Melanic Epipedon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 20

Chapter 4
Melanic Epipedon

4.1 Introduction

The melanic epipedon is defined as a surface horizon with (1) a cumulative thickness
of 30 cm or more, (2) andic properties throughout, (3) a moist color value and chroma
of 2 or less, (4) a melanic index of 1.7 or less, and (5) a SOC concentration of 6 % of
more (weighted average) and 4 % or more SOC in all layers.
Andic properties originate from weathering of tephra and other parent materials
enriched in volcanic glass, including basalt. More specifically, andic materials have
a low bulk density ( 0.90 g/cm 3 ), a high phosphate retention, and abundant

oxalate-extractable Al and Fe and volcanic glass. The melanic index was developed
to distinguish andic materials with high concentrations of organic C derived from
root residues of graminaceous plants. These materials have high concentrations of
organic C, dark colors, and an abundance of extractable type A humic acids (Soil
Survey Staff 1999 ).

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4.2 Properties of Soils with a Melanic Epipedon

Figure 4.1 shows a melanic epipedon in a Melanudand soil derived from basalt-rich
beach sands along the Oregon coast. The data in Table 4.1 clearly show the
major properties of the melanic epipedon (in bold face). The mean thickness
of the melanic epipedon for the five pedons is 48 cm, the average bulk density is
0.9 g/cm 3 , the average organic C is 9.0 %, and the mean phosphate fixation is 93 %.
The soils are enriched in amorphous and organic (oxalate-extractable) forms of
Fe and Al.