Soil is defined as the uppermost surface layer of loose or unconsolidated materials which overlie the crustal rock on which 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 involve accumulation of decaying vegetables and animal matter on the ground surface. The accumulated mass slowly breaks down to form humus which is major components of the soil. 
  • Eluviation. This is mechanical wash down of fine mineral particles like those of clay from the upper layer of the soil into middle of lower layers. The materials are moved in suspension by water which is percolating downward . Eluviation can take place in the the same layer when the water percolate horizontally. The upper layer from which materials have been eluviated is called eluvial zone
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  • Leaching . This is the removal of soluble mineral matter in the solution from the upper horizon (layer) to lower horizon of soil. This process can also be referred to as chemical eluviation . it is common in wet climates. Leaching causes the top soil to be deficient in minerals , especially the bases because since they easily dissolve in water.
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  •    Illuviation. This is deposition or accumulation of materials that has been washed down from the upper layer to the lower horizon of the the soil trough the process of eluviation . Illuviaton sometimes leads to the formation of hard pan 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 ordinary optical microscope. The tropical soils of plateau north of lake Victoria in east Africa are characterized by this hard pan.
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  • Precipitation. This is the formation of solid matters in in the the sub soil. the the solid matters 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 downward movement of materials in the soil which is very similar to leaching . however cheluviation occur through the influence of organic agents which are also referred to as chelating agents . The process involve plant acids rather than mere water as the case with leaching.

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    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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