Diagnostic surface horizons (Epipedons)

Diagnostic horizons can contain all or part of more than one master horizon. An epipedon is the surface, or uppermost soil horizon. It may be thinner than the soil profile A horizon, or include the E or part or all of the B horizon. Epipedons derived from bedrock lack rock structure and are normally darkened by organic matter.

1. Mollic epipedon (Latin: mollis – soft)

This epipedon is a soft dark grassland soil. Its organic carbon content is 0.6% or more. Its base saturation is 50% percent or more. It measures a minimum of 18 cm thick if not directly above a petrocalcic horizon or duripan and contains less than 250 ppm P2O5. Moist three month of more each year, it cannot have both hard consistence and massive structure.

2. Anthropic epipedon

While similar to the mollic epipedon with or without a 50% base saturation and soil is moist three months or more over 8 to 10 years. It is commonly found in fields cultivated over long periods of time.

3. Histic epipedon (Gk: Histos – tissue)

The organic horizon is water saturated long enough for residual conditions to occur unless artificially drained. It is 20 to 60 cm thick and has a low bulk density often less than 1g/cm3. If the soil has not been plowed, it contains 12% or more organic carbon with no clay and 18 percent or more organic carbon with 60% or more clay. When the soil has been plowed, the organic carbon content is from 8% with no clay to 16% with 60% or more clay.

4. Melanic epipedon

This thick black surface horizon with a high organic matter content formed in volcanic ejecta.. has a minimum thickness of 30 cm, contains 6% or more organic carbon, and has volcanic mineral-like allophane throughout.

5. Ochric epipedon (Greek: Ochros – pale)

This epipedon does not meet the definitions of any other surface horizon. It has either too thin or too light in colour or too low in organic matter. This epipedon might be hard and massive when dry due to organic matter content.

6. Plaggen epipedon

This man-made horizon is 50 cm or more thick and has resulted from centuries of accumulation of sod, straw, and manure, for example, it commonly contains artificial such as pottery and blocks.

7. Umbric epipedon

Mollic-like in thickness, organic carbon content, color, P2O5 content, consistence, and structure. This epipedon has less than 50% base saturation.