Geoscience Reference
In-Depth Information
Chapter 22
Spodic Horizon
22.1
Introduction
Spodosols are one of the most studied soil orders in ST. The Web of Science
contains 459 citations of Spodosols and 1,436 citations of Podzols. There are
446 references to the podzolization process. In ST the spodic horizon (from
Greek
spodos
, wood ash) is defined as an illuvial layer underlaying an O, A, Ap,
or E horizon with
2.5 cm in thickness. Spodic
materials defined simplistically include the following: a pH value in water (1:1) of
5.9 or less and a SOC content of 0.6 % or more, and, depending on whether or not it
is overlain by an albic horizon, strong colors (hues 5YR-10YR; values 5 or less;
chromas 5 or less), cementation of SOM and Al in
85 % spodic materials in a layer
>
>
Fe
percentages (by ammonium oxalate) total 0.50 or more, with half that amount or
less in an overlying eluvial horizon, and an optical-density-of-oxalate-extract
(ODOE) value of 0.25 or more, with half that amount or less in the overlying
eluvial horizon.
The WRB has similar requirements and simple field identification: “a spodic
horizon normally underlies an albic horizon and has brownish-black to reddish-
brown colors. Spodic horizons can also be characterized by the presence of a thin
iron pan, when weakly developed by the presence of organic pellets, or by the
accumulation of Fe in lamellar form” (p. 36). Spodosols, commonly called Podzols,
are recognized at the highest level in soil classification systems of Russia, Canada,
China, Brazil, and Australia, among other countries.
>
50 % of the pedon, Al +
½
22.2 Properties of Soils with a Spodic Horizon
Spodic horizon are designed in ST as Bh, Bhs, and Bs horizons, reflecting
pedogenic translocation of humus and sesquioxides of Fe and Al. Figure
22.1
shows a spodic horizon underlaying an albic horizon in a Typic Haplorthods in
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