An Introduction to Environmental Chemistry

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.