Introduction to Soil Chemistry

organic compounds 61
Table 3.3. Functional Groups Derived from the Acid Functional Group
Family Name Composition Structure and IUPAC Name
Acid halides Carbon, hydrogen, single bonds
with at least one —COXa in
the molecule (may include
double and triple bonds and
other structures) Propanoyl chloride
Anhydrides Carbon, hydrogen, single bonds
with at least one —COOOC—
in the molecule (may include
double and triple bonds
and other structures) Ethanoic anhydride
H
H
C
O
C N H
H
H
C
H
H
C
H
O
C
Cl
H O O H
H C
H
C O C
H
H
H
Amides Carbon, hydrogen, single bonds
with at least one —CONH 2 b
in the molecule (may include
double and triple bonds and
other structures) Ethanamide
Esters Carbon, hydrogen, single bonds
O
with at least one —COOC—
in the molecule (may include
double and triple bonds and
other structures)
CH3C
O CH3 Methyl ethanoate
a X is always used as a general representation for any of the halogens.
b Hydrogens on the nitrogen can be substituted with alkyl groups.
In addition to methane, other simple organic compounds will be found in
soil from two different sources. They can be either exuded from roots into the
rhizosphere of plants or derived from the decomposition of any organic matter
present. During decomposition all organic matter is broken down into smaller
and smaller organic molecules until it is finally completely converted into
carbon dioxide, water, and humus. 2 The same is true for anaerobic decomposition,
except that one of the final decomposition products is methane. Thus,
at any given time, intermediate decomposition products from these two
sources can be found in the soil solution.
Plant roots excrete a mixture of simple and complex compounds and materials,
which differ for different plant species. First, simple acids, both fatty and
amino; sugars; and phenolic compounds are commonly detected as exudates
from plant roots. Each of the these groups may be composed of a complex
mixture of compounds; thus, the most common amino acids can be found along
with many common sugars. In addition, a high-molecular-weight compound
called mucigel is secreted by root tip cells and is thought to lubricate root penetration
into soil. Mucigel is somewhat slower to decompose but can also be
2 Although seldom mentioned, this process is characterized by the release of energy that is utilized
by the organism carrying out the decomposition.
H