Geoscience Reference
In-Depth Information
All decomposition, transformation, condensation, and polymerization processes
are also infl uenced by the mineral part of the soil. Soil particles infl uence the rate of
all these processes. We note that lignins, early stages of humus, are very weakly
transformed in sand, moderately in loams, and very intensively in clays. The initial
products of early stages of humifi cation are readily linked to clay minerals. Generally
we are inclined to say that each of the various structures of silicates is an instructor
teaching organic compounds how to form the long-chain linkage by polymerization.
Many of these chains are directly joined to clay minerals, while some are enrolled
into small balls and connected through the charged parts of the reactive compounds
to the crystal lattice of clay minerals. Such linkage assists in slowing down the
decomposition rate of humic substances. The clay-humic associations form a very
useful habitat for microbial communities since they offer food substrate and protec-
tion through a great specifi c surface in ranges of tens to several hundreds of m
2
per
gram of soil. Microorganisms express their gratitude by excreting polysaccharides
and proteins that stabilize the organo-mineral associations. The strength of bonding
increases with the decreased size of soil particles. The special forms of structure of
individual components in humic acids and humins also play an important role - the
older and more “mature” they are, the stronger is the bonding. As a result, soil
humic substances stabilize organo-mineral associations more in subsoil layers than
those associations in the topsoil. All of these aspects are also important in our under-
standing of soil structure that we will discuss in later chapters (Fig.
5.14
).
