Mollic Epipedon - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 42

calcification to be the dominant process. Calcification leads to the formation of a
stable, black type of humified organic matter, referred to as Ca humates, in the
A horizon and often the upper part of the B horizon. Ca-humate compounds are
resistant to decomposition (Oades 1988 ). The presence of smectites aids in the
calcification process by providing a continuous supply of base cations (St. Arnaud
and Sudom 1981 ).
Pennock et al. ( 2011 ) explained the genesis of chernozemic A horizon (mollic
epipedon) in Canada Chernozems (Mollisols). Bioturbation involving roots and
other organisms plays an important role in maintaining SOC at depths in excess of
1 m. Anderson ( 1979 , 1987 ) highlighted pedogenic processes in the Great Plains
grasslands, including organic matter formation and transformation, carbonate
weathering and pH, clay formation, and translocation.
In Fig. 5.3 the characteristics of importance to the mollic epipedon include the
minimum SOC content, base saturation, and thickness and color requirements.
Inputs into these soils have come from wet and dry deposition, litterfall, and
manures (both organic and inorganic); outputs occur through erosion, SOC decom-
position, product removal (if cropped), and leaching losses of clay and carbonates.
In the mollic epipedons, there are transfers from one part to the other part of the
epipedon, which may be caused by biological activity or transfers induced by water
movement. The transformation of materials include mineralization of SOM (both
dead and alive), exudation of acids or Ca from the roots, weathering of primary and
secondary minerals plant nutrients, and the formation of Ca humates by which the
SOM has some protection against decomposition.
5.7 Summary
Mollic epipedons are ubiquitous in soils of the USA, covering about 3.27 million
km 2 . Mollic epipedons are found primarily in Mollisols, but they also occur in
Andisols and Vertisols. Alfisols, Entisols, and Inceptisols contain mollic subgroups
or intergrades that may or may not meet the strict definition of a mollic epipedon. The
mean thickness of the mollic epipedon or intergrade is greatest in Vertisols (72 cm),
followed by Mollisols and Andisols (48 and 43, cm, respectively), and Entisols,
Alfisols, and Inceptisols (20 cm). The mean unweighted average properties of more
than 250 mollic epipedons from the six orders include a Munsell moist color value of
3, a chroma of 2, a SOC concentration of 2.2 %, and a base saturation of 87 %.
Soils with a mollic epipedon form primarily under grassland and savanna
vegetation but may occur under broad-leaved forest or even under mixed broad-
leaved and coniferous forest. Although loess and calcareous till are the dominant
parent materials from which mollic epipedons are derived, a surprising number of
soils with mollic subgroups are derived from alluvium or lacustrine materials. An
argillic horizon underlies the mollic epipedon in about two-thirds of the Mollisols
and 100 % of the Alfisols; however, a cambic horizon is most common in
Inceptisols, Andisols, and Vertisols.