Soil colloids: Seat of soil chemical and physical acidity

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310
How can using sewage effluent for irrigation contribute to the safe recharge of ground-
water aquifers? Why is it more difficult to restore productivity after logging a tropical
rain forest on Oxisols than a temperate forest on Alfisols? Why would a nuclear power
plant accident seriously contaminate food grown on some downwind soils, but not on
others? The answers to these and other environmental mysteries lie in the nature of the
smallest of soil particles, the clay and humus colloids. These particles are not just extra-
small fragments of rock and organic matter. They are highly reactive materials with
electrically charged surfaces. Because of their size and shape, they give the soil an enor-
mous amount of reactive surface area. It is the colloids, then, that allow the soil to
serve as nature’s great electrostatic chemical reactor.
Each tiny colloid particle carries a swarm of positively and negatively charged ions
(cations and anions) that is attracted to electrostatic charges on its surface. The ions are
held tightly enough by the soil colloids to greatly reduce their loss in drainage waters,
but loosely enough to allow plant roots access to the nutrients among them. Other
modes of adsorption bind ions more tightly so that they are no longer available for
plant uptake, reaction with the soil solution, or leaching loss to the environment. In
addition to plant nutrient ions, soil colloids also bind with water molecules, biomole-
cules (e.g., DNA or antibiotics), viruses, toxic metals, pesticides, and a host of other
mineral and organic substances. Hence, soil colloids greatly impact nearly all ecosystem
functions.
We shall see that different soils are endowed with different types of clays that, along
with humus, elicit very different types of physical and chemical behaviors. Certain clay
minerals are much more reactive than others. Some are more dramatically influenced
than others by the acidity of the soil and other environmental factors. Studying
the soil colloids in some detail will deepen your understanding of soil architecture
(Chapter 4) and soil water (Chapters 5 and 6). Knowledge of the structure, origin, and
behavior of the different types of soil colloids will also help you understand soil chem-
ical and biological processes so you can make better decisions regarding the use of soil
resources.
8
THE COLLOIDAL
FRACTION:
SEAT OF SOIL
CHEMICAL AND
PHYSICAL ACTIVITY
The landscape of the clays is like—
the intricate folds of the womb—whose activity is to receive,
contain, enfold, and give birth.
—WILLIAM BRYANT LOGAN
Mica weathers to clay. (Serge Jolicoeur, Université de Moncton)
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 311
8.1 GENERAL PROPERTIES AND TYPES OF SOIL COLLOIDS
Size
The clay and humus particles in soils are referred to collectively as the colloidal frac-
tion because of their extremely small size and colloid-like behavior. Too small to be
seen with an ordinary light microscope, they can be made visible only with an electron
microscope. Particles behave as colloids if they are less than about 1 μm (0.000001
meter) in diameter, although some soil scientists consider 2 μm to mark the upper
boundary of the colloidal fraction to coincide with the definition of the clay particle
size fraction.
Surface Area
As discussed in Section 4.2, the smaller the size of the particles in a given mass of soil,
the greater the surface area exposed for adsorption, catalysis, precipitation, microbial
colonization, and other surface phenomena. Because of their small size, all soil colloids
expose a large external surface area per unit mass, more than 1000 times the surface
area of the same mass of sand particles. Some silicate clays also possess extensive
internal surface area between the layers of their platelike crystal units. To grasp the rel-
ative magnitude of the internal surface area, remember that these clays are structured
much like this book. If you were to paint the external surfaces of this book (the covers
and edges), a single brush of paint would do. However, to cover the internal surfaces
(both sides of each page in the book) you might need a very large can of paint.
The total surface area of soil colloids ranges from 10 m2/g for clays with only exter-
nal surfaces, to more than 800 m2/g for clays with extensive internal surfaces. To put
this in perspective, we can calculate that the surface area exposed within 1 ha (about
the size of a football field) of a 1.5-m-deep fine-textured soil (45% clay) might be as
great as 8,700,000 km2(the land area of the entire United States).
Surface Charges
The internal and external surfaces of soil colloids carry positive and/or negative elec-
trostatic charges. For most soil colloids, electronegative charges predominate, although
some mineral colloids in very acid soils have a net electropositive charge. As we shall
see in Sections 8.3 to 8.7, the amount and origin of surface charge differs greatly among
the different types of soil colloids and, in some cases, is influenced by changes in chem-
ical conditions, such as soil pH. The charges on the colloid surfaces attract or repulse
substances in the soil solution as well as neighboring colloid particles. These reactions,
in turn, greatly influence soil chemical and physical behavior.
Adsorption of Cations and Anions
Of particular significance is the attraction of positively charged ions (cations) to the
surfaces of negatively charged soil colloids. Each colloid particle attracts thousands of
Al3+, Ca2+, Mg2+, K+, H+, and Na+ions and lesser numbers of other cations. In moist
soils the cations exist in the hydrated state (surrounded by a shell of water molecules),
but for simplicity in this text, we will show just the cations (e.g., Ca2+ or H+) rather than
the hydrated forms (e.g., or the hydronium ion, . These hydrated
cations constantly vibrate about in a swarm near the colloid surface, held there by elec-
trostatic attraction to the colloid’s negative charges. Frequently, an individual cation
will break away from the swarm and move out to the soil solution. When this happens,
another cation of equal charge will simultaneously move in from the soil solution and
take its place. This process of cation exchange will be discussed in detail (Section 8.8)
because of its fundamental importance in nutrient cycling and other environmental
processes. The cations swarming about near the colloidal surface are said to be
adsorbed (loosely held) on the colloid surface. Because these cations can exchange
places with those moving freely about in the soil solution, the term exchangeable ions
is also used to refer to the ions in this adsorbed state.
The colloid with its adsorbed cations is sometimes described as an ionic double
layer in which the negatively charged colloid acts as a huge anion constituting the
inner ionic layer, and the swarm of adsorbed cations constitutes the outer ionic layer
H3O+)
Ca1H2O262+
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312 Chapter Eight
Al3+ Al3+
Ca2+
K+
H+
Ca2+ Al3+
Al3+ Ca2+ H+H+
H+
H+
H+
H+
K+
Mg2+
Ca2+
K+
K+
SO42–
NO3–
CI–
CI–
Ca2+
H+
Ca2+
K+
Al3+
Al3+ Mg2+ Al3+
K+
H+
Mg2+
Mg2+
Mg2+
Mg2+
Al3+
Al3+
Ca2+
Ca2+
Ca2+
K+
K+
H+
H+
Al3+ Ca2+
Ca2+ Na+
Al3+
Ca2+
Ca2+
Al3+
Ca2+ K+H+
Al3+ Ca2+ H+
K+
Mg2+
Mg2+
Ca2+
K+
Ca2+ Na2+
Ca2+ Al3+
Al3+ Ca2+ H+
K+
Mg2+
Mg2+
Ca2+
K+
Al3+ Ca2+
Ca2+
Mg2+
K+
Internal surfaces
Cation exchange
Cations
in solution
Anions and
cations
in solution
Adsorbed
cations
Ionic double layer
Negatively
charged
colloid
External surfaces
Clay particle
External surfaces
FIGURE 8.1 Simplified representa-
tion of a silicate clay crystal, its
complement of adsorbed cations,
and ions in the surrounding soil
solution. The enlarged view (right)
shows that the clay comprises sheet-
like layers with both external and
internal negatively charged surfaces.
The negatively charged particle acts
as a huge anion and a swarm of pos-
itively charged cations is adsorbed
to it because of attraction between
charges of opposite sign. Cation
concentration decreases with dis-
tance from the clay. Anions (such as
Cl-, NO3
-, and SO42-), which are
repulsed by the negative charges,
can be found in the bulk soil solu-
tion farthest from the clay (far right).
Some clays (not shown) also exhibit
positive charges that can attract
anions.
(Figure 8.1). Because cations from the soil solution are constantly trading places with
those that are adsorbed to the colloid, the ionic composition of the soil solution reflects
that of the adsorbed swarm. For example, if Ca2+ and Mg2+ dominate the exchangeable
ions, they will also dominate the soil solution. Under natural conditions, the propor-
tions of specific cations present are largely influenced by the soil parent material and
the degree to which the climate has promoted the loss of cations by leaching (see
Section 8.9).
Anions such as Cl-, , and (also surrounded by water molecules, though,
again, we do not show these water shells) may also be attracted to certain soil colloids
that have positive charges on their surfaces. While adsorption of exchangeable anions
is not as extensive as that for exchangeable cations, we shall see (Section 8.11) that it is
an important mechanism for holding negatively charged constituents, especially in
acid subsoils. When thinking about the colloids in soil, we should always keep in mind
that they carry with them a complement of exchangeable cations and anions, along
with certain other more tightly bound ions and molecules.
Adsorption of Water
In addition to adsorbing cations and anions, soil colloids attract and hold a large num-
ber of water molecules. Generally, the greater the external surface area of the soil col-
loids, the greater the amount of water held when the soil is air-dry (Figure 8.2). While
this water may not be available for plant uptake (see Section 5.8), it does play a role in
the survival of soil microorganisms, especially bacteria. The charges on the internal and
external colloid surfaces attract the oppositely charged end of the polar water molecule.
Some water molecules are attracted to the exchangeable cations, each of which is
hydrated with a shell of water molecules. Water adsorbed between the clay layers can
cause the layers to move apart, making the clay more plastic and swelling its volume.
Colloids that adsorb a great deal of water may make soil unsuitable for construction
SO42-
NO3-
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 313
01020304050 70
External surface area, m2/g
Air-dry water content, g/g
60 80 90
0.00
0.05
0.10
0.15
0.20
Clinoptilolite
Montmorillonite
Pyrophyllite
Kaolinite
FIGURE 8.2 The amount of water held in air-dry clay as influenced by the
external surface area of the clay. The clays were dried in low-humidity air
for 48 hours at 20 °C. The names refer to four silicate clay minerals that are
characterized by differing amounts of external surface area per unit mass.
Of the four, kaolinite and montmorillonite are by far the most common in
soils and are discussed in detail in Section 8.2. [Drawn from data in Morra
et al. (1998)]
1For a review of the structures and properties of the clays, see Meunier (2005), and for properties of clay
and humus in soils, see Dixon and Weed (1989).
purposes (see Sections 4.9 and 8.14). As a soil colloid dries, any water between the lay-
ers is removed, and the layers are brought closer together.
Types of Soil Colloids1
Soils contain numerous types of colloids, each with its particular composition, struc-
ture, and properties (Table 8.1). The colloids most important in soils can be grouped in
four major types:
CRYSTALLINE SILICATE CLAYS.These clays are the dominant type in most soils (except in
Andisols, Oxisols, and Histosols—see Chapter 3). Their crystalline structure is layered
much like pages in a book (clearly visible in Figure 8.3a). Each layer (page) consists of
two to four sheets of closely packed and tightly bonded oxygen, silicon, and aluminum
atoms. Although all are predominately negatively charged, silicate clay minerals differ
widely with regard to their particle shapes (kaolinite, afine-grained mica, and a
smectite are shown in Figure 8.3a–c), intensity of charge, stickiness, plasticity, and
swelling behavior.
TABLE 8.1 Major Properties of Selected Soil Colloids
Surface area, m2/g
Interlayer Net chargeb,
Colloid Type Size, μm Shape External Internal spacinga, nm cmolc/kg
Smectite 2:1 silicate 0.01–1.0 Flakes 80–150 550–650 1.0–2.0 -80 to -150
Vermiculite 2:1 silicate 0.1–0.5 Plates, flakes 70–120 600–700 1.0–1.5 -100 to -200
Fine mica 2:1 silicate 0.2–2.0 Flakes 70–175 — 1.0 -10 to -40
Chlorite 2:1 silicate 0.1–2.0 Variable 70–100 — 1.41 -10 to -40
Kaolinite 1:1 silicate 0.1–5.0 Hexagonal crystals 5–30 — 0.72 -1 to -15
Gibbsite Al-oxide <0.1 Hexagonal crystals 80–200 — 0.48 +10 to -5
Goethite Fe-oxide <0.1 Variable 100–300 — 0.42 +20 to -5
Allophane & Noncrystalline <0.1 Hollow spheres 100–1000 — — +20 to -150
Imogolite silicates or tubes
Humus Organic 0.1–1.0 Amorphous Variablec——-100 to -500
aFrom the top of one layer to the next similar layer, 1 nm = 10-9m = 10 Å.
bCentimoles of unbalanced or net charge per kilogram of colloid (cmolc/kg), a measure of ion exchange capacity (see Section 8.9).
cIt is very difficult to determine the surface area of organic matter. Different procedures give values ranging from 20 to 800 m2/g.
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314 Chapter Eight
NONCRYSTALLINE SILICATE CLAYS.These clays also consist mainly of tightly bonded silicon,
aluminum, and oxygen atoms, but they do not exhibit ordered, crystalline sheets. The
two principal clays of this type, allophane and imogolite, usually form from volcanic
ash and are characteristic of Andisols (Section 3.7). They have high amounts of both
positive and negative charge, and high water-holding capacities. Although malleable
(plastic) when wet, they exhibit a very low degree of stickiness. Allophane and imogo-
lite are also known for their extremely high capacities to strongly adsorb phosphate
and other anions, especially under acid conditions.
0.5 μm
(
a
)
0.5 μm
(b)
2 μm
(d)
0.5 μm
(c)
FIGURE 8.3 Crystals of three silicate clay minerals and a photomicrograph of humic acid found in soils. (a) Kaolinite from Illinois (note
hexagonal crystal at upper right). (b) A fine-grained mica from Wisconsin. (c) Montmorillonite (a smectite group mineral) from
Wyoming. (d) Fulvic acid (a humic acid) from Georgia. [(a)–(c) Courtesy of Dr. Bruce F. Bohor, Illinois State Geological Survey; (d) from
Dr. Kim H. Tan, University of Georgia; used with permission of Soil Science Society of America]
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IRON AND ALUMINUM OXIDES.These are found in many soils, but are especially important in
the more highly weathered soils of warm, humid regions (e.g., Ultisols and Oxisols).
They consist mainly of either iron or aluminum atoms coordinated with oxygen atoms
(the latter are often associated with hydrogen ions to make hydroxyl groups). Some,
like gibbsite (an Al-oxide) and goethite (an Fe-oxide) consist of crystalline sheets.
Other oxide minerals are noncrystalline, often occurring as amorphous coatings on
soil particles. The oxide colloids are relatively low in plasticity and stickiness. Their net
charge ranges from slightly negative to moderately positive. Although for simplicity we
will use term Fe,Al oxides for this group, many are actually hydroxides or oxyhydrox-
ides because of the presence of hydrogen ions.
ORGANIC (HUMUS). Organic colloids are important in nearly all soils, especially in the
upper parts of the soil profile. Humus colloids are not minerals, nor are they crystalline
(Figure 8.3d). Instead, they consist of convoluted chains and rings of carbon atoms
bonded to hydrogen, oxygen, and nitrogen. Humus particles are often among the
smallest of soil colloids and exhibit very high capacities to adsorb water, but almost no
plasticity or stickiness. Because humus is noncohesive, soils composed mainly of
humus (Histosols) have very little bearing strength and are unsuitable for making build-
ing or road foundations. Humus has high amounts of both negative and positive
charge per unit mass, but the net charge is always negative and varies with soil pH. The
negative charge on humus is extremely high in neutral to alkaline soils.
8.2 FUNDAMENTALS OF LAYER SILICATE CLAY STRUCTURE2
To see why soils rich in one silicate clay mineral, say kaolinite, behave so very differ-
ently from soils dominated by another silicate clay, say montmorillonite, it is necessary
to understand the main structural features of the silicate clay minerals. We will begin
by examining the main building blocks from which the layer silicates are constructed,
then consider the particular arrangements that give rise to the critically important sur-
face charges.
Silicon Tetrahedral and Aluminum-Magnesium Octahedral Sheets2
The most important silicate clays are known as phyllosilicates (Greek phyllon, leaf)
because of their leaflike or planar structure. As shown in Figure 8.4, they are composed
of two kinds of horizontal sheets.
TETRAHEDRAL SHEETS.This kind of sheet consists of two planes of oxygens with mainly sil-
icon in the spaces between the oxygens. The basic building block for the tetrahedral
sheet is a unit composed of one silicon atom surrounded by four oxygen atoms. It is
called a tetrahedron because (as shown in Figure 8.4, top left) the oxygens define the
apices of a four-sided geometric solid that resembles a pyramid (having three “sides”
and a bottom). An interlocking array of such tetrahedra, each sharing its basal oxygens
with its neighbor, give rise to a tetrahedral sheet.
OCTAHEDRAL SHEETS.Six oxygen atoms coordinating with a central aluminum or magne-
sium atom form the shape of an eight-sided geometric solid, or octahedron. Numerous
octahedra linked together horizontally constitute the octahedral sheet. If three Mg2+
atoms are coordinated with (and balance the charges on) the six oxygens/hydroxyls,
then the sheet is called a trioctahedral sheet. If, instead, the six oxygens/hydroxyls are
coordinated with two Al3+ atoms, then the sheet is called dioctahedral. Note that the
distinction is based on the number of metal atoms required to satisfy the six negative
charges from the oxygen/hydroxyls (see Figure 8.5, left and middle). As we will see
later, numerous intergrades are possible in which both 2+ and 3+ cations are present.
The tetrahedral and octahedral sheets are the fundamental structural units of sili-
cate clays. Two to four of these sheets may be stacked together in sandwich-like
THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 315
2The authors are indebted to Dr. Darrel G. Schultze of Purdue University for kindly providing the struc-
tural models for the silicate clay minerals.
Three-dimensional rotatable
models of silicate clay
minerals and their building
blocks:
http://www.soils1.cses.vt.edu/
MJE/VR_exports/intro.shtml
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316 Chapter Eight
Si plane
O, OH plane
O plane
Al, Mg plane Layer
O plane
O, OH plane
Si plane
Si plane
O, OH plane
O plane
Al, Mg plane
O plane
O, OH plane
Si plane
Interlayer Crystal
Adsorbed cations
and water
Interlayer
Interlayer
Layer
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
(b)
Aluminum,
magnesium
Hydroxide
Silicon
(a)
Apical oxygen
Oxygen
Basal oxygens
FIGURE 8.4 The basic molecular and structural components of silicate clays. (a) A single tetrahedron, a four-sided building block com-
posed of a silicon ion surrounded by four oxygen atoms; and a single eight-sided octahedron, in which an aluminum (or magnesium) ion
is surrounded by six hydroxy groups or oxygen atoms. (b) In clay crystals thousands of these tetrahedral and octahedral building blocks
are connected to give planes of silicon and aluminum (or magnesium) ions. These planes alternate with planes of oxygen atoms and
hydroxy groups. Note that apical oxygen atoms are common to adjoining tetrahedral and octahedral sheets. The silicon plane and asso-
ciated oxygen-hydroxy planes make up a tetrahedral sheet. Similarly, the aluminum-magnesium plane and associated oxygen-hydroxy
planes constitute the octahedral sheet. Different combinations of tetrahedral and octahedral sheets are termed layers. In some silicate clays
these layers are separated by interlayers in which water and adsorbed cations are found. Many layers are found in each crystal.
2–
2–
2–
2–
2–
2–
2+ 2+ 2+
2+
3–
Net = 0
Trioctahedral
(3 cations)
Dioctahedral
(2 cations)
Dioctahedral
with isomorphic
substitution
3–
6+
+
+
+
++
++
2–2–
2– 3+
Net = 0
2–
2–
2–
2–
3–
3–
6+
Net = 1–
3–
3–
5+
+
+
++
++
2–2– 2–
2–
2–
2–
2–
2–
2+
+
+
++
++
2–2–
Oxygen Hydrogen Aluminum Magnesium
or iron
3+ 3+3+
2–
2–
FIGURE 8.5 Simplified diagrams of
octahedral sheets in silicate minerals
illustrating tri- and dioctahedral struc-
tures and isomorphous substitution.
For each oxygen atom, one of the two
charges is balanced by a + charge
from either a H+(making a hydroxyl
group) or from a Si atom in the tetra-
hedral sheet (not shown, but repre-
sented by a +). In a trioctahedral sheet
(left)three out of three octahedral
positions are occupied by a metal
cation with a 2+charge (typically
either Fe2+or Mg2+). The + and -
charges are balanced so there is no net
charge. Biotite is an example of trioc-
tahedral clay. In a dioctahedral sheet
(middle) only two out of three octahe-
dral positions are occupied, but by a
metal cation with a 3+charge (Al3+is
most common). Again, the + and -
charges are balanced so there is no
net charge on the octahedral sheet.
Muscovite is an example of dioctahe-
dral clay. In the dioctahedral sheet
shown on the right, a Mg2+atom
occupies one of the positions nor-
mally occupied by an Al3+atom, thus
leaving a 1-net charge on the sheet.
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 317
arrangements, with adjacent sheets strongly bound together by sharing some of the
same oxygen atoms (see Figure 8.4). The specific nature and combination of sheets in
these layers vary from one type of clay to another and largely control the physical and
chemical properties exhibited. The relationship between planes, sheets, and layers
shown in Figure 8.4 should be carefully studied.
Isomorphous Substitution
The structural arrangements just described suggest a very simple relationship among
the elements making up silicate clays. In nature, however, clays have formulas that are
more complex. During the weathering of rocks and minerals, many different elements
are present in the weathering solution. As clay minerals or their precursors crystallize,
cations of comparable size (see Table 8.2) may substitute for silicon, aluminum, and
magnesium ions in the respective tetrahedral and octahedral sheets.
Note from Table 8.2 that aluminum is only slightly larger than silicon. Consequently,
aluminum can fit into the center of the tetrahedron in the place of the silicon without
much change in the basic structure of the crystal. This process by which one element
fills a position usually filled by another of similar size is called isomorphous substi-
tution. This phenomenon is responsible for much of the variability in the nature of
silicate clays.
Isomorphous substitution can also occur in the octahedral sheets. For example, iron
and zinc ions are not much different in size from aluminum and magnesium ions (Table
8.2). Any of these ions can fit in the central position of an octahedra. In some layer sili-
cates, isomorphous substitution occurs in both tetrahedral and in octahedral sheets.
Source of Charges
Isomorphous substitution is of vital importance because it is the primary source of both
negative and positive charges of silicate clays. For example, the Mg2+ ion is only slightly
larger than the Al3+ ion, but it has one less positive charge. If a Mg2+ ion substitutes for
an Al3+ ion in a dioctahedral sheet, there will be insufficient positive charges to balance
the negative charges from the oxygens; hence, the lattice is left with a 1-net charge
(see Figure 8.5, right). Similarly, every Al3+ that substitutes for a Si4+ in a tetrahedral
sheet creates a net negative charge at that site because the negative charges from the
four oxygens will be only partially balanced. In a trioctahedral sheet, if an Al3+ substi-
tutes for the usual Mg2+ or Fe2+, then a net positive charge is created. The net charge
associated with a clay crystal is the sum of the positive and negative charges. In most
silicate clays, the negative charges predominate (as will be discussed in Section 8.8). As
we shall see (Sections 8.3 and 8.6), additional, more temporary charges can also
develop on the edges of the tetrahedral and octahedral surfaces.
8.3 MINERALOGICAL ORGANIZATION OF SILICATE CLAYS
Based on the number and arrangement of tetrahedral (Si) and octahedral (Al, Mg, Fe)
sheets contained in the crystal units or layers, crystalline clays may be classed into two
main groups: 1:1 silicate clays, in which each layer contains one tetrahedral and one
TABLE 8.2 Ionic Radii and Location of Elements Found in Silicate Clays
Ion Radius, nm (10
-
9m) Found in
Si4+ 0.042
Al3+ 0.051 Tetrahedral sheet
Fe3+ 0.064
Mg2+ 0.066 Octahedral sheet
Zn2+ 0.074 Exchange or interlayer sites
Fe2+ 0.076
Na+0.095
Ca2+ 0.099
K+0.133
O2-0.140 Both sheets
OH-0.155
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318 Chapter Eight
octahedral sheet, and 2:1 silicate clays, in which each layer has one octahedral sheet
sandwiched between two tetrahedral sheets.
1:1-Type Silicate Clays
The 1:1 silicate clays include kaolinite, halloysite, nacrite, and dickite. To illustrate the
properties of 1:1 silicate clays we will focus on kaolinite, which is by far the most com-
mon in soils.
As implied by the term 1:1 silicate clay, each kaolinite layer consists of one silicon
tetrahedral sheet and one aluminum octahedral sheet. The two types of sheets are
tightly held together because the apical oxygen atom (the oxygen atom that forms the
apex or tip of the “pyramid”) in each tetrahedron also forms a bottom corner of one or
more of the octahedra in the adjoining sheet (Figure 8.6). Note that because a kaolinite
crystal layer consists of these two sheets, it exposes a plane of oxygen atoms on the bot-
tom surface, but a plane of hydroxyls on the upper surface.
This arrangement has two very important consequences. First, as will be discussed
in Section 8.6, where the hydroxyl plane is exposed on the clay particle surface,
removal or addition of hydrogen ions can produce either positive or negative charges,
depending on the pH of the soil. The exposed hydroxylated surface can also react
with and strongly bind specific anions. Second, when the layers consisting of alternat-
ing tetrahedral and octahedral sheets are stacked on top of one another, the hydroxyls
of the octahedral sheet in one layer are adjacent to the basal oxygens of the tetrahedral
sheet of the next layer. Therefore, adjacent layers are bound together by hydrogen
bonding (see Section 5.1).
Because of the interlayer hydrogen bonding, the structure of kaolinite is fixed, and
no expansion can occur between the layers when the clay is wetted. Cations and
water generally do not enter between the structural layers of a 1:1 mineral particle.
The effective surface of kaolinite is thus restricted to its outer faces or external surface
area. This fact and the lack of significant isomorphous substitution in this mineral
account for the relatively small capacity of kaolinite to adsorb exchangeable cations
(see Table 8.1).
Kaolinite crystals are usually hexagonal in shape (see Figure 8.3a) and larger
than most other clays (Table 8.1). In contrast to some 2:1 silicate clays, 1:1 clays like
kaolinite exhibit less plasticity, stickiness, cohesion, shrinkage, and swelling and
can also hold less water than other clays (Figure 8.2). Because of these properties, soils
dominated by 1:1 clays are relatively easy to cultivate for agriculture and, with
proper nutrient management, can be quite productive. Kaolinite-containing soils
are well suited for use in roadbeds and building foundations (Plate 43). The nonex-
panding 1:1 structure also makes kaolinite clays useful for making bricks and ceramics
(see Box 8.1).
Al
OH
O
OOSi
Apical
O
Al
OH OH
O, OH
Si
Si
Octahedral sheet
Tetrahedral sheet
Al
FIGURE 8.6 Models of the 1:1-type clay kaolinite. The primary elements of the octahedral (upper left) and tetrahe-
dral (lower left) sheets are depicted as they might appear separately. In the crystal structure, however, these sheets
are held together by common apical oxygen atoms. Note that each layer consists of alternating octahedral and
tetrahedral sheets—hence, the designation 1:1. The octahedral and tetrahedral sheets are bound together (center) by
mutually shared (apical) oxygen atoms. The result is a layer with hydroxyls on one surface and oxygens on the
other. To permit us to view the front silicon atoms, we have not shown some basal oxygen atoms that are normally
present. The diagram at right shows the bonds between atoms. The kaolinite mineral is comprised of a series of
these flat layers tightly held together with no interlayer spaces.
Interactive 3-D model of
Kaolinite structure:
http://www.soils.wise.edu/
virtual_museum/kaolinite/
index.html
Kaolin clay used for pest
control:
http://www.nysaes.cornell.
edu/pp/resourceguide/mfs/
07kaolin.php
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 318

THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 319
Expanding 2:1-Type Silicate Clays
The four general groups of 2:1 silicate clays are characterized by one octahedral sheet
sandwiched between two tetrahedral sheets. Two of these groups, smectite and vermic-
ulite, include expanding-type minerals; the other two, fine-grained micas (illite) and
chlorite, are relatively nonexpanding.
SMECTITE GROUP.The flakelike crystals of smectites (see Figure 8.3c) have a high amount
of mostly negative charge resulting from isomorphous substitution. Most of the charge
derives from Mg2+ ions substituted in the Al3+ positions of the octahedral sheet, but some
also derives from substitution of Al3+ ions for Si4+ in the tetrahedral sheets (Figure 8.9).
Kaolinite, the most common of the 1:1 clay minerals, has
been used for thousands of years to make pottery, roofing
tiles, and bricks. The basic processes have changed little to
this day. The clayey material is saturated with water,
kneaded and molded or thrown on a potter’s wheel to
obtain the desired shape, and then hardened by drying or
firing (Figures 8.7, 8.8). The mass of cohering clay platelets
hardens irreversibly when fired and the nonexpanding
nature of kaolinite allows it to be fired without cracking
from shrinkage. The heat also changes the typical gray color
of the soil material to “brick red” because of the irreversible
oxidation and crystallization of the iron-oxyhydroxides that
often coat soil kaolinite particles. In contrast, kaolinite
mined from pure deposits fires to a light, creamy color.
Kaolinite is not as plastic (moldable) as some other clays,
however, and so is usually mixed with more plastic types of
clays for making pottery.
It was in seventh-century China that pure kaolinite
deposits were first used to make objects of a translucent, lightweight, and strong ceramic called porcelain. The
name kaolinite derives from the Chinese words kan and ling, meaning “high ridge,” as the material was first
mined from a hillside in Kiangsi Province. The Chinese held a monopoly on porcelain-making technology
(hence the term china for porcelain dishes) until the early 1700s. English colonists, in what is now Georgia in the
United States, noted outcrops of white kaolinite clay in areas of rather unproductive soil. The colonialists soon
were exporting this kaolinite as the main ingredient for making porcelain in England, where the now-famous
pottery was first manufactured from the Georgia kaolinite clay.
The market for pure, white kaolinite clay greatly expanded
when paper manufacturers started using kaolinite clay to make
sizing, the coating that makes high-quality papers smoother,
whiter, and more printable. Other industrial uses now include
paint pigments, fillers in plastic manufacture, and ceramic materi-
als used for electrical insulation and heat shielding (as on the belly
of the space shuttle). The kaolinite in kaopectin-type medications
lines the stomach walls and inactivates diarrhea-causing bacteria
by adsorbing them on the clay particle surfaces. A development
that is likely to increase the demand for kaolinite is its use as a
spray-on coating for fruit tree leaves that provides nontoxic pro-
tection from insect pests and fungal diseases. All these uses have
made industrial kaolinite clay mining a big business in Georgia.
Unfortunately, surface mining of kaolinite to meet these needs
can cause both environmental and social disruptions.
BOX 8.1
KAOLINITE CLAY—THE STORY OF WHITE GOLDa
FIGURE 8.7 Kaolinitic clay soil is dug, molded, dried,
stacked to form a kiln, and fired to make bricks.
(Courtesy of R. Weil)
FIGURE 8.8 Kaolinite clay in African pottery and
early 19th-century English china (inset). (Courtesy
of R. Weil)
aFor more on social aspects, see Seabrook, 1995.
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 319

320 Chapter Eight
Because of these substitutions, the capacity to adsorb cations is very high—about 20 to
40 times that of kaolinite.
In contrast to kaolinite, smectites have a 2:1 structure that exposes a layer of oxygen
atoms at both the top and bottom planes. Therefore, adjacent layers are only loosely
bound to each other by very weak oxygen-to-oxygen and cation-to-oxygen linkages and
the space between is variable (Figure 8.9). The internal surface area exposed between the
layers by far exceeds the external surface area of these minerals and contributes to the
very high total specific surface area (Table 8.1). Exchangeable cations and associated
water molecules are attracted to the spaces between the interlayer spaces.
Flakelike smectite crystals tend to pile upon one another, forming wavy stacks that
contain many extremely small ultramicropores (see Table 4.6). When soils high in smec-
tite are wetted, adsorption of water in these ultramicropores leads to severe swelling;
when they are dried, the soils shrink in volume (see Section 8.14). The expansion upon
wetting contributes to the high degree of plasticity, stickiness, and cohesion that make
smectitic soils very difficult to cultivate or excavate. Wide cracks commonly appear
during the drying of smectite-dominated soils (such as Vertisols, Figure 3.22). The
shrink/swell behavior makes smectitic soils quite undesirable for most construction
activities, but they are well suited for a number of applications that require a
high adsorptive capacity and the ability to form seals of very low permeability (see
Section 8.14). Montmorillonite is the most prominent of the smectites in soils,
although others are also found.
VERMICULITE GROUP.The most common vermiculites are 2:1-type minerals in which
the octahedral sheet is aluminum dominated (dioctahedral), but some magnesium-
dominated (trioctahedral) vermiculites also exist. The tetrahedral sheets of most vermi-
culites have considerable substitution of aluminum in the silicon positions, giving rise
to a cation exchange capacity that usually exceeds that of all other silicate clays, includ-
ing smectites (Table 8.1).
Al, Mg
Al/Mg
Hydrated
exchangeable
cation
Exchangeable
cations
Variable
spacing
O
O
OOHSi
O, OH
O, OH
Si
Si
Al, Mg
O
O
O, OH
O, OH
Si
Si
O
Al/Mg
SiOH
Water
FIGURE 8.9 Model of two crystal layers and an interlayer characteristic of montmorillonite, a smectite
expanding-lattice 2:1-type clay mineral. Each layer is made up of an octahedral sheet sandwiched
between two tetrahedral sheets with shared apical oxygen atoms. There is little attraction between oxy-
gen atoms in the bottom tetrahedral sheet of one unit and those in the top tetrahedral sheet of another.
This permits a variable space between layers, which is occupied by water and exchangeable cations. The
internal surface area thus exposed far exceeds the surface around the outside of the crystal. Note that
magnesium has replaced aluminum in some sites of the octahedral sheet. Likewise, some silicon atoms
in the tetrahedral sheet may be replaced by aluminum (not shown). These substitutions give rise to a
negative charge, which accounts for the high cation exchange capacity of this clay mineral. A ball-and-
stick model of the atoms and chemical bonds is at the right.
Interactive 3-D model of
Smectite structure:
http://virtual-museum.soils.wisc.
edu/soil_smectite/index.html
Safe storage for nuclear wastes:
the Swedish approach using
Bentonite clay:
http://www.skb.se/templates/
SKBPage____8776.aspx
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 321
The interlayer spaces of vermiculites usually contain strongly adsorbed water mole-
cules, Al-hydroxy ions, and cations such as magnesium (Figure 8.10). However, these
interlayer constituents act primarily as bridges to hold the units together, rather
than wedges driving them apart. The degree of swelling and shrinkage is, therefore,
considerably less for vermiculites than for smectites. For this reason, vermiculites are
considered limited-expansion clays, expanding more than kaolinite, but much less
than the smectites.
Nonexpanding 2:1 Silicate Minerals
The main nonexpanding 2:1 minerals are the fine-grained micas and the chlorites.
We will discuss the fine-grained micas first.
MICA GROUP.Biotite and muscovite are examples of unweathered micas typically found
in the sand and silt fractions. The more weathered fine-grained micas, such as illite
and glauconite, are found in the clay fraction of soils. Their 2:1-type structures are
quite similar to those of their unweathered cousins. Unlike in smectites, the main
source of charge in fine-grained micas is the substitution of Al3+ in about 20% of the
Si4+ sites in the tetrahedral sheets. This results in a high net negative charge in the tetra-
hedral sheet, even higher than that found in vermiculites. The negative charge attracts
cations, among which potassium (K+) is just the right size to fit snugly into certain
hexagonal “holes” between the tetrahedral oxygen groups (Figures 8.10 and 8.11) and
thereby get very close to the negatively charged sites. By their mutual attraction for the
K+ions in between, adjacent layers in fine-grained micas are strongly bound together.
Hence, the fine-grained micas are quite nonexpansive. Because of their nonexpansive
character, the fine-grained micas are more like kaolinite than smectites with regard to
their capacity to adsorb water and their degree of plasticity and stickiness.
CHLORITES.In most soil chlorites, iron or magnesium, rather than aluminum, occupy
many of the octahedral sites. Commonly, a magnesium-dominated trioctahedral
hydroxide sheet is sandwiched in between adjacent 2:1 layers (Figure 8.10). Thus, chlo-
rite is sometimes said to have a 2:1:1 structure. Chlorites are nonexpansive because the
Water molecules,
miscellaneous cations
Smectite (2:1)
Expanding
(max. swelling)
Kaolinite (1:1)
Nonexpanding
(no swelling)
1–2
nm Water molecules,
Mg2+ and other ions
Vermiculite (2:1)
Expanding
(some swelling)
1.0–
1.5
nm
Fine-grained mica (2:1)
Nonexpanding
(min. swelling)
1.0
nm
K+
Chlorite (2:1)
Nonexpanding
(min. swelling)
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Octahedral sheet
Hydroxide sheet
Tetrahedral sheet
1.4
nm
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Tetrahedral sheet
Octahedral sheet
0.7
nm
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet
Octahedral sheet
Tetrahedral sheet Tetrahedral sheet
FIGURE 8.10 Schematic drawing illustrating the organization of tetrahedral and octahedral sheets in one 1:1-type mineral (kaolinite)
and four 2:1-type minerals. The octahedral sheets in each of the 2:1-type clays can be either aluminum dominated (dioctahedral) or mag-
nesium dominated (trioctahedral). However, in most chlorites the trioctahedral sheets are dominant while the dioctahedral sheets are
generally most prominent in the other three 2:1 types. Note that kaolinite is nonexpanding, the layers being held together by hydrogen
bonds. Maximum interlayer expansion is found in smectite, with somewhat less expansion in vermiculite because of the moderate bind-
ing power of numerous Mg2+ ions. Fine-grained mica and chlorite do not expand because K+ions (fine-grained mica) or an octahedral-like
sheet of hydroxides of Al, Mg, Fe, and so forth (chlorite) tightly bind the 2:1 layers together. The interlayer spacings are shown in
nanometers (1 nm = 10-9m).
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 321

322 Chapter Eight
hydroxylated surfaces of an intervening Mg-octahedral sheet are hydrogen-bonded to
the oxygen atoms of the two adjacent tetrahedral sheets, binding the layers tightly
together. The colloidal properties of the chlorites are therefore quite similar to those of
the fine-grained micas (Table 8.1).
X-Ray Diffraction Analysis
Until the invention of suitable means of investigating mineral structure at the atomic
level, it was thought that clays consisted of inert mineral fragments coated in amorphous
gels of iron oxides. Now, the relative amounts of various types of clay minerals present in
a soil can be determined by a procedure called X-ray diffraction analysis, which mea-
sures the distance between layers (the d-spacing) in the mineral structure (see Box 8.2).
8.4 STRUCTURAL CHARACTERISTICS OF NONSILICATE COLLOIDS
Iron and Aluminum Oxides
These clays consist of modified octahedral sheets with either iron (Fe3+) or aluminum
(Al3+) in the cation positions. They have neither tetrahedral sheets nor silicon in their
structures. Isomorphous substitution by ions of varying charge rarely occurs, so these
clays do not have a large negative charge. The small amount of net charge these clays
possess (positive and negative) is caused by the removal or addition of hydrogen ions at
the surface oxy-hydroxyl groups. The presence of these bound oxygen and hydroxyl
groups enables the surfaces of these clays to strongly adsorb and combine with anions
such as phosphate or arsenate. The oxide clays are nonexpansive and generally exhibit
relatively little stickiness, plasticity, and cation adsorption. They make quite stable
materials for construction purposes.
Gibbsite [Al(OH)3], the most common soil aluminum oxide, is a prominent con-
stituent of highly weathered soils (e.g., Oxisols and Ultisols). Figure 8.13 shows gibbsite
to consist of a series of aluminum octahedral sheets linked to one another by hydrogen-
bonding between their hydroxyls. Note that a plane of hydroxyls is exposed at the
upper and lower surfaces of gibbsite crystals. These hydroxylated surfaces can strongly
adsorb certain anions.
Other oxide-type clays have iron instead of aluminum in the central cation posi-
tions, and their octahedral structures are somewhat distorted and less regular than that
of gibbsite. Goethite (FeOOH) and ferrihydrite (Fe2O3·nH2O) are common iron oxide
Si
Al
OH
O
K
O
OH
Si
Al
Al
O
K
OH
Si
FIGURE 8.11 Model of a 2:1-type nonexpanding lattice mineral of the fine-grained mica group. The
general constitution of the layers is similar to that in the smectites, one octahedral sheet between two
tetrahedral sheets. However, potassium ions are tightly held between layers, giving the mineral a more
or less rigid type of structure that prevents the movement of water and cations into the space between
layers. The internal surface and cation exchange capacity of fine-grained micas are thus far below those
of the smectites.
Use of X-ray diffraction
in art conservation and
detecting art fraud:
http://www.cci-icc.gc.ca/
about-cci/cci-in-action/view-
document_e.aspx?Type_ID=
6&Document_ID=101
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 322

THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 323
When a thin smear of clay powder or paste is slowly rotated in an X-ray beam of a particular wavelength, the
X-ray waves reflecting off parallel planes of atoms in the clay crystal create a diffraction pattern that can be
detected by X-ray diffraction analysis. The detected X-ray energy is low when the waves bouncing off different
layers are out of phase and therefore cancel each other out. The detected energy is magnified when the waves
reflected from two layers are in phase (synchronous) and therefore reinforce each other (Figure 8.12, left).
These reinforced waves create peaks of
energy, which are recorded on a graph
called a diffractogram.
When incoming waves strike two (or
more) parallel surfaces, the angle of inci-
dence theta (θ) will determine how much
farther the waves striking the second
layer must travel compared to those strik-
ing the first layer. Bragg’s law tells us there
is a specific angle that will cause the
waves striking the second layer to travel
an additional distance exactly equal to
their wavelength. Therefore, the waves
reflected off the second layer will be in
phase with those reflected off the first
layer. The distance (d, in nm) between lay-
ers is calculated from Bragg’s law:
n

 2dsin (8.1)
where nis an integer,

is the known
wavelength (in nm) of the X-rays, and θis
the glancing angle that causes the waves
reflected from different layers to rein-
force each other.
Layers at specific distances apart characterize the crystal structure for each clay mineral (note the
nanometer d-spacings shown in Figure 8.10), so one can determine which minerals are present by examining
the specific angles that cause reinforced X-ray energy peaks. The angle at which a peak forms is indicative of
the layer spacing. The sizes of the X-ray peaks are semiquantitatively related to the relative amount of a spe-
cific mineral present. X-ray diffraction can best identify clay structures if the clay is given special pretreat-
ments by saturating it with specific cations, washing it with various solvents, or heating it to remove interlayer
impurities.
Figure 8.12 (right) shows two diffractograms that suggest the presence of mainly fine-grained mica, vermi-
culite, and kaolinite clays. The diffractograms shown are from a long-term experiment in the San Demas
Mountains of southern California. Scientists began by digging special pits (called lysimeters) fit with drains
that allowed leaching water to be collected. Each pit was then uniformly packed full with soil material of
known composition. Oak trees (Quercus dumosa) were planted in some of the lysimeters. After 41 years, the
surface layer of soil was sampled and compared to archived samples of the original soil material. Both X-ray
diffractograms showed a distinct peak for the 1.41-nm interlayer spacing characteristic of vermiculite.
However, a strong peak for 1.0-nm spacing (characteristic of mica) appeared only in the soil under oak forest.
Chemical analyses showed that the oak trees had increased the amount of potassium in the A horizon by
cycling this element up from the deeper layers (see Section 2.5). Together, these two diffractograms illustrate
(1) the utility of X-ray diffraction as a tool for identifying clay minerals and (2) the chemical alteration processes
by which one clay mineral may be transformed into another. The latter topic will be further explored in
Section 8.6.
BOX 8.2
X-RAYS UNLOCK THE MYSTERIES
OF CRYSTALLINE CLAY STRUCTURE
2 4 6 8 10 12 16
Degrees, 2θ
14
0.71 nm
Soil clay after 41
years of oak
vegetation
Soil clay from original
archived soil sample
1.00 nm (Mica)
1.41 nm
1.41 nm (Vermiculite)
0.71 nm
(Kaolinite)
X-ray
source
d spacing Clay
crystal
X-ray
detector
Rotating
angle
FIGURE 8.12 A simplified diagram of X-ray diffraction used to iden-
tify clay minerals (left) and two X-ray diffractograms (right) showing
peaks indicative of vermiculite, fine-grained mica, and kaolinite clay
minerals. [Diffractograms redrawn from Tice et al. (1996)]
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 323

324 Chapter Eight
O
2–
O
2–
O
A1 A1
HHH
OO
H H
2–
O
2–
O
A1 A1
HH
OO
H H
2–
O
H
O
H
0.48 nm
O
H
O
H
O
O
2–
O
2–
O
A1 A1
HHH
OO
H H H
2–
O
2–
O
A1 A1
HH
OO
H H
2–
O
H
O
H
O
H
O
H
O
H
FIGURE 8.13 A simplified diagram showing the
structure of gibbsite, an aluminum oxide clay com-
mon in highly weathered soils. This clay consists
of dioctahedral sheets (two are shown) that are
hydrogen-bonded together. Other oxide-type clays
have iron instead of aluminum in the octahedral
positions, and their structures are somewhat less
regular and crystalline than that shown for gibb-
site. The surface plane of covalently bonded
hydroxyls gives this, and similar clays, the capacity
to strongly adsorb certain anions (see Section 8.8).
(CH
3
)
0–3
(CH
3
)
0–2
(CH
3
)
0–5
(CH
3
)
0–2
(CH
3
O)
0–3
(CH
3
)
0–2
(CH
3
)
0–5
CH
2
OH
OHO
(CH
3
)
0–4
(CH
3
)
0–4
OCH
3
(CH
3
)
0–2
O
O
HO
HO
HO
HO
HO
HO
HO
O
OHO
HO
OH O
O
O
O
O
O
OH
OH
OH
OH
N
H
N
H
CN
O
OH
OH
OHO
O
OH
OH
OH
OH
OH
OH O
O
OH
O
O
O
O
OO
O
OH
HO
HO CN
O
O
O
O
O
OH
OH
OH
OO
O
O
OH
OH
OH
OOH OH
OH
HO
OH
O
OH
OH OH
OH
O
O
O
N
HO
O
O
O
D
OH OH
O
FIGURE 8.14 A possible structure
for humic acid, a primary con-
stituent of colloidal humus in soils.
Careful inspection will reveal the
presence of many of the active –OH
groups illustrated in Figure 8.15, as
well as certain nitrogen- and sulfur-
containing groups. [From Schulten
and Schnitzer (1993) with kind per-
mission of Springer-Verlag Publishers]
clays in temperate regions, accounting for the yellow-brown colors of many soils.
Hematite (Fe2O3) is common in drier environments and gives redder colors to well-
drained soils, especially in hot, dry climates.
In many soils, iron and aluminum oxide minerals are mixed with silicate clays. The
oxides may form coatings on the external surfaces of the silicate clays, or they may
occur as “islands” in the interlayer spaces of such 2:1 clays as vermiculites and smec-
tites. In either case, the presence of iron and aluminum oxides can substantially alter
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 325
the colloidal behavior of the associated silicate clays by masking charge sites, interfer-
ing with shrinkage and swelling, and providing anion-retentive surfaces.
Humus
As mentioned in Section 8.1, humus is a noncrystalline organic substance. It consists of
very large organic molecules whose chemical composition varies considerably, but gen-
erally contains 40 to 60% C, 30 to 50% O, 3 to 7% H, and 1 to 5% N. The molecular
weights of humic acids, a major type of colloidal humus, range from 10,000 to 100,000
g/mol. Identification of the actual structure of humus colloids is very difficult. A pro-
posed structure typical of humic acid is shown in Figure 8.14. Note that it contains a
very complex series of carbon chains and ring structures, with numerous chemically
active functional groups throughout. Figure 8.15 provides a simplified diagram to illus-
trate the three main types of -OH groups thought to be responsible for the high
amount of charge associated with these colloids. Negative or positive charges on the
humus colloid develop as H+ions are either lost or gained by these groups. Both cations
and anions are therefore attracted to and adsorbed by the humus colloid. The negative
sites always outnumber the positive ones, and a very large net negative charge is associ-
ated with humus (Table 8.1). Because of its great surface area and many hydrophilic
(water-loving) groups, humus can adsorb very large amounts of water per unit mass.
However, humus also contains many hydrophobic sites and therefore can strongly
adsorb a wide range of hydrophobic, nonpolar organic compounds (see Section 8.12).
Because of its extraordinary influence on soil properties and behavior, we will delve
much more deeply into the nature and function of soil humus in Chapter 12.
8.5 GENESIS AND GEOGRAPHIC DISTRIBUTION OF SOIL COLLOIDS
Genesis of Colloids
The silicate clays are developed from the weathering of a wide variety of minerals by at
least two distinct processes: (1) a slight physical and chemical alteration of certain pri-
mary minerals, and (2) a decomposition of primary minerals with the subsequent
recrystallization of certain of their products into the silicate clays. These processes will
each be given brief consideration.
ALTERATION.The changes that occur as muscovite mica is altered to fine-grained mica
represent a good example of alteration. Muscovite is a dioctahedral 2:1-type primary
mineral with a nonexpanding crystal structure and a formula of KAl2(Si3Al)O10(OH)2.
As weathering occurs, the mineral is broken down in size to the colloidal range. Part of
the interlayer potassium is lost, and some silicon along with such cations as Ca2+ or
Mg2+ are added from weathering solutions. The net result is a less rigid crystal structure
and the availability of free electronegative charges at sites formerly occupied by the
All about humic substances:
http://www.ar.wroc.pl/
~weber/humic.htm#start
OH
OH
OH
C
C
O
OO–
O
–
O
–
O–
OH2+
Alcoholic
hydroxyl
group
Large complex organic humus
molecule consisting of chains
and rings of mainly carbon and
hydrogen atoms
Phenolic
hydroxyl
group
Carboxylic
group
FIGURE 8.15 A simplified diagram showing the
principal chemical groups responsible for the high
amount of negative charge on humus colloids.
The three groups highlighted all include –OH that
can lose its hydrogen ion by dissociation and
thus become negatively charged. Note that the
carboxylic, phenolic, and alcoholic groups on the
right side of the diagram are shown in their disas-
sociated state, while those on the left side still have
their associated hydrogen ions. Note also that asso-
ciation with a second hydrogen ion causes a site to
exhibit a net positive charge. (Diagram courtesy of
R. Weil)
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 325

326 Chapter Eight
fixed interlayer potassium. The fine mica colloid that emerges still has a 2:1-type structure,
having only been altered in the process.
RECRYSTALLIZATION.This process involves the complete breakdown of the crystal structure
and recrystallization of clay minerals from products of this breakdown. It is the result of
much more intense weathering than that required for the alteration process just described.
An example of recrystallization is the formation of kaolinite (a 1:1-type clay min-
eral) from solutions containing soluble aluminum and silicon that came from the
breakdown of primary minerals having a 2:1-type structure. Such recrystallization
makes possible the formation of more than one kind of clay from a given primary min-
eral. The specific clay mineral that forms depends on weathering conditions and the
specific ions present in the weathering solution as crystallization occurs.
RELATIVE STAGES OF WEATHERING.Specific conditions conducive to the formation of impor-
tant clay types are shown in Figure 8.16. Note that fine-grained micas and magnesium-
rich chlorites represent earlier weathering stages of the silicates, and kaolinite and
(ultimately) iron and aluminum oxides the most advanced stages. The smectites (e.g.,
montmorillonite) represent intermediate stages. Different weathering stages may occur
across climatic zones or across horizons within a single profile. As noted in Section 2.1,
silicon tends to be lost as weathering progresses, leaving a lower Si:Al ratio in more
highly weathered soil horizons.
MIXED AND INTERSTRATIFIED LAYERS.In a given soil, it is common to find several silicate
clay minerals in an intimate mixture. In fact, the properties and compositions of
some mineral colloids are intermediate between those of the well-defined minerals
described in Section 8.3. For example, a mixed layer or interstratified clay mineral in
Microcline
Orthoclase
Others
Muscovite
Micas
Primary
chlorite
Soda lime
Others
Hornblende
Augite
Feldspars
Biotite
High in Mg, Ca, Na, Fe High in K
Primary aluminosilicates
Smectite
(montmorillonite) KaoliniteVermiculite
Clay
chlorite
Fine-grained
micas (illite)
Oxides of
Fe and Al
–Mg
–Mg –Si –Si
–Mg
+K
–K
–K
–K
–K
+H2O
–K
+H2O
–Mg
Slow removal of bases
Much Mg in weathering zone
Rapid removal of bases
Rapid removal of bases
Hot wet climates (–Si)
Hot wet climates (–Si)
FIGURE 8.16 General conditions for the formation of the various layer silicate clays and oxides of iron and aluminum. Fine-
grained micas, chlorite, and vermiculite are formed through rather mild weathering of primary aluminosilicate minerals,
whereas kaolinite and oxides of iron and aluminum are products of much more intense weathering. Conditions of interme-
diate weathering intensity encourage the formation of smectite. In each case silicate clay genesis is accompanied by the
removal in solution of such elements as K, Na, Ca, and Mg. Several members of this weathering series may be present in a sin-
gle soil profile, with the less weathered clay in the C horizon and the more weathered clay minerals in the B or A horizons.
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 327
which some layers are more like mica and some more like vermiculite might be called
fine-grained mica-vermiculite.
IRON AND ALUMINUM OXIDES.Iron oxides often are produced by the weathering of iron-
containing primary silicate minerals or by precipitation of iron from soil solutions. Under
aerated weathering conditions, divalent iron (Fe2+) oxidizes rapidly to trivalent iron (Fe3+),
either while still within the structure of primary minerals or after its release into the soil
solution. The Fe3+ forms stable oxides and hydroxides by reacting with oxygen atoms and
water. The yellow-brown colored goethite (FeOOH) is the most stable iron oxide under
most conditions and therefore tends to be the dominant iron oxide in most soils, espe-
cially in temperate, humid regions. The red-colored hematite (Fe2O3) tends to form under
drier, warmer, more oxidized conditions. It can also be inherited from such soil parent
materials as red shales. Other iron oxides precipitate under wet, poorly oxygenated condi-
tions. Interestingly, certain highly weathered soils contain significant amounts of
maghematite, a magnetic iron oxide that forms in surface horizons under the influence
of the heat from brush fires. If a water suspension of such a soil is stirred with a magnetic
stir-bar, this iron oxide can be readily observed as tiny particles clinging to the magnet.
Aluminum oxides, mainly gibbsite [Al(OH)3], are produced by strong weathering
environments in which acid leaching rapidly removes the Si released from the break-
down of primary and secondary silicate minerals. During weathering, hydrogen ions
replace the K+, Mg2+, and other such ions in the crystal, causing the framework to break
down and releasing the silicon and aluminum. The aluminum released by the break-
down of dark-colored, iron-rich rocks such as gabbro and basalt often forms gibbsite
directly. The weathering of light-colored rocks such as granite and gneiss may first
produce 1:1 silicate minerals such as kaolinite or halloysite, which yield gibbsite upon
further weathering. Gibbsite is extremely stable in soils and typically represents the
most advanced stage of weathering in soils.
ALLOPHANE AND IMOGOLITE.Relatively little is known of factors influencing the formation
of allophane and imogolite. While they are commonly associated with materials of vol-
canic origin, they are also formed from igneous rocks and are found in some Spodosols.
Apparently, volcanic ashes release significant quantities of Si(OH)xand Al(OH)xmateri-
als that precipitate as gels in a relatively short period of time. These minerals are gener-
ally poorly crystalline in nature, imogolite being the product of a more advanced state
of weathering than that which produces allophane. Both types of minerals have a pro-
nounced capacity to strongly retain anions as well as to bind with humus, protecting it
from decomposition.
HUMUS.The breakdown and alteration of plant residues by microorganisms and the
concurrent synthesis of new, more stable, organic compounds results in the formation
of the dark-colored colloidal organic material called humus (see Section 12.4 for details).
The various organic structural units associated with the decay and synthesis provide
charged sites for the attraction of both cations and anions.
Distribution of Clays by Geography and Soil Order
The clay of any particular soil is generally made up of a mixture of different colloidal
minerals. In a given soil, the mixture may vary from horizon to horizon, because the kind
of clay that develops depends not only on climatic influences and profile conditions but
also on the nature of the parent material. The situation may be further complicated by
the presence in the parent material itself of clays that were formed under a preceding and
perhaps an entirely different type of climatic regime. Nevertheless, some broad general-
izations are possible.
Table 8.3 shows the dominant clay minerals in different soil orders, descriptions of
which were given in Chapter 3. The well-drained and highly-weathered Oxisols and
Ultisols of warm humid and subhumid tropics tend to be dominated by kaolinite, along
with oxides of iron and aluminum. The smectite, vermiculite, and fine-grained mica
groups are more prominent in Alfisols, Mollisols, and Vertisols, where weathering is less
intense. Where the parent material is high in micas, fine-grained micas such as illite are
apt to be formed. Parent materials that are high in metallic cations (particularly mag-
nesium) or are subject to restricted drainage, which discourages the leaching of these
cations, encourage smectite formation.
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328 Chapter Eight
8.6 SOURCES OF CHARGES ON SOIL COLLOIDS
There are two major sources of charges on soil colloids: (1) hydroxyls and other func-
tional groups on the surfaces of the colloidal particles that by releasing or accepting H+
ions can provide either negative or positive charges, and (2) the charge imbalance
brought about by the isomorphous substitution in some clay crystal structures of one
cation by another of similar size but differing in charge.
All colloids, organic or inorganic, exhibit the surface charges associated with OH-
groups, charges that are largely pH dependent. Most of the charges associated with
humus, 1:1-type clays, the oxides of iron and aluminum, and allophane are of this
type. In the case of the 2:1-type clays, however, these surface charges are comple-
mented by a much larger number of charges emanating from the isomorphous substi-
tution of one cation for another in the octahedral and/or tetrahedral sheets. Since these
charges are not dependent on the pH, they are termed permanent or constant
charges. We will consider these constant charges first.
Constant Charges on Silicate Clays
We noted in Section 8.2 that isomorphous substitution could be the source of both neg-
ative and positive charges. Examples of specific substitutions will now be considered.
NEGATIVE CHARGES.A net negative charge is found in minerals where there has been an
isomorphous substitution of a lower-charged ion (e.g., Mg2+) for a higher-charged ion
(e.g., Al3+). Such substitution commonly occurs in some aluminum-dominated diocta-
hedral sheets. As shown in Figure 8.5 (right), this leaves an unsatisfied negative charge.
The substitution of Mg2+ for Al3+ is an important source of the negative charge on the
smectite, vermiculite, and chlorite clay micelles.
A second example is the substitution of an Al3+ for an Si4+ in the tetrahedral sheet,
which also leaves one unsatisfied negative charge from the tetrahedral oxygen atoms.
Such a substitution is common in several of the important soil silicate clay minerals,
such as the fine-grained micas, vermiculites, and even some smectites.
POSITIVE CHARGES.Isomorphous substitution can also be a source of positive charges if
the substituting cation has a higher charge than the ion for which it substitutes. In a tri-
octahedral sheet, there are three magnesium ions surrounded by oxygen and hydroxy
groups, and the sheet has no charge (review Figure 8.5). However, if an Al3+ ion substi-
tutes for one of the Mg2+ ions, a positive charge results.
Such positive charges are characteristic of the trioctahedral hydroxide sheet in the
interlayer of clay minerals such as chlorites, a charge that is overbalanced by negative
charges in the tetrahedral sheet. Indeed, in several 2:1-type silicate clays, including
chlorites and smectites, substitutions in both the tetrahedral and octahedral sheets can
occur. The net charge in these clays is the balance between the negative and positive
charges. In all 2:1-type silicate clays, however, the net charge is negative since those
TABLE 8.3 Prominent Occurrence of Clay Minerals in Different Soil Orders in the United States
and Typical Locations for These Soils
General Typical Fine-
Soil weathering location Fe, Al grained
orderaintensity in U.S. oxides Kaolinite Smectite mica Vermiculite Chlorite Intergrades
Aridisols Low Dry areas XX XX X X
VertisolsbAlabama, Texas XXX X
Mollisols Kansas, Iowa X XX X X X X
Alfisols Ohio, New York X X X X X X
Spodosols New England X X
Ultisols Southeast XX XXX X X X
Oxisols High Hawaii, Puerto Rico XX XXX
aSee Chapter 3 for soil descriptions.
bBy definition these soils have swelling-type clays, which account for the dominance of smectites.
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 329
substitutions leading to negative charges far outweigh those producing positive charges
(see Figure 8.17).
pH-Dependent Charges
The second source of charges noted on some layer silicate clays (e.g., kaolinite) and on
humus, allophane, and Fe, Al oxides, is dependent on the soil pH and consequently is
termed variable or pH-dependent. Both negative and positive charges come from this
source.
NEGATIVE CHARGES.The pH-dependent charges are associated primarily with hydroxyl
(OH) groups on the surfaces of the inorganic and organic colloids. Broken edges of
mineral colloids also generate pH-dependent charges (see Figure 8.18). The OH groups
or oxygen atoms are attached to iron and/or aluminum in the inorganic colloids (e.g.,
Al—OH) and to the carbon in humus (e.g., —C—OH). Under moderately acid condi-
tions, there is little or no charge on these particles, but as the pH increases, the hydro-
gen dissociates from the colloid OH group, and negative charges result.
(8.2)
As indicated by the arrows, such reactions are reversible. If the pH increases, more
OH-ions are available to force the reactions to the right, and the negative charge on
the particle surfaces increases. If the pH is lowered, OH-ion concentrations are
reduced, the reaction goes back to the left, and the negative charge is reduced.
Another source of increased negative charges as the pH is increased is the removal of
positively charged complex aluminum hydroxy ions [e.g., Al(OH)2+]. At low pH levels,
these ions block negative sites on the silicate clays (e.g., vermiculite) and make them
unavailable for cation exchange. As the pH is raised, the Al(OH)2+ions react with the
OH-ion in the soil solution to form insoluble Al(OH)3, thereby freeing the negatively
charged sites.
(8.3)
Al (OH)2
 Al(OH)2
 OH
Negative charge
site is freed
No charge
Negative charged
site is blocked
Al (OH)2
 Al(OH)3
Colloid  Ca(OH)2Colloid Ca2
H
H
Clay wiped
off (+) wire
FIGURE 8.17 Simple demonstration of the nega-
tively charged nature of clay. Wires connected to the
(-) and (+) terminals of a 9-volt battery are dipped
for a few minutes in a suspension of clayey soil in
water. The wires are then wiped on a piece of paper
(inset), showing that the wire on the (+) terminal has
attracted the clay while the (-) wire has not.
[Adapted from Weil (2005)]
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330 Chapter Eight
POSITIVE CHARGES.Under moderate to extreme acid soil conditions, some silicate clays
and Fe, Al oxides may develop positive charges by protonation—the attachment of H+
ions to the surface OH groups (Figure 8.18, right).
Since a mixture of humus and several inorganic colloids is usually found in soil, it is
not surprising that positive and negative charges may be exhibited at the same time.
In most soils of temperate regions, the negative charges far exceed the positive ones
(Table 8.4). However, in some acid soils high in Fe, Al oxides or allophane, the overall
net charge may be positive. The effect of soil pH on positive and negative charges on
such soils is illustrated in Figure 8.19.
The charge characteristics of selected soil colloids are shown in Table 8.4. Note the
high percentage of constant negative charges in some 2:1-type clays (e.g., smectites
Net surface charge = –3
2–
2–
2–
2–
–1
–1
pH = 7
–1
/2(B)
(A)
(C)
–1
/2
+
++
+
2–2–
3+ 3+
4+ 4+
4+
2–
2–
2–2–
2–2– 2–
Net surface charge = +1
2–
2–
2–
2–
0
pH = 4
+1
/2
+1
/2
++
+
2–2–
3+ 3+
4+ 4+
2–
2–
2–2–
2–2– 2–
+
+
+
+
Net surface charge = –1
2–
2–
2–
2–
–1
pH = 5.5
Crystal
edge
–1
/2
+1
/2
++
+
2–2–
3+ 3+
4+ 4+
2–
2–
2–2–
2–2– 2–
+
+
+
2– 3+
Oxygen Hydrogen Aluminum Silicon
FIGURE 8.18 How pH-dependent charges develop at the broken edge of a kaolinite crystal.
Three sources of net negative surface charge at a high pH are illustrated (left): (A) One (–1) charge
from octahedral oxygen that has lost its H+ion by dissociation (the H broke away from the sur-
face hydroxyl group and escaped into the soil solution). Note that such dissociation can generate
negative charges all along the surface hydroxyl plane, not just at a broken edge. (B) One half (–1⁄2) charge
from each octahedral oxygen that would normally be sharing its electrons with a second alu-
minum. (C) One (–1) charge from a tetrahedral oxygen atom that would normally be balanced by
bonding to another silicon if it were not at the broken edge. The middle and right diagrams show
the effect of acidification (lowering the pH), which increases the activity of H+ions in the soil
solution. At the lowest pH shown (right), all of the edge oxygens have an associated H+ion, giv-
ing rise to a net positive charge on the crystal. These mechanisms of charge generation are simi-
lar to those illustrated for humus in Figure 8.15.
TABLE 8.4 Charge Characteristics of Representative Colloids Showing
Comparative Levels of Permanent (Constant) and pH-Dependent
Negative Charges as Well as pH-Dependent Positive Charges
Negative charge
Total at pH 7, Constant, pH dependent, Positive charge,
Colloid type cmolc/kg % % cmolc/kg
Organic 200 10 90 0
Smectite 100 95 5 0
Vermiculite 150 95 5 0
Fine-grained micas 30 80 20 0
Chlorite 30 80 20 0
Kaolinite 8 5 95 2
Gibbsite (Al) 4 0 100 5
Goethite (Fe) 4 0 100 5
Allophane 30 10 90 15
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 331
and vermiculites). Humus, kaolinite, allophane and Fe, Al oxides have mostly variable
(pH-dependent) negative charges and exhibit modest positive charges at low pH values.
The negative and positive charges on soil colloids are of vital importance to the behav-
ior of soils in nature, especially with regard to the adsorption of oppositely charged ions
from the soil solution. This subject will be taken up next.
8.7 ADSORPTION OF CATIONS AND ANIONS
In soil, the negative and positive surface charges on the colloids attract and hold a com-
plex swarm of cations and anions. Table 8.5 lists some important cations and anions.
The adsorption of these ions by soil colloids greatly affects their biological availability
and mobility, thereby influencing both soil fertility and environmental quality. Note
that the soil solution and colloidal surfaces in most soils are dominated mainly by just
a few of the cations and anions, the others being found in much smaller amounts or
45673
+5
+4
+3
+2
+1
0
–1
–2
–3
–4
–5
Surface charge (cmolc/kg)
Soil pH
Negative charge
(cation exchange
capacity)
Net charge
Point of zero charge
Positive charge
(anion exchange
capacity)
FIGURE 8.19 Relationship between soil pH and positive and neg-
ative charges on an Oxisol surface horizon in Malaysia. The nega-
tive charges (cation exchange capacity) increase and the positive
charges (anion exchange capacity) decrease with increasing soil
pH. The point of zero charge is about pH 4.4. [Redrawn from
Shamshuddin and Ismail (1995)]
TABLE 8.5 Selected Cations and Anions Commonly Adsorbed to Soil Colloids and Important in Plant Nutrition
and Environmental Quality
The listed ions form inner- and/or outer-sphere complexes with soil colloids. Ions marked by an asterisk (*)
are among those that predominate in most soil solutions. Many other ions may be important in certain situations.
Cation Formula Comments Anion Formula Comments
Ammonium NH4+Plant nutrient Arsenate AsO43-Toxic to animals
Aluminum Al3+ etc.aToxic to many plants Borate B(OH)4
-Plant nutrient, can be toxic
Calcium* Ca2+ Plant nutrient Bicarbonate HCO3
-Toxic in high-pH soils
Cadmium Cd2+ Toxic pollutant Carbonate* CO32-Forms weak acid
Cesium Cs+Radioactive contaminant Chromate CrO42-Toxic pollutant
Copper Cu2+ Plant nutrient, toxic pollutant Chloride* Cl-Plant nutrient, toxic in large amounts
Hydrogen* H+Causes acidity Fluoride Fl-Toxic, natural and pollutant
Iron Fe2+ Plant nutrient Hydroxyl* OH-Alkalinity factor
Lead Pb2+ Toxic to animals, plants Nitrate* NO3
-Plant nutrient, pollutant in water
Magnesium* Mg2+ Plant nutrient Molybdate MoO42-Plant nutrient, can be toxic
Manganese Mn2+ Plant nutrient Phosphate HPO42-Plant nutrient, water pollutant
Nickel Ni2+ Plant nutrient, toxic pollutant Selenate SeO42-Animal nutrient and toxic pollutant
Potassium* K+Plant nutrient Selenite SeO32-Animal nutrient and toxic pollutant
Sodium* Na+Used by animals, some plants, Silicate* SiO44-Mineral weathering product, used by
can damage soil plants
Strontium Sr2+ Radioactive contaminant Sulfate* SO42-Plant nutrient
Zinc Zn2+ Plant nutrient, toxic pollutant Sulfide S2-In anaerobic soils, forms acid on oxidation
aImportant aluminum cations include Al3+, AlOH2+, and Al(OH)2+.
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332 Chapter Eight
only in special situations such as contaminated soils. In Figure 8.1 ion adsorption was
illustrated in a simplified manner, showing positive cations held on the negatively
charged surfaces of a soil colloid. Actually, both cations and anions are usually attracted
to the same colloid. In temperate-region soils, anions are commonly adsorbed in much
smaller quantities than cations because these soils generally contain predominately
2:1-type silicate clays on which negative charges predominate. In the tropics, where
soils are more highly weathered, acid, and rich in 1:1 clays and Fe, Al oxides, the
amount of negative charge on the colloids is not so high, and positive charges are more
abundant. Therefore, the adsorption of anions is more prominent in these soils.
Figure 8.20 shows how both cations and anions may be attracted to the same col-
loid if it has both positively and negatively charged sites. This figure also illustrates that
adsorption of ions on colloidal surfaces occurs by the formation of two quite different
general types of colloid-ion complexes referred to as outer-sphere and inner-sphere
complexes.
Outer- and Inner-Sphere Complexes
Remembering that water molecules surround (hydrate) the cations and anions in the
soil solution, we can visualize that in an outer-sphere complex water molecules form a
bridge between the adsorbed ion and the charged colloid surface. Sometimes several
layers of water molecules are involved. Thus, the ion itself never comes close enough to
the colloid surface to form a bond with a specific charged site. Instead, the ion is only
H
H
Hydroxylated
surface of
kaolinite
or gibbsite
(3)
Inner-sphere
complex
(1)
Diffuse ions
(2)
Outer-sphere
complex
Positively
charged site
+
O
2–2–
A1 A1
OO
H H
2–2–
A1 A1
OO
H
O
–– –
A1 A1
H
2–
O
H
2–
A1 A1
O
O
2–
O
H
H
O
P
Ca2+
O
H
2–
O
H
Negatively
charged sites
H
O
H
H
O
H
O
H
O
H
H
O
H
H
H
O
H
H
Mg2+
H
O
H
H
O
H
O
H
O
H
H
O
H
H
H
O
H
H
CI–
H
O
H
H
O
H
O
H
O
H
H
O
H
H
H
O
O
O
H
HO–
P
O
2–
O
H
2–
O
FIGURE 8.20 A diagrammatic representation of the adsorption of ions on a colloid by the for-
mation of outer-sphere and inner-sphere complexes. (1) Water molecules surround diffuse cations
and anions (such as the Mg2+, Cl-, and HPO4
-shown) in the soil solution. (2) In an outer-sphere
complex (such as the adsorbed Ca2+ ion shown), water molecules form a bridge between the
adsorbed cation and the charged colloid surface. (3) In the case of an inner-sphere complex
(such as the adsorbed H2PO4
-anion shown), no water molecules intervene, and the cation or
anion binds directly with the metal atom (aluminum in this case) in the colloid structure. Outer-
sphere complexes typify easily exchangeable ions that satisfy, in a general way, the net charge on
the colloid surface. Inner-sphere complexes, on the other hand, are not easily replaced from the
colloid surface, as they represent strong bonding of specific ions to specific sites on the colloid.
Adsorption on an exposed hydroxylated surface octahedral sheet, such as that in kaolinite
or gibbsite, is shown. In this example, all the charges originate with the dissociation of H+ions
from surface hydroxyl groups. Although not shown in this example, permanent charges from
isomorphous substitution in the interior structure of a colloid could also cause adsorption of
outer-sphere complexes. In other colloids (not shown), charged silica tetrahedral surfaces form
inner- and outer-sphere complexes by similar mechanisms. (Diagram courtesy of R. Weil)
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 333
weakly held by electrostatic attraction, the charge on the oscillating hydrated ion
balancing, in a general way, an excess charge of opposite sign on the colloid surface.
Ions in an outer-sphere complex are therefore easily replaced by other similarly
charged ions.
In contrast, adsorption via formation of an inner-sphere complex does not involve
any intervening water molecules. Therefore, one or more direct bonds are formed
between the adsorbed ion and the atoms in the colloid surface. One example already
discussed is the case of the K+ions that fit so snugly into the spaces between silicon
tetrahedra in a mica crystal (see Figure 8.11). Since there are no intervening water mol-
ecules, the K+ions are directly bonded by sharing electrons with the negatively charged
tetrahedral oxygen atoms. Similarly, strong inner-sphere complexes may be formed by
reactions of Cu2+ or Ni2+ with the oxygen atoms in silica tetrahedra.
Another important example, this time involving an anion, occurs when a
ion is directly bonded by shared electrons with the octahedral aluminum in the colloid
structure (Figure 8.20). Other ions cannot easily replace an ion held in an inner-sphere
complex because this type of adsorption involves relatively strong bonds that are
dependent on the compatible nature of specific ions and specific sites on the colloid.
Figure 8.20 illustrates only two examples of adsorption complexes on one type of
colloid. In other colloids, charged silica tetrahedral surfaces form inner- and outer-
sphere complexes by mechanisms similar to those shown in the figure. Permanent
charges from isomorphous substitution in the interior structure of a colloid (not shown
in Figure 8.20) can also cause adsorption of outer-sphere complexes.
8.8 CATION EXCHANGE REACTIONS
Let us consider the case of an outer-sphere complex between a negatively charged col-
loid surface and a hydrated cation (such as the Ca2+ ion shown in Figure 8.20). Such an
outer-sphere complex is only loosely held together by electrostatic attraction, and the
adsorbed ion remains in constant motion near the colloid surface. There are moments
(microseconds) when the adsorbed cation is located a bit farther than average from the
colloid surface. This moment provides an opportunity for another hydrated cation
from the soil solution (say the Mg2+ ion shown in Figure 8.20) to diffuse into a position
a bit closer to the negative site on the colloid. The instant this occurs, the second ion
would replace the first ion, freeing the formerly adsorbed ion to diffuse out into the soil
solution. In this manner, an exchange of cations between the adsorbed and diffuse
state takes place. As mentioned in Section 8.1, this process is referred to as cation
exchange. If a hydrated anion similarly replaces another hydrated anion at a positively
charged colloid site, the process is called anion exchange. The ions held in outer-
sphere complexes from which they can be replaced by exchange reactions are said to be
exchangeable cations or anions. As a group, all the colloids in a soil, inorganic and
organic, capable of holding exchangeable cations or anions are termed the cation or
anion exchangeable complex.
Principles Governing Cation Exchange Reactions
REVERSIBILITY.We can illustrate the process of cation exchange using a simple reaction in
which a hydrogen ion (perhaps generated by organic matter decomposition—see
Section 9.1) displaces a sodium ion from its adsorbed state on a colloid surface:
(8.4)
The reaction takes place rapidly and, as shown by the double arrows, the reaction is
reversible. It will go to the left if sodium is added to the system. This reversibility is a
fundamental principle of cation exchange.
CHARGE EQUIVALENCE.Another basic principle of cation exchange reactions is that the
exchange is chemically equivalent; that is, it takes place on a charge-for-charge basis.
Therefore, although one H+ion exchanged with one Na+ion in the reaction just shown,
it would require two singly charged H+ions to exchange with or replace one divalent
ColloidColloid Na H
(soil
solution)
H Na
(soil
solution)
H2PO4+
Animation explaining cation
exchange:
http://hintze-online.com/
sos/1997/Articles/Art5/
animat2.dcr
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334 Chapter Eight
Ca2+ ion. If the reaction is reversed, one Ca2+ ion will displace two H+ions. In other
words, two charges from one cation species replace two charges from the other:
(8.5)
Note that by this principle, it would require three Na+ions to replace a single Al3+ ion,
and so on.
RATIO LAW.Consider an exchange reaction between two similar cations, say Ca2+ and
Mg2+. If there are a large number of Ca2+ ions adsorbed on a colloid and some Mg2+ is
added to the soil solution, the added Mg2+ ions will begin displacing the Ca2+ from the
colloid. This will bring more Ca2+ into the soil solution and these Ca2+ ions will, in
turn, displace some of the Mg2+ back off the colloid. Theoretically, these exchanges will
continue back and forth until equilibrium is reached. At this point, there will be no fur-
ther net change in the number of adsorbed Ca2+ and Mg2+ ions (although the
exchanges will continue each balancing the other). The ratio law tells us that, at equi-
librium, the ratio of Ca2+ to Mg2+ on the colloid will be the same as the ratio of Ca2+ to
Mg2+ in the solution and both will be the same as the ratio in the overall system. To
illustrate this concept, assume that 20 Ca2+ ions are initially adsorbed on a soil colloid
and either 5 or 80 Mg2+ ions are added to the system:
(8.6)
(8.7)
If the two exchanging ions are not of the same charge (for example, K+exchanging
with Mg2+), the reaction becomes somewhat more complicated and a modified version
of the ratio law would apply. We will not go into any more detail on this point, except
to note that the reactions just given suggest that in order to completely replace one
element with a second element by cation exchange, an overwhelming amount of the
second ion must be added. This fact must be considered when using displacement to
measure the amounts of exchangeable ions or exchange sites in soils (see Section 8.9).
Up to this point, our discussion of exchange reactions has assumed that both ionic
species (elements) exchanging places take part in the exchange reaction in exactly the
same way. This assumption must be modified to take into account three additional
factors if we are to understand how exchange reactions actually proceed in nature.
ANION EFFECTS ON MASS ACTION.In the reactions just discussed, we have not mentioned
the anions that always accompany cations in solution. We also showed the exchange
reactions as being completely reversible, with an equal chance of proceeding to the
right or the left. In reality, the laws of mass action tell us that an exchange reaction will
be more likely to proceed to the right if the released ion is prevented from reacting in
the reverse direction. This may be accomplished if the released cation on the right side
of the reaction either precipitates, volatilizes, or strongly associates with an anion. In each
case, most of the displaced cations will be removed from solution and so will not be
able to reverse the exchange. To illustrate this concept, consider the displacement of H+
ions on an acid colloid by Ca2+ ions added to the soil solution, either as calcium chlo-
ride or as calcium carbonate:
(8.8)
(8.9)
ColloidColloid  CaCO3
(added) Water (gas)
Ca2 H2O CO2
H
H
ColloidColloid  CaCl2
(added) (dissociated ions)
Ca2 2 H 2 Cl
H
H
ColloidColloid 20 Ca2 80 Mg2
(soil solution) (soil solution)
 64 Mg2 16 Ca2
Ratio: 1 Ca:4 Mg
4 Ca2
16 Mg2
ColloidColloid 20 Ca2 5 Mg2
(soil solution) (soil solution)
 1 Mg2 4Ca
2
Ratio: 4 Ca:1 Mg
16 Ca2
4 Mg2
ColloidColloid Ca2 2 H
(soil solution) (soil solution)
 Ca2
H
H
Detailed tutorial on CEC,
M.J. Eick, VA Tech:
http://www.soils1.cses.vt.edu/
MJE/shockwave/cec_demo/
version1.1/cec.shtml
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 334

THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 335
In the first reaction, CaCl2is added, but relatively little calcium will end up on the col-
loid because the displaced hydrogen ions remain active in the solution and can reverse
the reaction, thus displacing calcium back off the colloid. In the second reaction, where
CaCO3is added, when a hydrogen ion is displaced off the colloid, it combines with an
oxygen atom from the CaCO3to form water. Furthermore, the CO2produced is a gas,
which can volatilize out of the solution and leave the system. The removal of these
products pulls the reaction to the right. Therefore, much more Ca will be adsorbed on
the colloid and much more H displaced if CaCO3, rather than an equivalent amount of
CaCl2, is added to the hydrogen-dominated soil. This principle explains why CaCO3(in
the form of limestone) is effective in neutralizing an acid soil, while calcium chloride is
not (see Section 9.11).
CATION SELECTIVITY.Up to now, we have assumed that both cation species taking part in
the exchange reaction are held with equal tenacity by the colloid and therefore have an
equal chance of displacing each other. In reality, some cations are held much more
tightly than others and so are less likely to be displaced from the colloid. In general, the
higher the charge and the smaller the hydrated radius of the cation, the more strongly
it will adsorb to the colloid. Because exchangeable adsorption involves creation of an
outer-sphere complex, it is the ion’s hydrated radius, not the ionic radius, that affects
the strength of adsorption. For example, Na+is very weakly held because, while it has a
relatively small ionic radius (Table 8.2), it carries a large shell of water, giving it a rela-
tively large hydrated radius of 79 nm (Table 8.6). The order of strength of adsorption for
selected cations is:
Al3+ > Sr2+ > Ca2+ > Mg2+ > Cs+> K+= NH4+> Na+> Li+
The less tightly held cations oscillate farther from the colloid surface and therefore are
the most likely to be displaced into the soil solution and carried away by leaching. This
series therefore explains why the soil colloids are dominated by Al3+ (and other alu-
minum ions) and Ca2+ in humid regions and by Ca2+ in drier regions, even though the
weathering of minerals in many parent materials provides relatively larger amounts of
K+, Mg2+, and Na+(see Section 8.10). The strength of adsorption of the H+ion is diffi-
cult to determine because hydrogen-dominated mineral colloids break down to form
aluminum-saturated colloids.
The relative strengths of adsorption order may be altered on certain colloids whose
properties favor adsorption of particular cations. An important example of such colloidal
“preference” for specific cations is the very high affinity for K+ions (and the similarly
sized and Cs+ions) exhibited by vermiculite and fine-grained micas (Section 8.3),
which attract these ions to inter-tetrahedral spaces exposed at weathered crystal edges.
The influence of different colloids on the adsorption of specific cations impacts the avail-
ability of cations for leaching or plant uptake (see Section 14.15 and Box 8.3). Certain
metals such as copper, mercury, and lead have very high selective affinities for sites on
humus and iron oxide colloids, making most soils quite efficient at removing these
potential pollutants from water leaching through the profile.
NH4+
TABLE 8.6 Some Exchangeable Ions, Their Hydrated Size, and Their Expected Replacement by
NH4+Ions
Among ions of a given charge, the larger the hydrated radius, the more easily it is replaced.
Hydrated ionic Likely replacement of ion initially saturating a
Element Ion radius, (nm)akaolinite clay if cmolcNH4+added = CEC of the soil, (%)b
Lithium Li+103 80
Sodium Na+79 67
Ammonium NH4+54 50
Potassium K+53 49
Rubidium Rb+51 48
Cesium Cs+50 47
Magnesium Mg2+ 108 31
Calcium Ca2+ 96 29
Strontium Sr2+ 96 29
Barium Ba2+ 88 26
aNot to be confused with nonhydrated radii (Table 8.2), hydrated radii are from Evangelou and Phillips (2005).
bBased on empirical data from various sources and assumes no special affinity by kaolinite for any of the listed ions.
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 335

336 Chapter Eight
COMPLEMENTARY CATIONS.In soils, colloids are always surrounded by many different
adsorbed cation species. The likelihood that a given adsorbed cation will be displaced
from a colloid is influenced by how strongly its neighboring cations are adsorbed to the
colloid surface. For example, consider an adsorbed Mg2+ ion. An ion diffusing in from
the soil solution is more likely to displace one of the neighboring ions rather than the
Mg2+ ion, if the neighboring adsorbed ions (sometimes called complementary ions)
are loosely held. If they are tightly held, then the chances are greater that the Mg2+ ion
will be displaced. In Section 8.10, we shall discuss the influence of complementary ions
on the availability of nutrient cations for plant uptake.
A nuclear accident can contaminate the environment with radioactive isotopes such as cesium (137Cs) and stron-
tium (90Sr) that readily move into the food chain because of their chemical similarity to K and Ca, respectively (see
periodic table in Appendix B). If taken into the body, the radiation released by these radioisotopes causes a high
incidence of cancer. Contamination of soil with radioisotopes can lead to their uptake by agricultural crops and
hence the contamination of human food supplies (see Section 18.11). The 1986 meltdown of a nuclear power plant
in Chernobyl spewed about 7 tons of radioactive material, contaminating millions of hectares of land, including
some 4 million hectares in Europe outside the immediate area in Belarus, Ukraine, and Russia. World health
experts estimate that this radiation may cause between 4,000 and 60,000 more people to die of cancer than would
have died if the accident had not occurred. In the event of such widespread soil contamination, safeguarding the
food supply requires the ability to predict which soils are likely to produce contaminated vegetables, cattle fodder,
and dairy products. One obvious factor to consider is the amount of radioactive contamination in a soil. However,
the ratio of 137Cs in the soil to 137Cs in plants varies by four orders of magnitude from one soil to another. In other
words, even if two soils have the same 137Cs concentration, the food produced on one soil may be 10,000 times
more 137Cs-contaminated than that produced on the other soil. This is because 137Cs availability for plant uptake is
largely governed by cation exchange reactions in the soil. If the soil has little capacity to adsorb cations, most of
the 137Cs will remain in the soil solution, where roots will easily take it up. Soils with more clay and humus will
adsorb much of the 137Cs, releasing into the soil solution such previously adsorbed cations as K+and Ca2+.
A study in Belgium found that the risk of 137Cs uptake by plants was closely related to soil clay content in
northern Belgium, but only weakly related in southern Belgium (see Figure 8.21). Differences in the types of clays
present in soils from each region explain this finding. Apparently, in northern Belgium (but not in southern
Belgium), soils have formed mainly in eolian parent materials rich in dioctahedral mica. The clay colloids in the
northern Belgium soils therefore are dominated by dioctahedral fine-grained mica and vermiculite, both miner-
als that exhibit a high selectivity for the K+(as well as for the similarly sized and Cs+) ions that fit so well into
their inter-layer spaces. The 137Cs+ions in the soil solution preferentially displace Ca2+ and other ions from the
colloids. These soils are therefore capable of “intercept-
ing” most of the 137Cs+before plants can take it up.
However, because K+ions are equally attracted to these
clays, repeated additions of potassium fertilizer to these
soils would release much of the adsorbed 137Cs+by cation
exchange, making it available again for plant uptake.
NH4
+
BOX 8.3
CATION EXCHANGE AND FOOD
CONTAMINATION BY NUCLEAR FALLOUT
0 100 200 300 400
Clay content of soil, g/kg
Radiocesium interception index, molc/kg
0
40
80
120
Northern Belgium
Southern Belgium
FIGURE 8.21 Influence of clay content on the ability of soils
to reduce plant uptake of radioactive cesium (Radiocesium
Interception Index). The soils were sampled in pastures used to
feed dairy cows and produce milk in several regions of south-
ern (×) and northern (•) Belgium. In southern Belgium, little
relationship existed between the index and the soil clay con-
tents. However, a strong relationship existed in the northern
regions, where micaceous parent materials are common and
soils are dominated by dioctahedral fine-grained mica and ver-
miculite clays. [Redrawn from data in Waegeneers et al. (1999)]
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 336

THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 337
In soils, cation exchange reactions follow the basic principles just illustrated.
However, because there are many different cations, both adsorbed on the cation
exchange complex and free in the soil solution, the overall reactions are much more
complicated than the simple two-species interactions we have discussed so far. One
additional example will illustrate the combined influence of charge equivalence, ion
selectivity, and complementary ions in cation exchange.
(8.10)
Note that 13 charges from displaced a total of 13 charges from the initially
adsorbed cations. Also note that a much larger proportion of loosely held Na+, K+, and
Mg2+ were exchanged rather than the more tightly held Al3+ and Ca2+.If more of the
complementary ions had been Al3+, then more of the Ca2+ would have been displaced
and released to the soil solution, where it would have been easily leached or taken up
by plants. Although the example merely approximates the ratio law, it does show that
only a portion of the added ions were adsorbed. Leaching loss of the adsorbed
ions would be retarded, but they could still serve as a source of nitrogen for plant
roots by further exchange reactions.
8.9 CATION EXCHANGE CAPACITY
Previous sections have dealt qualitatively with exchange reactions. We now turn to a
consideration of the quantitative cation exchange capacity (CEC). This property is
defined simply as the sum total of the exchangeable cations that a soil can absorb.
Means of Expression
The cation exchange capacity (CEC) is expressed as the number of moles of positive
charge adsorbed per unit mass. In order to be able to deal with whole numbers of a con-
venient size, many publications, including this textbook, report CEC values in centi-
moles of charge per kilogram (cmolc/kg). Some publications still use the older unit, mil-
liequivalents per 100 grams (me/100 g), which gives the same value as cmolc/kg
(1 me/100 g = 1 cmolc/kg). A particular soil may have a CEC of 15 cmolc/kg, indicating
that 1 kg of the soil can hold 15 cmolcof H+ions, for example, and can exchange this
number of charges from H+ions for the same number of charges from any other cation.
This means of expression emphasizes that exchange reactions take place on a charge-
for-charge (not an ion-for-ion) basis. The concept of a mole of charges and its use in
CEC calculations are reviewed in Box 8.4.
Methods of Determining CEC
The CEC is an important soil chemical property that is used for classifying soils in Soil
Taxonomy (e.g., in defining an Oxic, Mollic, or Kandic diagnostic horizon, Section 3.2) and
for assessing their fertility and environmental behavior. Several different standard meth-
ods can be used to determine the CEC of a soil. In general, a concentrated solution of a
particular exchanger cation (for example, Ba2+, , or Sr2+) is used to leach the soil sam-
ple. This provides an overwhelming number of the exchanger cations that can completely
replace all the exchangeable cations initially in the soil. Then, the CEC can be determined
by measuring either the number of exchanger cations adsorbed (Figure 8.22d) or the
amounts of each of the displaced elements originally held on the exchange complex
(usually Ca2+, Al3+, Mg2+, K+, and Na+, Figure 8.22b). See Box 8.5 for detailed calculations.
BUFFER CEC METHODS.The CEC procedure often calls for use of a solution buffered to
maintain a certain pH (usually either pH 7.0 using ammonium as the exchanger cation
or pH 8.2 using barium as the exchanger cation). If the native soil pH is less than the
pH of the buffered solution, then these methods measure not only the cation exchange
sites active at the pH of the particular soil, but also any pH-dependent exchange sites
(see Section 8.6) that would become negatively charged at pH 7.0 or 8.2.
NH4+
NH4
+
NH4
+
NH4
+
Colloid Colloid
 20 NH4
 20 Cl 4 K 3 Na 2 Mg2 Ca2 7 NH4
 20 C1
20 Ca2
5 K
10 Mg2
3 Na
10 Al3
19 Ca2
1 K
8 Mg2
13 NH4

10 Al3
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 337

338 Chapter Eight
One mole of any atom, molecule, or charge is defined as 6.02 1023 (Avogadro’s number) of atoms, molecules,
or charges. Thus, 6.02 1023 negative charges associated with the soil colloidal complex would attract 1 mole of
positive charge from adsorbed cations such as Ca2+, Mg2+, and H+. The number of moles of the positive charge
provided by the adsorbed cations in any soil gives us a measure of the cation exchange capacity (CEC) of that soil.
The CEC of soils commonly varies from 0.03 to 0.5 mole of positive charge per kilogram. To express the CEC
in whole numbers, the charge is usually indicated in centimoles per kilogram (cmolc/kg) of soil. Since there are
100 centimoles in 1 mole, the preceding range of CEC of soils is 3 to 50 cmolc/kg.
CALCULATING MASS FROM MOLES
Using the mole concept, it is easy to relate the mole charges to the mass of ions or compounds involved in
cation or anion exchange. Consider, for example, the exchange that takes place when adsorbed sodium ions in
an alkaline arid-region soil are replaced by hydrogen ions:
If 1 cmolcof adsorbed Na+ions per kilogram of soil were replaced by H+ions in this reaction, how many grams of
Na+ions would be replaced?
Since the Na+ion is singly charged, the mass of Na+needed to provide 1 mole of charge (1 molc) is the gram
atomic weight of sodium, or 23 g (see periodic table in Appendix B). The mass providing 1 centimole of charge
(cmolc) is 1/100 of this amount; thus, the mass of the 1 cmolcNa+replaced is 0.23 g Na+
/kg soil. The 0.23 g Na+
would be replaced by only 0.01 g H, which is the mass of 1 cmolcof this much lighter element.
Another example is the replacement of H+ions when hydrated lime [Ca(OH)2] is added to an acid soil. This
time assume that 2 cmolcH+/kg soil is replaced by the Ca(OH)2, which reacts with the acid soil as follows:
Since the Ca2+ ion in each molecule of Ca(OH)2has two positive charges, the mass of Ca(OH)2needed to
replace 1 mole of charge from the H+ions is only one-half of the gram molecular weight of this compound, or
74/2 = 37 g. A comparable figure for 1 centimole is 37/100, or 0.37 grams. The mass of Ca(OH)2needed to replace
2 cmolcH+/kg soil is:
2 cmolcCa(OH)2/kg ×0.37 g Ca(OH)2/cmolc= 0.74 g Ca(OH)2/kg soil
The 0.74 g Ca(OH)2/kg soil can be converted to the amount of Ca(OH)2needed to replace 2 cmolcH+/kg
from the surface 15 cm of 1 ha of field soil, remembering from Chapter 4 (Section 4.7, footnote 11) that this
depth of soil typically weighs 2 million kg/ha.
0.74 g/kg ×2×106kg = 1.48 ×106g; 1.48 ×103kg; or 1.48 Mg
CHARGE AND CHEMICAL EQUIVALENCY
In each preceding example, the number of charges provided by the replacing ion is equivalent to the number
associated with the ion being replaced. Thus, 1 mole of negative charges attracts 1 mole of positive charges
whether the charges come from H+, K+, Na+, , Ca2+, Mg2+, Al3+, or any other cation. Keep in mind, however,
that only one-half the atomic weights of divalent cations, such as Ca2+ or Mg2+, and only one-third the atomic
weight of trivalent Al3+ are needed to provide 1 mole of charge. This chemical equivalency principle applies to
both cation and anion exchange.
NH4
+
Colloid  Ca(OH)2Colloid Ca2 2H2O
H
H
Colloid Na HColloid H Na
BOX 8.4
CHEMICAL EXPRESSION OF CATION EXCHANGE
EFFECTIVE CEC (UNBUFFERED). Alternatively, the CEC procedure may use unbuffered solu-
tions to allow the exchange to take place at the actual pH of the soil. The buffered
methods ( at pH 7.0 or Ba2+ at pH 8.2) measure the potential or maximum cation
exchange capacity of a soil. The unbuffered method measures only the effective cation
NH4+
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 338

THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 339
(a)
Mg2+ H+
Al3+
H+
(b) (c)
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH
01
01
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH4
+
NH4
+NH4
+
NH4
+
NH4
+NH4
+
NH4
+
NH4
+NH4
+
NH4
+
NH4
+NH4
+
K+K+K+K+
K+K+K+K+
K+K+K
++
++K+
K+K+K+K+
K+K+K+K+
K+K+K+K+
(d)
Ca
Ca
2+
2+
Mg
Mg
2+
2+
H
+Al
Al
3+
3+ H
+
K
+Ca
Ca
2+
2+
Ca2+ Mg2+
H+Al3+ H+
K+Ca2+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH
NH4+
NH4
+
NH4
+NH4
+
NH4
+
NH4
+NH4
+
NH4
+
NH4
+NH4
+
NH4+
NH4
+
Solution
Soil
Porous
filter
plate
NH4+
solution
K+
solution
Ca2+
K+
Ca2+
NH4
+
FIGURE 8.22 Illustration of a method for determining the cation exchange capacity of soils.
(a) A given mass of soil containing a variety of exchangeable cations is leached with an
ammonium (NH4+) salt solution. (b) The NH4+ions replace the other adsorbed cations, which
are leached into the container below. (c) After the excess NH4+salt solution is removed with
an organic solvent, such as alcohol, a K+salt solution is used to replace and leach the
adsorbed NH4
+ions. (d) The amount of NH4+released and washed into the lower container
can be determined, thereby measuring the chemical equivalent of the cation exchange capac-
ity (i.e., the negative charge on the soil colloids). (Diagram courtesy of R. Weil)
Most procedures for measuring soil CEC use cation exchange reactions similar to those illustrated in Figure
8.22. The various cations initially adsorbed by the soil colloids are replaced by exposing the soil sample to a salt
solution containing an overwhelming number of cations of a single element. The element chosen is usually one
not found in large quantities on the soil exchange complex (e.g., , Ba2+, or Sr2+). Examine Figure 8.22 and
assume that 100 g (0.1 kg) of dry soil was placed in the funnel and that 400 mL (0.4 L) of solution was added at
each step. There are methods by which one can estimate the CEC of the soil.
METHOD 1: MEASURE ALL THE CATIONS ORIGINALLY HELD ON THE EXCHANGE COMPLEX
After leaching the soil with 0.4 L of solution, all the exchangeable cations shown in the soil sample in
Figure 8.22awere displaced off the colloids and washed into the beaker (Figure 8.22b) along with the excess
ions. The solution in this beaker (b) was analyzed for Ca, Mg, K, Al, and H with the following results: 200
mg/L Ca2+, 60 mg/L Mg2+, 97.5 mg/L K+, 5 mg/L H+, and 67.5 mg/L Al3+. Because only 0.4 L of solution was collected
from the soil sample and the soil sample weighed only 0.1 kg, these results can be multiplied by 0.4 and 10 to
give the amounts of each ion collected in mg/kg soil. As an example we can show the calculation for Ca2+ as:
200 mg Ca2+
L*0.4 L
sample *10 samples
kg soil =800 mg Ca2+
kg soil
NH4
+
NH4
+
NH4
+
BOX 8.5
CALCULATING SOIL CEC FROM LAB DATA
(continued )
M08_BRAD9383_14_SE_C08.QXD 6/5/08 12:23 PM Page 339

340 Chapter Eight
BOX 8.5 (Cont.)
CALCULATING SOIL CEC FROM LAB DATA
Similar calculations show that the concentration of exchangeable cations in the soil were: 800 mg Ca2+/kg, 240 mg
Mg2+/kg, 390 mg K+/kg, 20 mg H+/kg, and 270 mg Al3+/kg. The CEC is normally expressed as the cmol of charge per
kg of soil (cmolc/kg). Since the atomic weight of an element is defined as the grams per mole of that element, we
now turn to the periodic table in Appendix B for the atomic weights (g/mol) needed to convert our mg/kg con-
centrations into mol/kg for each cation species measured. We divide the mg/kg soil (from above) by 1000 to give
g/kg and then divide by the atomic weight to give mol/kg soil. We then multiply the mol by 100 to give the cmolc/kg
soil (see Box 8.4).
For example, for Ca2+ the atomic weight is approximately 40 g/mol, so we calculate the cmol of exchangeable
Ca2+ in 1 kg of our soil as follows:
Repeating this calculation provides the following results: 2 cmol Ca2+/kg, 1 cmol Mg2+/kg, 1 cmol K+/kg, 2 cmol
H+/kg, and 1 cmol Al3+/kg. We now must multiply the cmol/kg for each element by the valence of the ion to con-
vert to the cmol of charge (cmolc/ kg) from that element. Using Ca2+ again as an example:
Repeating this calculation provides the following results: 4 cmolcCa2+/kg, 2 cmolcMg2+/kg, 1 cmolcK+/kg, 2
cmolcH+/kg, and 3 cmolcAl3+/kg. Assuming the five elements measured account for nearly all the exchangeable
cations, the sum of their charges ( ) equals the CEC of our soil: 12 cmolc/kg.
METHOD 2: MEASURE ALL THE IONS IN THE FINAL LEACHATE
Assume that after washing out and discarding all excess (nonexchangeable) ions, the remaining exchange-
able ions were replaced by K+ions and washed into beaker d(Figure 8.22). Therefore the number of
charges from ions in beaker dis equal to the CEC of the soil. Note that by this method, the lab needs to
determine only one element. Assume the concentration in beaker dto be 540 mg . As in method 1
(above), because only 0.4 L of solution was collected from the soil sample and the soil sample weighed only 0.1
kg, these results can be calculated as follows to give the amount of ions collected in mg/kg soil:
The periodic table in Appendix B provides the atomic weights needed to convert our mg concentration
into mol . We divide the mg/kg (from above) by 1000 to give g/kg. Using atomic weights from the periodic
table (Appendix B) we calculate that 1 mol of = 18 g [14g/mol N + 4(1 g/mol H) = 18 g/mol ]. We then
divide the g/kg by the 18 g/mol to give the moles of . We then multiply the moles by 100 to give the
cmolc/ kg (see Box 8.4). We can therefore calculate the cmol of exchangeable in 1 kg of our soil as follows:
Since each ion carries only 1 charge, the cmol of charge from (cmolc/kg) is the same as the cmol
Therefore the CEC of our soil = 12 cmolc/kg.[If you happened to count the ions in Figure 8.22, you may have
noticed that the concentrations chosen here are those that would apply if each ion shown actually represented
1 cmol of ions.]
12 cmol NH4
+
kg soil *
1 cmolc from NH4
+
cmol NH4
+=
12 cmolc from NH4
+
kg soil
NH4
+/kg:
NH4
+
NH4
+
2160 mg NH4
+
kg soil *
1 mol NH4
+
18 g *1 g
1000 mg *100 cmol
mol =
12 cmol NH4
+
kg soil
NH4
+
NH4
+
NH4
+
NH4
+
NH4
+
NH4
+/kg
NH4
+/kg
540 mg NH4
+
L
*0.4 L
sample
*10 samples
kg soil =
2160 mg NH4
+
kg soil
NH4
+
NH4
+/LNH4
+
NH4
+
NH4
+
NH4
+
NH4
+
4+2+1+2+3=12
2 cmol Ca2+
kg soil *
2 cmolc from Ca2+
cmol Ca2+=
4 cmolc from Ca2+
kg soil
800 mg Ca2+
kg soil *1 g
1000 mg *1 mol Ca2+
40 g *100 cmol
mol =2 cmol Ca2+
kg soil
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 341
exchange capacity (ECEC), which can hold exchangeable cations at the pH of the soil
as sampled. As the different methods may yield significantly different values of CEC, it
is important that the method used be known when comparing soils based on their
CEC. This is especially significant if the soil pH is much below the buffer pH used.
Cation Exchange Capacities of Soils
The cation exchange capacity (CEC) of a given soil horizon is determined by the rela-
tive amounts of different colloids in that soil and by the CEC of each of these colloids.
Figure 8.23 illustrates the common range in CEC among different soils and other
organic and inorganic exchange materials. Note that sandy soils, which are generally
low in all colloidal material, have low CECs compared to those exhibited by silt loams
and clay loams. Also note the very high CECs associated with humus compared to
those exhibited by the inorganic clays, especially kaolinite and Fe, Al oxides. The CEC
coming from humus generally plays a very prominent role, and sometimes a dominant
one, in cation exchange reactions in A horizons. For example, in a clayey Ultisol (pH =
5.5) containing 2.5% humus and 30% kaolinite, about 75% of the CEC is associated
with humus. Figure 8.24 illustrates the contribution of organic matter to the CEC of
various forested soils and how that contribution increases at higher soil pH levels.
Using the CEC range from Figure 8.23 it is possible to estimate the CEC of a soil if
the quantities of the different soil colloids in the soil are known (see Box 8.6).
Data in Table 8.7 show the average CEC values for seven different soil orders. Note
the very high CEC for the Histosols, verifying the high CEC of the organic colloids. The
Vertisols, which are very high in swelling-type clays (mostly smectite), had the highest
average CEC of the mineral soils. Next came the Aridisols and Mollisols, which are also
commonly high in 2:1-type clays. The Ultisols, whose clays are dominantly kaolinite
and hydrous oxides of iron and aluminum, had relatively low CEC values. Despite large
variations in soil organic matter and texture, these data appear to reflect the quantities
and kinds of soil colloids found in the soils.
Soil
humus
Finished
compost Vertisols
Loamy sands
Sandy loams
Fe, Al
oxides
Silt loams
Clay
loams
Fine-grained
micas and
chlorites
Vermiculites
Smectites
Histosols
250
225
200
175
150
125
100
75
50
25
0
Cation exchange capacity (cmolc/kg exchanger)
Kaolinite
FIGURE 8.23 Ranges in the cation exchange
capacities (at pH 7) that are typical of a variety of
soils and soil materials. The high CEC of humus
shows why this colloid plays such a prominent
role in most soils and especially those high in
kaolinite and Fe, Al oxides, and clays that have
low CECs. (Diagram courtesy of R. Weil)
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342 Chapter Eight
Organic C, g/kg
010
40
30 pH = 8.2
slope = 0.50
pH = 5.0–5.5
slope = 0.26
pH = 4.0–4.5
slope = 0.13
20
10
0
20 30 40 50 60 70 80
Cation exchange capacity, cmolc /kg
FIGURE 8.24 Soil organic carbon (SOC) contributes
markedly to the cation exchange capacity (CEC) of
soils and more so at higher pH levels. In this graph,
the sloping lines indicate the relationship between
increasing SOC and CEC. The greater slopes for the
higher pH soils indicate that the CEC of humus
increases with increased pH. At pH 4.0 to 4.5, every 1 g
increase in organic C contributed to the CEC an addi-
tional 0.13 cmolc/kg soil. At the pH 5.0 to 5.5 level,
this contribution was twice as great (0.26 cmolc) and at
pH 8.2 the contribution was about four times as great
(0.50 cmolc). At each pH level the data represent
soils with similar clay contents, but with SOC con-
tents that varied because of differences in soil depth or
land use. [Data sources: squares and filled triangles,
Krishnaswamy and Richter (2002); circles, Bayer et al.
(2001); open triangles, Sullivan et al. (2006)]
Data on clay mineralogy and cation exchange capacity are rather time-consuming to obtain and not always avail-
able. Fortunately, one can often estimate one of these types of data from the other, assuming that data on soil
pH, clay content, and organic matter (OM) level are available.
1. Example of Estimating CEC from Data on Mineralogy
Assume you know that a cultivated Mollisol in Iowa contains 20% clay and 4% organic matter (OM) and its
pH = 7.0. The dominant clays in Mollisols are likely 2:1 types such as vermiculite and smectite (see Table
8.3). We estimate the average CEC of the clays of these types to be about 100 cmolc/kg clay (Tables 8.1 and
8.4). At pH 7.0 the CEC of OM is about 200 cmolc/kg (Table 8.4). Since 1 kg of this soil has 0.20 kg (20%) of
clay and 0.04 kg (4%) of OM, we can calculate the CEC associated with each of these sources.
From the clays in this Mollisol:
From the OM in this Mollisol:
The total CEC of this Mollisol:
2. Example of Estimating the Clay Mineralogy from Information on CEC
Assume you know that a soil contains 60% clay and 4% organic matter and the pH = 4.2. You also know the
CEC is 5.8 cmolc/kg. You want to estimate the types of clays present. At pH 4.2 the CEC of the organic mat-
ter would be comparatively low, about 100 cmolc/kg (Figure 8.21). Therefore we estimate:
The remaining portion of the CEC contributed by the clay can be estimated as:
Since this 1.8 cmolc/kg soil is provided by 0.60 kg of clay (60% of 1 kg soil), we can estimate:
From Tables 8.1 and 8.4 we see that the Fe and Al oxides at pH 7 have a CEC of about 4 cmolc/kg clay. We
know (from Section 8.6) their CEC would be lower at pH 4. Likewise, kaolinite has a CEC of about 8
cmolc/kg clay at pH 7 and perhaps only about 4 cmolc/kg clay at pH 4. The CEC values for the other types
of clay listed in Table 8.4 are far higher. Therefore, it is reasonable to conclude that the clays in this Oxisol
consist mainly of Fe and Al oxides and kaolinite.
CEC of the pure clay =1.8 cmolc>kg soil *1 kg soil>0.60 kg clay =3 cmolc>kg clay
CEC from the clay in 1 kg soil =5.8 cmolc-4.0 cmolc=1.8 cmolc
CEC from OM in 1 kg soil =0.04 kg OM *100 cmolc>kg OM =4.0 cmolc
20 + 8 = 28 cmolc>kg soil
0.04 kg *200 cmolc>kg =8 cmolc
0.2 kg *100 cmolc>kg =20 cmolc
BOX 8.6
ESTIMATING CEC AND CLAY MINERALOGY
pH and Cation Exchange Capacity
In previous sections it was pointed out that the cation exchange capacity of most soils
increases with pH. At very low pH values, the cation exchange capacity is also generally
low (Figure 8.25). Under these conditions, only the permanent charges of the 2:1-type
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 343
clays (see Section 8.8) and a small portion of the pH-dependent charges of organic col-
loids, allophane, and some 1:1-type clays hold exchangeable ions. As the pH is raised,
the negative charges on some 1:1-type silicate clays, allophane, humus, and even Fe, Al
oxides increases, thereby increasing the cation exchange capacity. As noted earlier, to
obtain a measure of this maximum retentive capacity, the CEC is commonly deter-
mined at a pH of 7.0 or 8.2. At neutral or slightly alkaline pH, the CEC reflects most of
those pH-dependent charges as well as the permanent ones.
8.10 EXCHANGEABLE CATIONS IN FIELD SOILS
The specific exchangeable cations associated with soil colloids differ from one climatic
region to another—Ca2+, Al3+, complex aluminum hydroxy ions, and H+being most
prominent in humid regions, and Ca2+, Mg2+, and Na+dominating in low-rainfall areas
(Table 8.8). The cations that dominate the exchange complex have a marked influence
on soil properties.
In a given soil, the proportion of the cation exchange capacity satisfied by a partic-
ular cation is termed the saturation percentage for that cation. Thus, if 50% of the CEC
TABLE 8.7 The Average Cation Exchange Capacities (CEC) and pH
Values of More Than 3000 Surface Soil Samples Representing Seven
Different Soil Orders
Organic colloids give Histosols a very high CEC. Compare to data in
Tables 8.3 and 8.4 to see the relationship between the average CEC
and the main types of colloids in the other soil orders.
Soil order CEC, cmolc/kg pH
Ultisols 3.5 5.60
Alfisols 9.0 6.00
Spodosols 9.3 4.93
Aridisols 15.2 7.26
Mollisols 18.7 6.51
Vertisols 35.6 6.72
Histosols 128.0 5.50
From Holmgren et al. (1993).
Soil pH
Kaolinite clay
Permanent
charge
Smectite
clay
Effective CEC (cmolc/kg)
pH-dependent
charge
Humus
(organic colloid)
4.0
240
220
200
180
160
140
120
100
80
60
40
20
0
20
5.5 7.0 8.5
FIGURE 8.25 Influence of pH on representative
cation exchange capacities of two clay minerals and
humus. Below pH 6.0 the charge for smectite has a
fairly constant charge that is due mainly to isomor-
phic substitution and is considered permanent. Above
pH 6.0 the charge on smectite increases somewhat
with pH (shaded area) because of the ionization of
hydrogen from the exposed hydroxyl groups at crys-
tal edges. In contrast, the charges on kaolinite and
humus are all variable, increasing with increasing
pH. Humus carries a far greater number of charges
than kaolinite. At low pH kaolinite carries a net neg-
ative CEC because its positive charges outnumber
the negative ones.
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344 Chapter Eight
TABLE 8.8 Cation Exchange Properties Typical for Unamended Clay Loam Surface Soils in Different Climatic Regions
Note that soils with coarser textures would have less clay and organic matter and
therefore lower amounts of exchangeable cations and lower CEC values.
Warm, humid Cool, humid Semiarid Arid
region region region region
Property (Ultisols)a(Alfisols) (Ustolls) (Natrargids)b
Exchangeable H+and Al3+, cmolc/kg (% of CEC) 7.5 (75%) 5 (28%) 0 (0%) 0 (0%)
Exchangeable Ca2+, cmolc/kg (% of CEC) 2.0 (20%) 9 (50%) 17 (65%) 13 (50%)
Exchangeable Mg2+, cmolc/kg (% of CEC) 0.4 (4%) 3 (17%) 6 (23%) 5 (19%)
Exchangeable K+, cmolc/kg (% of CEC) 0.1 (1%) 1 (5%) 2 (8%) 3 (12%)
Exchangeable Na+, cmolc/kg (% of CEC) Tr 0.02 (0.1%) 1 (4%) 5 (19%)
Cation exchange capacity (CEC)c, cmolc/kg 10 18 26 26
Probable pH 4.5–5.0 5.0–5.5 7.0–8.0 8–10
Nonacid cations (% of CEC)d25% 68% 100% 100%
aSee Chapter 3 for explanation of soil group names.
bNatrargids are Aridisols with natric horizons. They are sodic soils, high in exchangeable sodium, as explained in Section 10.5.
cThe sum of all the exchangeable cations measured at the pH of the soil. This is termed the effective CEC or ECEC (see Section 8.9).
dTraditionally referred to as “base” saturation.
is satisfied by Ca2+ ions, the exchange complex is said to have a calcium saturation
percentage of 50.
This terminology is especially useful in identifying the relative proportions of
sources of acidity and alkalinity in the soil solution. Thus, the percentage saturation
with Al3+ and H+ions gives an indication of the acid conditions, while increases in the
percentage nonacid cation saturation (sometimes referred to as the base saturation
percentage3) indicate the tendency toward neutrality and alkalinity. These relation-
ships will be discussed further in Chapter 9.
Cation Saturation and Nutrient Availability
Exchangeable cations generally are available to both higher plants and microorgan-
isms. By cation exchange, hydrogen ions from the root hairs and microorganisms
replace nutrient cations from the exchange complex. The nutrient cations are forced
into the soil solution, where they can be assimilated by the adsorptive surfaces of roots
and soil organisms, or they may be removed by drainage water. Cation exchange reac-
tions affecting the mobility of organic and inorganic pollutants in soils will be dis-
cussed in Section 8.12. Here we focus on the plant nutrition aspects.
The percentage saturation of essential nutrient cations such as calcium and potas-
sium greatly influences the uptake of these elements by growing plants. For example, if
the percentage calcium saturation of a soil is high, the displacement of this cation is
comparatively easy and rapid. Thus, 6 cmol/kg of exchangeable calcium in a soil whose
exchange capacity is 8 cmol/kg (75% calcium saturation) probably would mean ready
availability, but 6 cmol/kg when the total exchange capacity of a soil is 30 cmol/kg
(20% calcium saturation) would produce lower availability. This is one reason that, for
calcium-loving plants such as alfalfa, the calcium saturation of at least part of the soil
should approach 80 to 85%.
Influence of Complementary Cations
A second factor influencing plant uptake of a given cation is the effect of the comple-
mentary ions held on the colloids. As was discussed in Section 8.8, the strength of
adsorption of common cations is in the following order for most colloids:
Al3+ > Sr2+ > Ca2+ > Mg2+ > Cs+> K+= NH4+> Na+> Li+
3Technically speaking, nonacid cations such as Ca2+, Mg2+, K+, and Na+are not bases. When adsorbed
by soil colloids in the place of H+ions, however, they reduce acidity and increase the soil pH. For that
reason, they are traditionally referred to as bases and the portion of the CEC that they satisfy is often
termed base saturation percentage.
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 345
Consider, for example, the relatively loosely held ion, K+. If the complementary ions
surrounding a K+ion are held tightly (that is, they oscillate very close to the colloid
surface), then a H+ion from a root is less likely to “find” a complementary ion and
more likely, instead, to “bump into” and replace a K+ion (see Figure 8.26). If, on the
other hand, the complementary ions are loosely held (oscillating quite far from the
colloid surface), then the H+is more likely to “bump into” and replace one of the com-
plementary ions and less likely to “find” the K+. Consequently, K+is more likely to be
replaced off the colloid if the complementary ions are mainly tightly held Al3+ or H+
(as in acid soils) than if they are mainly Mg2+ and Na+(as in neutral to alkaline soils).
This is why, at a given level of K+saturation, K+is more readily available for both
plant uptake and for leaching in acid soils than in neutral to alkaline soils (see also
Section 14.13).
There are also some nutrient antagonisms that in certain soils cause inhibition of
uptake of some cations by plants. For instance, potassium uptake by plants is limited by
high levels of calcium in some soils. Likewise, high potassium levels are known to limit
the uptake of magnesium even when significant quantities of magnesium are present
in the soil.
Effect of Type of Colloid
Differences exist in the tenacity with which several types of colloidal micelles hold spe-
cific cations and in the ease with which they exchange cations. At a given percentage
base saturation, smectites—which have a high charge density per unit of colloid surface—
hold calcium much more strongly than does kaolinite (low charge density). As a result,
smectite clays must be raised to about 70% base saturation before calcium will exchange
easily and rapidly enough to satisfy most plants. In contrast, a kaolinite clay exchanges
calcium much more readily, serving as a satisfactory source of this constituent at a much
lower percentage base saturation. The need to add limestone to the two soils will be
somewhat different, partly because of this factor.
8.11 ANION EXCHANGE
Anions are held by soil colloids in two major ways. First, they are held by anion adsorp-
tion mechanisms similar to those responsible for cation adsorption. Second, they may
actually react with surface oxides or hydroxides, forming more definitive inner-sphere
complexes. We shall consider anion adsorption first.
The basic principles of anion exchange are similar to those of cation exchange,
except that the charges on the colloids are positive and the exchange is among nega-
tively charged anions. The positive charges associated with the surfaces of kaolinite,
HH
H
Loosely held complementary ions
with large oscillation zones
H
Na
K
Na
Na
K
Tightly held complementary ions
with small oscillation zones
A1 A1 A1 A1 A1
Root exuding H ions to exchange
with cations on the colloid
Colloid Colloid
FIGURE 8.26 Effect of complementary ions on
the availability of a particular exchangeable nutri-
ent cation. The half spheres represent the zones in
which the ion oscillates, the more loosely held
ions moving within larger zones of oscillation. For
simplicity, the water molecules that hydrate each
ion are not shown. (Left) H+ions from the root are
more likely to encounter and exchange with
loosely held Na+ions rather than the more tightly
held K+ion. (Right) The likelihood that H+ions
from the root will encounter and exchange with a
K+ion is increased by the inaccessibility of the
neighboring tightly held Al3+ ions. The K+ion on
the right colloid is comparatively more vulnerable
to being replaced and sent into the soil solution
and is therefore more available for plant uptake or
leaching than the K+ion on the left colloid.
(Diagram courtesy of R. Weil)
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346 Chapter Eight
iron and aluminum oxides, and allophane attract anions such as and . A
simple example of an anion exchange reaction is as follows:
(8.11)
Just as in cation exchange, equivalent quantities of NO3
-and Cl-are exchanged; the
reaction can be reversed; and plant nutrients so released can be absorbed by plants.
In contrast to cation exchange capacities, anion exchange capacities of soils gener-
ally decrease with increasing pH. Figure 8.27 illustrates this fact for an Ultisol in
Georgia. In some very acid tropical soils that are high in kaolinite and iron and alu-
minum oxides, the anion exchange capacity may actually exceed the cation exchange
capacity.
Anion exchange is very important in making anions available for plant growth
while at the same time retarding the leaching of such anions from the soil. For exam-
ple, anion exchange restricts the loss of sulfates from subsoils in the southern United
States (see Figure 8.27 and Section 13.21). Even the leaching of nitrate may be retarded
by anion exchange in the subsoil of certain highly weathered soils of the humid trop-
ics. Similarly, the downward movement into groundwater of some charged organic pol-
lutants found in organic wastes can be retarded by such anion and/or cation exchange
reactions.
Inner-Sphere Complexes
Some anions, such as phosphates, arsenates, molybdates, and sulfates, can react with
particle surfaces, forming inner-sphere complexes (see Figure 8.20). For example, the
ion may react with the protonated hydroxyl group rather than remain as an
easily exchanged anion.
(8.12)
Al OH2
 H2PO4

(soil solid) (soil solid)(soil solution) (soil solution)
Al H2PO4 H2O
H2PO4
-
Colloid
(positively charged
soil solid)
(positively charged
soil solid)
(soil solution) (soil solution)
NO3
 ClColloid Cl NO3

NO3
-
SO42-
5674
0
4
3
2
1
Cation/anion exchange capacity (cmolc/kg)
Sulfate in leachate from soil (mmolc/L)
Soil pH Pore volume
123450
12
10
8
6
4
2
0
Soil pH = 6.56
Cation exchange
capacity
Anion exchange
capacity
Soil pH = 4.26
FIGURE 8.27 (Left) Effect of increasing the pH of subsoil material from an Ultisol from Georgia on the cation and anion exchange
capacities. Note the significant decrease in anion exchange capacity associated with the increased soil pH. When a column of the low-
pH material (pH 4.26) was leached with (right), little sulfate was removed from the soil. In contrast, similar leaching of a
column of the soil with the highest pH (6.56), where the anion exchange capacity had been reduced by half, resulted in anion exchange
of ions for ions and significant leaching of sulfate from the soil. The importance of anion adsorption in retarding move-
ment of specific anions or other negatively charged substances is illustrated. [Data from Bellini et al. (1996)]
SO42-
NO -
3
Ca1NO3)2
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 347
This reaction actually reduces the net positive charge on the soil colloid. Also, the
is held very tightly by the soil solids and is not readily available for plant
uptake.
Anion adsorption and exchange reactions regulate the mobility and availability of
many important ions. Together with cation exchange they largely determine the ability
of soils to hold nutrients in a form that is accessible to plants and to retard movement
of pollutants in the environment.
Weathering and CEC/AEC Levels
The range of CEC levels of different clay minerals shown in Figure 8.28 shows that clays
developed under mild weathering conditions (e.g., smectites and vermiculites) have
much higher CEC levels than those developed under more extreme weathering pres-
sures. Also shown are the AEC levels that, in turn, tend to be much higher in clays
developed under strong weathering conditions (e.g., kaolinite) than in those found
under more mild weathering. This generalized figure is helpful in obtaining a first
approximation of CEC and AEC levels in soils of different climatic regions. It must be
used with caution, however, since some soils high in 2:1-type clays are found in areas
currently undergoing intensive weathering. The nature of the parent material and the
time allowed for the weathering to occur also influence the clay types present and the
CEC/AEC relations.
8.12 SORPTION OF PESTICIDES AND GROUNDWATER CONTAMINATION
Soil colloids help control the movement of pesticides and other organic compounds
into groundwater. The retention of these chemicals by soil colloids can prevent their
downward movement through the soil or can delay that movement until the com-
pounds are broken down by soil microbes.
By accepting or releasing protons (H+ions), groups such as —OH, —NH2, and —COOH
in the chemical structure of some organic compounds provide positive or negative charges
that stimulate anion or cation exchange reactions. Other organic compounds participate
in inner-sphere complexation and adsorption reactions just as do the inorganic ions we
have discussed. However, it is more common for organic colloids to be absorbed within
the soil organic colloids by a process termed partitioning. The soil organic colloids tend
to act as a solvent for the applied chemicals, thereby partitioning their concentrations
between those held on the soil colloids and those left in the soil solution.
Since we seldom know for certain the exact involvement of the adsorption, com-
plexation, or partitioning processes, we use the general term sorption to describe the
H2PO4
-
Intermediate StrongMild
0
+10
+20
–40
–30
–20
–10
Charges on clay colloids (cmolc/kg)
Weathering intensity
Cation
exchange capacity
Mostly 1:
1-
type clays
Mostly Fe,
Al oxide clays
Mostly 2:
1-
type clays
Anion
exchange capacity
FIGURE 8.28 The effect of weathering intensity on the
charges on clay minerals and, in turn, on their cation and
anion exchange capacities (CECs and AECs). Note the high
CEC and very low AEC associated with mild weathering,
which has encouraged the formation of 2:1-type clays such
as fine-grained micas, vermiculites, and smectites. More
intense weathering destroys the 2:1-type clays and leads to
the formation of first kaolinite and then oxides of Fe and
Al. These have much lower CECs and considerably higher
AECs. Such changes in clay dominance account for the
curves shown.
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348 Chapter Eight
retention by soils of these organic compounds. Nonionic organic compounds are
hydrophobic, being repelled by water. As a result, moist clays contribute little to parti-
tioning since their adsorbed water molecules prevent the movement of the nonionic
organic chemicals into or around the clay particles. The hydrated metal cations (e.g.,
Ca2+) that are adsorbed on the surface of smectites can be replaced with large organic
cations, giving rise to what are termed organoclays. Such clay surfaces are more
friendly toward applied organic compounds, making it possible for the clay to partici-
pate in partitioning. Environmental scientists use this phenomenon by making smec-
tite organoclays that can remove organic contaminants from wastewaters and from
contaminated groundwaters (see Section 18.5).
Distribution Coefficients
The tendency of a pesticide or other organic compound to leach into the groundwater
is determined by the solubility of the compound and by the ratio of the amount of
chemical sorbed by the soil to that remaining in solution. This ratio is known as the
soil distribution coefficient Kd.
(8.13)
The Kdtherefore is typically expressed in units of L/kg. Researchers have found that the
Kdfor a given compound may vary widely depending on the nature of the soil in which
the compound is distributed. The variation is related mainly to the amount of organic
matter (organic carbon) in the soils. Therefore, most scientists prefer to use a similar
ratio that focuses on sorption by organic matter. This ratio is termed the organic carbon
distribution coefficient Koc:
(8.14)
The Koc can be calculated by dividing the Kdby the fraction of organic C (g/g) in the
soil. This relationship can be seen in Table 8.9, which shows both Kdand Koc for several
commonly used herbicides and metabolites. Higher Kdor Koc values indicate the chem-
ical is more tightly sorbed by the soil and therefore less susceptible to leaching and
movement to the groundwater. On the other hand, if the management objective is to
wash the chemical out of a soil, this will be more easily accomplished for chemicals
with lower coefficients. Equations 8.13 and 8.14 emphasize the importance of the sorb-
ing power of the soil colloidal complex, and especially of humus, in the management
of organic compounds added to soils.
Koc =mg chemical sorbed>kg organic carbon
mg chemical>L solution =
Kd
g org. C >g soil
Kd=mg chemical sorbed>kg soil
mg chemical/L solution
TABLE 8.9 Partitioning Coefficients for Soil (Kd) and for
Organic Carbon (Koc) for Several Widely Used Herbicides
Three of the listed compounds are metabolites that form when
microorganisms decompose Atrazine. Higher Kdor Koc values indicate
stronger attraction to the soil solids and lower susceptibility to
leaching loss. The values were measured for a particular soil (an
Ultisol in Virginia). Using the relationship between Kdand Koc, it can
be ascertained that this soil contained 0.013 g C/g soil (1.3%).
Herbicide KdKoc
Atrazine 1.82 140
Diethyl atrazine 0.99 80
Diisopropyl atrazine 1.66 128
Hydroxy atrazine 7.92 609
Metolachlor 2.47 190
Data from Seybold and Mersie (1996).
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 349
8.13 BINDING OF BIOMOLECULES TO CLAY AND HUMUS
The enormous surface area and charged sites on the clay and humus in soils attract and
bind many types of organic molecules. These molecules include such biologically active
substances as DNA (genetic code material), enzymes, antibiotics, toxins, and even
viruses. The initial attraction may be between charged colloidal surfaces and positively
or negatively charged functional groups on the biomolecule, similar to the ion
exchange reactions just discussed. The data in Figure 8.29 show that adsorption of bio-
molecules takes place rapidly (in a matter of minutes) and the amount adsorbed is
related to the type of clay mineral. The bond between the biomolecule and the colloid
is often quite strong so that the biomolecule cannot be easily removed by washing or
exchange reactions. In most cases, biomolecules bound to clays do not enter the inter-
layer spaces, but are attached to the outer planar surfaces and edges of the clay crystals.
The binding of biomolecules to soil colloids in this manner has important environ-
mental implications for two reasons. First, such binding usually protects the biomole-
cules from enzymatic attack, meaning that the molecules will persist in the soil much
longer than studies of unbound biomolecules might suggest. Apparently, interaction
with the charged colloid surface changes the three-dimensional shape of the biomole-
cule and its electron distribution so that enzymes, which would normally cleave (cut)
the biomolecule, are unable to recognize and react with their target sites. Second, it has
been shown that many biomolecules retain their biological activity in the bound state.
Toxins remain toxic to susceptible organisms, enzymes continue to catalyze reactions,
viruses can lyse (break open) cells or transfer genetic information to host cells, and
DNA strands retain the ability to transform the genetic code of living cells, even while
bound to colloidal surfaces and protected from decay.
Figure 8.30 reveals the nature of one type of organoclay complex involving DNA, a
long-chain organic biomolecule. Researchers believe the DNA is adsorbed by hydrogen-
bonding to negatively charged sites on clay crystal planar surfaces and by electrostatic
attraction of negatively charged DNA to positively charged sites on clay crystal edges.
Strands of DNA bound to clay or humus may survive intact in soils for long periods
because the adsorption on the colloid surfaces hinders microbial enzyme attack. One
0 4123
Time, h
Toxin adsorbed, μg/500 μg
0
100
400
300
200
500
Montmorillonite
Kaolinite
FIGURE 8.29 Adsorption of a toxin called Bt, the insecticidal protein produced by the soil
bacteria, Bacillus thuringiensis, and used to protect some crop plants. Highly active clay minerals
such as montmorillonite (a member of the smectite group of 2:1 clays) adsorb and bind much
larger amounts of these biomolecules than do low-activity clays such as kaolinite (a 1:1 mineral).
In both cases, the adsorption reaction was completed in 30 minutes or less. Since only 500 μg of
either clay mineral was used in the experiment, it appears that the clays adsorbed an amount of
the toxin equal to 30 to 80% of their mass. [Redrawn from Stotzky (2000)]
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350 Chapter Eight
end of each DNA strand seems to be bound to the colloid, with the other end extended
into the soil solution, where it can interact with living cells. Therefore, such bound DNA
has the potential to be taken up by microbial cells in the soil, which could then express
the genes contained in the DNA. The presence of such DNA is not detectable by the
usual chemical tests since it is not in a living cell and therefore not expressing its genes.
When genetically modified organisms (GMOs) are introduced to the soil environ-
ment, the cryptic (hidden) genes just described may represent an undetected potential
for transfer of genetic information to organisms for which it was not intended. A simi-
lar concern exists regarding plants genetically modified to produce such compounds as
human drugs (“pharma crops”) or insecticidal toxins. For example, millions of hectares
are planted each year with corn and cotton plants given a bacterial gene that codes for
production of the insecticidal toxin Bt (see Figure 8.29). The Bt toxin is released into
the soil by root excretion and decomposition of plant residues containing the toxin.
We know little about what effect the toxins may have on soil ecology if it accumulates
in soils in the colloid-bound, but still active state (see Sections 11.2 to 11.5).
Antibiotics comprise another class of important organic compounds that sorb to
soil colloids. These unique natural chemicals are irreplaceable life-saving compounds.
Think of the last time you were given a prescription for an antibiotic drug; had the drug
not ended your bacterial infection, the infection may have ended your life! Yet, in most
industrial countries, about 80% of antibiotics manufactured are not used to cure dis-
eases (in animals or humans) but are used in livestock feed to stimulate faster growth of
cattle, swine, and chickens. Not surprisingly, the animal manures produced on most
industrial-style farms have been found to be laden with antibiotics that have passed
through the animals’ digestive systems. When these manures are applied to farmland,
the antibiotics become sorbed on the soil colloids and may accumulate with repeated
manure applications. Apparently the sorption is very strong. For example, Kdvalues as
high as 2300 L kg–1 have been reported for the antibiotic tetracycline in some soils
(compare this Kdvalue to those of the herbicides listed in Table 8.9). However, increas-
ingly research shows that even though strong sorption to soil colloids may reduce their
efficacy somewhat, the soil-bound antibiotics still work against bacteria (Figure 8.31).
This revelation raises concerns that the huge amounts of antibiotics exposed in the
environment will select for resistant strains of “super bacteria” (including human
pathogens), which would then no longer be controllable by these (once) life-saving
drugs. It is clear that soil colloids and soil science have important roles to play with
respect to environmental health.
0.5 μm
0.5 μm
FIGURE 8.30 Scanning electron micrographs (SEMs) of DNA from Bacillus subtilis bound on kaolinite clay (left) and on
montmorillonite clay (right). Note the hexagonal crystal shape of the kaolinite and the flake-like shape of the montmo-
rillonite (compare to Figure 8.3aand c). The arrows point to strands of bound DNA. As indicated by the 0.5-μm bars, the
clays are magnified about 40,000 times. Research shows that, in soil, DNA bound to clay or humus is protected from
decomposition but retains the capability of transferring genetic information to living cells. [Images courtesy of
Dr. Guenther Stotzky, Dept. of Biology, New York University]
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 351
8.14 PHYSICAL IMPLICATIONS OF SWELLING-TYPE CLAYS
Engineering Hazards
Soil colloids differ widely in their physical properties, including plasticity, cohesion,
swelling, shrinkage, dispersion, and flocculation. These properties greatly influence the
usefulness of soils for both engineering and agricultural purposes. As discussed in
Sections 4.9 and 8.3, the tendency of certain clays to swell in volume when wetted is a
major concern for the construction of roads and foundations. The worst clays in this
regard are the smectites, which form wavy stacks or microscopic clay domains contain-
ing extremely small ultramicropores. These ultramicropores attract and hold large
quantities of water (Figure 8.32), accounting for much of the swelling and plasticity of
these clays. The coefficient of linear extensibility [COLE] and the plasticity index
(PI) are two measures of soil expansiveness. Both are much higher for smectitic soils
than kaolinitic ones (Table 8.10). Relatively pure, mined smectite clay (often sold as
500
60
50
40
30
20
Rate of Salmonella decline, %
Tetracycline adsorbed by soil, mg kg1 soil
Hubbard loamy sand
Webster clay loam
10
0
01000 1500 2000 2500
FIGURE 8.31 Antibiotics fed to livestock are known to appear in animal
manure. When such manure is applied to farmland, the antibiotics become
adsorbed to the soil colloids and therefore may accumulate in soils. The graph
shows that the number of Salmonella bacteria declined where more of the
tetracycline was present, even though it was sorbed by the soil colloids.
Adsorption to the Webster clay loam soil was apparently stronger than to the
Hubbard loamy sand, resulting in a slightly diminished level of antibiotic
activity in the clay loam. The researchers concluded that “even though antibi-
otics are tightly adsorbed by clay particles, they are still biologically active and
may influence the selection of antibiotic resistant bacteria in the terrestrial
environment.” [Redrawn from Chander et al. (2005)]
H
O
P
W
B
O
O
S
S
S
S
400 nm
FIGURE 8.32 Transmission electron micrograph showing
smectite clay (S) along with humus (H), bacterial cells (B),
cell walls (W), and polysaccharides (P). Stacks of parallel
crystals of smectite clay (S) form an open wavy clay
domain structure (dark) in which ultramicropores (O) are
visible as white areas. Water drawn into these ultramicro-
pores in the clay domains accounts for most of the swelling
of smectite clay upon wetting. It is less likely, as was
once thought, that water causes swelling by entering the
interlayers between smectite clay crystal units. Note that
the entire image is about 1 micron across. [Image courtesy
of M. Thompson, T. Pepper, and A. Carmo, Iowa State
University]
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352 Chapter Eight
Kaolinitic soil Montmorillonitic soil
Wet DryDry Wet
FIGURE 8.33 The different swelling tendencies of two types of clay are illustrated in the lower left. All four cylinders initially contained
dry, sieved clay soil, the two on the left from the B horizon of a soil high in kaolinite, the two on the right from one of a soil high in
montmorillonite. An equal amount of water was added to the two center cylinders. The kaolinitic soil settled a bit and was not able to
absorb all the water. The montmorillonitic soil swelled about 25% in volume and absorbed nearly all the added water. The scenes to the
right and above show a practical application of knowledge about these clay properties. Soils containing large quantities of smectite
undergo pronounced volume changes as the clay swells and shrinks with wetting and drying. Such soils (e.g., the California Vertisol
shown here) make very poor building sites. The normal-appearing homes (upper) are actually built on deep, reinforced-concrete pilings
(lower right) that rest on nonexpansive substrata. Construction of the 15 to 25 such pilings needed for each home more than doubles the
cost of construction. (Photos courtesy of R. Weil)
TABLE 8.10 Plastic and Liquid Limits of the Clayey B Horizons of Several Soils and Pure Smectite Clay
All soils listed are high in clay, but those dominated by smectite and other high-activity clays tend
to have the highest plasticity indices and coefficients of linear extensibility (COLE). Also,
note the effect of saturating cation on the liquid limit of pure smectite clay.
Clay Plastic Liquid Plasticity
Soil content, % Clay mineralogy limit, % limit, % index COLEa
Bashaw (Aquerts) 65 Smectitic 18 71 53 —
Jackland (Udalfs) 68 Smectitic 42 90 48 0.16
Waxpool (Aqualfs) 68 Smectitic 36 76 40 0.18
Kelly (Udalfs) 59 Vermiculitic 12 45 33 0.10
Creedmoor (Udults) 54 Mixed, semiactive 36 67 31 0.09
Cecil (Udults) 68 Kaolinitic 43 61 18 0.03
Davidson (Udults) 68 Kaolinitic 40 56 16 0.04
Slickrock (Udands) 45 Iron oxides 46 59 13 —
Brazil Oxisol (Aquox) 60 Kaolinitic 25 45 20 0.03
Na-saturated smectite 100 — — 950 — —
Ca-saturated smectite 100 — — 360 — —
Na-saturated kaolinite 100 — — 36 — —
aCoefficients of linear extensibility, see Section 4.9.
Alfisols and Ultisols data from Thomas et al. (2000); Oxisol data from USDA/NRCS; Udands and Aquerts data from McNabb (1979); pure clay data from
Warkentin (1961).
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 353
The physical and chemical properties of swelling-type clays make them extremely useful in certain environ-
mental engineering applications. A common use of swelling clays—especially a mined mixture of smectite clays
called bentonite—is as a sealant layer placed on the bottom and sides of ponds, waste lagoons, and landfill cells
(see Section 18.10). The clay material expands when wetted and forms a highly impermeable barrier to the
movement of water as well as organic and inorganic contaminants contained in the water. The contaminants are
thus held in the containment structure and prevented from polluting the groundwater.
A more exotic use for swelling clays is proposed in Sweden for the final repository of that country’s highly
radioactive and toxic nuclear power plant wastes. The plan is to place the wastes in large (about 5 m ×1 m) cop-
per canisters and bury them deep underground in chambers carved from solid rock. As a final defense against
leakage of the highly toxic material to the groundwater, the canisters will be surrounded by a thick buffer layer
of bentonite clay. The clay is packed dry around the canisters and is expected to absorb water to saturation dur-
ing the first century of storage, thus gradually swelling into a sticky, malleable mass that will fill any cavities or
cracks in the rock. The clay buffer will serve three protective functions: (1) cushion the canister against small (10
cm) movements in the rock formation, (2) form a seal of extremely low permeability to keep corrosive sub-
stances in the groundwater away from the canister, and (3) act as a highly efficient electrostatic filter to adsorb
and trap cationic radionuclides that might leak from the canister in some far future time.
Figure 8.34 shows how bentonite is used as a plug or sealant to prevent leakage around an environmental
groundwater monitoring well. For most of the well depth, the gap between the bore hole wall and the well tube
is back-filled with sand to support the
tube and allow vertical movement of
the groundwater to be sampled. About
30 cm below the soil surface, the space
around the well casing is filled instead
with air–dry granulated bentonite
(white substance being poured from
bucket in the photograph). As the ben-
tonite absorbs water, it swells markedly,
taking on an almost rubbery consis-
tency and forming an impermeable seal
that fits tightly against both the well cas-
ing and the soil bore hole wall. This seal
prevents contaminants from the soil
surface from leaking down the outside
of the well casing. In the case of ground-
water contaminated with volatile organ-
ics like gasoline, the bentonite also
prevents vapors from escaping without
being properly sampled.
Increasingly, environmental scien-
tists are using swelling-type clays for
the removal of organic chemicals from
water by partitioning. For example,
where there has been a spill of toxic
organic chemicals, a deep trench may be dug across the slope and back-filled with a slurry of swelling clay and
water to intercept a plume of polluted water. The swelling nature of the smectites prevents the rapid escape of
the contaminated water while the highly reactive colloid surfaces chemically sorb the contaminants, purifying
the groundwater as it slowly passes by. Chapter 18 takes a more detailed look at such “slurry walls” and other
soil technologies for cleaning the environment.
aFor more detailed information on environmental use of swelling clays, see Reid and Ulery (1998). For details of the Swedish nuclear repository use of bentonite,
see Swedish Nuclear Power Inspectorate (2005) and S.K.B. (2004).
BOX 8.7
ENVIRONMENTAL USES OF SWELLING-TYPE CLAYSa
Access cap
Concrete block
for strength
Bentonite clay seal
Sand fill between tube
and soil to allow
groundwater to move
Well casing to
sample
groundwater
Slotted casing
to allow
sampling
groundwater
Access cap
Concrete block
for strength
Bentonite clay seal
Sand fill between tube
and soil to allow
groundwater to move
Well casing to
sample
groundwater
Slotted casing
to allow
sampling
groundwater
FIGURE 8.34 Use of swelling clay as seal
for environmental monitoring well.
(Courtesy of R. Weil)
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354 Chapter Eight
“bentonite”), especially when saturated with Na+ions, can have far greater potential for
swelling and plasticity than the impure clays in soil.
Figure 8.33 gives an example of special steps needed to safely build homes on soils
dominated by smectitic clay. The cost of building homes on smectitic soils may be dou-
ble that of building on soils dominated by nonswelling clays, for which conventional
foundation designs can be safely used. If preventative design measures are not taken
during the construction of houses on smectite clays, homeowners will pay dearly in the
future. The building foundation is likely to move with the swelling and shrinking of
the soil, misaligning doors and windows and eventually cracking foundations, walls,
and pipes. The same swelling properties that make smectitic soils so problematic for
construction activities make them attractive for certain environmental applications
(see Box 8.7) and well-suited for siting ponds and lagoons or creating wetlands. This is
just one example of the critical role soil colloids play in determining the usefulness of
our soils.
8.15 CONCLUSION
The complex structures, enormous surface area (both internal and external), and
tremendous numbers of charges associated with soil colloids combine to make these
tiniest of soil particles the seats of chemical and physical activity in soils. The physical
activity of the colloids, their adsorption of water, swelling, shrinking, and cohesion are
discussed in detail in Chapters 4, 5, and 6. Here we focused on the chemical activity of
the colloids, activity that results largely from charged sites on or near colloid surfaces.
These charged sites attract oppositely charged ions and molecules from the soil solu-
tion. The negative sites attract positive ions (cations) such as Ca2+, Cu2+, K+, or Al3+ and
the positive sites attract negative ion (anions) such as Cl-, , , or .
Although both positive and negative charges occur on colloids, in most soils the
negative charges far outnumber the positive. Most elements dissolved from rocks by
weathering or added to soils in lime or fertilizer will eventually end up in the oceans,
but it is very fortunate for land plants and animals that attraction to soil colloids
greatly slows the journey. Colloidal attraction is a major mechanism by which soils
accumulate the stocks of nutrients necessary to support forests, crops, and, ultimately,
civilizations. This role is especially critical for forests when nutrient storage in plant
biomass is disrupted by fire or timber harvest. The colloidal attraction also enables soils
to act as effective filters, sinks, and exchangers, protecting groundwater and food
chains from excessive exposure to many pollutants.
When ions are attracted to a colloid, they may enter into two general types of rela-
tionships with the colloid surface. If the ion bonds directly to atoms of colloidal struc-
ture with no water molecules intervening, the relationship is termed an inner-sphere
complex. This type of reaction is quite specific and, once created, is not easily reversed.
In contrast, ions that keep their hydration shell of water molecules around them are
generally attracted to colloidal surfaces with excess opposite charge. However, the
attractive forces are transmitted through a chain of polar water molecules and are there-
fore weakened, and the interaction is less specific and quite easily reversed.
The latter type of adsorption is termed outer-sphere complexation. The adsorbed ion
and its shell of water molecules oscillate or move about within a zone of attraction. The
size of the oscillation zone depends on the strength of attraction between the particular
ion and the type of charged site. Ions in such a state of dynamic adsorption are termed
exchangeable ions because they break away from the colloid whenever another ion from
the solution moves in closer and takes over, neutralizing the colloid’s charges.
The replacement of one ion for another in the outer-sphere complex is termed ion
exchange. Except in certain highly weathered, subsurface horizons, cation exchange is
far greater than anion exchange. Cation and anion exchange reactions are reversible
and balanced charge-for-charge (rather than ion-for-ion). The extent of the reaction is
influenced by mass action, the relative charge and size of the hydrated ions, the nature
of the colloid, and the nature of the other (complementary) ions already adsorbed on
the colloid. Plant roots can exchange H+for nutrient cations or OH-ions for nutrient
anions.
The colloids in soils are both organic (humus) and mineral (clays) in nature. In most
surface soils, half or more of the charges are contributed by organic matter colloids,
HPO42-
NO3-
SO42-
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THE COLLODIAL FRACTION: SEAT OF SOIL CHEMICAL AND PHYSICAL ACTIVITY 355
while in most subsurface horizons, clays provide the majority of charges. The total
number of negative colloid charges per unit mass is termed the cation exchange capacity
(CEC). The CEC of different colloids varies from about 1 to over 200 cmolc/kg and that
of whole mineral soils commonly varies from about 1 to 40 cmolc/kg. The CEC of a soil,
as well as its capacity to strongly adsorb particular ions (such as K+or ), depends
on the amount of humus in the soil and on both the amount and type of clays present.
Low-activity clays (iron and aluminum oxides and 1:1-type silicate clays like kaolinite)
tend to dominate highly weathered soils of warm, humid regions. High-activity clays
(expanding 2:1 silicates like smectite and vermiculite, and nonexpanding 2:1 silicates
like fine-grained mica and chlorite) tend to dominate soils in cooler or drier regions
where weathering is less advanced. Most of the charge on humus and low-activity clays
is pH-dependent (becomes more negative as pH rises), while most of the charge on
high-activity clays is permanent.
The differing ability of soil colloids to adsorb ions and molecules is key to man-
aging soils, both for plant production and to understand how CEC may regulate
movement of both nutrients and toxins in the environment. Among the important
properties influenced by colloids is the acidity or alkalinity of the soil, the topic of
the next chapter.
STUDY QUESTIONS
1. Describe the soil colloidal complex, indicate its various components, and explain
how it tends to serve as a “bank” for plant nutrients.
2. How do you account for the difference in surface area associated with a grain of
kaolinite clay compared to that of montmorillonite, a smectite?
3. Contrast the difference in crystalline structure among kaolinite, smectites, fine-
grained micas, vermiculites, and chlorites.
4. There are two basic processes by which silicate clays are formed by weathering
of primary minerals. Which of these would likely be responsible for the forma-
tion of (1) fine-grained mica, and (2) kaolinite from muscovite mica? Explain.
5. If you wanted to find a soil high in kaolinite, where would you go? The same
for (1) smectite and (2) vermiculite?
6. Which of the silicate clay minerals would be most and least desired if one were
interested in (1) a good foundation for a building, (2) a high cation exchange
capacity, (3) an adequate source of potassium, and (4) a soil on which hard
clods form after plowing?
7. Which of the following would you expect to be most and least sticky and plastic
when wet: (1) a soil with significant sodium saturation in a semiarid area, (2) a
soil high in exchangeable calcium in a subhumid temperate area, or (3) a well-
weathered acid soil in the tropics? Explain your answer.
8. A soil contains 4% humus, 10% montmorillonite, 10% vermiculite, and 10%
Fe, Al oxides. What is its approximate cation exchange capacity?
9. Calculate the number of grams of Al3+ ions needed to replace 10 cmolcof Ca2+
ion from the exchange complex of 1 kg of soil.
10. A soil has been determined to contain the exchangeable cations in these
amounts: Ca2+ 9 cmolc, Mg2+ 3 cmolc, K+1 cmolc, Al3+ 3 cmolc. (a) What
is the CEC of this soil? (b) What is the aluminum saturation of this soil?
11. A 100 g sample of a soil has been determined to contain the exchangeable
cations in these amounts: Ca2+ 90 mg, Mg2+ 35 mg, K+28 mg, Al3+ 
60 mg. (a) What is the CEC of this soil? (b) What is the aluminum saturation of
this soil?
12. A 100 g sample of a soil was shaken with a strong solution of BaCl2buffered at
pH 8.2. The soil suspension was then filtered, the filtrate was discarded, and the
soil was thoroughly leached with distilled water to remove any nonexchange-
able Ba2+. Then the sample was shaken with a strong solution of MgCl2and
again filtered. The last filtrate was found to contain 10,520 mg of Mg2+ and
258 mg of Ba2+. What is the CEC of the soil?
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356 Chapter Eight
13. Explain the importance of Kdand Koc in assessing the potential pollution of
drainage water. Which of these expressions is likely to be most consistently
characteristic of the organic compounds in question regardless of the type of
soil involved? Explain.
14. An accident at a nuclear power plant has contaminated soil with strontium-90
(Sr2+), a dangerous radionuclide. Health officials order forages growing in the
area to be cut, baled, and destroyed. However, there is concern that as the for-
age plants regrow, they will take up the strontium from the soil and cows eating
this contaminated forage will excrete the strontium into their milk. You are the
only soil scientist assigned to a risk assessment team consisting mainly of dis-
tinguished physicians and statisticians. Write a brief memo to your colleagues
explaining how the properties of the soil in the area, especially those related to
cation exchange, could affect the risk of contaminating the milk supply.
15. Explain why there is environmental concern about the adsorption by soil col-
loids of such normally beneficial substances as antibiotic drugs and natural
insecticides.
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