Geoscience Reference
In-Depth Information
In order to further explain, let us consider the half-life of humus in medium fertile
soils developed in a mild climatic zone. Being roughly equal to 100 years, we learn
that from each 1 g of humus, only 0.5 g of humus remains after 100 years because
0.5 g of the original humus decomposed. During the next 100 years, the remaining
0.5 g of humus is the source for further decomposition: 0.25 g is decayed, while
0.25 g is left as the source of decomposition during the next 100 years, and so on.
The previous two sentences were merely an explanation of the mathematical term
half-life. An understanding and appreciation of its real value for humus depends
upon the type and quality of the parent organic material, on the environmental con-
ditions for microbiological life, on the soil texture, and generally on the chemical
composition of humus.
5.3.3
Humic Substances
We have mentioned the quality of humus, and since humic substances are the main
component of humus, we have to explain how to understand their quality. Even if
their structure is varied and extremely multifarious, they have several common fea-
tures. They originated by condensation and polymerization of nuclei formed by
cyclical hexagonal arrangements of C-C and C=C bonds or simply by aromatic
compounds mutually connected by bonds. Their nuclei carry the reaction groups
responsible for the outside charge of humic substance molecules. They are nega-
tively charged since the carboxylic groups -COOH and phenolic groups -OH have
the common property of their hydrogen being separated, or as we say, the H is dis-
sociated. We spoke earlier about the H bond and its dissociation. Here it means that
the negatively charged oxygen of the mentioned groups (see the missing H) causes
parts of humic substances to be negatively charged. Variable arrangements of these
basic components result in differences of physical and chemical properties of the
humic molecules (Fig.
5.13
).
The formation of a humic substance described above can be compared to the
steps leading from sheep wool to a hand-tailored suit of clothing. The fi ne, indi-
vidual fi bers of wool are fi rst spun into spools of yarn that are next woven into the
woolen fabric used for sewing the suit. Something could be spoiled on each step
starting from the type of sheep and quality of fi ber determined by its diameter,
crimp, color, and strength and ending with the tailor's skill. A similar change of the
quality of the fi nal product of humifi cation could happen. In addition to the conden-
sation procedure, humifi cation brings the products of decomposition into the pro-
cess, and these products individually persist or more frequently they enter
condensation and polymerization, or eventually copolymerization. These steps are
responsible for humic substances being a multicomponent mixture of organic poly-
mers. Among them is a small group called glomalins, which originate by decompo-
sition, transformation, and polymerization of decayed parts of plants. They appear
as a direct product of fungi living on roots of plants (more about it later on in
Sect.
6.2
dealing with soil structure). We should not forget that conditions of soil
