MLRA 137: Carolina and Georgia Sand Hills D.K. Cassel North Carolina State University
Major land resource area (MLRA 137) occupies a relatively small area of land compared to most of the other MLRAs and is occupied primarily by deep sandy soils that are droughty and difficult to manage intensely. As we will see, these soils have low water holding capacities and are subject to leaching losses.
Location and Landuse MLRA 137 is composed of land in a relatively narrow strip extending east northeast from the western boundary of Georgia across the entire state of Georgia, across the entire state of South Carolina, and about halfway across North Carolina (view of regional map). This strip of land separates MLRA 136, the Southern Piedmont, to the west, from MLRA 133A, the Southern Coastal Plains, to the east. Soils from both the Piedmont and Coastal Plain are found in MLRA 137. Much of the general information to follow was extracted from USDA (1981).
MLRA 137 occupies an area of 22,680,000 ha (55,606,400 acres). The strip of land is about 650 km (404 miles) long and varies in width from 6 to 80 km (4 to 50 miles), but is mostly less than 40 km wide. Natural vegetation is/was pine-oak with long leaf pine being the dominant species. Currently most of the land is under forest cover (Table 1) with pine and scrub oak being dominant. Pulpwood and lumber are the primary forest products. Turkey, blackjack, bluejack, and sand live oaks are also present. Various grasses and forbs provide ground cover in the forested and pasture areas.
In 1981, about 15% of the area was cropland, primarily corn and cotton, and about 5% was pasture. About one-sixth of the area is owned and used by the federal government for military posts such as Fort Bragg Military Reservation in North Carolina. Only a small percentage of the land is used for urban development and other purposes, although several very high quality, world famous golf courses have been developed. Some orchards and small commercial fruit and vegetable farms are scattered throughout the area.
Climate This region has an udic moisture regime with average annual precipitation ranging from 1150 to 1275 mm. Maximum precipitation occurs in the summer and the minimum is in winter. A thermic temperature regime exists and the average annual air temperature is 17 to 18°C. The average frost-free period ranges from 220 to 240 days.
Topography Elevation in MLRA 137 ranges from 50 to 200 m above sea level. The area is dissected with rolling to hilly uplands. Local relief is only a few meters, but some hills are 25 to 50 m above adjacent areas.
Hydrogeology Precipitation, perennial streams, and ground water are plentiful, however, low water holding capacity and rapid permeability of the soils severely limit plant growth unless irrigation water is used. The rolling topography gives rise to many seeps and springs.
Soils The major soil components making up 1% or more of the total area occupied by MLRA 137 are shown in Table 2. The components are listed in descending order of acreage. The area is dominated by Ultisols, but Psamments occupy about 13% of the area, while Inceptisols occupy about 8%. Psamments are sandy Entisols and have no zone of clay accumulation.
Psamments are sand to a depth of at least 100 cm and quartz is the most dominant mineral. Deep, sandy Quartzipsamments (Lakeland series) and Paleudults (Blanton and Troup series) occur on rolling and hilly slopes where the upper sandy layer is thick. Paleudults (Blanton, Dothan, and Fuquay series) occur where the upper sand strata are thinner and underlain by more clayey materials. Very poorly drained soils occurring along drainage ways are Psammaquents (Osier series; 2,805 ha) and Humaquepts (Rutledge series; 4,816 ha). The STATSGO soil map for MLRA 137 is shown in Fig. 1.
Water and Chemical Transport Except for the Lakeland series, little data on soil water and/or chemical transport obtained within MLRA 137 has been published. This is probably due to the limited acreage and the relatively minor agricultural role the soils within this MLRA have had in the past. Information in other chapters of this publication deal with several of the soils found in MLRA 137. These include Norfolk, Wagram, and Troup in MLRA 133A. The following information on the Blanton and Gilead series stems from unpublished data collected at North Carolina State University in the late 1970s in Southern Regional Research project S-125.
Blanton Series The Blanton soil series (loamy, siliceous, thermic, Grossarenic Paleudults) developed from unconsolidated marine sediments and occurs on nearly level sites. The sand content has a narrow range, 87 to 94%, throughout the 152-cm profile. The high bulk density values of 1.63 and 1.64 g cm-3 occurring in the E1 and E2 horizons are indicative of an induced tillage pan.
In situ field capacity of this sandy soil is nearly uniform with depth (Table 3), but attains its maximum value of 0.133 cm3 cm-3 in the E2 horizon where the percentage of sand is least and bulk density is greatest. The available water holding capacity (AWHC) is essentially constant with depth throughout the profile, being 0.07 to 0.08 cm3 cm-3.
The saturated water content of the Ap and Bt1 horizons of the Blanton series is 0.38 to 0.40 cm3 cm-3. Water drains or is released by the soil at very low soil water pressures. At the -10 kPa soil water pressure, the water content is only 0.08 to 0.09 cm3 cm-3(Fig. 2A). The water retention relationships for the remaining horizons of the sandy Blanton material are similar. This low water retention, which is typical of coarse sandy soils, causes the soil to be droughty.
Saturated hydraulic conductivity values for all seven horizons (Table 3), measured using undisturbed soil cores (Klute and Dirksen, 1986), exceed 12 cm h-1. In situ unsaturated hydraulic conductivity K(q) for the various horizons of the Blanton soil was measured using the instantaneous profile method (Green et al., 1986). The K(q) in the Ap horizon (0 to 15 cm), for example, decreases more than three orders of magnitude (from 1 to 0.0002 cm h-1) as volumetric water content decreases from 0.30 to 0.10 cm3 cm-3(Fig. 3). A similar relationship exists for the remaining six horizons.
Gilead Series The Gilead series (clayey, kaolinitic, thermic, Typic Fragiudults) is developed on nearly level to sloping land from unconsolidated marine sediments. Unpublished data for this series from Moore and Harnett counties in North Carolina are shown in Table 4. This soil is typified by a sandy surface layer less than 50 cm thick overlying finer textured material. The change in soil texture between the Ap or E horizons and the Bt1 is abrupt. For these two pedons, the clay content increases about 30% at the Bt boundary. An E horizon is present at the Harnett County site whereas it is absent at the Moore County site where the Bt horizon is closer to the soil surface. A tillage pan is present in the E horizon as indicated by the greater bulk density in the E horizon compared to the Ap horizon.
Water retention at the -33 and -1500 kPa values is much greater in the clayey Bt horizons compared to the Ap and E horizons (Table 4). Using these data to estimate the available water holding capacity of the various horizons above the BC horizon, we find the AWHC to be greater and more variable at the Harnett County site.
Saturated hydraulic conductivity is variable with depth at the Moore County site (Table 4). The Ksat value of 9.2 cm h-1 in the Ap horizon decreases to a value of 0.65 cm h-1 in the Bt2 horizon, and then increases in the parent material. A similar pattern in Ksat vs. depth occurs for the Gilead soil at the Harnett County site, but the Ksat values in the A, E, and Bt horizons were less than those for the Gilead profile in Moore County. These large differences in Ksat between the Ap or E and the Bt horizons often give rise to perched water tables, which in turn give rise to lateral subsurface water flow in response to the gravitational gradient.
Water retention curves for Gilead (Fig. 2B) differ from those of Blanton soil. For Gilead, the sandy Ap horizon has a total porosity of 0.46 cm3 cm-3compared to 0.33 cm3 cm-3in the compact Bt 3 horizon at the 45 to 61 cm depth. Water retention in the C horizon is similar to that in the Bt3 . Because the clayey Bt horizons have such low hydraulic conductivity values, the instantaneous profile method was ineffective to measure K(q) of the various horizons deeper than the E horizon. Field-measured K(q) relations for the Ap and E horizons are shown in Fig. 4. The K(q) at 0.33 cm3 cm-3water content is about 0.001 cm h-1 . The K(q) values at water contents less than 0.33 cm3 cm-3 were not measured because water moved into the Bt1 horizon too slowly.
Lakeland Series The Lakeland series (thermic, coated, Typic Quartzipsamments) (Table 5) consists of very deep, excessively drained, rapidly permeable soils that formed in thick deposits of eolian or marine sands. These soils are found on broad nearly level to very steep uplands, however, slope typically ranges from 0 to 12%. The sand extends to a depth of at least 2 m. Small pockets of light gray or white sand grains or yellow or brown mottles occur in some pedons. It appears that some profiles of Lakeland in MLRA 137 have thin layers or lenses of finer textured material alternating with the coarser sand in the lower part of the profile. The presence of these lenses can increase the water holding capacity of the soil and decreases the saturated hydraulic conductivity..
These soils are excessively drained and the depth to the seasonal water table exceeds 2 m. Natural vegetation consists of drought resistant trees such as blackjack, turkey oak, and longleaf pine. Some acreage of this series is used for high risk farming. Without irrigation water the chance of losing a crop to drought is high due to the low water holding capacity of the soil. The risk of leaching agricultural chemicals out of the root zone is high.
The unsaturated hydraulic conductivity as a function of soil water content for six depth increments of Lakeland in Hoffman, North Carolina is shown in Fig. 5. Much additional soil physical property data for the Lakeland series is available in Southern Cooperative Series Bulletin 262 (Dane et al., 1983).
Water Management and Solute Transport Implications Efficient water management is a challenge for the deep sandy soils in MLRA 137. Water from rainfall or irrigation infiltrates the soil rapidly thereby causing runoff and soil erosion to be minimal. However, the low AWHC associated with each soil horizon retains only a small amount of infiltrating water. Thus, the infiltrating water moves down below the rooting zone of most field, tree, and pasture crops. Therefore, this soil is drougthy and agricultural crops are grown at considerable risk unless irrigated. If the soil is irrigated, a high level of water management is required to minimize percolation losses of water below the root zone.
Due to the low AWHC, high infiltration rate, rapid drainage, and low cation exchange capacity (CEC), leaching losses of chemicals can occur . Due to the very low CEC, both cations and anions are subject to leaching to depths below 1 m. Water and solutes moving vertically downward in these soils eventually encounter a less permeable layer thus giving rise to lateral transport. Some of this downward moving water may eventually intercept an impermeable layer and be transported horizontally until it surfaces at springs and seeps.
Literature Cited Dane, J.H., D.K. Cassel, J.M. Davidson, W.L. Pollans, and V.L. Quisenberry. 1983. Physical characteristics of soils of the Southern Region-Troup and Lakeland. p. 258. Cooperative Series Bulletin 262. Auburn, Alabama.
Green, R.E., L.R. Ahuja, and S.K. Chong. 1986. Hydraulic conductivity, diffusivity, and sorptivity of unsaturated soils: Field methods. In: A. Klute (ed.). Methods of Soil Analysis, Part 1, Agronomy 9. 2nd ed. p 771-798. Am. Soc. Agron. Madison, Wisconsin.
Klute, A., and C. Dirksen. 1986. Hydraulic conductivity and diffusivity: Laboratory methods. In: A. Klute (ed.). Methods of Soil Analysis, Part 1, Agronomy 9. 2nd ed. pp. 687-734. Am. Soc. Agron. Madison, Wisconsin.
USDA. 1981. Land resource regions and major land resource areas of the United States. Agriculture Handbook 296. p. 156. U.S. Government Printing Office. Washington, D.C.