Download Soil Forming Processes

Soil Forming Processes
Introduction
Soil forming processes are determined by climate and organisms
(both plants and animals) acting on the local geological surface
materials over time under the influence of the slope of the land and
human activities. The interaction between these factors initiates a
variety of processes including biologically driven accumulation and
destruction of organic matter, transformation of substances, the
migration and translocation of the products of soil formation which
all together gradually change geological materials into a soil with
distinct and well defined horizons.
Some soil types are deep or have thick organic layers that allow roots
to penetrate easily, others have strongly cemented horizons or acid
subsoil that inhibit rooting. These differences arise from the interaction
of local environmental processes acting upon the soil fabric.
Orientated stones
Frost heave
Sorting
FROST ACTION IN SOILS
Cryoturbation
Thawing
Cementation
Time is an important consideration since the soil forming factors
must act for a considerable period to develop a mature soil profile
with well expressed horizons. Some soil forming processes, such
as gleying (water-logging), might occur within a few years while
podzolization (the downward migration of aluminium and iron and
subsequent immobilisation at depth) might take centuries.
Finally, humans play an important role in soil formation by manuring,
irrigating, draining, liming and ploughing the land. More details can
be found in any standard soil science text book [18].
The dominating soil processes in northern latitudes can be grouped in to:
•
cryogenic process, where the role of ice and cold temperature
are important;
•
the accumulation of organic matter, including peat formation;
•
weathering;
•
brunification;
•
leaching, clay movement and destruction;
•
podzolization;
•
gleying (waterlogging);
•
salinisation (high salt levels).
This collage illustrates the diverse results of frost action in soil. The
result of frost action is heavily dependent on soil texture. These features
often coincide or appear in close proximity to one another. (CT)
Cryoturbation
An important consequence of frost-heave processes are
cryoturbated soils that includes the displacement and mixing of
soil materials, frost sorting (which separates fine materials from
coarse materials) and oriented stones (see above). On the soil
surface cryoturbation results in patterned ground, frost-heaved
stones and boulders.
SORTED POLYGON
VEGETATED
Cryogenic processes
The specific cryogenic processes that affect the genesis of
circumpolar soils are frost heave, cryoturbation (frost churning),
thermal cracking, and ice build-up. All these process also contribute
to the development of patterned ground
Other soil-forming processes, as mentioned above and described
later, can leave an imprint on these soils.
Frost heave
UNVEGETATED
VEGETATED
F
As a consequence of the cold, dry climates of the northern
circumpolar region, cryogenic processes, which lead to the
formation of permafrost-affected soils, dominate soil genesis.
The presence and mobility of unfrozen soil water drives this process as it
migrates along the thermal gradient towards the freezing front in the soil.
Cryoturbation occurs to varying degrees in most permafrostaffected soils and is characterised by deformed soil horizons and the
mixing of materials within the soil. Cryoturbation is often unaffected
by ‘conventional’ soil forming processes. Therefore, the features of
cryoturbation are often preserved in soil which allows past climatic
features to be recognized.
Cjj1
Cjj2
3
3
1
Cjj1
6
Bwjj
Cjj2
Bcjj
Cjj2
4
Cjj3
cm
20
2
Oa
5
5
The graphic above is an idealized sketch of an ice-wedge
polygon showing the main features of cryoturbation including
(1) irregular horizons, (2) deformation of textural bands, (3)
broken horizons, (4) involutions, (5) the accumulation of fibrous
or partially decomposed organic matter on top of the permafrost
table, (6) oriented stones, (7) silt caps from vertical sorting and
(8) the upwarping of sediments adjacent to ice wedges. [15]
7
10
0
10
20
permafrost table
Wfm
ice wedges
8
8
8
When the ground freezes, soil material can be displaced as a result
of a process known as ‘frost heave’. One of the most common
mechanisms that drives frost heave is the migration of water from
warm to cold areas because ice molecules have a lower energy state
than liquid water. Since the system tends towards a lower energy
level, the free water flows towards the developing segregated ice
forms (ice lenses, ice crystals and vein ice). Because these ice forms
develop parallel to the freezing front and continuously increase in
size, the result is an upward movement of the ground (frost heave).
This process depends on temperature, the availability of water, and
the texture of the soil.
If the soil is underlain by a layer of permafrost (or by some other
impermeable barrier such as hard massive rock) and at the same time,
bounded by a lateral obstruction such as ice wedges, tremendous
cryostatic pressure can build up when the soil freezes. This pressure
leads to dramatic structural changes in the soil and may result in
the upward or downward movement of rock fragments within the
profile. This movement may cause a bulging (differential heave) of
the surface or even the expulsion of material from the soil (e.g. the
frost heaved boulder in the above photograph).
Frost-heave causes fine particles to be separated from coarse
fragments and stone (frost sorting). In addition, stones in the soil
become orientated in a uniform direction (oriented stones).
This photograph from the Canadian Arctic illustrates several features of cryoturbation (frost churning).
These include irregular and broken soil horizons and involutions. Over time, cryoturbation can move
organic matter from the surface in to the subsoil. This process can have strong implications for climate
change (this aspect will be considered further in the section on carbon cycle and climate change). (ISRIC)
Soils in Northern Latitudes | Soil Atlas of the Northern Circumpolar Region
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