Geoscience Reference
In-Depth Information
activity. Especially in the last 10,000 years as agriculture was practiced and accom-
panied more and more by industrial and civilized ways of life, virtually all desired
features of our comfortable life brought about a lot of wastes and soluble com-
pounds into the environment, notably into soils. As these dissolved materials of
diverse chemical composition arrive inside soil pores, the cations attracted and kept
on the surfaces of clay minerals and humins are changed and redistributed. Those
cations are therefore called exchangeable cations, and the soil capacity to bind them
is described as cation exchange capacity, CEC, that is usually measured and reported
in milliequivalents per gram of soil, meq/g. The term milliequivalent describes the
weight of a substance in milligrams divided by its valence. CEC is closely corre-
lated to the specifi c surface (m
2
/g). Values of CEC for principal groups of clay
minerals are given in Sect.
5.2.2
. The range of CEC in soil textural classes varies
from units to tens of meq/g. Sand has a CEC less than 10 meq/g, sandy loam
10-15 meq/g, loam 15-20 meq/g, and clay loam 20-30 meq/g, and clays have the
largest range 20-40 meq/g because they are dominated by specifi c minerals. The
negative charge of humins created primarily by the separation of H
+
from carboxyl
or phenolic OH
−
causes the CEC of humins to be smaller than that of smectites.
The specifi c exchangeable cations actually bound to soil particles depend not
only upon the percentage of individual compounds in the soil water but also upon
the diversity of individual cations. For example, among exchangeable cations, Na
+
has the weakest binding while Fe
3+
has the strongest binding. The adsorbing strength
of other cations increases in the sequence of K
+
, NH
4
+
, Ca
2+
, Mg
2+
, and Al
3+
. The pH
of the soil has a great infl uence upon the CEC and the relative proportions of indi-
vidual adsorbed cations. When we measure the CEC of a soil, because we do not
usually separate the organic from its inorganic components, we denote all of its
sorption-causing materials as a sorption complex.
When the soil is changing toward acidic conditions, the ratio between concentra-
tions of H
+
and OH
−
ions is also changing with H
+
concentrations beginning to
increase and eventually prevailing. At such times, with a proportion of exchange
positions being occupied by H
+
, the CEC will not be fully occupied by basic cations
such as Ca
2+
, Mg
2+
, K
+
, and Na
+
. The ratio between these adsorbed basic cations and
CEC is the base saturation percentage. High values of this base saturation percent-
age indicate that the soil is a favorable medium for the production of cultural plants.
Low values of base saturation indicate that acid conditions and associated processes
should be ameliorated by liming accompanied by fertilization of ingredients con-
taining basic cations. Without applying both steps in the sequence, fi rst liming and
then fertilization, the acid processes cannot be stopped. Soils in mild climatic
regions have a tendency to gradually acidify because they receive a natural supply
of free H
+
ions at higher rates than their bases are released by natural processes. The
higher the intensity of agriculture, the higher is the “export” of bases by harvesting.
Hence, the disproportion between “export and import” of bases increases. In humid
tropical regions, the natural processes and outwashing of bases are so intensive that
soils are strongly acidic without any human contribution. Extreme values of pH
have a major impact upon the availability of important plant nutrients. Because the
entry of phosphorus and molybdenum into soil water is strongly reduced under low
