Agriculture Reference
In-Depth Information
Box 3.4
Moles and Moles of Charge in Soil Chemistry
A mole of an element or compound is its molecular weight in grams, more
commonly referred to as the molar mass. The standard against which all substances
are measured is the stable isotope of carbon (
12
C). On this scale, the molar mass of
hydrogen (H) is 1 g, calcium (Ca) is 40 g, and aluminum (Al) is 27 g.
The recommended unit of charged mass for cations, anions, and charged
surfacesisthemoleofcharge,whichisequaltothemolarmassdividedbythe
ionic charge. It follows that
• ForH
+
, because its ionic charge is +1, a mole of charge is 1/1 = 1 g.
• ForCa
2+
, because its ionic charge is +2, a mole of charge is 40/2 = 20 g.
• ForAl
3+
, because its ionic charge is +3, a mole of charge is 27/3 = 9 g.
For clay minerals, oxides, and organic matter, the most appropriate unit is
the centimole of charge (+) or (-) per kilogram (abbreviated cmol (+)/kg). For
example, the
CEC
of clay is expressed in cmols (+) per kg because
CEC
is measured
by the moles of cation charge adsorbed by the clay. The
CEC
expressed in this way
is the same as that expressed in the obsolete units, still used in some laboratories, of
milliequivalents(mEq)per100g.
Table 3.3
Some Features of the Common Clay Minerals
Specific
surface area
(m
2
/g)
CEC
(cmol
(+)/kg)
Clay mineral type
Other salient features
Kaolinite
5-25
5-40
Strong bonding between crystal layers;
interlayer spaces not accessed by cations
or water; relatively large crystals and
minimal swelling
Illite
20-40
100-200
K
+
ions fit snugly into holes between
opposing crystal surfaces and hold
the layers together; as K
+
is lost by
weathering and replaced by partially
hydrated Ca
2+
andMg
2+
, the crystal
expands slightly
Vermiculite
150-160
300-500
This is the end product of illite weathering
in which K
+
ions have been replaced by
Ca
2+
andMg
2+
; limited swelling
Montmorillonite
100-120
750
a
Crystal layers are held in roughly parallel
alignment when Ca
2+
andMg
2+
are
present; swelling increases as these ions
are replaced by Na
+
and dispersion can
occur (see box 4.3, chapter 4)
Note. CEC
= cation exchange capacity.
a
For montmorillonite saturated with Na
+
ions.
