Location and Extent
This major land resource area occurs in eastern Arkansas, eastern Louisiana,
western Mississippi, southeast Missouri and western Tennessee, western
Kentucky and covers approximately 97,913 km2. It consists of flood plains
and low terraces of the Mississippi River south of its confluence with
the Ohio River.
The annual precipitation ranges from 1150 to 1650 mm, increasing from
north to south. In most of MLRA 131 maximum precipitation occurs in winter
and early spring, decreasing gradually to a minimum in autumn. Along the
Gulf Coast, maximum precipitation occurs during midsummer and early autumn.
Snowfall is negligible. The annual air temperature ranges from 14 to 21o
C increasing from north to south. The precipitation, streamflow and aquifers
supply moderate to large quantities of potable groundwater. The Mississippi
River crosses the area from north to south. Oxbow lakes and bayous are
extensive throughout MLRA 131. Potable groundwater is not available in
extreme southern Louisiana.
The Southern Mississippi Valley Alluvium is commonly called the Delta,
however, it is not a delta in a geologic sense, but is a series of river
deposits in a very wide valley (Guccione, 1993). These sediments are late
Tertiary and Quaternary age deposits of the Mississippi River, which dip
very slightly to the south in the same direction as the flow of the Mississippi
River (Saucier, 1994).
The topography in MLRA 131 has been developing since the Quaternary
Period by deposition of river sediments, eolian sand dunes, and loess in
various parts of the valley (Guccione, 1993). According to Guccione (1993)
when the Mississippi River first formed it flowed in a wide, shallow valley
and deposited sand and gravel in a series of channels that migrated across
the valley. Later, the Mississippi River eroded more deeply into the Tertiary
sediments along the west margin of the valley, and the Ohio River eroded
more deeply into the Tertiary sediments along the east margin of the valley.
There are two prominent ridges in MLRA 131. Crowley’s Ridge, which is
composed of Tertiary sediments overlain by Mississippi River gravels, was
originally left as a divide between the Mississippi River to the west and
the Ohio River to the east. Periodically the wind blew sand and dust from
these river valleys. The sand was moved a short distance from the channels
and was deposited as sand dunes. The dust was lighter and could be blown
greater distances. The dust was deposited as loess on Crowley’s Ridge and
on the older terraces. Crowley’s Ridge extends from southeastern Missouri
to east-central Arkansas and may be as much as 50 m+ above the surrounding
plain. Another prominent feature is the Macon Ridge, located in southeastern
Arkansas and northeastern Louisiana. This ridge is composed mostly of loess
mantled over alluvium and is 10 - 20 m above the flood plain.
Both the Mississippi and the Ohio Rivers deposited thick sand in their
valleys when the glaciers to the north melted. Later, the rivers eroded
some of this sandy alluvium and formed a newer and lower flood plain in
a portion of the valleys. The older and higher flood-plain surfaces that
have been left are terraces. Finally, the Mississippi River cut through
Crowley’s Ridge, joined the Ohio River at Cairo, Illinois, and began to
flow down the eastern side of the ridge. About 10,000 years ago the Mississippi
River switched from a braided river to the meandering river that forms
the state boundaries today. The river migrates rapidly by eroding on the
outside of a river bend and depositing point bars on the inside of the
river bend. Abundant oxbow lakes mark old positions of the channel that
have been abandoned when the river cut through a narrow piece of land separating
two meander bends.
Elevation is at sea level in the south and increases gradually to about
200m in the north. The area consists of level to gently sloping broad flood
plains and low terraces. Most of the area is relatively flat with significant
areas in swamps and freshwater lakes in Arkansas and Louisiana.
According to the USDA-SCS (1981) most of MLRA 131 is in farms containing
on average about 55% cropland, 35% woodland and 7% pasture. The remaining
3% is used for miscellaneous purposes. Cropland composes about 75% of the
acreage in the north and less than 25% in the south. The proportion of
forest land varies inversely with that planted to crops; the proportion
of pasture is a little higher in the south. This MLRA is an important cropland
area with rice, soybean, cotton, grain sorghum and wheat grown by highly
mechanized production methods. Corn is an increasingly important crop in
Arkansas and Mississippi whereas sugarcane is an important crop in southern
Water management is an important component of crop production in MLRA
131. Controlling surface water by land shaping and artificially draining
wet areas are major concerns in management of excessive water. Irrigation
is an important water management strategy in rice, soybean, cotton and
corn production in Arkansas (Scott, et al., 1998), Mississippi and northern
Dominant soils in MLRA 131 belong to the Aquepts, Aqualfs, Aquents,
Udolls, Aquerts and Udalfs (Fig. 1). These are deep, medium- and fine-textured
soils that have a udic or aquic moisture regime, a thermic temperature
regime, and montmorillonitic or mixed mineralogy. Selected properties that
relate to water and solute transport in the dominant soils in this MLRA
are given in Table 1. Most soils in MLRA
are poorly or somewhat poorly drained. The wettest areas that are not artificially
drained remain in forests, which are important for hardwood timber production
and wildlife habitat.
The soils in MLRA 131 are derived from parent materials deposited mainly
during Holocene geological time. The alluvium is a mixture of materials
washed from many kinds of soils, rocks and unconsolidated sediments from
24 states ranging from Montana to Pennsylvania, deposited by the Mississippi
and Ohio Rivers and, in part, reworked by local tributaries of the Mississippi
River. The wide ranges in texture of the alluvium result from differences
in the site of deposition. When a river overflowed and spread over the
flood plain, the coarse sediments were dropped first, therefore sandy and
loamy sediments were deposited in bands along the channel. Thus, low ridges
known as natural levees were formed. Soils such as Beulah, Crevasse, Dubbs
and Robinsonville formed on the higher parts of these ridges. Finer sediments
that are high in clay were deposited on the lower parts of natural levees
as the flood waters spread and velocity decreased. Soils such as Commerce,
Dundee, and Mhoon soils formed in these sediments. Where the water was
left standing as shallow lakes or backswamps, the clays and finer silts
settled. Soils such as Alligator, Bowdre, Earle, Forestdale, Sharkey and
Tunica formed in these sediments.
This simple pattern of sediment distribution is not always found along
the Mississippi River, because through centuries, the river channel has
meandered back and forth across the flood plain. Sometimes the river channel
cut out all or part of natural levees, and at other times sandy or loamy
sediments were deposited on top of slack-water clay, or slack-water clay
on top of sandy or loamy sediments. The normal pattern of sediment distribution
from a single channel has been severely truncated in many places, and more
recent alluvium have been superimposed. Soils such as Bowdre, Earle, and
Tunica formed in these kinds of materials. Bowdre soils were formed in
thin beds of clayey sediments over coarse sediments, and Tunica soils were
formed in somewhat thicker beds of clayey sediments over coarser sediments.
Several equilibrium sorption studies have been conducted using the
“batch” method in the laboratory to examine the influence of soil characteristics
on the retention of pesticides. The influences of the soils found in MLRA
131 on sorption of pesticides are summarized below.
Goetz (1990) evaluated the sorption of Imazethapyr and Imazaquin by
two soils in Arkansas. Imazaquin and Imazethapyr are herbicides used for
selective weed control in soybeans. The soils included in his study were
the Sharkey silty clay and the Gallion silt loam. Selected physical and
chemical properties of the two soils are presented in Table
The Freundlich equation was fit to the experimental data using least
squares linear regression analysis and the Freundlich constants were determined.
The Freundlich sorption constant K is an index of the sorption capacity
of the soil and can be used to compare sorption on the two soils when the
slopes of the regression lines are approximately equal.
Goetz (1990) found that the Freundlich K values for Imazethapyr were
0.747 and 0.203 for the Sharkey and Gallion soils, respectively, and were
0.109 and 0.055 for Imazaquin for the same two soils. Thus, sorption by
these soils of Imazethapyr was greater than of Imazaquin. The slopes of
the sorption regression lines were similar and slightly greater than one
for both herbicides on both soils. Goetz’s results indicated that these
two soils had a higher sorption potential for Imazethapyr than for Imazaquin
and that the Gallion silt loam sorbed significantly less of these herbicides
than the Sharkey silty clay.
Johnson (1989) determined the relationships between soil organic matter
and sorption and herbicidal activity of Metribuzin, Fluometuron and Imazaquin
by 14 soils in MLRA 131. Metribuzin and Imazaquin are used to control broadleaved
weeds in soybeans whereas Fluometuron is used to control annual grass and
broadleaved weeds in cotton. The physical and chemical properties of the
soils used in his sorption study are given in Table
3. Equilibrium sorption of these herbicides by these soils was compared
by determining values of Kd, which relate the amount of herbicide sorbed
(µg/g) to the amount in solution (µg/ml) at equilibrium. The
sorption results for the three herbicides by the 14 soils are given in
4. In general, values of Kd for the three herbicides were quite variable
in these alluvial soils. For a given soil, Fluometuron tended to be sorbed
to a greater extent than Metribuzin and Imazaquin. The Beulah fine sand
had the lowest Kd and the Sharkey clay sample obtained from Crittenden
County, Arkansas had the highest Kd. Even within the clayey Sharkey series
considerable variability was found in Kd, not all of which could be explained
by calculating values of Koc for each herbicide and soil sample. Koc assumes
that the sorption is a function only of organic C.
Strebe (1995) conducted sorption and mobility studies of Flumetsulam
on several surface and subsurface soils found in MLRA 131 in Arkansas,
Louisiana and Mississippi. Selected physical and chemical properties are
given in Table 5. For the surface soils,
the Bosket loam exhibited the lowest and the Mhoon silty clay loam the
highest percent organic carbon. The Mhoon surface soil also had the highest
Kd value. The Dundee subsurface in Mississippi had the highest Kd and Koc
values for this herbicide. The results of these studies indicate that under
equilibrium conditions sorption of organic pesticides by soils in MLRA
131 varies with soil characteristics. There also appears to be considerable
variability in sorption within given soil series. Organic matter content
is not the only soil characteristic that governs the amounts of these organic
compounds sorbed by these soils.
Water and Solute
Movement in the Field
Several field studies have been conducted to characterize the transport
of water and solutes in soils in MLRA 131. We present the results of several
of these studies conducted in Arkansas and Louisiana.
Dundee soils are deep, somewhat poorly drained soils formed in thinly
stratified loamy alluvial sediments. They are found on nearly level to
gently sloping natural levees and low terraces that border former channels
of the Mississippi River and its tributaries. The redistribution of the
conservative water-soluble tracer bromide anion was examined in a bare
Dundee silt loam near Clarkedale, Arkansas (H. D. Scott, unpublished data).
As given in Table 1, Dundee soils are
classified as fine-silty, somewhat poorly drained and moderately slowly
permeable. The morphological profile description and selected physical
and chemical characteristics at the study site are given in Tables
6 and 7, respectively. These data show that the lower portion of the
Dundee profile, i.e. the BEg horizons, contains significantly higher clay
contents and lower sand contents than the surface, which were reflected
in the lower saturated hydraulic conductivities (Ksat). Because the plowpan
(Ap2, 11 - 21 cm) has a higher bulk density and platy structure resulting
from compaction due to extensive tillage, Ksat was also low in this horizon.
The field site had been in continuous cotton production for over 20 years.
The redistribution of bromide in the Dundee profile is shown in Fig.
2. The percent of bromide applied that remained in the 1.0 m profile is
presented as a function of cumulative rainfall. These results indicate
that the amount of bromide in the profile decreased with rainfall and suggested
that a significant proportion of the applied Br moved through the Dundee
soil and beyond the root zone with the percolating water. After 112 cm
of rainfall over a 330 day period, about 57% of the applied Br remained
in the soil profile and this has serious implications on the accumulation
and redistribution of salts in the profile. Most of this remaining Br was
found in the BEg horizons.
Sharkey soils are the dominant soil map unit in MLRA 131 with at least
1.2 million hectares (Petry and Switzer, 1996). According to the data presented
in Table 1, Sharkey soils are mapped on
almost 1.9 million hectares. They extend throughout the Delta from the
Gulf of Mexico to Kentucky. They are formed in clayey alluvium from Mississippi
River sediments and occur primarily on the Mississippi River alluvial plain
at low elevations. 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 its tributaries.
Geographically associated soils include Bowdre, Commerce, Mhoon, and Newellton
soils along with Barbary, Fausse, Iberia, and Tunica soils.
The clayey, sticky and plastic nature of Sharkey soils gave rise to
the usage of the terms “gumbo” and “buckshot.” Sharkey soils have slope
gradients between 0 to 5% with short slopes that typically occur as parallel
ridges and swales. They are typically dark gray to gray with brownish,
yellowish, or reddish mottles. They are classed as poorly drained with
slow or very slow surface runoff and permeability. They have expansive
clays that develop large cracks during droughty summer months. Sharkey
soils are designated as prime farmland and as a hydric soil (Petry and
Switzer, 1996). We present the results of two field studies on the transport
of water and chemicals on two sites mapped as Sharkey soils.
The movement of the conservative tracer bromide in a Sharkey soil was
examined by Scott (1999, unpublished data). The site had been in soybean
and corn production for the previous five years. A brief profile description
at the site is given in Table 8 and selected
physical and chemical properties of the soil at the site are given in Table
9. The soil properties at this site were strongly influenced by the
New Madrid earthquakes in 1811 and 1812 when sand was extruded from below
and deposited on the soil surface. Subsequently, the sand was incorporated
in the soil profile during cropping. The bulk densities, an indication
of compaction, near the soil surface are quite high which probably is due
to the presence of extensive amounts of sand. The hydraulic properties
of the soil are extensively influenced by the clay content and smectitic
type of clay. The soil swells upon wetting and shrinks during drying. As
a result the Ksat, as determined by the constant head method on undisturbed
soil cores, is practically zero at saturation. In the field, infiltration
of water in Sharkey soils is relatively rapid when the cracks are prominent
and near zero after the soil surface swells and seals over. After 160 cm
of rainfall and 477 days, about 59% of the applied Br remained in the top
1 m of the profile (Fig. 3). This indicates that movement of solutes, such
as Br, will be retarded in this fine textured, small porous soil unless
the transport occurs with the water infiltrating and redistributing through
the cracks and secondary fissures.
Bulk densities of the Sharkey soil decreased slightly with depth in
the profile. More important is the greater variation of bulk density in
the lower portions of the profile (> 60 cm). The presence of extensive
shrinkage cracks is perhaps the primary reason for such variation in the
profile. Bulk densities less than 1 g cm-3 were also encountered,
which are indicative of high pore space resulting from swelling and shrinkage.
Laboratory Ksat values with depth as measured using undisturbed soil cores
indicated extensive variability among all horizons. In fact, two and three
orders of magnitude variations were measured (103 to 101
cm h-1). Although several unrealistically high Ksat values were
measured, it was found that for several cores there was no flow. Results
from in situ unsaturated K measurements based on the instantaneous profile
method provided K values in the range of 10-4 and 10-2
cm h-1 for the top soil layers (Romkens, et al. 1986).
Sharkey soils are heavy-textured soils, which can be characterized by
shallow water tables and subject to extensive swelling during periods of
water infiltration and redistribution and shrinkage during drying. Very
little work has been conducted to describe the leaching behavior of agricultural
chemicals in such soils in the field. It is postulated that in these soils
two major transport processes of dissolved chemicals are dominant: one
is the movement through the soil matrix and the other is a rapid bypass
flow through soil macropores or shrinkage-swelling cracks. The latter is
referred to as preferential flow. In Sharkey soils, preferential or macropore
flow is perhaps the dominant water flow path to subsurface layers and ultimately
to the water table.
Johnson et al. (1995) conducted a study to quantify the mobility of
atrazine and nitrates in a Sharkey clay soil (Table
10) at St. Gabriel, LA in the presence of a shallow water table, and
to determine evidence of preferential flow patterns on the mobility of
agricultural chemicals in such soils. They also quantified the movement
of atrazine and nitrate in a field site of Sharkey clay planted to sugarcane.
Subsurface drains at 1 m depth provided leachate and a series of suction
lysimeters allowed sampling of the soil solution. Within a few hours of
the onset of rain, atrazine and nitrate were observed at high concentrations
in the drainage effluent. Atrazine breakthrough to the drains was consistently
observed to occur within 30 minutes of the onset of rain during the 1991
summer season. Such intervals were short in relation to measured soil hydraulic
conductivity and is indicative of the dominance of preferential water flow
in this soil. The leaching behavior of nitrate during 1991 and 1992 followed
a similar pattern to that of atrazine despite the fact that nitrate is
not subject to retention by the soil matrix. Subsequent rainfall events
result in progressively smaller atrazine and nitrate peaks. Maximum (or
peak) concentrations in drainage effluent for atrazine ranged from 60 to
80 ppb and for nitrate form 40 to 70 ppm. Despite the observed high peak
concentrations, total atrazine and nitrate leaching losses, based on mass
balance calculations, did not exceed 1.2 and 8%, respectively.
The findings of Johnson et al. (1995) have significant implications
on management practices of sugarcane on Sharkey clay soils in southern
Louisiana. Concentrations of nitrate-N in the drainage water were below
the lifetime health advisory limit in drinking water of 10 ppm. The only
exceptions were observed after N fertilizations where peak concentrations
did not exceed 17 ppm for two growing seasons. Therefore, we conclude that
based on 1991 and 1992 effluent results of nitrate-N, recommended fertilizer
N applied to this soil did not contribute to elevated N levels in the shallow
Olivier soils are fine, silty, mixed, thermic Aquic Fragiudalfs occurring
on nearly level to slightly convex stream divides and gently sloping areas
along streams in the lower Mississippi River basin in Louisiana and southern
Mississippi. Slopes range from 0 to 5% and were formed in loess of 1.2
to 6 m thickness. The Olivier soils are somewhat poorly drained and have
a moderately slow permeability in the fragipan. A water table is perched
above the fragipan at depths of 0.3 to 0.75 m for most of the winter months.
11 contains a description of a profile of Olivier soil in Louisiana.
The textural composition and the saturated hydraulic conductivities
from undisturbed soil cores and for different soil depths are given in
12 (Romkens et al., l986). The assumption often made in describing
water flow in layered or stratified soils is that the soil is homogeneous
and isotropic. This assumption is often relaxed by assuming that the soil
consists of several soil layers, each homogeneous and isotropic. Fragipan
soils such as Olivier differ from soil containing stratification due to
sedimentary processes. They typically contain polygonal peds with long
vertical dimensions separated and completely surrounded by bleached grey
seams. Such a structural configuration is expected to influence water flow
patterns and may render different Ksat values in different directions.
Therefore, undisturbed core samples were obtained in vertical and horizontal
directions from surface and subsurface horizons of an Olivier soil in order
to test for anisotropy of Ksat, bulk density, and soil water content at
0.3 bar or 0.03 MPa suctions (Dabney and Selim, 1987). Ksat values within
the Ap horizon did not differ in horizontal and vertical directions. However,
within the Btx1 horizon, measured Ksat values were three times greater
in the vertical than the horizontal direction. This was attributed to the
primarily vertical orientation of flow restricted zones within the fragipans.
Soil bulk density and moisture content differed between surface and subsurface
horizons but were not influenced by direction of core sampling. The results
of this study have relevance to water flow models and sampling methods
for fragipan soils.
In another study, the spatial variability of surface soil moisture in
space at field capacity and over a wide range of suctions were investigated
on an Olivier soil near Baton Rouge, Louisiana (Burden and Selim, 1989).
Measurements of soil moisture at field capacity, saturation and 0.005,
0.01, 0.03, 0.1, and 1.5 MPa suctions were performed on undisturbed soil
cores which were sampled at 30 cm spacing from the soil surface along an
80 m transect. The soil cores were further used for laboratory measurements
of Ksat, bulk density and particle size fractions. Statistical properties
of measured soil parameters along the transect are given in Table
13. As the suction increased, the coefficient of variation (CV) for
the corresponding moisture content increased. Lowest CV’s were obtained
for bulk density and the silt fractions, whereas highest CV was for measured
K along the transect. Semivariogram analysis indicated extensive spatial
structure for soil moisture data sets at most suctions with 50% of the
sample variance attributed to spatial variation. However, lack of spatial
structure, i.e. pure nugget effect was obtained for moisture data sets
at low suctions (0.005 and 0.01 MPa). This semivariogram finding was consistent
with results based on autocorrelation analyses. Moreover, no clear patterns
were observed for the range of spatial influence or length of correlation
for soil moisture with increasing suctions. We concluded that the extent
of spatial structure for soil moisture was not influenced by the degree
of tension in the 0 to 1.5 MPa range. In addition, cross-correlograms results
indicated that optimum correlations were obtained for field capacity and
at 0.03 MPa and for field capacity and bulk density observations. Poorly
defined ranges with significance but low correlations were obtained for
field capacity and Ksat and for field capacity and the silt fraction.
In a separate study, in situ mechanical impedance (MI) was measured
with soil depth using a cone penetrometer on the fragipan - Olivier soil
(Selim et al., 1987). A tillage pan was found at the base of Ap horizon
followed by a fragipan (>0.3 m) extending to a depth of 1 m. The fragipan
exhibited visible variation in matrix color and the presence of numerous
grey tongues or seams in between polygon units. Soil MI was measured at
equal intervals of 0.3 m along an 80 m transect. Table
14 summarizes the MI values versus soil depth along the transect. Bulk
density and soil moisture content (using a neutron probe) were also measured
along the transect (see Table 15). Highest
values for mean MI were encountered in the tillage pan and Btx3 layers
whereas lowest values were for Btx1 and Btx2. To describe the spatial variation
of measured observations semivariograms, cross-semivariograms, autocorrelations,
and kriging analyses were performed. Semivariogram results of MI indicated
limited spatial structure for all horizons. The nugget effect ranged from
47 to 77% of the sample variance indicating that considerably less than
half the variance is a result of spatial variation. Moreover, the range
of influence was limited to 2 to 4 m for Btx2 and 5 to 8 m for all other
layers. It is suggested that the random occurrence of grey seams and brittle
brown areas in the fragipan horizons significantly contributed to the lack
of observed spatial variability. Observed versus kriged (jack-knifed) MI
results showed limited correlation for the fragipan layers which is consistent
with autocorrelation results.
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soil water retention for a surface soil. Soil. Sci. 148:436-447.
Dabney, S.M. and H.M. Selim. 1987. Anisotropy of a fragipan soil: vertical
vs. horizontal hydraulic conductivity. Soil Sci. Soc. Am. J. 51:3-6.
Goetz, A.J. 1990. Sorption, mobility and degradation of Imazethapyr
in Arkansas soils. Unpublished Ph.D. Dissertation. Department of Agronomy,
University of Arkansas, Fayetteville.
Guccione, M.J. 1993. Geologic history of Arkansas through space and
time. The Arkansas and Regional Studies Center. University of Arkansas,
Johnson, D.H. 1989. Relation of soil humic and organic matter with the
activity of Metribuzin, Fluometuron, and Imazaquin. M. S. Thesis. Department
of Agronomy, University of Arkansas, Fayetteville.
Johnson, D.C., H.M. Selim, L. Ma, L. M. Southwick, and G.H. Willis.
1995. Movement of atrazine and nitrate in Sharkey clay soil: evidence of
preferential flow. Louisiana Agric. Exp. Stn. Bull. No. 846.
Pettry, D.E. and R.E. Switzer. 1996. Sharkey soils in Mississippi. Mississippi
Agric. and Forestry Exp. Stn. Bull. 1067.
Romkens, M.J., H.M. Selim, H.D. Scott, and F.D. Whisler. 1986. Physical
characteristics of soils in the southern region:
Captina, Gigger, Grenada, Loring, Olivier and Sharkey Series. Southern
Coop. Ser. Bull. 264.
Saucier, R.T. 1994. Geomorphology and quaternary geologic history of
the lower Mississippi Valley. U. S. Army Engineer. Waterways Experiment
Station. Vicksburg, MS.
Scott, H.D., J.A. Ferguson, L. Hanson, T. Fugitt and E. Smith. 1998.
Agricultural water management in the Mississippi Delta Region of Arkansas.
Arkansas Agricultural Experiment Station. Research Bulletin 959.
Selim, H.M., B.Y. Davidoff, H. Fluhler, and R. Schulin. 1987. Variability
of in situ measured mechanical impedance for a fragipan soil. Soil Sci.
Strebe, T.A. 1995. Adsorption, mobility, dissipation and persistence
of Flumetsulam in southern soils. M.S. Thesis. Department of Agronomy,
University of Arkansas, Fayetteville.
USDA-SCS. 1981. Land resource regions and major land resources areas
of the United States. Agriculture Handbook 296. Washington, D.C.