Introduction to Soil Chemistry

64 soil basics iii
using various extraction procedures. These procedures result in the isolation
of three or more “fractions”: humic acid, fluvic acid, and humin. Humic material
is isolated from soil by treating the soil with alkali. The insoluble material
remaining after this treatment is called humin. The alkali solution is
acidified to a pH of 1.0 and the precipitate is called humic acid while the
soluble components are called fluvic acid. A lot is known about these components;
for instance, they contain three carbon or propyl groups, aromatic
moieties with various and usually multiple functional groups, and variable
amounts of other components. The isolation of these components, however,
has not brought us much closer to a complete understanding of the structure
of humus. Applying other extraction procedures will allow the extraction and
isolation of other “fractions” of humus; however, humin, humic acid, and
fluvic acid are the three main components likely to be discussed in terms of
humus.
What is known is that humus is an extremely important component of soil.
Even small amounts can cause demonstrable differences in a soil’s CEC and
its other chemical and physical properties as well. It is active in binding soil
particles together to form peds, increases the soil water holding capacity, and
increases the absorptive capacity of soil for organic and inorganic constituents,
both natural and synthetic.As an example of the importance of humus in terms
of soil water, it could be noted that mineral soils absorb and hold 20–40% of
their weight in water, while some organic soils can hold 10 times this amount,
specifically, 200–400% of their weight in water. This is due in part to the fact
that organic soils are much lighter than mineral soils, but nevertheless this
increased water holding capacity is dramatic. Increased sorptive capacity is
reflected in the increased need for herbicides on soils high in organic matter.
Different soils contain different amounts of humus. Some tropical African
soils may contain less than 0.1% organic matter. At the other extreme organic
soils, such as Histosols, generally must have 20% or more organic carbon in
the upper 80cm, although this will vary somewhat depending on the conditions
of moisture, texture, and depth of the soil. Many agricultural soils contain
1–2% organic matter, although it is not unusual to find soils containing 10%
organic matter.
When developing a soil analytical method, it is essential that either the
method be applicable to soils of all organic matter contents or that variations
of the procedure be applicable to different soil organic matter contents be
developed [16,18].
3.7. ANALYSIS
There are a myirad of methods used for analyzing soil for animals, plants, and
microorganisms. All are well developed and easily carried out; however, some
are more useful when large numbers of samples are to be analyzed, while some
are subject to significant error [19–21].