Geoscience Reference
In-Depth Information
naming the observed structure. Such visual fi eld observations are sometimes sup-
plemented with penetration resistance measurements when we determine the pres-
sure necessary to push a cone-tipped rod or pin into the soil. Such measurements are
a vast improvement over those used a half-century or longer ago when a farmer
pushed his thumb into the soil and subjectively estimated the penetrability of the
soil. Today, we have instruments with a scale showing how many Pascals (more
precisely MPa or kPa) are applied to push the rod or pin down to a prescribed soil
depth. Except for sands, the higher the applied pressure, the poorer is the soil struc-
ture. Measurements of penetrability are sometimes accompanied by water infi ltra-
tion tests that are discussed in Sect. 10.1.1 . Many results of laboratory measurements
of soil physical properties are also related to soil structure, as, e.g., soil porosity or
hydraulic conductivity, Sects. 7.3 and 9.2 .
Up to now we described the features of macroaggregates recognized by the
naked eye. These large aggregates, composed of microaggregates ranging in size
from 10 to 250
m, are studied on thin sections. An undisturbed small soil block is
fi rst saturated with epoxy resin and heated, and after the glue-like resin solidifi es,
the hard soil block is cut in a special machine. The cut side is polished and placed
on a glass slide. Then the soil is cut on the opposite side and trimmed to about 30 or
50
μ
m (equal to 0.03-0.05 mm). Light from a special microscope can be success-
fully transmitted through the 30- to 50-micron thin section. Inasmuch as different
minerals have different optical properties, most minerals can be easily identifi ed
using polarized light. From these thin sections, we can study details of the edges of
silicates and their structural arrangement, the forms of Fe and Al oxides and their
coverage of particles, or how humus is bound to particles. More detailed studies are
possible using an electron microscope to examine the scattering of neutrons or
X-rays. Scattering occurs exclusively at the interfaces between the solid matrix and
pores containing a different material, such as air or water, which differs substan-
tially in its ability to scatter the radiation. From such microscopic studies, we learn
how various arrangements of microaggregates form different sizes and shapes of
macroaggregates. And from illustrations of intricate pore space distributions, we
readily understand and appreciate the key role of microaggregates as they spatially
protect humins and humic acids from microbial decomposition.
μ
6.2
Magic of Soil Aggregation
The fi rst prerequisite for the origin of soil aggregation and soil structure is the exis-
tence of an attractive force between soil particles, especially those of clay minerals
and soil humic substances. The surfaces of both - clay minerals and humic sub-
stances - carry a negative charge. Hence, both attract positively charged ions, the
cations from the water solution that always exists in soil pores. If bivalent cations
dominate the solution, they are preferably attracted to the negatively charged clay
minerals and humic substances causing their concentration near the solid particle
surfaces to be higher than that in the solution further from the particle surfaces. The
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