Soil Taxonomy A Basic System of Soil Classification for Making and Interpreting Soil Surveys

  Sand

detailed definition of what is a mineral soil (soil type affects nutrient availability)

Soil in this text is a natural body comprised of solids
(minerals and organic matter), liquid, and gases that occurs on
the land surface, occupies space, and is characterized by one or
both of the following: horizons, or layers, that are
distinguishable from the initial material as a result of additions,
losses, transfers, and transformations of energy and matter or
the ability to support rooted plants in a natural environment.
This definition is expanded from the previous version of Soil
Taxonomy to include soils in areas of Antarctica where
pedogenesis occurs but where the climate is too harsh to
support the higher plant forms.
The upper limit of soil is the boundary between soil and air,
shallow water, live plants, or plant materials that have not begun
to decompose Soil, as defined in this text, does not need to have discernible
horizons, although the presence or absence of horizons and
their nature are of extreme importance in soil classification.
Plants can be grown under glass in pots filled with earthy
materials, such as peat or sand, or even in water. Under proper
conditions all these media are productive for plants, but they are
nonsoil here in the sense that they cannot be classified in the
same system that is used for the soils of a survey area, county,
or even nation. Plants even grow on trees, but trees are regarded
as nonsoil.
Soil has many properties that fluctuate with the seasons. It
may be alternately cold and warm or dry and moist. Biological
activity is slowed or stopped if the soil becomes too cold or too
dry. The soil receives flushes of organic matter when leaves fall
or grasses die. Soil is not static. The pH, soluble salts, amount of
organic matter and carbon-nitrogen ratio, numbers of microorganisms, soil fauna, temperature, and moisture all change with
the seasons as well as with more extended periods of time. Soil
must be viewed from both the short-term and long-term
perspective.

CHAPTER 3
Differentiae for Mineral Soils1
and Organic Soils

Soil taxonomy differentiates between mineral soils and
organic soils. To do this, first, it is necessary to
distinguish mineral soil material from organic soil material.
Second, it is necessary to define the minimum part of a soil
that should be mineral if a soil is to be classified as a mineral
soil and the minimum part that should be organic if the soil is
to be classified as an organic soil.
Nearly all soils contain more than traces of both mineral
and organic components in some horizons, but most soils are
dominantly one or the other. The horizons that are less than
about 20 to 35 percent organic matter, by weight, have
properties that are more nearly those of mineral than of organic
soils. Even with this separation, the volume of organic matter
at the upper limit exceeds that of the mineral material in the
fine-earth fraction.
Mineral Soil Material
Mineral soil material (less than 2.0 mm in diameter) either:
1. Is saturated with water for less than 30 days (cumulative)
per year in normal years and contains less than 20 percent (by
weight) organic carbon; or
2. Is saturated with water for 30 days or more cumulative in
normal years (or is artificially drained) and, excluding live
roots, has an organic carbon content (by weight) of:
a. Less than 18 percent if the mineral fraction contains 60
percent or more clay; or
b. Less than 12 percent if the mineral fraction contains no
clay; or
c. Less than 12 + (clay percentage multiplied by 0.1)
percent if the mineral fraction contains less than 60 percent
clay.
Organic Soil Material
Soil material that contains more than the amounts of
organic carbon described above for mineral soil material is
considered organic soil material.
In the definition of mineral soil material above, material
that has more organic carbon than in item 1 is intended to
include what has been called litter or an O horizon. Material
that has more organic carbon than in item 2 has been called
peat or muck. Not all organic soil material accumulates in or
under water. Leaf litter may rest on a lithic contact and support
forest vegetation. The soil in this situation is organic only in
the sense that the mineral fraction is appreciably less than half
the weight and is only a small percentage of the volume of the
soil.

ather than the organic soils defined later as Histosols.
If a soil has both organic and mineral horizons, the relative
thickness of the organic and mineral soil materials must be
considered. At some point one must decide that the mineral
horizons are more important. This point is arbitrary and
depends in part on the nature of the materials. A thick layer of
sphagnum has a very low bulk density and contains less
organic matter than a thinner layer of well-decomposed muck.
It is much easier to measure the thickness of layers in the field
than it is to determine tons of organic matter per hectare. The
definition of a mineral soil, therefore, is based on the thickness
of the horizons, or layers, but the limits of thickness must vary
with the kinds of materials. The definition that follows is
intended to classify as mineral soils those that have both thick
mineral soil layers and no more organic material than the
amount permitted in the histic epipedon, which is defined in
chapter 4.
In the determination of whether a soil is organic or mineral,
the thickness of horizons is measured from the surface of the soil whether that is the surface of a mineral or an organic
horizon, unless the soil is buried as defined in chapter 1. Thus,
any O horizon at the surface is considered an organic horizon
if it meets the requirements of organic soil material as defined
later, and its thickness is added to that of any other organic
horizons to determine the total thickness of organic soil
materials.

Definition of Mineral Soils
Mineral soils are soils that have either of the following:
1. Mineral soil materials that meet one or more of the
following:
a. Overlie cindery, fragmental, or pumiceous materials
and/or have voids2
that are filled with 10 percent or less
organic materials and directly below these materials have
either a densic, lithic, or paralithic contact; or
b. When added with underlying cindery, fragmental, or
pumiceous materials, total more than 10 cm between the soil
surface and a depth of 50 cm; or
c. Constitute more than one-third of the total thickness of
the soil to a densic, lithic, or paralithic contact or have a
total thickness of more than 10 cm; or
d. If they are saturated with water for 30 days or more per
year in normal years (or are artificially drained) and have
organic materials with an upper boundary within
40 cm of the soil surface, have a total thickness of either:
(1) Less than 60 cm if three-fourths or more of their
volume consists of moss fibers or if their bulk density,
moist, is less than 0.1 g/cm3
; or
(2) Less than 40 cm if they consist either of sapric or
hemic materials, or of fibric materials with less than
three-fourths (by volume) moss fibers and a bulk density,
moist, of 0.1 g/cm3
or more; or
2. More than 20 percent, by volume, mineral soil materials
from the soil surface to a depth of 50 cm or to a glacic layer or
a densic, lithic, or paralithic contact, whichever is shallowest;
and
a. Permafrost within 100 cm of the soil surface; or
b. Gelic materials within 100 cm of the soil surface and
permafrost within 200 cm of the soil surface.
Definition of Organic Soils
Organic soils have organic soil materials that:
1. Do not have andic soil properties in 60 percent or more of
the thickness between the soil surface and either a depth of 60
cm or a densic, lithic, or paralithic contact or duripan if
shallower; and
2. Meet one or more of the following:
a. Overlie cindery, fragmental, or pumiceous materials
and/or fill their interstices2 and directly below these
materials have a densic, lithic, or paralithic contact; or
b. When added with the underlying cindery, fragmental, or
pumiceous materials, total 40 cm or more between the soil
surface and a depth of 50 cm; or
c. Constitute two-thirds or more of the total thickness of
the soil to a densic, lithic, or paralithic contact and have no
mineral horizons or have mineral horizons with a total
thickness of 10 cm or less; or
d. Are saturated with water for 30 days or more per year in
normal years (or are artificially drained), have an upper
boundary within 40 cm of the soil surface, and have a total
thickness of either:
(1) 60 cm or more if three-fourths or more of their
volume consists of moss fibers or if their bulk density,
moist, is less than 0.1 g/cm3
; or
(2) 40 cm or more if they consist either of sapric or
hemic materials, or of fibric materials with less than
three-fourths (by volume) moss fibers and a bulk density,
moist, of 0.1 g/cm3
or more; or
e. Are 80 percent or more, by volume, from the soil
surface to a depth of 50 cm or to a glacic layer or a densic,
lithic, or paralithic contact, whichever is shallowest.
It is a general rule that a soil is classified as an organic soil
(Histosol) if more than half of the upper 80 cm (32 in) of the
soil is organic or if organic soil material of any thickness rests
on rock or on fragmental material having interstices filled with
organic materials.

Horizons and Characteristics
Diagnostic for Both Mineral and
Organic Soils
Following are descriptions of the horizons and
characteristics diagnostic for both mineral and organic soils

Aquic Conditions2
Soils with aquic (L. aqua, water) conditions are those that
currently undergo continuous or periodic saturation and
reduction. The presence of these conditions is indicated by
redoximorphic features, except in Histosols and Histels, and
can be verified by measuring saturation and reduction, except
in artificially drained soils. Artificial drainage is defined here
as the removal of free water from soils having aquic conditions
by surface mounding, ditches, or subsurface tiles to the extent
that water table levels are changed significantly in connection
with specific types of land use. In the keys, artificially drained
soils are included with soils that have aquic conditions.
Elements of aquic conditions are as follows:
1. Saturation is characterized by zero or positive pressure in
the soil water and can generally be determined by observing
free water in an unlined auger hole. Problems may arise,
however, in clayey soils with peds, where an unlined auger
hole may fill with water flowing along faces of peds while the
soil matrix is and remains unsaturated (bypass flow). Such free
water may incorrectly suggest the presence of a water table,

The duration of saturation required for creating aquic
conditions varies, depending on the soil environment, and is
not specified.
Three types of saturation are defined:
a. Endosaturation.—The soil is saturated with water in all
layers from the upper boundary of saturation to a depth of
200 cm or more from the mineral soil surface.
b. Episaturation.—The 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 a depth of 200 cm, below the saturated
layer. The zone of saturation, i.e., the water table, is perched
on top of a relatively impermeable layer.
c. Anthric saturation.—This term refers to a special kind
of aquic conditions that occur in soils that are cultivated and
irrigated (flood irrigation). Soils with anthraquic conditions
must meet the requirements for aquic conditions and in
addition have both of the following:
(1) A tilled surface layer and a directly underlying
slowly permeable layer that has, for 3 months or more in
normal years, both:
(a) Saturation and reduction; and
(b) Chroma of 2 or less in the matrix; and
(2) A subsurface horizon with one or more of the
following:
(a) Redox depletions with a color value, moist, of 4
or more and chroma of 2 or less in macropores; or
(b) Redox concentrations of iron; or
(c) 2 times or more the amount of iron (by dithionite
citrate) contained in the tilled surface layer.
2. The degree of reduction in a soil can be characterized by
the direct measurement of redox potentials. Direct
measurements should take into account chemical equilibria as
expressed by stability diagrams in standard soil textbooks.

Reduction and oxidation processes are also a function of soil
pH

The duration of reduction required for creating aquic
conditions is not specified.
3. Redoximorphic features associated with wetness result
from alternating periods of reduction and oxidation of iron and
manganese compounds in the soil. Reduction occurs during
saturation with water, and oxidation occurs when the soil is not
saturated. The reduced iron and manganese ions are mobile
and may be transported by water as it moves through the soil.
Certain redox patterns occur as a function of the patterns in
which the ion-carrying water moves through the soil and as a
function of the location of aerated zones in the soil. Redox
patterns are also affected by the fact that manganese is reduced
more rapidly than iron, while iron oxidizes more rapidly upon
aeration. Characteristic color patterns are created by these
processes. The reduced iron and manganese ions may be
removed from a soil if vertical or lateral fluxes of water occur,
in which case there is no iron or manganese precipitation in
that soil. Wherever the iron and manganese are oxidized and
precipitated, they form either soft masses or hard concretions
or nodules. Movement of iron and manganese as a result of
redox processes in a soil may result in redoximorphic features
that are defined as follows:
a. Redox concentrations.—These are zones of apparent
accumulation of Fe-Mn oxides, including:
(1) Nodules and concretions, which are cemented
bodies that can be removed from the soil intact.

Concretions are distinguished from nodules on the basis
of internal organization. A concretion typically has
concentric layers that are visible to the naked eye.
Nodules do not have visible organized internal structure.
Boundaries commonly are diffuse if formed in situ and
sharp after pedoturbation. Sharp boundaries may be relict
features in some soils; and
(2) Masses, which are noncemented concentrations of
substances within the soil matrix; and
(3) Pore linings, i.e., zones of accumulation along
pores that may be either coatings on pore surfaces
or impregnations from the matrix adjacent to the
pores.
b. Redox depletions.—These are zones of low chroma
(chromas less than those in the matrix) where either Fe-Mn
oxides alone or both Fe-Mn oxides and clay have been
stripped out, including:
(1) Iron depletions, i.e., zones that contain low amounts
of Fe and Mn oxides but have a clay content similar to
that of the adjacent matrix (often referred to as albans or
neoalbans); and
(2) Clay depletions, i.e., zones that contain low
amounts of Fe, Mn, and clay (often referred to as silt
coatings or skeletans).
c. Reduced matrix.—This is a soil matrix that has low
chroma in situ but undergoes a change in hue or chroma
within 30 minutes after the soil material has been exposed
to air.
d. In soils that have no visible redoximorphic features, a
reaction to an alpha,alpha-dipyridyl solution satisfies the
requirement for redoximorphic features

Anthraquic conditions are a variant of episaturation and
are associated with controlled flooding (for such crops as
wetland rice and cranberries), which causes reduction
processes in the saturated, puddled surface soil and oxidation of reduced and mobilized iron and manganese in the unsaturated
subsoil.

Soil Moisture Regimes
It has been conventional to identify three soil moisture
regimes. In one, the soil is saturated. In another, the amount of
water is enough to cause leaching. In the third, no leaching
occurs. In the leaching regime, some water moves through the
soil at some time during the year and moves on down to the
moist substratum. In the nonleaching regime, water moves into
the soil but is withdrawn by evapotranspiration, leaving
precipitated carbonates and more soluble salts. Between these
two regimes, there is another possible one in which there is
alternation from year to year; leaching occurs in some years but
not in all. For consideration of the losses of soluble materials
or their accumulation in horizons with k, y, or z suffixes, these
concepts are adequate. For the understanding of biological
processes, they leave much to be desired. A soil can be subject
to leaching during winter, when it is too cold for optimum
biological activity, and it can be too dry during most of the
summer for significant biological activity

 

Water can be suspended by capillary
forces in a dry sandy soil (Rode, 1965). When the suspended
water exceeds a critical limit, it drains rapidly at some points
into lower soil layers. Water can be suspended in a dry or moist
soil if pore sizes increase with increasing depth, and similar
leaks may occur.

Aquic moisture regime.—The aquic (L. aqua, water)
moisture regime is a reducing regime in a soil that is virtually
free of dissolved oxygen because it is saturated by water. Some
soils are saturated with water at times while dissolved oxygen
is present, either because the water is moving or because the
environment is unfavorable for micro-organisms (e.g., if the
temperature is less than 1 o
C); such a regime is not considered
aquic.
It is not known how long a soil must be saturated before it is
said to have an aquic moisture regime, but the duration must
be at least a few days, because it is implicit in the concept that
dissolved oxygen is virtually absent. Because dissolved oxygen
is removed from ground water by respiration of microorganisms, roots, and soil fauna, it is also implicit in the
concept that the soil temperature is above biologic zero for
some time while the soil is saturated. Biologic zero is defined
as 5 o
C in this taxonomy. In some of the very cold regions of
the world, however, biological activity occurs at temperatures
below 5 o
C.
Very commonly, the level of ground water fluctuates with
the seasons; it is highest in the rainy season or in fall, winter,
or spring if cold weather virtually stops evapotranspiration.
There are soils, however, in which the ground water is always
at or very close to the surface. Examples are soils in tidal
marshes or in closed, landlocked depressions fed by perennial
streams. Such soils are considered to have a peraquic moisture
regime. The distinction between the aquic moisture regime and
the peraquic moisture regime is not closely defined because
neither regime is used as a criterion for taxa above the series
level. These terms can be used in descriptions of taxa. Some
soils with an aquic moisture regime also have a xeric, ustic, or
aridic (torric) regime.
Although the aquic and peraquic moisture regimes are
not used as either criteria or formative elements for taxa,
they are used in taxon descriptions as an aid in understanding
genesis. The formative term “aqu” refers to aquic conditions,
not an aquic moisture regime. Some soils included in the
“Aqu” suborders may have aquic or peraquic moisture
regimes.

Udic moisture regime.—The udic (L. udus, humid)
moisture regime is one in which the soil moisture control
section is not dry in any part for as long as 90 cumulative days
in normal years. If the mean annual soil temperature is lower
than 22 o
C and if the mean winter and mean summer soil
temperatures at a depth of 50 cm from the soil surface differ by
6 o
C or more, the soil moisture control section, in normal
years, is dry in all parts for less than 45 consecutive days
in the 4 months following the summer solstice. In addition,
the udic moisture regime requires, except for short periods, a
three-phase system, solid-liquid-gas, in part or all of the soil
moisture control section when the soil temperature is above
5 o
C.
The udic moisture regime is common to the soils of humid
climates that have well distributed rainfall; have enough rain
in summer so that the amount of stored moisture plus rainfall
is approximately equal to, or exceeds, the amount of
evapotranspiration; or have adequate winter rains to recharge
the soils and cool, foggy summers, as in coastal areas. Water
moves downward through the soils at some time in normal
years.
In climates where precipitation exceeds evapotranspiration in
all months of normal years, the moisture tension rarely reaches
100 kPa in the soil moisture control section, although there are
occasional brief periods when some stored moisture is used. The water moves through the soil in all months when it is not frozen. Such an extremely wet moisture regime is
called perudic (L. per, throughout in time, and L. udus,
humid). In the names of most taxa, the formative element “ud”
is used to indicate either a udic or a perudic regime; the
formative element “per” is used in selected taxa. The
distinction between perudic and udic can always be made
at the series level.

Ustic moisture regime.—The ustic (L. ustus, burnt;
implying dryness) moisture regime is intermediate between
the aridic regime and the udic regime. Its concept is one of
moisture that is limited but is present at a time when
conditions are suitable for plant growth. The concept of
the ustic moisture regime is not applied to soils that have
permafrost or a cryic soil temperature regime (defined
below).
If the mean annual soil temperature is 22 o
C or higher or if
the mean summer and winter soil temperatures differ by less
than 6 o
C at a depth of 50 cm below the soil surface, the soil
moisture control section in areas of the ustic moisture regime is
dry in some or all parts for 90 or more cumulative days in
normal years. It is moist, however, in some part either for more
than 180 cumulative days per year or for 90 or more
consecutive days.
If the mean annual soil temperature is lower than 22 o
C and if the mean summer and winter soil temperatures differ by 6 o
C
or more at a depth of 50 cm from the soil surface, the soil
moisture control section in areas of the ustic moisture regime is
dry in some or all parts for 90 or more cumulative days in
normal years, but it is not dry in all parts for more than half of
the cumulative days when the soil temperature at a depth of 50
cm is higher than 5 o
C. If in normal years the moisture control
section is moist in all parts for 45 or more consecutive days in
the 4 months following the winter solstice, the moisture control
section is dry in all parts for less than 45 consecutive days in
the 4 months following the summer solstice. Diagram 13
illustrates an area with an ustic soil moisture regime and a
thermic soil temperature regime. Diagram 14 illustrates an
area with an ustic soil moisture regime and a mesic soil
temperature regime.
In tropical and subtropical regions that have a monsoon
climate with either one or two dry seasons, summer and winter
seasons have little meaning. In those regions the moisture
regime is ustic if there is at least one rainy season of 3 months
or more. In temperate regions of subhumid or semiarid
climates, the rainy seasons are usually spring and summer or
spring and fall, but never winter. Native plants are mostly
annuals or plants that have a dormant period while the soil is
dry.

Soil Temperature Regimes
The temperature of a soil is one of its important properties.
Within limits, temperature controls the possibilities for plant
growth and for soil formation. Below the freezing point, there
is no biotic activity, water no longer moves as a liquid, and,
unless there is frost heaving, time stands still for the soil.
Between temperatures of 0 and 5 o
C, root growth of most plant
species and germination of most seeds are impossible. A
horizon as cold as 5 o
C is a thermal pan to the roots of most
plants.
Biological processes in the soil are controlled in large
measure by soil temperature and moisture. Each plant species
has its own temperature requirements. In the Antarctic, for
example, there is a microscopic plant that grows only at
temperatures below 7 o
C, temperatures at which most other
plants are inactive. At the other extreme, germination of seeds
of many tropical plant species requires a soil temperature of
24 o
C or higher. Plant species have one or more soil
temperature requirements that are met by the soils of their
native environment. Similarly, soil fauna have temperature
requirements for survival. Soil temperature, therefore, has an
important influence on biological, chemical, and physical
processes in the soil and on the adaptation of introduced plant
species The mean annual soil temperature is related most closely
to the mean annual air temperature, but this relationship is
affected to some extent by the amount and distribution of
rain, the amount of snow, the protection provided by shade
and by O horizons in forests, the slope aspect and gradient,
and irrigation. Other factors, such as soil color, texture,
and content of organic matter, have negligible effects (Smith
et al., 1964).

Daily fluctuations.—Daily changes in air temperature
have a significant effect on the temperature of soil horizons
to a depth of about 50 cm. The fluctuations may be very
large, particularly in soils of dry climates where the daily
range in temperature of the upper 2.5 cm of the soils may
approach 55 o
C. At the other extreme, under melting snow,
the temperature at the soil surface may be constant
throughout the day. In a few places in high mountains
very near the Equator, the soils have virtually constant
temperature.
Daily fluctuations in soil temperature are affected by
clouds, vegetation, length of day, soil color, slope, soil
moisture, air circulation near the ground, and the temperature
of any rain that falls. Moisture can be exceedingly important
in reducing fluctuations in soil temperature. The specific heat
of water is roughly 4 times that of a dry surface horizon, and the
specific heat of a medium textured surface horizon at field
capacity is roughly one-half more than that at the wilting
point. Water increases the thermal conductivity of soils, and
it can also absorb or liberate heat by freezing and thawing
or by evaporating and condensing. All of these effects of
soil water reduce fluctuations in soil temperature at the
surface.
Fluctuations caused by changes in weather.—Soil
temperatures also fluctuate during short periods of belowaverage or above-average air temperatures. The fluctuations
caused by weather extend to a greater depth than those of the
diurnal cycle. Periods of high or low temperature tend to last
a few days to a week in most of the United States. Like
weather patterns in general, however, they occur at irregular
intervals.
The soil temperature at a depth of 50 cm shows almost no daily fluctuation, but it reflects short-time weather patterns. Data
indicate that at shallow depths soil temperatures reflect daily
fluctuations in temperature.

Sulfuric Horizon
Brackish water sediments frequently contain pyrite (rarely
marcasite), which is an iron sulfide. Pyrite forms from the
microbial decomposition of organic matter. Sulfur released
from the organic matter combines with the iron to crystallize
FeS. Characteristically, the pyrite crystals occur as nests or
framboids composed of bipyramidal crystals of pyrite. In an
oxidizing environment, pyrite oxidizes and the products of
oxidation are jarosite and sulfuric acid. The jarosite may
undergo slow hydrolysis, leading to further production of
sulfuric acid. Iron is precipitated as a reddish ochre precipitate,
commonly ferrihydrite, which later may crystallize as
maghemite, goethite, and even hematite. If free aluminum is
present, alunite may crystallize in addition to jarosite. The
jarosite has a straw-yellow color and frequently lines pores in
the soil. Jarosite concentrations are among the indicators of a
sulfuric horizon A sulfuric horizon forms as a result of drainage (most
commonly artificial drainage) and oxidation of sulfide-rich or
organic soil materials. It can form in areas where sulfidic
materials have been exposed as a result of surface mining,
dredging, or other earth-moving operations. A sulfuric horizon
is detrimental to most plants.Required Characteristics
The sulfuric (L. sulfur) horizon is 15 cm or more thick and is
composed of either mineral or organic soil material that has a pH
Horizons and Characteristics Diagnostic for the Higher Categories 113
value of 3.5 or less (1:1 by weight in water or in a minimum of
water to permit measurement) and shows evidence that the low
pH value is caused by sulfuric acid. The evidence is one or more
of the following:
1. Jarosite concentrations; or
2. Directly underlying sulfidic materials (defined above);
or
3. 0.05 percent or more water-soluble sulfate.

Literature Cited
Brewer, R. 1976. Fabric and Mineral Analysis of Soils.
Second edition. John Wiley and Sons, Inc. New York, New
York.
Chang, Jen Hu. 1958a. Ground Temperature. I. Blue Hill
Meteorol. Observ. Harvard Univ.
Chang, Jen Hu. 1958b. Ground Temperature. II. Blue Hill
Meteorol. Observ. Harvard Univ.
Childs, C.W. 1981. Field Test for Ferrous Iron and FerricOrganic Complexes (on Exchange Sites or in Water-Soluble
Forms) in Soils. Austr. J. of Soil Res. 19: 175-180.
Grossman, R.B., and F.J. Carlisle. 1969. Fragipan Soils of the
Eastern United States. Advan. Agron. 21: 237-279.
Institut National pour l’etude Agronomique du Congo Belge
(I.N.E.A.C.). 1953. Bulletin Climatologique annuel du Congo
Belge et du Duanda-Urundi. Annee. Bur. Climatol. 7.
Jensen, M.E., G.H. Simonson, and R.E. Keane. 1989. Soil
Temperature and Moisture Regime Relationships Within Some
Rangelands of the Great Basin. Soil Sci. 147: 134-139.
Mather, J.R., ed. 1964. Average Climatic Water Balance Data
of the Continents, Parts V-VII. C.W. Thornthwaite Assoc. Lab.
Climatol. Publ., Vol. XVII, No. 1-3.
Mather, J.R., ed. 1965. Average Climatic Water Balance Data
of the Continents, Part VIII. C.W. Thornthwaite Assoc. Lab.
Climatol. Publ., Vol. XVIII, No. 2.
Molga, M. 1958. Agricultural Meteorology. Part II. Outline
of Agrometeorological Problems. Translated (from Polish)
reprint of Part II, pp. 218-517, by Centralny Instytut
Informacji. Naukowo-Technicznej i Ekonomicznej, Warsaw.
1962.
Moore, J.P., and C.L. Ping. 1989. Classification of Permafrost
Soils. Soil Surv. Horiz. 30: 98-104.
Pons, L.J., and I.S. Zonneveld. 1965. Soil Ripening and Soil
Classification. Initial Soil Formation in Alluvial Deposits and
a Classification of the Resulting Soils. Int. Inst. Land Reclam.
and Impr. Pub. 13. Wageningen, The Netherlands.
Rode, A.A. 1965. Theory of Soil Moisture. Vol. 1. Moisture
Properties of Soils and Movement of Soil Moisture. pp. 159-
202. (Translated from Russian, 1969). Israel Program Sci.
Transl., Jerusalem.
Shur, Y.L., G.J. Michaelson, and C.L. Ping. 1993. International
Correlation Meeting on Permafrost-Affected Soils. Suppl. Data
to the Guidebook—Alaska Portion.
Smith, G.D., D.F. Newhall, and L.H. Robbins. 1964. SoilTemperature Regimes, Their Characteristics and Predictability.
U.S. Dep. Agric., Soil Conserv. Serv, SCS-TP-144.
United States Department of Agriculture, Natural Resources
Conservation Service. 1998. Keys to Soil Taxonomy. Eighth
edition. Soil Surv. Staff.
United States Department of Agriculture, Soil Conservation
Service. 1975. Soil Taxonomy: A Basic System of Soil
Classification for Making and Interpreting Soil Surveys. Soil
Surv. Staff. U.S. Dep. Agric. Handb. 436.
United States Department of Agriculture, Soil Conservation
Service. 1993. Soil Survey Manual. Soil Surv. Div. Staff. U.S.
Dep. Agric. Handb. 18.


CHAPTER 6
The Categories of Soil Taxonomy

 

Entisols
The unique properties common to Entisols are dominance of
mineral soil materials and absence of distinct pedogenic
horizons. The absence of features of any major set of soilforming processes is itself an important distinction. There can
be no accessory characteristics. Entisols are soils in the sense
that they support plants, but they may be in any climate and
under any vegetation. The absence of pedogenic horizons may
be the result of an inert parent material, such as quartz sand, in
which horizons do not readily form; slowly soluble, hard rock,
such as limestone, which leaves little residue; insufficient time
for horizons to form, as in recent deposits of ash or alluvium;
occurrence on slopes where the rate of erosion exceeds the rate
of formation of pedogenic horizons; recent mixing of horizons
by animals or by plowing to a depth of 1 or 2 m; or the spoils
from deep excavations.

Inceptisols
Inceptisols have a wide range in characteristics and occur in
a wide variety of climates. They can form in almost any
environment, except for an arid environment, and the
comparable differences in vegetation are great. Inceptisols can
grade toward any other soil order and occur on a variety of
landforms. The unique properties of Inceptisols are a
combination of water available to plants for more than half the
year or more than 3 consecutive months during a warm season
and one or more pedogenic horizons of alteration or
concentration with little accumulation of translocated materials
other than carbonates or amorphous silica. In addition,
Inceptisols do not have one or more of the unique properties of
Mollisols, which are a thick, dark surface horizon and a high
calcium supply, or the unique property of Andisols, which is
the dominance of short-range-order minerals or Al-humus
complexes.

Suborders
Sixty-four suborders currently are recognized. The
differentiae for the suborders vary with the order but can be
illustrated by examples from two orders. The Entisol order has
five suborders that distinguish the major reasons for absence of
horizon differentiation. One suborder includes soils that have
aquic conditions. These are the soils in areas of marshy recent
alluvium and the soils of coastal marshes that are saturated
with water and have a blue or green hue close to the surface.
This suborder segregates the wet varieties. A

A fourth suborder includes sands that may
range from recent to old. If old, they either lack the building
blocks for pedogenic horizons or do not have enough moisture.
Although the reasons for absence of horizons in the sands vary,
the sands have many common physical properties, such as a
low capacity for moisture retention, high hydraulic
conductivity, and susceptibility to soil blowing. T

stopped at page 163 out of 900 – declining in relevance

 


https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_051232.pdf

LEAVE A COMMENT