Introduction
Pedogenesis can
be defined as the process of soil development. Late
in the 19th century, scientists Hilgard in the United
States and the Russian Dukuchaev both suggested independently
that pedogenesis was principally controlled by climate
and vegetation. This idea was based on the observation
that comparable soils developed in spatially separate
areas when their climate and vegetation were similar.
In the 1940s, Hans Jenny extended these ideas based
on the observations of many subsequent studies examining
the processes involved in the formation of soils. Jenny
believed that the kinds of soils that develop in a
particular area are largely determined by five interrelated
factors: climate; living
organisms; parent
material; topography;
and time (Figure
10u-1).
| Figure 10u-1: The
development of a soil is influenced by five interrelated
factors: organisms, topography, time, parent material,
and climate. |
Climate plays
a very important role in the genesis of a soil. On
the global scale, there is an obvious correlation
between major soil types and the Köppen climatic
classification systems major climatic types. At regional
and local scales, climate becomes less important
in soil formation. Instead, pedogenesis is more influenced
by factors like parent material, topography, vegetation,
and time. The two most important climatic variables
influencing soil formation are temperature and moisture. Temperature has
a direct influence on the weathering of
bedrock to produce mineral particles. Rates of bedrock
weathering generally increase with higher temperatures.
Temperature also influences the activity of soil microorganisms,
the frequency and magnitude of soil chemical reactions,
and the rate of plant growth. Moisture levels in most
soils are primarily controlled by the addition of water
via precipitation minus
the losses due to evapotranspiration.
If additions of water from precipitation surpass losses
from evapotranspiration, moisture levels in a soil tend
to be high. If the water loss due to evapotranspiration
exceeds inputs from precipitation, moisture levels in
a soil tend to be low. High moisture availability in
a soil promotes the weathering of bedrock and sediments,
chemical reactions, and plant growth. The availability
of moisture also has an influence on soil pH and
the decomposition of organic
matter.
Living Organisms have
a role in a number of processes involved in pedogenesis
including organic matter accumulation, profile mixing,
and biogeochemical
nutrient cycling. Under equilibrium conditions,
vegetation and soil are closely linked with each other
through nutrient cycling. The cycling of nitrogen and
carbon in soils is almost completely controlled by the
presence of animals and plants. Through litterfall and the process
of decomposition,
organisms add humus and
nutrients to the soil which influences soil structure
and fertility. Surface vegetation also protects the upper
layers of a soil from erosion by
way of binding the soils surface and reducing the speed
of moving wind and water across the ground surface.
Parent Material refers
to the rock and mineral materials
from which the soils develop. These materials can be
derived from residual sediment due to the weathering
of bedrock or from sediment transported into an area
by way of the erosive forces of wind, water, or ice.
Pedogenesis is often faster on transported sediments
because the weathering of parent material usually takes
a long period of time. The influence of parent material
on pedogenesis is usually related to soil texture, soil
chemistry, and nutrient cycling.
Topography generally
modifies the development of soil on a local or regional
scale. Pedogenesis is primarily influenced by topography's
effect on microclimate and drainage. Soils developing
on moderate to gentle slopes are often better drained
than soils found at the bottom of valleys. Good drainage
enhances an number of pedogenic processes of illuviation and eluviation that
are responsible for the development of soil horizons.
Under conditions of poor drainage, soils tend to be immature.
Steep topographic gradients inhibit the development of
soils because of erosion. Erosion can retard the development
through the continued removal of surface sediments. Soil
microclimate is also influenced by topography. In the
Northern Hemisphere, south facing slopes tend to be warmer
and drier than north facing slopes. This difference results
in the soils of the two areas being different in terms
of depth, texture, biological activity, and soil
profile development.
Time influences
the temporal consequences of all of the factors described
above. Many soil processes become steady
state overtime when a soil reaches maturity.
Pedogenic processes in young soils are usually under
active modification through negative and positive
feedback mechanisms in attempt to achieve equilibrium.
Principal Pedogenic Processes
A large number of processes
are responsible for the formation of soils. This fact
is evident by the large number of different types of
soils that have been classified by soil scientists
(see topic 10v).
However, at the macro-scale we can suggest that there
are five main principal pedogenic processes acting
on soils. These processes are laterization, podzolization, calcification, salinization,
and gleization.
Laterization is
a pedogenic process common to soils found in tropical
and subtropical environments. High temperatures and heavy
precipitation result in the rapid weathering of
rocks and minerals. Movements of large amounts of water
through the soil cause eluviation and leaching to
occur. Almost all of the by products of weathering, very
simple small compounds or nutrient ions,
are translocated out of the soil profile by leaching
if not taken up by plants for nutrition. The two exceptions
to this process are iron and aluminum compounds. Iron
oxides give tropical soils their unique reddish coloring.
Heavy leaching also causes these soils to have an acidic pH because
of the net loss of base cations.
Podzolization is
associated with humid cold mid-latitude climates and coniferous vegetation.
Decomposition of coniferous litter and heavy summer precipitation
create a soil solution that is strongly acidic. This acidic
soil solution enhances the processes of eluviation and
leaching causing the removal of soluble base cations and
aluminum and iron compounds from the A horizon.
This process creates a sub-layer in the A horizon that
is white to gray in color and composed of silica sand.
Calcification occurs
when evapotranspiration exceeds precipitation causing
the upward movement of dissolved alkaline salts from
the groundwater. At the same time, the movement of rain
water causes a downward movement of the salts. The net
result is the deposition of the translocated cations
in the B horizon.
In some cases, these deposits can form a hard layer called caliche.
The most common substance involved in this process is calcium
carbonate. Calcification is common in the prairie grasslands.
Salinization is
a process that functions in the similar way to calcification.
It differs from calcification in that the salt deposits
occur at or very near the soil surface. Salinization
also takes place in much drier climates.
Gleization is
a pedogenic process associated with poor drainage. This
process involves the accumulations of organic matter
in the upper layers of the soil. In lower horizons, mineral
layers are stained blue-gray because of the chemical reduction of
iron.