Environmental Engineering Reference
In-Depth Information
Plate 18.4
Black tropical soil (FAO Pellic Vertisol) composed
of expanding 2:1 montmorillonite clay minerals. Repeated
expansion and shrinkage due to wetting and drying cause
thorough mixing and overturning of the soil. Scale in 10 cm
units.
Photo: Ken Atkinson.
(Ca
2+
, Mg
2+
) and for trivalents one-third (Fe
3+
, Al
3+
). Since the amounts involved are very
small, the term
milliequivalent
(EW/1000) is used. The value me 100 g
−1
therefore
represents the number of milligrams of particular elements which can be held by 100 g of
a particular soil. The average electric charges (CEC) on the common clay minerals are
given in Table 18.6.
The net negative charge on the clay colloids is balanced by
exchangeable cations
which are attracted to the surface of the clay particles. These are positively charged ions
in the soil solution (H
+
, Ca
2+
, Mg
2+
, K
+
, Na
+
). They are termed 'exchangeable' because
one cation can be readily replaced by another of equal valence, or by two of half the
