The soil forming factors result in the formation of layers within the soil from the surface down to varying depths. These layers are called soil horizons. The combination of these layers in a sequence from the surface down represent a soil profile, or a vertical cross section of all the soil horizons at a particular location.

The soil horizons, or natural layers of the soil, vary in thickness, depending upon location, and have somewhat irregular boundaries. However, all the boundaries generally parallel the earth’s surface. The uppermost layers have been changed the most, while the deepest layers are more similar to the original parent material. Exceptions to this vertical aging process occur when soil material is transported and deposited on the surface of previously formed soil profiles. Soil horizons can sometimes be easily identified and at other times be very gradual and faint. In older, well-developed soils, as many as five or six master horizons may be found in the soil profile. Younger, less-developed soils may have only two master horizons. Master horizons that may be found in soils include O, A, E, B, C, and R.

O horizon –When found, it is generally the uppermost layer of the soil and is predominantly made up of organic material. It consists of leaves, needles, twigs, mosses, lichens, and other accumulations of organic matter in various stages of decay. This horizon is typically found in wetlands and forested area, but is not present in cultivated fields due to soil mixing during cultivation.

A horizon –Commonly called topsoil, this horizon is darker colored than lower horizons because it contains more organic matter, and it is often the most fertile layer of the soil. This is the layer that is plowed in cultivated fields and is where most root activity occurs. As rainwater percolates downward through the A horizon it dissolves minerals, and they are leached from the topsoil.

E horizon –This horizon is characterized by its light color or bleached appearance. This is a zone which has been strongly leached. The main feature of this horizon is the loss of clay, iron, aluminum, humus, or some combination of these, which migrated downward as water passed through and a concentration of sand and silt particles. This horizon is commonly found in older, well-developed soils of woodlands. It is rarely found in cultivated areas, since plowing usually mixes this horizon with the A horizon.

B horizon –Commonly called subsoil, this layer is usually lighter in color than the A horizon due to its lower content of organic matter. It is the zone of accumulation for materials leached from the A and E horizons. In older, well-developed soils, this horizon commonly has the highest clay content.

C horizon –This horizon is the transition layer between soil and parent material. It is less weathered than the upper horizons and contains partially disintegrated or weathered parent material from the underlying bedrock or transported to the area by glaciers, wind, or water.

R horizon –This horizon is the bedrock. As the bedrock weathers, it contributes parent material to the C horizon above. Bedrock can be within a few inches of the surface or many feet below the surface.