Geoscience Reference
In-Depth Information
with dead remnants, while others are by-products of humifi cation. As a result, in the
humic top horizon of soils, we fi nd vitamins, enzymes, and even some antibiotics
that infl uence in many ways not only living soil organisms but also plant roots and
growth rates of plants. These multiple infl uences are usually positive and only in
some instances cause a negative effect. Phenols and a variety of organic acids
appearing mainly in the acid medium of waterlogged soils cause the growth of
plants to slow down. Their action also slows down the transformation of plant rem-
nants and acts unfavorably upon microbial life. Fortunately and quite the opposite,
that sort of humifi cation also leads to the origin of humic acids that act positively to
increase the permeability of cell membranes and therefore enhances the acceptance
of nitrogen and other important nutritional elements by the plant root system.
Humifi cation is not limited to merely the decomposition of organic matter. Once
it starts, large organic molecules are transformed and cut into smaller molecules.
One portion of them is again synthesized into more complex organic molecules
under the contributing action of microorganisms. The remaining portion does not
enter into any new synthesis, but continues to be decomposed. The portion of newly
synthesized molecules also accompanies this decomposition. In other words, the
continuation of decomposition is realized in new different ways than it was during
the initial process. Some processes appear to run in a circle. Simpler organic com-
pounds are created due to the decomposition of very complex organic materials.
Parts of these simpler compounds enter a new synthesis resulting in formation of a
group of various complex compounds that have substantially different properties
than the “mother” compounds. The processes of decomposition and synthesis are
repeated, and with their repetition, all of the properties of the newly formed com-
pounds differ more and more from the initial organic matter. With the origin of new
compounds not being restricted just to synthesis, other concomitant processes
formed chains that are coiled into a ball at the molecular scale. All such processes
are accompanied by the release of simple inorganic compounds that are dissolved in
soil water. And if any of these solutes were plant nutrients, they were immediately
doled out for absorption by plant roots. Hence, groups of humic substances are
formed as a mixture of many molecules, many of which have an aromatic nucleus
with phenolic, quinone, and carboxyl components linked together.
We have demonstrated in a very simplifi ed way that the products of humifi cation,
mixtures of stable polymolecular material, are suffi ciently resistant against further
decomposition to prevent their return to cycles of decomposition and resynthesis.
Being out of the reach of these processes, they form a new family of soil material
even though it is not completely saved from further decomposition. Their continued
decomposition rate is an order of magnitude slower than the decomposition rate of
the original raw organic material. In order to more clearly express the decay of
organic substances in soil, we use the concept of half-life. One half-life is the time
required for the decomposition of one-half the mass of the original organic material.
This sentence can be expressed mathematically as a process described by an expo-
nential equation. Average half-life values of soil humus manifest very broad ranges,
from years to hundreds of years. Their values depend upon the various components
of humus. Generally, higher half-life values indicate better qualities of soil humus.
