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Michigan Department of Agriculture </mda> 	Michigan.gov </>


*What Are Horizons?*

Soils are deposited in or developed into layers. These layers, called
horizons, can be seen where roads have been cut through hills, where
ditches have been dug, or in other areas where the soil is exposed.

Where soil forming factors are favorable, five or six master horizons
may make up a mineral soil profile. Each master horizon is subdivided
into specific layers that have a unique identity. The thickness of each
layer varies with location. Under disturbed conditions, such as
intensive agriculture, or where erosion is severe, not all horizons will
be present. Young soils, such as bottomland soils, have fewer major
horizons.

The uppermost layer generally is an organic horizon, or O horizon. It
consists of fresh and decaying plant residue from such sources as
leaves, needles, twigs, moss, lichens, and other organic material
accumulations. Some organic materials were deposited under water.
Subdivisions of Oa, Oe, and Oi are used to identify different levels of
decomposition, The characteristic dark color of the O horizon is the
result of humus formation from the decomposition of organic matter.

Below the O horizon is the A horizon, or topsoil. The A horizon is
mainly mineral material. It is generally darker than the lower horizons
because of greater amounts of decomposed or humified organic matter.
This horizon is where most root activity occurs and is usually the most
productive layer of the soil. It may be referred to as a surface layer
in a soil survey.

The E horizon, or subsurface layer, generally is bleached or white-ish
in appearance. As water moves down through this horizon, soluble
minerals and nutrients dissolve and some dissolved materials are washed
(leached) out The main feature of this horizon is the loss of silicate
clay, iron, aluminum, humus, or some combination of these, leaving a
concentration of sand and silt particles. In cultivated areas, the O, A,
E, and possibly the upper part of the B horizons are incorporated into
an Ale horizon, or plow layer.

Below the A or E horizon is the B horizon, or subsoil. The B horizon is
usually lighter colored, denser, and lower in organic matter than the A
horizon. It commonly is the zone where leached materials accumulate. The
8 horizon is further defined by the materials that make up the
accumulation, such as "h" in the form of "Bh", which identifies that
humus has accumulated. Other illuvlal concentrations or accumulations
include iron, aluminum, clay, or carbonates. Soil not having
recognizable concentrations within 8 horizons but show color or
structural differences from adjacent horizons is designated "Bw".

Still deeper is the C horizon, or substratum. The C horizon may consist
of less clay, or other less weathered sediments. Partially disintegrated
parent material and mineral particles are in this horizon. C horizons
described as "2C" consist of different material, usually of an older age
than horizons which overlay it. C horizons may also be called "parent
material".

The lowest horizon, the R horizon, is bedrock. Bedrock can be within a
few inches of the surface or many feet below the surface. Where bedrock
is very deep and below normal depths of observation, an R horizon is not
described.