Soil Profile

Earth Science

What is a soil profile?

• A soil horizon is a layer generally parallel to the soil surface, whose physical characteristics differ from the layers above and beneath.

• Each soil type usually has three or four horizons. Horizons are defined in most cases by obvious physical features, chiefly color and texture

Horizon O Organic

• Litterfall, plant litter, leaf litter, tree litter, soil litter, or duff, is dead plant material, such as leaves, bark, needles, and twigs, that has fallen to the ground.

• This detritus or dead organic material and its constituent nutrients are added to the top layer of soil, commonly known as the litter layer or O horizon ("O" for "organic")

Horizon A Topsoil

• Topsoil is the upper, outermost layer of soil, usually the top 2 inches (5.1 cm) to 8 inches (20 cm).

• It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.

Horizon B Subsoil

• Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil it is composed of a variable mixture of small particles such as sand, silt and/or clay, but it lacks the organic matter and humus content of topsoil.

• As it is lacking in dark humus, subsoil is usually paler in color than the overlying topsoil.

• It may contain the deeper roots of some plants, such as trees, but a majority of plant roots lie within the surface topsoil.

Horizon C Regolith

• The regolith includes all of the weathered material within the profile and is very rocky.

• The regolith has two components: the solum and the saprolite. The solum includes the upper horizons with the most weathered portion of the profile.

• The saprolite is the least weathered portion that lies directly above the solid, consolidated bedrock but beneath the regolith.

Horizon R Bedrock

• Granite, basalt, quartzite, and indurated limestone or sandstone are examples of bedrock designated R. R layers are cemented, and excavation difficulty exceeds high.

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• The R layer is sufficiently coherent when moist to make hand-digging with a spade impractical, although it may be chipped or scraped. Some R layers can be ripped with heavy power equipment.