Melanic Epipedon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 26

4.7 Summary

The melanic epipedon is a surface horizon that is thick ( 30 cm), has andic

properties throughout, has dark value and chroma of 2 or less, has a melanic

index of 1.7 or less, and has abundant SOC concentration of ( 6 %). The melanic

>

0.90 g/cm 3 ) and a high P fixation value

epipedon often has a low bulk density (

<

( 90 %). There are only 18 soil series with a melanic epipedon, and they are

differentiated at the great-group level within the Andisols. Soils with a melanic
epipedon occur nearly exclusively in western WA and western OR and cover an
area of about 595 km 2 . The key factors affecting the development of the melanic
epipedon are highly weathered Si-rich parent materials, abundant

>

rainfall

( 1,500 mm/yr), a cool soil climate, and vegetation such as conifer forests that

produce abundant organic C. The melanic epipedon is formed from the complex-
ation of humic acids with amorphous and organic-bound Al. These compounds
have a net positive charge that enable fixation of phosphate and other anions.

>

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