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.
Organic accumulation.
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.
Eluviation.
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
Leaching
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 wet climates. Leaching causes the topsoil to be deficient in minerals, especially the bases because since they easily dissolve in water.
Illuviation.
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.
Precipitation.
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.
Cheluviation.
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.
Organic sorting
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.
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