Soil Formation and Classification


(Conservation Currents, Northern Virginia Soil and Water Conservation District, June 2004)

Do you know the difference between soil and dirt? Dirt is what you find under your fingernails. Soil is what you find under your feet.

The National Cooperative Soil Survey identifies and maps over 20,000 different kinds of soil in the United States. Most soils are given a name, which generally comes from the locale where the soil was first mapped. For example, the soil named Dulles was first mapped in the Dulles area of Loudoun County. Fairfax Silt Loam was first mapped in Fairfax County. Named soils are referred to as soil series.

Soil survey reports in Virginia include the soil survey maps and the names and descriptions of the soils in the county. These soil survey reports are published by the National Cooperative Soil Survey and are available to everyone.

The USDA Natural Resources Conservation Service (NRCS) and the Northern Virginia Soil and Water Conservation District are in the process of updating and completing the soil survey for Fairfax County.

NRCS Soil Scientist David Harper is the project leader for the Fairfax County Soil Survey project. “A noteworthy addition to the soil survey update,” says Harper, “is the study of many tentative urban soil types that will aid in the general planning, use, and management of areas that have been disturbed by heavy machinery. This process has just begun, and final conclusions have not been made. In many cases with disturbed soils, the soil properties are highly variable, and in most cases site specific investigations will continue to be needed.”

Soils are named and classified on the basis of physical and chemical properties in their horizons (layers). “Soil Taxonomy” uses color, texture, structure, and other properties of the first two meters of the soil. This information is used to key the soil into a classification system to help people use soil information. This system also provides a common language for scientists.
Soils and their horizons differ from one another, depending on how and when they formed. Soil scientists use five soil forming factors to explain how soils form and to help them predict where different soils may occur.

Soil Forming Factors

Parent material

Many soils in our area weather directly from the underlying rocks. These “residual” soils have the same general chemistry as the original rocks. In other areas of the U.S., soils may form in materials that have moved in from elsewhere. Materials may have moved many miles or only a few feet. The material in which soils form is called “parent material.” In the lower part of the soils, these materials may be relatively unchanged from when they were deposited by moving water, ice, or wind.

Sediments along rivers have different textures, depending on whether the stream moves quickly or slowly. Fast-moving water leaves gravel, rocks, and sand. Slow-moving water and lakes leave fine textured material (clay and silt) when sediments in the water settle out.

Climate

Soils vary, depending on the climate. Temperature and moisture amounts cause different patterns of weathering and leaching. Wind redistributes sand and other particles especially in arid regions. The amount, intensity, timing, and kind of precipitation influence soil formation. Seasonal and daily changes in temperature affect moisture effectiveness, biological activity, rates of chemical reactions, and kinds of vegetation.

Topography

Slope and exposure affect the moisture and temperature of soil. Steep slopes facing the sun are warmer, just like the south-facing side of a house. Steep soils may be eroded and lose their topsoil as they form. Thus, they may be thinner than the more nearly level soils that receive deposits from areas upslope. Deeper, darker colored soils may be expected on the bottom land.

Biological factors

Plants, animals, micro-organisms, and humans affect soil formation. Animals and micro-organisms mix soils and form burrows and pores. Plant roots open channels in the soils. Different types of roots have different effects on soils. Grass roots are “fibrous” near the soil surface and easily decompose, adding organic matter. Taproots open pathways through dense layers. Micro-organisms affect chemical exchanges between roots and soil. Humans can mix the soil so extensively that the soil material is again considered parent material.

The native vegetation depends on climate, topography, and biological factors plus many soil factors such as soil density, depth, chemistry, temperature, and moisture. Leaves from plants fall to the surface and decompose on the soil. Organisms decompose these leaves and mix them with the upper part of the soil. Trees and shrubs have large roots that may grow to considerable depths.

Time

Time for all these factors to interact with the soil is also a factor. Over time, soils exhibit features that reflect the other forming factors. Soil formation processes are continuous. Recently deposited material, such as the deposition from a flood, has not had time to exhibit features brought about by soil development activities, such as climate and biological factors. The previous soil surface and underlying horizons become buried. The time clock resets for these soils. Terraces above the active floodplain, while genetically similar to the floodplain, are older land surfaces and exhibit more development features.

These soil forming factors continue to affect soils even on “stable” landscapes. Materials are deposited on their surface, and materials are blown or washed away from the surface. Additions, removals, and alterations are slow or rapid, depending on climate, landscape position, and biological activity.

For more information about the soil survey, call NVSWCD Soil Scientist Dan Schwartz at 703-324-1422, TTY 711 or send him an e-mail.


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