Explanation
Soil Formation and Classification:
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. Named soils are
referred to as soil series.
Soil survey reports include the soil survey maps and the names and descriptions
of the soils in a report area. These soil survey reports are published by the National
Cooperative Soil Survey and are available to everyone.
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 surface two meters deep 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 factors to explain how soils form and to help
them predict where different soils may occur. The scientists also allow for additions
and removal of soil material and for activities and changes within the soil that
continue each day.
Parent material:
Few soils weather directly from the underlying rocks. These "residual"
soils have the same general chemistry as the original rocks. More commonly, soils
form in materials that have moved in from elsewhere. Materials may have moved many
miles or only a few feet. Windblown "loess" is common in the Midwest. It buries
"glacial till" in many areas. Glacial till is material ground up and moved by a
glacier. 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 aspect 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, exhibits no features from
soil development activities. 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.
When mapping soils, a soil scientist looks for areas with similar soil-forming factors
to find similar soils. The colors, texture, structure, and other properties are
described. Soils with the same kind of properties are given taxonomic names. A common
soil in the Midwest reflects the temperate, humid climate and native prairie vegetation
with a thick, nearly black surface layer. This layer is high in organic matter from
decomposing grass. It is called a "mollic epipedon." It is one of several types
of surface horizons that we call "epipedons." Soils in the desert commonly have
an "ochric" epipedon that is light colored and low in organic matter. Subsurface
horizons also are used in soil classification. Many forested areas have a subsurface
horizon with an accumulation of clay called an "argillic" horizon.
