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V. Processes and Formation of Oxisols
ed to water that is undersaturated with silica experience a loss of silica. Silica loss
is to be expected in the surface of almost all soils as rainwater inﬁltrates and moves
through the surface layer. Dissolution of silica from any of the silicate minerals is
time dependent (Wilding et al., 1977). As inﬁltrated water moves downward in the
soil, silica is dissolved. The amount of silica removed depends on the residence
time around the silicate mineral and the type of mineral. If the inﬁltrated water is
taken up by actively growing plants, some silica is biocycled but any layer of soil
subject to the downward movement of water that is undersaturated with respect to
the solubility of the silicate mineral present will experience a net loss of silica. Net
silica loss results in a lowering of the Si/Al ratio and kaolinite enrichment of the
clay fraction (Carvalho et al., 1983).
Desilication and the formation of gibbsite, halloysite, and kaolinite take place
on the initial weathering of granite rock and clay suites with the mineralogical
composition of oxic horizons are formed in saprolite 5 to 15 m below the soil surface in udic SMRs (Eswaran and Bin, 1978a,b,c; Calvert et al., 1980a,b). Such
saprolite material needs only to be disturbed by pedoturbation processes to be recognized as kandic horizons or, if lacking weatherable sand and silt, oxic horizons.
Quartz solubility is between 3 and 7 mg literϪ1, with the rate at which it dissolves increasing as the particle size decreases accounting for the almost universal lack of clay-sized quartz in soil materials subject to leaching. Sand-sized
quartz, with less surface area per unit weight, is dissolved less rapidly, accounting
for quartz sand stability in leached soil material. Noting this behavior of silica, a
mechanism of kaolinite-rich clay and quartz-rich sand material can accumulate in
gently sloping landscapes where the surface material is subjected to minimal erosion and redeposition after the transport of short distances. If the soil environment
is not subjected to a reducing condition, iron released from iron silicates is concentrated as iron oxides by the dissolution and removal of silica. Almost all of the
iron in Oxisols is present as iron oxides, with the iron-bearing silicates having been
weathered. Oxisols found on stable landforms and formed in apparently translocated sediments often have relatively high iron oxide contents. The rapid accumulation of eroded surface sediments desilicated previously perhaps accounts for
the presence of Oxisols in ﬂood plains as reported in Sierra Leone (Odell et al.,
Oxisols are often best known because of the many red, red-yellow, and yellow
hues various pedons exhibit. Dark red hues are indicative of hematite mineral,
whereas yellow colors indicate geothite. Mixtures of these minerals (Bigham et
al., 1978) obtain intermediate colors. Macedo and Bryant (1987, 1989) found that
hematite is reduced more easily, and thus dissolved, than geothite. Even short periods of reduction around decaying roots will, over time, cause the removal of
hematite, leaving geothite as the dominate iron oxide in the more yellow Oxisols.
If an Oxisol or oxic material is subjected to prolonged periods of reduction, the
iron oxides are removed as in gray or gley Aquoxs, which are present in poorly
drained depressions with high water tables.
S. W. BUOL AND H. ESWARAN
Although Oxisol pedons contain more organic carbon than most other mineral
soils (Sanchez et al., 1982; Sanchez, 1987; Eswaran et al., 1993, 1995; Lepsch et
al., 1994), much of the organic carbon in the subsoil appears to be relatively unavailable to soil microbes. Couto et al. (1985) observed Ustoxs that developed no
low chroma (2 or less) whereas saturated conditions were observed throughout
much of the year. They subjected samples to saturated conditions and observed
that surface (0 to 40 cm) horizon material was reduced, but subsurface (40 to 80
cm) samples, some containing as much as 12.5 g kgϪ1 organic carbon, did not become reduced until sucrose was added to provide an energy (available carbon)
source for the microbes.
Areas of Oxisol formation must have minimal rejuvenation of weatherable minerals in the pedon. Rapid burial of Oxisol pedons by volcanic ash or eolian deposits obviously result in a different soil. Slow recharge of an area by aerosols is
often detected less easily because pedoturbation processes, especially the activity
of ants and termites, tend to homogenize accumulating sediments. Areas of Oxisols are frequently rejuvenated by erosion, which removes oxic material and
exposes relatively unweathered material at the land surface (Ruhe, 1956). Relatively level and stable surfaces of Oxisols, surrounded by erosional geomorphic surfaces of Inceptisols, Ultisols or Alﬁsols, are common features of many
Oxisol-dominated areas (Camargo et al., 1981; Lepsch and Buol, 1988). The nature of the soils on the associated erosional surfaces depends in large part on the
nature of the material being exposed. While Inceptisols and Ultisols are perhaps
the most common soils within areas dominated by Oxisols, signiﬁcant areas of
Mollisols and Alﬁsols are present where the exposed material is calcareous or basic rock (Lepsch et al., 1977b).
The presence of kandic horizons on side slopes below nearly level landscapes
of Oxisols is frequently observed in thick formations of oxic materials. On sloping landscapes, usually where slopes exceed 8 to 10%, pedons are present that have
clay content increases with depth and kandic horizons, whereas pedons on more
level land have little clay content increase with depth. Clay skins are frequently
present in these kandic horizons but are absent in the Oxisols. Beinroth et al.
(1974) in Hawaii attributed this development to shear processes associated with
creep movement on the slopes that disrupted interparticle bonds of the stable oxic
material, allowing the clay to be mobilized for lessivage. In Brazil, Moniz and
Buol (1982) observed the same relationship but attributed it to seasonal interﬂow
of water from the above surface that caused saturation, accompanied by the complete relaxation of interaggregate tension and the formation of blocky structure of
lower hydraulic conductivity than the granular structure. The anisotropic hydraulic
conductivity within the soil proﬁles on the lower slopes would be responsible for
microsite reduction of iron oxides, thus freeing silicate clays for lessivage and clay
skin formation on the surface of the blocky peds and increased clay content in the
VI. SOIL–LANDSCAPE RELATIONS
A. SUR AMERICANA AND ASSOCIATED SURFACES IN BRAZIL
Central Brazil, centered at the capital Brasilia, is a classic example of landscape
resulting from multiple erosion cycles (Feuer, 1956). The area forms the major divide between the watersheds of the Amazon and Parana rivers. Precambrian crystalline metamorphic rocks underlie the area, but the soils are formed in younger
Neocenozoic superﬁcial deposits. Layers of gravelly stone lines are occasionally
present in pedons (Fig. 1). Stone lines are most frequent below escarpments and
decrease in thickness with distance from the escarpment (Cline and Buol, 1973).
Gravel in the stone lines is quartz, ironstone (laterite), or mixtures of the two. Ironstone gravel is more abundant as distance from the center of the divide increases
and quartz is more abundant near the divide center. The broad plateau features have
plinthite formations near their perimeters that harden into ironstone as erosion
cause the scarps to retreat. Ironstone gravel is derived from this ironstone and is
deposited on lower planation surfaces. Near the center of the divide the quartz
gravel can be attributed to a few Gondwana remnants that rise above the highest
planation surface (Lepsch and Buol, 1988).
The edaphic savanna or cerrado vegetation in central Brazil is maintained by
the nutrient-poor, acid Oxisols, as attested to by the presence of semideciduous
forests on rare occurrences of basalt or other basic rock that give rise to higher
base-saturated soils (Camargo et al., 1981). Many of these naturally forested areas are no longer visible because local inhabitants, mainly gold miners prior to present farmers, sought these more chemically fertile areas and cleared the trees to
grow food crops.
Although the central Brazilian planation surfaces are characteristic of the Oxisol–geomorphic associations, smaller areas are present throughout central Brazil,
with Oxisols formed in polycyclic (remanie) reworked superﬁcial deposits and
studied by several authors cited by Lepsch and Buol (1988). Where the reworked
deposits have been derived from basalt or other basic rock Eutrustoxs and Eutrudoxs, often associated with Mollisols and Alﬁsols, are present (Carvalho et al.,
1983). In these areas, Eutrustoxs or Eutrudoxs in udic SMRs are most often present on the more gentle slopes below Acrustoxs (acrudoxs). Mollisols and Alﬁsols
are often present frequently on the steeper slopes where they may be forming in
saprolite from basalt or other basic rock rather than from the superﬁcial deposits.
B. LOWER AMAZON BASIN
Oxisols dominate the landscape in the lower Amazon basin but few are present
west of Manaus between the Negro and Madeira rivers (Camargo et al., 1981). Most
S. W. BUOL AND H. ESWARAN
of these Oxisols are identiﬁed as yellow and red-yellow latosols (Latosolo Amarelo and Vermelho-Amarelo Latossolo) of acid reaction (distroﬁco). In Soil Taxonomy, Xanthic subgroups were established to identify their distinctive pale yellow
color. Although limited research has been conducted, these Oxisols have lower contents of iron than present in many Oxisols. This probably accounts for the lower
phosphorus ﬁxation experience when fertilizer P is applied. It is probable that the
deposition of kaolinitic clay in the basin occurred during the time the Andean mountains were being formed and the basin was forced to reverse its discharge from a
westward direction and eventually drain into the Atlantic ocean. Saturated conditions in this delta-like environment dictated that iron oxides were reduced and removed. Xanthic Oxisols are formed in clayey, kaolinitic sediments on nearly level
surfaces dissected by modern erosional stream valleys. Limited observations have
found that the kaolinitic sediments often overlie white sand deposits. Where the sand
deposits are exposed in the river valleys, Spodosols and Quartzipsamments, depending on the depth to a spodic horizon that forms at the water table, have formed.
C. CENTRAL ZAIRE BASIN
Extensive areas dominated by Oxisols and other soils with low activity clay
mineralogy, primarily Ultisols, are present in central Africa. Ruhe (1956) determined that many of the soils formed in transported material most probably of Tertiary age. Eswaran et al. (1975) conﬁrmed the transported nature of these materials with micromorphologic studies and the presence of gravel deposits, stone lines,
below the sediment, and the underlying mica schist. Sys (1972) compiled data
from 230 pedons in the region, not all Oxisols, and Smith et al. (1975) classiﬁed
the Sys pedons according to Soil Taxonomy (Soil Survey Staff, 1975). This work
provides a valuable link to the Institut National pour les Etudes Agronomiques au
Congo (INEAC) soil classiﬁcation system (Tavernier and Sys, 1965) used by many
authors and Soil Taxonomy.
Many volcanic mountains bound the Zaire basin on the east. Therefore, many
of the soils bordering that area have been enriched by either direct ash fall or alluvium containing volcanic materials (Matungulu, 1992). It is probable that much
of the fertility accorded higher elevations in the eastern part of the basin can be attributed to these more recent basaltic materials. The Kalahari desert lies to the
south of the basin, and sand from that area is suspected to have inﬂuenced soil textures in the southwest part of the basin (Sys, 1983; Kalima and Spaargaren, 1988).
D. TERTIARY SURFACES OF AFRICA
Old stable land surfaces are characteristic of Oxisol landscapes. It is often not
possible to determine with certainty that these surfaces are of Tertiary age, but most
authors place them as older than Pliocene or Pleistocene. In Kenya, Oxisols are
found in udic and ustic soil moisture regimes, but many have aridic soil moisture
regimes (Muchena and Sombroek, 1983). Where formed from materials derived
from schist, base saturation is low, but a higher base saturation (“Eutr” great
groups) is often present where the parent material has been derived from basic
rocks. Oxisols are identiﬁed well into the isomesic soil temperature regime, with
sites in Burundi having mean annual temperatures of 11ЊC (Opdecamp and Sottiaux, 1983). The cooler Oxisols often have umbric epipedons and some, with a
higher base saturation, have mollic epipedons. Sombric horizons are also commonly present (Frankart, 1983).
E. LOCALIZED FORMATIONS ON BASIC ROCKS
Buurman and Soepraptohardjo (1980) described Acrudoxs on stable landscapes
on lower foot slopes in Sulawesi, Indonesia. Associated soils on steeper slopes
were Rhodudalfs and Inceptisols. These soils formed on materials with an abundance of iron-containing primary minerals are often bright red to dusky red in color. Local farmers call them “tanah Merah” (red soils). Their ease of cultivation and
general resistance to erosion are properties liked by farmers. However, their low
fertility status, particularly their propensity to ﬁx added phosphorous, makes them
difﬁcult to manage. Local farmers use heavy application of farmyard manure for
annual crops or interplant with tree crops.
Similar occurrences of Oxisols have been reported on the volcanic islands of the
Paciﬁc and the Caribbean. In Fiji and western Samoa, Hapludoxs are present with
acrudoxs on the toe slopes of volcanic terrain. In New Caledonia, Acrudoxs are
formed on ultrabasic rocks; these rocks also yield copper and nickel. In Malaysia,
Oxisols are formed on basaltic or andesitic parent materials. The basaltic deposits
are about 8 million years old as shown by K-Ar dating (Eswaran and Paramanathan, 1977). A characteristic feature of these Oxisols formed on basic and ultrabasic rocks is the absence of a distinct saprolite zone. At contact with the underlying rock is a rim, a few centimeters thick, that is weathered and frequently
rich in smectite. The overlying solum is composed of kaolinite, gibbsite, and
goethite clays. Many of these soils on the basic rocks are chocolate colored and
were referred to as “chocolate soils” by local inhabitants. In many of the islands
of the Paciﬁc, coincidentally, cocoa is the dominant crop on such soils.
F. OCCURRENCE ON RECENT AND SUBRECENT ALLUVIUM
Oxisols are present on recent river alluvium where the material deposited is rich
in kaolinite and contains few weatherable minerals. Odell et al. (1974) found this
in the river systems of Sierra Leone, west Africa, where the upland soils were pri-
S. W. BUOL AND H. ESWARAN
marily Alﬁsols and Ultisols, and attributed the lack of weatherable minerals and
the low-cation exchange capacity to preweathering of parent material.
Riverine alluvium is generally the parent material for many of the Aquoxs. Upland
soils associated with these Aquoxs are usually well-drained Oxisols or Alﬁsols and
Ultisols belonging to Kandi great groups. In bauxitic areas, as in Malaysia, Aquoxs
are formed on the alluvium from the bauxite deposits. In these soils, rounded gibbsitic
nodules are present and are probably deposited as alluvium (Eswaran et al., 1977).
VII. FEATURES AND PROPERTIES
A. MINERALOGY AND MICROMORPHOLOGY
Weathering and transformation of primary rock-forming minerals and the subsequent alteration of the secondary clay minerals eventually leads to a mineral
suite that is, for most practical purposes, “resistant” (M. P. F. Fontes, 1988; M. R.
Fontes, 1990). The resistant minerals are frequently the oxide or oxyhydrate forms
of the metal elements such as iron, aluminum, and silica. The silicate clay mineral associated with these is usually kaolinite. The common iron minerals are
goethite and hematite with maghemite being present in soils derived from basic or
ultrabasic rocks (Herbillon, 1980). The aluminum mineral is generally gibbsite;
boehmite has been reported but is not normally present in Oxisols (Eswaran et al.,
1977). Anatase, rutile, and other very resistant minerals are present in small
amounts in the heavy sand fraction but much of the sand is quartz (Herbillon,
1988). These relatively resistant minerals also undergo slow weathering, as shown
by the etch surfaces of the minerals when observed under the high magniﬁcations
of the scanning electron microscope (Eswaran et al., 1977).
The clay fraction (Ͻ2 m) is composed largely of kaolinite, which is generally disordered and has a higher amount of lattice iron (Fripiat and Gastuche, 1952)
than those from geologic deposits (Herbillon et al., 1976). Inclusion of iron or aluminum in the lattice of secondary minerals takes place during the mineral formation stage and results in properties different from the purer form of the mineral
(Fey and Le Roux, 1976). Aluminum substitution in goethite has been established
since the ﬁrst observations of Norrish and Taylor (1961). Aluminum substitution
was found to vary from 17 to 36 mol % for goethite, 6–26 mol % for hematite,
and 16–26 mol % in maghemite (Fontes, 1988). Partly resulting from substitutions
and also due to the lower crystallinity of the minerals, Oxisol clays have a higher
speciﬁc surface area with an associated higher pH-dependent negative and positive charge when compared to their purer and better crystallized forms (Fey and
Le Roux, 1976). The surface area of Oxisol kaolinite is about 60 m2gϪ1, approximately four times greater than pure kaolinite.
The relative proportions of kaolinite to gibbsite in Oxisols are quite variable.
Kaolinite:gibbsite ratios of 0.5 to 3.14 are present in Oxisols derived from maﬁc
(basalt) rocks, schist, and clayey sediments, but pedons with ratios above 9.2 and
pedons with no gibbsite are present in Bauru sandstone within the Triangulo
Mineiro region of Minas Gerais State, Brazil (Fontes, 1988).
Fontes (1990) found that humic acid extracted from Humic and Typic Hapludoxs contained crystalline, highly Al-substituted goethite particles 10–15 nm in diameter. While the surface area of the humic acid/goethite complex is 1 m2gϪ1 or
less, the surface area of the goethite is 100 m2gϪ1 or more. She concluded that ligand-exchange mechanisms and Coulombic attraction were responsible. Phosphate
adsorption on the goethite was reduced greatly in humic acid/goethite complexes.
Another mineral that has been reported in the surface horizons of Oxisols is aluminuous chlorite or hydroxy interlayered mineral (HIM). Some suspect that it is
inherited from the rock and preserved in the soil (Flach et al., 1969), whereas Lelong and Millot (1966) subscribed to a process they term as retrogenesis, whereby a mica mineral is formed through soil processes. In a review of the literature,
Barnhisel (1977) attributed these very resistant clays to either incomplete degradation of chlorite or depositions of hydroxy material within the interlayers of
expansible layer silicates. Greater HIM abundance near the surface and lack of
chlorite in subsoils would indicate that the hydroxy interlayering process is more
probable in most Oxisols.
The presence of amorphous or short range ordered minerals has been speculated for a long time due to the high speciﬁc surface areas and the presence of a pHdependent charge. Opaque masses in transmission electron microscopy were considered as the amorphous component, but Segalen (1968) and Gallez et al. (1976)
showed that this form is a minor component. More recent nuclear magnetic resonance studies and other supporting data show the presence of cryptocrystalline
goethite and hematite in the clay fraction. Unlike iron minerals, aluminum minerals are usually very well crystallized and present dominantly in the ﬁne-silt fraction (Eswaran et al., 1977).
The characteristic mineral association—disordered kaolinite with sesquioxides—imparts many of the deﬁning characteristics of Oxisols. These include stable aggregate structure, nutrient-holding characteristics, water-holding characteristics, and general response to management. These features are elaborated later.
Other soils may have one or more of these characteristics, but the combination of
all these characteristics throughout the soil proﬁle distinguishes Oxisols from other soils.
The microfabric characteristics also show major differences from other soils and
have been well documented by Stoops (1968), Buol and Eswaran (1978), and others. Clayey Oxisols with a low sand content have a homogeneous fabric with few
modiﬁcations of plasma. Stress-induced features are absent and cutanic features
resulting from clay translocation, such as ferriargillans, are rare. This uniform ma-
S. W. BUOL AND H. ESWARAN
trix may extend to depths of several meters in Oxisols formed from sediments.
Relics of pedogenesis, from which the transformations may be interpreted, may be
present. Such relics include weathered rock fragments, nodules and concretions of
iron with or without manganese, embedded cutans in which the clay aggregates
have lost their optical properties, and preserved biological features.
There are Oxisols where pedogenesis results in speciﬁc horizons, and in thin
sections of these soils, special features may be observed. Many Oxisols on old geomorphic surfaces are formed on materials with laterite or petroplinthite fragments. The petroplinthites are generally composed of well-crystallized goethite
(Eswaran and Raghumohan, 1973) and may also have inclusions of manganese and
iron minerals (Eswaran et al., 1978). The petroplinthite may incorporate fragments
of ferriargillans that are fossilized features, indicating that the petroplinthite was
formed in another environment and during another period (Eswaran et al., 1978).
Gibbsitic nodules are also frequent in such soils and the gibbsite crystals are usually well crystallized (Eswaran et al., 1977). A range of other features may be present in Oxisols and these are well documented (Brewer, 1964; Stoops, 1968; Bennema et al., 1970; Comerma and Chirinos, 1976; Eswaran and Tavernier, 1980).
Oxisols with isothermic soil temperature regimes often have organic-enriched
horizons extending to a meter or more in depth. Apart from making the fabric
opaque, the high amounts of humus do not impart other features to the soil.
When a water table is present, the redoximorphic condition results in a net removal or reorganization of the iron. In many Aquoxs, the lower horizons are
bleached as the soil material is devoid of staining iron and have low contents of
dithionite–citrate–bicarbonate (DCB) extractable iron (FeDCB) (Table V). Under
certain conditions, the reorganization of iron results in the formation of plinthite,
with the fabric showing iron-rich and iron-impoverished zones. In the zone of water table ﬂuctuation, nodules and concretions of iron with or without manganese
also form. The plinthite, nodules, and concretions are all precursors of petroplinthite
(Eswaran and Raghumohan, 1973) that form on the permanent lowering of the water table as rivers entrench and hydrology of the landscape is altered. Hardening of
plinthite to petroplinthite takes place in situ as the water table deepens. As erosion
of the landscape progresses, the petroplinthite is exposed and the most resistant nodules and concretions behave as gravel when transported and deposited in stone lines
at a new site (Alexander and Cady, 1962; Cline and Buol, 1973).
By deﬁnition, the weatherable mineral content of the 50- to 200-m fraction in
the oxic horizon must contain less than 10% weatherable minerals. Feldspars,
feldspathoids, ferromagnesian minerals, glass, micas, zeolites, and apatite are considered weatherable minerals (Soil Survey Staff, 1996). The determination of
weatherable mineral content is most often made by optical grain counts (National
Soil Survey Center, 1996). Also, a total elemental analysis ﬁnding less than 25
cmol of Ca2+, Mg2+, K+, and Na+ kgϪ1 soil offers a viable estimate of weatherable mineral content less than 10% (Herbillon, 1988).
B. STRUCTURE AND CONSISTENCE
Oxic horizons, being sandy loam or ﬁner by deﬁnition, usually have a minimum
of 15% clay and may contain 80% or more clay (Table IV). Despite the wide range
of clay content that Oxisols may have, their physical properties are determined by
the sesquioxides and the poorly ordered kaolinite mineralogy. In a slightly dry
state, the soil has a very friable to friable consistence (Soil Survey Division Staff,
1993). The ﬂoury ﬂow of the soil material between the ﬁngers was an early observed property of the soil. Some form of subangular blocky structure may be discerned in the oxic horizon in the ﬁeld, but the grade of blocky structure is very
weak. The blocky structural elements break abruptly when slight pressure is applied by the thumb and foreﬁnger, revealing a strong ﬁne and very ﬁne granular
structure (Soil Survey Division Staff, 1993).
The microfabric of the ﬁne and very ﬁne granular materials is very porous and
is well established by water retention studies. The high porosity leads to a low bulk
density, which is generally between 1.0 and 1.3 Mg mϪ3 (El Swaify, 1980). The
high sesquioxide content also imparts other features. Oxic material with a high oxide content is generally not very sticky. Even the quartz grains appear clean with
little or no soil material sticking to them. However, Xanthic Hapludoxs with low
oxide content and more than 60% clay, most of which is less than 1 m, are very
sticky and very plastic. Oxide-rich material is also hydrophobic to some extent.
Vigorous rubbing for several minutes is required to destroy the very ﬁne granules
that feel like sand when particle-size estimates are made by “hand texturing” in
the ﬁeld (Cline and Buol, 1973). Water moves rapidly through the large pores between the ﬁne and very ﬁne granules. The combination of high porosity and low
wettability makes the soil resistant to erosion. Oxisol landscapes are stable unless
poor management initiates erosion and soil loss.
Due to the low shrink–swell capacity of the material, biologic features in the soil
are generally well preserved. Evidence of worm and other soil faunal activities is
preserved and may be seen in the ﬁeld or in thin sections. In some Oxisols, the preponderance of such relict features has caused some to suggest that such soils are continuously biologically reworked (Stoops, 1968), speciﬁcally by termites. They have
also used this to explain the absence of other pedological features, such as cutans.
C. CHEMISTRY AND PHYSICS
The mineralogy of Oxisols, particularly those with high amounts of sesquioxides, speciﬁcally iron oxides and oxyhydrates in the ﬁne earth fraction, imparts
some unique properties to these soils. These properties are used as deﬁning characteristics of Acric (Acr) great groups. Aluminum oxyhydrate minerals, such as
gibbsite, are inert and do not appear to contribute to these special features.
Physical Properties of Selected Pedons
Water retention (%)
Humic Rhodic Eutrustox
30 – 48
40 – 64
Particle size (%)
With increasing iron oxyhydrate content, usually about 10% Fe2O3, and when
the organic matter content is low, the soils attain a net positive charge. Organic
matter has a very high negative charge and a very low zero point of net charge
(ZPNC) and, when present in high amounts, imparts a net negative charge to the
soil. For this reason, the surface horizons of Oxisols always have a net negative
charge and if there are sufﬁcient positive charge minerals in the soil, the net positive charge is expressed in deeper layers (Van Raij and Peech, 1972).
⌬pH, which is the difference between pH in 1 M KCl solution and pH in H2O,
is used to express the sign of the charge. When ⌬pH is negative, the net charge is
negative and vice versa. The pH at ZPNC, termed pHo, is a more reliable measure
but is tedious to measure and so ⌬pH is used as a surrogate value (Uehara and Gillman, 1980; Uehara, 1995). Table V shows pH values and ⌬pH, and Fig. 3 shows
a plot of pH values in H2O, KCl, and pHo with depth in four kinds of Oxisols.
When ⌬pH is zero, the system has a net zero charge, and this point differentiates
the soil into two distinct parts. The upper part of the soil, where organic matter
contents are greater, as in the case of the Anionic Acrudox and Typic Acraquox in
Fig. 3 and Table V, the soil has a net negative charge and can be considered as a
cation exchanger. Fertilizer elements such as Ca2+ and K+ are exchanged by the
H+ ion on the exchange surfaces and are retained for plant uptake. However, in
horizons where ⌬pH is positive, the soil behaves as an anion exchanger. Here the
cations may leach readily and anions such as nitrates and sulfates are captured. In
some Oxisols, anion exchange may extend to a depth of several meters. The positive charge of subsoils minimizes the leaching of NOϪ
3 and the potential contamination of groundwater. This is an important feature of Oxisol landscapes. In other soils, nitrate leaching and pollution of the aquatic system are environmental
concerns. Erosion and loss of organic matter may result in the positive charge being expressed closer to the surface.
D. SOIL COLOR
Oxisols come in all colors. Although hues of red, red-yellow, and yellow are
most common, the poorly drained Oxisols subjected to gleization are gray, reﬂecting the color of kaolinite, quartz, and gibbsite. Some Oxisols are nearly black
due to the high contents of organic matter. Bigham et al. (1978) found that DCB
iron was most concentrated in the ﬁne clay fraction (2 m) of oxic and kandic horizons. Proportions of hematite and goethite in the ﬁne clay fraction were indicated
by color hue. No hematite was found in samples of 7.5 or 10 YR hue. Hues between 2.5 and 5 YR contained between 25 and 75% of both hematite and goethite.
Soil hue of 5 R contained more than 75% of its iron as hematite and less than 25%
as goethite. Resende (1976) found that the 10 YR hue of an Oxisol changed to a 5
YR hue when 1% by weight of ﬁnely powdered hematite was added. Eswaran and