic acid and phospholipids.
Properties of Humus
- The tiny colloidal particles are composed of C,H, and O
.
- The colloidal particles are negatively charged (-OH, -COOH or phenolic
groups), has very high surface area, higher CEC (150 – 300 cmol/kg), 4
5 times higher WHC than that of silicate clays.
-
Humus has a very favorable effect on aggregate formation and stability.
-
Impart black colour to soils.
Cation exchange reactions are similar to those occurring with silicate clays
Clay – Humus Complex
Humus, the organic amorphous colloid supplies both basic and acidic ions
which is transitory and ultimately disappears from soil. Clay, the inorganic
crystalline colloid supplies chiefly the basic nutrient ions is more or less stable.
Both these colloids form the soil colloidal complex and are extremely active and
form important sources of plant nutrients.
It is believed that humus and clay exist in the soil as clay – humus complex,
the two being held together by cations like Ca, Fe, etc. Depending upon the nature
of binding cation, two types of Clay – humus complex have been recognized. The
colloidal complex bound by Ca ions is more stable and is responsible for the
favorable physical condition of the soil, particularly its structure. The other type
where Fe acts as the binding agent creates a poor physical condition of the soils.
Maintenance of Humus (Soil organic matter)
Maintenance of humus at a higher level is difficult.
This is due to the reason that the loss of carbon from the soil increases as
the organic matter content is raised.
No attempt should be made to increase the organic matter content over that
the soil plant-climate control mechanism can permit.
There is a strong linkage between soil Nitrogen and soil organic matter.
To maintain adequate level of organic matter in the soil the N level should
be maintained by inclusion of legumes in crop rotation and judicious
application of N fertilizers.
Loss of N from the soil is also to be minimized.
To maintain the organic matter level continuous addition of organic
materials is essential.
These include animal manure, organic wastes and crop residues.
Incorporation of green manure will add to the organic carbon level of soil.
Ensuring vigorous crop growth by removing constraints in crop production
will result in addition of root and top residues to the soil.
Conservation tillage (minimum tillage) should be followed to the extent
possible.
This will reduce the decay of residues.
Keeping the land fallow to encourage natural vegetation is also advisable.
Summary Cheat Sheet
Quick Recall Points
- Humification transforms residues into stable organic fractions.
- Major humus fractions include humic acid, fulvic acid, and humin.
- Humus supports CEC, aggregation, and long-term fertility.
Exam Traps
- Humus is not identical to fresh organic residue.
- Fulvic acids are generally more soluble than humic acids.
References
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References
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