The Birth of Soils - Soil: The Skin of the Planet Earth - page 65

Another important feature of humic substances is that they form a habitat for
microbes and other microorganisms. Some humic materials are much more impor-
tant than others, while some play a trivial or even a hindering, noncompatible role.
Because humic substances form more or less chains and balls on the scale of mol-
ecules, they can behave as fi lms that cover or wrap around soil particles or as small
balls they can be glued to soil particles owing to various potential bonds. The
smaller are the mineral particles, the more intensive is the adhesion of humic sub-
stances. When they associate with clay minerals, they exert their maximum capacity
to adhere to a mineral surface.
Humic substances are classifi ed into three chemical groups based on solubility
and other physical and chemical properties. Fulvic acids are the humic fractions that
are lowest in molecular size and molecular weight. They have the lowest cation
exchange capacity and are lightest in color. Fulvic acids are soluble in both acidic
and alkaline solutions and are more susceptible to microbial degradation than other
types of humic substances. Humic acids are medium in molecular weight and their
color is darker. They are soluble in alkaline solutions but insoluble in acidic solu-
tions. They possess an intermediate resistance to microbial degradation. Humins are
dark colored, have the highest molecular weight and highest cation exchange capac-
ity, are insoluble in both acidic and alkaline solutions, and are the most resistant to
further microbial degradation. All three groups are relatively stable in soils. Fulvic
acids have the lowest half-life ranging from 10 to 50 years, while the longest half-
life reaching up to several centuries belongs to humins. Soils with black color in
their top horizons belong to the most fertile soils. Owing to their black color, cher-
nozem is their name (the Russian word for “black earth”). Their humus has high
content of humins.
Fig. 5.14 Model of the
humic acid as a simplifi ed
organic polymer