It is an organic layer formed by pedological proses (Decomposition ) of organic matter and the upper boundary shall start from 0 cm to 2 cm.
An O layer may be on the surface of a mineral soil, or it may be at any depth below the surface if it is buried. A horizon formed by the illuviation of organic material into a mineral subsoil is not an O horizon, although some horizons that have formed in this manner contain considerable amounts of organic matter.
with my appreciations and regards.
Yours faithful
Prof. Dr. Abdulellah A. Abughanem
Head of Soil & Water Department
Faculty of Agriculture ,Sana'a University
Republic of Yemen, P.O. Box 13511Sana’a
Tel.00967-1441912 Mobile :00967-777188264