This document discusses humus formation and its role in nutrient cycling. It defines humus and explains that it is formed through a complex biochemical process as organic residues in soil are broken down and transformed by microorganisms and chemical reactions. Several theories for humus formation are described, including the lignin-protein theory, polyphenol theory, and sugar-amine condensation. Humus improves soil properties, supports nutrient cycling by holding nutrients, and acts as a reservoir for microbes. It plays an important role in plant nutrient cycles by releasing nutrients as organic matter is decomposed. The document outlines fractionation of organic matter and interactions between plants, soil and the atmosphere in nutrient cycling.

1. Process Of Humus Formation
&
Its Role In Nutrient Cycling
Submitted by-
Sunil Kumar Meher
M.Sc.(Ag)
Dept of Soil Science
Guided By-
Dr. R.N. Singh
Prof. Dept of Soil Science
IGKVV, Raipur
Soil Biology & Biochemistry
SOILS- 506 (2+1)

2. Outlines
1. What Is HUMUS ??
2. Importance Of Soil Humus
3. Process Of Humification
4. The Formation Of Humic Substances
5. The Lignin- Protein Theory
6. The Polyphenol Theory
7. Sugar-amine Condensation
8. Fractionation Of Organic Matter
9. Role Of Humus In Plant Nutrient Cycling
10. Basic Plant Nutrient Cycle
11. Interaction Of Plant-soil-atmosphere In Plant Nutrient Cycling
12. References

3.  Humus is defined as a complex and rather resistant mixture of brown
or dark brown amorphous and colloidal substances modified from
the original tissues or synthesized by the various soil organisms” .
 It is a natural body having variable heterogeneous mass.
 Soil Humus is also known as “Soil skin/Flesh”.
 The formation of humus is strictly a biochemical process and rather
complicated one.
 Humification of organic residues incorporating the soil depends upon
their chemical composition and soil conditions influencing the activity
of soil micro-organisms. On reaching the soil, organic residues of
plant and animal origin undergo diverse changes brought about by
various factors.

5. Importance Of Soil Humus
• The process that converts raw organic matter into humus feeds the soil population
of microorganisms and other creatures, thus maintains high and healthy levels
of soil life.
• Improve soil physical condition by lowering Bulk Density of soil and increasing soil
WHC, Infiltration Rate, improved structure, aggregates etc..
• Help in Plant Nutrition as its CEC is 150-300 cmol/ kg of Humus, Highest among
any soil colloids it store cations on its negatively charged surface.
• Effective humus and stable humus are further sources of nutrients to microbes,
the former provides a readily available supply, and the latter acts as a longer-term
storage reservoir.
• Decomposition of dead plant material causes complex organic compounds to be
slowly oxidized (lignin-like humus) or to break down into simpler forms
(sugars and amino sugars, aliphatics, and phenolics organic acids), which are
further transformed into microbial biomass (microbial humus) or are reorganized,
and further oxidized, into humic assemblages (fulvic and humic acids), which bind
to clay minerals and metal hydroxides.

6. Continue..
• There has been a long debate about the ability of plants to uptake
humic substances from their root systems and to metabolize them.
There is now a consensus about how humus plays a hormonal role
rather than simply a nutritional role in plant physiology.
• Humus is a colloidal substance, and increases the soil's cation
exchange capacity, hence its ability to store nutrients by chelation.
While these nutrient cations are accessible to plants, they are held
in the soil safe from being leached by rain or irrigation.
• Humus can hold the equivalent of 80–90% of its weight in moisture,
and therefore increases the soil's capacity to withstand drought
conditions.
• The biochemical structure of humus enables it to moderate or
buffer excessive acid or alkaline soil conditions.

7.  Some of these changes take place without the participation of
microorganisms which are as follows:
 (a) Destructive changes due to the physical action of natural factors
(rainfall, wind etc.) and to the action of man (soil cultivation).
 (b) Changes in the chemical nature of organic residues under the
direct action of water, light, air and reaction of the medium, e.g. the
oxidation of fats and resins in light, the oxidation of aromatic
compounds under alkaline soil conditions etc.
 (c) Changes resulting from the effect of tissue enzymes, whose
action, in dead cells, has a uni-directional, pre-dominantly oxidative
character (e.g. oxidation of tannins, polyphenols and other aromatic
amino acids etc.) forming complex dark-coloured condensation
products.

8. Humus formation from the residues of plant and animal is two-
stage process:-
1. The decomposition of the original components of tissues and their
conversion by micro-organisms into simpler chemical compounds
and partially to products of complete mineralization (CO , NO , NO ,
NH , CH , H O etc.).
2. The synthesis of organic compounds with the formation of high
molecular-weight humus substances of specific nature. Micro-
organisms participate mainly in the first stage of humus formation the
decomposition of the fresh organic materials and the synthetic
activity is limited to the re-synthesis of bacterial plasma. In the
second stage of humus formation i.e. the condensation reactions
(physico-chemical reactions) can take place without the participation
of microorganisms.
 Humus or soil organic matter consists of two major types of
compounds-
Un-humified and Humified residues of plant and
animal.
 The un-humified substances are represented by certain organic

9. The Formation Of Humic
Substances•The formation of humic substances is one of the least understood aspects
of humus chemistry and one of the most intriguing. Studies on this subject
are of long-standing and continued research can be justified on theoretical
and practical grounds.
Several pathways exist for the formation of humic substances during the
decay of plantand animal remains in soil,
•Classical theory, popularized by Waksman, is that humic substances
represent modified lignins (pathway 1) but the majority of present-day
investigators favor a mechanism involving quinones (pathway 2 and 3).
•In practice all four pathways must be considered as likely mechanisms for
the synthesis of humic and fulvic acids in nature, including sugar-amine
condensation (pathway 4).
•This four pathways may operate in all soils, but not to the same extent or in
the same order of importance.
•A lignin pathway may predominate in poorly drained soils and wet
sediments (swamps, etc.) whereas synthesis from polyphenols may be of
considerable importance in certain forest soils. The frequent and sharp
fluctuations in temperature, moisture and irradiation in terrestrial surface
soils under a harsh continental climate may favor humus synthesis by sugar-
amine condensation.

11. Pathway 1 - The lignin- protein theory
(Walksmann Classical Theory- 1932)
•For many years it was thought that
humic substances were derived from
lignin(pathway1).
•According to this theory, lignin is
incompletely utilized by microorganism
and the residuum becomes part of the
soil humus.
•Modification in lignin include loss of
methoxyl (OCH3) groups with the
generation of ortho-hydroxyphenols
and oxidation of aliphatic side chains to
form COOH groups.
•The modified material is subject to
further unknown changes to yield first
humic acids and then fulvic acids.
•This pathway, be of considerable
importance in certain forest soils.

12. •In pathway 3 lignin still plays an important
role in humus synthesis, but in a different
way.In this case phenolic aldehydes and
acids released from lignin during
microbiological attack undergo enzymatic
conversion to quinones, which polymerize
in the presence or absence of amino
compounds to form humic like
macromolecules.
•Pathway 2 is somewhat similar to
pathway 3 except that the polyphenols are
synthesized by microbes from non-lignin C
sources (e.g.celulose).
•The polyphenols are then enzymatically
oxidized to quinones and converted to
humic substances.
•As noted earlier, the classical theory of
Waksman is now considered obsolete by
many investigators.
•According to current concepts quinones
of lignin origin, together with those
synthesized by microorganisms, are the
major building blocks from which humic
Pathway 2 And 3 - The Polyphenol Theory
(Flaig & Kononova Concepts- 1964)

13.  The concept of humus formation as described by Flaig is given
below:
1. Lignin, freed of its linkage with cellulose during decomposition of
plant residues, is subjected to oxidative splitting with formation of
primary structural units (derivatives of phenyl-propane).
2. The side-chains of the lignin-building units are oxidised, de-
methylation occurs, and the resulting polyphenols are converted
to quinones by poly-phenol-oxidase enzymes.
3. Quinones arising from lignin (as well as from other sources)
react with N-containing compounds to form dark-coloured
polymers.
A. Flaig’s Concept of Humus Formation:

14. There are three stages leading to the formation of humic
substances which are as follows:
 Stage 1:
Fungi attack simple carbohydrates and parts of the protein and
cellulose in the medullary rays, cortex of plant residues.
 Stage 2:
Cellulose of the xylem is decomposed by aerobic myxobacteria.
Polyphenols synthesized by the myxobacteria are oxidised to
quinones by poly-phenol-oxidase enzymes, and the quinones
subsequently react with N compounds to form brown humic
substances.
 Stage 3:
Lignin is decomposed. Phenols released during decay also
serve as source materials for humus synthesis.
B. Kononova’s Concept of Humus Formation:

15. • The concept that humus is formed from
sugars dates back to the early days of humus
chemistry.
• According to this concept reducing sugars
and amino acids, formed as by-products of
microbial metabolism, undergo non-enzymatic
polymerization to form brown nitrogenous
polymers of the type produced during
dehydration of certain food products at
moderate temperatures.
• A major objection to this theory is that the
reaction proceeds rather slowly at the
temperatures found under normal soil
conditions.
• However, drastic and frequent changes in the
soil environment (freezing and thawing, wetting
and drying), together with the intermixing of
reactants with mineral material having catalytic
properties, may facilitate condensation.
• An attractive feature of the theory is that the
reactants (sugars, amino acids etc.) are
produced in abundance through the activities of
Pathway 4 - Sugar-amine condensation
(Maillard Concept- 1911)

16. Fractionation of Organic Matter

18. Role of Humus in Plant Nutrient Cycling
• The basic nutrient cycle usually describes the outstanding role of soil
organic matter. Cycling of many plant nutrients, especially N, P, S, and
micronutrients, are similar to the Carbon Cycle.
• Plant residues, grain green manure, farmyard manure and other
substances are incorporated to the soil. This organic matter pool of carbon
compounds serve as food for bacteria, fungi, and other decomposers. As
organic matter is decomposed to simpler compounds, plant nutrients are
released in available forms for root uptake and the cycle begins again.
• Plant-available macronutrients such as N, P, K, Ca, Mg, S and
micronutrients are also released when soil minerals dissolve.
• As soil organic matter contains large number of microbes(Bacteria, Fungi,
Algae, Protozoa etc) these organisms decompose all incorporated organic
matter readily so generally soil organic matter constitute about-
• 95% of total soil N
• 25-70% of total soil P
• 70-85% of total soil S

21. Atmosphere
Interaction of Plant-Soil-Atmosphere in Plant Nutrient Cycling ;-

22. References-
• Books-
1. Fundamentals of Soil Science, ISSS
2. Introductory Soil Science ( Dr D.K. Das)
• Internet-
1. rosealchemy.org
2. scirp.org
3. tankonyvtar.hu/en (Nutrient management)
4. researchgate.net
5. karnet.up.wroc.pl/

23. THANK YOU