O horizon

1-Oi, Oe, and Oa are different genetic horizon designations based on the state of decomposition in the organic materials and their rubbed fiber content.

2-For your example, if the surface horizon designated as "Oi" is at least slightly decomposed in part of its two centimeter thickness, then the soil surface (the ?0? depth of measurement) is at the depth where you observe the slight decomposition in the organic material. If this Oi horizon is slightly decomposed throughout its entire thickness, then the correct horizon sequence and depths are:

Oi 0-2 cm<br>A 2-20 cm
B(?) 20-50 cm
C 50-80 cm

Note that horizon thicknesses recorded above the "0" depth of measurement (e.g., 2 to 0) are obsolete and no longer permitted. Any thickness of surficial plant material that is undecomposed (e.g., fresh leaf fall) is considered a surface feature of the site but not a horizon of the soil.