Soil Horizons (layers)
Soil is made comprised of distinct horizontal layers called soil horizons.
Horizons in soil range from organic rich surface layers (humus and topsoil horizons)
to underlying layers high in rock content (subsoil, regolith, and bedrock).
There are many different types of soils and each soil has unique characteristics
including color, texture, structure, firmness, and mineral composition. The depth
and drainage properties of soils can also vary even within short distances. These
soil properties are very important because they determine the suitability of land
for particular uses (homes, septic systems, plant growth, landfills, etc.).
Soil scientists classify soils into 12 major soil orders or types of soil: Alfisols,
Aridisols, Entisols, Histosols, Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols,
Gelisols, Andisols, and Vertisols.
Click to view the Munsell Color Chart.
Click on the soil layers to read the characteristics of the soil horizons.
Simple Definition
A surface horizon that is a mix of minerals and organic matter (leaf litter
and humus). The organic matter is the dominant material in the O horizon.
Advanced Definition
This layer is dominated by a high content of organic matter. O horizon
formation is favored by large inputs of vegetative materials and
saturated soil conditions. Saturation reduces organic matter
decomposition processes; wetlands soils often have thick O horizons
due to saturation and high vegetative input. However, O horizons also
form in soils that are not saturated (forest floors). (SSSA, 1996)
Simple Definition
A surface horizon that is a mix of mineral and organic matter.
Unlike the O horizon, minerals are the dominant material.
Advanced Definition
Mineral horizons which have formed at the surface or below and
O horizon. The exhibit obliteration of all or much of the original
rock structure and show one or both of the following: (1) an accumulation
of humified organic matter intimately mixed with the mineral fraction
and not dominated by properties of E or B horizons (defined below), or
(2) properties resulting from cultivation, pasture, or similar types of
land disturbance. (SSSA, 1996)
Before the prairies were settled, the A-Horizon is estimated to have
been 30 inches or more in depth. It is estimated that Missouri has
only 10 inches of A-Horizon soil remaining in the prairie regions of
the state since the settlement of the prairie. Over the same period,
it is estimated that 1 foot of soil has accumulated along the streams.
Simple Definition
A subsurface horizon bleached in color (white or gray) due to loss of
mineral coloring agents (iron and aluminum oxides).
Advanced Definition
Mineral horizons exhibiting a loss of silicate clays, iron, or aluminum,
or some combination of these losses, resulting in a concentration of
sand and silt size particles. These horizons are often bleached in
appearance and they exhibit obliteration of all or much of the original
rock structure. (SSSA, 1996)
Simple Definition
A subsoil horizon showing evidence of inorganic or organic material
accumulation.
Advanced Definition
Horizons formed below the O, A, and E horizons. B horizons are
dominated by the obliteration of all or much of the original
rock structure and show one or more of the following: (1) illuvial
(movement into a horizon usually through percolation of soils
drainage water) concentration of silicate clay, iron, aluminum,
humus, carbonates, gypsum, or silica alone or in combination;
(2) evidence of removal of carbonates; (3) residual concentration
of iron and aluminum oxides (sesquioxides); (4) coatings of
sesquioxides that make the horizons lower in color value,
higher in chroma, and redder in hue than overlying and
underlying horizons; (5) alteration which forms silicate
clays or liberates oxides or both, and which forms granular,
blocky, or prismatic structure; or (6) brittleness. (SSSA, 1996)
Simple Definition
Unconsolidated parent material that has undergone little soil weathering.
Advanced Definition
Horizons, excluding hard bedrock, that are little affected by
soil development processes and lack the properties of O, A, E,
or B horizons. Most C horizons are mineral layers. The material
of C horizons may be either like or unlike the material from which
the solum has presumably formed. (SSSA, 1996)
Hard Bedrock.
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