You are here
Soil is a critical support system for the natural system, providing water and nutrients to trees and other plants and playing an integral role in all other biological systems. Influenced by several factors, the formation of the region’s soil is dependent on topography, geology, climate, vegetation, elevation, and time—it takes as long as 250,000 years to produce three feet of soil. The parent material is predominantly broken-down crystalline rocks composed of varying degrees of mica content and metasedimentary rocks. Soil nutrient availability is affected by varying precipitation levels in the region, which range widely from 40–80 inches annually. The interaction of these factors has led to unique and highly variable soil patterns and properties across the mountain region.
Soil also is a critical support system for the human system. Early settlers in the Southern Appalachians looked for good soil in fertile valleys to support their farms; different types of soil yielded good clay for pottery and ceramics, while others contained good components for making cement and concrete.
Soil and highly weathered rock derived from the underlying bedrock is generally thinner in the Blue Ridge than in the Piedmont. Also, soil on mountainsides is eroded and moved downslope by water and gravity more readily than on the flatter Piedmont slopes. This transported soil forms large, deep deposits at the foot of many mountain slopes. The deposits can yield productive forest soils and sources of groundwater, and can also indicate where future landslides may deposit material.
Soil types are further categorized by permeability, porosity, and texture. Permeability is how quickly water moves through soil and the ability of water to be held for plant use. Permeability is partially affected by porosity, which is the amount of empty space between soil particles. Soil texture (the proportion of sand, silt, and clay) is an important determinant of drainage, water-holding capacity, aeration, susceptibility to erosion, organic matter content, and buffering capacity. These factors mean that different soils are able to hold and transport different amounts of groundwater. In the Blue Ridge province, soils are generally thinner and not continuous enough to provide reliable groundwater sources. Shallow wells into these layers perform well when there is plenty of rainfall, but usually dry up during extended periods of drought.
Inceptisols, Ultisols, and Alfisols
Three major soil systems are found in the Southern Appalachians, where their differing properties relate primarily to elevation change:
- Mountain System Soils (Inceptisols). Inceptisols are soils of cool to warm, humid, and subhumid regions, and are shown in orange on the accompanying map. The Southern Appalachian Mountains represent the largest area of these soils in the United States. The cooler climates inhibit microbial activity, and steep slopes allow erosion and leaching of these soils.
- Broad Basins, River Terraces, and Floodplain System (Ultisols). Ultisols are soils that have an abundance of clay, are relatively humus-poor (low organic content), and do not naturally drain well. They are shown in yellow on the map. Historically, most of these soils supported mixed coniferous and hardwood forest vegetation. Some are now used as cropland or pasture, but usually require the use of soil amendments for them to remain productive. This system is characterized by wide valleys and low, rounded hills with few steep slopes. Easy access, a temperate climate, and pleasant landscapes make these areas desirable for development and have historically been the region’s urban, industrial, and agricultural centers. Although prone to flooding in the river basins, these soils are ideal for agriculture, as higher elevation soils have shifted downward over time, increasing soil productivity.
- Alfisols. These soils are more common in the Ridge and Valley than in other ecoregions of the Southern Appalachians, and are shown in green on the map. They have a good soil moisture and temperature regime and are considered good agricultural soils. They are dominantly well-drained and have a clay-enriched subsoil. Alfisols range from shallow soil horizons on sandstone and shale ridges to very deep soils in valleys and on large limestone formations.