Oxic Horizon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 213

19.7 Summary

The oxic horizon must be 30 cm thick, have a texture of sandy loam or finer,

>

contain 10 % weatherable minerals, lack rock structures, have a diffuse upper

boundary within a vertical distance of 15 cm, have a minimal clay increase with
depth, and have an apparent CEC of

<

16 cmol(+)/kg clay. Very few studies of soils
containing an oxic horizon have been conducted in the USA. There are 64 soil
series with an oxic horizon in the NRCS database; they occur in three orders
(Andisols, Inceptisols, and Oxisols), eight suborders, 18 great groups, and 34 sub-
groups. Although Oxisols account for 23 % of the soil cover worldwide, they
account for only 0.02 % of the land area (1,666 km 2 ) in the USA, primarily in
HI, PR, and Micronesia. Soils with an oxic horizon tend to be well drained despite
receiving abundant rainfall. They are derived from strongly weathered parent
materials of pre-Quaternary age under the influence of tropical savanna and
rainforest vegetation. Soils of the intertropical regions undergo a series of processes
in which Al and Fe are concentrated and Si is lost in the profile as a result primary
and secondary mineral weathering that

<

involves desilication, fersiallitization,

ferrallitization, and ferrugination.

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