An Introduction to Environmental Chemistry

The mollisol (mollic gleysol) in Plate 4.2, for example, has a Bg but no Ag horizon.

Gley horizons have a mottled red-brown/grey appearance on account of both

reduced (Fe^2 +,grey) and oxidized (Fe^3 +,red/brown) iron species being present

together. Both iron species are present because of the continuous change in redox

conditions (Box 4.3) within the soil profile as the water table rises and falls.

The geochemical cycling of iron species in gley horizons (Fig. 4.25) is medi-

ated strongly by microbiological reactions (Section 4.6.5). Most permanent

groundwater has a high Ca^2 +concentration, near neutral pH and very low dis-

solved oxygen (Eh near zero). Under these conditions iron is stable in the reduced

mineral FeCO 3 called siderite (see Box 5.4). However, due to the ingress of

118 Chapter Four

O 2 CO 2

O 2

CO 2

Al

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Fe3+

Bg

Mottled

red/brown and

grey horizon

diagnostic of

gleysols

Ah horizon in

Fig. 4.24 not

diagnostic of

gleysols

O X I C R E D U C E D

Water

table

A

Fe (HCO 3 )2(aq)

O

X

I

C

FeCO3(S)

Fe (OH)3(S)

Water

table

B

Grey colours

Red/brown colours

Fe3+

Fig. 4.25Schematic diagram of a soil with a gley horizon based on the mollisol (mollic gleysol) in Plate 4.2(c).
The upper Ah horizon contains oxygen and carbon dioxide in the soil atmosphere, while the lower Bg horizon
is very low in oxygen. Under conditions of high water table (A) and low oxygen, Fe^2 +species are stable and soil
colours are grey. A fall in water table (B) allows further ingress of oxygen into the Bg horizon causing oxidation
of Fe^2 +species and precipitation of iron oxides as red/brown patches (mottles). The master horizons are
typically centimetres to tens of centimetres thick.