(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.
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.
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.
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.
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 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 or send him an
e-mail.
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