Download 24 Soil-forming processes

Soil-forming processes
Principle processes
Source of organic matter
The specific properties of an individual soil are determined by
pedological processes that operate during its lifetime. These
biological, chemical and physical actions add, transform, move
(translocate) and destroy or remove material within the soil. It is
important to recognise that soil-forming processes can evolve and
change over time in response to factors such as climatic variability
and land use. Due to their old age, many African soils exhibit several
distinct and different phases of soil formation.
Accumulation of organic matter
Weathering
Below the soil we usually find solid rock or unconsolidated
sediments in (or on) which the soil has developed (see page 14). In
reality, all sediments are derived from solid rock by a process known
as weathering. Weathering proceeds by a physical destruction of
the rock structure which in turn facilitates chemical changes to the
constituent minerals.
Biological activity
Movement
of clay
In principle, there are two main types of weathering: physical and
chemical. Some people also consider biological weathering but, in
reality, it is a manifestation of physical and chemical actions (see
below).
Leaching
Physical Weathering
In physical weathering, rocks disintegrate without changing their
chemical composition. Typical examples of these processes are
the splitting of rocks by the daily warming of the sun and cooling
during the night (typical of desert environments) or by the repeated
freezing and thawing of water (when water freezes, its volume
increases by 10%, causing tremendous pressures if it occurs in
confined spaces such as crevices in rocks).
Physical weathering produces a layer of loose material which covers
the underlying solid rock. This material is known as regolith and can
vary from a few milimetres to tens of metres thick. Regolith layers
in some parts of west Africa have been found to be more than 150
m thick. There is often a sharp boundary between the bottom of
the regolith and the bedrock. This narrow zone is known as the
weathering front and is the focus of active weathering.
Weathering front
Parent material
A schematic of key soil-forming processes. The dark colour of the upper part of this soil profile indicates
that significant amounts of organic matter have accumulated in the topsoil through the decay of vegetation
and root material. The lighter colour between 20 and 40 cm is due to a combination of the leaching of
mobile iron and loss of clays by percolating rainwater. In the subsoil, the iron has coated soil particles
with a thin, reddish film. The bedrock from which the soil has developed and the weathering front is
clearly visible at the base of the profile. Biological processes are generally more active in the topsoil. (EM)
Chemical Weathering
Chemical weathering is a gradual and constant process. It is driven
primarily by the reaction between water or an acid and elements
within the parent material which lead to the creation of secondary
minerals from the original compounds present in the rock. Chemical
weathering is much stronger if temperatures and humidity are both
high (e.g. in the humid tropics).
Water is the key factor in chemical weathering. Most people are
unaware that rainfall is slightly acidic with a pH of around 5.6 in
unpolluted environments. Atmospheric carbon dioxide dissolves in
the rainwater to produce a weak carbonic acid. Some minerals, due
to their natural solubility (e.g. evaporites such as highly soluble salts
and gypsum) or inherent instability relative to surface conditions (e.g.
primary silicate minerals such as feldspar, mica, augite, hornblende
and olivine), will slowly dissolve to form secondary products such
as clay minerals (e.g. kaolinite, illite, vermiculite and smectite), iron
and aluminium (hydr)oxides, carbonates and nutrients such as
calcium and potassium.
The destruction of bedrock through both physical and chemical weathering is
clearly visible in this photograph from South Africa. The red rock is a dolerite
dyke that has intruded into granite (visible to the left). Weathering processes
have caused blocks of granite to break away while the dolerite breaks down
more easily to produce the distinctive red coloured finer material that is
accumulating at the foot of the slope. It is very evident that the dolerite is
weathering more rapidly than the granite. The dolerite is receding into the cliff
face and approximately 15 cm of soil has developed above it while bare rock is
still visible on the surface of the granite. (EM)
Minerals vs Nutrients
A mineral is a naturally occurring solid substance formed through
geochemical processes with a characteristic chemical composition.
Rocks are composed of several minerals.
Nutrients are chemical elements required by organisms to live and
grow. Nutrients can be produced by the organism or must be taken in
from its environment. Plants absorb nutrients from dissolved minerals
in the soil which, in turn, are consumed by herbivores and then by
the people who eat the herbivores. In this way, minerals move up the
food chain.
24
Soil Atlas of Africa | Introduction
Another chemical process involves the simultaneous loss (referred
to as oxidation) and gain (referred to as reduction) of electrons in
substances. These exchanges are referred to as redox reactions. As
materials become oxidised, the unbalanced charge will degrade a
material's structural composition.
Biological weathering is caused by the activities of living organisms and
has both physical and chemical aspects. Examples of physical biological
weathering include the loosening of rock by roots growing into cracks
and burrowing creatures such as termites that mix or churn the soil.
Chemical biological weathering can be caused by bacterial activity
or by strong organic acids from plant roots or litter. A recent study
demonstrated a 3 – 4 fold increase in weathering rate under lichencovered surfaces compared to recently exposed bare rock surfaces.
Biological weathering factors in Africa are highly significant.
One of the most well-known solution weathering processes is decalcification which occurs on parent material such as limestone
and chalk that are rich in calcium carbonate. Rain combines with
carbon dioxide or an organic acid to form a weak carbonic acid
which then reacts with calcium carbonate in the limestone to form
calcium bicarbonate, which is then removed. This process can be
even stronger if gases such as sulphur dioxide and nitrogen oxides
are present in the atmosphere. These oxides react in the rainwater
to produce stronger acids with a pH as low as 3).
Water molecules can break up into positively charged hydronium
(H3O+) and negatively charged hydroxyl (OH-) particles (see
Glossary for ion and cation). These small and mobile particles
can actually penetrate the crystal lattice of silicate and carbonate
minerals. Those with positive charge upset the balanced state of
the mineral in question causing various cations to be released into
the soil.
This process is known as hydrolysis and is one of the underlying
factors of soil fertility (see page 32).
A clear example of a weathering front on limestone in Tigray,
Ethiopia. The photographs shows the breakdown of the underlying
bedrock along vertical joints and horizontal bedding planes. (JD)