Agriculture Reference
In-Depth Information
Functional acidic groups in soil
organic matter (including humus)
•
carboxyl group
COO
-
+ H
+
COOH
•
phenolic group
Example of carboxyl and phenolic-OH groups
Figure 2.15
O
-
H
+
OH
in soil organic matter.
portant outcome of humification is that the number of acidic groups—carboxyl
and phenolic groups (fig. 2.15)—increases per unit weight, thus contributing to
the cation exchange capacity. This slow polymerization of phenolic compounds
is analogous to the polymerization reactions, and “softening” of the tannins, that
occur as a wine matures.
Because of the diversity of complex molecules formed during humification,
identification of the chemical structure of humus has proved difficult. Much of
the humus is strongly adsorbed to soil minerals, so ultrasonic vibration and strong
solvents are needed to extract it. The treatment is severe, and it is possible that
the compounds extracted are not the same as those in the soil originally. A widely
used empirical procedure for extracting humic compounds in strong alkali fol-
lowed by acid produces the chemical fractions
humic acid
(HA),
fulvic acid
(FA),
and
humins
. More details are given in box 2.7.
Box 2.7
Composition and Properties of Humic Fractions
The “core” structure of the HA and FA fractions is composed of aliphatic (a
long hydrocarbon chain) and aromatic groups covalently bonded to form high
molecular weight polymers, with much branching and folding. Polysaccharides and
protein materials occupy the spaces in the folded and branched macromolecules,
but they do not contribute more than about 20% of the total mass. These
molecules may be covalently bonded or held by electrostatic attraction or
hydrogen-bonding.
Approximately 35-45% of the HA fraction is aromatic, consisting mainly of
single ring structures that are highly substituted, particularly with carboxyl (-
COOH) and phenolic-OH groups of varying acid strength. Long aliphatic chains,
also with carboxyl groups, link the aromatic structures or exist as separate side
chains attached to the rings. Approximately 25% of the FA fraction is aromatic.
FA molecules are smaller, more highly charged, and more polar than HA
molecules. Repulsion of negative charges causes the FA molecules to be more linear
than the randomly coiled HA molecules. FA molecules are soluble under acidic
conditions and susceptible to leaching.
The steric arrangement of different functional groups (-COOH, phenolic-
OH, and carbonyl C
O) facilitates the complexing of metal cations. The
nonextractable (insoluble) humins are thought to be HA-type compounds that are
strongly adsorbed, or precipitated, on mineral surfaces as metal salts or chelates.
