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VI. Relationship to Soil Properties
AMORPHOUS CLAY CONSTITUENTS OF SOILS
and the volume and nature of the washing media; adsorption is strongly
influenced by pH (Birrell and Gradwell, 1956; Birrell, 1961a) (Table I ) .
Conventional methods for determination of the exchange capacity therefore
require some revision. Perhaps the least ambiguous method is to determine
the cation and anion exchange capacity (CEC and AEC) by equilibrating
the soil sample with a dilute salt solution containing the index ions at an
appropriate pH and then measuring the retention of these ions without
removing the excess salt by washing (Schofield, 1949; Wada and Ataka,
1958; Wada and Harada, 1969; Chichester et al., 1970; van Raij and
Peech, 1972). Most of the values described here have been obtained by
such procedures unless otherwise specified.
Van Raij and Peech (1972) determined the charge characteristics of
two highly weathered Oxisols and one Alfisol from Brazil containing gibbsite, iron oxides, and kaolinite as major clay constituents. These soils develop both pH-dependent negative and positive charges in an amount less
than 10 and 5 me per 100 g soil, respectively, for the pH range 4 to 7
and at electrolyte concentration of 0.01 N ; they exhibit a point of zero
charge at pH 3-6. Barber and Rowel1 (1972) obtained similar data for
an iron-rich kaolinitic soil. They explained the variations in charge characteristics in terms of the presence of a small permanent negative charge,
pH dependent positive and negative charge influenced by the indifferent
electrolyte concentration, and the overlap of diffuse double layers causing
a neutralization of charge. In addition to this, Atkinson et al. (1967) presented evidence for ion-pair formation on iron oxides, which would inhibit
the development of diffuse double layers. Shifts of the isoelectric point in
more than 2 pH units toward an alkaline side were found for sesquioxide
minerals with the increase both in the degree of their hydration and disordering in their structural organization (Parks, 1965).
The pH-dependent CEC also occurs for chloritized 2 : 1 layer silicates,
where permanent negative charge is blocked by positively charged hydroxyaluminum groups. Accessibility to exchange sites is restored by deprotonation of the latter on addition of base (de Villiers and Jackson, 1967;
Chichester et al., 1970). Sawhney et al. (1970) thus assigned the irreversible portion of the pH-dependent CEC to the interlayers and coatings, and
the reversible portion to organic matter, and found the pH-dependent CEC
of several acid soils to be primarily due to weakly dissociated organic matter groups. The negative charge development for imogolite, allophane, and
synthetic sesquioxides was found to be reversible (Sawhney and Norrish,
1971; Harada and Wada, 1973).
Exchange capacity data for allophane and imogolite of established purity
has been very scarce. The CEC values of 135 for allophane (Si0,/A1,03
ratio = 2.0) and 30 me per 100 g for imogolite were obtained by equili-
KOJI WADA AND M. E. HARWARD
Effects of Changing the Conditions for the Measurement and Predrying of Samples on
Cation Exchange Capacity (CEC) and Anion Exchange Capacity (AEC) Values of
Imogolite-Allophane, Allophane, Halloysite, and Montmorillonitea
Cation and anion exchange material
Changes in conditions
for measurement and
predrying of samples
Cation species: N H 4
Predrying of sample
Evacuated over P205
Heated a t 150°C
(Si02/A1203; (Si02/A1203; Halloy- morillonite
- and f respectively stand for about a 20% increase and decrease, and less than
a 20% change in CEC and AEC, in comparison with their reference values, when conditions
for the measurement and predrying of samples were changed as specified in the first column
from those for the reference measurement. The reference CEC and AEC values were
measured for air-dried samples by determining the cation and anion retention from 0.05
N Ca(CH&00)2 or CaClz of p H 7.0 a t 10-25°C. N D means not determined.
References (1) Wada and Harada (1969); (2) Wada and Ataka (1958); (3) Harada and
Wada (1973); (4) Wada and Harada (1971); (5) Harada and Wacla, (1972).
brating with 0.05 N NaCH,COO at pH 7.0 (T. Henmi and K. Wada, unpublished). The CEC increases with the increasing SiO,/A1,0:+ ratio for
soil clays containing allophane and imogolite ( Wada, 1967). The point
of zero net charge was found at about pH 7 for the soils containing imogolite and allophane, while the CEC was much higher than the AEC at the
same pH for weathered pumice containing allophane with a Si0,/A120,
ratio 2.0 (Wada and Harada, 1969). The large affinity and capacity of allophanic clays for anion retention were illustrated in studies by Singh and
Kanehiro (1969) and Kinjo and Pratt (1971a,b) on nitrate adsorption
by subsoils of Andepts in comparison with other soils.
Table I illustrates the effects of changing the conditions on the CEC
and AEC values measured for materials of different clay mineral compositions. The results demonstrate the greater effect of conditions on assessing
AMORPHOUS CLAY CONSTITUENTS OF SOILS
electric charges of soils containing allophane and imogolite compared with
crystalline layer silicates. The data have implications to methods employed.
The observed effect of pH and electrolyte concentration on both the CEC
and AEC of imogolite and allophane (Table I) was accounted for by their
effects on the proton dissociation from and the uptake by the functional
OH groups bonded to silicon and aluminum atoms, respectively (Iimura,
1966; Wada, 1967). Wada and Harada (1969) ascribed a gradual decrease in CEC with the decreasing concentration below 0.05 N to hydrolysis of exchangeable cation, while the relatively rapid increase in CEC at
concentrations above 0.1 N were attributed to a non-coulombic adsorption
of cation-anion pairs. By analogy to coprecipitated silica and alumina, Birre11 (1961a) suggested physical adsorption of salts by allophane. There
is a clear-cut difference in the hydrolysis of adsorbed cations between
the soils containing imogolite-allophane and those of weathered pumice containing allophane with a Si0,/A1,03 ratio of 2.0. This may suggest a difference in the mechanism of development of negative charge between imogolite and allophane, and may be correlated with the absence of aluminum
in 4-fold coordination in imogolite and its presence in allophane (T. Henmi
and K. Wada, unpublished). Another possible interpretation involves the
presence or absence of overlap of diffuse double layer at lower electrolyte
concentrations as proposed by Schofield (1949) and others.
Different values of the CEC of clay materials depending on the cation
species have often been reported. The effect is more manifested for imogolite-allophane and least for montmorillonite (Table I). Since the proton
is involved in the development of its negative charge, the observed phenomenon may be interpreted broadly in terms of relative bonding energies
(strength of adsorption) between the proton and the index cation being
added. Therefore, this will vary with valence and effective radius of the
cation. Approximately, the CEC decreased in the order: Ba > Ca > K,
Mg > NH,, for imogolite-allophane, and the ratio of the lowest to the
highest CEC was 0.5 to 0.6 (Wada and Harada, 1969). Different values
of the AEC depending on the anion species were also found for imogolite
and allophane (Wada and Tsuji, 1973). The AEC increased in the order:
NO, < C1 < CH3CO0<< SO,. The much higher value for sulfate anion
was interpreted in terms of specific adsorption (Section VI, A, 2).
Quite remarkably, the CEC of imogolite-allophane and allophane was
found to increase when the temperature of the solution was raised, whereas
a slight decrease was found in the AEC (Table I). The increased CEC
at higher temperature in a neutral 1 N acetate solution, was only partly
reduced by lowering the temperature again (Wada and Harada, 1971).
It was proposed that this large CEC increase includes reactions by which
some “bound” functional groups are set free for ionization and/or by which
KOJI WADA AND M. E. HARWARD
an AI-OH-AI bonding is formed between two terminal AI-H,O groups
(Harada and Wada, 1973).
The effect of previous drying or heating of the samples containing imogolite and/or allophane shows another interesting feature (Table I). Egawa
et al. (1959) found that the CEC decreased remarkably for an allophane
on air-drying or heating at 105OC when the CEC was measured using concentrated, e.g., 2.5 N , NH,CH,COO and 80% C,H,OH for washing. A
significant increase was, however, found after the same treatment when
the CEC was determined with 0.3 N NH,CH,COO and 80% C,H,OH.
The data shown in Table I indicate that the negative charge could develop
on dehydration, possibly including changes in the coordination of surface
aluminum atoms. The changes of surface functional groups resulting in increase of net negative charge upon drying may also be inferred from the
changes in electrophoretic mobility of allophanic clays or more simply from
the changes in pH of allophanic soils suspended in salt solutions. Watanabe
(1961, 1962) observed that the isoelectric point of allophane with a
SiO2/A1,O, ratio of 1.11 was lowered from pH 6.8 to 4.1 by air drying.
Sadzawka et al. (1 972) observed that the pH of Chilean volcanic ash soils
suspended in 0.01 A4 Na,SO, is lowered by about 0.5 pH unit or more
when the soils were dried at 110°C.
Imogolite and allophane have weak acid properties in soils as expected
from the pH-dependence of the negative charge developed on them.
Yoshida (1970, 1971 ) confirmed this by treating allophane, imogolite, and
crystalline layer silicate clays with 1 N AlCI, and determining exchangeable
aluminum and hydrogen on these clays. All the exchange sites on allophane
and imogolite were occupied by hydrogen but not aluminum, whereas more
than 60% of exchange sites in crystalline layer silicates were occupied by
aluminum. The cation selectivity of soils in cation-exchange reactions is
markedly influenced by the nature of the cation-exchange material. Yoshida (1961) determined the ratio of exchangeable NH, to Ca after treating
the soils with 1 : 1 mixture of 1 N ammonium and calcium acetates. The
soils in which humus and/or allophane predominated gave ratios in the
range of 0.2 to 0.4, whereas those predominated by crystalline layer
silicates gave ratios in the range of 1.4 to 4.
2. Sorption of Cation and Anion
A strong sorption of certain anions such as phosphate in soils is usually
associated with amorphous ahminosilicates and hydrous iron and aluminum oxides. The reaction is primarily attributed to aluminum and iron
atoms present on the clay surface, and the high reactivity of soil amorphous
constituents is correlated with their large aluminum and iron specific surface area.
AMORPHOUS CLAY CONSTITUENTS OF SOILS
Recent studies of anion adsorption by hydrous iron and aluminum oxides
have shown that “specific” adsorption in addition to “nonspecific” adsorption of anions normally occurs in soils (Hingston et al., 1967, 1968). The
“nonspecific” adsorption refers to adsorption of anions by simple coulombic interaction with positive charges on Al-OH,+ or Fe-OH,+ groups, while
the specific adsorption refers to incorporation of anion in the coordination
shell of an iron or aluminum atom as a ligand. The anion thus bound cannot be displaced from the soil simply by leaching with a solution containing
a nonspecifically adsorbed anion, such as chloride. The process is strongly
pH dependent, and a maximum or inflection occurs in the adsorption maximum-pH curve at pH values corresponding to the pK values of the acid
species formed by the anion. Silicate, arsenate, fluoride, ortho-, pyro-, and
tripolyphosphates, selenite, and molybdate are specifically adsorbed. Specific adsorption of molybdate by some crystalline and amorphous soil clays
was studied by Theng (1971 ).
The high value of phosphate absorption obtained by a conventional
method (more than 1500 mg as P,O, by 100 g of air-dry soil from 200
ml of 2.5% ammonium phosphate solution at pH 7 ) has often been used
in Japan for roughly distinguishing soils derived from volcanic ash and
other parent materials. The greater role in this phosphate sorption has
recently been assigned to the dithionite-citrate soluble sesquioxidic components rather than allophane (Kato, 1970a,b; Miyauchi and Nakano, 19711.
Difficulty in obtaining and defining an adsorption maximum at a pH was
also reported for adsorption of phosphate by volcanic ash soils (Tsukada
et al., 1967; Miyauchi and Nakano, 1971). This is probably due to overlapping of the reaction with a phosphate-induced decomposition of clay
minerals and with precipitation of complex or discrete new aluminum and
iron phosphates. The rapidness of the latter reaction for allophane was
demonstrated by its transformation to taranakite-like minerals in weak acid,
ammonium, or potassium phosphate solutions (Wada, 1959; Birrell,
3 . Interaction with Organic Conipounds
There is ample evidence which indicates that the level of organic matter
in soils is affected by the presence of amorphous inorganic material (see
Section V) . Recent advances in the study of possible mechanisms of interaction between humic and fulvic acids and soil clays have been reviewed
by Greenland ( 1971) .
The high ratio of the pyrophosphate or oxalate soluble aluminum and
iron to dithionite-citrate soluble iron and aluminum was obtained for
Podzol B horizons (Bascomb, 1968; McKeague, 1967; Blume and Schwertmann, 1969). The high solubility of aluminum and iron in the pyrophos-
KOJI WADA AND M. E. HARWARD
phate solution was interpreted as indicating that aluminum and iron or
hydroxy-aluminum and iron ions form organic complexes (McKeague et
al., 1971). Schnitzer (1969) explained the dissolution and precipitation
of organic matter in the Podzol A and B horizons by change in the solubility
of Fe(II1)- and aluminum-fulvic acid complexes. While 1 :1 molar
Fe(II1)-fulvic acid complexes were completely water soluble, 6 : 1 molar
Fe( 111)- and aluminum-fulvic acid complexes were water insolubIe. By
differential thermogravimetric analysis, the ironpan in the Humic Podzol
from Newfoundland who also identified as being essentially a 6 : 1 molar
Fe (111)-fulvic acid complex. In these complexes, the formation of electrovalent bonds between negatively charged carboxyl groups and positively
charged aluminum and iron ions and or hydroxy-aluminum and iron ions
was inferred from infrared spectroscopy.
Accumulation of humus constitutes one of the striking features of Andosols. It is not uncommon for the carbon content of the surface soil to be
20% within 5000 years. This occurs under a warm, humid climate and
grass vegetation, and at well-drained sites. The role of allophane and related materials in accumulation of organic matter in Andosols has, therefore, been studied by a number of investigators. Recent investigations may
be classified under the following four groups. The first is concerned with
possible effects of allophane on the action of enzymes such as protease
and amylases (Aomine and Kobayashi, 1964, 1966; Kobayashi and
Aomine, 1967). These authors pointed out the importance of protective
action of allophane against biotic degradation of organic materials which
become incorporated in the soils. The second is concerned with a catalytic
effect of allophane in an oxidative polycondensation of phenolic units,
which results in formation of stable skeletons in soil humic materials. The
greater catalytic effect of allophane and sesquioxides compared with crystalline layer silicates was demonstrated for a chestnut tannin-containing substance (Kyuma and Kawaguchi 1964) and pyrogallol (Kumada and Kato,
1970). The third possibility is that allophane acts as a source of aluminum and/or iron which form insoluble humates. The fourth is concerned
with the adsorption of humic materials. Kobo and Fujisawa (1964)
studied adsorption of humic acids extracted from soils by various clays.
Adsorption studies by Inoue and Wada (1968, 1971a,b) with an extract
of humified clover showed that allophane and imogolite have a much
greater sorption capacity than crystalline layer silicates. The importance
of this adsorption in the overall accumulation of humus was inferred from
a soil incubation experiment (Wada and Inoue, 1967).
In all these studies, the preformation of allophane in the soil has implicitly been assumed. As described in Section V, C, the importance of sesquioxidic constituents, rather than allophane, in accumulation of organic mat-
AMORPHOUS CLAY CONSTITUENTS OF SOILS
ter in some volcanic ash soils has been increasingly recognized. In fact, the
ratio of pyrophosphate soluble aluminum to dithionite-citrate soluble
aluminum as high as 1.0 was found for such soils in which little or no allophane was detected by selective dissolution and infrared spectroscopy (K.
Wada and T. Highashi, unpublished).
The importance of “free iron oxide” in affecting physical properties such
as aggregate formation has often been mentioned. Deshpande et al. (1968)
subjected “red” soils to different dissolution treatments and observed
changes in the physical properties. They suggested that the acid-soluble
“active” aluminum oxide rather than iron oxide is important in aggregate
formation in red soils.
It has been established that the physical properties of soils having a
high content of amorphous clay material, specifically allophane, are unique.
Birrell (1 964) and Forsythe et al. (1969) summarized features of physical
properties of Ando soils, and Sherman et al. (1964) summarized those
of Hydrol-Humic Latosols, which were thought to contain amorphous
aluminum, iron, and titanium hydrous oxides as well as allophane.
Soils containing a high content of allophane and imogolite have low bulk
densities. Values as low as 0.25 and 0.3 were found even for such volcanic
ash soils, which contain little organic matter and develop under a temperate, humid climate (Wada and Aomine, 1973). In Hawaii, Sherman et
al. (1964) reported that the ferruginous aluminous soils have bulk densities
as low as 0.09 with an average range between 0.3 and 0.5, while the ferruginous soils have a bulk density of 0.8 to 1.0. Soils developed on the
Mazama ash in Oregon commonly have bulk densities in the range of 0.7
A very high natural moisture content is a common feature of soils described above, and is associated with a very low bulk density value. The
moisture contents on an oven-dried basis for volcanic ash soils in the Kanto
district range from 80 to 180% and are mostly from 100 to 140% (Suyama
and Oya, 1965). The subsoils compared with the surface soils have high
amounts of both available or free water ( p F 2.5-4.2 or 1/3 to 15 bar)
and nonavailable or nonfree water. Water retention, liquid limit, and plasticity index show a marked decrease when the samples have been previously air-dried (Birrell, 1952; Suyama and Oya, 1965; Tada, 1969). There
seems to be a critical moisture content below which this effect of predrying
of the sample appears both for the liquid limit and plasticity limit values.
KOJI WADA AND M. E. HARWARD
The change is, however, not parallel, and is much more pronounced for
the liquid limit. Wells and Furkert (1972) inferred that an irreversible
change takes place in water status of natural allophane by a simple
The tendency toward irreversible drying of soils containing allophane
is manifested in formation of stable aggregates. Kubota (1972) found that
there is a critical pF value, which may vary in a range from pF 3 to 4,
depending on the natural moisture status of the soils. Sherman et al.
(1964) reported that the Akaka soils undergo a tremendous loss of volume
on drying and will not rehydrate. The original samples, which showed little
or no crystallinity, underwent a change of state to form crystalline gibbsite,
poorly crystallized iron oxide, and allophane after dehydration. They attributed the change of soil properties to crystallization of hydrous oxides
from the corresponding gel of cryptocrystalline materials.
The difficulty in determination of particle size distribution for volcanic
ash soils has often been mentioned. The high content of organic matter
combined with amorphous inorganic constituents in volcanic ash soils poses
a problem. Undesirable effects of H,O, treatment resulting in a partial destruction of the clay constituents were discussed by Mitchell et al. ( 1964).
Lavkulich and Wiens (1970) compared effects of H,O, and NaOCl treatments for removal of organic matter on selected mineral constituents. It
was found that NaOCl extracted more organic matter with less destruction of the oxides than procedures employing H,O,. Harris (1973)
reported that the clay yield from Parkdale soils containing considerable
amounts of allophane was 14 % less in average after NaOCl treatment
than after H,O, treatment.
Treatment of these soils at pH 10 (NaOH) or at pH 4 (HCl) after
peroxidation and washing with water and with the aid of ultrasonic or sonic
oscillations seems to have been partially accepted as a means for dispersion
of the clay fraction. The alkaline medium is used for soils in which crystalline layer silicates predominate in the clay fraction, while the acid medium
is used for those containing allophane and imogolite in substantial amounts.
Matsuo (1964) claimed that sodium metaphosphate can be used as efficiently for dispersion of volcanic ash soils as other soils in Japan, but
neither his nor other subsequently published data on highly allophanic soils
support his conclusion (Kobo and Oba, 1965; Sherman et al., 1964;
Miyazawa, 1966). Adsorption of metaphosphate in an appreciable amount
by amorphous clay constituents (Yoshinaga and Yamaguchi, 1970a) also
is a disadvantage in the use of metaphosphate at recommended concentrations such as 0.032 to 0.02 M . Kobo and Oba (1965) and Kanno and
Arimura (1967) showed that when sonic oscillation is used, the removal
of iron oxide as a pretreatment is not necessary for dispersion of volcanic
ash soils. The use of sonic or ultrasonic oscillation for mechanical analysis
has, however, some disadvantages as pointed out by Watson (1971).
AMORPHOUS CLAY CONSTITUENTS OF SOILS
Birrell (1966) proposed to indirectly determine clay contents in soils containing allophane by measuring adsorption of nitrogen, acetic acid, and
Aomine and Egashira ( 1968 ) tested effectiveness of various electrolytes
as flocculants for allophanic clays in comparison with montmorillonite. The
flocculation of allophane is primarily determined by the valence of anions
and that of montmorillonite by the valence of cations. This probably reflects their predominating positive and negative charges. From this observation, they proposed to use the ratio of flocculation value of CaCI, to that
of Na,SO, for differentiation of predominating surface charge of soil colloids. The values 20-30 were obtained for soil clays in which allophane
and imogolite predominated while the values 0.04-0.12 were found for
those with crystalline layer silicates or humus.
The void ratios of fine-textured volcanic ash soils are in the range from
2 to 5, and mostly from 3 to 4. These values are higher than 0.8 to 1.0
for sandy alluvial soils and 1.5 to 2.5 for clayey alluvial soils. A peculiarity
common to volcanic ash soils subjected to the standard consolidation test
is that preconsolidation load (e.g., 20-40 tons/m?) far exceeds the present
overburden pressure or even the bearing capacity (10-15 tons/m2), Since
there is no geologic evidence for this preconsolidation, one might imagine
the same effect being produced by aggregation of soil particles with allophane and related colloids, possibly through drying during weathering of
volcanic ash (Birrell, 1964; Suyama and Oya, 1965).
An anomalous compaction behavior has also been noted for volcanic
ash soils. The soil normally gives a single compaction curve with a welldefined maximum, which is primarily determined by the density and the
size distribution of soil particles. The compaction curve shows the dry
density values attained at the respective moisture contents when the soil
is subjected to the same compaction procedure. However, a volcanic ash
soil gives a variety of compaction curves, and hence, the maximum dry
density and the optimum moisture at which the former is attained varies
depending upon the initial moisture content of the sample. Kuno (quoted
by Suyama and Oya, 1965) for example, reported a variation of the maximum dry density from 1.02 to 0.58 tons/m3 in association with the corresponding variation of the optimum moisture content from 60 to 135%,
when the initial moisture content of a volcanic ash soil from the Kanto
district has been changed from 40 to 130%. Another anomaly which was
noted for the allophanic soil is that the clear maximum in the dry density
curve was attained only on the moistening cycle but not on the drying
cycle. This behavior was again attributed to the fact that allophane can
contain a relatively high amount of nonfree water and that there seems to
be no sharp boundary between the nonfree and free water. It is clear that
building structures that involve soils with allophanic materials, particularly
those disturbed and compacted, make special care necessary.
KOJI WADA AND M. E. HARWARD
CURRENT STATUS AND
Historically, problems have been encountered in the use and management of soils high in amorphous constituents. This is true for the United
States as well as other Pacific r i d countries such as Japan and New Zealand.
The problems involve failure to recognize and evaluate the importance of
the amorphous components. Laboratory analyses either have not been
oriented to these materials or the wide range in sensitivity of the amorphous
materials to the index tests has not been recognized. To a large degree
the problems encountered involve a high degree of porosity in an undisturbed state, large water-holding capacities with the tendency for the clays
to be saturated or nearly so in nature, and rearrangement of the gel structure or matrix upon manipulation. Although not always understood, some
of the relationships to engineering properties have been at least recognized. In view of this it is somewhat surprising that soil scientists
have not done more to relate the nature and occurrence of amorphous
materials to problems of soil management which involve dispersion, flocculation, aggregation, infiltration, erosion, and landscape stability. Perhaps
this is partially due to our overzealous efforts to “clean-up” the sample
prior to the analysis and obtain data more like those of standard or reference specimens which are easier to interpret. In doing so, we may have removed or modified components so that the samples no longer reflect the
properties of the systems in situ.
A significant accomplishment toward the recognition and evaluation of
amorphous components has been made by Jones and Uehara (1973). They
have perfected techniques for transmission electron microscopy which show
the presence of gel-hulls that enclose the bridge between crystalline particles. Their success is due to a combination of factors including absence
of chemicals and use only of mild sonification for dispersion, use of holey
substrate for mounting of specimens, operation of the equipment at high
voltage and low current to minimize effects of the beam, and exposure
of an area only long enough to focus and obtain a film record. Subsequent
interpretations were then made from photo micrographs. These procedures
permitted them to detect gellike materials in aluminosilicate systems, high
aluminum soils and on quartz surfaces.
In view of previous experience with relationships of amorphous constituents to engineering properties, relationships to landscape stability are to
be expected. In a study of a watershed in the Cascade Range, Youngberg
et al. (1973) found amorphous inorganic colloids to be dominant components of readily dispersible soils which were also major contributors to
stream turbidity. Amorphous components tended to remain in suspension
in the reservoir longer than smectite clay minerals. Similar studies of a
AMORPHOUS CLAY CONSTITUENTS OF SOILS
watershed on older geologic formations and under a different climate indicate that samples which contain a combination of smectite and amorphous
clays tend to remain in suspension and contribute to continued turbidity
of the reservoirs (Silvernale et al., 1973). In view of current interest in
preservation of the environment and our natural resources, efforts to develop similar relationships should be increased.
There are different classes of amorphous clay constituents in soils. They
have a common characteristic due to structural randomness, but react differently in physical and chemical reactions in soils because of differences
in chemical composition and structure.
The amorphous clay constituents in soils derived from volcanic ash have
been studied most extensively and intensively, simply because they are predominating in the clay fraction and have predominating influence on the
physical and chemical properties of the soils. Considerable progress has
been made in characterization of these amorphous clay constituents, specifically allophane, imogolite, and opaline silica, by application of selective
dissolution techniques combined with physical methods as represented by
infrared spectroscopy and electron microscopy.
Amorphous constituents have a significant effect on the properties of
soils. Unique physical properties associated with these components include
high water holding capacity, slippery but nonsticky consistence, high Atterberg limits, greater values for liquid and plastic limits on undried than dried
samples, low bulk densities, high void ratios, and large preconsolidation
loads, and anomalous compaction behaviors. Some of the more important
chemical properties include ability to retain large amounts of organic matter, pH-dependent cation and anion exchange capacities, large phosphate
fixation capacities, weak strength of ion adsorption, and high pH-low base
saturation relationships. Knowledge about structure, morphology, and
charge characteristics of allophane and imogolite has provided a basis for
interpreting their effect on the soil properties which pose a number of problems in practical management and use of soil systems.
A better understanding of the processes of soil development from volcanic ash has been obtained by applying advanced methods of mineralogical analysis, and by relating the formation and transformation of the amorphous clay constituents to crystalline clay constituents and to accumulation
of organic matter.
One of the problems left to future studies is characterization of amorphous sesquioxidic constituents, which are probably associated either with
allophane or with organic matter. There are indications that these relatively