An Introduction to Environmental Chemistry

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different conditions to those described for argillic horizons, the materials being
removed and deposited are generally metal ions and/or organic matter rather than
clay particles. Spodosols are sandy forest soils, forming beneath a horizon of tree
litter, (horizon ‘L’ in Fig. 4.24). In pine forests, for example, large annual inputs
of dead pine needles cause an accumulation of organic matter in both the litter
horizon ‘L’ and the organic-rich ‘Ah’ horizon. The low pH in these horizons,
resulting from the release of organic acids, often limits organic matter process-
ing by soil organisms allowing thick accumulations to form.
Spodosols are typified by a prominent eluvated horizon (E) that has lost many
of its metal ions, particularly iron, manganese and aluminium. This horizon shows
clearly in Plate 4.2 as the grey Ea horizon. Rather insoluble metal ions like iron
and aluminium are leached because their solubility has been enhanced by a ‘com-
plexing’ or chelating agent. In spodosols the ‘free’ metal ions are chelated (see Box

The Chemistry of Continental Solids 115

Entisols
(little or no profile
development)

Inceptisols
(beginning of
B horizon
development)

Oxisols
(oxides)

Histisols
(organic)

Andisols
(volcanic ash)

Aridisols
(dry)

Vertisols
(swelling clays)

Alfisols
(mildly base-
poor clays)

Mollisols
(soft, dark)

Ultisols
(base-poor clays)

Spodosls
(spodic horizon)

Gelisols
(permafrost)

Desert shrubs,
grasses, dry

High base status,
high CEC clays
smectite, dry season

Moist, mildly acidic
fertile forests

Semi-arid to moist
grasslands, >0.6%
SOM in thick upper
horizon

Wet tropical and
subtropical forests, acid
silicate and Fe, Al oxides

Cool, wet, sandy acid,
coniferous forest

Very cold permafrost

Mild weathering,
various conditions

Deep accumulation of organic
materials, wet conditions

Mild weathering on volcanic
ejecta

Slight

Degree of weathering and soil development
Intermediate Strong

Wet, tropical
forest, extreme
weathering,
low activity
clays, Fe, Al
oxides

Fig. 4.22Diagram relating the 12 USDA soil orders to degree of weathering, degree of soil development,
climate and vegetation conditions. SOM, soil organic matter. Approximate FAO names where different are:
entisol (arenosol, fluvisol, regosol); inceptisol (cambisol); andisol (andosol); ardisol (xerosols); alfisol (luvisols);
ultisol (acrisol); spodosol (podzol); oxisol (ferralsol). Gelisol and mollisol have no simple FAO equivalent.
Modified from Brady and Weil (2002), reprinted by permission of Pearson Education Inc., Upper Saddle
River, N J.

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