Geoscience Reference
In-Depth Information
to the mass of its solid phase is 6 cm 2 /g = 0.0006 m 2 /g. Let us further suppose that
we have a special saw to cut this cube into smaller pieces without producing any
sawdust. Hence, sawing the original cube into cubes having an edge of 0.1 mm
produces 10 6 cubic particles each having the surface 0.6 mm 2 , and the specifi c sur-
face of all these small cubes will be 0.06 m 2 /g. If we continue to ideally saw each of
the cubes into much smaller cubes having an edge of 1
m, the specifi c surface of
all of these micro-cubes will equal 6 m 2 /g. It is therefore entirely logical and not
surprising that clay minerals have specifi c surfaces ranging between m 2 /g up to
several hundreds of m 2 /g. Let us recap some earlier written data. The specifi c sur-
faces of clay minerals range between tens and hundreds of m 2 /g. Kaolinites have the
smallest specifi c surface - always below 15 m 2 /g. And smectites and montmoril-
lonites have the largest specifi c surfaces, usually slightly below 400 m 2 /g. Their
large internal surface responsible for such high values is a result of their octahedral
layer being sandwiched between two tetrahedral layers. In contrast, owing to the
partial binding of these triple layers by K + ions, the specifi c surface of illites is dis-
tinctly reduced to values usually between 40 and 90 m 2 /g.
The specifi c surface of a soil depends upon its texture and the type of clay miner-
als within its clay particles. It usually has a very high value that ranges between 20
and 90 m 2 /g. Excluding the consideration of sands, the soil-specifi c surface supply-
ing essential elements and nutrients to roots of plants growing on a square meter
area of an “average” fi eld soil is about 20 km 2 . Recognizing that the specifi c surface
of real soils also depends upon the content and quality of humus, its magnitude typi-
cally ranges from 10 to 40 km 2 except of sands where it sinks well below 10 km 2 .
Hence, the fi rst approximation of the specifi c surface of the root-containing soil
layer below an area of 1 m 2 in a farmer's fi eld is 20 km 2 . If this value together with
its uncertainty owing to spatial variations of soil texture, clay mineral composition,
and humus content is extrapolated from 1 m 2 to the size of an average farm in the
USA (approximately 440 acres), we obtain the specifi c surface of the root-containing
soil layer that ranges roughly from 20 to 60 million km 2 . Although diffi cult to imag-
ine, we now realize that the specifi c area of soil particles within the root zone of
only one average US farm is even larger than all of the continental area of North
America occupied by the USA. Without a doubt, and across all continents, the soil-
specifi c surface is a very important quantitative characteristic having an infl uential
role in many processes decisive for soil fertility, for the role of the soil in hydrologic
cycle, for plant and animal life across the landscape, and generally for the quality of
the global ecosystem.
We have already explained in Sect. 5.2.2 ( The Finest Minerals ) that a clay min-
eral surface is never chemically neutral and carries a negative charge due to the
substitution of cations inside the tetrahedral and octahedral confi gurations of its
crystal lattice. The negative charges of the crystal surface attract cations present in
the soil water residing within soil pores. The broken bonds of the surface of the
crystal lattice also contribute to the magnitude of the negative charge by behaving
like tentacles that capture cations from the soil water. Humic substances behave
similarly. The cations and generally all soluble compounds residing in soil water
have their origin in mineral weathering, organic matter humifi cation, and animal
μ
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