Soil covers the earth's surface as a continuum, except on bare rock, in areas of perpetual frost or deep water, or on the bare ice of glaciers. In this sense, soil has a thickness that is determined by the rooting depth of plants.
Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, translocations, and transformations (soil forming processes) or the ability to support rooted plants in a natural environment. This is the soil that we want to classify.
Our understanding and management of soils would be difficult if we did not know how one soil differs from another. Through classification we can create a universal language of soils that enhances communication among users of soils around the world.
Soils are classified as natural bodies, much like we classify birds, trees, or insects. We use the profile characteristics to classify soils and thus we must divide the soil into parts that can be quantified. We can than place these organized units into groupings that have similar properties. When groups of soils have similar properties the soils can be placed into the same category or taxon. However, soils change gradually over the landscape and the zone where one soil stops and another soil starts may be one to 20 meters wide. This makes mapping soils a real challenge.
The first step in classifying soils is looking at the characteristics of the surface and the second step is to determine the characteristics of the subsurface. Note that surface and subsurface do not refer to specific horizons though surface is mostly A horizons and subsurface is mostly B horizons.
The upper surface of a soil is the boundary between the soil and air, shallow water, live plants, or plant materials that have not begun to decompose.
The epipedon is a horizon (or horizons) that forms at or near the surface. It is darkened by organic matter or shows evidence of eluviation. An epipedon is not the same as an A horizon. The epipedons may include part or all of an illuvial B horizon if the darkening by organic matter extends from the soil surface into the B horizon. All soils must have one of the following epipedons.
Eight epipedons are recognized but only six occur naturally. The anthropic and plaggen are the result of intensive human use.
Mollic
Epipedon: The mollic is a dark, colored mineral horizon. It
is thick (generally > 25 cm) and has a high base status ( > 50% base
saturation & B.S.=% of bases on the CEC see Unit
12-2& Unit12-3 ). These epipedons are
characteristic of soils developed under native prairie.
Umbric Epipedon: The umbric has the same characteristics as the mollic except the % base saturation is less than 50% due to leaching of basic cations. The umbric develops in areas with higher rainfall than the mollic.
Melanic Epipedon: The melanic epipedon is a mineral horizon that is very black in color due to its high organic matter content. It is characteristic of soils developed from volcanic ash.
Histic Epipedon :
The histic epipedon is a layer of organic soil that is
naturally saturated with water.
Folistic Epipedon: The folistic epipedon is like the histic except it is not saturated with water for more than 30 days.
Ochric
Epipedon: The ochric fails to meet the definitions for any of
the other epipedons. Thus it is too light, too thin or too low in organic
matter. If a soil's epipedon does not meet any of the criteria for the other
seven epipedons it is ochric.
For Minnesota, the most important epipedons are Mollic, Ochric, and Histic.
Many subsurfaces are used to characterize different soils in Soil Taxonomy. The ones listed below are the most common for soils in Minnesota and the U.S.. Each diagnostic horizon provides a characteristic that helps place a soil in its proper class in the system.
Argillic Horizon : The argillic is a horizon that accumulates clay that has translocated from above. The clays are found as coatings or clay skins on the surface of peds. The Bt horizon identifies the argillic horizon. The soil above with the ochric epipedon has an argillic horizon at the Bt.
Natric Horizon : 
The natric horizon is like the argillic
with the addition that sodium has accumulated also to where there is > 15 %
exchangeable sodium. The horizon will also have columnar soil structure in the
Btn.
Kandic Horizon: The kandic horizon has an accumulation of Fe and Al oxides as well as kaolinite clays. Clays skins do not need to be evident. The low activity of the clay is shown by the low cation-holding capacities ( < 16 meq/100g) . These horizons form in areas of intense weathering.
Oxic Horizon : the oxic horizon is a highly weathered subsurface horizon that is very high in Fe and Al oxides. The oxic horizon only has 10% weatherable minerals in the sand, silt or clay components, all the other minerals have weathered to elements. This is very different from other soils which have weatherable minerals in the 50-90% range. Oxic horizons are found mostly in humid tropical and subtropical regions and is designated as the Bo horizon.
Spodic Horizon : The spodic horizon is
an illuvial horizon characterized by the accumulation of colloidal organic
matter, aluminum oxide and iron. It is commonly found in highly leached forest
soils of cool humid climate, on sandy parent materials and is given the Bhs, or
Bs horizon designation. In Minnesota the spodic is found below the E horizon
and is a red colored loamy sand or sand.
Albic Horizon : The albic horizon is a light colored eluvial horizon that is low in clay and other oxides which have been removed by leaching ( E horizon in soil with Ochric above). The color of the horizon is the often white or light tan. Where these horizons finger down into the argillic the diagnostic feature of Glossic is identified. This interfingering is a common feature of soils in northern Minnesota and is identified with the E/B horizon).
Calcic Horizon: The calcic horizon is an
illuvial accumulation of carbonates (mostly CaCO3). The carbonates
will react with HCl giving off CO2 or bubbles. They are white
chalk-like nodules or filaments. The Calcic horizon will be identified in the
profile as the Bk horizon
Cambic Horizon: The cambic horizon is a slightly altered layer that has not undergone enough illuviation to become argillic. Cambic horizons have developed different color or structure from what the parent material (C horizon) had before pedogenesis. The Bw horizon designation is used for the cambic horizon . A cambic horizon must have a sandy loam or finer texture, it can not have a texture of sand or loamy sand.
NONE: Not all soils need to have a
diagnostic subsurface (unlike the epipedon), because they may have not
undergone very much soils development. Thus a soil may only have the diagnostic
epipedon to place it in a soil order.
Summary:
Soils have diagnostic features that allow us to place them in categories. We use the surface characteristics (epipedon) as the first determining factor. Second, we look at the subsoil characteristics to further define the category that the soil will be placed in. In Chapter 2 we will identify the 12 soil orders that identify all the soils of the world.
Go to Next Chapter - Chapter 2)
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Prepared by tcooper@umn.edu