Under high soil pH conditions, damaged soil clays with exposed OH groups
in octahedral layers may dissociate, forming a negative charge, which is neutral-
ized by other cations in the soil water, for example:
eqn. 4.18
Micrometre-sized clay minerals have a large surface area : volume ratio and con-
stitute a significant sink for some anions and cations in soil environments. Despite
this, in smectite clays, surface ion exchange is much less important than inter-
layer site exchange (Table 4.10). The cation exchange capacity (CEC) of a soil
provides a useful indication of the availability of essential trace metals (which are
fundamental for plant health) and is often considered the best index of soil fer-
tility. In soils it is usually impossible to calculate the amount of ion exchange
caused by clay minerals alone, since other components, especially organic matter,
have significant CECs (Table 4.10). Organic matter provides a significant pro-
portion of exchange sites in many soils, created by the dissociation of acidic
or alcoholic/phenolic functional groups at pH above 5. The high CEC of soil
organic matter (150–500 meq/100 gsoil; Table 4.10) means that organic matter
content, rather than clay mineral content, often provides the most significant con-
tribution to a soil’s CEC. Addition of organic matter to soil is thus an easy and
effective means of increasing the CEC, and thereby the soil fertility.4.9 Soil structure and classification
As a result of the various factors and processes outlined in Sections 4.6.1–4.6.5,
over time soils develop stable and diagnostic features, many of which are recog-Clay–mineral–O-–H()s++K()+-aqªclay–mineral O K– – ()s H+()aq112 Chapter Four
Table 4.10Representative cation exchange capacity, CEC (meq* 100 g-^1 dry wt), for various
soil materials. Table adapted from Pedology, Weathering and Geomorphological Researchby P. W.
Birkeland, copyright 1974 by Oxford University Press, Inc. Used by permission of Oxford
University Press, Inc.
Non-clay
materials CEC Clay minerals CEC Cation exchange site
Quartz, feldspars 1–2
Hydrous oxides 4 Kaolinite 3–15 Edge effects
of Al and Fe
Illite 10–40 Mainly edge effects, plus some
interlayer
Chlorite 10–10
Organic matter 150–500 Smectite 80–150 Mainly interlayer plus some
edge effects
* A milliequivalent (meq) is the charge carried by 1.008 milligrams (mg) of H+(i.e. the atomic mass of
hydrogen), or the charge carried by any ion (measured in mg l-^1 ), divided by its relative atomic mass and
multiplied by the numerical value of the charge. If, for example, divalent Ca^2 +displaces H+, it occupies the
charged sites of 2H+ions. Thus the amount of Ca^2 +required to displace 1 meq of H+is 40 (atomic mass of
Ca) divided by 2 (charge)=20 mg, i.e. the mass of 1 meq of Ca^2 +.