to ascertain if this soil is properly classified as hydric.
The Sharkey soil
series was studied because of the vast acreage it comprises over a
broad region and its importance to society. The study was conducted
from 1991 to 1995.
The Sharkey soil series
was established in Yazoo County, Mississippi in 1901, and it is one
of the oldest soils recognized in the United States. Sharkey is the
dominant soil mapped in Mississippi, comprising about one million
acres. It is also a dominant soil in the nation, with at least 3 million
acres mapped. Many of the original theories concerning the origin
and properties of Sharkey soils have persisted since it was established.
These soils were initially recognized as clayey, expansive soils occurring
on nearly level topography on lower parts of natural levees, terraces,
and flood plains of the Mississippi River and tributaries. Their clayey,
sticky, and plastic nature gave rise to usage of the terms "gumbo"
and "buckshot" when referring to Sharkey soils.
occur in the Southern Mississippi Valley Alluvium major land resource
area (MLRA 131) and extend from the Gulf of Mexico to Kentucky. These
soils are extensive in Arkansas, Kentucky, Louisiana, Mississippi,
Missouri, and Tennessee on the Mississippi River flood plain and terraces.
Slope gradients are 0 to 5%, with short slopes that typically occur
as parallel ridges and swales. Sharkey soils formed in clayey alluvium
of recent Holocene age and have clay contents of 60 to 90% in the
subsoils. Soil reaction ranges from strongly acid in surface horizons
to moderately alkaline in the subsoil. Sharkey soils are typically
dark gray to gray, with brownish, yellowish, or reddish mottles. These
soils are currently classed as poorly drained with slow surface runoff
and permeability. The expansive clay develops large cracks each year.
The Sharkey series is designated prime farmland and hydric soil.
as poorly drained in soil surveys, several phases of Sharkey soils
were mapped indicating a range of drainage or wetness. In the county
where the Sharkey series was established, the latest Yazoo County
Soil Survey (Scott et al., 1975) mapped Sharkey clay depressional
phase (11,500 acres) and Sharkey clay (60,790 acres). Sharkey depressional
phase was much wetter than other phases of the same soil series.
In other Delta
counties, such as Washington (Morris, 1961), Sharkey clay, level phase,
0 to ½% slopes, was mapped on broad flats or in slightly depressed
areas (36,630 acres). Sharkey clay, nearly level phase, ½ to
2% slopes, comprised 100,460 acres or 21.6% of Washington County.
Other Sharkey units in the county were Sharkey clay, gently sloping
phase, 2 to 5% slopes, which included areas with slopes up to 8%;
Sharkey silty clay loam, nearly level phase, ½ to 2% slopes;
and Sharkey very fine sandy loam, nearly level overwash phase, ½
to 2% slopes.
The soil surveys
recognized the effects of topography on soil wetness and used different
phases of Sharkey to depict landscape and drainage differences that
were important to land use and management (Appendix
The soil surveys
of Mississippi Delta counties delineated associated soils wetter than
Sharkey in depressions, low swales, and drains as the Dowling series.
Low, wet areas flooded much of the time were mapped Swamp, and frequently
flooded soils near streams were mapped Alluvial Soils (Morris, 1961).
In recent years, Dowling soils were office-correlated into the Sharkey
series. Nearly a half-million acres of wetter Dowling soils were correlated
to Sharkey soils (Appendix Table 2).
Delta counties were largely mapped and published prior to adoption
of Soil Taxonomy (USDA, 1975). The soils were reclassified without
additional field mapping and studies. Office correlations merged Dowling
and Alluvial soils with very limited or no additional field data of
a modern nature. Hence, the drainage and wetness concepts of the Sharkey
soils became more general and nondefinitive. Extensive agricultural
production data for various crops provided a factual data base for
classifying soils, including Sharkey, as prime farmland soils. However,
no comparable data existed to provide a framework for classifying
Sharkey as a hydric soil.
The Sharkey series
was classed hydric (SCS, 1987) based upon criteria (2B3), "water
table at less than 1.5 feet from the surface for a significant period
(usually a week or more) during the growing season if permeability
is less than 6.0 in/h in any layer within 20 inches." Other phases
of Sharkey soils classed as hydric were: Sharkey commonly flooded
(criteria 2B2, 4); Sharkey, overwash (2B2); Sharkey, ponded (2B2,
3). The list of hydric soils was created by computer using criteria
developed by the National Technical Committee for Hydric Soils (SCS,
1987). Water table criteria for Sharkey soils mapped in Mississippi
have not been clearly established by field validation over a temporal
were classified as Grumusols prior to Soil Taxonomy, based on shrinking,
swelling, and cracking properties. In the Tunica County, Mississippi
Soil Survey completed in 1942, Simonson (1956) reported the organic
matter content in the A1 horizon was common to many Grumusols. Humic
Gley soils were wetter and had very high organic matter contents in
surface horizons because of wet, reduced conditions. After 1965, Sharkey
soils were reclassified without field studies as very-fine, montmorillonitic,
nonacid, thermic Vertic Haplaquepts in spite of numerous data indicating
a Vertisol classification (Holmes and Hearn, 1942; Bruce et al., 1958;
Fowlkes et al., 1956; Morris, 1961). The Washington County Soil Survey
(Morris, 1958) reported cracks in Sharkey soils 1 to 5 inches wide
extending several feet in depth.
of Sharkey soils are used intensely for production of soybeans, rice,
wheat, cotton, grain sorghum, oats, catfish, hay crops, and pasture.
Uncleared areas remain in forest.
The study sites
are in the Southern Mississippi Valley Alluvium Major Land Resource
Area (MLRA-131), commonly referred to as the Delta (Figure
1). The Delta comprises about 36,130 square miles (93,600 km2)
in Arkansas, Kentucky, Louisiana, Mississippi, Missouri, and Tennessee
(USDA, 1981) and is one of the largest contiguous agricultural areas
in the United States.
Delta is an elliptical-shaped physiographic region comprising the
western part of Mississippi (Figure
2). The area is bounded on the west by the Mississippi River,
and it abruptly meets the loessial bluffs, which rise above the Delta
on the east. The largest part of the Delta extends from Memphis, Tennessee
to Vicksburg, Mississippi, a distance of about 200 miles. The Delta
is about 75 miles wide at its widest point in the state. The area
is nearly level, which is typical of large flood plains. The general
slope extends to the south. Elevations range from 217 feet near Memphis
to about 94 feet above mean sea level at Vicksburg. In addition to
the Mississippi River, the main streams are the Yazoo, Big and Little
Sunflower, Tallahatchie, and Cold Water Rivers. Abandoned stream meanders
and oxbow lakes are common.
River Valley was formed over the last 1.5 million years through a
series of down-cuttings and subsequent refillings directly related
to advancing and recycling continental glaciation (Saucier, 1974).
The last change from braided to meandering stream conditions occurred
10,000 to 12,000 years ago. Meander belts reflect the previous course
changes of the Mississippi River.
The climate of
the Mississippi Delta is warm and humid, with hot summers and moderate
winters. The mean annual temperature is about 63 °F, and the mean
annual rainfall is about 51 inches. The area generally has 220 to
260 frost-free days (Pettry, 1977).
The Delta's soils
are very productive under proper management, and they are suited to
a wide range of crops. One of the early technical studies (Holmes
and Hearn, 1942) stressed the agricultural importance of the Mississippi
alluvial soils and compared them in importance to soils of the Tigris
and Euphrates Rivers and the birth of civilization.
The factors of
soil formation are parent material, climate, organisms, time, and
relief (Jenny, 1941). The length of time a material has been in place
and under the influence of local climate and vegetation often determines
the kind of soil found. Recently deposited alluvium usually shows
little development or formation of soil horizons.
Parent material is "the
physical body of soil and its associated chemical and mineralogical
properties at the starting point of a particular set of other soil-forming
factors" (Buol, Hole, and McCracken, 1973). Generally, parent
material exerts greater influence on younger soils than on older landscapes.
The original parent material becomes less recognizable as weathering
and soil formation proceed.
Delta parent material is dominantly alluvium deposited by the Mississippi
River and its tributaries. The sediments originated in the vast, diverse
Mississippi drainage system, which comprises a large area of the United
States. Consequently, the materials have diverse mineral suites because
of their heterogeneous origin and differential stages of weathering,
and previous pedogenic development. The surficial materials are primarily
two types. The first, sands and loamy deposits, occurs as old natural
levees of previous stream beds. The other type of sediment consists
of clayey materials located in "slackwater" areas (interfluves)
between streams. Water velocity decreases as it overflows its banks
and moves away from the stream bed. This results in heavier sandier
materials being deposited near the stream and clayey sediments being
carried in suspension until a low velocity is achieved and they are
deposited. The slackwater areas are usually lower in elevation than
stream-side deposits (Logan, 1916) and act as a sink for finer-textured
The present surface
of the Delta was deposited in recent times in the Holocene geologic
timeframe. Material was deposited on a broad scale by the Mississippi
River until the levee on the Mississippi side of the river was completed.
Local flooding and sedimentation continue to occur in "backwater"
areas, where drainage waters flowing into the Mississippi River back
up during high flow periods.
the land surface were deposited during and after the advances of Wisconsin
glaciers, the latest active in the North Central States about 11,000
years ago (Arnold and Libby, 1951). The present surface of the Mankato
drift has been exposed about 8,000 years, and the soils in the Mississippi
Delta counties could be slightly older (Simonson, 1956).
The Delta is
characterized by level relief and low hydraulic gradients. Small differences
in elevation (microrelief) have a major impact on water movement.
A few inches difference in relief in the Delta has a major impact
on water movement and soil development.
The climate of
the Delta is warm and humid, with hot summers and mild winters. The
temperature and precipitation are conducive to intense weathering.
The region was
primarily a hardwood forest with intermittent swamps and bayous before
it was cleared for cropland. Diverse species, including oaks, gums,
ash, hickory, black willow, and cypress, covered this region. A limited
area of forest cover still exists in the Mississippi Delta.
Study sites were
located in forested areas in Washington County, Mississippi on the
MAFES Delta Branch Experiment Station Forest near Stoneville, and
in Percy Quinn State Park, about 25 miles south of Stoneville (Figure
3). The sites were selected in natural wooded areas representative
of the Sharkey series with typical bottomland hardwood vegetation.
The areas had not been previously cleared or cultivated. Two research
sites were installed February 1991 at each location about 0.5 mile
apart in two phases of the Sharkey series (Table
The Delta Experimental
Forest sites were about 10 miles east and the Percy Quinn Park sites
about 8 miles east of the Mississippi River (Figure
3). The sites were not subject to flooding.
piezometers were installed 40 inches apart at 10, 20, 40, and 120
inches depth. Piezometers were constructed of 3-inch diameter polyvinylchloride
(PVC) tubes permeated with 0.125-inch diameter holes. Washed gravel
was placed in the bottom of the holes and piezometers were driven
and fitted snugly into 3-inch diameter auger holes. A clay seal was
packed around the piezometer at the soil surface. A vented cap covered
the piezometers, which extended about 6 inches above the soil surface.
An unlined 3-inch diameter bore hole was drilled to 120 inches about
40 feet from the piezometers and the surface was covered. Piezometers
were installed in February and March 1991. No water was encountered
during installation at any of the depths. Water levels were measured
monthly or more frequently for the duration of the study. Piezometers
were pumped dry and allowed to equilibrate to verify water levels.
Soils were examined
by hand auger in transects to locate representative pedons for detailed
characterization and evaluate spatial variability. Soil pits were
excavated by hand shovels. Landscape elements were determined at each
site. Soil morphological parameters were determined (USDA, 1994),
including horizonation and depth, Munsell color, texture, structure,
consistence, mottling and redoximorphic features, presence of concretions,
topography and thickness of horizon boundaries, and size and distribution
core samples were taken in selected horizons for determination of
bulk density, saturated hydraulic conductivity, and moisture retention.
Duplicate core samples were taken from selected horizons within a
one-meter distance of a contiguous pedon by cutting back the face
of the pit and exposing each horizon from the surface to the bottom
of the pit.
was determined on 10-foot intervals in directional transects with
a transit level. Soil microdepressions were measured by rigid steel
tape and transit level.
content was monitored gravimetrically on a temporal basis in 10-inch
increments from the surface using 100-gram auger samples. Soil temperature
was measured in the epipedon with a soil thermometer (ReoTemp Instrument
levels and temperature of soil water were measured with a YSI model
58 dissolved oxygen meter (Yellow Springs Instrument Co., Inc.). A
submersible stirring oxygen probe was lowered into the piezometers
containing water to determine oxygen levels.
Fresh soil peds
from the surface through the solum were tested with ,
for nonvisible redoximorphic features (soluble iron) on a temporal
basis (Soil Survey Staff, 1994).
Soil bulk density
was determined by the method described by Blake (1995) on nondisturbed
cores taken with a double-cylinder sampler. The inner cylinder of
known volume was dried in the oven at 110 °C for 24 hours and
weighed. The bulk density was calculated by the following formula:
method (Klute, 1965) was used on the nondisturbed cores. The cores
were saturated in standing water for 48 hours and then placed on a
constant head permeameter rack to equilibrate for one hour. Water
passing through the cores was measured at 10-minute intervals for
a total of five measurements. Values reported are the average value
of five observations.
membrane method (Richard, 1949) was used with the natural aggregates
of the nondisturbed cores. The cores were saturated with water for
24 hours on a presoaked ceramic porous plate. Pressures of 0.03, 0.1,
0.3, 0.6, and 1.5 MPa were maintained until equilibrium was achieved.
The moisture contents were calculated on an oven-dry basis.
were air-dried in the laboratory, crushed with a wood cylinder, and
sieved through a No. 10 sieve to remove coarse fragments larger than
2 mm (USDA, 1992). Particle size distribution was determined by hydrometer
method and sieving (Day, 1965). Organic matter was determined by wet
combustion procedure (Allison, 1935). Extractible acidity was determined
by the barium chloride-triethanolamine method (Peech, 1965). Exchangeable
aluminum was determined in KC1 extractions following the procedure
of Yuan (1959). Exchangeable cations were extracted with neutral N
NH4OAC and determined by atomic absorption spectrophotometry
(USDA, 1992). Soil pH was measured in water and 0.1 N KC1 using a
1:1 soil-to-liquid ratio. Iron was fractionated using the method of
Gamble and Daniels (1972). Total Fe was analyzed by HF and HC1O4
digestion in Pt crucibles (Jackson, 1982). Total sulfur was determined
on soil ground to pass a 60-mesh sieve with a sulfur analyzer (Model
LECO SC 132).
of selected horizons were separated by centrifugal centrifugation.
They were analyzed by x-ray diffraction (Jackson, 1956) with a Norelco
Geiger counter spectrophotometer using Cu K
radiation and a Ni filter. Mineral type and content were estimated
from basal spacings and x-ray peak intensity. Microscopic examinations
were made of soil peds using conventional light microscopy. Coefficient
of linear extensibility (COLE) was determined on < 2 mm extruded
soil paste (Shafer and Singer, 1976) where:
wet - length dry
Sharkey pedons had ochric epipedons and cambic subsurface horizons
(Tables 2, 3,
4, and 5).
Surfaces were dark to very dark grayish-brown in hues of 10YR with
values of 3 to 4 and chromas of 2. The upper subsoil was dark to very
dark gray in hues of 10YR with values of 4 to 5 and chromas of 1 to
2 with strong brown mottles. Soil color became brighter with increased
depth. Site 1 was yellowish-red in 5YR hues below 120 inches depth
and highly mottled above this depth. Site 2 had pale brown color with
3 chromas below 36 inches. Site 3 was brownish-yellow to strong brown
below 60 inches, and Site 4 was grayish-brown below 50 inches.
soils had well-developed angular blocky structure in the upper sola.
The subsoil had compound structure consisting of coarse prismatic
parting to angular and subangular blocky structure. Pockets and cracks
were evident in the upper sola, with common pressure faces on peds.
The surface horizons contained many roots, which extended to depths
of 40 inches and greater and promoted structural development. The
roots also created many macrovoids in the surface horizon. The soils
had friable to firm consistency in the surface horizon and firm subsoils,
which were sticky and plastic when moist.
slickensides were very prominent features in the Sharkey pedons, reflecting
the shrinking and swelling properties, except for Site 2, which had
loam textures at 36 inches. The slickensides became wider with depth
and were prominent to depths of 60 inches and greater. The soils had
cracks at the surface each year of the study. The cracks ranged to
3 inches wide and extended to depths of 3 feet and greater. The cracks
were visible from May to October, and they would close and reopen
after significant precipitation events. The forest litter tended to
obscure the surface cracks, which were exposed when the litter was
(CaSO4 2H2O) crystals were prominent features
at 40 to 70 inches in the subsoils of sites 1, 3, and 4. The gypsum
occurred in a clay matrix as clusters in veins and pockets with individual
crystals ranging to 4 mm and larger. The crystal habit of the gypsum
was both tabular and fibrous. Round, black concretions occurred intermingled
with gypsum at 50 to 65 inches in Site 1. The concretions occurred
at different depths in Sites 3 and 4.
features of the Sharkey soils at the four sites corresponded very
closely with previous descriptions and mapping concepts (Brown et
al., 1970; Schumacher et al., 1988; Bruce et al., 1958; Holmes and
Hearn, 1942; Morris, 1961; Rogers, 1958; Wynn, 1959). The earlier
descriptions of Sharkey were limited by the relatively shallow depths
at which they were examined. Recognition of brighter colors with increasing
depth requires deeper examination (> 60 inches). Also, the common
occurrence of gypsum in the subsoils and pronounced compound structure
had not been previously recognized and stressed. Cracks that extend
to the surface, large intersecting slickensides, and compound structure
are distinctive features of the Sharkey pedons.
All the sites
had clay textures in the epipedon (Appendix
Table 3). Sites 1, 3, and 4 had clay contents exceeding 60% to
depths of 60 inches (Figure 4). Site 2
had loamy materials below 30 inches with clay contents ranging to
72% in the upper sola (Figure 4).
Site 1 had clay
contents exceeding 60% to depths of 100 inches, with less than 5%
sand to 90 inches, and silt contents less than 30% in the upper 30
inches. Site 2 had average clay content of 56% in the upper 30 inches,
with less than 20% sand (Figure 5), and
silt contents of 20 to 44%. Clay contents exceeded 40% to depths of
70 inches in Site 3, with less than 10% sand in the upper 50 inches,
and silt contents of 23 to 13%. Site 4 had greater than 68% clay in
the upper 80 inches, accompanied by sand contents less than 5%, and
silt contents of 26 to 13%.
Fine clay dominated
the clay fraction in all sites with fine clay (<0.2 µ)/coarse clay
(2 to 0.2 µ) ratios ranging from 1.2 to 1.84. There was no indication
of fine clay accumulation with increased depth suggesting no eluviation/illuviation.
The sand was dominated by very fine and fine fractions reflecting
low energy deposition.
are presented in Appendix Table 4.
Organic matter decreased regularly with depth (Figure
6). Maximum contents exceeded 3% and occurred in the thin ochric
epipedon. Soil pH levels were very strongly acid in the surface horizons
and increased with depth to slightly alkaline levels at 60 inches
depth (Figure 7). Calcium was the dominant
exchangeable cation and Ca/Mg levels were less than 2. Cation exchange
capacities exceeded 50 cmolc kg-1 in the upper 70 inches
of Sites 1, 3, and 4 and were greater than 30 cmolc kg-1
in Site 2. Higher exchangeable acidity levels in the upper 20 inches
correspond to lower pH levels reflecting the effects of organic matter
and weathering. Exchangeable Al3+ was very low or not detectable.
Total S levels
corresponded to the presence of gypsum crystals with maximum values
occurring in subsoil Bssyg horizons of Sites 1, 3, and 4 (Figure
8). Sulfur contents were lower in Site 2 where no gypsum was detected.
The gypsum appears to be authigenic and probably formed by precipitation
and crystallation in an oxidized environment.
levels were extremely low in the surface horizons (Table
6) because of a dense root mat and presence of large macrovoids.
Values ranged from 0.77 g cm-3 in Site 2 to 1.01 g cm-3
in Site 3 in the 0- to 6-inch surface horizon. Bulk densities increased
slightly in the subsoil but were relatively low because of the high
content of montmorillonitic clay. The higher values in the subsoil
of Site 2 reflect the loamy textures and lower montmorillonite clay
retention data for selected depths are presented in Table
7. Terms applicable to soil moisture retention are: field capacity,
permanent wilting point, and available water capacity. Field capacity
is the amount of water in the soil after excess gravitational water
has drained and the downward water movement has decreased (Veihmeyer
and Hendrickson, 1931). Field capacity is defined as the moisture
content at one-third (1/3) atmospheric tension (0.03 MPa). Permanent
wilting point occurs at 15 atmospheres (1.5 MPa) and is the moisture
content of the soil where plants wilt and cannot recover even under
saturated conditions. Available water represents the soil water that
plants can withdraw, and it is defined as the difference between field
capacity and permanent wilting point.
retention values were very similar for the four sites except for lower
values in Site 2. The available water was greater in the surface horizons
because of higher organic matter content. The high moisture retention
at field capacity (0.03 MPa) and permanent wilting point (1.5 MPa)
is typical of soils with high montmorillonite clay contents.
is the moisture content of the soil when all the pores are filled
with water, and it was measured in the laboratory by the Direct Method
(Gardner, 1965). Saturation values in the surface horizons ranged
from 74 to 88% at all four sites with variation caused by differences
in the amount of organic matter and degree of decomposition. The high
moisture contents at saturation are due to the high clay content,
organic matter, and expansive montmorillonite clay.
in the surface horizon was high because of the presence of roots and
associated macrovoids (Table 8).
The process of taking cores of the root-matted surface horizon also
may have created additional fissures. Very low hydraulic conductivity
was measured in the clayey subsoil horizons. Soil cores were not taken
in volumes containing cracks and krotovinas.
dominates the fine clay fraction (<0.2 µ) at all sites. The coarse
clay fraction (2 to 0.2µ) consists of montmorillonite, illite, kaolinite,
and quartz. The silt fraction contains quartz, feldspars, and mica.
The COLE values of the
upper 60 inches of Sites 1, 3, 4, and the upper 30 inches of Site
2 far exceeded the value of 0.09 considered minimum for Vertisols
and ranged to 0.28. The COLE values decreased in the loamy subsoil
of Site 2 to values less than 0.02. Total clay and montmorillonite
contents have been shown to be highly correlated to COLE values (Karathanasis
and Hajek, 1985). The high values indicate the very expansive nature
of these cracking soils.
Peds from surface
and subsoil horizons from each site were examined under reflected
light at 50 to 150X. Observations of root-soil matrix in surface horizons
revealed no oxidized rhizopheres. Faunal pellets were common in the
surface horizons. The soil had very tight adhesion to the roots. Scattered
flecks of 5YR and 7.5YR hues were mixed throughout the matrix. Pressure
faces on ped surfaces gave the appearance of "scales." The
gray matrix had a dull, waxy appearance. Slickenside surfaces were
polished and striated and appeared to have a thin coating of colloidal
organic matter. In deeper horizons, gypsum crystals had a very sharp
boundary with the surrounding matrix. Some crystals had a thin CaCO3
effloresced coating. Concretions were embedded in random patterns
in deep subsoil horizons with gradual and sharp boundaries with the
oxalate (Feo) and dithionite-citrate-bicarbonate (Fed)
extracted Fe are presented in Table 9.
The oxalate extraction dissolves the amorphous Fe, and the dithionite
extraction dissolves the crystalline Fe (McKeague and Day, 1966).
Amorphous Feo was dominant in the upper sola of all sites
and dominant at all depths in Site 1. Both Feo and Fed
levels in Site 2 dropped sharply in the loamy subsoil materials.
(Perchloric acid digestion) levels (Table
10) revealed the Sharkey soils have abundant Fe content. Maximum
Fet values were 44,800 ppm in Site 1, 39,400 ppm in Site
2, 46,800 ppm in Site 3, and 51,000 ppm in Site 4. Fet
contents decreased in the loamy materials of Site 2 subsoil. The Fe
that exists as a structural component of silicates may be estimated
by the difference between total Fet and dithionite extractable
Fe = (Fet - Fed) according to Blume and Schwertman
(1969). Santos et al. (1986) reported the presence of iron-rich montmorillonite
in the fine clay fraction in three Boralfs. Other researchers have
reported the main phyllosilicate clay in selected soils was comprised
of Fe-rich montmorillinite that contained less Fe than nontronite
(Mermut et al., 1984).
for exchangeable Fe (ferrous) during wetter winter and spring seasons
revealed only trace levels (<1 ppm) in the Sharkey pedons (Table
11). In saturated reduced soils, soluble ferrous iron would tend
to displace other cations on the soil exchange complex resulting in
significant exchangeable Fe (Gotoh and Patrick, 1974; Richardson and
Hole, 1979). Gotoh and Patrick (1974) detected 1,065 to 1,979 ppm
exchangeable Fe under controlled reduced conditions in the laboratory
using Crowley soil. The lack of exchangeable Fe suggests oxidized
Fanning (1976) reported that permanent soil wetness may lead to complete
loss of Fe and Mn by leaching. McDaniel and Bush (1991) quantified
the differential movement of Fe and Mn in saturated soils and suggested
the relationship could be used to infer the saturated status of soils.
Dithionite extractable Mn contents of the Sharkey pedons (Table
12) were similar to levels of upland Vertisols in the state. Higher
levels occurred at depths of 40 inches in Sites 1, 3, and 4, and at
the textural break in Site 2. Mn levels tended to coincide with presence
of concretions in the subsoil. The Mn levels indicate solubility and
removal of Mn has not occurred in the Sharkey soils.
Surface and subsurface
waters were periodically sampled and analyzed during the study with
emphasis on ferrous iron levels. Average levels of ferrous iron were
as follows for January sampling:
H2O at 110 inches
Soil water contained
extremely low Fe levels, and runoff waters were also very low in ferrous
Fe throughout the duration of the study.
The Sharkey soils
have dominant gray colors but contain high levels of Fe similar to
well-drained Vertisols in upland positions. The content and form of
Fe in soils have long been related to soil color, which has been used
to infer drainage and wetness. Soils with low chromas (2 or less)
are often considered gleyed. Early workers (Bloomfield, 1950, 1951)
suggested gleyed soil color resulted from unmasking of soil material
by the removal of iron oxide coatings to expose the mineral grains.
Daniels et al. (1961) suggested soils with high Munsell values and
low chromas usually have little free iron, and may have small amounts
of weatherable iron-bearing materials. These researchers further stated
that coloration patterns and redoximorphic features are difficult
to understand in these soils. Later work by Bloomfield (1952) reported
soils can have significant amounts of extractable Fe and yet be gleyed
(chroma 2 or less). Bloomfield suggested the iron was present as an
organoferrous complex adsorbed on the mineral surface. Recent research
(Dobos et al., 1990) attributed the lower chromas to hematite dissolution
allowing goethite and nonoxide minerals to influence color strongly.
Daniels et al.
(1961) noted that neutral and gley hues relate to the presence of
ferrous iron. Various researchers (Maubach et al., 1994; Daniels et
al., 1961) have reported that ferrous iron occurs only in reduced
or waterlogged soils depleted of oxygen. The accepted field test (SCS,
1994) to determine the presence of ferrous iron and reduced conditions
uses an indicator reaction. The Keys to Soil Taxonomy (SCS, 1994)
state, "A freshly broken surface of a field-wet soil sample is
treated with , -1-dipyridyl
in neutral 1-normal ammonium-acetate solution. The appearance of a
strong red color on the freshly broken surface indicates the presence
of reduced iron ions."
soil peds from the surface to 70 inches depth were tested seasonally
with , -1-dipyridyl
solution for the presence of ferrous Fe as demonstrated in Table
13. Tests were repeated frequently during the wetter winter and
spring months. No positive reactions were detected throughout the
total study. Tests were conducted at surface moisture contents as
high as 78% with negative results (data not presented). These tests
indicate the Sharkey soils did not have reduced conditions producing
soluble ferrous iron and agree with analyses indicating 0 to trace
levels of exchangeable Fe.
One of the very
difficult and complex aspects of soil color analysis is determining
if the color is a reflection of the parent material, relic from previous
environmental conditions, or due to current weathering and pedogenesis.
One of the early comprehensive studies of the Mississippi River Alluvium
(Brown, 1970) reported some of the soil color characteristics were
believed to be inherited from the parent materials. This research
considered the red colors to be relict from the Permian Red Bed materials
deposited by rivers and resistant to color change. A more recent study
of Louisiana delta soils (Schumaker et al., 1988) recognized that
soil color could be inherited directly from initial sediments or created
during weathering processes. Scientific explanations of red colors
persisting in delta soils as relic and resistant to change must also
consider gray clayey soils that occur in far greater proportion and
The Sharkey sites
were not subject to flooding and water input to the soil system is
due to precipitation. No other state, except Louisiana, in the continental
United States receives as much annual precipitation per square mile
land area as Mississippi (Way and Walker, 1986). Despite the extensive
precipitation, evaporation exceeds rainfall 7 months of the year resulting
in a small surplus (Figure 9). During the
5-year study, normal, dry, and wet years were experienced (Appendix
occurred in the Sharkey soils when evaporation exceeded precipitation.
Large surface cracks typically appeared in May and periodically closed
and opened at the surface until early November. The cracks would temporarily
close at the surface after precipitation events of 0.5 to 1 inch and
reopen within 1 to 3 days. The precipitation affected only the upper
surface 3 to 4 inches, with moisture contents largely unaffected below
those depths in the hot summer months.
The open cracks
served as open conduit for air and water entry into the subsoils.
The cracks exposed a large volume of soil to atmospheric air equilibrium
about half of the year.
moisture contents over the 5-year study exhibited a very consistent
distribution pattern (Figure 10) despite
variable precipitation. Highest moisture contents occurred in the
surface horizon and decreased with increasing depth. Sites 1, 3, and
4, which contained more clay, exhibited very little subsoil moisture
variation. Site 2, which had a loamy substratum, had the lowest moisture
contents and exhibited the greatest variation between surface and
subsoil portions of the soil profile. The average moisture contents
in the surface horizon ranged from 47 to 51%.
effects on soil moisture contents were limited to the surface horizons
(Figure 11). Soil moisture was highest
in the spring and winter seasons and lowest during summer and fall
seasons with spring > winter > summer > fall. The surface
soil moisture levels coincide with seasonal precipitation and evaporation.
the average soil moisture content range over the 5-year study indicates
80% of the total variation occurred in the upper 20 inches (50 cm)
as illustrated in Figure 12, which includes
all four sites. When Site 2 with the loamy substratum is excluded,
the range of variation is smaller (Figure 13).
in soil moisture with increased depths over the 5-year study was not
anticipated for the sites separated by a considerable distance. The
pronounced lack of seasonal variation in subsoil moisture levels over
the extended study was very revealing. Field observations via soil
auger readily detected the moisture gradient, with increased dryness
consistently observed in subsoils.
of soil moisture in the surface horizons 24 hours after a significant
precipitation event is shown in Figure 14.
The soil moisture content in the surface ranged from 49% to 53% over
a horizontal distance of 21m (70 feet). The sites had small depressions,
which were pronounced at Sites 1 and 3 of the Sharkey to 0 to ½%
slopes. Although not recognized during the soil surveys, some people
have recently referred to these depressions as gilgai features. These
features are not evident in cleared, cultivated areas of Sharkey soils.
Tree throws during the study produced very similar micro-topographical
features. The depressions tend to temporarily collect water during
precipitation events. The soil moisture 24 hours after rainfall at
a representative micro-depression at site 1 is shown in Figure
15. Moisture contents were similar below 25-cm (10-inch) soil
depths with variation confined to the surface. Similar data were obtained
at Sites 2, 3, and 4.
table depths in Sites 1, 3, and 4 over the 5-year study were about
3m (120 inches) as shown in Figure 16.
However, this was the maximum depth of the piezometers and they were
dry most months, so the actual water tables were deeper. Site 2, with
the loamy substratum, had the highest water table and exhibited the
widest fluctuations. The water table in Site 3 came within 45 inches
(112 cm) in December 1991, within 49 inches in April 1994 for short
duration, and within 29 inches (72 cm) for a very brief period in
July 1994 after intensive precipitation for 2 weeks. The water table
in Site 3 dropped from 29 inches on July 28, 1994, to 50 inches depth
on August 4, 1994, to 120 inches 2 weeks later. The water table in
Site 2 came within 7 inches of the surface during the same period,
and dropped rapidly to 97 inches in 6 days.
Water table measurements
by Soil Conservation Service-USDA soil scientists in forested and
cultivated Sharkey soils near the study sites in Washington County
showed very similar water table levels (Figure
of high water table levels for short durations in the Sharkey site
with a loamy substratum was not expected. The water levels dropped
quickly in the loamy subsoil. Dissolved oxygen levels in the fluctuating
groundwater indicated it was dynamic and charged with oxygen.
for dissolved oxygen were limited by the lack of water tables in the
piezometers. Most of the measurements were made in Site 2 and adjacent
drains and puddles. The dissolved oxygen levels decreased with increased
depth as shown in Figure 18. Dissolved
oxygen was always present in all water measured. Zero oxygen contents
were never encountered during the study.
In studies with
other gray soils, Cogger et al. (1992) reported dissolved oxygen levels
remained high enough in ground water to maintain oxidizing conditions.
Ransom and Smeck (1986) measured 02 levels in soil water
and did not correlate redox potential (Eh) with dissolved
12 inches wide, 12 inches long, and 12 inches deep were made at Sites
3 and 4 on September 12, 1991. Subsoil (Bt horizon) from a yellowish-red
clayey, montmorillonitic Wilcox soil, and a red, fine-loamy, kaolinitic
Lucedale soil were implanted in the excavation. The implanted soils
were covered with A horizon (0 to 4 inches) and forest litter. Selected
properties of the implanted soils were as follows:
soils were excavated 3 years after burial for field and laboratory
analyses. The implanted soils had "welded" to the surrounding
Sharkey soil and roots had extended into the soil mass. The soil hue,
value, and chroma had not changed. No physical or chemical differences
were detected in field or laboratory analyses. Three years of burial
in the Sharkey pedosphere did not alter the implanted soils. No reduction
or migration of iron or bases was detectable. Negative reactions were
obtained with , -1-dipyridyl
on the freshly broken peds of the exhumed soils, and no ferrous iron
was detected in water extracts or exchangeable form with ammonium
(1kg) from each site were subjected to intense reduction under controlled
laboratory conditions for 27 months. The soil was covered with H2O
with a sealed gas trap to permit CO2 discharge but no O2
entry, and sucrose was added as an energy source for microorganisms.
The water column was periodically removed for analyses while maintaining
a reduced condition on the soil.
Ferrous Fe and
Mn were released from the soil after 33 days reduction. Ferrous Fe
levels ranged from 890 ppm for Site 4 peds to 411 ppm for Site 2.
Soluble Mn ranged from 61 ppm (Site 4) to 14 ppm (Site 2). A significant
color change in the soil peds was detected after 7 months reduction
to Blue Green (BG) and neutral hues. The color change became more
pronounced with time. The soil peds still exhibited structural features
after 27 months immersion conditions. The peds were removed after
27 months for analyses. Bright red reactions were pronounced when
solution was placed on the peds after 27 months reduction. The positive
reactions, indicating ferrous iron, were the only positive reactions
obtained with Sharkey soils during the study. The reduction study
clearly demonstrates that Sharkey peds will release ferrous iron and
change from gray to blue green colors.
Richardson (1994) reported the presence of ferrous Fe in groundwater
discharge zones of reduced soils. Daniels et al. (1961) noted neutral
and gley hues relate to the presence of ferrous Fe. They reported
ferrous Fe occurs only in reduced or waterlogged soils with depleted
oxygen supplies. It was only in the controlled laboratory reduction
studies that ferrous Fe was detected.
Simonson (1956), "only the major differences in the original
vegetation are reflected to any extent in the soils, probably because
of the general youth of the land surface." Apparently, the Delta
region originally had a dense forest broken by occasional cane breaks
(Simonson, 1956). Heavy stands of cypress comprised the swampy areas,
and hardwood stands occupied the better-drained soils and many of
the wet ones. According to Simonson (1956), trees on the slight ridges
were chiefly hickory, pecan, post oak, blackgum, and winged elm. In
the swales and low places (not swampy), Tupelo gum, sweetgum, soft
elm, green ash, hackberry, overcup oak, and willow oak occupied the
John D. Hodges,
Professor of Forestry at Mississippi State University, directed a
vegetative survey of the four Sharkey Study Sites in Washington County
September 1993. At each site, four 0.1-acre vegetation quadrants (N,
S, E, W) were established to measure overstory and midstory vegetation,
and 0.01-acre plots were used to measure understory. General characteristics
of the vegetative cover are as follows:
Age 45 to 60 years; basal area 75 to 80 ft2, willow oak,
ash, sugarberry, honey locust, elm, ash, persimmon.
Age 45 to 60 years; basal area 80 ft2; average diameter
16 inches; sweetgum dominant, water oak, willow oak, nuttall oak,
sweet pecan, dogwood, mulberry, red maple, box elder.
Age 50 to 65 years; basal area 85 to 90 ft2; willow oak
dominant, nuttall oak, american elm, sugarberry, sweetgum, overcup
Age 50 to 65 years; basal area 80 to 90 ft2; willow oak
dominant, nuttall oak, persimmon.
The total species
by site on a per acre basis are as follows:
reflect total number species per acre present at each site.
bNumbers in parentheses reflect total number of individuals
per acre present at each site.
survey indicated relatively small differences among the sites (Appendix
Table 6). Facultative (FAC) and Facultative Wet (FACW) species
comprised 96.7% of Sites 1 and 3, and 99.3% of Sites 2 and 4. Obligate
species were overcup oak, nuttall oak, water hickory, and bitter pecan.
Only two bitter pecan saplings were detected on Sites 2 and 4.
counted in the survey, poison ivy ground cover was lush and abundant
on the Sharkey soils but disappeared abruptly on the adjacent wetter
Dowling soils. The Dowling soils commonly contained sedges, cypress,
and overcup oak (data not presented).
and laboratory studies over 59 months of four Sharkey soils in Washington
County, Mississippi revealed the following definitive soil properties.
- Contained prominent
intersecting slickensides within 40 inches (100 cm) of the soil
- Contained greater
than 30% clay between the surface and 7.2 inches (18 cm) depth,
and greater than 30% weighted average clay content between depths
of 7.2 inches (18 cm) and 20 inches (50 cm).
- Exhibited cracks
each summer of the study period with widths of 3 inches and greater
that extended to depths of 3 feet and greater.
- Possessed coefficient
of linear extensibility (COLE) greater than 0.09 in the clayey horizons.
- The clay fraction
was dominated by montmorillonite with fine clay (<0.2 µ) exceeding
coarse clay (2-0.2 µ).
- Base saturation
levels exceeded 50% with Ca the dominant exchangeable cation, and
high cation exchange capacities.
- Contained high
Fe levels comparable to upland well-drained Vertisols dominated
by ferric (oxidized) forms.
- Contained gypsum
(CaSO4 2H2O) crystals at subsoil depths below
40 inches that apparently formed under oxidized conditions.
- Did not have
field indicators of hydric soils, and did not have Munsell values
> 5 with chromas < 2 in the upper 10 inches
- Soil colors
became brighter with increasing depths.
Data in this study
clearly indicate the Sharkey soil series should be classified as Vertisols.
Reviews of previous soil descriptions and supporting data also clearly
indicate a Vertisol classification. The Sharkey soils exhibit maximal
properties definitive for Vertisols with pedogenic expression comparable
to soils typifying the Vertisol order on a global basis.
moisture and water table measurements over the 5-year study encompassed
normal, wet, and dry years. The hydrologic data indicated the following
- Average soil
moisture of surface horizons ranged from 47 to 51%. The clayey,
montmorillonitic horizons had high moisture retention at field capacity
(0.03 MPa) with 8 to 10% moisture differential between field capacity
and permanent wilting point.
- Average soil
moisture contents decreased with increased depth and subsoils exhibited
relatively small seasonal variations.
- Seasonal moisture
changes were largely limited to surface horizons with 80% of the
total change occurring in the upper 20 inches (50 cm).
- Soil moisture
was highest in spring and winter seasons and lowest in summer and
fall seasons, with spring > winter > summer > fall.
- Average water
table depths were below 100 inches (250 cm) in pedons with clay
textures extending to 60 inches depth. The site with loamy textures
below 30 inches exhibited the highest water table for very brief
duration and had widest fluctuations.
- The Sharkey
pedons did not have seasonal water tables within 18 inches.
- Soil water
had essentially no ferrous Fe indicating lack of reduced conditions.
- All tests with
were negative during all seasons indicating lack of ferrous iron.
- Dissolved oxygen
was present in all soil water measurements and tended to decrease
with increased depth.
midstory, and understory vegetation was dominated by Facultative
and Facultative Wet species.
- Small differences
(richness x diversity) existed among sites.
- Obligate species
comprised less than 1% of Sites 2, 4 and less than 3.3% of Sites
- Poison ivy
was abundant on the Sharkey sites but absent on adjacent Dowling
- Sedges and
cypress were common on adjacent Dowling soils but absent on the
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