Geoscience Reference
In-Depth Information
i.e. the topographic factor of soil formation. The soil
profile of each member of the catena is related to every
other member of that catena, and its individual soils are
like individual links in a chain. Milne recognized two
types of catena. In the first the parent material is uniform
and differences between soils in the catena result from
different surface and subsurface processes down the slope.
In the second type one parent material is superimposed
on another, and the slope thus cuts across them both. The
upper part of the catena is on one parent material, and
the lower slope on another. Hence a parent material or
geological factor is added to slope effects. In Britain, as in
most countries, both types of catena are well represented.
An example of the first type is the effect of slopes
giving soils with different drainage conditions and water
contents. The sequence of soils formed along the transect
from a hill crest to the adjacent valley bottom is the
hydrological sequence
. An example on an acid parent
material is shown in
Figure 18.17
.
Freely draining soils in
the upper part of the slope have bright, well oxidized
colours. Progressively less well drained and increasingly
gleyed soils occupy the lower positions of the catena.
Greyish subsoil with ochreous mottling denotes imperfect
drainage. A continuously gleyed horizon with ochreous
mottles shows poor drainage, whilst surface peat overlying
a blue-grey horizon reflects very poor drainage. Such
sequences are common on uniform parent materials,
especially on glacial tills. In Scotland the Soil Survey maps
hydrologic sequences as
soil associations
when formed on
uniform parent materials in the landscape.
The movement of water is the principal reason for the
differences in soils downslope in humid temperate
regions. Subsurface lateral flow is more important than
overland flow. Cations and anions will be carried in the
water and will thus tend to accumulate relatively in
downslope locations. The increased content of cations
downslope causes a parallel increase in the pH of the
mineral horizons below the surface peat. Often pH can
reach 7ยท0 in the Humic Gleysol and the Eutric (i.e. base-
rich) Histosol. There is also increased weathering
by hydrolysis because of the increased wetness. Any
ferromagnesian minerals in the parent material will
PLACIC
PODZOL
ORTHIC
PODZOL
GLEYIC
CAMBISOL
EUTRIC
GLEYSOL
HUMIC
GLEYSOL
EUTRIC
HISTOSOL
FAO
Peaty
podzol
with pan
Non-
calcareous
gley
SOIL
SURVEY
SCOTLAND
Brown
earth with
gleying
Peaty
gley
Basin
(fen) peat
Brown
podzolic
HORIZONS
Peat
Mor
humus
Eluvial
Thin iron
pan
Illuvial iron
Organo-
mineral
Drainage
class
Weathered
B
EXCESSIVE
FREE
IMPERFECT
POOR
VERY
POOR
Gley
mottles
Blue
gley
Glacial till
