Agriculture Reference
In-Depth Information
Box 1.3 The Processes of Gleying and Mottling
The weathering of Fe-containing minerals releases ferric (Fe
3+
) and ferrous (Fe
2+
)
ions, which are in a reversible equilibrium (a redox reaction) that depends on the
availability of oxygen (O
2
). High O
2
concentrations (characteristically aerobic
conditions) favor Fe
3+
compounds, particularly the oxides (general formula Fe
2
O
3
.
nH
2
O, where “n” is a variable number), which are insoluble and orange-red (“rusty”)
in color. Low O
2
concentrations (anaerobic conditions) favor Fe
2+
compounds,
which are more soluble than Fe
3+
compounds and give a blue-gray color to the soil.
Soils that are waterlogged most of the time are predominantly blue-gray in color and
have little or no structure. This condition is described as “gleyed” and typically occurs
in clay subsoils that remain saturated for most of the year. Figure B1.3.1 shows an
example of a vineyard where the vines have died from soil waterlogging.
Figure B1.3.1
A vineyard soil
with an impermeable subsoil in
Virginia. The vines died from
waterlogging. (Photo courtesy
of Brad Johnston, Warrenton,
Virginia.)
However, in soils where waterlogging is periodic (e.g., occurring mainly
during wet periods in winter) the soil matrix is not uniformly gleyed but instead
shows “mottling.” Mottling occurs when dissolved Fe
2+
ions migrate to pockets of
higher O
2
concentration where they are oxidized to Fe
3+
and precipitate as Fe oxide.
The rust-colored Fe oxide mottles show up against the gley-colored background,
(continued)
