Geoscience Reference
In-Depth Information
Chapter 5
Mollic Epipedon
5.1
Introduction
The mollic epipedon is a key diagnostic epipedon in Soil Taxonomy (Soil Survey
Staff 2010 ) and is recognized in many other soil classification schemes as black
soil, Chernozems, chestnut soils, Brunizems, Phaeozems, and Kastanozems. The
origin of the mollic epipedon is only partially understood; however, the relation
between Mollisols and grassland or steppe has been recognized for more than a
century (Shantz 1923 ). Soils containing a mollic epipedon are among the world's
most productive soils (Liu et al. 2012 ). The thickness and high soil organic carbon
(SOC) contents of the mollic epipedon mean that these soils have sequestered large
amounts of C over long periods of time. In agricultural areas, the SOC content has
decreased somewhere between 30 and 50 % from soil erosion and increased
decomposition and cropping (Mann 1985 ; Mikhailova et al. 2000 ; Liu et al. 2010 ;
Fenton 2012 ; David et al. 2009 ). As a result some Mollisols no longer have a mollic
epipedon (Veenstra and Burras 2012 ; Fenton 2012 ).
In Soil Taxonomy (ST), the mollic epipedon has four (simplified) key require-
ments: (i) the moist Munsell color value and chroma must be 3, (ii) the SOC must
be
50 %, and (iv) the minimum thickness
of mollic colors is at least 10, 18, or 25 cm, depending on the depth and texture of
the solum (Soil Survey Staff 2010 ). A mollic epipedon is required for a soil to be
classified as a Mollisol, but a mollic epipedon or mollic intergrade may occur in
other orders. Intergrade subgroups are transitional forms to other orders, suborders,
or great groups, for example, Vertic Hapludolls or Mollic Paleudalfs.
A mollic horizon is also recognized in the World Reference Base System, except
that the thickness requirement is 20 cm and an added requirement is that the depth
to secondary carbonates must be within 50 cm of the base of the mollic horizon
(IUSS Working Group WRB 2007 ).
0.6 %, (iii) the base saturation must be
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