Geoscience Reference
In-Depth Information
guish two other kinds of reactions in soil science, namely, active and exchangeable
reactions.
An active reaction in soil science is determined after soil is mixed with neutral
water in a ratio 1:2.5. We deal with exchangeable reaction in the next subchapter. In
order to provide a comparative scale, we list here a few values of pH in solutions
known in our everyday life. The acid in an accumulator or battery for a car has a pH
below 1. Our gastric juices have pH = 2.0. Citron juice has pH = 2.4. Vinegar's pH is
2.9. Ceylon tea has pH = 5.5 and milk's pH is 6.5. Our blood has a pH of about 7.4
and seawater about 8.0, and the pH in soap can reach a value up to 10.
Water contained in soil pores is never pure because at each and every moment, it
always contains dissolved mineral substances. The weathering of rocks continu-
ously produces simple soluble mineral substances, and even humifi cation culmi-
nates with simple dissolved mineral products being added once again into water.
Hence, the primary liquid in a soil is actually an aqueous solution of inorganic and
organic substances that soil scientists commonly refer to as
soil water
. In other
words, soil water is the appropriate name for the liquid phase of a natural soil
because it implicitly acknowledges the fact that it is not pure and always contains
soluble materials without being specifi ed. Among those diverse, ever-present solu-
ble materials are essential plant nutrients. A logical question would be: Why are
they not fl ushed out of the soil during the infi ltration of rainwater? A quick answer:
soil does not behave like a strainer. With the great majority of soil pores having sizes
well below 1 mm, water is retained in the same manner as if it were in the narrow
part of an eyedropper being retained by capillarity. In the next chapter we shall
explain more about it, about the forces retaining water in the soil, about the nature
of soil water, and the movements of water with and without its solutes inside the
soil.
When we summarize all of these soil processes, we fi nd a superfi cial resem-
blance with an endless carnival where masks are frequently changed and the selec-
tion of dancing partners sometimes appears limitless. With more detailed,
comprehensive linkages of observations, we recognize and eventually understand
the very strict rules of that soil carnival.
7.5
Specifi c Surface of the Soil on the Farmer's Field
The walls of soil pores are in contact mainly with liquid soil water, and only rarely
is there a direct contact between the solid phase of the pore wall and the gaseous
constituents of a soil. Various chemical and biological processes occurring within
the liquid soil water infl uence the solid phase of the entire soil including all of its
pore walls. The soil-specifi c surface and its reactions with neighboring fi lms of soil
water play an important role in soil evolution and in relationships between the soil
and the fl ora and fauna that completely depend upon the soil during their life cycles.
Let us imagine a cube having edges that measure 1 cm. If the density of the
cube's material is 1 g/cm
3
, its surface area is 6 cm
2
, and the specifi c surface related
