2.1 Types of
Soil organic components
Types of organic compounds
Soil organic matter (SOM) can be of plant, animal, or microbial
origin and may be relatively fresh or highly decomposed and
transformed. In this course, terms
“soil organic matter” and “humus” are considered
of many definitions of soil organic matter states that “it is
a complex and rather resistant mixture of brown or dark brown
amorphous and colloidal organic compounds that result from
microbial decomposition and synthesis and has chemical and
physical properties of great significance to soils and
species of fungi and bacteria soil particles decompose soil
organic matter. The fungi and bacteria that can break down the
woody tissues and cellulose of intact leaves, stems and dead
roots of plants perform the function of decomposition of organic
matter that often builds humus and returns nutrient back to the
soil. When the species that are capable of decomposition are
lacking, leaves and other organic matter remain intact and the
food energy they contain are not released to drive the foodweb.
may vary greatly in their organic matter contents. For example,
a typical prairie grassland soil (Chernozem) may contain 5-6%
SOM (by weight) in surface horizons, sandy desert soil may have
<1% of SOM, while Organic soils (by definition) contain
>30% SOM (by weight).
simple test that allows qualitative comparison of soil organic
matter is an important soil component because it:
a) holds soil particles together and stabilizes the
soil, thus reducing the risk of erosion;
b) aids crop growth by improving the soil's ability
to store and transmit air and water;
c) stores and supplies many nutrients needed for the
growth of plants and soil organisms;
d) prevents or minimizes soil
e) retains carbon from the
f) reduces the negative environmental effects of
pesticides, heavy metals, and many other pollutants.
physically and chemically heterogeneous mixture of materials
that make up SOM varies substantially in terms of quantity and resistance to biological decomposition.
components that are inherited from plant and animal residues
entering the soil. Primary components are sometimes referred to
as non-humic substances.
These compounds are relatively easily decomposed by
microorganisms and they persist in soil for a brief
time (e.g. several months or years). They make about
20-30% of total SOM. These include
carbohydrates and several derivatives (monosaccharides,
and polisaccharides such as cellulose, hemiscellulose)
amino acids and several derivatives (proteins)
lignin (condensed polyphenil-propane structures of
extremely large molecular weight, very resistant to microbial
lipids (a class of organic compounds which is a
convenient analytical group rather than a specific type of
compound. They include fats, oils, waxes, phospholipids, and
a variety of other compounds.
also includes secondary
compounds, compounds formed within the soil by breaking down
organic structures and synthesizing new ones. The secondary
components include carbohydrates, amino acids, lipids and
others, more or less resembling many primary components.
In addition, secondary components also include humic
substances, which are rather different from most primary
substances are products of biochemical decomposition. They are
complex substances of high molecular weight, which are resistant
to further decomposition. Consequently they tend to accumulate
in the soil. Most are
dark and are hence responsible for the dark soil color that is
commonly associated with soils of high organic matter content.
Humic substances make up 60-80% of total SOM.
substances can be classified based on their behavior in acids
and bases. There are three general groups of humic substances:
(1) fulvic acids, (2)
humic acids, and (3) humin.
These three groups are nondescript mixtures of many chemical
compounds and should not be considered as distinct chemical
Humins are particles with large molecular weight, relatively small
specific surface area, relatively low number of carboxyl groups,
and are inactive.
Humic acids are smaller in size then humins (approximately
colloid-sized) and have more carboxyl groups than humins.
Fulvic acids are the smallest in size among humic substances, and
have a large number of carboxyl groups per unit mass.
For this reason they are the most active among humic
The humic substances posses carboxyl groups (*R-COOH), which
give them their acidic character and make them effective in
buffering soil pH:
R-COO- + H+
*R represents any number of organic compounds.
common range of soil pH the overall charge on SOM is negative,
due to dissociation of hydrogen ions from carboxyl and other
percentage of the humus which occurs in the various humic
fractions varies considerably from one soil type to another. The
humus of forest soils is characterized by a high content of
fulvic acids while the humus of peat and grassland soils is high
in humic acids (see figure). The humic acid / fulvic acid ratio
usually, but not always, decreases with increasing depth.