SCSB# 395

MLRA 133B: Western Coastal Plain
H.D. Scott1 and L.B. Ward2
1University of Arkansas and 2USDA-NRCS Little Rock

Chapter Contents

The Western Coastal Plain major land resource area covers approximately 140,640 km2 in southern Arkansas, western Louisiana, southeastern Oklahoma, and northeastern Texas (USDA-SCS, 1981). Recent digital MLRA data indicates that the Western Coastal Plains occupies about 137,276 km2 or 33,920,900 acres.

Increasing from northwest to southeast, annual precipitation ranges from 1,025 to 1,350 mm. Maximum precipitation occurs in the spring and early summer, while minimum precipitation occurs during late summer and autumn months. The average annual air temperature is between 16°C and 20°C. The average freeze free period increases from north to south and ranges from 200 to 270 days. Precipitation, perennial streams, and ground water provide an abundance of water, however, droughts often occur during the summer months. In times of greater precipitation accumulation, wet soils are sometimes drained before crop use. Municipal water supplies and recreation sites are usually provided by large reservoirs located on major streams, however, numerous deep wells have been recently drilled mostly for urban uses. Water from ponds and wells is directed to farm production.

Geology, Elevation, and Topography
The geology of MLRA 133B was given by Guccione (1993). The sandstone and shale rocks dip slightly to the south. These rocks were deposited in the shallow Gulf of Mexico during the Cretaceous Period through the Cenozoic Period, and later in stream valleys. As the Gulf of Mexico receded to the south during the middle Cenozoic Era, rivers migrated across the entire coastal plain and deposited sediment throughout the region. Subsequently, the dendritic streams eroded, and in the upper parts, were restricted to distinct valleys. Between the valleys the bedrock has been subject to weathering and erosion. The Western Coastal Plains consists of level to nearly level flood plains, level to gently sloping terraces, and nearly level to steep uplands. Increasing from south to north, the elevation ranges from 25 m to 200 m above sea level.

Forests and woodlands occupy approximately one-half to three-fourths of this area. The remainder is devoted to farmland, one-sixth being used for cropland. Large forestry corporations and the federal government (designated as national forests) own a few large tracts within this area.

Natural vegetation is pine-hardwood forests. Dominant herbaceous species include Little bluestem and Pinhole bluestem. Other major grasses are beaked panicum, longleaf uniola, spike uniola, and yellow indiangrass. Many low-growing species of panicums, paspalums, and perennial forbs contribute to the total annual yield. Commonly grown crops include corn, grain sorghum, oats, soybeans, peanuts, rice, and vegetables.

Fig. 1. STATSGO soils of the Western Coastal Plain MLRA 133B.

Most soils in MLRA 133B developed from clayey loamy or sandy marine sediments. They range from poorly drained to somewhat excessively drained. Dominant soils are Paleudalfs, Hapludalfs, Paleudults, and Hapludults. These are very deep, loamy and clayey soils with thermic temperature regimes, an udic moisture regime, siliceous, smectitic or mixed mineralogy. Other common soils found in MLRA 133B include Glossudalfs, Glossiqualfs, Kandiudults, Endoaquults, Paleaquults, Fragiudults Fluvaquents, Udifluvents, Endoaquepts, and Dystrudepts. STATSGO soils are depicted in Fig. 1. Soil series mapped with more than 100,000 acres in MLRA 133B are presented in Table 1.

The soils in MLRA 133B vary widely in drainage, frequency of flooding, slopes and position on the landscape. Generally, the soils can be divided into three groups based on their landscape position.

The soils on flood plains consist of very deep, level to nearly level, poorly drained to well drained loamy soils with slow to moderate permeability. These soils are subject to frequent flooding and were formed in loamy alluvial sediments. Included in this group are soil series such as Guyton, Ouachita, Luka, Mantachie, and Nahatche. These soils are poorly suited to crops and pasture because of flooding and seasonal wetness and are mainly used for woodlands.

Soils on terraces tend to be very deep, poorly drained to somewhat excessively drained, and slowly to moderately rapidly permeable, level to nearly level and have loamy and clayey textures. Soil series included in this group are the Amy, Annona, Bienville, Cahaba, Caddo, Wrightsville, Smithton, and Pheba. These soils are mainly used for woodlands, however, a few of the nearly level to gently sloping areas are used for pastures.

Soils on the uplands consist of somewhat excessively drained to somewhat poorly drained, very slowly to moderately permeable, nearly level to steep, and have loamy and clayey textures. Soil series included in this group are the Cuthbert, Darco, Malbis, Kirvin, Ruston, Smithdale, Sacul, Savannah, Sawyer, Saffell, Wolfpen, and Woodtell. Seasonal wetness, droughtiness, and potential aluminum toxicity are the main limitations to agricultural uses. Steepness of slope, moderate to very slow permeability, and moderate to very high shrink-swell potential are limitations.

Water and Solute Transport
The movement of water and the conservative tracer bromide (Br) in two soils in MLRA 133B were examined in the field (H.D. Scott, unpublished data). Both sites were in pasture and located near Hope, Arkansas. Nearly flat 10 m by 10 m plots, which were bounded with Al flashing, were constructed to restrict runoff and runon. Br was broadcast applied to the square plots at a rate of 2.5 kg of KBr in about 10 L of water. The grass was killed with a herbicide and maintained vegetation free throughout the study. The soil profile was gravimetrically sampled in 10-cm increments at 24 known locations within each plot. The Br was extracted from the soil and the Br concentrations determined with an anion chromatograph. The concentrations of Br within each depth interval and sampling time were geometrically averaged.

Study Site 1
The soil at this site classifies as a Fine-loamy, siliceous, thermic Plinthic Fragiudult. It contains a fragipan beginning at 95 cm and contains more than 5% plinthite in the lower part of the profile. The soil was formed in marine or stream deposits, consisting of thick beds of loamy sediments. The site location and a brief soil description are given in Table 2.

Selected physical and chemical properties of the soil at Study Site 1 are given in Table 3. These data indicate that in the A and transition BE horizons the soil profile has a sandy texture, with moderately high values of Ksat and bulk density. The B horizons contain considerably more clay and have lower values of Ksat but somewhat higher values of bulk density. The soil pH is acidic and with the exception of the Ap horizon has organic carbon contents of less than 1%.

The geometric mean concentration distributions of Br in the soil for three samplings are shown in Fig. 2. The cumulative rainfall was 15.5, 55.7, and 100.5 cm at the sampling times of 64, 155, and 299 days after application, respectively.

Fig. 2. Distribution of bromide concentrations at Study Site 1.

The shapes of the concentration distribution curves show that Br was readily redistributed within the soil at Study Site 1 by the rain that infiltrated the profile. Of particular interest is that the peak Br concentration was found at the 45-cm depth after only 15.5 cm of rainfall. This indicated that the Br anion was redistributed from the soil surface and rapidly moved through the sandy loam A and B horizons by the infiltrating water. After almost 300 days and 100 cm of rainfall after application, the Br concentrations were relatively uniform throughout the soil profile and at concentrations less than 1 mg kg-1.

Mass balances of Br were calculated in the 1-m profile at each sampling (Table 4). The mass of Br decreased with increases in rainfall and time. The average rates of loss, which were greatest during the first sampling period, decreased dramatically during the second and third sampling period. By the end of the third sampling period, almost 95% of the applied Br was lost from the 1-m profile.

Study Site 2
The soil at Study Site 2 classifies as a Fine, mixed, thermic Oxyaquic Hapludalf. The soil developed in two parent materials. The upper part is loamy, marine sediments and the lower part is Cretaceous-aged, marly clays. The site location and brief description is given in Table 5.

Selected physical and chemical properties of the soil at Study Site 2 are given in Table 6. In comparison with the soil at Site 1, this soil has considerably more clay and water retained at the three pressures and lower values of Ksat, bulk density, pH, and percent organic carbon.

The geometric mean concentration distributions of Br in the Site 2 soil at three samplings are shown in Fig. 3. The cumulative rainfall was 15.5, 55.7, and 100.5 cm at the three samplings, respectively. The elapsed time since application of Br was 64, 155, and 299 days for the respective sampling dates.

Fig. 3. Distribution of bromide concentrations at Study Site 2.

The data show that Br was readily redistributed within the Site 2 profile with the rain. However, the redistribution of Br within this soil profile was not as rapid as within the Site 1 soil. This slower redistribution of Br in the Site 2 soil can be attributed primarily to the lower permeabilities and finer textures, particularly in the B horizons as compared to the site 1 soil. For example, at the first sampling, the cumulative rainfall was 15.5 cm and the highest Br concentration occurred near the soil surface. After almost 300 days the Br concentrations had decreased throughout the profile with the highest Br concentrations remaining near the soil surface.

Mass balances of Br were calculated in the 1-m profile at each sampling (Table 7). The mass of Br decreased with increased rainfall and time. The average rates of loss were greatest during the first sampling period and lowest during the second sampling period. By the end of the third sampling period almost 85% of the applied Br was lost from the 1-m profile.

Literature Cited
Guccione, M.J. 1993. Geologic history of Arkansas through time and space. Copyright. M.J. Guccione. University of Arkansas, Fayetteville.

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

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