Geoscience Reference
In-Depth Information
8.6 Genesis of Folistic and Histic Epipedons
The dominant processes occurring in soils with a folistic epipedon are littering,
decomposition, humification, ripening, and mineralization (Fox and Tarnocai
2011 ). A primary characteristic of Folists is that they are subject to aerobic rather
than anaerobic processes which are common in most Histosols. However, the key
characteristic is that organic matter addition exceeds decomposition and loss so that
a folistic horizon is formed. Despite that plant biomass may be high on Folists,
litterfall deposition and N-mineralization rates are often quite low. For example,
Pastor et al. ( 1984 ) recorded values of 0.45 Mg litterfall/ha/yr and 29 kg N/ha/yr for
eastern hemlock ( Tsuga canadensis ) on a Folist on Blackhawk Island, Wisconsin.
These values were the lowest of eight forest cover types on the island.
The dominant process in soils with a histic epipedon is paludification. This term
pertains primarily to the deep (
40 cm) accumulation of organic matter (histic
materials) on the landscape, usually in marshy areas. Ripening is a subprocess of
paludization and refers to chemical, physical, and biological changes following
drainage and aeration of organic materials (Pons and Van Der Molen 1973 ).
Gleization is often manifest in the mineral subsoils of Histosols and refers to the
presence of aquic conditions. These are evidenced by reductimorphic or
redoximorphic features such as mottles, gleying, etc. (Bouma 1983 ; Blume and
Schlichting 1985 ). The effect of reduction and oxidation processes has focused on
Fe and Mn compounds since these results in visible morphological features that
have been used for predicting an aquic soil-moisture regime.
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8.7 Summary
Histic and folistic epipedons are the only surface horizons comprised dominantly of
organic materials. Whereas the histic epipedon occurs under saturated conditions,
the folistic epipedon is saturated for less than 30 consecutive days. Histic and
folistic epipedons are
30 % SOC, range from acid
to basic, and contain high rubbed and unrubbed fiber contents. Organic soils occur
in nearly all of the 50 states and account for 165,000 km 2 , 1.8 % of the land area of
the USA. Key factors in the development of organic soils include a climate where
precipitation or run on is greater than evapotranspiration and peatland vegetation.
Whereas soils with a histic epipedon tend to develop under anaerobic conditions in
depressions, soils with a folistic epipedon occur under coniferous vegetation on
steep slopes with porous bedrock. The dominant processes leading to the develop-
ment of the folistic epipedon include littering, decomposition, humification, ripen-
ing, and mineralization. In contrast, the two dominant processes in soils with a
histic epipedon are paludification and gleization.
20 cm thick, often contain
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