Soil formation (holiday homework)
Next to water and air, soil is most essential to our very existence on the earth. Soils are the basis of
support for most life, and a source of nutrients for marine life and fresh water.
As a natural resource, soil is of immense value to man. In the agricultural orient the distribution and
density of population have conformed to the persisting patterns of soil fertility and productivity.
Soil is basically broken-down rock materials and consists of decomposed rock debris and decayed
organic matter(humus) which have been produced by weathering.
According to Arthur Holmes From a geological point of view soil may be defined as the surface layer
of the mantle of rock-waste in which the physical and chemical processes of weathering co-operate
in intimate association with biological proc-esses (Principles of Physical Geology, Second Edition
Completely Revised, the English Language Book Society & Nelson).
Arthur N. Strahler and Alan H. Strahler defines soil as a natural surface layer con-taining living matter
and supporting or capable of supporting plants. (Modern Physical Geography, John Wiley & Sons,
Inc., Newyork).
Chester R. Longwell & Richard F. Flint defines soil as-Soil is that part of the regolith that will support
rooted plants. (Introduction to Physical Geology, 2nd Edition, John Wiley & Sons, Inc. New York,
London). From the above definitions, it may be stated that-
Soil is the surface layer of the earth restricted to land, consisting of a layer of broken-down, fine and
loose rock material, produced by the weathering processes, mixed with decayed vegetation and
other organic matter.
Soil may be found on top of the parent rock or at some distance from it after transport and supports
vegetation.
The science which deals with the study of the soil is known as Pedology and the process of soil
formation is called pedogenesis.
The Soil Profile
The pedologists have distinguished a number of layers or horizons within the soil. A vertical section
made through a soil reveals a series of more or less distinct layers.
These layers from the surface down to the unchanged parent material is called the soil-profile, which
is character-istic of the residual soil (i.e. the soil developed on top of the parent rock), where there is
a gradual transition from the top soils to the partially decayed rock and finally to the unaltered rock).
But a transported soil does not show any such characteristic.
A simple soil-profile shows three distinct layers designated as A, B and C- layer. The upper layer
containing most of the organic material is called the A-layer or horizon, which is commonly known as
the top- soil.
This is the horizon of maximum biological activity.
The layer below the A-horizon is the B-horizon, which is poor in organic content and rich in clay. This
layer is regarded commonly as 'sub soil' Mineral matter removed from the A-horizon through
solution are precipitated in the B-horizon.
High concentration of clay minerals in the B-horizon may be due to mechanical removal of colloidal
clays in suspension by the descending soil water.
Deposition of the ferro-humus material along with the silt and clay particles sometimes form a layer
which is dense, tough and well cemented and is called a hard pan or clay pan.
The C-horizon lies below the B-horizon.
This horizon contains remnants of the parent material and is little affected by biologic activity.
However, it is affected by physical and chemical processes. This horizon grades downward into the
unaltered parent rock. Bedrock underlying the C-horizon is designated as R- horizon.
Soil Formation
The natural processes of soil formation are very slow; and are due to a combination of several
factors such as 1. Parent rock material, 2. Cli-mate, 3. Plant and animal life, 4. Local topography 5.
Time etc.
Most of these factors are interdependent. The processes of soil formation are most intimately
associated with the weathering processes and the factors indicated above also determine the
characteristics of the soil.
1. Parent rock material
These are the bedrocks on which the soil develops. The processes of mechanical disintegration and
chemical decomposition which constitute weathering proceed side by side result-ing in the breaking
down of the bedrock into a mixture of soluble and insoluble materials.
While the soluble matter is removed in water, the insoluble residue forms a framework for the
development of soil. Since the parent material or the bedrock is composed entirely of materials, it
provides the inorganic constituents to the soil.
The mineralogical composition of the bed rock from which the soil is derived, determines the rate of
physical and chemical weathering as well as the composition of the soil. As we know, rocks
composed of resistant minerals are slowly affected by weathering processes.
Texture and structure of the parent material also determine the rate of weathring and the degree of
retention of plant nutrients in the soil.
2. Climate
Climatic conditions control weathering to a great extent. Climate of any particular region comprises
elements like temperature, moisture including both humidity and precipitation, wind, air pressure,
evaporation, and transpiration.
Rainfall, as we know, is primarily dependent on the moisture content in the air and it varies from
place to place with respect to its intensity, total amount in its distribution throughout the year.
Heavy rainfall causes downward movement of water and soluble materials in the soil and the
process is known as leaching. Leaching produces two distinct zones as:-
(i) Zone of eluviation (i.e. washed out) in the A-horizon and (ii) Zone of illuviation (i.e. washed in) in
the B-horizon. Climate gives rise to different soil types from the same parent material and also
widely different parent materials may produce similar soils in one climatic context.
Apart from rainfall, temperature plays a significant role in the weathering process. Temperature and
moisture not only affect the rate of chemical weathering but also of bacterial activity, on the parent
rock material in the process of soil-formation.
3. Plant and Animal life
Many pedologists believe that soil is a biological phenomenon and that the plants play the leading
role in the process of soil formation.
Plants promote disintegration of rocks by the growth of trees in cracks or joints, thus wedging off
large and small fragments of rocks. Roots often penetrate into the crevices of rocks exert on
expanding force on the side walls.
The biochemical activity of plants includes the extraction of various mineral substances, water and
the necessary elements of nutri-tion on the one hand, and on their death and decomposition they
contribute towards the accumulation of organic matter in the soil, on the other.
Dead plants contribute to the humus content of the soil and the process of humification releases
carbon dioxide and organic acids together with traces of ammonia and nitric acid etc. which often
speed up the decomposition of the mineral matter and accelerates soil-formation.
The microflora such as bacteria, algae and fungi contributes sig-nificantly to soil formation. But the
activities of both bacteria and fungi are related to climatic conditions. In cold climates bacterial
activity is limited whereas it is very intense and rapid in warm, moist climates.
Bacteria are also involved in the nitrogen and sulphur cycles. Even the bacteria sometimes cause the
quicker decay of neighbouring rock sur-faces.
Burrowing animals are effective soil makers. They make the soil and softer rocks porous and spongy
and thus make them more readily susceptible to weathering and erosion. It has been estimated that
earthworms completely work over a soil layer of 6 to 12 iches thick every 50 years.
They extract vegetable matter from the soil by eating their way through it. As the soil passes through
their bodies, it is subjected to mechanical and chemical modification.
4. Local topography
It affects the character of the soil profile. True soils with a full profile can develop only on fairly flat
surfaces where erosion is slow; whereas on steep slopes the profile never becomes completely
developed as erosion removes the products of weathering as soon as they form.
On flat upland surfaces, a thick soil is formed, often with a layer of clay, but it is well leached as
uplands also attract heavier rainfall. On fiat-bottom lands in the flood plains, there is dark coloured,
thick soils, since the flat bottom lands are poorly drained.
5. Time
The development of a matured soil profile requires time. Soils are less well-developed if the soilforming processes have not been in op-eration for an adequate time period for a fully developed
soil-profile, in most places, it needs several thousands years.