SCSB# 395
MLRA 131: Southern Mississippi Valley Alluvium
H.D. Scott1, H.M. Selim2, and L.B. Ward3
1University of Arkansas, 2Lousiana State University, and 3USDA-NRCS Little Rock

Chapter Contents

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.

Geology, Topography and Elevation
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 Louisiana.

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 Louisiana.

Fig. 1. STATSGO soils of the Southern Mississippi Valley Alluvium MLRA 131.

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.

Equilibrium Sorption of Pesticides
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 2.

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 Table 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.

Fig 2. Concentration distribution of Br at three sampling times in a Dundee silt loam in Arkansas.

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.

Arkansas site:
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.

Fig. 3. Concentration distributions of Br at three samplings in a Sharkey soil in Arkansas.

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.

Louisiana Site:
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 water table.

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. Table 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 Table 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.

Literature Cited
Burden, D.S. and H.M. Selim. 1989. Correlation of spatially variable 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, Fayetteville.

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. 144:442-452.

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.

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