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Alfisols
Alfisols
Summary:
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Vegetation: deciduous forest (prairie, grassland)
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Climate: thermic or warmer, mesic or cooler
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Soil moisture regime: erratic soil moisture regime
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Major soil property: medium to high base saturation
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Diagnostic horizons: albic, argillic (natric, kandic)
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Epipedon: ochric (mollic, umbric)
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Major processes: weathering, eluviation/illuviation
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Alfisols - Environmental Conditions
Climate: The climatic conditions under which Alfisols form are thermic
or warmer and mesic or cooler. Therefore, most Alfisols are in temperate
regions, but these soils are also extensive in tropical and subtropical
zones. Alfisols can occur generally in zones with a temperature range from
below 0 oC to above 22
o C. Important for the development of Alfisols is the change
between periods of high moisture content and high soil temperature, to break
down the primary mineral components and to leach the weathered products,
and low moisture content and low soil temperatures, which permit the precipitation
or accumulation of the weathered products. Most Alfisols have an udic, ustic,
or xerix moisture regime, and many have aquic conditions, but they are not
known to have a perudic moisture regime. The suborder of Aqualfs requires
higher soil moisture conditions compared to the development of other suborders
of the Alfisols.
Vegetation: Most Alfisols are formed under broadleaf deciduous forest,
but they occur also under grassland and prairie vegetation. In forested
ecosystems, the trees deliver the bulk of their annual production of organic
matter aboveground, which is different from grassland soils. In those
ecosystems the organic matter is enriched by the huge root system of the
grass or prairie cover. While present vegetation may be deciduous forest,
earlier vegetation may have been grass or conifers.
Relief: In most Alfisols the drainage is not restricted with
the water table occuring below the solum during major portions of the nonfrozen
period. For instance, the suborder of Aqualfs is often functionally related
to landscape position. Alfisols develop under several drainage conditions
ranging from excessive on hill crest and steep slopes (e.g Lithic Hapludalfs)
to poorly drained footslopes and level plains (e.g. Albaqualfs). Alfisols
do not develop on very steep slopes, alluvial floodplains, and very poorly
drained depressions. High elevations combined with limited rainfall favor
Alfisol formation in the tropics.
Parent Material: The parent material has a major impact on the formation
of clay minerals within soil. The resistance to weathering and the composition
of primary minerals determine in combination with the other soil forming
factors which clay minerals are formed. Generally, a wide variety of clay
minerals ranging from kaolinites, hydrous micas, montmorillonites to vermiculites
can occur. It should be stressed that several clay minerals do have a potential
to adsorb exchangeable bases (high cation exchange capacity), which is
a criteria that should be met to qualify for an Alfisol. Most Alfisols are
present on relative old landscapes (beginning Holocene or older) wherever
the supply of primary minerals is plentiful.
Time: Most Alfisols need a longer period of time for development.
Several studies postulated that the time to develop Alfisols is at least
200 y, where an argillic horizon is approached, to 1000 y for a clear expression
of an Alfisol profile, and even longer periods, depending on the other
soil forming factors.
Alfisols - Processes
The weathering of primary mineral components is a prerequisite
for further processes to form Alfisols. Water is the master ingredient to
accelerate physical and chemical weathering, particularly for hydration,
hydrolysis, and oxidation. If the primary minerals are weathering in an alkaline
environment then carbonates often initially dominate the weathering products.
The release of H+ ions for Ca
2+ , Mg2+, and a variety
of other cations, from the roots of vegetation fosters also the process of
weathering. At the same time, under forest vegetation, most profiles show
Ca 2+ and Mg2+
higher in amount in the surface horizon than in horizons below. This
may be attributed to recycling through leaf fall and decay. On the other
hand, lower Ca 2+ and Mg
2+ values in the lower horizons of Alfisol solum can be an
indication of more intense weathering.
The litter is decomposed to form an A horizon (decomposition,
humification, mineralization). Under deciduous forest often
an O and A horizon is found. There is relatively little accumulation of
organic matter in the mineral horizons due to cycling of nutrients in the
upper horizons. Biocycling of nutrients from B horizons to A and O horizons
is an important process in most forested Alfisols. This explains the high
content of bases (Ca, Mg, and K) in the ochric epipedon.
Eluviation of clay (in organic and inorganic form) from the A and
E horizons in the initial material, of clay formed by mineral weathering,
and of clay progressively added in eolian material is a dominant process
in the formation of Alfisols. The eluviated material is illuviated in the
underlying B horizon (illuviation), i.e., an argillic horizon is
formed. Therefore, particularly the E horizon, is depleted in organic
colloids, clay minerals, and / or oxides and hydroxides, i.e., an albic diagnostic
horizon is formed. The process of clay translocation is also called lessivage
. An erratic moisture regime favors the formation of an argillic horizon,
because the processes of weathering and translocation are supported by
percolation water and the precipitation of the translocated material by
dry moisture conditions.
The details of eluviation and illuviation can highlight the complexity
of a variety of sub-processes involved in the development of Alfisols.
Leaching of carbonates from the top layers appear to be a prerequisite
before clay can migrate. The presence of exchangeable calcium (from calcium
carbonate) flocculates clay particles, creating particles that are too
large to be transported in suspension. Removal of the calcium leaves the
solum in a condition favorable for the dispersion of clay particles. When
the clay particles are dispersed in an aqueous suspension translocation
from the A and E horizons into the B horizon occurs with or without
aid of complexing organic compounds, and possibly by migration of Si,
Fe and Al under the influence of percolating water. Fine clays move more
readily than coarse clay, therefore, the fine clay to total clay ratios
are typically higher in the B horizon (0.6 - 0.8) than in the A and E horizons
(0.3 - 0.6). Freshly formed clays tend to move more readily than older
clays. The influence of organic matter on the transport phenomena of clay
colloids in soils has been stressed by many authors. Organic matter is known
to act as an electron donor for the reduction and solubilization of iron
oxides which are leached. Sesquioxides do act as a cohesion agent. Furthermore,
the presence of organic acids tends to destabilize the soil micro-aggregates
and produce dispersible clays which are subsequently leached.
Argillans (clay coatings) are formed in the B horizon, which are often
fewer in the upper B compared to the lower B horizon(s). This can be explained
by shrink-swell cycles (freezing-thawing, wetting-drying), soil creep,
and biologic mixing, which are more intense in the upper horizon. The
precipitation of clays, often with sequioxides and organic matter,
in the argillic horizon may be brought about by (i) depletion of percolating
waters through sorption by peds, (ii) swelling shuts of voids and consequent
slowing of percolating water, (iii) sieve action by clogging of fine pores,
(iv) flocculation of the negatively charged clay by positively charged iron
oxides in the Bt horizon or by calcium in the higher-base saturation lower
solum, and (v) low pH which favors flocculation. The accumulation of clay
may be masked by other processes such as pedoturbation.
Additionally, there might be in situ formation of clay minerals
in the B horizon by weathering of primary minerals such as feldspars,
micas, and ferromagnesian minerals, or by neosynthesis from illuvial weathering
products. In young Alfisols the illuviation is the dominant process for
the formation of an argillic horizon, whereas through time the in situ formation
of clays within the argillic horizon becomes more dominant.
If the accumulation of clay materials in the Bt horizon is high
it results in a decrease of percolation and subsequent waterlogging
(reducing, anerobic environmental conditions). The slower permeability
also favors the in situ weathering of primary minerals to clays. For example,
Palexeralfs form on earlier-Pleistocene deposits when clay accumulation
and slow permeability is sufficient to cause perching of a seasonal water
table in the winter. Under such conditions iron oxide concretions form in
horizons affected by a perched water table above dense B horizons.
In most Alfisols there is also a removal of Fe and Al from the
E horizon to the B horizon. This can be attributed to the cheluviation
of metal ions and organic colloids that form metal-organic complexes
which are translocated.
Alfisols - Properties
Characteristics of Alfisols:
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On uncultivated sites: A very thin O horizon is common; on cultivated
sites: no O horizon
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Thin A (less than 15 cm), weakly expressed crumb or granular structure
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Moderate thin E horizon (15 - 25 cm), platy structure, light-colored
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B horizon, usually with several subdivisions, which is normally between
25 - 75 cm thick, moderate to strong angular or subangular blocky structure,
a lower case 't' is used to denote for an accumulation of silicate clay
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Characteristics of the albic diagnostic horizon:
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High in silt-size and larger particles
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High amount of stable minerals such as quartz, tourmaline and rutile
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Absence of organic matter
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Particles are not aggregated
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Higher pH compared to the argillic horizon (pH 6.5 - 7.0)
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Higher Eh compared to the argillic horizon
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Low cation exchange capacity
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Platy structure
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Characteristics of the argillic diagnostic horizon:
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Accumulation of clay (organic and mineral colloids). The illuviated
materials are deposited on structural aggregates, along root channels and
on the surfaces of coarser particles (e.g. argillans)
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Accumulation of iron and aluminum oxides (partly adsorbed to clay
minerals)
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The colloidal organic matter is mostly in the form of organo-clay complexes
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Lower pH compared to the albic horizon (pH 4.5 - 6.0)
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Lower Eh compared to the albic horizon
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High cation exchange capacity
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Blocky structure
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Alfisols - Classification
The requirements to qualify for an Alfisol are the following:
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High base status: > 35 % base saturation at a depth of 125 cm below
the upper boundary of the argillic, natric, or kandic horizon
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An argillic horizon that is not under a spodic or oxic horizon
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Any soil temperature regime is allowed, except pergelic
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The suborders of Alfisols are distinguished by soil temperature and soil
moisture (Figure 1.). The suborders, great groups, and subgroups of Alfisols
are described in the
Keys to Soil Taxonomy
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Figure 1. Diagram showing some relationships between suborders of Alfisols.
Aqualfs: They have aquic conditions for some time in most years
within 50 cm of the mineral horizon and redoximorphic features in the upper
12.5 cm of the argillic, natric, or kandic horizon. Their appearance is
normally controlled by gray redox depletions and higher-chroma redox concentrations.
In some, ground water is near the surface during a considerable part of
the year but drops to depths below the argillic (or natric, kandic horizon)
in another part of the year. In others, the ground water may be deep most
of the year but horizons that have low hydraulic conductivity restrict
the downward movement of water and extend the period of saturation. Aqualfs
occur in many parts of the world, mostly in small areas in deposits of late-Pleistocene
age, where they occupy depression areas or low-gradient landscapes subject
to seasonal high water tables. Nearly all Aqualfs are believed to have
supported forests at some time in the past. Most Aqualfs, except those
that have a frigid or cryic temperature regime, have some artificial drainage
or other water control and are cultivated. Rice is a common crop on Aqualfs
that have a thermic or warmer temperature regime.
Boralfs: Boralfs are the more or less freely drained Alfisols of
cold regions. They have a cryic temperature regime and an udic moisture regime
is considered normal. Boralfs are not extensive. They form in North America,
eastern Europe, and Asia above 49o
north latitude and in some high mountains south of that latitude. In the
mountains, they tend to form below the Spodosols or Inceptisols. Most of
them are or have been under a coniferous forest. Characteristically, Boralfs
have an O horizon, an albic horizon, and an argillic horizon. A thin A horizon
is present in some. In regions of the least rainfall, they are neutral or
slightly acid in all horizons and a Bk horizon may underlie the argillic horizon.
In many of the more humid areas of their occurrence, the lower part of the
albic horizon and the upper part of the argillic horizon are strongly or
very strongly acid. Boralfs in the U.S. generally developed in Pleistocene
deposits, mostly Wisconsinan age and under forest.
Udalfs: Udalfs are the more or less frequently drained Alfisols
that have udic moisture regime and a frigid, mesic, isomesic, or warmer
temperature regime. They are principally but not entirely on late-Pleistocene
deposits and erosion surfaces of about the same age. Some of the Udalfs
that are on older surfaces are underlain by limestone or other calcareous
sediments. Udalfs are very extensive in the United States and in western
Europe. All of them are believed to have had forest vegetation at some time
during development. Most Udalfs with a mesic or warmer temperature regime
have or had a deciduous forest vegetation and many of the frigid temperature
regime have or had mixed coniferous and deciduous trees. Many Udalfs have
been cleared of forests and intensively farmed, and as a result of erosion
many now have only an argillic or a kandic horizon below the Ap horizon that
is mostly material part of the argillic or kandic horizon. Others are on stable
surfaces and retain most of their eluvial horizon above the argillic or
kandic horizon. Normally, the undisturbed soil has a thin A horizon darkened
by humus. A few Udalfs have a natric horizon. Others have a fragipan in or
below the argillic or kandic horizon.
Ustalfs: They have an ustic moisture regime and a frigid, mesic,
isomesic, or warmer temperature regime. They do not have, near the soil surface,
both redoximorphic features with low chroma and aquic moisture regime for
some time in normal years or artificial drainage. Moisture moves through
most of these soils to deeper layers only in occasional years. If there are
carbonates in the parent material or in the dust that settles on the surface,
they tend to have a Bk or a calcic horizon below or in the argillic or kandic
horizon. The dry season or seasons are pronounced enough that trees are either
deciduous or xerophytic. Many of these soils have or have had a savanna vegetation
and some were grasslands. Most of these soils are used for cropland of for
grazing land. Ustalfs are the Alfisols of subhumid to semiarid regions. Sorghum,
wheat, and cotton are common crops. Droughts are common. They occur in the
United States mostly on the southern Great Plains. They are common in Africa,
India, South America, Austalia, and southeastern Asia. The Ustalfs may be
on erosion surfaces or deposits of late Wisconsian age, but many occur on
old surfaces. In those soils the minerals have been strongly weathered, possibly
in an environment more humid than the present one. At least, the clays in
many of these older soils are kaolinitic. The base saturation in them at
present probably reflects additions of bases in dust and rain.
Xeralfs: They have xeric moisture regime common of regions that
have Mediterranean climate. They are dry for extended periods in summer,
but in winter, moisture moves through the soil to deeper layers in at least
occasional years, if not in normal years. Small grains, and other annuals
are common crops where there is no irrigation. Grapes and olives are also
common crops where the climate is thermic. With irrigation, a wide variety
of crops can be grown. The Xeralfs formed in South Africa, Chile, Western
Australia, Southern Australia and the Western United States. Most border
the Mediterranean Sea or lie to the east of an ocean in midlatitudes.
In the world as a whole, the Xeralfs are not extensive soils, but in the
regions where they occur, they are extensive. The vegetation, before the
soils were farmed, was a mixture of annual grasses, forbs, and woody shrubs
on the warmest and driest Xeralfs and coniferous forest on the coolest
and most moist Xeralfs. Xeralfs formed on surfaces that are different ages.
Some formed on erosion surfaces or in deposits of late-Wisconsinan age,
and some, as in Australia, are on old surfaces and have characteristics
that probably reflect an environment greatly different from the present
one. It is common in the oldest Xeralfs that the boudary between the A
and B horizons is very abrupt. The epipedon of some Xeralfs is hard and
massive when dry.
Great groups and subgroups are classified according to following features:
Alfisol may have (i) a fragipan (e.g. Fragixeralfs, Fragiaquic Paleudalfs),
(ii) a duripan (e.g. Durixeralfs, Durudands, Durustands ), (iii) a kandic
horizon (e.g. Kandiaqualfs), (iv) a natric horizon (e.g. Natraqualfs),
(v) a salic horizon (e.g. Salidic Natrustalfs), (vi) a calcic horizon (e.g.
Calcic Rhodoxeralfs), (vii) a petrocalcic horizon (e.g. Petrocalcic Natrustalfs),
(viii) or plinthite horizon (e.g. Plinthustalfs, Plinthic Paleustalfs).
(i) Fragipans are found in some great groups of Alfisols. It is postulated
that the majority of fragipans have developed nearly concurrent with the
argillic horizon, sometimes as a part of it, in other cases immediately
below. The dense, brittle character of the fragipan is attributed to various
cementing agents such as silicate clays, oxides of iron, manganese and
aluminum, and colloid silica. These are weathering products of the upper
horizons, which are translocated and accumulated in lower horizons. The
phenomena of large polygonal cracking commonly observed in the fragipan
zone suggests a time of desiccation, probably on a recurring basis, with
accumulated in-filling.
Recent research on the formation of fragipans suggest that the 'hydroconsolidation
process', i.e., a structure collapse when loaded and wetted may contributed
to fragipan formation (Bryant, 1989; Assallay et al., 1998). The classic
occurences of hydroconsolidation are in loess soils with a clay content
of 5 to 30 %. Fragipans occur more or less at a constant depth of about
40 to 80 cm below the soil surface.
(ii) In some Alfisols there is a duripan, i.e., a horizon of silica cementation.
For example, the processes to form duripans are the slow weathering of
feldspars and ferromagnesian minerals in older landscapes or rapid weathering
of volcanic glass.
(iii) A kandic horizon is a subsoil diagnostic horizon having a clay increase
relative to overlying horizons and low activity clays, with <= 16 cmol/kg
clay CEC.
(iv) In soils with high Na content the sodium ion is important in the
dispersion and mobilization of clay. Under such environmental conditions
natric horizons can form, where pH may be as high as 10 or 11. Sodium is
a cation which is weakly absorbed and is leached easily. Soil layers high
in sodium are dispersed when wet, and show a low permeability and low aeration.
Natric horizons are expressed in the great groups of Alfisols, for example,
in Natrixeralfs, Natrudalfs, or Natrustalfs. (v) Salic horizons are enriched
in secondary soluble salt such that the electrical conductivity exceeds 30
dS/m more than 90 days each year.
(vi) A calcic horizon is a mineral soil horizon of secondary carbonate
enrichment that is more than 15 cm thick, has a CaCO
3 equivalent of > 150 g/kg. (vii) If a horizon of indurated
carbonates occur the formed diagnostic horizon is called petrocalcic.
In general, a shift to a drier regime with periods of evaporation would
contribute to carbonate accumulation.
(viii) Plinthite is a weakly-cemented iron-rich, humus poor mixture of
clay with other diluents that commonly occurs as dark red redox concentrations
that form platy, polygonal, or reticulate patterns. Plinthite changes
irreversibly to ironstone hardpans or irregular aggregates on exposure
to repeated wetting and drying.
A tonguing of the A horizon into the B horizon is also found in some Alfisols.
The matrix of the tongues is similar to that of the eluvial horizon. It
may be initiated by tree-root penetration and decay. These soils are classified
in the great groups of Alfisols, such as, Glossaqualfs, Glossocryalfs,
Glossudalfs or in the subgroups of Alfisols, such as Glossaquic Paleudalfs,
Glossaquic Natrudalfs, or Glossic Natraqualfs.
Alfisols with vertic soil characteristics, i.e., cracks that are
5 mm or more wide through a thickness of 30 cm or more for some time in
most years, and slickensides or wedge-shaped aggregates in a layer 15 cm
or more thick that has its upper boundary within 125 cm of the mineral
soil surface; or a linear extensibility of 6.0 cm or more between the mineral
soil surface and either a depth of 100 cm or a densic, lithic, or paralithic
contact, whichever is shallower (e.g. Vertic Natraqualfs).
'Albic' materials, i.e., soil materials with a color white to gray mainly
due to the color of primary sand and silt particles and from which clay
and/or free iron oxides have been removed, is used to define Alfisols at
the subgroup level (e.g. Albic Natraqualfs).
Alfisols with recognizable bioturbation such as filled animal burrows,
wormholes, or casts are named 'Vermic' (e.g. Vermic Natraqualfs, Vermic
Fragiaqualfs).
Soil color is used to define 'Aeric' - chroma of 2 or more and no redox
depletions (e.g. Aeric Kandiaqualfs), 'Udollic' - color value moist of
3 or less (e.g. Udollic Albaqualfs), and 'Rhodic' - a hue of 2.5YR or redder
and a value (moist) of 3 or less (e.g Rhodic Kandiustalfs) characteristics
of Alfisols.
Epipedons are also used to distinguish Alfisols at the subgroup level:
'Mollic' (e.g. Mollic Natraqualfs), 'Umbric' (e.g. Umbric Fragiaqualfs),
or 'Histic' epipedon (e.g. Histic Glossaqualfs). 'Humic' is used for Alfisols
with high organic matter content (e.g. Humic Fragiaqualfs).
Soil texture is used to classify Alfisols at the subgroup level: 'Arenic'
or 'Grossarenic' show a sandy or sandy-skeletal particle-size class (e.g.
Arenic Kandiaqualfs, Grossarenic Kandiaqualfs), 'Psammentic' subgroups
show a sandy particle-size class throughout the argillic horizon (e.g.
Psammentic Cryoboralfs).
Soils formed in volcanic parent material with low bulk densities (<
1.0 g/cm3) and more than 35 % fragments
coarser 2.0 mm are denoted by 'Andic', 'Aquandic', or 'Vitrandic' (e.g.
Andic Palexeralfs, Aquandic Albaqualfs, Vitrandic Fragiudalfs).
Alfisols that have episaturation, i.e., when a soil is saturated with
water in one or more layers within 200 cm of the mineral soil surface and
also has one or more unsaturated layers with an upper boundary above 200
cm, below the saturated layers(s) the prefix 'Epi' (e.g. Epiaqualfs) is used.
Alfisols with wet soil moisture conditions and redox depletions with a chroma
of 2 or less (e.g. Aquic Paleboralfs) are named 'Aquic' or 'Oxyaquic' when
soils are saturated with water, in one or more layers within 100 cm of the
mineral soil surface, for 1 month or more per year in 6 or more out of 10
years (e.g. Oxyaquic Paleboralfs).
Alfisols which are shallow are classified as 'Lithic' (e.g. Lithic Cryoboralfs)
and soils which show less pronounced characteristics of an Alfisol are
classified as 'Inceptic' (e.g. Inceptic Fragixeralfs).
Alfisols with high base saturation are named 'Eutr' (e.g. Eutroboralfs,
Eutric Glossocryalfs). If the base saturation (by sum of cations) is less
than 75 % throughout the argillic horizon the prefix 'Ultic' is used for
classification (e.g. Ultic Paleustalfs).
A special feature is the presence of lamellae, which are subhorizons (two
or more), each with an overlying eluvial horizon. The lamellae layers
are of pedogenic origin. Alfisols with these features are classified as
'Lamellic' (e.g. Lamellic Eutroboralfs). Alfisols which are relatively
old soils showing pronounced characteristics to qualify for this order
are denoted by 'Pale' (e.g. Paleustalfs).
Soil temperature regimes are used to classify Alfisols at the great group
and subgroup level: 'Cryic' (e.g. ), 'Xeric' (e.g. Xeric Palecryalfs),
'Ustic' (e.g. ), 'Aridic' (e.g. Aridic Kandiustalfs), 'Udic' (Udic Paleustalfs),
'Torrertic' (e.g. Torrertic Natrustalfs).
Alfisols - Distinguishing Characteristics
If the Entisols are considered of soils in the stage of minimum organization
the Alfisols show a higher degree of organization. Weathering and eluviation
/ illuviation altered Entisols or Inceptisols to form Alfisols. Transitions
between areas of Alfisols and Spodosols lie in ecotones between mixed
deciduous forest and coniferous forest. The Ustalfs tend to form a belt
between the Aridisols of arid regions and the Udalfs, Ultisols, Oxisols,
and Inceptisols of humid regions. A lower content of organic matter in
the surface horizon distinguishes the Alfisols from the Mollisols, which
develop under grassland or prairie. The soil moisture is not high enough
to accumulate organic matter to form Histosols. A pergelic soil temperature
regime would develop Gelisols. Other soil orders with argillic horizons
are Ultisols, Mollisols, and Aridisols.
References
Assallay, A.M., I. Jefferson, C.D.F. Rogers, and I.J. Smalley. 1998. Fragipan
formation in loess soils: development of the Bryant hydroconsolidation
hypothesis. Geoderma 83: 1-16.
Bryant, R.B., 1989. Physical processes of fragipan formation. In: Smeck,
N.E., Ciolkosz I. (Eds.). Fragipans: Their occurrence, classification and
genesis. Soil Sci. Soc. Am. Apec. Publ. 24: 141-150.
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Figure 1. Diagram showing some relationships between suborders of Alfisols.