SCSB# 395

MLRA 134: Southern Mississippi Valley Silty Uplands
H.D. Scott1, H.M. Selim2, and A.B. Johnson3
1University of Arkansas, 2Lousiana State University, and 3Alcorn State University

Chapter Contents

Location, Extent, and Landuse
MLRA 134 occurs in eastern Arkansas, western Kentucky, western Tennessee, eastern Louisiana, and western Mississippi. It covers about 51,410 km2 (USDA-SCS 1981). The digital version indicates that this MLRA covers about 74,094 km2.

Most of MLRA 134 is in farms with about 35% in cropland, which consists of primarily cotton, rice, soybeans, corn and wheat. About 16% of the area is in pasture or hay and about 46% is in forest of mixed pine and hardwoods. Lumber is the major forest product and some pulpwood is harvested. About 3% of the area is used for urban development or other purposes. Dairying is an important enterprise locally. A small percent of the area is federally owned, and about 3% of the area is used for urban development or other purposes.

The climate of the region is similar to that of MLRA 131. Annual precipitation ranges from 1,150 to 1,525 mm, increasing from north to south. Maximum precipitation occurs during winter and in spring, decreasing gradually through summer to autumn except for a moderate increase in midsummer. The average annual air temperature ranges from 16 to 20oC, increasing from north to south. The freeze-free period is 200 days or more in most of the region but is as long as 290 days in the southernmost part.

Geology and Elevation
The sharply dissected plains have a thick loess mantle that is underlain by unconsolidated sand, silt and clay, mainly of marine origin. Valley sides are hilly to steep, especially in the west. The intervening ridges are mostly narrow and rolling, but some of the interfluves between the upper reaches of the valleys are broad and flat. Stream valleys are narrow in the upper reaches but broaden rapidly downstream and have wide, flat flood plains and meandering stream channels. Local relief ranges from nearly level to more than 25 m. The elevation ranges from 25 to 100 m.

Fig. 1. STATSGO soils of the Southern Mississippi Valley Silty Uplands in MLRA 134.

The Udalfs are the dominant soils in MLRA 134 (Table 1). The STATSGO soils are presented in Fig. 1. They are deep, medium textured soils that have a thermic temperature regime, an udic moisture regime, and mixed mineralogy. Well drained, nearly level to very steep Hapludalfs such as the Memphis series occur on the uplands whereas the moderately well drained, nearly level to strongly sloping Fragiudalfs such as the Grenada and Loring series occur on ridge tops, side slopes, and terraces. On the flood plains, somewhat poorly drained Fragiudalfs such as the Calloway, poorly drained Udifluvents such as the Morganfield and Vicksburg soils, moderately well drained Udifluvents such as the Adler and Collins soils and somewhat poorly drained Fluvaquents such as the Falaya and Gillsburg series are found.

In the east where the loess mantle thins, well drained Paleudalfs such as the Lexington soils, moderately well drained Fragiudalfs such as the Providence soils, and well drained Hapludults such as the Brandon series, occur on gently sloping to steep ridgetops and side slopes. Well drained Dystrochrepts such as the Ariel series, moderately well drained Udifluvents such as the Collins series, moderately well drained Dystrochrepts such as the Oaklimeter series and somewhat poorly drained Fluvaquents such as the Falaya occur on the flood plain.

Water and Solute Transport
Examples are presented where the transport of water and the conservative solute bromide (Br) are presented in field soils in Arkansas, Louisiana and Mississippi. In the field, the conservative tracer Br was applied as KBr to bare soil surfaces and determinations were made of the Br redistribution in the soil profile over time.

Stuttgart Soils
Stuttgart soils are very deep, moderately well to somewhat poorly drained, very slowly permeable soils formed in alluvium. They occur on level to gently sloping prairie terraces along the current and former channels of the Arkansas River in the Lower Mississippi Valley. Slopes range from 0 to 8 percent; runoff is slow to medium. This soil is associated with Crowley (DeWitt) soils and is similar to those soils in all respects except color and drainage. These soils are used extensively for rice and soybean production. The morphological description of the Stuttgart soil at the study site is presented in Table 2.

Physical characteristics of the Stuttgart soil near Stuttgart Arkansas are given in Table 3. These indicate that Stuttgart soils have a silt loam over clay texture. The saturated hydraulic conductivity (Ksat) determined from undisturbed cores is essentially negligible in the lower portions of the B horizon, primarily due to the high silty clay content and elevated Na concentrations. The clay mineralogy is dominantly montmorillonite, and thus, shrinking upon drying and swelling upon wetting is important to the hydrology and solute transport in this soil.

The movement of the conservative tracer Br in and through the Stuttgart soil is reflected in the temporal mass balances of Br in the profile to a depth of 1.0 m (Fig. 2). These results showed that about 41% of the Br applied to the bare Stuttgart soil remained in the profile after 91 cm of rainfall and 178 days. The rate of loss of Br from the profile varied during the experiment. The highest loss rate occurred during the initial 13 cm of rainfall and was 0.54 % of Br per day. This was about three times higher than toward the end of the study of 0.17% of Br per day. These results indicated that transport of anions in and through the Stuttgart soil is restricted, which could have serious implications to the accumulations of salts in the profile.

Fig. 2. Bromide concentration distributions in the Stuttgart soil profile at two sampling times.

Gigger Soil Series
The Gigger soil series are fine silty, mixed, thermic, Typic Fragiudalfs occurring on nearly level to sloping stream terraces of the late Pleistocene age in the lower Mississippi River Basin (Louisiana). They are formed in a thin mantle of loess over loamy stream terrace sediments, have slopes from 1% to 8%, and have a seasonal high water table perched above the fragipan during winter and early spring. Gigger soils are moderately well drained.

The morphological description of a Gigger soil profile at the study site in Louisiana is given as follows:

Ap (0-15 cm): Dark grayish-brown silt loam with weak, medium granular structure.
B21t (15-20 cm): Light grayish-brown silt loam with weak, fine subangular, blocky structure; hardpan was encountered in some samples.
B22t (28043 cm): Yellowish-brown silt loam; medium subangular blocky structure.
Bx1 (43-63 cm): Light gray silt loam; medium subangular structure; irregular boundary.
Bx1 (63-90 cm): Yellowish-brown silt loam; medium-prismatic, medium subangular blocky structure; granular silt seams (discontinuous).
Bx3 (90-133 cm): Yellowish-brown silt loam, weak, prismatic, medium subangular blocky structure; granular wavy boundaries.
B23t (133-153 cm): Dark brown loam; weak, coarse subangular blocky structure with black stains and thin discontinuous black films on faces of peds.

Water Relations
Bulk density data for Gigger soil indicated a higher value for the fragipan layers compared to the Ap and B21t layers. This observation is consistent with other fragipan (Grenada and Olivier) soils. The textural composition and the saturated hydraulic conductivities from undisturbed soil cores and for different soil depths are given in Table 4 (Romkens et al. 1986). An increase in clay content with depth is observed. Saturated hydraulic conductivities measured in the laboratory on undisturbed soil cores showed distinct differences for the various horizons. Most striking is that an order of magnitude or greater decrease in Ksat of the top soil layers (Ap and B21t) compared to the fragipan layers. Field measured soil water suction vs. moisture content results from two in situ plots provided a limited range of water retention for lower soil depth. Moreover, field measured retention results were consistently lower in water content values than laboratory-determined retention relations. The differences in water content values ranged from 0.05-0.07 m3 m-3 . In situ K values vs. suction is difficult to measure for the fragipan layers with most values in the 10-4 to 10-3 cm h-1 range. With the exception of the top layer in the fragipoan (Bx1), the change in soil moisture suction was negligible over time, thus K values were not obtained.

Grenada Soil Series
Grenada soils are fine silty, mixed, thermic, Glossic Fragiudalfs consisting of moderately well-drained soil with distinct A'2 horizons, which have tongues or interfingers of gray silt loam extending into an underlying fragipan. These soils have developed in silty, loessial material on broad upland ridges and stream terraces, and are found among the bluffs along the valleys of the Mississippi River and its tributaries in western Kentucky, western Tennessee, Mississippi, eastern Arkansas, and northeastern Louisiana. The Grenada soils are moderately permeable in layers above the fragipan and slowly permeable in the fragipan.

The description of a Grenada soil profile at the study site in Louisiana is given below:
Ap (0-15 cm): Dark grayish-brown silt loam; medium granular structure.
A2 (15-30 cm): Light grayish-brown with weak to fine subangular blocky structure. Hardpan was encountered in some samples.
B2 (30-42 cm): Yellowish-brown with medium subangular block structure.
A2 (43-56 cm): Light gray with medium subangular blocky structure; gradual wavy and often irregular boundaries.
Bx1 (56-88 cm): Yellowish-brown gray with medium prismatic, subangular blocky, gray silt seams throughout the horizon.
Bx2 (88-138 cm): Yellowish-brown with weak prismatic medium subangular blocky structure with light gray silt coatings.

Water Relations
The textural composition and the saturated hydraulic conductivities from undisturbed soil cores and for different soil depths are given in Table 5 (Romkens et al. 1986). Larger bulk density values were encountered in the B2 horizon. Moreover, a noticeable decrease in bulk density was found between the B2 and A'2 horizons. The presence of the gray silt seams in the fragipan layers, which was observed during profile description (58 to 107 cm), did not cause an increase in variability of the bulk density values. Results of laboratory measured saturated hydraulic conductivities (Ksat) on undisturbed soil cores for various soil depths are presented in Table 5. Small Ksat values were found in the surface horizons (0 to 43 cm), whereas the greatest variability in the measured Ksat values was found in the deeper soil horizons (43 to 107 cm). The latter variability of Ksat probably was due to the presence of gray seams. These seams, which were frequently found to be discontinuous through the fragipan horizons, are often more conducive to water flow than is the surrounding soil material. The soil cores used in Ksat measurements were only 3 cm in length, and seams in some cores were continuous, which resulted in more variability in the measured Ksat. Also, the presence of discontinuous rather than continuous gray seams in the soil cores probably resulted in larger Ksat values.

Laboratory-measured as well as predicted soil water retention relationships using van Genuchten model (CTXFIT) for individual soil horizons were extremely successful. The associated parameters qr, a , and h from the van Genuchten model were obtained by least square fit of the experimental data (see Table 6). The results also indicate a gradual increase in q at 15 bars of water suction as soil depth increased. The smallest values for q at 15 bars were encountered in the Ap surface horizon of all plots. The range of suction where the greatest decrease in soil water content occurred was between 100 and 1,000 cm. This decrease in water content was fairly steep for the surface horizons. In contrast, the fragipan horizons (Bx1 and Bx2) yielded moderate changes in slope of the water retention curves. Model predictions of q vs. suction exhibited good agreement with experimental data for all three Grenada plots.

Memphis Soil Series
The Memphis study site in Mississippi was located in Church Hill, 20 miles south of Alcorn State University. The specific location was a pasture containing Memphis silt loam. A 100 m2 plot with 0% slope was selected. Vegetation on the plot was controlled with glyphosate (Roundup). A solution of a conservative tracer, potassium bromide (KBr) was applied with a hand sprayer evenly 12 days after glyphosate application. Application was made to provide the rate of 2.5 kg of KBr and yielding 1678.8 g of Br-. Samples were taken within 24 hours with a soil probe in 10-cm increments to the depth of 120 cm after a 2.54-cm or higher precipitation event over 313-day period. At least three samples were taken diagonally across the plot for every sampling period and air dried for Br- analysis. Twenty mL of deionized water was added to 10 g aliquot of each soil sample and agitated on a benchtop shaker for 2 hrs to extract Br-. The samples were placed in a centrifuge and allowed to spin at 300 rpm for 1 hour. Supernatant from each sample was placed in a vial and Br- analyzed with a DX 500 Ion Chromatography System containing an IonPac column (Dionex Corporation, Sunnyvale, California). Flow rate for this system was set to 2 mL min-1 . Bromide data obtained were expressed in mg kg-1 (on dry soil basis).

A pit was dug 100 m east of the plot for soil profile characterization. Profile morphological characterization was made by horizon and samples were taken in 10-cm increments to the depth of 120 cm to measure bulk density, water retention, saturated hydraulic conductivity (Ksat), particle size distribution, organic carbon (OC), pH, and electrical conductivity (EC). Nitrate-nitrogen (NO3 -N) and elemental analyses were made at similar depth intervals. All physical and chemical analyses were made using standard protocols.

Water Transport and Soil Physical Properties
The physical properties of the Memphis soil (described in Table 7) at the study site are given in Table 8. Bulk density was higher from 20 to 90 cm. Neither particle size nor bulk density influenced Ksat in the profile. Soil water retention at -33 and -1500 kPa was higher from 10 to 60 cm. This may have been due to the relatively higher clay content (5% more clay) in the upper 60 cm.

The soil chemical properties of the Memphis soil at the study site are shown in Table 9. As expected, OC was highest in the top 10 cm and gradually decreased with depth. Soil pH was consistently moderately acid throughout the profile. The amount of calcium was highest in the profile with manganese (Mn) having the smallest amount. High EC values were observed from the 70 to 120 cm depth. The same trend was observed for the amount of magnesium (Mg) and sodium (Na) in the profile.

Bromide Transport
Bromide recovered in the profile ranged from 77.6 to 18.1% (Table 10). The total amount of precipitation during the study period was 1027 mm with the highest precipitation event occurring 213 days following application (456 mm).

Concentration profiles of Br are presented in Figs. 3 and 4. The highest concentration was observed at the 10-cm depth 12 days after application. Thirty-nine days after application, peak concentration was located between 60 to 80 cm in the profile (Fig. 3). Peak concentrations were about 2.5 times lower at days 78 and 203 than days 12 and 39 (Fig. 4). Location of Br- at days 78 and 203 was between 30 and 50 cm. The locations of peak concentration of Br on various days following application are shown in Fig. 5. Depths of peak concentration fluctuated in the profile, however, Br concentration decreased with time.

Fig. 3. Mean Br - concentration profile on days 12 and 31 following application. Bar represents standard deviation of the mean.

Fig. 4. Mean Br - concentration profile on days 78 and 203 following application. Bar represents standard deviation of the mean.

Fig. 5. Depth of peak resident concentration (a), rainfall (b), and cumulative rainfall (c) following Br - application.

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.

Johnson, D.C., H.M. Selim, L. Ma, L.M. Southwick, and G.H. Willis. 1995. p. 24. Movement of atrazine and nitrate in sharkey clay soil: Evidence of preferential flow. Louisiana Agric. Exp. Stat. Bull. no. 846.

Petty, D.E., and R.E. Switzer. 1996. Sharkey soils in Mississippi. Mississippi Agric. and Forstry Exp. Sta. Bull. No. 1067 (37p).

Romkens, M.J., H.M. Selim, H.D. Scott, and F.D. Whisler. l986. Physical characteristics of soils in the southern region; Captina, Gigger, Grenada, Loring, Olivier, and Sharkey Series. Southern Coop. Ser. Bull. 264.

Selim, H.M., B.Y. Davidoff, H. Flühler, and R. Schulin. 1987. Variability of in situ measured mechanical impedance for a fragipan soil. Soil Sci. 144:442-452.

USDA-SCS. 1981. Land Resource Regions and Major Land Resource Areas of the United States. Agriculture Handbook 296. Washington, DC.

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