SCSB# 395
MLRA 153A: Atlantic Coast Flatwoods
D.K. Cassel
North Carolina State University

Chapter Contents

MLRA 153A (Atlantic Coast Flatwoods) is one of eight MLRAs comprising the Atlantic and Gulf Coast Lowland Forest and Crop Region. Associated with MLRA 153A on the Atlantic Coastal Plain are MLRAs 153B (Tidewater Area) and 153C (Mid-Atlantic Coastal Plain). These three MLRAs have elevations less than 25 m giving rise to poorly drained soils. Soils to the west of MLRA 153A, for the most part, occur in MLRA 133A, the Southern Coastal Plain, a component of the South Atlantic and Gulf Slope Cash Crops, Forest, and Livestock Region. Much of the general information for MLRA 153A which follows was extracted from USDA (1981).

MLRA 153A consists of level lowlands interspersed with swamps, estuaries, and lagoons. The soils are poorly drained and have minimal local relief. Slopes are long and have low gradients across broad interfluves. Texture of the marine and fluvial sediment parent material is variable. Soils are acidic and have low base saturation. Histosols are common in closed depressions and in large swampy areas.

Location and Landuse
MLRA 153A extends in a continuous belt along the Atlantic Coast from northern Florida to Virginia (Fig. 1). In northern Florida and southern Georgia, MLRA 153A extends inland from the seacoast as much as 175 km, but the width of the remainder of the belt typically ranges from 50 to 75 km. In South Carolina, the northern part of North Carolina, and the southern part of Virginia, small belts of MLRA 153B (Tidewater Area) lie between the sea coast and MLRA 153A. MLRA 153A occupies an area of 8,447,600 ha (20, 874,014 acres) (Table 1). Natural vegetation is pine-oak forest. Loblolly pine and upland oaks occur on uplands, and water tupelo, sweetgum, oaks, and swamp blackgum occur in the lowlands. Much of the original timber has been harvested and pine plantations have been established. Various grasses and forbs adopted to poorly drained soils provide understory. Based on 1981 land use data, about 30% of the land was devoted to agriculture and 70% to forest. More recent data by the USDA Forest Service (1994) states that about 40% of the land has been cleared for agricultural use. Most of the forest holdings are large, being owned by companies such as Weyerhauser. Pulpwood is the main product, but lumber and naval stores are also harvested. About 20% of the area was cropland in 1981. Cropping intensity is greater in the northern section than in the south. Cultivated fields tend to be large with the main agronomic crops being corn, soybeans, and wheat. Some smaller fields are planted to tobacco and peanuts (on the sandier fields). Small fields of vegetables such as potato and special niche vegetable crops are also grown. Some poultry, but little livestock is grown.

Fig.1. STATSGO soils in Major Land Resource Area 153A (from SCS-USDA, 1981).

This area has an udic moisture regime with average annual precipitation ranging from 1025 to 1400 mm. Maximum precipitation occurs during the summer months. All of the area except the northern extreme has a thermic temperature regime with mean annual temperature ranging from 13 to 21°C. The average frost-free period is 200 to 280 days.

Elevation in MLRA 153A ranges from sea level to 25 m above sea level. The area is occupied with level lowlands interspersed with waterways, swamps and lagoons. These lowlands have low gradients across broad interfluves creating poor to very poorly drained soils.

The predominant landform is a flat, weakly dissected alluvial plain. It was created by deposition of continental sediments on a submerged, shallow continental shelf, which was later exposed by gradual sea level subsidence. Rainfall, perennial streams, and ground water provide adequate, if not excessive, water during most of the year. The flat nature of the land, the low elevation, and the presence of many waterways, swamps, and lagoons, and the presence of closed depressions, combine to create water tables close to the soil surface throughout much of the year. Unless artificial drains are installed, the high water tables and the resulting poor drainage severely limit field equipment activities and restrict crop root growth. Many of the soils require artificial drainage, both surface and/or subsurface, before they can be used for growing crops. Some of the sandier soils need irrigation during doughty periods, but it often is not economical to invest in irrigation equipment. During the past two decades the use of drainage systems with flashboard risers allow soils to be drained during wet periods and sub-irrigated during dry periods. Water for industry, municipal, and domestic uses is obtained primarily from wells.

Over 290 soil series and numerous complexes occur within MLRA 153A. Soils having restricted drainage are dominant. The seventeen most common soils, each of which occupy 1% or more of the land area in MLRA 153A, are listed in Table 2. Aquults are prevalent. These soils are deep and medium to fine textured. Rains, Lynchburg, Pelham, and Coxville are the most common Paleaquults. Umbric Paleaquults such as Pantego are found in the wetter sites. Hapludults and Paleudults (Goldsboro and Norfolk) are found on higher, better drained sites. Larger areas of Haplaquods (Leon and Mascotte) are also common. Other common soils are Blanton, Lakeland, Bayboro, and Paxville.

Water and Chemical Transport
Many of the soils in MLRA 153A have high water tables and poor drainage. Surface drainage is also poor for much of the area because the slopes are small. Artificial drainage is needed in order to farm many of these soils. Underground drains in Lumbee sandy loam (fine-loamy over sandy or sandy skeletal, siliceious, subactive, thermic Typic Endoaquults) spaced 16 m apart were adequate to drain the soil and supply sub-irrigation water, but when the drains were 30 m apart, water did not move from the drains into the soil rapidly enough to supply water to the crop during dry periods (Skaggs et al., 1972). Few studies have evaluated chemical transport through these soils. Below are limited hydrological data for the following soil series: Aycock, Craven, Leon, Pelham, Portsmouth, and Rains.

Aycock Series
The Aycock series (fine-silty, siliceous, thermic Typic Paleudults) consists of very deep, well drained, moderately or moderately slowly permeable soils formed in loamy marine sediments. These soils are found on uplands on slopes ranging from 0 to 6%. Solum thickness is greater than 150 cm, but the lower B and C horizons in many pedons contain plinthite nodules. Runoff is slow to medium. Much of this soil is used to grow cotton, corn, soybeans, tobacco, peanuts, truck crops, and small grains. The remainder is under forest or pastured. Aycock also occurs in MLRA 133A. The instantaneous profile method was used to determine the unsaturated hydraulic conductivity as a function of soil depth for Aycock near Rocky Mount, NC. Data in Table 3 show that bulk density was similar throughout the profile except for a slight tillage pan at the 15 to 23 cm depth. In general, saturated hydraulic conductivity decreases as clay content increases in the soil profile. In situ field capacity, measured several days after drainage of the nearly saturated soil began, lies between water contents measured at pressures of -10 kPa and -33 kPa. Regression models describing the relationship between unsaturated hydraulic conductivity and volumetric soil water content are presented in Table 4.

Craven Series
The Craven series (fine, mixed, subactive, thermic Aquic Hapludults) consists of moderately well drained, slowly permeable soils that formed in clayey Pleistocene sediments. These soils are found on nearly level to sloping uplands at 8 m above sea level or higher. Solum thickness can exceed 120 cm. Runoff is medium or rapid and the thick argillic horizon causes slow permeability. The soil is extremely to strongly acid unless the soil has been limed for agricultural crops. Exum and Goldsboro soils are similar to the Craven series but have less clay. Craven is also found in MLRA 133A. Data for Craven collected near Vanceboro, North Carolina are shown in Table 5. Sand content is somewhat variable with depth in the profile, but silt plus clay content ranges from 78 to 58%. Saturated hydraulic conductivity decreases as silt plus clay content increases; hydraulic conductivity values range from 2.0 to 0.1 cm h-1 in the 152 cm deep profile. In situ field capacity measured using the instantaneous profile method lies between water contents measured on undisturbed soil cores at pressures of -10 and -33 kPa.

Leon Series
The Leon series (sandy, siliceous, thermic Aeric Alaquods) consists of very deep, poorly drained and very poorly drained, sandy soils with a Bh horizon within 76 cm of the soil surface. These soils formed in sandy marine sediments and occur on flatwoods, depressions, low uplands, and stream terraces. Slope ranges from 0 to 5%. Texture of the surface horizon ranges from sand to fine muck with all other horizons except the Bh being sand. Texture of the Bh horizon can range from loamy sand to loamy fine sand. In some pedons a black to dark indurated transitional layer between the E2 and Bh1 horizon may be present. Runoff is slow and ponding of the soil surface often occurs during wet periods. Permeability ranges from moderately slow to rapid in the various horizons. Depending on landscape position, the water table can be as shallow as 15 cm during the winter months or as deep as 150 cm in dry periods. These soils are used for forestry, range and pasture. If water table control is used, some crops can be grown. Leon is also found in MLRAs 133A, 138, 152A, 153B, and 153C. Saturated hydraulic conductivity and soil water retention data for Leon soil near Bridgeton in Craven County, North Carolina, are shown in Table 6. Saturated hydraulic conductivity of the A, E1, and E2 horizons are extremely rapid, but decreases two orders of magnitude in the Bh1 horizon.

Pelham series
The Pelham series (loamy, siliceous, subactive, thermic Arenic Paleaquults) consists of very deep, poorly drained, moderately permeable soil that formed in unconsolidated Coastal Plain sediments. It occurs on nearly level broad inter-fluves to very gently sloping, poorly drained areas, toe slopes, depressions, and drainageways. Elevation ranges from 5 to 135 m and slope ranges from 0 to 5%. Although these soils have moderate permeability, runoff is slow, and they are poorly drained. Some areas of this soil are subject to seasonal ponding. Depth to water table on undrained soil typically extends as deep as 0.5 m during dry periods. Native vegetation is pine-decidious forest. Most of the soil is suited for forestry although some is placed in pasture, but little has been used for growing agricultural crops. Pelham is also found in MLRAs 133A, 133B, and 153B. Selected soil physical and hydraulic properties for Pelham loamy sand, measured on undisturbed 7.6 cm high by 7.6 cm long soil cores, were reported by Shirmohammadi et al. (1991). The purpose of the study was to evaluate the depth and separation distance of drains for growing blueberries. The system has to function as a drain during wet periods and as a supplier of sub-irrigation water during droughty periods. Bulk density ranged from 1.47 in the surface to 1.73 g cm-3 in the 70 to 125 cm depth. Average soil water content throughout the profile at soil water pressure heads of 0, 60, 100, 200, 300, 400, and 500 cm were 0.36, 0.28, 0.22, 0.14, 0.11, 0.10, and 0.095 cm3 cm-3 , respectively. Saturated vertical hydraulic conductivity ranged from a high of 6.3 cm h-1 in the 0 to 20 cm depth to a low of 0.8 cm h-1 in the lower portion of the profile. Unsaturated hydraulic conductivity (Fig. 2), was estimated using laboratory-measured saturated hydraulic conductivity values and the soil water retention curves. For this site, drain spacing of 20 m and placement at the 1 m depth were sufficient to manage the drainage-subirrigation water regime for this soil.

Fig. 2. Computed unsaturated hydraulic conductivity for Pelham loamy sand (from Shirmohammadi et al., 1991).

Portsmouth series
The Portsmouth series (fine-loamy over sandy or sandy-skeletal, mixed, thermic Typic Umbraquults) is very poorly drained, moderately permeable, and formed in loamy textured marine and fluvial sediments. The loamy textural horizons are 60 to 100 cm thick over contrasting sandy horizons. The soil is located on nearly level flats and slight depressions, generally at elevations below 7 to 8 m. Slope ranges from 0 to 2% The soil is very poorly drained; runoff is slow and water often ponds on the soil surface. Permeability of the solum is moderate but the underlying sandy material has rapid or very rapid permeability. Unless drains have been installed, the water table is often at or near the soil surface in winter and early spring. Portsmouth also occurs in MLRAs 133A, 153B, and 153C. Soil properties of 150 soil cores taken from a tilled 7.4 ha field of Portsmouth in the Tar River Valley near Conetoe, NC, were evaluated by Anderson and Cassel (1986). The soil cores were taken from positions in the field that encompassed the visually observed range in color of the soil surface and minor changes in elevation. Each soil core was further divided into 15-cm-long by 6.6-cm-dia soil cores of the A, Btg, and Bg horizons. The soil water characteristic was determined by sequential desorption of the saturated intact soil cores, saturated hydraulic conductivity was measured using the constant head method (Klute, 1965), and soil-water diffusivity was determined on each core using the hot-air method described by Arya et al.(1975). Unsaturated hydraulic conductivity at selected soil water pressures was computed using soil water diffusivity data and the inverse of the van Genuchten equation (van Genuchten, 1979) for the measured soil water characteristic. Means and coefficients of variation for the 150 measurements for each soil property for each horizon are given in Table 7. This soil is quite sandy and bulk density attained its minimum of 1.30 g cm-3 in the soil surface. Mean saturated hydraulic conductivity was lognormally distributed in all three horizons with the value being lowest in the Bg horizon, 1500 mm d-1 , whereas the CV for saturated hydraulic conductivity was highest, 3300%, in the Btg horizon. The CV for unsaturated hydraulic conductivity increased with depth for the A horizon but had no distinct pattern for the Btg and Bg horizons.

Rains series
The Rains series (fine-loamy, siliceous, semiactive, thermic, Typic Paleudults) consists of very deep, poorly drained, moderately permeable soils that formed in thick loamy sediments on marine terraces. Solum thickness exceeds 150 cm and the soil is acid throughout unless lime applications have raised pH in the surface horizon. These soils occur on nearly level flats (0 to 2% slope) and slight depressions. The soil is poorly drained, but the soil has moderate permeability. Due to its nearly level position on the landscape, runoff is slow. Rains also occurs in MLRAs 133A, 137, and 153B. Data for a tilled Rains soil from the Lower Coastal Plain Tobacco Research Station at Kinston, North Carolina is presented in Table 8. (The soil surface is just a bit too coarse to fit the modal concept of Rains.) The bulk density of this soil below 23 cm exceeds 1.7 g cm-3. In situ unsaturated hydraulic conductivity as a function of volumetric soil water content determined by the instantaneous profile method is given in Table 9.

Literature Cited
Anderson, S.H., and D.K. Cassel. 1986. Statistical and autoregressive analysis of soil physical properties of Portsmouth sandy loam. Soil Sci. Soc. Am. J. 50:1096-1104.

Arya, L.M., D.A. Farrell, and G.R. Blake. 1975. A field study of soil water depletion patterns in presence of growing soybean roots: I. Determination of hydraulic properties of soil. Soil Sci. Soc. Am. Proc. 39:424-430.

Klute, A. 1965. Laboratory measurement of hydraulic conductivity of saturated soil. In: C.A. Black et al. (eds.). Methods of soil analysis, Part 1. Agronomy 9:210-220.

van Genuchten, M. Th. 1979. Calculating the unsaturated hydraulic conductivity with a new closed form analytical model. Res. Rep. no. 78-WR-08. Princeton University, Princeton, New Jersey.

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