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 processes (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 containing 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 characteristic 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.
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. Climate, 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...
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