Historically, the first soil surveys in western North Carolina were cooperative endeavors to classify soils according to their characteristics, both internal and external, with special emphasis on the features that influence the production of crop plants, grasses and trees (Goldston 1948). The United States Department of Agriculture in cooperation with the North Carolina Agricultural Experiment Station and the Tennessee Valley Authority conducted most of the initial soil surveys in the 1940s and 1950s. Most of these soil surveys were published at a 1:48,000 scale (Perkins 1947). The soil classification system of this era had a strong agricultural bias, and most of the soil scientists field time and effort were spent classifying, mapping, and interpreting the agricultural soils in valleys and along the major streams in western North Carolina.
Modern soil surveys, beginning in 1981, are very different documents from soil surveys produced in the 1940s and 1950s. The major areas of difference are kind of photography, scale of photography, in-depth study of geology and geologic behavior, shading phenomena, new series creation, and innovative soil map unit design.
Soil maps in western North Carolina are being produced on 1:12,000-scale ortho photos. These maps are true to scale and are compatible with the soil-digitizing product that most soil survey users are requesting. The 1:12,000 scale allows the field soil scientist to delineate significant areas, as small as one acre. Small areas of landscape features, such as prime farmland, hydric soils, and rock outcrops, can be accurately located on 1:12,000-scale soil maps. In addition, the 1:12,000-scale ortho photos allow field soil scientists to delineate narrow ridgetops. Top of ridges are common sites for roads and are desirable areas for home sites. Some ridgetop delineations are covered with old-growth forests that have little commercial value but which are quite valuable to groups wanting to preserve old-growth forests. Ridgetop delineations also help map users orient themselves correctly on the map (Figure 1).
In-depth studies of common geological formations in western North Carolina are part of the modern soil surveys. Three major separations based on the type of rock are made in most new soil surveys. These geologic groups are crystalline rocks high in mica content, crystalline rocks low in mica content, and metasedimentary rocks.
Crystalline rocks, such as mica schist and mica gneiss, are high in mica content. This rock tends to produce soils and saprolite high in mica content. Soils and underlying saprolite erode easily and are difficult to compact when used as earthen foundation material. Soils such as Fannin and Chandler have a high mica content and are poor as engineering materials (Figure 2). Micaceous rock tends to weather deeply. Most micaceous saprolite commonly extends tens of feet below the soil surface.
The relationship of soils to metasedimentary rock formations in western North Carolina is difficult to interpret. Metasedimentary rocks generally are composed of thin beds that dip at some angle from the horizontal. When slopes parallel the bedding dip, soils are very susceptible to landslides. Some thin beds contain sulfur compounds and produce a yellowish leachate during road building. This leachate is very acid (Figure 3). When this leachate enters nearby streams, fish kills and other aquatic damage commonly occur. The map unit descriptions in modern soil surveys discuss this soil/geologic problem and discuss possible solutions.
|
|
|
Figure 2. Roads built of micaceous soil material are subject to landslides. |
Figure 3. Acid leachate stains of metasedimentary rock. |
Generally in western North Carolina, soils on south- to west-facing slopes have surface layers that are lighter in color and contain less organic matter than north- to east-facing slopes. In certain areas, the phenomenon of shading occurs when a higher mountain to the west casts a shadow on mountains of lower elevation to the east (Figure 4). Shading from the direct rays of the afternoon sun on south- to west-facing slopes of lower mountains lowers the maximum daily soil temperature, slows biological activity and causes more organic matter to accumulate. These shaded areas have tree species and productivity similar to north- to east-facing slopes. This is important information to foresters in timber management and road building. Roads on unshaded south- to west-facing cut slopes are more difficult to vegetate and expensive to maintain due to the frequent freeze-thaw cycles during the winter. Shaded south- to west-facing slopes are not as frequently subjected to freeze-thaw cycles and are less difficult to vegetate and maintain.
Modern soil surveys conducted from 1983 through 1990 established 50 new soil series in an attempt to classify, map, and interpret soils encountered in western North Carolina. Soils such as Nikwasi and Hemphill in the floodplains near toeslopes are subject to wetland preservation regulations. Ellijay soils are formed from an ultramafic rock that is very high in magnesium content. Ellijay soils have a calcium-magnesium imbalance and are difficult to manage for crops. In the high mountains, the Wayah soil series was accurately classified as an Inceptisol. However, its high content of organic matter in the surface layer also gives this soil properties shared with Andisols. The taxonomic establishment of new soil series, along with old established soil series, gives the soil scientist in western North Carolina the basic information needed to develop soil map unit names, descriptions, and other interpretive materials.
Soil map unit designation can be a challenging task in some areas of western North Carolina. Soil scientists quickly observed that in some areas of the same soil series, trees of commercial value grew, though in other areas the timber is of very poor quality. This contrast is related to meteorological events. The west side and ridgetops of north-south- trending mountains are barriers to the prevailing westerly wind. During the winter season, trees on these sites experience tremendous ice damage and wind shear. A map unit modifier, "windswept," has been established for the west side and ridgetops of prominent north-south trending mountains where tree growth is severely affected by wind and ice (Figure 5).
The "windswept" map unit helps foresters identify noncommercial forest areas. Summer home owners quickly learn that ice and wind shear also remove shingles from rooftops as well as tree limbs.
Soil survey work in the mountains is not yet complete. In 1998, a memorandum of understanding was signed by the National Park Service and the Natural Resources Conservation Service to produce a detailed soil survey of the Great Smoky Mountains National Park at a 1:24,000 scale. Many of our established series will be used in the park, but we also know that this 550,000-acre piece of real estate also contains soils that will require establishing additional soil series and some unique map unit designs. We have learned that some high mountain areas burned repeatedly in the early 1900s did not revegetate with forest cover. These areas, known as "balds," lack the thick dark surface layers common in nonburned areas. Burned areas are easily identified on color infrared aerial photography as pale green (Figure 6). To accurately identify these areas, and thus enhance the value of our soil maps to National Park managers whose primary interest is vegetation, we may need the soil map unit modifier "burned" added to our growing list of map unit modifiers.
Goldston EF, Davis WA, Croom CW. 1948. Soil survey of Jackson County, North Carolina. Washington (DC): US Department of Agriculture, Bureau of Plant Industry, Soils, and Agricultural Engineering. 87+ p. (Series 1938; 19).
Perkins SO, Gettys W. 1947. Soil survey of Swain County, North Carolina. Washington (DC): US Department of Agriculture, Bureau of Plant Industry, Soils, and Agricultural Engineering. 65+ p. (Series 1937; 18).
