Leaching is the removal of materials in solution or suspension downwards as water moves vertically through the soil body by the force of gravity. Leaching results in movements of soluble and suspended materials in water percolation.
Leaching involves two forms:
Elluviation – which is washing out of materials in solution or suspension from the overlying parts of the soil body; and
Illuviation – which is the accumulation of materials taken from the overlying parts of the soil body.
Soil profile refers to the vertical arrangement of the various soil layers from the top layer down to the parent rock or bottom layer.
It is a vertical section through the soil horizons extending into the parent material or the bedrock.
It describes the sections downwards through the soil which comprises differing characteristics in terms of texture, color, mineral composition, the ratio of the combination of organic and inorganic matter, hardness, and rate of weathering.
The different layers are referred to as horizons.
A soil horizon is a well-defined layer within the soil profile parallel to the local round surface. There are four main horizons namely:
A horizon, B horizon, C horizon, and D horizon. Each horizon has different physical and chemical properties, which result from various soil-forming processes such as weathering, the introduction of humus, and the movement of minerals.
This is the topmost/ surface layer of the soil comprising organic matter.
The constituents of this layer include un-decomposed litter, decomposing organic matter, and humus.
This is also known as the topsoil and it is rich in organic matter which organic matter accounts for dark colour.
Leaching and Eluviation may at times impoverish the topsoil.
This is known as subsoil. Nutrients removed from the A horizon through leaching and Eluviation accumulate or are deposited in this horizon.
The process of plant nutrients precipitating or accumulating in this horizon is known as illuviation.
This horizon may also be characterized by hardpans due to the accumulation of large quantities of clay and other nutrients.
This consists of partially weathered rock, this is because weathering and other soil-forming processes may not effectively operate at this depth.
This consists of solid parent rock or unweathered rock or fresh parent material. It is also known as the bedrock.
It has no soil particles but has potential for future soil formation
Soil is defined as the uppermost surface layer of loose or unconsolidated materials which overlie the crustal rock on which the plant grows.
Soil can also be defined as a mixture of minerals, organic matter, gases, liquids, and myriads organisms.
The following are the processes that are involved in soil formation.
This process involves the accumulation of decaying vegetables and animal matter on the ground surface. The accumulated mass slowly breaks down to form humus which is a major component of the soil.
This is a mechanical wash down of fine mineral particles like those of clay from the upper layer of the soil into the middle of lower layers.
The materials are moved in suspension by water which is percolating downward.
Eluviation can take place in the same layer when the water percolates horizontally.
The upper layer from which materials have been eluviated is called the eluvial zone
This is the removal of soluble mineral matter in the solution from the upper horizon (layer) to the lower horizon of soil.
This process can also be referred to as chemical eluviation.
It is common in wetclimates. Leaching causes the topsoil to be deficient in minerals, especially the bases because since they easily dissolve in water.
This is the deposition or accumulation of materials that have been washed down from the upper layer to the lower horizon of the soil through the process of eluviation.
Illuviaton sometimes leads to the formation of a hardpan or crust of laterite.
The materials deposited in the lower horizon include colloids, salts, and other mineral particles.
Colloids are very tiny particles of humus (organic colloids) and minerals especially clay.
They are so tiny that they can not be seen using an ordinary optical microscope.
The tropical soils of the plateau north of Lake Victoria in East Africa are characterized by this hardpan.
This is the formation of solid matter in the subsoil.
The solid matter is formed from the solution washed down from the upper layers through leaching. eluviation, leaching, illuviation, and precipitation take place at the same time and within the same soil under the same climatic conditions.
This is the downward movement of materials in the soil which is very similar to leaching.
However, cheluviation occurs through the influence of organic agents which are also referred to as chelating agents.
The process involves plant acids rather than mere water as the case with leaching.
This is the separation of materials, usually of different sizes, through organic influence.
It involves changes in sizes of soil particles, enrichment of the soil with organic matters such as hummus, and movement of mineral elements in the solution that is leaching.
To sum up, soil formation involves seven processes which are organic accumulation, eluviation, leaching, illuviation, precipitation, cheluviation, and organic sorting. All these processes are related to each other.