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Humus has a characteristic black or dark brown color, due to an accumulation of organic soil carbon | carbon.

Soil scientists use the capital letters O, A, B, C, and E to identify the master horizons, and lowercase letters for distinctions of these horizons.

Most soils have three major horizons -- the surface horizon ( A ), the subsoil ( B ), and the substratum ( C ).

Some soils have an organic horizon ( O ) on the surface, but this horizon can also be buried.

The master horizon, E, is used for subsurface horizons that have a significant loss of minerals ( eluviation ).

Hard bedrock, which is not soil, uses the letter R.