A simplified representation of the chemical nature
and reactions of soil humus
F. J. Stevenson* and R. A. Olsen
ABSTRACT
An understanding of the chemistry and reactions of humic
acids and related substances is crucial to an understanding of
the nature and properties of soils. A comprehensible structural
representation of humic substances is presented and the simplified model is used to illustrate: (i) the way in which stable
humus is formed from lignin; (ii) forms of organic N in soils;
(iii) the interaction of humus with clay minerals; and (iv) the
important roles played by soil organic matter, including contribution to the cation-exchange capacity, binding of pesticides,
and formation of complexes with micronutrient cations. The
approach is of value for teaching soil science at the undergraduate level.
T
HE BULK of the organic matter in most soils consists
of a series of brown- to black-colored substances
referred to by such names as humic acids, fulvic acids,
and humins. In classical terminology, humic acid is the
material extracted from soil by alkaline solutions and
which precipitates upon acidification; fulvic acid is the
extracted material that remains in solution upon acidification. A portion of the soil organic matter cannot be
F.J. Stevenson, Dep. of Agronomy, 1102 S. Goodwin Ave., Univ. of
Illinois, Urbana, IL 61801; R.A. Olsen, Plant and Soil Science Dep.,
Montana State Univ., Bozeman, MT 59797. Received 22 Aug. 1988.
"Corresponding author.
Published in J. Agron. Educ. 18:84-88 (1989).
84
J. Agron. Educ., Vol. 18, no. 2, 1989
solubilized by base extraction and is called humin. The
various fractions are not specific compounds for which
discrete structural formulas can be given. Rather, they
represent an extremely heterogeneous mixture of substances that range in molecular weight from as low as
several hundred for fulvic acids to over 300 000 for humic
acids and humin (Stevenson, 1982).
The inconvenience and awkwardness of writing structural formulas and reactions for high-molecular-weight
organic molecules is readily apparent. In some textbooks
on soil chemistry, one or more "type" structures are given
for humic and/or fulvic acids; in other cases, empirical
formulas are provided (e.g., C2.2H2.2O). More frequently, humic subtances are merely described as being exceedingly complex and of unknown composition. With respect
to both teaching and learning, this state of affairs is unfortunate. In order to explain the nature and properties
of humic substances to students, it would be advantageous to express their basic chemical structure in terms
of a simple structural unit or building block, and to use
this structural unit to demonstrate the important chemical reactions that humus undergoes in soil.
In this paper, a simple dimeric unit is proposed as a
structural representation of humic substances for instructional purposes. The simplified model is then used to illustrate the more important aspects of humus chemistry
and reactions. Using this model, some precision is
sacrificed for clarity, simplicity, and usefulness. The treatment is oriented more towards the undergraduate student
rather than to graduate students or researchers.
BASIC CONCEPTS OF STRUCTURAL UNITS
IN HUMIC SUBSTANCES
Basically, humicsubstances in soil are high-molecularweight compounds (macromolecules) that have been
chemically structured through the linking together of individual building blocks or monomers(i.e., quinones and
phenolic acids). To some extent, chemical reactions of
the macromoleculesare similar to those of the monomers
from which they were formed, primarily because they
both contain similar types of reactive functional groups
(i.e., COOH
and phenolic OH). The functional groups
are attached to a frameworkor matrix that can be visualized as being comparableto a six-sided chain-like fence,
except for being linked together in a more haphazard way.
Also, the humus matrix contains imperfections and
brokenlinks. In humus,the six-sided basic unit is the benzene or aromatic ring to which functional groups and various aliphatic substances are attached.
I
lignin can be reasonably represented by the formula
(C6-C3)n,
where n is the number of monomers linked
together to form the highly polymerized molecule.
The breakdownof lignin by soil microorganisms leads
to the release of the C6-Ca units (Stevenson, 1982).
(C6-C3)
(-COOH)
(-OH)
(-NH2)
(-NH-)
(-N=)
( = O)
The benzene ring contains six positions for attachment
of functional groups or side chains. As a consequence,
a large number of different monomersare theoretically
possible.
Lignin as a Source of Building Blocks
A major source of monomers for humus synthesis is
the lignin from crop residues. Like humus, lignin is a
high-molecular-weight macromolecule whose building
blocks also contain an aromatic ring. However,the aromatic ring of lignin contains a Ca side chain (i.e., lignin
is made up of C6-Ca monomers, where C6 refers to the
benzene ring and Ca to the propane side chain). Thus,
C6-C3
A common C6-C3 monomerin lignin is coniferyl alcohol. In soil, this compoundcan be transformed to
phenols (i.e., phenolic acids) and quinones, as shown
Fig. 1.
It should be realized that these sequences are only two
of manythat can occur for the lignin-derived monomers.
The transformations are typically brought about through
the action of enzymes produced by microorganisms. For
example, the conversion of phenols to quinones (the last
step shownin each pathway) is catalyzed by polyphenol
oxidase enzymes. The phenols and quinones are highly
reactive and readily combine one with another, or with
other biochemical compounds(i.e., amino acids), to form
polymeric molecules of increasingly higher molecular
weight.
A typical structural unit (dimer) formed by combination of two individual building blocks is illustrated helow, where an amino group (-NH2) has been attached
one of the monomers.
COOH
Each corner of the ring consists of a C atom and each
dash extending from the ring represents an H atom or
a reactive functional group. Functional groups attached
to the ring readily undergo chemical interactions with
other soil components. Amongsome of the more commonfunctional groups of humic substances are the following (Stevenson, 1982).
Carboxyl
Hydroxyl
a. Aliphatic
b. Phenolic
Amine
a. Primary
b. Secondary
c. Tertiary
Car bonyl
n = n
\O~
O
COOH
~
OH
OH
NH
2
II
A matrix, like the chain-like fence mentioned earlier,
is formed by addition of other monomersto Structure
II. Because the types of monomers are numerous, and
the ways in which they combine is great, structures of
~H20H
COOH
CH
CH
CH
CH
COOH
COOH
0
OH
OH
OH
OH
0
Coniferyl alcohol
I
alternate
(hydroxyquinone)
COOH
pathway ~
O
(quinone)
~
OH
0
Fig. 1. Biochemicalconversion of coniferyl alcohol, a lignin building
unit, to phenols (i.e., phenolic acids) and quinones.
J. Agron. Educ., Vol. 18, no. 2, 1989 85
the resulting macromoleculesare exceedingly variable and
complex. Stevenson (1982) appropriately compared the
process of humussynthesis to a game of cards. If each
card in the deck (pool of precursor molecules) represents
a different structural unit, the odds of getting the same
hand (structure) twice are extremely small.
sidered along with Item 4, the model provides an understanding of the ability of humusto resist decomposition
in soil.
The Dimer Concept
Wesuggest that the relevant chemistry of humic substances can best be learned or taught by regarding the
central core of the macromolecules as consisting of the
dimer, represented by Structure II. This dimer is but one
of manythat would exist in humic substances but it is
adequate for instructional purposes. The dimer can also
be represented by using dashes to show the attachment
of H and functional groups and the letter R to indicate
an additional monomer(s).
resistant
core
of
humic
substances
4. The tendency of humus to be bonded to clay and
other silicate surfaces can easily be illustrated, viz.,
through linkages with polyvalent cations (IV) and by
bonding (V).
CLAY ,
M(polyvalent
: cation)
O
(H-bonding)
\c¢ o o~
III
APPLICATION OF THE DIMER CONCEPT TO
AN UNDERSTANDING OF SOIL PROCESSES
Advantages and uses of the simplified structural
representations for instructional purposes are:
1. Thedimeric structures (II and III) are readily visualized by the student and can easily be remembered and
drawn. Dimer II, which has a molecular weight of approximately 250, includes the more important functional groups found in humic substances. The C/N ratio of
the dimer is 14:1, which is of the order of that found for
soil humus.
The undergraduate student can relate to Dimeric Structure II easier than to the more elaborate and highly empirical chemical structures often shown for humic and
fulvic acids (Stevenson, 1982; Aiken et al., 1985; Frimo
mel and Christman, 1988). By relating Dimer II with III,
the student gains an appreciation of the heterogeneous
nature of humicsubstances (i.e., not all dimers will have
the same suite of reactive functional groups).
2. As has already been discussed (Fig. l), the origin
of muchof humuscan best be indicated by reference to
lignin as being composed of C6-C3 monomers, which
upon
degradation
leads to the formation of C
6
monomersthat serve as precursors for humus synthesis.
OH
NH
2
OH
IV
V
The ability of humusto resist decompositionin soil can
be accounted for in two ways: chemical resistance, as
noted in Item 3, and physical protection through interactions with clay minerals to form clay-metal-organic (IV)
and clay-organic (V) complexes.
5. The dimer can be used to illustrate the bonding between soil organic matter and pesticides. This interaction
is of interest to agronomistsbecause application rates for
pesticides must often be adjusted upward on soils rich
in organic matter due to adsorption by the organic fraction. The reaction is also of interest to environmentalists
who are concerned about contamination of ground and
surface waters. A representative structure is shownbelow.
NH--C-- O-- C.I’I (herbicide)
: -~ H-bond
O\
COOH
C~ O
OH
2
NH
(C6-C3)n ~ nC6-C3 --- nC6 building blocks for
Lignin
humus synthesis
3. The simplified representation can be used to illustrate the ways in which individual monomers can be
linked together to form a six-sided chain-like matrix that
is relatively resistant to microbial degradation. Whencon86
J. Agron. Educ.,
Vol. 18, no. 2, 1989
NH
VI
6. The dimer is also useful for illustrating the association of proteins, carbohydrates, and other aliphatic constituents to humic substances. Recent work using a
technique called ~3C-nuclear magnetic resonance (NMR)
indicates that appreciable amountsof the C in somehumic and fulvic acids occurs as alkyl (-CH2-) structures.
Accordingly, one or more groups on the aromatic ring
(indicated by the dashes in III) maybe an aliphatic constituent. Someproposed structures for humic substances
(Stevenson, 1982; Aiken etal., 1985; Frimmeland Christman, 1988) include an aliphatic component attached to
the aromatic ring.
Although humic substances from all environments have
manycharacteristics in common,those of lake sediments
(as well as streams) are morealiphatic in nature than those
of soils (Aiken et al., 1985, 142-209). Humicand fulvic
acids, from whatever source, invariably contain somecarbohydrate material (possibly as polysaccharides); these
constituents may be bound to the central core though Hbonding in much the same manner as noted in VI for
pesticides.
Waksman’
s (1936) lignin-protein theory of humusformation was at one time widely accepted in scientific circles and is still extensively quoted in textbooks on soils.
Whereasthe theory is no longer regarded as acceptable
in its original form, the idea that proteinaceoussubstances
are attached to the central core of the humic molecule
is probably valid. The linkage of protein (or peptide) with
the dimeric structure is illustrated below.
COOH
\0
0
OH
COOH
0
II
NH-CH-C-NH-CH-C-NHI
VII
I
R1
R2
protein or peptide
The attached protein (or peptide), while partially stabilized, wouldbe subjected to microbial attack with release
of N to available mineral forms (see Item 8). In the
process, the terminal -NH2group may remain attached
to the central core (Structure II), thus accounting for
some of the unknownN in humic substances (see also
Item 7).
7. Reference can be made to the dimer as being
representative of stored forms of organic N in soils.
Whereas Dimeric Structure II shows N as an -NH2
group, some of the N can exist as a linkage between
monomeric units.
8. Conversion of humusN, such as from VII, to available mineral forms (NH4+, NO~-) can be used as part of
a discussion of N cycling in the soil-plant-microbial system, as illustrated below.
+
ORGANICN --~ NH3 ~- NH~ -"
\
/
+H+ \
ammonific~tion
NO,~ -~ NO~
/
nitrification
9. By consideration of the acidic properties of COOH
and phenolic-OHgroups, the dimer facilitates discussion
of the buffering capacity of soil organic matter, the lime
requirement of acid soils, and related topics. The reaction of added base (MOH)with COOHgroups in the
H + form, for example, can be written as:
COOH
CO0-+M
H20
\O
\O
10. The dimer can conveniently be used to illustrate
the known influence of pH on the cation exchange capacity (CEC)of the soil, namely, as the pH of the soil
increases so does the CEC,due to an increase in negative charges on the soil organic colloids, as illustrated by
the sequences shownin Fig. 2. The diagram can be used
to showthe influence of massaction in shifting the reaction either to the left or to the right.
11. Use of the dimer serves to explain whysoil organic matter exhibits anion exchangecapacity at low pH(acid
soils) but little or no capacity in near neutral or alkaline
soils. The following reaction shows the formation of a
positively charged site on soil organic matter.
COOH
COO-
+
÷ H
\O
\O
--~
~
\O
VIII
/~OH
+
H÷
\O /~O-
Fig. 2. Ionization
reactionsof acidicfunctional
groups
in humic
substances.Thetop reaction(formation
of COOoccursat slightly
acidicto neutralpH’s;the bottom
reaction(formation
of -O-)occursat near-neutral
andalkalinepH’s.
J. Agron. Educ.,
Vol. 18,
no. 2, 1989 87
rf
12. The dimer can be used to illustrate the ability of
humus to form stable complexes with trace elements and
heavy metals (M). Chelation can occur when the Ocontaining functional groups are in adjacent positions on
the aromatic ring, as shown below.
COOH
2H+
13. Humus has typically been represented by the empirical formula C2.2H2.2O. One advantage of using the
dimer to represent humic substances is that N can be
shown to be an integral part of the molecule, as it should
be.
14. The dimer concept can be used to explain why organic matter that is saturated with Na + is highly
charged (negatively), highly dissociated, and dispersed,
such as in black alkali soils. Conversely, when saturated
with Ca 2+ or other di- or trivalent cations, cross-linking
occurs by a mechanism similar to that shown for the claymetal-organic complex (see Structure IV), the cations
Humus
(Decomposition products of organic residues)
| Nonhumlc substances
I Humlc sub stances |
(Known classes of organic compounds)
(Plgmented polymers)
Fulvic acid
(Oden)
Crenic acid
Apocrenlc acid
(Berz ellus)
Light yellow
Yellow-brown
Humlc acid
(Berzelius)
Brown humic acids Gray humic acids
(Spr nger)
Dark brown
Gray-black
- - - - - - - - - - Increase in degree of polymerization- - - - - - - - - - - - - - —*•
2,000?— - - - - Increase In molecular weight- - - - - - - - - - - - - —»-300,000?
45%- — - - - - - Increase in carbon content - — - — - — — — - — - - — -- — —^62%
48%- - - - - - - decrease in oxygen content - - - - - - - - - - - - - - - —«^30%
1,400--- -- -decrease in exchange acidity- - - - - - - - - - - - - - —»^500
Fig. 3. Classification and chemical properties of humic substances.
Additional fractions of humic and fulvic acids are shown (Stevenson, 1982). Humin (not included in the diagram) is insoluble in alkali
and may consist, in part, of humic acids intimately bound to clay
minerals. As noted in the text, some of the nonhumic substances (biochemicals) may be attached to the central core of humic and fulvic
acids.
88
J. Agron. Educ., Vol. 18, no. 2, 1989
themselves are largely unionized, and the colloids are in
a more flocculated state (i.e., Ca-humuates are insoluble).
15. The dimeric structure has an oxidation number for
C near zero, which is about correct for soil organic C.
Accordingly, it provides a basis for showing the potential for organic C to accumulate under reducing conditions (e.g., flooded soil), or to be oxidized to CO2 in
well-aerated soils.
16. The concept that humic substances consist of a
heterogeneous mixture of molecules (system of polymers)
is useful in explaining to students the relationship between
fulvic acids (low molecular weight, soluble in mineral
acids) and humic acids (high molecular weight, insoluble in mineral acids). As can be seen from Fig. 3, degree
of polymerization, C and O contents, and exchange acidity all change systematically with increasing molecular
weight.
17. The dimer provides a convenient model for illustrating the ease with which functional groups and side
chains on the aromatic ring can be altered, particularly
under the influence of enzymatic changes induced by
microorganisms. Examples were shown earlier and others
can be found in the literature (Stevenson, 1982).
The above examples are by no means all-inclusive but
they do illustrate the usefulness of the dimer concept for
learning and/or teaching humus chemistry at the undergraduate level. The dimer concept also bridges the crucial gap between the precursors of humus (lignin
decomposition products) and formation of the humus
matrix.
SUMMARY
A simple and useful model is presented for the basic
chemical structure of humic substances. The model is
based on a simple dimer to which major reactive functional groups known to occur in humus can conveniently be shown. The dimer approach facilitates an
understanding of the conversion of lignin to humus by
focusing attention upon the dimer as a crucial intermediate decomposition product. By reference to the dimer,
the relative inertness of the humus matrix to microbial
attack is made understandable; furthermore, simple reactions can be used to illustrate the important roles played
by organic matter in soil. By consideration of associated
functional groups, much of the chemical and physical behavior of soil organic colloids can easily be explained.
Accordingly, the model is of value for both the teaching
and learning of the chemical nature and behavior of soil
organic matter.