Mollic Epipedon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 30

5.3 Classification of Soils with a Mollic Epipedon
or Intergrade
Soils with mollic epipedons occur predominantly in the Mollisol order (~6,900 soil
series); however, soils with a mollic epipedon or within mollic subgroups occur in
the Alfisols (421 soil series), Vertisols (149), Entisols (87), Andisols (53), and
Inceptisols (30 soil series) (Table 5.2 ). Although Gelisols containing mollic
epipedons are recognized in ST, there are no Mollorthels or Molliturbels in the
SSURGO database.
The Mollisol order contains eight suborders, 35 great groups, and 309 subgroups.
Dominant soil subgroups include Typic Argiustolls (211 soil series), Aridic
Argiustolls (208), Vitrandic Haploxerolls (147), Pachic Argiustolls (144),
Vidrandic Argixerolls (140) (Table 5.2 ). Some common Mollisols from Wisconsin
are shown in Fig. 5.3 . Three-quarters of the Mollisol soil series have a mesic or
frigid soil-temperature regime (STR); 64 % have an ustic or xeric soil-moisture
regime (SMR); 72 % have a mixed mineralogy; and 60 % have a fine-loamy,
loamy-skeletal, or fine-particle-size class (PSC; Table 5.3 ).
Alfisols with mollic subgroups occur in five suborders, 14 great groups, and
23 subgroups (Table 5.2 ). The most represented subgroups in the Alfisols include
Mollic Hapludalfs (107 soil series), Mollic Haploxeralfs (80), and Mollic
Endoaqualfs (36 soil series); these three subgroups account for more than half of
the Alfisols with mollic epipedons. These soils do not have a true mollic epipedon;
rather, they commonly have an ochric epipedon, because the mollic epipedon must
constitute one-third or more of the total thickness between the mineral soil surface
and a diagnostic subsurface horizon. Seventy-four percent of the Alfisol soil series
in mollic subgroups have a mesic or thermic STR and a fine-loamy, fine, or fine-
silty texture, 66 % have a udic or aquic SMR, and 92 % are in mixed or smectitic
soil mineral classes (Table 5.3 ).
Mollic epipedons are not taxonomically defined in the Vertisols and Andisols,
but examination of the soil series within these orders in the SSURGO database
yielded 149 series within the Vertisols (36 % of the total in that order) and 53 soil
series in the Andisol order (6.8 % of the total series in that order). Vertisol soil
series with a mollic epipedon occur in six suborders, 15 great groups, and 41 sub-
groups; and Andisol soil series with a mollic epipedon occur in four suborders, six
great groups, and eight subgroups (Table 5.2 ). Sixty-eight percent of the Vertisol
soil series with a mollic epipedon have a mesic or thermic STR; 60 % have an ustic
or udic SMR; 94 % are in smectitic mineral classes; and 83 % are in fine PSC
(Table 5.3 ). Sixty-two percent of the Andisol soil series with a mollic epipedon
have a frigid/cryic STR; 83 % have an ustic or xeric SMR; 94 % are in glassy or
amorphic mineral classes; and 74 % have an ashy or medial PSC.
The Entisol order contains three suborders, nine great groups, and ten subgroups
of soils with mollic intergrades, and the Inceptisol order has soils with mollic
intergrades in only one suborder, the Aquepts, two great groups, and two subgroups
(Table 5.2 ). Seventy-nine percent of the Entisol soil series in mollic subgroups have